Energy Recovery

First Edition, 2011

ISBN 978-93-81157-51-0

© All rights reserved.

Published by: The English Press 4735/22 Prakashdeep Bldg, Ansari Road, Darya Ganj, Delhi - 110002 Email: [email protected] 

Table of Contents Chapter 1- Introduction to Energy Recovery Chapter 2 - Regenerative Brake Chapter 3 - Energy Recovery Ventilation Chapter 4 - Energy Recycling Chapter 5 - Heat Recovery Steam Generator Chapter 6 - Waste Heat Recovery Unit Chapter 7 - Heat Recovery Ventilation Chapter 8 - Other Energy Recovery Systems & Techniques

Chapter- 1

Introduction to Energy Recovery

Energy recovery includes any technique or method of minimizing the input of energy to an overall system by the exchange of energy from one sub-system of the overall system with another. The energy can be in any form in either subsystem, but most energy recovery systems exchange thermal energy in either sensible or latent form.

Principle A common utilization of this principle is in systems which have an exhaust stream or waste stream which is transferred from the system to its surroundings. Some of the energy in that flow of material (often gasesous or liquid) may be transferred to the makeup or input material flow. This input mass flow often comes from the system's surroundings, which, being at ambient conditions, are at a lower temperature than the waste stream. This temperature differential allows heat transfer and thus energy transfer, or in this case, recovery. Thermal energy is often recovered from liquid or gaseous waste streams to fresh make-up air and water intakes in buildings, such as for the HVAC systems, or process systems.

System approach Energy consumption is a key part of most human activities. This consumption involves converting one energy system to another, for example: The conversion of mechanical energy to electrical energy, which can then power computers, light, motors etc. The input energy propels the work and is mostly converted to heat or follows the product in the process as output energy. Energy recovery systems harvest the output power and providing this as input power to the same or another process. An energy recovery system will close this energy cycle to prevent the input power from being released back to nature and rather be used in other forms of desired work.

Examples of energy recovery

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Heat recovery is implemented in heat sources like e.g. a steel mill. Heated cooling water from the process is sold for heating of homes, shops and offices in the surrounding area. Regenerative brake is used in electric cars, trains, heavy cranes etc where the energy consumed when elevating the potential is returned to the electric supplier when released. Active pressure reduction systems where the differentioal pressure in a preassurized fluid flow is recovered rather than converted to heat in a pressure reduction valve and released. Energy recovery ventilation Energy recycling Water heat recycling Heat recovery ventilation Heat recovery steam generator Heat Regenerative Cyclone Engine Hydrogen turboexpander-generator Thermal diode Thermal oxidizer Thermoelectric Modules Waste heat recovery units

Environmental impact There is a large potential for energy recovery in compact systems like large industries and utilities. Together with Energy conservation it should be possible to dramatically reduce the world energy consumption. The effect of this will then be: • • • • • •

Reduced number of coal fired power plants Reduced airborne particles, NOx and CO2 - improved air quality Slowing or reducing climate change Lower fuel bills on transport Longer availability of crude oil Change of industries and economies not fully researched

In 2008 Tom Casten, chairman of Recycled Energy Development, said that "We think we could make about 19 to 20 percent of U.S. electricity with heat that is currently thrown away by industry." A 2007 Department of Energy study found the potential for 135,000 megawatts of combined heat and power (which uses energy recovery) in the U.S., and a Lawrence Berkley National Laboratory study identified about 64,000 megawatts that could be obtained from industrial waste energy, not counting CHP. These studies suggest about 200,000 megawatts—or 20% -- of total power capacity that could come from energy recycling in the U.S. Widespread use of energy recycling could therefore reduce global warming emissions by an estimated 20 percent. Indeed, as of 2005, about 42 percent of

U.S. greenhouse gas pollution came from the production of electricity and 27 percent from the production of heat. It is, however, difficult to quantify the environmental impact of a global energy recovery implementation in some sectors. The main impediments are •

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Lack of efficient technologies for private homes. Heat recovery systems in private homes can have an efficiency as low as 30% or less. It may be more realistic to use energy conservation like insulation or improved buildings. Many areas are more dependant on forced cooling and a system for extracting heat from dwellings to be used for other uses are not widely available. Ineffective infrastructure. Heat recovery in particular need a short distance from producer to consumer to be viable. A solution may be to move a large consumer to the vicinity of the producer. This may have other complications. Transport sector not ready. With the transport sector using about 20% of the energy supply, most of the energy is spent on overcoming gravity and friction. Electric cars seems with regenerative breaking seems to be the best candidate for energy recovery. Wind systems on ships is under development. Very little work on the airline industry is known in this field.

Chapter- 2

Regenerative Brake

Mechanism for regenerative brake on the roof of a Škoda Astra tram.

A regenerative brake is an energy recovery mechanism which slows a vehicle by converting its kinetic energy into another form, which can be either used immediately or stored until needed. This contrasts with conventional braking systems, where the excess kinetic energy is converted to heat by friction in the brake linings and therefore wasted. The most common form of regenerative brake involves using an electric motor as an electric generator. In electric railways the generated electricity is fed back into the supply system, whereas in battery electric and hybrid electric vehicles, the energy is stored in a battery or bank of capacitors for later use. Energy may also be stored by compressing air or in a rotating flywheel.

The motor as a generator Vehicles driven by electric motors use the motor as a generator when using regenerative braking: it is operated as a generator during braking and its output is supplied to an electrical load; the transfer of energy to the load provides the braking effect. Regenerative braking is used on hybrid gas/electric automobiles to recoup some of the energy lost during stopping. This energy is saved in a storage battery and used later to power the motor whenever the car is in electric mode. Early examples of this system were the front-wheel drive conversions of horse-drawn cabs by Louis Antoine Krieger (1868-1951). The Krieger electric landaulet had a drive motor in each front wheel with a second set of parallel windings (bifilar coil) for regenerative braking. An Energy Regeneration Brake was developed in 1967 for the AMC Amitron. This was a completely battery powered urban concept car whose batteries were recharged by regenerative braking, thus increasing the range of the automobile. Many modern hybrid and electric vehicles use this technique to extend the range of the battery pack. Examples include the Toyota Prius, Honda Insight, the Vectrix electric maxi-scooter, and the Chevrolet Volt.

Limitations Traditional friction-based braking is used in conjunction with mechanical regenerative braking for the following reasons: •

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The regenerative braking effect drops off at lower speeds; therefore the friction brake is still required in order to bring the vehicle to a complete halt. Physical locking of the rotor is also required to prevent vehicles from rolling down hills. The friction brake is a necessary back-up in the event of failure of the regenerative brake.

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Most road vehicles with regenerative braking only have power on some wheels (as in a two-wheel drive car) and regenerative braking power only applies to such wheels, so in order to provide controlled braking under difficult conditions (such as in wet roads) friction based braking is necessary on the other wheels. The amount of electrical energy capable of dissipation is limited by either the capacity of the supply system to absorb this energy or on the state of charge of the battery or capacitors. No regenerative braking effect can occur if another electrical component on the same supply system is not currently drawing power and if the battery or capacitors are already charged. For this reason, it is normal to also incorporate dynamic braking to absorb the excess energy. Under emergency braking it is desirable that the braking force exerted be the maximum allowed by the friction between the wheels and the surface without slipping, over the entire speed range from the vehicle's maximum speed down to zero. The maximum force available for acceleration is typically much less than this except in the case of extreme high-performance vehicles. Therefore, the power required to be dissipated by the braking system under emergency braking conditions may be many times the maximum power which is delivered under acceleration. Traction motors sized to handle the drive power may not be able to cope with the extra load and the battery may not be able to accept charge at a sufficiently high rate. Friction braking is required to absorb the surplus energy in order to allow an acceptable emergency braking performance.

For these reasons there is typically the need to control the regenerative braking and match the friction and regenerative braking to produce the desired total braking output. The GM EV-1 was the first commercial car to do this. Engineers Abraham Farag and Loren Majersik were issued two patents for this 'brake-by-wire' technology.

Electric railway vehicle operation During braking, the traction motor connections are altered to turn them into electrical generators. The motor fields are connected across the main traction generator (MG) and the motor armatures are connected across the load. The MG now excites the motor fields. The rolling locomotive or multiple unit wheels turn the motor armatures, and the motors act as generators, either sending the generated current through onboard resistors (dynamic braking) or back into the supply (regenerative braking). For a given direction of travel, current flow through the motor armatures during braking will be opposite to that during motoring. Therefore, the motor exerts torque in a direction that is opposite from the rolling direction. Braking effort is proportional to the product of the magnetic strength of the field windings, times that of the armature windings. Savings of 17% are claimed for Virgin Trains Pendolinos. There is also less wear on friction braking components. The Delhi Metro saved around 90,000 tons of carbon dioxide (CO2) from being released into the atmosphere by regenerating 112,500

megawatt hours of electricity through the use of regenerative braking systems between 2004 and 2007. It is expected that the Delhi Metro will save over 100,000 tons of CO2 from being emitted per year once its phase II is complete through the use of regenerative braking. Another form of simple, yet effective regenerative braking is used on the London underground which is achieved by having small slopes leading up and down from stations. The train is slowed by the climb, and then leaves down a slope, so kinetic energy is converted to "stored" potential energy in the station.

Comparison of dynamic and regenerative brakes Dynamic brakes ("rheostatic brakes" in the UK), unlike regenerative brakes, dissipate the electric energy as heat by passing the current through large banks of variable resistors. Vehicles that use dynamic brakes include forklifts, Diesel-electric locomotives, and streetcars. This heat can be used to warm the vehicle interior, or dissipated externally by large radiator-like cowls to house the resistor banks. The main disadvantage of regenerative brakes when compared with dynamic brakes is the need to closely match the generated current with the supply characteristics and increased maintenance cost of the lines. With DC supplies, this requires that the voltage be closely controlled. Only with the development of power electronics has this been possible with AC supplies, where the supply frequency must also be matched (this mainly applies to locomotives where an AC supply is rectified for DC motors). A small number of mountain railways have used 3-phase power supplies and 3-phase induction motors. This results in a near constant speed for all trains as the motors rotate with the supply frequency both when motoring and braking.

Kinetic Energy Recovery Systems Kinetic Energy Recovery Systems (KERS) were used for the motor sport Formula One's 2009 season, and under development for road vehicles. However, KERS was abandoned for the 2010 Formula One season. The Formula One Teams that used Kinetic Energy Recovery Systems in the 2009 season are Ferrari, Renault, BMW, and McLaren. One of the main reasons that not all cars use KERS is because it adds an extra 25 kilograms of weight, while not adding to the total car weight, it does incur a penalty particularly seen in the qualifying rounds, as it raises the car's center of gravity, and reduces the amount of ballast that is available to balance the car so that it is more predictable when turning. FIA rules also limit the exploitation of the system. Eventually, during the season, Renault and BMW stopped using the system. Williams is developing a flywheel-KERS system. The concept of transferring the vehicle’s kinetic energy using Flywheel energy storage was postulated by physicist Richard Feynman in the 1950s and is exemplified in complex high end systems such as the Zytek, Flybrid, Torotrak and Xtrac used in F1 and simple,

easily manufactured and integrated differential based systems such as the Cambridge Passenger/Commercial Vehicle Kinetic Energy Recovery System (CPC-KERS) Xtrac and Flybrid are both licensees of Torotrak's technologies, which employ a small and sophisticated ancillary gearbox incorporating a continuously variable transmission (CVT). The CPC-KERS is similar as it also forms part of the driveline assembly. However, the whole mechanism including the flywheel sits entirely in the vehicle’s hub (looking like a drum brake). In the CPC-KERS, a differential replaces the CVT and transfers torque between the flywheel, drive wheel and road wheel.

Use in motor sport

History

A Flybrid Systems Kinetic Energy Recovery System The first of these systems to be revealed was the Flybrid. This system weighs 24 kg and has an energy capacity of 400 kJ after allowing for internal losses. A maximum power boost of 60 kW (81.6 PS, 80.4 HP) for 6.67 s is available. The 240 mm diameter flywheel weighs 5.0 kg and revolves at up to 64,500 rpm. Maximum torque is 18 Nm (13.3 ftlbs). The system occupies a volume of 13 litres.

Two minor incidents have been reported during testing of KERS systems in 2008. The first occurred when the Red Bull Racing team tested their KERS battery for the first time in July: it malfunctioned and caused a fire scare that led to the team's factory being evacuated. The second was less than a week later when a BMW Sauber mechanic was given an electric shock when he touched Christian Klien's KERS-equipped car during a test at the Jerez circuit.

FIA

A KERS flywheel Formula One have stated that they support responsible solutions to the world's environmental challenges, and the FIA allowed the use of 81 hp (60 kW; 82 PS) KERS in

the regulations for the 2009 Formula One season. Teams began testing systems in 2008: energy can either be stored as mechanical energy (as in a flywheel) or as electrical energy (as in a battery or supercapacitor). Due to high cost, FOTA teams agreed to drop KERS from the 2010 season onwards, but this is still an open issue as Williams F1 said it will use KERS in 2010 and changes to the regulations must be agreed by all teams. Vodafone McLaren Mercedes became the first team to win a F1 GP using a KERS equipped car when Lewis Hamilton won the Hungarian Grand Prix on July 26, 2009. Their second KERS equipped car finished fifth. At the following race Lewis Hamilton became the first driver to take pole position with a KERS car, his team mate, Heikki Kovalainen qualifying second. This was also the first instance of an all KERS front row. On August 30, 2009, Kimi Räikkönen won the Belgian Grand Prix with his KERS equipped Ferrari. It was the first time that KERS contributed directly to a race victory, with second placed Giancarlo Fisichella claiming "Actually, I was quicker than Kimi. He only took me because of KERS at the beginning". New rules for the 2011 F1 season raise the minimum weight limit of the car and driver by 20kg to 640kg. This is to prepare the way for a return of the KERS power-boost and energy storage systems which featured in 2009. Although KERS is still legal in F1, for the 2010 season all the teams have agreed not to use it. As of 2013 it is possible that F1 will require KERS again as FIA rules force F1 to use greener methods to power their cars. The proposal is to increase the power storage from 60KW to 120KW. The FIA release of the 2011 rule changes with both FOTA, FOM and the FIA have agreed that F1 will have KERS for the 2011 season at 80hp, with this value rising in 2013 to coincide with smaller engines. This is still optional as it was in the 2009 season and can be battery or flywheel based.

Autopart makers Bosch Motorsport Service is developing a KERS for use in motor racing. These electricity storage systems for hybrid and engine functions include a lithium-ion battery with scalable capacity or a flywheel, a four to eight kilogram electric motor [with a maximum power level of 60 kW (81 hp)], as well as the KERS controller for power and battery management. Bosch also offers a range of electric hybrid systems for commercial and light-duty applications.

Carmakers Automakers including Honda have been testing KERS systems. At the 2008 1000 km of Silverstone, Peugeot Sport unveiled the Peugeot 908 HY, a hybrid electric variant of the diesel 908, with KERS. Peugeot plans to campaign the car in the 2009 Le Mans Series season, although it will not be capable of scoring championship points. Vodafone McLaren Mercedes began testing of their KERS in September 2008 at the Jerez test track in preparation for the 2009 F1 season, although at that time it was not yet known if they would be operating an electrical or mechanical system. In November 2008 it was announced that Freescale Semiconductor would collaborate with McLaren Electronic Systems to further develop its KERS for McLaren's Formula One car from

2010 onwards. Both parties believed this collaboration would improve McLaren's KERS system and help the system filter down to road car technology. Toyota has used a supercapacitor for regeneration on Supra HV-R hybrid race car that won the 24 Hours of Tokachi race in July 2007.

Motorcycles KTM racing boss Harald Bartol has revealed that the factory raced with a secret Kinetic Energy Recovery System (KERS) fitted to Tommy Koyama's motorcycle during the 2008 season-ending 125cc Valencian Grand Prix. This was illegal and against the rules. so they were later banned from doing it afterwards.

Races Automobile Club de l'Ouest, the organizer behind the annual 24 Hours of Le Mans event and the Le Mans Series is currently "studying specific rules for LMP1 that will be equipped with a kinetic energy recovery system. " Peugeot was the first manufacturer to unveil a fully functioning LMP1 car in the form of the 908 HY at the 2008 Autosport 1000 km race at Silverstone.

Use in compressed air cars Regenerative brakes could be employed in compressed air cars to refill the air tank during braking.

Chapter- 3

Energy Recovery Ventilation

Energy Recovery Ventilation (ERV) is the energy recovery process of exchanging the energy contained in normally exhausted building or space air and using it to treat, precondition, the incoming outdoor ventilation air in residential and commercial HVAC systems. During the warmer seasons the system will pre-cool and dehumidify while humidifying and pre-heating in the cooler seasons. The benefit of using energy recovery is the ability to meet the ASHRAE ventilation & energy standards, while improving indoor air quality, and reducing total HVAC equipment capacity. This technology, as expected, has not only demonstrated an effective means of reducing energy cost and heating and cooling loads, but has allowed for the scaling down of equipment. Additionally, this system will allow for the indoor environment to maintain a relative humidity of an appealing 40% to 50% range. This range can be maintained under essentially all conditions. The only energy penalty is the power needed for the blower to overcome the pressure drop in the system.

Methods of transfer An Energy Recovery Ventilator (ERV) is a type of air-to-air heat exchanger that not only can transfer sensible heat but also latent heat. Since both temperature and moisture is transferred, ERVs can be considered total enthalpic devices. On the other hand, a Heat Recovery Ventilator (HRV) is limited to only transferring sensible heat. HRVs can be considered sensible only devices because they only exchange sensible heat. It should be known that all ERVs are HRVs, but not all HRVs are ERVs. Throughout the cooling season, the system works to cool and dehumidify the incoming, outside air. This is accomplished by the system simply taking the rejected heat and sending it into the exhaust airstream. Sequentially, this air cools the condenser coil at a lower temperature than if the rejected heat had not entered the exhaust airstream. During the heating seasons, the system works in reverse. Instead of discharging the heat into the exhaust airstream, the system draws heat from the exhaust airstream in order to pre-heat the incoming air. At this stage, the air passes through a primary unit and then into a space. With this type of system, it is normal, during the cooling seasons, for the exhaust

air to be cooler than the ventilation air and, during the heating seasons, warmer than the ventilation air. It is this reason the system works very efficiently and effectively. The Coefficient of Performance (COP) will increase as the conditions become more extreme (i.e., more hot and humid for cooling and colder for heating).

