Status of not-in-kind refrigeration technologies for household space conditioning, water heating and food refrigeration

Status of not-in-kind refrigeration technologies for household space conditioning, water heating and food refrigeration

International Journal of Sustainable Built Environment (2012) 1, 85–101 Gulf Organisation for Research and Development International Journal of Sust...

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International Journal of Sustainable Built Environment (2012) 1, 85–101

Gulf Organisation for Research and Development

International Journal of Sustainable Built Environment SciVerse ScienceDirect www.sciencedirect.com

Review

Status of not-in-kind refrigeration technologies for household space conditioning, water heating and food refrigeration Pradeep Bansal ⇑, Edward Vineyard, Omar Abdelaziz Building Equipment Program, Oak Ridge National Laboratory (ORNL), One Bethel Valley Road, P.O. Box 2008, Oak Ridge, TN 37831-6070, USA Received 2 February 2012; accepted 19 July 2012

Abstract This paper presents a review of the next generation not-in-kind technologies to replace conventional vapor compression refrigeration technology for household applications. Such technologies are sought to provide energy savings or other environmental benefits for space conditioning, water heating and refrigeration for domestic use. These alternative technologies include: thermoacoustic refrigeration, thermoelectric refrigeration, thermotunneling, magnetic refrigeration, Stirling cycle refrigeration, pulse tube refrigeration, Malone cycle refrigeration, absorption refrigeration, adsorption refrigeration, and compressor driven metal hydride heat pumps. Furthermore, heat pump water heating and integrated heat pump systems are also discussed due to their significant energy saving potential for water heating and space conditioning in households. The paper provides a snapshot of the future R&D needs for each of the technologies along with the associated barriers. Both thermoelectric and magnetic technologies look relatively attractive due to recent developments in the materials and prototypes being manufactured. Ó 2012 The Gulf Organisation for Research and Development. Production and hosting by Elsevier B.V. All rights reserved. Keywords: Efficiency; Thermoacoustics; Thermoelectricity; Stirling; Magnetic refrigerator

Contents 1. 2. 3. 4. 5.

Introduction . . . . . . . . . . . . . . . . . . . . . . Thermoacoustic refrigeration . . . . . . . . . . Thermoelectric refrigeration . . . . . . . . . . . Thermotunneling (thermionic) refrigeration Magnetic refrigeration . . . . . . . . . . . . . . .

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⇑ Corresponding author.

E-mail addresses: [email protected], [email protected] (P. Bansal). Peer review under responsibility of The Gulf Organisation for Research and Development.

Production and hosting by Elsevier

2212-6090/$ - see front matter Ó 2012 The Gulf Organisation for Research and Development. Production and hosting by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ijsbe.2012.07.003

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Nomenclature AMRR active magnetic regenerative refrigeration CCHP combined cooling heating and power CD-MHHP compressor driven metal hydride heat pumps COP coefficient of performance HX heat exchanger HPWH heat pump water heater i electric current [A] IHPS integrated heat pump system K thermal conductivity [Wm1 K1] MCE magneto caloric effect NIK not-in-kind PTR pulse tube refrigerator Q heat transfer rate [W] T temperature [K]

6. 7. 8. 9. 10. 11. 12.

13. 14.

VS Z

variable speed figure of merit

Greek symbols a seebeck coefficient [VK1] D difference q electrical resistivity [X-m] Subscripts C cold H hot adiabatic adiabatic process L low temperature R room temperature

Stirling cycle refrigeration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Pulse tube refrigerator (PTR) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Malone refrigeration. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Absorption refrigeration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Adsorption refrigeration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Compressor-driven metal hydride heat pump . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Developments in water heating and space conditioining. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.1. Heat pump water heater (HPWH) using transcritical CO2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.2. Integrated heat pump systems (IHPS) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Overall assessment of NIK technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction The vapor compression refrigeration has remained practically a predominant technology for well over 100 years. The fundamental principle is to use liquid–vapor and vapor–liquid phase transitions to transfer heat from a low temperature state to a higher temperature state. It is desirable to have these phase transitions occur at room temperature. The ideal refrigerant for the vapor compression systems should be non-toxic, noncorrosive, efficient, cost effective and more importantly environmentally benign. There is a general trend of increasing demand for heating, cooling and refrigeration services world-wide. This will eventually lead to the increase in related CO2 emissions. This trend could be alleviated by the performance enhancement of current heat pumping technologies and/ or the development of new energy efficient technologies. In this context, the current paper reviews emerging not-in-kind technologies (NIK) that offer the potential

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for significant energy savings and environmental benefits compared to existing technologies. In addition, the status of emerging technologies that are useful in a household, including space conditioning, water heating and refrigeration, are discussed. There have been a few integrated reviews of alternative technologies in the open literature. Fischer et al. (1994) presented one of the earliest and most comprehensive summaries of not-in-kind technologies. This was then updated by Fischer and Labinov (2000) with emphasis on economic impact and potential commercialization. Lately there has been a flurry of activity (Radermacher et al., 2007; Dieckmann et al., 2007) in this area, where Navigant Consulting Inc. (2009) provided an overview of some of the alternative technologies targeting energy savings for commercial refrigeration applications. This was followed by a report from Brown et al. (2010) that assessed the prospects of thermoelectric, thermionic, thermotunneling, thermoacoustic and magnetic refrigeration for space cooling and food

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refrigeration applications. Most recently, Tassou et al. (2010) provided a broader view of emerging technologies for food refrigeration applications. This paper evaluates the status of ten not-in-kind heat pump technologies relevant to domestic applications, namely thermoacoustic refrigeration, thermoelectric refrigeration, thermotunneling, magnetic refrigeration, Stirling cycle refrigeration, pulse tube refrigeration, Malone cycle refrigeration, absorption refrigeration, adsorption refrigeration, and compressor driven metal hydride heat pumps. The paper also discusses the development of heat pump water heating and integrated heat pump systems and their respective impact on energy consumption in households. In addition, the paper presents assessments of potential benefits from alternative technologies and a brief summary of the R&D opportunities that could develop such technologies further. Potential barriers to implement these technologies in the marketplace are discussed along with options for each technology to achieve significant improvements in energy efficiency or other environmental benefits for their application in space conditioning, water heating and refrigeration in households.

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2. Thermoacoustic refrigeration Thermoacoustic cooling is a technology that uses highamplitude sound waves in a pressurized gas to generate a temperature gradient across a stationary element called the stack (Newman et al., 2006). A thermoacoustic device is placed inside a sealed pressure vessel consisting of an acoustic driver (e.g. a loudspeaker) that generates a highamplitude sound wave, and hence large temperature and pressure oscillations into a resonator containing a regenerator or stack. The sound wave may be generated using either thermal or mechanical energy. This cycle is shown in Fig. 1(A), and consists of four principal components: 1. A “stack” of porous material, parallel plates, or spiral rolls of thin sheets, 2. Hot and cold heat exchangers with large area to volume ratio, 3. A rigid and sealed tube that may incorporate a Helmholtz resonator to shorten the device and minimize losses, and 4. An acoustic energy source.

Fig. 1. (A) Schematic of a thermoacoustic refrigerator. (B) Working principle of a thermoacoustic refrigerator from Largrangian viewpoint.

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The sound wave causes the gas to compress and expand adiabatically, which results in the gas to heat up and cool down respectively. Heat is transferred from the working fluid (i.e. gas) to the stack near the phase of greatest compression and from the stack to the gas parcel near the phase of greatest expansion. The heat is then respectively dissipated to and received from an external fluid through a heat exchanger placed at each end of the stack. The standingwave device, such as shown in Fig. 1, generates useful cooling by pumping heat from the cold heat exchanger to the hot heat exchanger. Fig. 1(B) shows an idealized thermoacoustic refrigeration cycle, consisting of four processes:

Some continuing major difficulties in achieving higher efficiencies with acoustic refrigerators have been the relatively low power density (Brown et al., 2010), low cooling capacities, large physical size, heat conduction between the heat exchangers and hence poor performance of the heat exchangers (Wetzel and Herman, 1997). Design and control of compact heat exchangers in oscillating flow presents a unique challenge for thermoacoustic refrigeration units with large capacities. Due to these deficiencies, thermoacoustic refrigeration will continue to be a non-competitive technology for domestic applications in the foreseeable future. 3. Thermoelectric refrigeration

 1–2: gas parcel is compressed adiabatically while being displaced toward the velocity node  2–3: gas parcel is further compressed while heat is transferred to the stack  3–4: gas parcel is expanded adiabatically while being displaced toward the pressure node  4–1: gas parcel is further expanded while heat is absorbed from the stack The complete cycle described above resembles a series of Brayton cycles grouped together. Thermoacoustic refrigerators employ environmentally friendly refrigerants, usually a mixture of perfect gases, such as xenon and helium. The stack is typically fairly short, on the order of few centimeters, and is made of a material that does not conduct heat well but has high heat capacity (e.g. ceramic). Although the concept of thermoacoustic refrigeration has been around for a while, there is still no commercial system available except for few examples of advanced developments (Wollan and Swift, 2001; Naluai, 2002; Poese et al., 2004; Hotta et al., 2009). Tijani et al. (2002) achieved a temperature of 65 °C (85 °F) from an optimized thermoacoustic refrigerator. An early prototype thermoacoustic refrigerator (Swift, 1988) achieved 3 W of cooling at a temperature of 29 °C (20 °F) and a sink temperature of 25 °C (77 °F). Another prototype thermoacoustic refrigeration unit designed for an ice-cream freezer (Poese et al., 2004; PSU, 2012) with a cooling capacity of 119 W at 24.6 °C (12.3 °F) and a COP of 0.81, was still well below vapor compression system performance. Other early prototypes achieved cooling capacities from 20 W (Garrett et al., 1993; Berhow, 1994) to as high as 10 kW (Garrett, 2002) in a unit designed for air-conditioning applications. A recent study by Nsofor and Ali (2009) found that, for a given frequency, there exists an optimum pressure that results in the maximum temperature difference, which in turn yields in the maximum possible cooling load. The simulation/optimization study of standing-wave thermoacoustic coolers by Paek et al. (2007) suggests that maximum second law efficiency increases with temperature span and reaches a maximum for temperature lifts of around 80 °C (144 °F). Zink et al. (2010) presented a study showing the environmental motivation for thermoacoustic refrigeration with other benefits being low cost and high reliability.

Thermoelectric refrigeration is based on the observation first made by Peltier (1834) that a direct electric current, i, passing through a circuit formed by two dissimilar conductors or semiconductors, A and B, will cause a temperature difference to develop at the junctions of the two conductors. A refrigeration effect develops at the cold junction, and heat is rejected at the hot junction. The heat produced or absorbed at each junction can be given by: Q ¼ ðaA  aB Þ  i  T

ð1Þ

where a is known as the Seebeck coefficient and is the property (positive or negative) of the material, i the electrical current supplied to the thermoelectric device and T is the absolute temperature of the junction. The absolute Seebeck coefficient, a, for metals does not exceed 50 lV per K (ASHRAE, 1981). However, a can be much higher for semiconductor materials. The highest a (250 lV per K) is achieved from alloys of Tellurium (Te) doped with antimony tri-iodide (SbI3) to produce an “ntype” semiconductor and with excess Te to make a “p-type” semiconductor. In the cooling mode, direct current passes from the n- to p-type semiconductor materials. The temperature TC of the conductor decreases and the heat is absorbed from the space to be cooled. This occurs when electrons pass

Fig. 2. Schematics of thermoelectric refrigeration cycle in cooling mode.