Efficiency The efficiency of an ERV system is the ratio of energy transferred between the two air streams compared with the total energy transported through the heat exchanger. With the variety of products on the market, efficiency is unquestionably going to vary from product to product. Some of these systems have been known to have heat exchange efficiencies as high as 70-80% while others have as low as 50%. Even though this lower figure is preferable to the basic HVAC system, it is not up to par with the rest of its class. Studies are being done to increase the heat transfer efficiency to 90%. The use of modern low-cost gas-phase heat exchanger technology will allow for significant improvements in efficiency. The use of high conductivity porous material is believed to produce an exchange effectiveness in excess of 90%. By exceeding a 90% effective rate, an improvement of up to 5 factors in energy loss can be seen. The Home Ventilation Institute (HVI) has developed a standard test for any and all unites manufactured within the United States. Regardless, not all have been tested. It is imperative to investigate efficiency claims, comparing data produced by HVI as well as that produced by the manufacturer. (Note: all unites sold in Canada are placed through the R-2000 program, a standard test synonymous to the HVI test).

Types of energy recovery devices Energy Recovery Devices Type of Transfer Rotary Enthalpy Wheel Total & Sensible Fixed Plate Total** & Sensible Heat Pipe Sensible Run Around Loop Sensible Thermosiphon Sensible Twin Towers Sensible **Total Energy Exchange only available on Hygroscopic units and Condensate Return units

Rotary air-to-air enthalpy wheel

The rotating wheel heat exchanger is composed of a rotating cylinder filled with an air permeable material resulting in a large surface area. The surface area is the medium for the sensible energy transfer. As the wheel rotates between the ventilation and exhaust air streams it picks up heat energy and releases it into the colder air stream. The driving force behind the exchange is the difference in temperatures between the opposing air streams which is also called the thermal gradient. Typical media used consists of polymer, aluminum, and synthetic fiber. The Enthalpy Exchange is accomplished through the use of desiccants. Desiccants transfer moisture through the process of adsorption which is predominately driven by the difference in the partial pressure of vapor within the opposing air-streams. Typical desiccants consist of Silica Gel, and molecular sieves. Though very effective in its energy recovery, rotary enthalpy wheels have the common characteristic of high static pressures and poor durability. Therefore they are not as practical for energy savings purposes, and should only be considered for a cheaper alternative - in comparison to other ERVs - for situations where increased fresh outdoor ventilation is required. High static pressures result in increased fan power lowering the net energy savings of an installation. As for durability, rotary enthalpy wheels are normally guaranteed for no longer than 1 year, and the characteristic lifetime is about 5 years. Some companies, like the 1983-started finnish Enervent Oy, provide warranties for two years which is extendable up to five years for ERVs with integrated heat pump. There are additional disadvantages to the use of the heat wheel. Initial costs are higher due to the power needed for the fan to overcome its resistance. The system demands that the two air streams be adjacent to one another, they consistently be maintained, and they have filtration. Weekly maintenance is focused on the required rotating mechanism. Further up-keeping of the fill medium is also required. Colder climates may call for an increase in services. Caution must be taken when providing all upkeep, specifically if additional services are required, because cross-contamination (of air streams) can occur.

Plate heat exchanger Fixed plate heat exchangers have no moving parts. Plates consist of alternating layers of plates that are separated and sealed. Typical flow is cross current and since the majority of plates are solid and non permeable, sensible only transfer is the result. The tempering of incoming fresh air is done by a heat or energy recovery core. In this case, the core is made of aluminum or plastic plates. Humidity levels are adjusted through the transferring of water vapor. This is done with a rotating wheel either containing a desiccant material or permeable plates. Enthalpy plates were introduced 2006 by Paul, a special company for ventilation systems for passive houses. A crosscurrent countercurrent air to air heat exchanger built with a humidity permeable material. Polymer fixed-plate countercurrent energy recovery ventilators were introduced in 1998 by Building Performance Equipment (BPE), a

residential, commercial, and industrial air-to-air energy recovery manufacturer. These heat exchangers can be both introduced as a retrofit for increased energy savings and fresh air as well as an alternative to new construction. In new construction situations, energy recovery will effectively reduce the required heating/cooling capacity of the system. The percentage of the total energy saved will depend on the efficiency of the device (up to 90%) and the latitude of the building. A result of an ERV is that the HVAC install's initial cost is lower and the overall energy consumed by the building is lower as well.

Chapter- 4

Energy Recycling

Energy recycling is the energy recovery process of utilizing energy that would normally be wasted, usually by converting it into electricity or thermal energy. Undertaken at manufacturing facilities, power plants, and large institutions such as hospitals and universities, it significantly increases efficiency, thereby reducing energy costs and greenhouse gas pollution simultaneously. The process is noted for its potential to mitigate global warming profitably. This work is usually done in the form of combined heat and power (also called cogeneration) or waste heat recovery.

Forms of energy recycling Waste heat recovery is a process that captures excess heat that would normally be discharged at manufacturing facilities and converts it into electricity and steam. A "waste heat recovery boiler" contains a series of water-filled tubes placed throughout the area where heat is released. When high-temperature heat meets the boiler, steam is produced, which in turn powers a turbine that creates electricity. This process is similar to that of other fired boilers, but in this case, waste heat replaces a traditional flame. No fossil fuels are used in this process. Metals, glass, pulp and paper, silicon and other production plants are typical locations where waste heat recovery can be effective. Waste heat recovery from air conditioning is also used as an alternative to wasting heat to the atmosphere from chiller plants. Heat recovered in summer from chiller plants is stored in Thermalbanks in the ground and recycled back to the same building in winter via a heat pump to provide heating without burning fossil fuels. This elegant approach saves energy - and carbon - in both seasons by recycling summer heat for winter use. Cogeneration (also combined heat and power, CHP) is the use of a heat engine or a power station to simultaneously generate both electricity and useful heat. All power plants must emit a certain amount of heat during electricity generation. This can be into the natural environment through cooling towers, flue gas, or by other means. By contrast CHP captures some or all of the by-product heat for heating purposes, either very close to the plant, or—especially in Scandinavia and eastern Europe—as hot water

for district heating with temperatures ranging from approximately 80 to 130 °C. This is also called Combined Heat and Power District Heating or CHPDH. Small CHP plants are an example of decentralized energy. In the United States, Con Edison distributes 30 billion pounds of 350 °F/180 °C steam each year through its seven cogeneration plants to 100,000 buildings in Manhattan—the biggest steam district in the United States. The peak delivery is 10 million pounds per hour (corresponding to approx. 2.5 GW) This steam distribution system is the reason for the steaming manholes often seen in "gritty" New York movies. Other major cogeneration companies in the U.S. include Recycled Energy Development and leading advocates include Tom Casten and Amory Lovins. By-product heat at moderate temperatures (212-356°F/100-180°C) can also be used in absorption chillers for cooling. A plant producing electricity, heat and cold is sometimes called trigeneration or more generally: polygeneration plant. Cogeneration is a thermodynamically efficient use of fuel. In separate production of electricity some energy must be rejected as waste heat, but in cogeneration this thermal energy is put to good use.

Overview

Masnedø CHP power station in Denmark. This station burns straw as fuel. The adjacent greenhouses are heated by district heating from the plant. Thermal power plants (including those that use fissile elements or burn coal, petroleum, or natural gas), and heat engines in general, do not convert all of their thermal energy into electricity. In most heat engines, a bit more than half is lost as excess heat (see: Second law of thermodynamics and Carnot's theorem). By capturing the excess heat, CHP uses heat that would be wasted in a conventional power plant, potentially reaching an efficiency of up to 89%, compared with 55% for the best conventional plants. This means that less fuel needs to be consumed to produce the same amount of useful energy.

Some tri-cycle plants have used a combined cycle in which several thermodynamic cycles produced electricity, and then a heating system was used as a condenser of the power plant's bottoming cycle. For example, the RU-25 MHD generator in Moscow heated a boiler for a conventional steam powerplant, whose condensate was then used for space heat. A more modern system might use a gas turbine powered by natural gas, whose exhaust powers a steam plant, whose condensate provides heat. Tri-cycle plants can have thermal efficiencies above 80%. The viability of CHP (sometimes termed utilisation factor), especially in smaller CHP installations, depends upon a good baseload of operation, both in terms of an on-site (or near site) electrical demand and heat demand. In practice, an exact match between the heat and electricity needs rarely exists. A CHP plant can either meet the need for heat (heat driven operation) or be run as a power plant with some use of its waste heat. The latter being least advantageous in terms of its utilisation factor and thus overall efficiency. The viability can be greatly increased where opportunities for Trigeneration exist. In such cases the heat from the CHP plant is also used as a primary energy source to deliver cooling by means of an absorption chiller. CHP is most efficient when the heat can be used on site or very close to it. Overall efficiency is reduced when the heat must be transported over longer distances. This requires heavily insulated pipes, which are expensive and inefficient; whereas electricity can be transmitted along a comparatively simple wire, and over much longer distances for the same energy loss. A car engine becomes a CHP plant in winter, when the reject heat is useful for warming the interior of the vehicle. This example illustrates the point that deployment of CHP depends on heat uses in the vicinity of the heat engine. Cogeneration plants are commonly found in district heating systems of cities, hospitals, prisons, oil refineries, paper mills, wastewater treatment plants, thermal enhanced oil recovery wells and industrial plants with large heating needs. Thermally enhanced oil recovery (TEOR) plants often produce a substantial amount of excess electricity. After generating electricity, these plants pump leftover steam into heavy oil wells so that the oil will flow more easily, increasing production. TEOR cogeneration plants in Kern County, California produce so much electricity that it cannot all be used locally and is transmitted to Los Angeles. CHP is one of the most cost efficient methods of reducing carbon emissions of heating in cold climates.

Types of plants Topping cycle plants primarily produce electricity from a steam turbine. The exhausted steam is then condensed, and the low temperature heat released from this condensation is utilized for e.g. district heating or water desalination.

Bottoming cycle plants produce high temperature heat for industrial processes, then a waste heat recovery boiler feeds an electrical plant. Bottoming cycle plants are only used when the industrial process requires very high temperatures, such as furnaces for glass and metal manufacturing, so they are less common. Large cogeneration systems provide heating water and power for an industrial site or an entire town. Common CHP plant types are: • •

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Gas turbine CHP plants using the waste heat in the flue gas of gas turbines. The gaseous fuel used is typically natural gas Gas engine CHP plants (in the US "gaseous fuelled") use a reciprocating gas engine which is generally more competitive than a gas turbine up to about 5 MW. The gaseous fuel used is normally natural gas. These plants are generally manufactured as fully packaged units that can be installed within a plantroom or external plant compound with simple connections to the site's gas supply and electrical distribution and heating systems. Biofuel engine CHP plants use an adapted reciprocating gas engine or diesel engine, depending upon which biofuel is being used, and are otherwise very similar in design to a Gas engine CHP plant. The advantage of using a biofuel is one of reduced hydrocarbon fuel consumption and thus reduced carbon emissions. These plants are generally manufactured as fully packaged units that can be installed within a plantroom or external plant compound with simple connections to the site's electrical distribution and heating systems. Another variant is the wood gasifier CHP plant whereby a wood pellet or wood chip biofuel is gasified in a zero oxygen high temperature environment; the resulting gas is then used to power the gas engine. Combined cycle power plants adapted for CHP Steam turbine CHP plants that use the heating system as the steam condenser for the steam turbine. Molten-carbonate fuel cells have a hot exhaust, very suitable for heating. Nuclear Power

Smaller cogeneration units may use a reciprocating engine or Stirling engine. The heat is removed from the exhaust and the radiator. These systems are popular in small sizes because small gas and diesel engines are less expensive than small gas- or oil-fired steam-electric plants. Some cogeneration plants are fired by biomass, or industrial and municipal waste.

Heat Recovery Steam Generators A Heat Recovery Steam Generator or HRSG is a steam boiler that uses hot exhaust gases from the gas turbines or reciprocating engines in a CHP plant to heat up water and generate steam. This steam in turn drives a steam turbine and/or is used in industrial processes that require heat.

HRSGs used in the CHP industry are distinguished from conventional steam generators by the following main features: •

The HRSG is designed based upon the specific features of the gas turbine or reciprocating engine that it will be coupled to.

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Since the exhaust gas temperature is relatively low, heat transmission is accomplished mainly through convection.

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The exhaust gas velocity is limited by the need to keep head losses down. Thus, the transmission coefficient is low, which calls for a large heating surface area.

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Since the temperature difference between the hot gases and the fluid to be heated (steam or water) is low, and with the heat transmission coefficient being low as well, the evaporator and economizer are designed with plate fin heat exchangers.

Costs Typically for gas engined plant, the fully installed cost per kW electrical, is around £400/kW, which is comparable with large central power stations.

History

Cogeneration in Europe

A cogeneration thermal power plant in Ferrera Erbognone (PV), Italy Europe has actively incorporated cogeneration into its energy policy via the CHP Directive. In September 2008 at a hearing of the European Parliament’s Urban Lodgment Intergroup, Energy Commissioner Andris Piebalgs is quoted as saying, “security of supply really starts with energy efficiency.” Energy efficiency and cogeneration are recognized in the opening paragraphs of the European Union’s Cogeneration Directive 2004/08/EC. This directive intends to support cogeneration and establish a method for calculating cogeneration abilities per country. The development of cogeneration has been very uneven over the years and has been dominated throughout the last decades by national circumstances. As a whole, the European Union currently generates 11% of its electricity using cogeneration, saving Europe an estimated 35 Mtoe per annum a day. However, there is large difference between Member States with variations of the energy savings between 2% and 60%. Europe has the three countries with the world’s most intensive cogeneration economies: Denmark, the Netherlands and Finland. Other European countries are also making great efforts to increase their efficiency. Germany reported that at present, over 50% of the country’s total electricity demand could be provided through cogeneration. So far Germany has set the target to double its

electricity cogeneration from 12.5% of the country’s electricity to 25% of the country’s electricity by 2020 and has passed supporting legislation accordingly in “Federal Ministry of Economics and Technology, (BMWi), Germany, August 2007. The UK is also actively supporting combined heat and power. In light of UK’s goal to achieve a 60% reduction in carbon dioxide emissions by 2050, the government has set the target to source at least 15% of its government electricity use from CHP by 2010. Other UK measures to encourage CHP growth are financial incentives, grant support, a greater regulatory framework, and government leadership and partnership. According to the IEA 2008 modeling of cogeneration expansion for the G8 countries, expansion of cogeneration in France, Germany, Italy and the UK alone would effectively double the existing primary fuel savings by 2030. This would increase Europe’s savings from today’s 155.69 Twh to 465 Twh in 2030. It would also result in a 16% to 29% increase in each country’s total cogenerated electricity by 2030. Governments are being assisted in their CHP endeavors by organizations like COGEN Europe who serve as an information hub for the most recent updates within Europe’s energy policy. COGEN is Europe’s umbrella organization representing the interests of the cogeneration industry, users of the technology and promoting its benefits in the EU and the wider Europe. The association is backed by the key players in the industry including gas and electricity companies, ESCOs, equipment suppliers, consultancies, national promotion organisations, financial and other service companies.

Cogeneration in the United States

A 250 MW cogeneration plant in Cambridge, Massachusetts Perhaps the first modern use of energy recycling was done by Thomas Edison. His 1882 Pearl Street Station, the world’s first commercial power plant, was a combined heat and power plant, producing both electricity and thermal energy while using waste heat to warm neighboring buildings. Recycling allowed Edison’s plant to achieve approximately 50 percent efficiency. By the early 1900s, regulations emerged to promote rural electrification through the construction of centralized plants managed by regional utilities. These regulations not only promoted electrification throughout the countryside, but they also discouraged decentralized power generation, such as cogeneration. As Recycled Energy Development CEO Sean Casten testified to Congress, they even went so far as to make it illegal for non-utilities to sell power. By 1978, Congress recognized that efficiency at central power plants had stagnated and sought to encourage improved efficiency with the Public Utility Regulatory Policies Act (PURPA), which encouraged utilities to buy power from other energy producers.

The U.S. DOE has an aggressive goal of having CHP comprise of 20% of the US generation capacity by the year 2030. Eight Clean Energy Application Centers have been established across the nation whose mission is to develop the required technology application knowledge and educational infrastructure necessary to lead “clean energy” (Combined Heat and Power, Waste Heat Recovery and District Energy) technologies as viable energy options and reduce any perceived risks associated with their implementation. The focus of the Application Centers is to provide an outreach and technology deployment program for end users, policy makers, utilities, & industry stakeholders

Percentage of US energy produced by cogeneration Cogeneration plants proliferated, soon producing about 8 percent of all energy in the U.S. However, the bill left implementation and enforcement up to individual states, resulting in little or nothing being done in many parts of the country. In 2008 Tom Casten, chairman of Recycled Energy Development, said that "We think we could make about 19 to 20 percent of U.S. electricity with heat that is currently thrown away by industry." Outside the U.S., energy recycling is more common. Denmark is probably the most active energy recycler, obtaining about 55% of its energy from cogeneration and waste heat recovery. Other large countries, including Germany, Russia, and India, also obtain a much higher share of their energy from decentralized sources.

MicroCHP "Micro cogeneration" is a so called distributed energy resource (DER). The installation is usually less than 5 kWe in a house or small business. Instead of burning fuel to merely heat space or water, some of the energy is converted to electricity in addition to heat. This electricity can be used within the home or business or, if permitted by the grid management, sold back into the electric power grid. In a comparison by Claverton Energy Research Group, it was found that in the UK case where heat would otherwise be produced by burning fossil fuels in a boiler, micro cogeneration is a more cost effective mean of reducing CO2 emissions than photovoltaics.