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from a low energy level in the p-type material through an interconnecting conductor to a higher energy level in the n-type material. This heat is then rejected to the surroundings at TH. This phenomenon is illustrated in Fig. 2. The advantages of thermoelectric refrigeration are that it has no moving parts, no CFCs or other fluids that are hazardous to the environment (Riffat and Ma, 2003), high reliability, reduced weight, and flexible operation. In order to achieve the maximum COP of the cycle, given by Eq. (2), TH and TC (being respectively the absolute temperatures at the hot and cold junctions), should respectively be as low and as high as possible, while Z (called the ‘figure of merit’ defined by Eq. (3) – a temperature dependent property of each material) should be as high as possible qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi   C 1 þ Z T H þT  TT HC TC 2 COPmax ¼  qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ð2Þ TH  TC C 1 þ Z T H þT þ 1 2 2

ðap  an Þ Z ¼ pffiffiffiffiffiffiffiffiffiffiffi pffiffiffiffiffiffiffiffiffiffiffi2 K p qp  K n qn

ð3Þ

Higher performance requires materials with high difference in a’s, low thermal conductivity K, and high electrical conductivity (or low q). However, this is intrinsically contradictory. Thermoelectric modules, based on commercially available materials that have a ZT [T is the average of TH and TC] of about 1, cannot compete in efficiency with traditional vapor compression systems (Yang et al., 2008) when operating at a relatively large temperature lift (TH  TC), e.g., 30 °C (54 °F). However, the efficiency of

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thermoelectric modules increases rapidly with decreasing temperature lift, where it may have some advantage over traditional vapor compression systems. A ZT with a value of 9 and above is required to produce energy efficient cooling units. At an absolute temperature of 300 K (27 °C or 80 °F), ZT = 1 would correspond to a disappointing figure of merit Z = 0.0033. The best ZT materials are found in heavily doped semi-conductors. BiTe3 (p-type)/Sb2Te3 (n-type) super lattices are reported to have ZT of 2.5 around room temperature. A significant ZT increase has been reported in bulk materials made from nano crystalline powders of p-type BiSbTe, with a ZT peak of 1.4 at 100 °C (212 °F) (Yang et al., 2008). Significant advancements are taking place in the development of thermoelectric nano composites, resulting in higher ZT values (Lan et al., 2010). Although ZT of thermoelectric modules has increased significantly in recent years, their practical applications are still limited. To date, reported thermoelectric system efficiency could not compete with conventional vapor compression technology. Fig. 3 depicts the theoretical COP of different thermoelectric materials as well as the Carnot COP and the COP for a conventional vapor compression system using R134a as a working fluid. All thermoelectric materials are less efficient than vapor compression system except for the single molecule devices (Finch et al., 2009; Alexandrov and Bratkovsky, 2010). Fig. 3 shows that the efficiency of a thermoelectric device exceeds the efficiency of the vapor compression only when the temperature lift is less than 5 °C (9 °F). Vian and Astrain (2009) built a thermoelectric domestic refrigerator with a single food compartment (of 0.225 m3)

Fig. 3. COP of thermoelectric modules for different materials at TH = 300 K compared to Carnot and vapor compression system (using R134a) COPs.

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Fig. 4. Advantage of thermionic phenomenon.

maintained at 5 °C (41 °F). A COP of 0.45 was demonstrated at a temperature lift of 19 °C (34.2 °F). Such performance is much below that of conventional vapor compression technology. However, in order to achieve better COP, Yang et al. (2008) proposed a hybrid system by using a low temperature lift thermoelectric subcooler to increase the subcooling temperature in a vapor compression system. This capitalizes on the fact that the thermoelectric COP is higher for small temperature lifts such as 5 °C (9 °F). Other niche applications for thermoelectric refrigeration include mobile coolers that are quiet and vibration free. They are also widely used as replacements for wine cabinets, mini-refrigerators, and water coolers (Navigant Consulting Inc., 2009; Bansal and Martin, 2000). Despite numerous advantages of thermoelectric refrigeration, low figure of merit hinders its wide scale deployment. An order of magnitude increase in the ‘figure of merit’ is required for thermoelectric refrigeration to compete with the energy efficiency of the ‘state-of-the-art’ vapor compression technologies. Molecular thermoelectric devices have great potential energy efficiency; however these cannot be produced economically at large scale with current fabrication technologies. Furthermore, current fabrication and assembly technologies result in a high thermal contact resistance that causes the temperature lift to increase, thereby dramatically reducing the energy efficiency. Efforts are needed to integrate thermoelectric devices with heat exchangers to eliminate the contact resistance. It is unlikely for thermoelectric refrigeration to compete with vapor compression technology for household applications in the foreseeable future.

retically, under an applied electrical potential, hot electrons emitted by the cathode are at a higher energy level than those that are left behind, which reduces the average energy level (temperature) of the cathode. Since the electrons being absorbed on the other side of the gap are at a higher energy level than those in the electrode, the average energy level is increased, and the electrode (i.e. anode) is heated. The new type of materials called electrides that require only small amount of energy to emit electrons at lower temperatures, make this technology attractive. The major advantage of thermionic over thermoelectric refrigeration is the elimination of the conduction heat transfer mode as shown in Fig. 4. A number of studies have been carried out on thermotunneling (e.g. Dillner, 2008, 2010; O’Dwyer et al., 2009; Weaver et al., 2007; Shakouri, 2006; Hishinuma et al., 2001; Ulrich et al., 2001; Kenny et al., 1996; Mahan, 1994). O’Dwyer et al. (2009) suggested that the most promising way to develop room temperature vacuum thermionic refrigerators is to combine new low work function emitter materials with the nanometer gap techniques. Dillner (2010) calculated an upper limit for the dimensionless thermoelectric figure of merit attainable by thermotunneling as p2/12, which suggests that thermotunneling cannot outperform the state-of-the-art thermoelectric materials. It is unlikely for thermotunneling to be an energy saving technology for household applications in the near future. Considerable R&D would be required including the development of cost effective low work function surfaces, with typically less than 0.3 eV (O’Dwyer et al., 2006). In addition, the requirement for extremely small inter-electrode spacing (nanometer-sized gaps) presents a unique challenge for large-scale manufacturing.

4. Thermotunneling (thermionic) refrigeration 5. Magnetic refrigeration There is a fine distinction between thermoelectric and thermionic cooling, where the former uses a flow of electrons through a pair of semiconductors in close physical contact, while the latter uses the flow of electrons between two electrodes (i.e. cathode and anode) that are separated by an extremely small gap (of the order of microns). Theo-

Magnetic refrigeration at room temperature is an emerging technology that exploits the magnetocaloric effect (MCE) found in solid-state refrigerants. These refrigerants are environmentally friendly since they have zero ozone depletion potential and zero global warming potential

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Fig. 5. Thermomagnetic cycle showing entropy-temperature diagram for Gd (Gd properties based on data from Jelinek et al., 1966 and Benford and Brown, 1981).

(Dettmer, 2006. The temperature or point, at which a ferromagnetic material loses its permanent magnetism and becomes paramagnetic, and exhibits its greatest MCE, is called the ‘Curie temperature or Curie point’. The MCE effect varies for different materials and can be intensified by increasing the magnetic field. MCE effect causes certain materials to warm adiabatically upon application of a magnetic field and cool when the field is removed, and is coupled to an external heat transfer fluid to accomplish the heat pumping effect. The MCE behavior depends on the type of material: ferromagnetic with magnetic domains below the Curie point, or paramagnetic without magnetic domains. Fig. 5 depicts a theoretical magnetocaloric refrigeration cycle using Gadolinium with a magnetic field of 7 Tesla (T). In the magnetic refrigeration cycle, randomly oriented magnetic spins in a paramagnetic material can be aligned via a magnetic field, resulting in an adiabatic rise in temperature and decrease in entropy. This phenomenon can be used in heat pumping applications to reject heat at higher temperatures (Hull and Uherka, 1989). This process is highly reversible since, upon removal of the magnetic field, the magnetic spins return to their randomized state, resulting in an adiabatic decrease in temperature but increase in entropy. The processes involved in magnetocaloric refrigeration are summarized below:  (A–B) Randomly oriented magnetic spins align after applying a magnetic field (H) along an isentropic process increasing the magnetocaloric material temperature by DTadiabatic, AB.  (B–C) Excess heat is rejected to ambient maintaining constant magnetic field H.  (C–D) When the magnetic field is turned off, the spin moments re-randomize and the temperature is reduced by DTadiabatic, CD following an isentropic process.  (D–A) The magnetocaloric material absorbs heat from the refrigerated volume. This raises its temperature and the cycle continues.

The magnetic refrigeration technology, using active magnetic regenerator (AMR) cycle, is claimed to have the potential of higher energy efficiency than the current vapor compression technology (Russek and Zimm, 2006), however, no competitive system is commercially available todate for room temperature applications. An AMR cycle uses magnetic material (or refrigerant) both as a thermal storage medium as well as a means to convert magnetic work to net heat transfer. The solid material is cycled through a low and high magnetic field, while exchanging energy with a heat transfer fluid (e.g. glycol water) oscillating through the void space of the AMR. An effective regenerator has high surface area per unit volume, high conductivity and low pressure drop. A prototype rotary magnetic refrigerator built by Astronautics Corporation of America Inc., Milwaukee, USA is shown in Fig. 6. There has been a vigorous research activity related to this technology in the last decade where an exponential increase in publications has been seen; exceeding 250 in 2007 (Gschneidner and Pecharsky, 2008). As a result, a number of prototypes have emerged (Hiraro et al., 2010; Muller et al., 2010; Zimm et al., 1998, 2006, 2007; Pecharsky and Gschneidner, 2006; Hirano, 2003; Zimm, 2003). Subsequently, new designs for magnetic refrigeration components and systems have evolved that use compact devices and water-based heat transfer fluids. Yu et al. (2010) reviewed near room temperature magnetic refrigeration prototypes showing 41 working prototypes, 11 of which were demonstrated in 2009. Yu et al. (2010) reviewed near room temperature magnetic refrigeration prototypes and patents and noted that there were 41 prototypes and almost 135 patents were issued during 1997–2009. At Thermag conference in Grenoble during September 17–20, 2012, 29 prototypes were presented in varying sizes from a few Watts to 2 kW that employed rare earth alloys such as LaFeCoSi, LaFeMnSiH, LaFeSiH, MnFePas and MnFePGe (Bruck et al., 2012). The recent invigoration in patents applications and prototypes of magnetic refrigeration

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Fig. 6. Rotary magnetic refrigerator from Astronautics Corporation of America Inc., Milwaukee, Wisconsin (after Zimm, 2003 and Navigant Consulting Inc., 2009).

depicts advancing nature of this technology and the underlying demand. The research theme in recent studies is focused on developing better magnetocaloric materials, cycles, magnets and working prototypes, as highlighted in some of the exemplary references (Bruck et al., 2012; Bahl et al., 2010; Bjork et al., 2010; Russek et al., 2010; Kim et al., 2007; Phan and Yu, 2007; Allab et al., 2006; Basso et al., 2006; Bingfeng et al., 2006; Gao et al., 2006; Tagliafico et al., 2006; Zimm et al., 2006). Smaller temperature differences are more feasible for magnetic refrigeration technology due to the limited temperature difference of magnetocaloric materials, while cascade systems are more desirable for higher temperature differences (Kitanovski and Egolf, 2009). A layered regenerator bed from several magnetic refrigeration materials (Rowe and Tura, 2006) that have Curie temperatures tailored to the local regenerator temperature in active magnetic regenerative refrigeration (AMRR) can result in maximizing the MCE (Engelbrecht et al., 2006, 2007). Room temperature applications require materials with a Curie temperature around 22 °C (71.6 °F). Gadolinium and Gadolinium alloys exhibit large MCE around this temperature. They are, therefore, among the most widely used materials for room temperature refrigeration and space cooling applications. These materials undergo secondorder phase transitions and do not exhibit magnetic or thermal hysteresis, the physics of which is discussed by Basso et al. (2006). By using such materials and applying a 2 T magnetic field, researchers have demonstrated temperature lifts of 5 °C (9 °F). Higher magnetic fields result in larger temperature lifts, but at higher cost and lower efficiency. Most prototypes rely on the use of the active magnetic regenerative cycle to provide high temperature lift for air-conditioning and refrigeration applications. Recent research on materials that exhibit a large entropy change, such as Gd5(SixGe1  x)4, La(FexSi1  x)13Hx and MnFeP1  xAsx alloys, provide acceptable performance for near room temperature applications. These materials are called giant magnetocaloric effect materials (Pecharsky and Gschneidner, 2006).