MiniCHP "Mini cogeneration" is a so called distributed energy resource (DER). The installation is usually more than 5 kWe and less than 500 kWe in a building or medium sized business. In this size range the viability or utilisation factor of the CHP plant is very important to consider since it will greatly affect the efficiency and cost effectiveness (payback) of the CHP plant. The utilisation factor is essentially the calculated hours of operation of the CHP plant expressed as a percentage of the total number of hours in a year. If less than 40% then the application of CHP is considered to be unviable. To be viable a good baseload for electrical demand and heat demand must exist. Such baseloads arise where

building occupation or process activities are extended or continuous in operation. This typically includes for hospitals, prisons, manufacturing processes, swimming pools, airports, hotels, apartment blocks, etc. Current (2007) Micro- and MiniCHP installations use five different technologies: microturbines, internal combustion engines, stirling engines, closed cycle steam engines and fuel cells. One author indicates that MicroCHP based on Stirling engines is the most cost effective of the so called microgeneration technologies in abating carbon emissions; however, advances in reciprocation engine technology are adding efficiency to CHP plant, particularly in the biogas field. MiniCHP has a large role to play in the field of CO2 reduction from buildings where more than 14% of emissions can be saved by 2010 using CHP in buildings according to the author.

Current system Both waste heat recovery and CHP constitute "decentralized" energy production, which is in contrast to traditional "centralized" power generated at large power plants run by regional utilities. The “centralized” system has an average efficiency of 34 percent, requiring about three units of fuel to produce one unit of power. By capturing both heat and power, CHP and waste heat recovery projects have higher efficiencies. A 2007 Department of Energy study found the potential for 135,000 megawatts of CHP in the U.S., and a Lawrence Berkley National Laboratory study identified about 64,000 megawatts that could be obtained from industrial waste energy, not counting CHP. These studies suggest about 200,000 megawatts—or 20% -- of total power capacity that could come from energy recycling in the U.S. Widespread use of energy recycling could therefore reduce global warming emissions by an estimated 20 percent. Indeed, as of 2005, about 42 percent of U.S. greenhouse gas pollution came from the production of electricity and 27 percent from the production of heat. Advocates contend that recycled energy costs less and has lower emissions than most other energy options in current use.

History Perhaps the first modern use of energy recycling was done by Thomas Edison. His 1882 Pearl Street Station, the world’s first commercial power plant, was a CHP plant, producing both electricity and thermal energy while using waste heat to warm neighboring buildings. Recycling allowed Edison’s plant to achieve approximately 50 percent efficiency. By the early 1900s, regulations emerged to promote rural electrification through the construction of centralized plants managed by regional utilities. These regulations not only promoted electrification throughout the countryside, but they also discouraged

decentralized power generation, such as CHP. They even went so far as to make it illegal for non-utilities to sell power. By 1978, Congress recognized that efficiency at central power plants had stagnated and sought to encourage improved efficiency with the Public Utility Regulatory Policies Act (PURPA), which encouraged utilities to buy power from other energy producers. CHP plants proliferated, soon producing about 8 percent of all energy in the U.S. However, the bill left implementation and enforcement up to individual states, resulting in little or nothing being done in many parts of the country. In 2008 Tom Casten, chairman of Recycled Energy Development, said that "We think we could make about 19 to 20 percent of U.S. electricity with heat that is currently thrown away by industry." Outside the U.S., energy recycling is more common. Denmark is probably the most active energy recycler, obtaining about 55% of its energy from CHP and waste heat recovery. Other large countries, including Germany, Russia, and India, also obtain a much higher share of their energy from decentralized sources.

Chapter- 5

Heat Recovery Steam Generator

Modular HRSG A heat recovery steam generator or HRSG is an energy recovery heat exchanger that recovers heat from a hot gas stream. It produces steam that can be used in a process or used to drive a steam turbine.

General Usage A common application for an HRSG is in a combined-cycle power station, where hot exhaust from a gas turbine is fed to an HRSG to generate steam which in turn drives a steam turbine. This combination produces electricity more efficiently than either the gas turbine or steam turbine alone. Another application for an HRSG is in diesel engine combined cycle power plants, where hot exhaust from a diesel engine, as primary source of energy, is fed to an HRSG to generate steam which in turn drives a steam turbine. The HRSG is also an important component in cogeneration plants. Cogeneration plants typically have a higher overall efficiency in comparison to a combined cycle plant. This is due to the loss of energy associated with the steam turbine.

Modular HRSGs

Modular HRSG GA HRSGs consist of three major components: the Evaporator, Superheater, and Economizer. The different components are put together to meet the operating requirements of the unit. Modular HRSGs can be categorized by a number of ways such as direction of exhaust gases flow or number of pressure levels. Based on the flow of exhaust gases, HRSGs are categorized into vertical and horizontal types. In horizontal type HRSGs, exhaust gas

flows horizontally over vertical tubes whereas in vertical type HRSGs, exhaust gas flow vertically over horizontal tubes. Based on pressure levels, HRSGs can be categorized into single pressure and multi pressure. Single pressure HRSGs have only one steam drum and steam is generated at single pressure level whereas multi pressure HRSGs employ two (double pressure) or three (triple pressure) steam drums. As such triple pressure HRSGs consist of three sections: an LP (low pressure) section, a reheat/IP (intermediate pressure) section, and an HP (high pressure) section. Each section has a steam drum and an evaporator section where water is converted to steam. This steam then passes through superheaters to raise the temperature and pressure past the saturation point.

Packaged HRSGs Packaged HRSGs are designed to be shipped as a fully assembled unit from the factory. They can be used in waste heat or turbine (usually under 20 MW) applications. The packaged HRSG can have a water cooled furnace which allows for higher supplemental firing and better overall efficiency.

Variations Some HRSGs include supplemental, or duct firing. These additional burners provide additional energy to the HRSG, which produces more steam and hence increases the output of the steam turbine. Generally, duct firing provides electrical output at lower capital cost. It is therefore often utilized for peaking operations. HRSGs can also have diverter valves to regulate in the inlet flow into the HRSG. This allows the gas turbine to continue to operate when there is no steam demand or if the HRSG needs to be taken offline. Emissions controls may also be located in the HRSG. Some may contain a Selective Catalytic Reduction system to reduce nitrogen oxides (a large contributor to the formation of smog and acid rain) and/or a catalyst to remove carbon monoxide. The inclusion of an SCR dramatically effects the layout of the HRSG. NOx catalyst performs best in temperatures between 650 °F (340 °C) and 750 °F (400 °C). This usually means that the evaporator section of the HRSG will have to be split and the SCR placed in between the two sections. Some low temperature NOx catalysts have recently come to market that allows for the SCR to be placed between the Evaporator and Economizer sections (350 °F - 500 °F (175 °C - 260 °C)).

Packaged HRSG

Once-Through Steam Generators (OTSG) A specialized type of HRSG without boiler drums is the Once Through Steam Generator. In this design, the inlet feedwater follows a continuous path without segmented sections for economizers, evaporators and superheaters. This provides a high degree of flexibility as the sections are allowed to grow or contract based on the heat load being received from the gas turbine. The absence of drums allows for quick changes in steam production and fewer variables to control, and is ideal for cycling and base load operation.

Applications • • •

•

Heat recovery can be used extensively in energy projects. In the energy-rich Persian Gulf region, the steam from the HRSG is used for desalination plants. Universities are ideal candidates for HRSG applications. They can use a gas turbine to produce high reliability electricity for campus use. The HRSG can recover the heat from the gas turbine to produce steam/hot water for district heating or cooling. Large ocean vessels (e.g. Emma Maersk) make use of heat recovery

Manufacturers • •

Victory Energy () Nooter/Eriksen ()

• • • • • • • • • • •

NEM () CMI Energy () Doosan Heavy Industries & Construction () BHEL 3DCAD (I) PVT LTD () Thermax Ltd () Aalborg Engineering A/S () DKME () Innovative Steam Technologies () Larsen & Toubro Ltd () Rentech Boiler Inc.

Block diagram

Block diagram of IGCC power plant, which utilizes the HRSG

Chapter- 6

Waste Heat Recovery Unit

A waste heat recovery unit (WHRU) is an energy recovery heat exchanger that recovers heat from hot streams with potential high energy content, such as hot flue gases from a diesel generator or steam from cooling towers or even waste water from different cooling processes such as in steel cooling.

Principle Heat recovery units Waste heat found in the exhaust gas of various processes or even from the exhaust stream of a conditioning unit can be used to preheat the incoming gas. This is one of the basic methods for recovery of waste heat. Many steel making plants use this process as an economic method to increase the production of the plant with lower fuel demand. There are many different commercial recovery units for the transferring of energy from hot medium space to lower one: 1. Recuperators: This name is given to different types of heat exchanger that the exhaust gases are passed through, consisting of metal tubes that carry the inlet gas and thus preheating the gas before entering the process. The heat wheel is an example which operates on the same principle as a solar air conditioning unit. 2. Regenerators: This is an industrial unit that reuses the same stream after processing. In this type of heat recovery, the heat is regenerated and reused in the process. 3. Heat pipe exchanger: Heat pipes are one of the best thermal conductors. They have the ability to transfer heat hundred times more than copper. Heat pipes are mainly known in renewable energy technology as being used in evacuated tube collectors. The heat pipe is mainly used in space, process or air heating, in waste heat from a process is being transferred to the surrounding due to its transfer mechanism.

4. Economizer: In case of process boilers, waste heat in the exhaust gas is passed along a recuperator that carries the inlet fluid for the boiler and thus decreases thermal energy intake of the inlet fluid. 5. Heat pumps: Using an organic fluid that boils at a low temperature means that energy could be regenerated from waste fluids.

Recuperator A recuperator is a special purpose counter-flow energy recovery heat exchanger used to recover waste heat from exhaust gases. In many types of processes, combustion is used to generate heat, and the recuperator serves to recuperate, or reclaim this heat, in order to reuse or recycle it. The term recuperator refers as well to liquid-liquid counterflow heat exchangers used for heat recovery in the chemical and refinery industries and in closed processes such as ammonia-water or LiBr-water absorption refrigeration cycles. Other forms of heat or enthalpy recovery include the regenerative heat exchanger, the heat wheel, and the enthalpy wheel. Recuperators are often used in association with the burner portion of a heat engine, to increase the overall efficiency. For example, in a gas turbine engine, air is compressed, mixed with fuel, which is then burned and used to drive a turbine. The recuperator transfers some of the waste heat in the exhaust to the compressed air, thus preheating it before entering the fuel burner stage. Since the gases have been pre-heated, less fuel is needed to heat the gases up to the turbine inlet temperature. By recovering some of the energy usually lost as waste heat, the recuperator can make a heat engine or gas turbine significantly more efficient.

Rotating recuperator During the automotive industry's interest in gas turbines for vehicle propulsion (around 1965), Chrysler invented a unique recuperator that consisted of a rotary drum constructed from corrugated metal (similar in appearance to corrugated cardboard). This drum was continuously rotated by reduction gears driven by the turbine. The hot exhaust gasses were directed through a portion of the device, which would then rotate to a section that conducted the induction air, where this intake air was heated. This recovery of the heat of combustion significantly increased the efficiency of the turbine engine. This engine proved impractical for an automotive application due to its poor low-rpm torque. Even such an efficient engine, if large enough to deliver the proper performance, would have a low average fuel economy. Such an engine may at some future time be attractive when combined with an electric motor in a hybrid vehicle owing to its robust longevity and an ability to burn a wide variety of liquid fuels.

Regenerative heat exchanger

A regenerative heat exchanger, or more commonly a regenerator, is a type of heat exchanger where the flow through the heat exchanger is cyclical and periodically changes direction. It is similar to a countercurrent heat exchanger. However, a regenerator mixes the two fluid flows while a countercurrent exchanger maintains them separated. The temperature profile remains at a nearly constant temperature, and this includes the fluid entering and exiting each end.

Five Cowper's regenerative heat exchanger placed in series.

Regenerative heat exchangers use counter-current exchange to minimize loss of heat while permitting flow. In regenerative heat exchangers, the fluid on either side of the heat exchanger is nearly always the same fluid. The fluid is cycled through the heat exchanger, often reaching high temperatures. The fluid may go through an external processing step, and then it is flowed back through the heat exchanger in the opposite direction for further processing. Usually the application will use this process cyclically or repetitively. Thus, in

regenerative heat exchangers, a fluid incoming to a process is heated using the energy contained in the fluid exiting this process. The regenerative heat exchanger gives a considerable net savings in energy, since most of the heat energy is reclaimed nearly in a thermodynamically reversible way. This type of heat exchanger can have a thermal efficiency of over 90%, transferring almost all the relative heat energy from one flow direction to the other. Only a small amount of extra heat energy needs to be added at the hot end, and dissipated at the cold end, even to maintain very high or very low temperatures.

History The regenerator was invented by Rev. Robert Stirling in 1816, and is commonly found as a component of his Stirling engine. The simplest Stirlings, and most models, use a less efficient but simpler to construct, displacer instead.

Types of regenerators In rotary regenerators the matrix rotates continuously through two counter-flowing streams of fluid. In this way, the two streams are mostly separated but the seals are generally not perfect. Only one stream flows through each section of the matrix at a time; however, over the course of a rotation, both streams eventually flow through all sections of the matrix in succession. Each portion of the matrix will be nearly isothermal, since the rotation is perpendicular to both the temperature gradient and flow direction, and not through them. The two fluid streams flow counter-current. The fluid temperatures vary across the flow area; however the local stream temperatures are not a function of time. In a fixed matrix regenerator, a single fluid stream has cyclical, reversible flow; it is said to flow "counter-current". This regenerator may be part of a valveless system, such as a Stirling engine. In another configuration, the fluid is ducted through valves to different matrices in alternate operating periods Ph and Pc resulting in outlet temperatures that vary with time. Another type of regenerator is called a micro scale regenerative heat exchanger. It has a multilayer grating structure in which each layer is offset from the adjacent layer by half a cell which has an opening along both axes perpendicular to the flow axis. Each layer is a composite structure of two sublayers, one of a high thermal conductivity material and another of a low thermal conductivity material. When a hot fluid flows through the cell, heat from the fluid is transferred to the cell wells, and stored there. When the fluid flow reverses direction, heat is transferred from the cell walls back to the fluid. A third type of regenerator is called a "Rothemuhle" regenerator. This type has a fixed matrix in a disk shape, and streams of fluid are ducted through rotating hoods. The Rothemuhle regenerator is used as an air preheater in some power generating plants. The thermal design of this regenerator is the same as of other types of regenerators.

Biology We use our nose and throat as a regenerative heat exchanger when we breathe. The cooler air coming in is warmed, so that it reaches the lungs as warm air. On the way back out, this warmed air deposits much of its heat back onto the sides of the nasal passages, so that these passages are then ready to warm the next batch of air coming in.

Cryogenics Regenerator heat exchangers are made up of materials with high volumetric heat capacity and low thermal conductivity in the longitudinal (flow) direction. At cryogenic (very low) temperatures around 20 K, the specific heat of metals are low, and so a regenerator must be larger for a given heat load.

Advantages of regenerators The advantages of a regenerator over a recuperating (counter-flowing) heat exchanger is that it has a much higher surface area for a given volume, which provides a reduced exchanger volume for a given energy density, effectiveness and pressure drop. This makes a regenerator more economical in terms of materials and manufacturing, compared to an equivalent recuperator. The design of inlet and outlet headers used to distribute hot and cold fluids in the matrix is much simpler in counter flow regenerators than recuperators. The reason behind this is that both streams flow in different sections for a rotary regenerator and one fluid enters and leaves one matrix at a time in a fixed-matrix regenerator. Furthermore flow sectors for hot and cold fluids in rotary regenerators can be designed to optimize pressure drop in the fluids. The matrix surfaces of regenerators also have self-cleaning characteristics, reducing fluid-side fouling and corrosion. Finally properties such as small surface density and counter-flow arrangement of regenerators make it ideal for gas-gas heat exchange applications requiring effectiveness exceeding 85%. The heat transfer coefficient is much lower for gases than for liquids, thus the enormous surface area in a regenerator greatly increases heat transfer.

Disadvantages of regenerators The major disadvantage of a regenerator is that there is always some mixing of the fluid streams, and they can not be completely separated. There is an unavoidable carryover of a small fraction of one fluid stream into the other. In the rotary regenerator, the carryover fluid is trapped inside the radial seal and in the matrix, and in a fixed-matrix regenerator, the carryover fluid is the fluid that remains in the void volume of the matrix. This small fraction will mix with the other stream in the following half-cycle. Therefore regenerators are only used when it is acceptable for the two fluid streams to be mixed. Mixed flow is common for gas-to-gas heat and/or energy transfer applications, and less common in

liquid or phase-changing fluids since fluid contamination is often prohibited with liquid flows.

Heat pipe

A laptop heat pipe system A heat pipe is a heat transfer mechanism that combines the principles of both thermal conductivity and phase transition to efficiently manage the transfer of heat between two solid interfaces. At the hot interface within a heat pipe, which is typically at a very low pressure, a liquid in contact with a thermally conductive solid surface turns into a vapor by absorbing the heat of that surface. The vapor condenses back into a liquid at the cold interface, releasing the latent heat. The liquid then returns to the hot interface through either capillary action or gravity action where it evaporates once more and repeats the cycle. In addition, the internal pressure of the heat pipe can be set or adjusted to facilitate the phase change depending on the demands of the working conditions of the thermally managed system.

Structure, design and construction

Diagram showing components and mechanism for a heat pipe containing a wick

Cut-away view of a 500 µm thick flat heat pipe, with a thin planar capillary (aqua colored)

Thin flat heat pipe (heat spreader) with remote heat sink and fan A typical heat pipe consists of a sealed pipe or tube made of a material with high thermal conductivity such as copper or aluminum at both hot and cold ends. A vacuum pump is used to remove all air from the empty heat pipe, and then the pipe is filled with a fraction of a percent by volume of working fluid (or coolant) chosen to match the operating temperature. Examples of such fluids include water, ethanol, acetone, sodium, or mercury. Due to the partial vacuum that is near or below the vapor pressure of the fluid, some of the fluid will be in the liquid phase and some will be in the gas phase. The use of a vacuum eliminates the need for the working gas to diffuse through any other gas and so the bulk transfer of the vapor to the cold end of the heat pipe is at the speed of the moving molecules. In this sense, the only practical limit to the rate of heat transfer is the speed with which the gas can be condensed to a liquid at the cold end. Inside the pipe's walls, an optional wick structure exerts a capillary pressure on the liquid phase of the working fluid. This is typically a sintered metal powder or a series of grooves parallel to the pipe axis, but it may be any material capable of exerting capillary pressure on the condensed liquid to wick it back to the heated end. The heat pipe may not need a wick structure if gravity or some other source of acceleration is sufficient to overcome surface tension and cause the condensed liquid to flow back to the heated end.

A heat pipe is not a thermosiphon, because there is no siphon. Thermosiphons transfer heat by single-phase convection. Heat pipes contain no mechanical moving parts and typically require no maintenance, though non-condensing gases (that diffuse through the pipe's walls, result from breakdown of the working fluid, or exist as impurities in the materials) may eventually reduce the pipe's effectiveness at transferring heat. This is significant when the working fluid's vapour pressure is low. The materials chosen depend on the temperature conditions in which the heat pipe must operate, with coolants ranging from liquid helium for extremely low temperature applications (2–4 K) to mercury (523–923 K) & sodium (873–1473 K) and even indium (2000–3000 K) for extremely high temperatures. The vast majority of heat pipes for low temperature applications use some combination of ammonia (213–373 K), alcohol (methanol (283–403 K) or ethanol (273–403 K)) or water (303–473 K) as working fluid. Since the heat pipe contains a vacuum, the working fluid will boil and hence take up latent heat at well below its boiling point at atmospheric pressure. Water, for instance, will boil at just above 273 K (0 degrees Celsius) and so can start to effectively transfer latent heat at this low temperature. The advantage of heat pipes over many other heat-dissipation mechanisms is their great efficiency in transferring heat. They are a fundamentally better heat conductor than an equivalent cross-section of solid copper (a heat sink alone, though simpler in design and construction, does not take advantage of the principle of matter phase transition). Some heat pipes have demonstrated a heat flux of more than 230 MW/m², nearly four times the heat flux at the surface of the sun. Active control of heat flux can be effected by adding a variable volume liquid reservoir to the evaporator section. Variable conductance heat pipes employ a large reservoir of inert immiscible gas attached to the condensing section. Varying the gas reservoir pressure changes the volume of gas charged to the condenser which in turn limits the area available for vapor condensation. Thus a wider range of heat fluxes and temperature gradients can be accommodated with a single design. A modified heat pipe with a reservoir having no capillary connection to the heat pipe wick at the evaporator end can also be used as a thermal diode. This heat pipe will transfer heat in one direction, acting as an insulator in the other.

Flat heat pipes Thin planar heat pipes (heat spreaders) have the same primary components as tubular heat pipes. These components are a hermetically sealed hollow vessel, a working fluid, and a closed-loop capillary recirculation system. Compared to a one-dimensional tubular heat pipe, the width of a two-dimensional heat pipe allows an adequate cross section for heat flow even with a very thin device. These

thin planar heat pipes are finding their way into “height sensitive” applications, such as notebook computers, and surface mount circuit board cores. It is possible to produce flat heat pipes as thin as 0.5 mm (thinner than a credit card).

Heat transfer

A heat sink (aluminium) with heat pipe (copper) Heat pipes employ evaporative cooling to transfer thermal energy from one point to another by the evaporation and condensation of a working fluid or coolant. Heat pipes rely on a temperature difference between the ends of the pipe, and cannot lower temperatures at either end beyond the ambient temperature (hence they tend to equalise the temperature within the pipe). When one end of the heat pipe is heated the working fluid inside the pipe at that end evaporates and increases the vapour pressure inside the cavity of the heat pipe. The latent heat of evaporation absorbed by the vaporisation of the working fluid reduces the temperature at the hot end of the pipe. The vapour pressure over the hot liquid working fluid at the hot end of the pipe is higher than the equilibrium vapour pressure over condensing working fluid at the cooler end of the pipe, and this pressure difference drives a rapid mass transfer to the condensing end where the excess vapour condenses, releases its latent heat, and warms the cool end of the pipe. Non-condensing gases (caused by contamination for instance) in the vapour impede the gas flow and reduce the effectiveness of the heat pipe, particularly at low temperatures, where vapour pressures are low. The velocity of molecules in a gas is approximately the speed of sound and in the absence of non condensing gases, this is the

upper velocity with which they could travel in the heat pipe. In practice, the speed of the vapour through the heat pipe is dependent on the rate of condensation at the cold end. The condensed working fluid then flows back to the hot end of the pipe. In the case of vertically-oriented heat pipes the fluid may be moved by the force of gravity. In the case of heat pipes containing wicks, the fluid is returned by capillary action. When making heat pipes, there is no need to create a vacuum in the pipe. One simply boils the working fluid in the heat pipe until the resulting vapour has purged the non condensing gases from the pipe and then seals the end. An interesting property of heat pipes is the temperature over which they are effective. Initially, it might be suspected that a water charged heat pipe would only work when the hot end reached the boiling point (100 °C) and steam was transferred to the cold end. However, the boiling point of water is dependent on absolute pressure inside the pipe. In an evacuated pipe, water will boil just slightly above its melting point (0 °C). The heat pipe will operate, therefore, when the hot end is just slightly warmer than the melting point of the working fluid. Similarly, a heat pipe with water as a working fluid can work well above the boiling point (100 °C), if the cold end is low enough in temperature to condense the fluid. The main reason for the effectiveness of heat pipes is the evaporation and condensation of the working fluid. The heat of vaporization greatly exceeds the sensible heat capacity. Using water as an example, the energy needed to evaporate one gram of water is equivalent to the amount of energy needed to raise the temperature of that same gram of water by 540 °C (hypothetically, if the water was under extremely high pressure so it didn't vaporize or freeze over this temperature range). Almost all of that energy is rapidly transferred to the "cold" end when the fluid condenses there, making a very effective heat transfer system with no moving parts.

Origins and research in the United States The general principle of heat pipes using gravity (commonly classified as two phase thermosiphons) dates back to the steam age. The modern concept for a capillary driven heat pipe was first suggested by R.S. Gaugler of General Motors in 1942 who patented the idea. The benefits of employing capillary action were independently developed and first demonstrated by George Grover at Los Alamos National Laboratory in 1963 and subsequently published in the Journal of Applied Physics in 1964. Grover noted in his notebook: "Heat transfer via capillary movement of fluids. The "pumping" action of surface tension forces may be sufficient to move liquids from a cold temperature zone to a high temperature zone (with subsequent return in vapor form using as the driving force, the difference in vapor pressure at the two temperatures) to be of interest in transferring heat from the hot to the cold zone. Such a closed system, requiring no external pumps, may be of particular interest in space reactors in moving heat from the reactor core to a radiating

system. In the absence of gravity, the forces must only be such as to overcome the capillary and the drag of the returning vapor through its channels." Between 1964 and 1966, RCA was the first corporation to undertake research and development of heat pipes for commercial applications (though their work was mostly funded by the US government). During the late 1960s NASA played a large role in heat pipe development by funding a significant amount of research on their applications and reliability in space flight following from Grover's suggestion. NASA’s attraction to heat pipe cooling systems was understandable given their low weight, high heat flux, and zero power draw. Their primary interest however was based on the fact that the system wouldn’t be adversely affected by operating in a zero gravity environment. The first application of heat pipes in the space program was in thermal equilibration of satellite transponders. As satellites orbit, one side is exposed to the direct radiation of the sun while the opposite side is completely dark and exposed to the deep cold of outer space. This causes severe discrepancies in the temperature (and thus reliability and accuracy) of the transponders. The heat pipe cooling system designed for this purpose managed the high heat fluxes and demonstrated flawless operation with and without the influence of gravity. The developed cooling system was the first description and usage of variable conductance heat pipes to actively regulate heat flow or evaporator temperature.

Corporate R&D Publications in 1967 and 1968 by Feldman, Eastman, & Katzoff first discussed applications of heat pipes to areas outside of government concern and that did not fall under the high temperature classification such as; air conditioning, engine cooling, and electronics cooling. These papers also made the first mentions of flexible, arterial, and flat plate heat pipes. 1969 publications introduced the concepts of the rotational heat pipe with its applications to turbine blade cooling and the first discussions of heat pipe applications to cryogenic processes. Starting in the 1980s Sony began incorporating heat pipes into the cooling schemes for some of its commercial electronic products in place of both forced convection and passive finned heat sinks. Initially they were used in tuners & amplifiers, soon spreading to other high heat flux electronics applications. During the late 1990s increasingly hot microcomputer CPUs spurred a threefold increase in the number of U.S. heat pipe patent applications. As heat pipes transferred from a specialized industrial heat transfer component to a consumer commodity most development and production moved from the U.S. to Asia. Modern CPU heat pipes are typically made from copper and use water as the working fluid.

Applications

Alaska pipeline support legs cooled by heat pipes to keep permafrost frozen. Grover and his colleagues were working on cooling systems for nuclear power cells for space craft, where extreme thermal conditions are found. Heat pipes have since been used extensively in spacecraft as a means for managing internal temperature conditions. Heat pipes are extensively used in many modern computer systems, where increased power requirements and subsequent increases in heat emission have resulted in greater demands on cooling systems. Heat pipes are typically used to move heat away from components such as CPUs and GPUs to heat sinks where thermal energy may be dissipated into the environment.

Solar Thermal Heat pipes are also being widely used in solar thermal water heating applications in combination with evacuated tube solar collector arrays. In these applications, distilled water is commonly used as the heat transfer fluid inside a sealed length of copper tubing that is located within an evacuated glass tube and oriented towards the sun. In solar thermal water heating applications, an evacuated tube collector can deliver up to 40% more efficiency compared to more traditional "flat plate" solar water heaters. Evacuated tube collectors eliminate the need for anti-freeze additives to be added as the vacuum helps prevent heat loss. These types of solar thermal water heaters are frost protected down to more than -3 °C and are being used in Antarctica to heat water.

Pipelines over permafrost Heat pipes are used to dissipate heat on the Trans-Alaska Pipeline System. Without them residual ground heat remaining in the oil, as well as that produced by friction and turbulence in the moving oil would conduct down the pipe's support legs. This would likely melt the permafrost on which the supports are anchored. This would cause the pipeline to sink and possibly sustain damage. To prevent this each vertical support member has been mounted with 4 vertical heat pipes.

Cooking Heat pipes have been designed to speed the cooking of roasts. The pipe is poked through the roast. One end of the pipe extends into the oven where it draws heat to the middle of the roast.

Limitations Heat pipes must be tuned to particular cooling conditions. The choice of pipe material, size and coolant all have an effect on the optimal temperatures in which heat pipes work. When heated above a certain temperature, all of the working fluid in the heat pipe will vaporize and the condensation process will cease to occur; in such conditions, the heat pipe's thermal conductivity is effectively reduced to the heat conduction properties of its solid metal casing alone. As most heat pipes are constructed of copper (a metal with high heat conductivity), an overheated heatpipe will generally continue to conduct heat at around 1/80 of the original conductivity. In addition, below a certain temperature, the working fluid will not undergo phase change, and the thermal conductivity will be reduced to that of the solid metal casing. One of the key criteria for the selection of a working fluid is the desired operational temperature range of the application. The lower temperature limit typically occurs a few degrees above the freezing point of the working fluid.

Most manufacturers cannot make a traditional heat pipe smaller than 3mm in diameter due to material limitations (though 1.6mm thin sheets can be fabricated). Experiments have been conducted with micro heat pipes, which use piping with sharp edges, such as triangular or rhombus-like tubing. In these cases, the sharp edges transfer the fluid through capillary action, and no wick is necessary.

Economizer Economizers (US), or economisers (UK/international), are mechanical devices intended to reduce energy consumption, or to perform another useful function like preheating a fluid. The term economizer is used for other purposes as well. Boiler, powerplant, and heating, ventilating, and air-conditioning (HVAC) uses are discussed here. In simple terms, an economizer is a heat exchanger.

Stirling engine Robert Stirling's innovative contribution to the design of hot air engines of 1816 was what he called the 'Economiser'. Now known as the regenerator, it stored heat from the hot portion of the engine as the air passed to the cold side, and released heat to the cooled air as it returned to the hot side. This innovation improved the efficiency of Stirling's engine enough to make it commercially successful in particular applications, and has since been a component of every air engine that is called a Stirling engine.

Boilers In boilers, economizers are heat exchange devices that heat fluids, usually water, up to but not normally beyond the boiling point of that fluid. Economizers are so named because they can make use of the enthalpy in fluid streams that are hot, but not hot enough to be used in a boiler, thereby recovering more useful enthalpy and improving the boiler's efficiency. They are a device fitted to a boiler which saves energy by using the exhaust gases from the boiler to preheat the cold water used to fill it (the feed water).

History The first successful design of economizer was used to increase the steam-raising efficiency of the boilers of stationary steam engines. It was patented by Edward Green in 1845, and since then has been known as Green's economizer. It consisted of an array of vertical cast iron tubes connected to a tank of water above and below, between which the boiler's exhaust gases passed. This is the reverse arrangement to that usually but not always seen in the fire tubes of a boiler; there the hot gases usually pass through tubes immersed in water, whereas in an economizer the water passes through tubes surrounded by hot gases. But this is but a detail. The important thing is this: In a boiler the burning gases heat the water to produce steam to drive an engine, whether piston or turbine. In an economizer, some of the heat energy that would otherwise all be lost to the atmosphere is

instead used to heat the water and/or air that will go into the boiler, thus saving fuel. The most successful feature of Green's design of economizer was its mechanical scraping apparatus, which was needed to keep the tubes free of deposits of soot. Economizers were eventually fitted to virtually all stationary steam engines in the decades following Green's invention. Some preserved stationary steam engine sites still have their Green's economizers although usually they are not used. One such preserved site is the Claymills Pumping Engines Trust in Staffordshire, England, which is in the process of restoring one set of economizers and the associated steam engine which drove them.

Powerplants Modern-day boilers, such as those in coal-fired power stations, are still fitted with economizers which are descendants of Green's original design. In this context they are often referred to as feedwater heaters and heat the condensate from turbines before it is pumped to the boilers. Economizers are commonly used as part of a heat recovery steam generator in a combined cycle power plant. In an HRSG, water passes through an economizer, then a boiler and then a superheater. The economizer also prevents flooding of the boiler with liquid water that is too cold to be boiled given the flow rates and design of the boiler. A common application of economizers in steam powerplants is to capture the waste heat from boiler stack gases (flue gas) and transfer it to the boiler feedwater. This raises the temperature of the boiler feedwater thus lowering the needed energy input, in turn reducing the firing rates to accomplish the rated boiler output. Economizers lower stack temperatures which may cause condensation of acidic combustion gases and serious equipment corrosion damage if care is not taken in their design and material selection.

HVAC Air-side economizers can save energy in buildings by using cool outside air as a means of cooling the indoor space. When the enthalpy of the outside air is less than the enthalpy of the recirculated air, conditioning the outside air is more energy efficient than conditioning recirculated air. When the outside air is both sufficiently cool and sufficiently dry (depending on the climate) the amount of enthalpy in the air is acceptable to the control, no additional conditioning of it is needed; this portion of the air-side economizer control scheme is called free cooling. Air-side economizers can reduce HVAC energy costs in cold and temperate climates while also potentially improving indoor air quality, but are most often not appropriate in hot and humid climates. With the appropriate controls economizers can be used in climates which experience various weather systems.

When the outside air's dry- and wet-bulb temperatures are low enough, water-side economizers use water cooled by a wet cooling tower or a dry cooler (also called fluid cooler) to cool buildings without operating a chiller. They are historically known as the strainer cycle, but the water-side economizer is not a true thermodynamic cycle. Also, instead of passing the cooling tower water through a strainer and then to the cooling coils, which causes their fouling, more often a plate-and-frame heat exchanger is inserted between the cooling tower and chilled water loops. Good controls, and valves or dampers, as well as maintenance, are needed to ensure proper operation of the air- and water-side economizers.

Refrigeration Another use of the term occurs in industrial refrigeration, specifically vapor-compression refrigeration. For example, for a walk-in freezer that is kept at -20°F, the main refrigeration components would include: an evaporator coil (a dense arrangement of pipes containing refrigerant and thin metal fins used to remove heat from inside the freezer), fans to blow air over the coil and around the box, an air-cooled condensing unit sited outdoors, and valves and piping. The condensing unit includes a compressor and a coil and fans to exchange heat with the ambient air. There are two types of economizers, flash and sub-cooling. A sub-cooling economizer is simply a type of heat exchanger that uses the cold gas leaving the evaporator coil to cool the high pressure liquid that is headed into the start of the evaporator coil via a thermal expansion valve. The gas is used to chill a chamber that has a series of pipes for the liquid running through it. The warmer gas then proceeds on to the compressor to be returned to a liquid form. The sub-cooling term refers to cooling the liquid below it's boiling point. 10 degrees of sub-cooling means it is 10 degrees colder than boiling at a given pressure. An example of the effect of the sub-cooler: 105°F liquid refrigerant leaving the compressor could be reduced to 50°F liquid.