Japan has launched a national project of developing a room temperature magnetic refrigerator with a COP exceeding 10 by using new materials and other innovations (Hiraro et al., 2010). The group fabricated a sample of Mn1 + dAs1  xSbx that has a magneto-caloric effect several times higher than Gd, while developing another magnetic material, Pr2Fe17 that has the same relative cooling power as that of Gd at 10% of the cost. A record COP of 4.6 was claimed to have been achieved by the Cooltech magnetic refrigeration prototype [Muller et al., 2010]. The prototype measures 230  300  250 mm3, weighs 34 kg, and uses 0.6 mm thick and 100 mm long magnetocaloric material strips. The device achieved minimum and maximum temperatures of 17 °C (1.4 °F) and +45 °C (113 °F) respectively. The system employs a permanent magnet with 1.6 T magnetic fields. The prototype achieved a 110 W cooling capacity between 13 °C (55.4 °F) and 43 °C (109.4 °F) (DT of 30 K). Despite all the above advancements, there is still no experimental data available in the open literature to compare magnetic refrigeration with vapor compression refrigeration technology. Various studies, including Kitanovski and Egolf (2010), have outlined major challenges facing the magnetocaloric technology, which include scarcity of magnetocaloric materials, high cost of materials and magnets, limitations of physical properties of materials, and time delay required to reach the required temperature lift. Although significant developments (Jung et al., 2012; Rowe, 2011; Tura and Rowe, 2011; Arnold et al., 2011) have occurred lately in the AMR devices, magnetic devices are still not able to compete with vapor compression systems. Some of the recent research efforts are devoted to synthesizing and characterizing properties of MCEs, and modeling and testing of AMRs including designing, building and testing of prototypes. 6. Stirling cycle refrigeration An ideal Stirling cooler is a reversed Stirling engine. It consists of a closed-cycle regenerative heat engine with a

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Fig. 7. Schematics of the working principle of Stirling cycle.

Fig. 8. Picture and schematics of a prototype Stirling refrigerator (after Otaka et al., 2002).

gaseous working fluid (generally He or H2). The cycle consists of two isothermal reversible processes and two constant volume reversible processes, as shown in Fig. 7. Like the Brayton cycle, the working fluid is moved between the hot and cold spaces through a regenerator by a system of displacers. The power piston is driven by an electric motor for a refrigeration device. Large flow rates are required to produce large capacities. Although Stirling cycle cryocoolers are commercially available for infrared sensors and high temperature superconducting devices, their application at room temperature is practically nonexistent due to a relatively low COP and high first cost. Otaka et al. (2002) designed, simulated, and tested a displacer-type or b-type Stirling cycle prototype refrigerator

with a 100 W capacity, as shown in Fig. 8, to operate between 40 °C (40 °F) and 30 °C (86 °F) at an operating frequency of 16.7 Hz and a sealing pressure within the refrigerator of less than 1.0 MPa. At a cooling temperature of 20 °C (4 °F), radiator temperature of 30 °C (86 °F), and mean pressure of 0.4 MPa, the cooling capacity of the refrigerator increased by 20% when hydrogen was used as a working fluid instead of helium. A free piston Stirling cooler prototype with a closed thermosyphon system and R134a refrigerant was integrated into a domestic refrigerator by Oguz and Ozkadi (2002). The prototype was tested at different refrigerant charges and voltage inputs to the cooler. It was found to consume approximately 30.5 W to maintain a cabinet

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Fig. 9. Schematics of a pulse tube refrigerator.

temperature of 5 °C (41 °F) at 25 °C (77 °F) ambient temperature. Luo et al. (2006) tested a thermoacoustic-Stirling heat engine driven refrigerator, which achieved a no-load temperature of 65 °C (85 °F) with a cooling capacity of 270 W at 20 °C (4 °F) and 405 W at 0 °C (32 °F). These results were quite encouraging for the application of thermoacoustic-Stirling refrigerator for food refrigeration and air-conditioning. Recently, Sun et al. (2009) tested a V-type Stirling cycle for domestic refrigeration with He and N2 as the working fluids and a cold end temperature of 35 °C (31 °F). Using He, cooling capacities up to 70 W were achieved with a COP of 0.8. Stirling cycle refrigeration may provide incremental energy savings in domestic applications. However, for Stirling cycle refrigeration to compete with vapor compression technologies, regenerator performance needs to improve substantially to achieve higher effectiveness, lower pressure drop, lower void volume and lower cost. Furthermore, cold and hot heat exchangers need to be designed with a higher heat transfer density and lower log mean temperature differences. 7. Pulse tube refrigerator (PTR) The pulse tube refrigerator (PTR) or pulse tube cryocooler is a developing technology that pumps heat through the compression and expansion of a gas. It offers several advantages over Stirling refrigerators, including no displacer and no mechanical vibrations. It can be made without any moving parts in the low temperature section of the device. The gas in-phase motion is achieved by the use of an orifice and a reservoir volume to store gas (Radebaugh, 2000). A PTR, as shown in Fig. 9, consists of eight main components – (i) a compressor that compresses the gas, typically He, to higher temperatures, (ii) a heat exchanger (HXH1) that rejects heat at room temperature cooling the gas, (iii) a porous regenerator (to absorb and discharge heat, from and to the gas, when it flows to the right and to the left respectively), (iv) another heat exchanger (HXL) absorbing the useful cooling power (QL) at low temperature TL, (v) a tube in which gas moves back and forth, (vi) a hot end heat exchanger (HXH2) rejecting heat to room temperature, (vii) an orifice as a flow resistance device, and (viii) a large buffer volume containing He gas.