Heat pump

Outdoor components of a residential air-source heat pump A heat pump is a machine or device that moves heat from one location (the 'source') at a lower temperature to another location (the 'sink' or 'heat sink') at a higher temperature using mechanical work or a high-temperature heat source. The difference between a heat pump and a normal air conditioner is that a heat pump can be used to provide heating or cooling. Even though the heat pump can heat, it still uses the same basic refrigeration cycle to do this. In other words a heat pump can change which coil is the condenser and which the evaporator. This is normally achieved by a reversing valve. In cooler climates it is common to have heat pumps that are designed only to provide heating. Common examples are food refrigerators and freezers, air conditioners, and reversiblecycle heat pumps for providing building space heating. In heating, ventilation, and air conditioning (HVAC) applications, a heat pump normally refers to a vapor-compression refrigeration device that includes a reversing valve and optimized heat exchangers so that the direction of heat flow may be reversed. Most commonly, heat pumps draw heat from the air or from the ground.

Overview Heat pumps have the ability to move heat energy from one environment to another, and in either direction. This allows the heat pump to both bring heat into an occupied space, and take it out. In the cooling mode a heat pump works the same as an ordinary air conditioner (A/C). A heat pump uses an intermediate fluid called a refrigerant which

absorbs heat as it vaporizes and releases the heat when it condenses. It uses an evaporator to absorb heat from inside an occupied space and rejects this heat to the outside through the condenser. The refrigerant flows outside of the space to be conditioned, where the condenser and compressor are located, while the evaporator is inside. The key component that makes a heat pump different from an A/C is the reversing valve. The reversing valve allows for the flow direction of the refrigerant to be changed. This allows the heat to be pumped in either direction. •

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In heating mode the outdoor coil becomes the evaporator, while the indoor becomes the condenser which absorbs the heat from the refrigerant and dissipates to the air flowing through it. The air outside even at 0 °C has heat energy in it. With the refrigerant flowing in the opposite direction the evaporator (outdoor coil) is absorbing the heat from the air and moving it inside. Once it picks up heat it is compressed and then sent to the condenser (indoor coil). The indoor coil then rejects the heat into the air handler, which moves the heated air through out the house. In cooling mode the outdoor coil is now the condenser. This makes the indoor coil now the evaporator. The indoor coil is now the evaporator in the sense that it is going to be used to absorb the heat from inside the enclosed space. The evaporator absorbs the heat from the inside, and takes it to the condenser where it is rejected into the outside air.

Operating principles Since the heat pump or refrigerator uses a certain amount of work to move the refrigerant, the amount of energy deposited on the hot side is greater than taken from the cold side. One common type of heat pump works by exploiting the physical properties of an evaporating and condensing fluid known as a refrigerant.

A simple stylized diagram of a heat pump's vapor-compression refrigeration cycle: 1) condenser, 2) expansion valve, 3) evaporator, 4) compressor. The working fluid, in its gaseous state, is pressurized and circulated through the system by a compressor. On the discharge side of the compressor, the now hot and highly pressurized vapor is cooled in a heat exchanger, called a condenser, until it condenses into a high pressure, moderate temperature liquid. The condensed refrigerant then passes through a pressure-lowering device also called a metering device like an expansion valve, capillary tube, or possibly a work-extracting device such as a turbine. The low pressure, liquid refrigerant leaving the expansion device enters another heat exchanger, the evaporator, in which the fluid absorbs heat and boils. The refrigerant then returns to the compressor and the cycle is repeated. In such a system it is essential that the refrigerant reach a sufficiently high temperature when compressed, since the second law of thermodynamics prevents heat from flowing from a cold fluid to a hot heat sink. Practically, this means the refrigerant must reach a temperature greater than the ambient around the high-temperature heat exchanger. Similarly, the fluid must reach a sufficiently low temperature when allowed to expand, or heat cannot flow from the cold region into the fluid, i.e. the fluid must be colder than the ambient around the cold-temperature heat exchanger. In particular, the pressure difference must be great enough for the fluid to condense at the hot side and still evaporate in the lower pressure region at the cold side. The greater the temperature difference, the greater the required pressure difference, and consequently the more energy needed to compress the fluid. Thus as with all heat pumps, the Coefficient of Performance (amount of heat moved per unit of input work required) decreases with increasing temperature difference. Insulation is used to reduce the work and energy required to achieve and maintain a lower temperature in the cooled space. Due to the variations required in temperatures and pressures, many different refrigerants are available. Refrigerators, air conditioners, and some heating systems are common applications that use this technology.

Heat sources Many heat pumps also use an auxiliary heat source for heating mode. This means that, even though the heat pump is the primary source of heat, another form is available as a back-up. Electricity, oil, or gas are the most common sources. This is put in place so that if the heat pump fails or can't provide enough heat, the auxiliary heat will kick on to make up the difference. Geothermal heat pumps use the ground and water to work as the condensers and evaporators. They work in the same manner as an air to air heat pump, but instead of indoor and outdoor coils they use the earth as natural evaporators and condensers. These

are very eco-friendly and are a cheaper alternative in the long run due to lower operating cost.

Applications In HVAC applications, a heat pump is typically a vapor-compression refrigeration device that includes a reversing valve and optimized heat exchangers so that the direction of heat flow may be reversed. The reversing valve switches the direction of refrigerant through the cycle and therefore the heat pump may deliver either heating or cooling to a building. In the cooler climates the default setting of the reversing valve is heating. The default setting in warmer climates is cooling. Because the two heat exchangers, the condenser and evaporator, must swap functions, they are optimized to perform adequately in both modes. As such, the efficiency of a reversible heat pump is typically slightly less than two separately optimized machines. In plumbing applications, a heat pump is sometimes used to heat or preheat water for swimming pools or domestic water heaters. In somewhat rare applications, both the heat extraction and addition capabilities of a single heat pump can be useful, and typically results in very effective use of the input energy. For example, when an air cooling need can be matched to a water heating load, a single heat pump can serve two useful purposes. That is, a heat pump domestic water heater located in the living area of a home could cool the home, reducing or eliminating the need for additional air conditioning. This installation would be best-suited to a climate that is warm or hot most of the year. Unfortunately, these situations are rare because the demand profiles for heating and cooling are often significantly different.

Refrigerants Until the 1990s, the refrigerants were often chlorofluorocarbons such as R-12 (dichlorodifluoromethane), one in a class of several refrigerants using the brand name Freon, a trademark of DuPont. Its manufacture was discontinued in 1995 because of the damage that CFCs cause to the ozone layer if released into the atmosphere. One widely adopted replacement refrigerant is the hydrofluorocarbon (HFC) known as R-134a (1,1,1,2-tetrafluoroethane). R-134a is not as efficient as the R-12 it replaced (in automotive applications) and therefore, more energy is required to operate systems utilizing R-134a than those using R-12. Other substances such as liquid R-717 ammonia are widely used in large-scale systems, or occasionally the less corrosive but more flammable propane or butane, can also be used. Since 2001, carbon dioxide, R-744, has increasingly been used, utilizing the transcritical cycle. In residential and commercial applications, the hydrochlorofluorocarbon (HCFC) R-22 is still widely used, however, HFC R-410A does not deplete the ozone layer and is being used more frequently. Hydrogen, helium, nitrogen, or plain air is used in the Stirling cycle, providing the maximum number of options in environmentally friendly

gases. More recent refrigerators are now exploiting the R600A which is isobutane, and does not deplete the ozone and is friendly to the environment. Dimethyl ether (DME) is also gaining popularity as a refrigerant.

Efficiency When comparing the performance of heat pumps, it is best to avoid the word "efficiency" which has a very specific thermodynamic definition. The term coefficient of performance (COP) is used to describe the ratio of useful heat movement to work input. Most vaporcompression heat pumps utilize electrically powered motors for their work input. However, in most vehicle applications, shaft work, via their internal combustion engines, provide the needed work. When used for heating a building on a mild day of say 10 °C, a typical air-source heat pump has a COP of 3 to 4, whereas a typical electric resistance heater has a COP of 1.0. That is, one joule of electrical energy will cause a resistance heater to produce one joule of useful heat, while under ideal conditions, one joule of electrical energy can cause a heat pump to move much more than one joule of heat from a cooler place to a warmer place. Note that the heat pump is more efficient on average in hotter climates than cooler ones, so when the weather is much warmer (in a desert city or southern city) the unit will perform better than average COP. Conversely in cold weather the COP approaches 1. Thus when there is a wide temperature differential on a hot day the COP is higher than average (better). When there is a high temperature differential on a cold day, e.g., when an air-source heat pump is used to heat a house on a very cold winter day of say 0 °C, it takes more work to move the same amount of heat indoors than on a mild day. Ultimately, due to Carnot efficiency limits, the heat pump's performance will approach 1.0 as the outdoor-to-indoor temperature difference increases for colder climates (temperature gets colder). This typically occurs around −18 °C (0 °F) outdoor temperature for air source heat pumps. Also, as the heat pump takes heat out of the air, some moisture in the outdoor air may condense and possibly freeze on the outdoor heat exchanger. The system must periodically melt this ice. In other words, when it is extremely cold outside, it is simpler, and wears the machine less, to heat using an electric-resistance heater than to strain an air-source heat pump. Geothermal heat pumps, on the other hand, are dependent upon the temperature underground, which is "mild" (typically 10 °C at a depth of more than 1.5m for the UK) all year round. Their COP is therefore is normally in the range of 4.0 to 5.0. The design of the evaporator and condenser heat exchangers is also very important to the overall efficiency of the heat pump. The heat exchange surface areas and the corresponding temperature differential (between the refrigerant and the air stream)

directly affect the operating pressures and hence the work the compressor has to do in order to provide the same heating or cooling effect. Generally the larger the heat exchanger the lower the temperature differential and the more efficient the system. Since heat exchangers are expensive, and the heat pump industry generally competes on price rather than efficiency, the drive towards more efficient heat pumps and air conditioners is often led by legislative measures on minimum efficiency standards. In cooling mode a heat pump's operating performance is described as its energy efficiency ratio (EER) or seasonal energy efficiency ratio (SEER), and both measures have units of BTU/(h·W) (1 BTU/(h·W) = 0.293 W/W). A larger EER number indicates better performance. The manufacturer's literature should provide both a COP to describe performance in heating mode and an EER or SEER to describe performance in cooling mode. Actual performance varies, however, and depends on many factors such as installation, temperature differences, site elevation, and maintenance. Heat pumps are more effective for heating than for cooling if the temperature difference is held equal. This is because the compressor's input energy is largely converted to useful heat when in heating mode, and is discharged along with the moved heat via the condenser. But for cooling, the condenser is normally outdoors, and the compressor's dissipated work is rejected rather than put to a useful purpose. For the same reason, opening a food refrigerator or freezer heats up the room rather than cooling it because its refrigeration cycle rejects heat to the indoor air. This heat includes the compressor's dissipated work as well as the heat removed from the inside of the appliance. The COP for a heat pump in a heating or cooling application, with steady-state operation, is:

where • • • •

ΔQcool is the amount of heat extracted from a cold reservoir at temperature Tcool, ΔQhot is the amount of heat delivered to a hot reservoir at temperature Thot, ΔA is the compressor's dissipated work. All temperatures are absolute temperatures usually measured in kelvins (K).

Types The two main types of heat pumps are compression heat pumps and absorption heat pumps. Compression heat pumps always operate on mechanical energy (through

electricity), while absorption heat pumps may also run on heat as an energy source (through electricity or burnable fuels). An absorption heat pump may be fueled by natural gas or LP gas, for example. While the Gas Utilization Efficiency in such a device, which is the ratio of the energy supplied to the energy consumed, may average only 1.5, that is better than a natural gas or LP gas furnace, which can only approach 1. Although an absorption heat pump may not be as efficient as an electric compression heat pump, an absorption heat pump fueled by natural gas may be advantageous in locations where electricity is relatively expensive and natural gas is relatively inexpensive. A natural gasfired absorption heat pump might also avoid the cost of an electrical service upgrade which is sometimes necessary for an electric heat pump installation. In the case of air-toair heat pumps, an absorption heat pump might also have an advantage in colder regions, due to a lower minimum operating temperature.ROBUR heat pumps comparison A number of sources have been used for the heat source for heating private and communal buildings. •

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air source heat pump (extracts heat from outside air) o air–air heat pump (transfers heat to inside air) o air–water heat pump (transfers heat to a tank of water) geothermal heat pump (extracts heat from the ground or similar sources) o geothermal–air heat pump (transfers heat to inside air) ƒ ground–air heat pump (ground as a source of heat) ƒ rock–air heat pump (rock as a source of heat) ƒ water–air heat pump (body of water as a source of heat) o geothermal–water heat pump (transfers heat to a tank of water) ƒ ground–water heat pump (ground as a source of heat) ƒ rock–water heat pump (rock as a source of heat) ƒ water–water heat pump (body of water as a source of heat)

Heat sources Most commonly, heat pumps draw heat from the air (outside or inside air) or from the ground (groundwater or soil). The heat drawn from the ground is in most cases stored solar heat, and it should not be confused with geothermal heat, though the latter will contribute in some small measure to all heat in the ground. Other heat sources include water; nearby streams and other natural water bodies have been used, and sometimes domestic waste water which is often warmer than the ambient temperature.

Air-source heat pumps Air source heat pumps are relatively easy (and inexpensive) to install and have therefore historically been the most widely used heat pump type. However, they suffer limitations due to their use of the outside air as a heat source or sink. The higher temperature differential during periods of extreme cold or heat leads to declining efficiency, as explained above. In mild weather, COP may be around 4.0, while at temperatures below around −8 °C (17 °F) an air-source heat pump can achieve a COP of 2.5 or better, which

is considerably more than the COP that may be achieved by conventional heating systems. The average COP over seasonal variation is typically 2.5-2.8, with exceptional models able to exceed 6.0 (2.8 kW).

Air source heat pumps for cold climates At least two manufacturers are selling heat pumps that maintain better heating performance at lower outside temperatures than conventional air source heat pumps. These low temperature optimized models make heat pumps more practical for cold climates. In areas where only one fossil fuel is currently available (e.g. heating oil; no natural gas pipes available) these heat pumps could be used as an alternative, supplemental heat source to reduce a building's direct dependence on fossil fuel. Depending on fuel and electricity prices, using the heat pump for heating may be less expensive than fossil fuel. A backup, fossil-fuel heat source may still be required for the coldest days. The heating performance of low temperature optimized heat pumps still declines as the temperature drops, but the threshold at which the decline starts is lower than conventional pumps, as shown in the following table (temperatures are approximate and may vary by manufacturer and model): Air Source Heat Pump Type Conventional Low Temp Optimized

Full heat output at or above this temperature 47 °F (8.3 °C)

Heat output down to 60% of maximum at 17 °F (-8.3 °C)

14 °F (-10 °C)

-13 °F (-25 °C)

Ground source heat pumps Ground source heat pumps, which are also referred to as Geothermal heat pumps, typically have higher efficiencies than air-source heat pumps. This is because they draw heat from the ground or groundwater which is at a relatively constant temperature all year round below a depth of about eight feet (2.5 m). This means that the temperature differential is lower, leading to higher efficiency. Ground-source heat pumps typically have COPs of 3.5-4.0 at the beginning of the heating season, with lower COPs as heat is drawn from the ground. The trade off for this improved performance is that a groundsource heat pump is more expensive to install due to the need for the digging of wells or trenches in which to place the pipes that carry the heat exchange fluid. When compared versus each other, groundwater heat pumps are generally more efficient than heat pumps using heat from the soil. Their efficiency can be further improved, by pumping summer heat into the ground. One way is to use ground water to cool the floors on hot days. Another way is to make large solar collectors, for instance by putting plastic pipes just under the roof tiles or in the tarmac of the parking lot. The most price effective way is to put a large air to water heat exchanger on the rooftop.

Solid state heat pumps In 1881, the German physicist Emil Warburg put a block of iron into a strong magnetic field and found that it increased very slightly in temperature. Some commercial ventures to implement this technology are underway, claiming to cut energy consumption by 40% compared to current domestic refrigerators. The process works as follows: Powdered gadolinium is moved into a magnetic field, heating the material by 2 to 5 °C (4 to 9 °F). The heat is removed by a circulating fluid. The material is then moved out of the magnetic field, reducing its temperature below its starting temperature. Solid state heat pumps using the Thermoelectric Effect have improved over time to the point where they are useful for certain refrigeration tasks. Commercially available technologies have efficiencies that are currently well below that of mechanical heat pumps, however this area of technology is currently the subject of active research in materials science. Near-solid-state heat pumps using Thermoacoustics are commonly used in cryogenic laboratories.

Heat to power units According to a report done by Energitics, Inc. for the DOE in November 2004 titled Technology Roadmap and several others done by the European commission, the majority of energy production from conventional and renewable resources are lost to the atmosphere due to onsite (equipment inefficiency and losses due to waste heat) and offsite (cable and transformers losses) losses, that sums to be around 66% loss in electricity value. Waste heat of different degrees could be found in final products of a certain process or as a by-product in industry such as the Slag in steelmaking plants. Units or devices that could recover the waste heat and transform it into electricity are called WHRUs or heat to power units. Such units, for example, uses an Organic Rankine Cycle with an organic fluid as the working fluid. The fluid has a lower boiling point than water to allow it to boil at low temperature, to for a superheated gas that could drive the blade of a turbine and thus a generator. Another unit like the Thermoelectric could be named a WHRU since they transform the change of heat between two plates directly into a small DC Power (Seebeck, Peltier, Thosmson effects) which could be amplified to produce a usable electric power. A WHRU is different from a Heat Recovery Steam Generator HRSG in the sense that the heating medium does not change phase.

Applications •

Waste Heat of low temperature range (0-120°C) could be used for the production of bio-fuel by growing of algae farms or could be used in greenhouses or even used in Eco-industrial parks.

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Waste Heat of medium (120-650°C) and high (>650°C) temperature could be used for the generation of electricity via different capturing processes.

Advantages and disadvantages of waste heat recovery The waste heat recovery process have no visible disadvantage on Ecology or Economy. In the contrary, these systems have many benefit which could be direct or indirect. • •

Direct benefits: The recovery process will add to the efficiency of the process and thus decrease the costs of fuel and energy consumption needed for that process. Indirect benerfits:

1. Reduction in Pollution: Thermal and air pollution will dramatically decrease since less flue gases of high temperature are emitted from the plant since most of the energy is recycled. 2. Reduction in the equipment sizes: As Fuel consumption reduces so the control and security equipment for handling the fuel decreases. Also, filtering equipment for the gas is no longer needed in large sizes. 3. Reduction in auxiliary energy consumption: Reduction in equipment sizes means another reduction in the energy fed to those systems like pumps, filters, fans,...etc

Chapter- 7

Heat Recovery Ventilation

Heat recovery ventilation, also known as HRV, Mechanical ventilation heat recovery, or MVHR, is an energy recovery ventilation system, using equipment known as a heat recovery ventilator, Heat exchanger, air exchanger or air-to-air exchanger, that employs a counter-flow heat exchanger (countercurrent heat exchange) between the inbound and outbound air flow. HRV provides fresh air and improved climate control, while also saving energy by reducing the heating (or cooling) requirements. Energy recovery ventilators (ERVs) are closely related, however ERVs also transfer the humidity level of the exhaust air to the intake air.

Benefits As building efficiency is improved with insulation and weatherstripping, buildings are intentionally made more air-tight, and consequently less well ventilated. Since all buildings require a source of fresh air, the need for HRVs has become obvious. While opening a window does provide ventilation, the building's heat and humidity will then be lost in the winter and gained in the summer, both of which are undesirable for the indoor climate and for energy efficiency, since the building's HVAC systems must compensate. HRV technology offers an optimal solution: fresh air, better climate control, and Energy efficiency - Sustainability.

Technology Heat recovery ventilation-HRVs and ERVs can be stand-alone devices that operate independently, or they can be built-in, or added to existing HVAC systems. For a small building in which nearly every room has an exterior wall, then the HRV/ERV device can be small and provide ventilation for a single room. A larger building would require either many small units, or a large central unit. The only requirements for the building are an air supply, either directly from an exterior wall or ducted to one, and an energy supply for air circulation, such as wind energy or electricity for a fan. When used with 'central' HVAC systems, then the system would be of the 'forced-air' type.

Air to air heat exchanger

Shell and tube heat exchanger

Fluid flow simulation for a shell and tube style exchanger; The shell inlet is at the top rear and outlet in the foreground at the bottom A shell and tube heat exchanger is a class of heat exchanger designs. It is the most common type of heat exchanger in oil refineries and other large chemical processes, and is suited for higher-pressure applications. As its name implies, this type of heat exchanger consists of a shell (a large pressure vessel) with a bundle of tubes inside it. One fluid runs through the tubes, and another fluid flows over the tubes (through the shell) to transfer

heat between the two fluids. The set of tubes is called a tube bundle, and may be composed by several types of tubes: plain, longitudinally finned, etc.

Theory and Application Two fluids, of different starting temperatures, flow through the heat exchanger. One flows through the tubes (the tube side) and the other flows outside the tubes but inside the shell (the shell side). Heat is transferred from one fluid to the other through the tube walls, either from tube side to shell side or vice versa. The fluids can be either liquids or gases on either the shell or the tube side. In order to transfer heat efficiently, a large heat transfer area should be used, leading to the use of many tubes. In this way, waste heat can be put to use. This is an efficient way to conserve energy. Heat exchangers with only one phase (liquid or gas) on each side can be called one-phase or single-phase heat exchangers. Two-phase heat exchangers can be used to heat a liquid to boil it into a gas (vapor), sometimes called boilers, or cool a vapor to condense it into a liquid (called condensers), with the phase change usually occurring on the shell side. Boilers in steam engine locomotives are typically large, usually cylindrically-shaped shell-and-tube heat exchangers. In large power plants with steam-driven turbines, shelland-tube surface condensers are used to condense the exhaust steam exiting the turbine into condensate water which is recycled back to be turned into steam in the steam generator.

Shell and tube heat exchanger design There can be many variations on the shell and tube design. Typically, the ends of each tube are connected to plenums (sometimes called water boxes) through holes in tubesheets. The tubes may be straight or bent in the shape of a U, called U-tubes.

In nuclear power plants called pressurized water reactors, large heat exchangers called steam generators are two-phase, shell-and-tube heat exchangers which typically have Utubes. They are used to boil water recycled from a surface condenser into steam to drive a turbine to produce power. Most shell-and-tube heat exchangers are either 1, 2, or 4 pass designs on the tube side. This refers to the number of times the fluid in the tubes passes through the fluid in the shell. In a single pass heat exchanger, the fluid goes in one end of each tube and out the other.

Surface condensers in power plants are often 1-pass straight-tube heat exchangers (see Surface condenser for diagram). Two and four pass designs are common because the fluid can enter and exit on the same side. This makes construction much simpler.

There are often baffles directing flow through the shell side so the fluid does not take a short cut through the shell side leaving ineffective low flow volumes. Counter current heat exchangers are most efficient because they allow the highest log mean temperature difference between the hot and cold streams. Many companies however do not use single pass heat exchangers because they can break easily in addition to being more expensive to build. Often multiple heat exchangers can be used to simulate the counter current flow of a single large exchanger.

Selection of tube material To be able to transfer heat well, the tube material should have good thermal conductivity. Because heat is transferred from a hot to a cold side through the tubes, there is a temperature difference through the width of the tubes. Because of the tendency of the tube material to thermally expand differently at various temperatures, thermal stresses occur during operation. This is in addition to any stress from high pressures from the fluids themselves. The tube material also should be compatible with both the shell and tube side fluids for long periods under the operating conditions (temperatures, pressures, pH, etc.) to minimize deterioration such as corrosion. All of these requirements call for careful selection of strong, thermally-conductive, corrosion-resistant, high quality tube materials, typically metals, including copper alloy, stainless steel, carbon steel, nonferrous copper alloy, Inconel, nickel, Hastelloy and titanium. Poor choice of tube material could result in a leak through a tube between the shell and tube sides causing fluid crosscontamination and possibly loss of pressure.

Plate heat exchanger

An interchangeable plate heat exchanger A plate heat exchanger is a type of heat exchanger that uses metal plates to transfer heat between two fluids. This has a major advantage over a conventional heat exchanger in that the fluids are exposed to a much larger surface area because the fluids spread out over the plates. This facilitates the transfer of heat, and greatly increases the speed of the temperature change. Plate heat exchangers are now common and very small brazed versions are used in the hot-water sections of millions of combination boilers. The high heat transfer efficiency for such a small physical size has increased the domestic hot

water (DHW) flowrate of combination boilers. The small plate heat exchanger has made a great impact in domestic heating and hot-water. Larger commercial versions use gaskets between the plates, smaller version tend to be brazed. The concept behind a heat exchanger is the use of pipes or other containment vessels to heat or cool one fluid by transferring heat between it and another fluid. In most cases, the exchanger consists of a coiled pipe containing one fluid that passes through a chamber containing another fluid. The walls of the pipe are usually made of metal, or another substance with a high thermal conductivity, to facilitate the interchange, whereas the outer casing of the larger chamber is made of a plastic or coated with thermal insulation, to discourage heat from escaping from the exchanger. The plate heat exchanger (PHE) was invented by Dr Richard Seligman in 1923 and revolutionised methods of indirect heating and cooling of fluids. Dr Richard Seligman founded APV in 1910 as the Aluminium Plant & Vessel Company Limited, a specialist fabricating firm supplying welded vessels to the brewery and vegetable oil trades.

Design of plate and frame heat exchangers

Schematic conceptual diagram of a plate and frame heat exchanger.

An individual plate for a heat exchanger The plate heat exchanger (PHE) is a specialized design well suited to transferring heat between medium- and low-pressure liquids. Welded, semi-welded and brazed heat exchangers are used for heat exchange between high-pressure fluids or where a more compact product is required. In place of a pipe passing through a chamber, there are instead two alternating chambers, usually thin in depth, separated at their largest surface by a corrugated metal plate. The plates used in a plate and frame heat exchanger are obtained by one piece pressing of metal plates. Stainless steel is a commonly used metal for the plates because of its ability to withstand high temperatures, its strength, and its corrosion resistance. The plates are often spaced by rubber sealing gaskets which are cemented into a section around the edge of the plates. The plates are pressed to form troughs at right angles to the direction of flow of the liquid which runs through the channels in the heat exchanger. These troughs are arranged so that they interlink with the other plates which forms the channel with gaps of 1.3–1.5 mm between the plates. The plates produce an extremely large surface area, which allows for the fastest possible transfer. Making each chamber thin ensures that the majority of the volume of the liquid contacts the plate, again aiding exchange. The troughs also create and maintain a turbulent flow in the liquid to maximize heat transfer in the exchanger. A high degree of turbulence can be obtained at low flow rates and high heat transfer coefficient can then be achieved. A plate heat exchanger consists of a series of thin, corrugated plates which are mentioned above. These plates are gasketed, welded or brazed together depending on the application of the heat exchanger. The plates are compressed together in a rigid frame to form an arrangement of parallel flow channels with alternating hot and cold fluids. As compared to shell and tube heat exchangers, the temperature approach in a plate heat exchangers may be as low as 1 °C whereas shell and tube heat exchangers require an approach of 5 °C or more. For the same amount of heat exchanged, the size of the plate

heat exchanger is smaller, because of the large heat transfer area afforded by the plates (the large area through which heat can travel). Expansion and reduction of the heat transfer area is possible in a plate heat exchanger.

Evaluating plate heat exchangers All plate heat exchangers look similar on the outside. The difference lies on the inside, in the details of the plate design and the sealing technologies used. Hence, when evaluating a plate heat exchanger, it is very important not only to explore the details of the product being supplied, but also to analyze the level of research and development carried out by the manufacturer and the post-commissioning service and spare parts availability. An important aspect to take into account when evaluating a heat exchanger are the forms of corrugation within the heat exchanger. There are two types: intermating and chevron corrugations. In general, greater heat transfer enhancement is produced from chevrons for a given increase in pressure drop and are more commonly used than intermating corrugations.

Advantages and disadvantages of plate heat exchangers Advantages •

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Compactness- The units in a plate heat exchanger occupy less floor space and floor loading by having a large surface area that is formed from a small volume. This in turn produces a high overall heat transfer coefficient due to the heat transfer associated with the narrow passages and corrugated surfaces. Flexibility- Changes can be made to heat exchanger performance by utilizing a wide range of fluids and conditions that can be modified to adapt to the various design specifications. These specifications can be matched with different plate corrugations. Low Fabrication Costs- Welded plates are relatively more expensive than pressed plates. Plate heat exchangers are made from pressed plates, which allow greater resistance to corrosion and chemical reactions. Ease of Cleaning- The heat exchanger can be easily dismantled for inspection and cleaning (especially in food processing) and the plates are also easily replaceable as they can be removed and replaced individually. Temperature Control- The plate heat exchanger can operate with relatively small temperature differences. This is an advantage when high temperatures must be avoided. Local overheating and possibility of stagnant zones can also be reduced by the form of the flow passage.

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The main weakness of the plate and frame heat exchanger is the necessity for the long gaskets which holds the plates together. Although these gaskets are seen as

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drawback, plate-and-frame heat exchangers have been successfully run at high temperatures and pressures. There is a potential for leakage. The leaks that occur are sent to the atmosphere and not between process streams. The pressure drop that occurs through a plate heat exchanger is relatively high and the running costs and capital of the pumping system should be considered. When loss of containment or loss of pressure occurs, it can take a long time to clean and reinitialise this type of exchanger as hundreds of plates are common in larger built. The narrow spacing between plates can become blocked by particulate contaminants in the fluid, for example oxide and sludge particles found in central heating systems. for the reason above most manufacturers will only guarantee their units for 12 months, furthermore replacement plate and gasket sets can be as much as the plate to buy initially.

Plate heat transfer equation The total rate of heat transfer between the hot and cold fluids passing through a plate heat exchanger may be expressed as: Q = UA∆Tm where U is the overall heat transfer coefficient, A is the total plate area, and ∆Tm is the temperature difference. U is dependent upon the heat transfer coefficients in the hot and cold streams.

Plate fin heat exchanger A plate-fin heat exchanger is a type of heat exchanger design that uses plates and finned chambers to transfer heat between fluids. It is often categorized as a compact heat exchanger to emphasise its relatively high heat transfer surface area to volume ratio. The plate-fin heat exchanger is widely used in many industries, including the aerospace industry for its compact size and lightweight properties, as well as in cryogenics where its ability to facilitate heat transfer with small temperature differences is utilized.

History Design of plate-fin heat exchangers Originally conceived by an Italian mechanic, Paolo Fruncillo. A plate-fin heat exchanger is made of layers of corrugated sheets separated by flat metal plates, typically aluminium, to create a series of finned chambers. Separate hot and cold fluid streams flow through alternating layers of the heat exchanger and are enclosed at the edges by side bars. Heat is transferred from one stream through the fin interface to the separator plate and through the next set of fins into the adjacent fluid. The fins also serve to increase the structural integrity of the heat exchanger and allow it to withstand high pressures while providing an extended surface area for heat transfer.

A high degree of flexibility is present in plate-fin heat exchanger design as they can operate with any combination of gas, liquid, and two-phase fluids. Heat transfer between multiple process streams is also accommodated, with a variety of fin heights and types as different entry and exit points available for each stream. The main four type of fins are: plain, which refer to simple straight-finned triangular or rectangular designs; herringbone, where the fins are placed sideways to provide a zig-zag path; and serrated and perforated which refer to cuts and perforations in the fins to augment flow distribution and improve heat transfer. A disadvantage of plate-fin heat exchangers is that they are prone to fouling due to their small flow channels. They also cannot be mechanically cleaned and require other cleaning procedures and proper filtration for operation with potentially-fouling streams.

Flow arrangement In a plate-fin heat exchanger, the fins are easily able to be rearranged. This allows for the two fluids to result in crossflow, counterflow, cross-counterflow or parallel flow. If the fins are designed well, the plate-fin heat exchanger can work in perfect countercurrent arrangement.

Layout Stacking Cost The cost of plate-fin heat exchangers is generally higher than conventional heat exchangers due to a higher level of detail required during manufacture. However, these costs can often be outweighed by the cost saving produced by the added heat transfer.

Areas of application Plate-fin heat exchangers are generally applied in industries where the fluids have little chances of fouling. The delicate design as well as the thin channels in the plate-fin heat exchanger make cleaning difficult or impossible. Applications of plate-fin heat exchangers include: • • • • •

Natural gas liquefaction Cryogenic air separation Ammonia production Offshore processing Nuclear engineering

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Syngas production

Ground-coupled heat exchanger A ground-coupled heat exchanger is an underground heat exchanger loop that can capture or dissipate heat to or from the ground. They use the Earth's near constant subterranean temperature to warm or cool air or other fluids for residential, agricultural or industrial uses. If building air is blown through the heat exchanger for heat recovery ventilation, they are called earth tubes (also known as earth cooling tubes or earth warming tubes) in Europe or earth-air heat exchangers (EAHE or EAHX) in North America. These systems are known by several other names, including: air-to-soil heat exchanger, earth channels, earth canals, earth-air tunnel systems, ground tube heat exchanger, hypocausts, subsoil heat exchangers, underground air pipes, and others. Earth tubes are often a viable and economical alternative or supplement to conventional central heating or air conditioning systems since there are no compressors, chemicals or burners and only blowers are required to move the air. These are used for either partial or full cooling and/or heating of facility ventilation air. Their use can help buildings meet the German Passive House standards or the North American LEED's (Leadership in Energy and Environmental Design) Green Building rating system. Earth-air heat exchangers have been used in agricultural facilities (animal buildings) and horticultural facilities (greenhouses) in the United States over the past several decades and have been used in conjunction with solar chimneys in hot arid areas for thousands of years, probably beginning in the Persian Empire. Implementation of these systems in Austria, Denmark, Germany, and India has become fairly common since the mid-1990s, and is slowly being adopted into North America. Ground-coupled heat exchanger may also use water or antifreeze as a heat transfer fluid, often in conjunction with a geothermal heat pump. See, for example downhole heat exchangers.

Design

Heat recovery ventilation, often including an earth-to-air heat exchanger, is essential to achieve the German passivhaus standard

Earth pipe before being covered with ground Earth-air heat exchangers can be analyzed for performance with several software applications using weather gage data. These software applications include GAEA, AWADUKT Thermo, EnergyPlus, L-EWTSim, WKM, and others. However, numerous earth-air heat exchanger systems have been designed and constructed improperly, and failed to meet design expectations. Earth-air heat exchangers appear best suited for air pretreatment rather than for full heating or cooling. Pretreatment of air for an air-source heat pump or ground-source heat pump often provides the best economic return on investment, with simple payback often achieved within one year after installation. Most systems are usually constructed from 100 to 600 mm (4 to 24 inch) diameter, smooth-walled (so they do not easily trap condensation moisture and mold), rigid or

semi-rigid plastic, plastic-coated metal pipes or plastic pipes coated with inner antimicrobial layers, buried 1.5 to 3 m (5 to 10 ft) underground where the ambient earth temperature is typically 10 to 23 °C (50-73 °F ) all year round in the temperate latitudes where most humans live. Ground temperature becomes more stable with depth, and between about 3 m and 12 m (10 ft and 40 ft) the soil is steadily at - or close to - the median annual air temperature. Smaller diameter tubes require more energy to move the air and have less earth contact surface area. Larger tubes permit a slower airflow, which also yields more efficient energy transfer and permits much higher volumes to be transferred, permitting more air exchanges in a shorter time period, when, for example, you want to clear the building of objectionable odors or smoke. It is more efficient to pull air through a long tube than to push it with a fan. A solar chimney can use natural convection (warm air rising) to create a vacuum to draw filtered passive cooling tube air through the largest diameter cooling tubes. Natural convection may be slower than using a solar-powered fan. Sharp 90-degree angles should be avoided in the construction of the tube - two 45-degree bends produce less-turbulent, more efficient air flow. While smooth-wall tubes are more efficient in moving the air, they are less efficient in transferring energy. There are three configurations, a closed loop design, an open 'fresh air' system or a combination: •

Closed loop system: Air from inside the home or structure is blown through a Ushaped loop(s) of typically 30 to 150 m (100 to 500 ft) of tube(s) where it is moderated to near earth temperature before returning to be distributed via ductwork throughout the home or structure. The closed loop system can be more effective (during air temperature extremes) than an open system, since it cools and recools the same air.