The PTR works on following four adiabatic compression and expansion processes in the pulse tube (de Waele, 2000): (i) Gas is compressed to high temperature, (ii) Gas at high temperature and pressure flows through the orifice to the reservoir, and rejects heat to the ambient through a heat exchanger (at room temperature TH), (iii) The piston moves up and expands the gas adiabatically in the pulse tube, (iv) This cold gas at low pressure in the pulse tube travels back through the cold heat exchanger at the low temperature TL (providing cooling capacity QL). The flow in either direction stops when the pressure in the tube is either lower (when moving forward) or higher (when moving backwards) than the average pressure in the tube. The PTR has a regenerator (made of a porous matrix) that precools the incoming high pressure gas before it reaches the cold end (and vice-versa) and a hot-end heat exchanger that rejects heat to room temperature. PTRs are commercially available for temperature applications between 196 °C (320.8 °F) down to 269 °C (466.7 °F), where their relative Carnot efficiency is steadily improving (Swift, 1997; Hu et al., 2010)]. However, the COP of PTR at room temperature is quite low and is unlikely to play a role in domestic refrigeration. 8. Malone refrigeration Malone refrigeration, invented in 1931 (Malone, 1931), uses a liquid without evaporation as the working fluid near its critical point, instead of the customary gas, in a regenerative or recuperative refrigeration cycle such as Stirling or Brayton cycles. Due to the inherent incompressible nature of liquid, this cycle has the advantage over gas cycle for achieving higher pressure change per unit volume (Malone, 1931). The machine can be driven externally to produce a refrigerating effect. In a refrigeration system, the Malone cycle uses the cooling associated with the expansion of a liquid, but without a phase change. One of the earliest papers studying the physics of the liquids working in heat engines was published in 1980 (Allen et al., 1980). Most of the preliminary research was conducted at the Los

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Alamos National Laboratory (USA) in the early 1990s (Swift, 1989, 1993; Swift and Brown, 1994), where one of the first Malone refrigerators (Swift, 1989) used liquid propylene (C3H6) in a double acting 4 cylinder Stirling configuration, followed by liquid CO2 (Swift and Brown, 1994). Although CO2 being an environmentally friendly refrigerant could make a good candidate for the Malone cycle, its critical temperature (31 °C, 88 °F) is too low for efficient operation in many HVAC/R applications. Other possible candidate fluids include methyl alcohol, ethyl alcohol, acetone or sulfur dioxide, which have higher critical temperatures, but present safety issues. Other fluids include sulfur hexafluoride and various fluorocarbons such as HFC134a. In general, these compounds have critical temperatures more suited to HVAC applications and critical pressures lower than CO2. Although high heat capacity liquids offer the advantage of reduced mass flow rate for good heat transfer, the main drawback of Malone refrigeration is that these liquids are unable to achieve the desired (or required) temperature change for efficient refrigeration. There has been practically no activity in the recent past on Malone refrigeration and it is unlikely for this technology to penetrate in the household applications in the near future. 9. Absorption refrigeration Absorption and adsorption refrigeration are thermally driven technologies that respectively use liquid and solid sorbents. These systems are popular in applications where demand side management is important and/or waste heat is readily available. An absorption cycle, shown in Fig. 10, utilizes a binary mixture of refrigerants such as ammonia–water or water–LiBr. The single effect cycle consists of an absorber, a generator or desorber, a condenser, an evaporator, and an electric solution pump, with the possibility of additional components, such as internal heat exchangers, to enhance efficiency. An external heat source, such as a gas burner in a direct fired system, steam or hot water in an indirect fired system, or waste heat, is used in the generator (or desorber). Heat absorbed in the generator allows the refrigerant to desorb from the absorbent, creating a high pressure vapor. In cases where a volatile absorbent is used (e.g. ammonia–water), a rectifier is needed to reduce the concentration of the volatile absorbent (e.g. water) in the vapor to the condenser. A number of advanced cycles have been proposed in the literature in order to improve the COP starting from single effect to GAX (Altenkirch and Tenckhoff, 1911), double-effect (Vliet et al., 1982), cycle with two absorbers (CMostofizadeh and Kulick, 1998), compression-absorption (Hulte´n and Berntsson, 1999), auto cascade (Chen, 2002), two stage absorption (Fan et al., 2007) and more recently an expander-compressor cycle (Hong et al., 2010) and several waste heat/renewable energy operated absorption systems (Wang et al., 2012). Single- and double-effect absorption chillers are commercially available for large scale applications, while absorption refrigerators are available for small

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capacity applications, such as mini-bar fridges, recreational vehicles, hotel rooms, and boats. Some of the advantages of small refrigerators include quiet operation and flexible use of any energy source such as gas, battery and electricity (Bansal and Martin, 2000). Although absorption coolers offer many advantages over vapor compression systems (e.g. environmentally friendly absorbent/refrigerant pairs, fewer moving parts), they are mainly limited to large scale applications and are not competitive for small scale applications due to system complexity, high cost and lower efficiency compared to vapor compression systems. 10. Adsorption refrigeration An adsorption system uses multiple beds of adsorbents such as silica-gel in a silica-gel water system, to provide continuous capacity, and does not use any mechanical energy but only thermal energy. An adsorption refrigeration system usually consists of four main components: a solid adsorbent bed, a condenser, an expansion valve and an evaporator. The solid adsorbent bed is linked to the evaporator. It desorbs refrigerant when heated and adsorbs refrigerant vapor when cooled such that the bed works like a thermal compressor to drive the refrigerant around the system to heat or cool a heat transfer fluid or to provide space heating or cooling. When the bed becomes saturated with refrigerant, it is isolated from the evaporator and connected to the condenser. The refrigerant vapor is condensed to a liquid, followed by expansion to a lower pressure in the evaporator where the low pressure refrigerant is vaporized producing the refrigeration effect (i.e. cooling the refrigerator air). When further heating no longer produces desorbed refrigerant from the adsorbent bed, the refrigerant vapor from the evaporator is reintroduced to the bed to complete the cycle. To obtain a continuous and stable cooling effect, generally two (or multiple) adsorbent beds are used, where one bed is heated during desorption while the other bed is cooled during adsorption. In order to achieve high efficiency, heat of adsorption needs to be recovered to provide part of the heat needed to regenerate the adsorbent. A recent literature review of conventional adsorption cycle was presented by Wang et al. (2010). Adsorptive beds of the chillers can be regenerated by low-grade temperatures using waste heat or solar energy as heat source. These chillers can also be employed in CCHP systems. The overall thermal and electrical efficiency in these systems can be above 70% (Wang et al., 2005). Some of the recent adsorption system performance enhancement technologies include heat pipes (Yang et al., 2006) and consolidated compound adsorbents (TamainotTelto and Critoph, 2003). Lu et al. (2006) designed a prototype icemaker with specific cooling power of 770 Wkg1 and a COP of 0.39, at 20 °C evaporation temperature. Adsorption systems are known to suffer from low coefficient of performance (COP) and low specific cooling power (Wang et al., 2010; Wang and Oliveira, 2006).

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working fluid, which is flammable and hence not suitable for domestic applications. Other challenges for using CDMHHP in domestic applications include high cost of metal hydrides, non-availability of suitable materials with fast reaction kinetics, and the need for improved hydrogen compressor technology. 12. Developments in water heating and space conditioining 12.1. Heat pump water heater (HPWH) using transcritical CO2 Fig. 10. Schematics of single-effect absorption cycle.

Although commercial adsorption systems for air-conditioning applications between 35 and 350 kW (Tassou et al., 2010: Mayekawa, 2012) are reported to be available, household scale systems are not yet commercially available. Wang et al. (2010) concluded that there is still a strong research need for advanced refrigerant/adsorbent pairs, advanced cycles and design to overcome the system transient effects. Their review acknowledged the lower system efficiency and lower specific capacity compared to absorption and vapor compression refrigeration systems. It is unlikely for adsorption refrigeration to be considered as a replacement for vapor compression refrigeration system in the near future. 11. Compressor-driven metal hydride heat pump Compressor driven metal hydride heat pumps (CDMHHP) are based on a modified adsorption heat pump system and use environmentally friendly refrigerants. The main difference is that the adsorption/desorption process is controlled via a low speed refrigerant compressor. The compressor imposes a pressure drop causing the refrigerant (hydrogen in this case) to desorb from the charged metal hydride bed and to be adsorbed in a second discharged reactor. The refrigerant is desorbed from the adsorbent (e.g. lanthanum pentanickle, LaNi5) at low pressure and temperature on the suction side and adsorbed by the LaNi5 on the high pressure side. The refrigerant flow direction and cycling can be controlled via three or four-way valves. Park et al. (2001) demonstrated the practical applicability of Zr0.9Ti0.1Cr0.55Fe1.45 hydride for air-conditioning systems by using an oil free compressor between 1 and 18 atm, and achieved a specific cooling power of 410 Wkg1 of alloy with a COP of 1.8. A schematic of this system is shown in Fig. 11, where Magnetto et al. (2006) reported a COP above 2.5 at ambient conditions between 21 °C (69.8 °F) and 35 °C (95 °F). Muthukumar and Groll (2010) concluded in their comprehensive review that compressor-driven metal hydride systems can compete with vapor compression technology; however, the major bottlenecks include the development of a low capacity hydrogen compressor and high cost of hydride alloys. CD-MHHP mainly uses hydrogen as the

The use of a vapor compression heat pump in water heating applications dates back to the early work of Wilkes and Reed (1937). Heat pump water heater technologies did not receive the required attention until the 1973 oil embargo (Dunning et al., 1978a,b). While US research focused on developing fluorinated carbon refrigerant based vapor compression HPWHs since early 2000, the Japanese researchers focused more on the development of a natural refrigerant HPWH (Hepbasli and Kalinci, 2009). Carbon dioxide proved to be among the top performing fluids due to several reasons, including its large temperature glide in transcritical cycle. State of the art Japanese HPWHs depend on the transcritical CO2 vapor compression cycle. Several types of compressors can be used, including scroll compressor (Hashimoto, 2006), single rotary compressor with brushless DC motor (Maeyama and Takahashi, 2007), or two-stage rotary compressor (Sanyo, 2010). In water heater applications, transcritical CO2 exiting the compressor flows in a gas cooler submerged inside the water heater storage unit, where water is heated up to 90 °C. An ejector or an expansion valve reduces the refrigerant pressure and temperature. It then flows back to the evaporator, completing the circuit. The CO2 HPWH is generally more expensive than conventional technology; however, according to (Kawashima, 2005) they offer 30% reduction in primary energy consumption and 50% reduction in CO2 emission compared with conventional combustion type boilers. The overall heat pump market is expected to grow by 8.1% in Japan (IIR, 2010). CO2 HPWHs are appealing in Japan, particularly when the COPs range between 3 and 4.9 (as compared with efficiencies for electric water heating at 1.0 and gas heating at 0.8 including pilot light losses). In order to achieve higher overall energy efficiencies and to expand the use in households, a multi-functional CO2 HPWH is being developed (KEPC, 2012) that combines a floor heater and bathroom heater/dryer. In addition, compact CO2 HPWH units that can be used on small lots in urban areas and in multihousehold dwellings are also being developed. 12.2. Integrated heat pump systems (IHPS) Low-energy and passive houses are designed with high levels of insulation and air-tightness, resulting in low space

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Fig. 11. Compressor driven heat pump operation (after Magnetto et al., 2006).

conditioning requirements. Domestic hot water (DHW) demand typically constitutes 50–85% of the total annual heating demand in Scandinavian residence (Stene, 2007), and up to 21% in US homes (Tomlinson et al., 2005). An integrated heat pump system (IHPS) for combined space heating and hot water heating can ideally meet both these requirements (Baxter et al., 2005) with reduced overall cost and better efficiency. IHPS can be designed to utilize different heat sources, such as bedrock, ground, exhaust ventilation air, ambient air, or a combination of exhaust ventilation air and ambient air. Although an IHPS will be instrumental in the promotion of the zero energy building concept, it faces the challenge of delivering efficient space heating and cooling, efficient water heating, and space dehumidification, particularly at times when latent heat loads are large. A conceptual design of IHPS, shown in Fig. 12, integrates space heating and cooling, water heating, ventilation, and humidity control (humidification and dehumidification) functions into a single unit. This concept consists of a modulating compressor, two variable-speed (VS) fans, and heat exchangers, including two air-to-refrigerant, one water-to-refrigerant, and one air-to-water to meet all the HVAC and water heating loads. The air-to-water HX uses excess hot water generated during the cooling and dehumidification modes to temper the ventilation air, as needed, to provide space-neutral conditions. The outdoor unit air source heat exchanger could be replaced by a ground source heat exchanger that would result in higher energy efficiency, but at a higher initial cost. Simulation results for IHPS indicate an approximate 50% reduction in energy use for space heating & cooling, water heating, dehumidification, and ventilation, compared to that of the base system (Rice et al., 2008). Research efforts are currently devoted to building and testing a ground source integrated heat pump (GSIHP) prototype at the Oak Ridge National Laboratory (USA) to convert this concept into a reality. Their GSIHP system was predicting to use up to 61% less energy than the baseline system while meeting total annual space conditioning and water heating loads (Rice et al., 2013). IHPS with CO2 as a working fluid can achieve a high COP due to the unique characteristics of the transcritical