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Open system: outside air is drawn from a filtered air intake (Minimum Efficiency Reporting Value MERV 8+ air filter is recommended). The cooling tubes are typically 30 m (100 ft) long (or more) of straight tube into the home. An open system combined with energy recovery ventilation can be nearly as efficient (8095%) as a closed loop, and ensures that entering fresh air is filtered and tempered.

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Combination system: This can be constructed with dampers that allow either closed or open operation, depending on fresh air ventilation requirements. Such a design, even in closed loop mode, could draw a quantity of fresh air when an air pressure drop is created by a solar chimney, clothes dryer, fireplace, kitchen or bathroom exhaust vents. It is better to draw in filtered passive cooling tube air than unconditioned outside air.

Single-pass earth air heat exchangers offer the potential for indoor air quality improvement over conventional systems by providing an increased supply of outdoor air. In some configurations of single-pass systems, a continuous supply of outdoor air is

provided. This type of system would usually include one or more ventilation heat recovery units.

Safety If humidity and associated mold colonization is not addressed in system design, occupants may face health risks. At some sites, the humidity in the earth tubes may be controlled simply by passive drainage if the water table is sufficiently deep and the soil has relatively high permeability. In situations where passive drainage is not feasible or needs to be augmented for further moisture reduction, active (dehumidifier) or passive (desiccant) systems may treat the air stream. Formal research indicates that earth-air heat exchangers reduce building ventilation air pollution. Rabindra (2004) states, “The tunnel [earth-Air heat exchanger] is found not to support the growth of bacteria and fungi; rather it is found to reduce the quantity of bacteria and fungi thus making the air safer for humans to inhale. It is therefore clear that the use of EAT [Earth Air Tunnel] not only helps save the energy but also helps reduce the air pollution by reducing bacteria and fungi.” Likewise, Flueckiger (1999) in a study of twelve earth-air heat exchangers varying in design, pipe material, size and age, stated, “This study was performed because of concerns of potential microbial growth in the buried pipes of ground-coupled air systems. The results however demonstrate, that no harmful growth occurs and that the airborne concentrations of viable spores and bacteria, with few exceptions, even decreases after passage through the pipe-system”, and further stated, “Based on these investigations the operation of ground-coupled earth-to-air heat exchangers is acceptable as long as regular controls are undertaken and if appropriate cleaning facilities are available”. Whether using earth tubes with or without antimicrobial material, it is extremely important that the underground cooling tubes have an excellent condensation drain and be installed at a 2-3 degree grade to ensure the constant removal of condensed water from the tubes. When implementing in a house without a basement on a flat lot, an external condensation tower can be installed at a depth lower than where the tube enters into the house and at a point close to the wall entry. The condensation tower installation requires the added use of a condensate pump in which to remove the water from the tower. For installations in houses with basements, the pipes are graded so that the condensation drain located within the house is at the lowest point. In either installation, the tube must continually slope towards either the condensation tower or the condensation drain. The inner surface of the tube, including all joints must be smooth to aid in the flow and removal of condensate. Corrugated or ribbed tubes and rough interior joints must not be used. Joints connecting the tubes together must be tight enough to prevent water or gas infiltration. In certain geographic areas, it is important that the joints prevent Radon gas infiltration. Porous materials like uncoated concrete tubes cannot be used. Ideally, Earth Tubes with antimicrobial inner layers should be used in installations to inhibit the potential growth of molds and bacteria within the tubes.

Effectiveness Implementations of earth-air heat exchangers for either partial or full cooling and/or heating of facility ventilation air have had mixed success. The literature is, unfortunately, well populated with over-generalizations about the applicability of these systems - both supportive and unsupportive. A key aspect of earth-air heat exchangers is the passive nature of operation and consideration of the wide variability of conditions in natural systems. Earth-air heat exchangers can be very cost effective in both up-front/capital costs as well as long-term operation and maintenance costs. However, this varies widely depending on the location latitude, altitude, ambient Earth temperature, climatic temperature-andrelative-humidity extremes, solar radiation, water table, soil type (thermal conductivity), soil moisture content and the efficiency of the building's exterior envelope design / insulation. Generally, dry-and-low-density soil with little or no ground shade will yield the least benefit, while dense damp soil with considerable shade should perform well. A slow drip watering system may improve thermal performance. Damp soil in contact with the cooling tube conducts heat more efficiently than dry soil. Earth cooling tubes are much less effective in hot humid climates (like Florida) where the ambient temperature of the earth approaches human comfort temperature. The higher the ambient temperature of the earth, the less effective they are for cooling and dehumidification. However, they can be used to partially cool and dehumidify the replacement fresh air intake for passive-solar thermal buffer zone areas like the laundry room, or a solarium / greenhouse, especially those with a hot tub, swim spa, or indoor swimming pool, where warm humid air is exhausted in the summer, and a supply of cooler drier replacement air is desired. Not all regions and sites are suitable for earth-air heat exchangers. Conditions which may hinder or preclude proper implementation include shallow bedrock, high water table, and insufficient space, among others. In some areas, only cooling or heating may be afforded by earth-air heat exchangers. In these areas, provision for thermal recharge of the ground must especially be considered. In dual function systems (both heating and cooling), the warm season provides ground thermal recharge for the cool season and the cool season provides ground thermal recharge for the warm season, though overtaxing the thermal reservoir must be considered even with dual function systems. Renata Limited, a prominent pharmaceutical company in Bangladesh, tried out a pilot project trying to find out whether they could use the Earth Air Tunnel technology to complement the conventional air conditioning system. Concrete pipes (total length 60 feet, inner diameter 9 inches, outer diameter 11 inches) were placed at a depth of 9 feet underground and a blower of 1.5 kW rated power was employed. The underground temperature at that depth was found to be around 28°C. The mean velocity of air in the tunnel was about 5 m/s. The Coefficient of Performance (COP) of the underground heat exchanger thus designed was poor ranging from 1.5-3. The results convinced the authorities that in hot and humid climates, it is unwise to implement the concept of Earth-

Air heat exchanger. The cooling medium (earth itself) being at a temperature approaching that of the ambient environment happens to be the root cause of the failure of such principles in hot, humid areas (parts of Southeast Asia, Florida in the U.S.A. etc.). However, investigators from places like Britain and Turkey have reported very encouraging COPs-well above 20. The underground temperature seems to be of prime importance when planning an Earth-Air heat exchanger.

Environmental impact In the context of today's diminishing fossil fuel reserves, increasing electrical costs, air pollution and global warming, properly-designed earth cooling tubes offer a sustainable alternative to reduce or eliminate the need for conventional compressor-based air conditioning systems, in non-tropical climates. They also provide the added benefit of controlled, filtered, temperate fresh air intake, which is especially valuable in tight, wellweatherized, efficient building envelopes.

Water to earth An alternative to the earth-to-air heat exchanger is the "water" to earth heat exchanger. This is typically similar to a geothermal heat pump tubing embedded horizontally in the soil (or could be a vertical sonde) to a similar depth of the earth-air heat exchanger. It uses approximately double the length of pipe of 35 mm diameter, e.g., around 80 m compared to an EAHX of 40 m. A heat exchanger coil is placed before the air inlet of the heat recovery ventilator. Typically a brine liquid (heavily salted water) is used as the heat exchanger fluid. Many European installations are now using this setup due to the ease of installation. No fall or drainage point is required and it is safe because of the reduced risk from mold.

Dynamic scraped surface heat exchanger The dynamic scraped surface heat exchanger (DSSHE) was designed to face some problems found in other types of heat exchangers. They increase heat transfer by: • • •

removing the fouling layers, increasing turbulence in case of high viscosity flow, avoiding the generation of ice and other process by-products.

DSSHEs incorporate an internal mechanism which periodically removes the product from the heat transfer wall.

Introduction

The most important technologies for indirect heat transfer use tubes (shell-and-tube exchangers) or flat surfaces (plate exchangers). Their goal is to exchange the maximum amount of heat per unit area by generating as much turbulence as possible below given pumping power limits. Typical approaches to achieve this consist of corrugating the tubes or plates or extending their surface with fins. However, these geometry conformation technologies, the calculation of optimum mass flows and other turbulence related factors become diminished when fouling appears, obliging designers to fit significantly larger heat transfer areas. There are several types of fouling, including particulate accumulation, precipitation (crystallization), sedimentation, generation of ice layers, etc. Another factor posing difficulties to heat transfer is viscosity. Highly viscous fluids tend to generate deep laminar flow, a condition with very poor heat transfer rates and high pressure losses involving a considerable pumping power, often exceeding the exchanger design limits. This problem becomes worsened frequently when processing nonnewtonian fluids. The dynamic scraped surface heat exchangers (DSSHE) have been designed to face the aforementioned problems. They increase heat transfer by: • • •

removing the fouling layers, increasing turbulence in case of high viscosity flow, avoiding the generation of ice and other process by-products.

Basic description The dynamic scraped surface heat exchangers incorporate an internal mechanism which periodically removes the product from the heat transfer wall. The product side is scraped by blades attached to a moving shaft or frame. The blades are made of a rigid plastic material to prevent damage to the scraped surface. This material is FDA approved in the case of food applications.

Types of dynamic scraped surface heat exchangers There are basically three types of DSSHEs depending on the arrangement of the blades: 1. Rotating, tubular DSSHEs. The shaft is placed parallel to the tube axis, not necessarily coincident, and spins at various frequencies, from a few dozen rpm to more than 1000 rpm. The number of blades oscillates between 1 and 4 and may take advantage of centrifugal forces to scrape the inner surface of the tube. Examples are the Waukesha Cherry-Burrell Votator II and the Alfa Laval Contherm. 2. Reciprocating, tubular DSSHEs. The shaft is concentric to the tube and moves longitudinally without rotating. The frequency spans between 10 and 60 strokes

per minute. The blades may vary in number and shape, from baffle-like arrangements to perforated disk configurations. 3. Rotating, plate DSSHEs. The blades wipe the external surface of circular plates arranged in series inside a shell. The heating/cooling fluid runs inside the plates. The frequency is about several dozen rpm. An example is the HRS Spiratube TSensation.

Evaluation of dynamic scraped surface heat exchangers Computational fluid dynamics (CFD) techniques are the standard tools to analyse and evaluate heat exchangers and similar equipment. However, for quick calculation purposes, the evaluation of DSSHEs are usually carried out with the help of ad-hoc (semi)empirical correlations based on the Buckingham π theorem: Fa = Fa(Re, Re', n, ...) for pressure loss and Nu = Nu(Re, Re', Pr, Fa, L/D, N, ...) for heat transfer, where Nu is the Nusselt number, Re is the standard Reynolds number based on the inner diameter of the tube, Re' is the specific Reynolds number based on the wiping frequency, Pr is the Prandtl number, Fa is the Fanning friction factor, L is the length of the tube, D is the inner diameter of the tube, n is the number of blades and the dots account for any other relevant dimensionless parameters.

Applications The range of applications covers a number of industries, including food, chemical, petrochemical and pharmaceutical. The DSSHEs are appropriate whenever products are prone to fouling, very viscous, particulate, heat sensitive or crystallizing.

Waste heat recovery unit A waste heat recovery unit (WHRU) is an energy recovery heat exchanger that recovers heat from hot streams with potential high energy content, such as hot flue gases from a diesel generator or steam from cooling towers or even waste water from different cooling processes such as in steel cooling.

Principle

Heat recovery units Waste heat found in the exhaust gas of various processes or even from the exhaust stream of a conditioning unit can be used to preheat the incoming gas. This is one of the basic methods for recovery of waste heat. Many steel making plants use this process as an economic method to increase the production of the plant with lower fuel demand. There are many different commercial recovery units for the transferring of energy from hot medium space to lower one: 1. Recuperators: This name is given to different types of heat exchanger that the exhaust gases are passed through, consisting of metal tubes that carry the inlet gas and thus preheating the gas before entering the process. The heat wheel is an example which operates on the same principle as a solar air conditioning unit. 2. Regenerators: This is an industrial unit that reuses the same stream after processing. In this type of heat recovery, the heat is regenerated and reused in the process. 3. Heat pipe exchanger: Heat pipes are one of the best thermal conductors. They have the ability to transfer heat hundred times more than copper. Heat pipes are mainly known in renewable energy technology as being used in evacuated tube collectors. The heat pipe is mainly used in space, process or air heating, in waste heat from a process is being transferred to the surrounding due to its transfer mechanism. 4. Economizer: In case of process boilers, waste heat in the exhaust gas is passed along a recuperator that carries the inlet fluid for the boiler and thus decreases thermal energy intake of the inlet fluid. 5. Heat pumps: Using an organic fluid that boils at a low temperature means that energy could be regenerated from waste fluids.

Heat to power units According to a report done by Energitics, Inc. for the DOE in November 2004 titled Technology Roadmap and several others done by the European commission, the majority of energy production from conventional and renewable resources are lost to the atmosphere due to onsite (equipment inefficiency and losses due to waste heat) and offsite (cable and transformers losses) losses, that sums to be around 66% loss in electricity value. Waste heat of different degrees could be found in final products of a certain process or as a by-product in industry such as the Slag in steelmaking plants. Units or devices that could recover the waste heat and transform it into electricity are called WHRUs or heat to power units. Such units, for example, uses an Organic Rankine Cycle with an organic fluid as the working fluid. The fluid has a lower boiling point than water to allow it to boil at low temperature, to for a superheated gas that could drive the blade of a turbine and thus a generator. Another unit like the Thermoelectric could be named a WHRU since they transform the change of heat between two plates directly into a small DC Power (Seebeck, Peltier, Thosmson effects) which could be amplified to produce a usable electric power.

A WHRU is different from a Heat Recovery Steam Generator HRSG in the sense that the heating medium does not change phase.

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Waste Heat of low temperature range (0-120°C) could be used for the production of bio-fuel by growing of algae farms or could be used in greenhouses or even used in Eco-industrial parks. Waste Heat of medium (120-650°C) and high (>650°C) temperature could be used for the generation of electricity via different capturing processes.

Advantages and disadvantages of waste heat recovery The waste heat recovery process have no visible disadvantage on Ecology or Economy. In the contrary, these systems have many benefit which could be direct or indirect. • •

Direct benefits: The recovery process will add to the efficiency of the process and thus decrease the costs of fuel and energy consumption needed for that process. Indirect benerfits:

1. Reduction in Pollution: Thermal and air pollution will dramatically decrease since less flue gases of high temperature are emitted from the plant since most of the energy is recycled. 2. Reduction in the equipment sizes: As Fuel consumption reduces so the control and security equipment for handling the fuel decreases. Also, filtering equipment for the gas is no longer needed in large sizes. 3. Reduction in auxiliary energy consumption: Reduction in equipment sizes means another reduction in the energy fed to those systems like pumps, filters, fans,...etc

Example During the past years, companies have developed many products for the recovery of the waste heat. A new concept is being introduced by Cyclone Power Technologies that uses an external combustion engine design for the waste heat recovery application.

Micro heat exchanger Micro heat exchangers, Micro-scale heat exchangers, or microstructured heat exchangers are heat exchangers in which (at least one) fluid flows in lateral confinements with typical dimensions below 1 mm. The most typical such confinement are microchannels, channels with a hydraulic diameter below 1 mm.

Background

Investigation of microscale thermal devices is motivated by the single phase internal flow correlation for convective heat transfer:

Where h is the heat transfer coefficient, Nuc is the Nusselt number, k is the thermal conductivity of the fluid and d is the hydraulic diameter of the channel or duct. In internal laminar flows, the Nusselt number becomes a constant. This is a result which can be arrived at analytically: For the case of a constant wall temperature, Nuc = 3.657 and for the case of constant heat flux Nuc = 4.364. As Reynolds number is proportional to hydraulic diameter, fluid flow in channels of small hydraulic diameter will predominantly be laminar in character. This correlation therefore indicates that the heat transfer coefficient increases as channel diameter decreases. Should the hydraulic diameter in forced convection be on the order of tens or hundreds of micrometres, an extremely high heat transfer coefficient should result. This hypothesis was initially investigated by Tuckerman and Pease . Their positive results have since fueled a hot field of research ranging from classical investigations of single channel heat transfer to more applied investigations in parallel micro-channel and micro scale plate fin heat exchangers. Recent work in the field has focused on the potential of two-phase flows at the micro-scale.

Classification of micro heat exchangers Just like "conventional" or "macro scale" heat exchangers, micro heat exchangers have either one or two fluidic passages. In the case of one passage, heat is transferred to the fluid from electrically powered heater cartridges, or removed from the fluid by electrically powered elements like Peltier chillers. In the case of two fluidic passages, micro heat exchangers are usually classified by the orientation of the fluid passages to another as "cross flow" or "counter flow" devices. If a chemical reaction is conducted inside a micro heat exchanger, the latter is also called a microreactor.

Moving bed heat exchanger Moving bed heat exchangers (also moving bed coolers) are stationary heat exchangers for bulk materials for continuous processes in chemical engineering.

Construction Moving bed heat exchangers essentially exist of a huge number of square tubes which are arranged in heat exchanger packages one above the other. The ends of the tubes are closed with end plates. Behind the plates are reversing chambers for the cooling or heating medium. The sides of the external tubes are equipped with steel plate strips which

hold the product in the shaft. To protect the environments or the product quality, doors that close the side walls can be fitted. Above and under the heat exchanger are feed respectively discharge hoppers. Different conveyor facilities for bulk materials, as for example conveying screws, bucket conveyors or similar are downstream systems.

Function The cooling or warming of the bulk materials in the Moving bed Cooler happens indirectly; via water, thermal oil or steam. The heating or cooling medium flows through the square tubes. Medium and product flow in cross countercurrent to each other. The coolers work according to the Moving Bed Principle. I.e. the product forms a product column which flows continuously down between the cooling pipes. A discharge bottom with variable openings regulates dwell time and flow rate.