Fig. 12. Conceptual design of an integrated heat pump system (after Tomlinson et al., 2005).

cycle with heat rejection in a gas cooler at a gliding CO2 temperature. A counter-flow CO2 gas cooler in combination with an external single-shell hot water tank and a low temperature heat distribution system, as shown in Fig. 13, can deliver domestic hot water in the required temperature range from 60 to 85 °C with a COP up to 20% better than the baseline system (Stene, 2007). 13. Overall assessment of NIK technologies It is clear that there is a growing interest in not-in-kind technologies for household applications in a quest for sustainable energy development. However, it is almost impossible to rank these technologies due to insufficient information being available in the open literature on their performance, size, reliability and cost compared to current vapor compression technologies. Thermoacoustic, Stirling, absorption and compressor driven metal hydride heat pumps are developing technologies, and may be classified in the medium and long term range of developments, while thermotunneling, Malone and adsorption refrigeration lie in the long term development range. The two emerging

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Fig. 13. Schematics of an integrated CO2 heat pump water heating system.

technologies that show promise are thermoelectric and magnetic refrigeration, where the latter is ahead due to the amount of international interest, research and development activity. The efficiency improvement for these two technologies is highly dependent on significant breakthroughs in materials development. These technologies will have to compete with conventional vapor compression at higher levels of efficiency. However, recent developments

in the area of linear compressors (F&P, 2010) will make it increasingly difficult for emerging technologies to displace evolving vapor compression technologies. Due to the varying sizes, varying operating conditions and varying methods for calculating performance of a specific system, comparing these systems quantitatively with each other and making recommendations is neither accurate nor can be justified. However, based on our review and

Table 1 Qualitative comparison of not-in-kind refrigeration technologies for household applications. Technology

Potential for energy efficiency

R&D status

Technical risks (low, med, high)

Time to commercialize (years)

Thermoacoustic Thermoelectric

Poor Promising

High Medium

Long term Medium term

Thermotunneling Magnetic Stirling cycle Pulse-tube refrigeration Malone cycle refrigeration Absorption

Poor Promising Poor Poor Theoretically good performance

Limited activity Well established, on-going material research No recent activity Strong activity Manufacturing issues Developed No reported activities

High Medium Medium Medium High

Long Long Long Long Long

Well established

Medium

Short term

Adsorption Compressor driven metal hydride Heat pump water heater using CO2 Integrated heat pumps

term term term term term

None – except when integrated with renewable energy None except for using renewable energy integration Poor

On-going

Medium

Medium term

No recent activities

Medium

Long term

High

Strong

Low

Short

High

Limited ongoing

Low

Short to medium

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compiled data, a qualitative and/or subjective analysis of these technologies is presented in Table 1, where ‘short term’, ‘medium term’ and ‘long term’ are respectively defined as within 5 years, less than 15 years and beyond 15 years. 14. Conclusions A review of NIK refrigeration technologies has been presented in this paper, where thermoelectric and magnetic refrigeration technologies show promise for energy efficiency improvements compared to vapor compression technology. However, these technologies are still developing due to current limitations posed by the state-of-the-art in materials research. A significant amount of research has recently been pursued in the area of magnetic refrigeration where fast developments are occurring both in new materials and system architecture. It is envisioned that magnetic refrigeration equipment may initially be costly, but the future of the technology may be promising. Technologies such as thermoacoustic refrigeration, absorption, and adsorption refrigeration have lower energy efficiency compared to vapor compression refrigeration. However, these have the advantage of flexibility in energy sources and can improve household energy efficiency when waste heat is available. Absorption is the most developed NIK, adsorption is currently available for large air-conditioning capacities, and thermoacoustic refrigeration is still developing. The thermotunneling refrigeration technology has advantage over thermoelectric refrigeration; however, materials and fabrication roadblocks limit its development. In a nutshell, significant breakthroughs are needed in materials research, fabrication technologies, and systems integration for both thermoelectric and magnetic refrigeration technologies to compete with conventional vapor compression technology. Substantial energy savings may be achieved through implementing heat pump technologies for water heating. Furthermore, domestic energy efficiency can be greatly improved through systems integration such as using an integrated heat pump system serving both air-conditioning and water heating loads. References Alexandrov, A.S., Bratkovsky, A.M., 2010. Giant thermopower and figure of merit of semiconducting polaronic nanolayers. Phys. Rev. B: Condens. Matter Mater. Phys. 81 (153204), 4. Allab, F., Kedous-Lebouc, A., Yonnet, J.P., Fournier, J.M., 2006. A magnetic field source system for magnetic refrigeration and its interaction with magnetocaloric material. Int. J. Refrig. 29, 1340–1347. Allen, P.C., Knight, W.R., Paulson, D.N., Wheatley, J.C., 1980. Principles of liquids working in heat engines. Proc. Nat. Acad. Sci. USA 77 (1), 39–43. Altenkirch, E., Tenckhoff, B., 1911. Absorptionkaeltemaschine Zur kontinuierlichen erzeugung von kaelte und waerme oder acuh von arbeit. German Patent 278076. Arnold, D.S., Tura, A., Rowe, A., 2011. Experimental analysis of a two material active magnetic regenerator. Int. J. Refrig. 34 (1), 178–191.

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