Applications Moving bed heat exchangers can be used for cooling or warming of all free-flowing bulk materials which correspond to the requirements of the apparatus, concerning grain size and angle of repose. The heat exchangers often can be found after rotary kilns and dryers to cool e.g. mineral (quartz sand, Ilmentit etc.) or chemical products (fertilizer, soda etc.). The entry temperatures of the products can reach up to 1200 °C.

Technical specifications Moving bed heat exchangers have a relatively compact construction. Because of the working principle they need only a small base. However, depending on their application they can build relatively high. Because of having only few moved parts they have low electrical requirements and are low-maintenance. Problems with noise or dust contamination of the environments do not occur.

Chapter- 8

Other Energy Recovery Systems & Techniques

Kinetic Energy Recovery Systems Kinetic Energy Recovery Systems (KERS) were used for the motor sport Formula One's 2009 season, and under development for road vehicles. However, KERS was abandoned for the 2010 Formula One season. The Formula One Teams that used Kinetic Energy Recovery Systems in the 2009 season are Ferrari, Renault, BMW, and McLaren. One of the main reasons that not all cars use KERS is because it adds an extra 25 kilograms of weight, while not adding to the total car weight, it does incur a penalty particularly seen in the qualifying rounds, as it raises the car's center of gravity, and reduces the amount of ballast that is available to balance the car so that it is more predictable when turning. FIA rules also limit the exploitation of the system. Eventually, during the season, Renault and BMW stopped using the system. Williams is developing a flywheel-KERS system. The concept of transferring the vehicle’s kinetic energy using Flywheel energy storage was postulated by physicist Richard Feynman in the 1950s and is exemplified in complex high end systems such as the Zytek, Flybrid, Torotrak and Xtrac used in F1 and simple, easily manufactured and integrated differential based systems such as the Cambridge Passenger/Commercial Vehicle Kinetic Energy Recovery System (CPC-KERS) Xtrac and Flybrid are both licensees of Torotrak's technologies, which employ a small and sophisticated ancillary gearbox incorporating a continuously variable transmission (CVT). The CPC-KERS is similar as it also forms part of the driveline assembly. However, the whole mechanism including the flywheel sits entirely in the vehicle’s hub (looking like a drum brake). In the CPC-KERS, a differential replaces the CVT and transfers torque between the flywheel, drive wheel and road wheel.

Use in motor sport

History

A Flybrid Systems Kinetic Energy Recovery System The first of these systems to be revealed was the Flybrid. This system weighs 24 kg and has an energy capacity of 400 kJ after allowing for internal losses. A maximum power boost of 60 kW (81.6 PS, 80.4 HP) for 6.67 s is available. The 240 mm diameter flywheel weighs 5.0 kg and revolves at up to 64,500 rpm. Maximum torque is 18 Nm (13.3 ftlbs). The system occupies a volume of 13 litres. Two minor incidents have been reported during testing of KERS systems in 2008. The first occurred when the Red Bull Racing team tested their KERS battery for the first time in July: it malfunctioned and caused a fire scare that led to the team's factory being evacuated. The second was less than a week later when a BMW Sauber mechanic was given an electric shock when he touched Christian Klien's KERS-equipped car during a test at the Jerez circuit.

FIA

A KERS flywheel Formula One have stated that they support responsible solutions to the world's environmental challenges, and the FIA allowed the use of 81 hp (60 kW; 82 PS) KERS in the regulations for the 2009 Formula One season. Teams began testing systems in 2008: energy can either be stored as mechanical energy (as in a flywheel) or as electrical energy (as in a battery or supercapacitor). Due to high cost, FOTA teams agreed to drop KERS from the 2010 season onwards, but this is still an open issue as Williams F1 said it will use KERS in 2010 and changes to the regulations must be agreed by all teams. Vodafone McLaren Mercedes became the first team to win a F1 GP using a KERS equipped car when Lewis Hamilton won the Hungarian Grand Prix on July 26, 2009. Their second KERS equipped car finished fifth. At the following race Lewis Hamilton became the first

driver to take pole position with a KERS car, his team mate, Heikki Kovalainen qualifying second. This was also the first instance of an all KERS front row. On August 30, 2009, Kimi Räikkönen won the Belgian Grand Prix with his KERS equipped Ferrari. It was the first time that KERS contributed directly to a race victory, with second placed Giancarlo Fisichella claiming "Actually, I was quicker than Kimi. He only took me because of KERS at the beginning". New rules for the 2011 F1 season raise the minimum weight limit of the car and driver by 20kg to 640kg. This is to prepare the way for a return of the KERS power-boost and energy storage systems which featured in 2009. Although KERS is still legal in F1, for the 2010 season all the teams have agreed not to use it. As of 2013 it is possible that F1 will require KERS again as FIA rules force F1 to use greener methods to power their cars. The proposal is to increase the power storage from 60KW to 120KW. The FIA release of the 2011 rule changes with both FOTA, FOM and the FIA have agreed that F1 will have KERS for the 2011 season at 80hp, with this value rising in 2013 to coincide with smaller engines. This is still optional as it was in the 2009 season and can be battery or flywheel based.

Autopart makers Bosch Motorsport Service is developing a KERS for use in motor racing. These electricity storage systems for hybrid and engine functions include a lithium-ion battery with scalable capacity or a flywheel, a four to eight kilogram electric motor [with a maximum power level of 60 kW (81 hp)], as well as the KERS controller for power and battery management. Bosch also offers a range of electric hybrid systems for commercial and light-duty applications.

Carmakers Automakers including Honda have been testing KERS systems. At the 2008 1000 km of Silverstone, Peugeot Sport unveiled the Peugeot 908 HY, a hybrid electric variant of the diesel 908, with KERS. Peugeot plans to campaign the car in the 2009 Le Mans Series season, although it will not be capable of scoring championship points. Vodafone McLaren Mercedes began testing of their KERS in September 2008 at the Jerez test track in preparation for the 2009 F1 season, although at that time it was not yet known if they would be operating an electrical or mechanical system. In November 2008 it was announced that Freescale Semiconductor would collaborate with McLaren Electronic Systems to further develop its KERS for McLaren's Formula One car from 2010 onwards. Both parties believed this collaboration would improve McLaren's KERS system and help the system filter down to road car technology. Toyota has used a supercapacitor for regeneration on Supra HV-R hybrid race car that won the 24 Hours of Tokachi race in July 2007.

Motorcycles KTM racing boss Harald Bartol has revealed that the factory raced with a secret Kinetic Energy Recovery System (KERS) fitted to Tommy Koyama's motorcycle during the 2008 season-ending 125cc Valencian Grand Prix. This was illegal and against the rules. so they were later banned from doing it afterwards.

Races Automobile Club de l'Ouest, the organizer behind the annual 24 Hours of Le Mans event and the Le Mans Series is currently "studying specific rules for LMP1 that will be equipped with a kinetic energy recovery system. " Peugeot was the first manufacturer to unveil a fully functioning LMP1 car in the form of the 908 HY at the 2008 Autosport 1000 km race at Silverstone.

Thermal diode The term thermal diode is sometimes used to describe a (possibly non-electrical) device which causes heat to flow preferentially in one direction. Or, the term may be used to describe an electrical (semiconductor) diode in reference to a thermal effect or function. Or the term may be used to describe both situations, where an electrical diode is used as a heat-pump or thermoelectric cooler.

One-way heat-flow A thermal diode can be: • • •

a heat engine which converts a heat difference directly into electric power. a heat engine working backwards as a refrigerator, such as a Stirling engine a type of Heat pipe which will only allow heat to flow from the evaporator to the condenser. When the condenser is hotter than the evaporator, the coolant in the heat pipe condenses in a reservoir at the evaporator end. This reservoir does not have a capillary connection with the condenser, preventing the liquid from returning to the condenser. During normal heat pipe operation, when the evaporator and reservoir become hotter than the condenser, the reservoir is emptied by evaporation, and heat is transferred to the condenser.

Due to the second law of thermodynamics a passive system cannot move heat from a cold source to a hot destination. But the law allows to avoid or minimize the flow in this direction. If the source is hotter than the destination heat flows with a low thermal resistance towards the destination.

Electrical diode thermal effect or function

•

a sensor device embedded on microprocessors used to monitor the temperature of the processor's die.

Thermoelectric heat-pump or cooler There are two types. One uses semiconductor, or less efficient metal, i.e. thermocouples, working on the principles of the Peltier-Seebeck effect. The other relies on vacuum tubes and the principles of thermionic emission.

Peltier devices •

a heat engine working backwards as a refrigerator, such as a Peltier device (diode)

Thermal oxidizer

A thermal oxidizer (or thermal oxidiser) is a process unit for air pollution control in many chemical plants that decomposes hazardous gases at a high temperature and releases them into the atmosphere.

Principle Thermal Oxidizers are typically used to destroy Hazardous Air Pollutants (HAPs) and Volatile Organic Compounds (VOCs) from industrial air streams. These pollutants are generally hyrdocarbon based and when destroyed via thermal combustion they are chemically changed to form CO2 and H2O.

Regenerative thermal oxidizer (RTO) One of today’s most widely accepted air pollution control technologies across the industry is a Regenerative Thermal Oxidizer, commonly referred to as a RTO. They are very versatile and extremely efficient – energy recovery efficiency can reach 95%. This is achieved through the storage of heat by dense ceramic stoneware. Regenerative Thermal Oxidizers are ideal in low VOC concentrations with larger process requirements. Systems can be used during long continuous 24 hour operations. There are currently many Regenerative Thermal Oxidizers on the market with the capabitlity of 99+% Volatile Organic Compound (VOC) destruction efficiencies. The ceramic heat exchanger(s) can be designed for thermal efficiencies as high as 97+%. Regenerative Thermal Oxidizers can be designed with multiple hot gas bypass systems, bake-out cycles, re-circulation heat exchangers and O2 monitoring to reduce carbon monoxide and nitrous oxide. Many environmental agencies are requiring 02 continuous monitoing for the control of these secondary gases. Higher VOC streams allow the RTO to operate at reduced or zero fuel usage, which makes these systems ideal for certain plant operations.

Regenerative catalytic oxidizer (RCO) In some applications, the use of Catalyst with the ceramic media helps allow oxidation at reduced temperatures. This can result in even lower operating costs compared to a Regenerative Thermal Oxidizer.

Ventilation air methane thermal oxidizer (VAMTOX) Ventilation Air Methane Thermal Oxidizers are used to destroy methane in the exhaust air of underground coal mine shafts. Methane is a greenhouse gas and, when destroyed via thermal combustion, is chemically altered to form CO2 and H2O. CO2 is 25 times less potent than methane when emitted into the atmosphere with regards to global warming. Concentrations of methane in mine ventilation exhaust air of coal and trona mines are very dilute; typically below 1% and often below 0.5%. VAMTOX units have a system of valves and dampers that direct the air flow across one or more ceramic filled bed(s). On start-up, the system preheats by raising the temperature of the heat exchanging ceramic material in the bed(s) at or above the auto-oxidation temperature of methane (1,000°C or 1,832°F), at which time the preheating system is turned off and mine exhaust air is introduced. Then the methane-filled air reaches the preheated bed(s), releasing the heat from combustion. This heat is then transferred back to the bed(s), thereby maintaining the temperature at or above what is necessary to support auto-thermal operation.

Thermal recuperative oxidizer

A less commonly used thermal oxidizer technology is a thermal recuperative oxidizer. Thermal recuperative oxidizers have a primary and/or secondary heat exchanger within the system. A primary heat exchanger preheats the incoming dirty air by recuperating heat from the exiting clean air. This is done by a shell and tube heat exchanger or a platetype exchanger. As the incoming air passes on one side of the metal tube or plate, hot clean air from the combustion chamber passes on the other side of the tube or plate and heat is transferred to the incoming air through the process of conduction using the metal as the medium of heat transfer. In a secondary heat exchanger the same concept applies for heat transfer, but the air being heated by the outgoing clean process stream is being returned to another part of the plant – perhaps back to the process.

Catalytic oxidizer Catalytic oxidation occurs through a chemical reaction between the VOC hydrocarbon molecules and a precious-metal catalyst bed that is internal to the oxidizer system. A catalyst is a substance that is used to accelerate the rate of a chemical reaction, allowing the reaction to occur in a normal temperature range of 550°F - 650°F (275°C to 350°C).

Direct fired thermal oxidizer - afterburner A direct-fired oxidizer is the simplest technology of thermal oxidation. A process stream is introduced into a firing box through or near the burner and enough residence time is provided to get the desired destruction removal efficiency (DRE) of the VOCs. Also called afterburners, these systems are the least capital intensive, but when applied incorrectly, the operating costs can be devastating because there is no form of heat recovery. These are best applied where there is a very high concentration of VOCs to act as the fuel source (instead of natural gas or oil) for complete combustion at the targeted operating temperature.

Thermoelectric generator Thermoelectric generators (also called thermogenerators) are devices which convert heat (temperature differences) directly into electrical energy, using a phenomenon called the "Seebeck effect" (or "thermoelectric effect"). Their typical efficiencies are around 510%. Older Seebeck-based devices used bimetallic junctions and were bulky while more recent devices use bismuth telluride (Bi2Te3) semiconductor p-n junctions and can have thicknesses in the millimeter range. These are solid state devices and unlike dynamos have no moving parts, with the occasional exception of a fan. Radioisotope thermoelectric generators can provide electric power for spacecraft. Automotive thermoelectric generators are proposed to recover usable energy from automobile waste heat.

Uses

Thermoelectric generators can be applied in a variety of situations. Usually, thermoelectric generators are used for small applications where heat engines (which are bulkier but more efficient) such as Stirling engines would not be possible. The use of waste heat in combustion engines promises to be a high volume application field. Not only the exhaust but also cooling agents are targeted. Two general problems exist in such devices: high output resistance and adverse thermal characteristics. In order to get a significant output voltage a very high Seebeck coefficient is needed (high V/°C). A common approach is to place many thermo-elements in series, causing the effective output resistance of a generator to be very high (>10kOhm). Thus power is only efficiently transferred to loads with high resistance; power is lost across the output resistance otherwise. A generator with very high output impedance is effectively a temperature sensor, not a generator. Secondly, because low thermal conductivity is required for a good TEG, this can severely dampen the heat dissipation of such a device (i.e. TEG's serve as poor heat sinks). For example it is not generally considered wise to place a TEG on an essential IC chip that requires cooling. Because of the low thermal conductivity of a TEG device, that IC is cooled at a slower rate. Cars produce waste heat, and harvesting it can increase the fuel efficiency of the car. Space probes to the outer solar system make use of the effect in radioisotope thermoelectric generators for electrical power. Solar cells use only the high frequency part of the radiation, while the low frequency heat energy is wasted. Several patents about the use of thermoelectric devices in tandem with solar cells have been filed. The idea is to increase the efficiency of the combined solar/thermoelectric system to convert the solar radiation into useful electricity. Thermoelectric modules can be used for energy recovery of otherwise wasted heat. The modules are efficient in use when collecting heat between 250 and 280 degrees Celsius and can convert 7.2 percent of the available heat into electricity. Currently, each device can produce 24 watts of energy though developments by the scientists at UC Berkeley suggest the output will improve. Choice in material may also change as availability and cost escalation of bismuth telluride may lead to the use of gallium arsenide for cars or silicon, which is under study by mechanical engineering professor, Agnes Maynard at UC Berkeley. Several companies have begun projects in installing large quantities of these thermoelectric devices.

Hydrogen turboexpander-generator

A hydrogen turboexpander-generator or generator loaded expander for hydrogen gas is an axial flow turbine or radial expander for energy recovery through which a high pressure hydrogen gas is expanded to produce work that is used to drive a electrical generator. It replaces the control valve or regulator where the pressure drops to the appropriate pressure for the low pressure network.

Description Per stage 200 bar is handled with up to 15,000 kW power and a maximum expansion ratio of 14, the generator loaded expander for hydrogen gas is fitted with automatic thrust balance, a dry gas seal and a programmable logic control with remote monitoring and diagnostics.

Application The hydrogen turboexpander-generators are used for hydrogen pipeline transport in combination with hydrogen compressors and for the recovery of energy in underground hydrogen storage. A variation are the compressor loaded turboexpanders which are used in the liquefaction of gases such as liquid hydrogen

Water heat recycling Water heat recycling (also known as drain water heat recovery, greywater heat recovery, or sometimes shower water heat recovery) is the use of a heat exchanger to recover energy and reuse heat from drain water from various activities such as dishwashing, clothes washing and especially showers. The technology is used to reduce primary energy consumption for water heating. Standard units save up to 60% of the heat energy that is otherwise lost down the drain when using the shower. The Drain Water Heat Recovery Industry and Manufacturers Association, the international trade organization, has set standards for the manufacturers, representatives and installers of this industry. Members of this association adhere to a strict set of guidelines. Drain Water Heat Recovery Association. The manufacturing members include EcoInnovation Technologies and WaterCycles. EcoInnovation Technologies has released a savings calculator in August 2010, this savings calculator that will revolutionize the way we think about drain water heat recovery. It gives the user many different parameters and variables to determine energy wasted, energy that could be potentially saved and the return on investment. The EcoInnovation calculator is free. The link is below. The technology is fully recognized in Canada and USA by LEED for homes, Energy Star for New Homes Canada. LEED credits are 1 per DWHR unit installed. Energy Star credits are 1 per DWHR installed on 2 showers and 1/2 a credit for 1 shower. Ontario has

a slightly different approach to Energy Star for Homes credits and DWHR. Details need to be verified through EnerQuality Corp.. It is important to note that only the list of Natural Resources Canada authorized models and manufactures are eligible for these points. Each manufacture has a list of the units permitted to participate in the LEED and Energy Star for Homes programs. Manufactures include Watercycles and EcoInnovation Technologies. Typical retail price for a domestic drain water heat recovery unit ranges from around $400 to $1,000. For a regular household, water heating is usually the second highest source of energy demand. The energy savings results in an average payback time for the initial investment of 2–10 years according to Natural Resources Canada, Canadian Center for Housing Technology and US DOE.