bansal status of not in kind refrigeration technologies for household space conditioning water...

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

Gulf Organisation for Research and Development

International Journal of Sustainable Built Environment

SciVerse ScienceDirectwww.sciencedirect.com

Review

Status of not-in-kind refrigeration technologies for householdspace 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 refrigerationtechnology for household applications. Such technologies are sought to provide energy savings or other environmental benefits for spaceconditioning, water heating and refrigeration for domestic use. These alternative technologies include: thermoacoustic refrigeration, ther-moelectric refrigeration, thermotunneling, magnetic refrigeration, Stirling cycle refrigeration, pulse tube refrigeration, Malone cyclerefrigeration, absorption refrigeration, adsorption refrigeration, and compressor driven metal hydride heat pumps. Furthermore, heatpump water heating and integrated heat pump systems are also discussed due to their significant energy saving potential for water heatingand space conditioning in households. The paper provides a snapshot of the future R&D needs for each of the technologies along withthe associated barriers. Both thermoelectric and magnetic technologies look relatively attractive due to recent developments in the mate-rials and prototypes being manufactured.

Keywords: Efficiency; Thermoacoustics; Thermoelectricity; Stirling; Magnetic refrigerator

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Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 862. Thermoacoustic refrigeration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 873. Thermoelectric refrigeration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 884. Thermotunneling (thermionic) refrigeration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 905. Magnetic refrigeration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90

2212-6090 � 2012 The Gulf Organisation for Research and Development. Production and hosting by Elsevier B.V.

http://dx.doi.org/10.1016/j.ijsbe.2012.07.003

⇑ Corresponding author.E-mail addresses: [email protected], [email protected]

(P. Bansal).

Peer review under responsibility of The Gulf Organisation for Researchand Development.

Production and hosting by Elsevier

Open access under CC BY-NC-ND license.


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Nomenclature

AMRR active magnetic regenerative refrigerationCCHP combined cooling heating and powerCD-MHHP compressor driven metal hydride heat

pumpsCOP coefficient of performanceHX heat exchangerHPWH heat pump water heateri electric current [A]IHPS integrated heat pump systemK thermal conductivity [Wm�1 K�1]MCE magneto caloric effectNIK not-in-kindPTR pulse tube refrigeratorQ heat transfer rate [W]T temperature [K]

VS variable speedZ figure of merit

Greek symbols

a seebeck coefficient [VK�1]D differenceq electrical resistivity [X-m]

Subscripts

C coldH hotadiabatic adiabatic processL low temperatureR room temperature

86 P. Bansal et al. / International Journal of Sustainable Built Environment 1 (2012) 85–101

6. Stirling cycle refrigeration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 927. Pulse tube refrigerator (PTR) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 948. Malone refrigeration. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 949. Absorption refrigeration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95

10. Adsorption refrigeration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9511. Compressor-driven metal hydride heat pump . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9612. Developments in water heating and space conditioining. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96

12.1. Heat pump water heater (HPWH) using transcritical CO2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9612.2. Integrated heat pump systems (IHPS) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96

13. Overall assessment of NIK technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9714. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99

References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99

1. Introduction

The vapor compression refrigeration has remained prac-tically a predominant technology for well over 100 years.The fundamental principle is to use liquid–vapor andvapor–liquid phase transitions to transfer heat from alow temperature state to a higher temperature state. It isdesirable to have these phase transitions occur at roomtemperature. The ideal refrigerant for the vapor compres-sion systems should be non-toxic, noncorrosive, efficient,cost effective and more importantly environmentallybenign. There is a general trend of increasing demand forheating, cooling and refrigeration services world-wide. Thiswill eventually lead to the increase in related CO2 emis-sions. This trend could be alleviated by the performanceenhancement of current heat pumping technologies and/or the development of new energy efficient technologies.In this context, the current paper reviews emergingnot-in-kind technologies (NIK) that offer the potential

for significant energy savings and environmental benefitscompared to existing technologies. In addition, the statusof emerging technologies that are useful in a household,including space conditioning, water heating and refrigera-tion, are discussed.

There have been a few integrated reviews of alternativetechnologies in the open literature. Fischer et al. (1994) pre-sented one of the earliest and most comprehensive summa-ries of not-in-kind technologies. This was then updated byFischer and Labinov (2000) with emphasis on economicimpact and potential commercialization. Lately there hasbeen a flurry of activity (Radermacher et al., 2007; Dieck-mann et al., 2007) in this area, where Navigant ConsultingInc. (2009) provided an overview of some of the alternativetechnologies targeting energy savings for commercialrefrigeration applications. This was followed by a reportfrom Brown et al. (2010) that assessed the prospects ofthermoelectric, thermionic, thermotunneling, thermoacou-stic and magnetic refrigeration for space cooling and food

P. Bansal et al. / International Journal of Sustainable Built Environment 1 (2012) 85–101 87

refrigeration applications. Most recently, Tassou et al.(2010) provided a broader view of emerging technologiesfor food refrigeration applications.

This paper evaluates the status of ten not-in-kind heatpump technologies relevant to domestic applications,namely thermoacoustic refrigeration, thermoelectric refrig-eration, thermotunneling, magnetic refrigeration, Stirlingcycle refrigeration, pulse tube refrigeration, Malone cyclerefrigeration, absorption refrigeration, adsorption refriger-ation, and compressor driven metal hydride heat pumps.The paper also discusses the development of heat pumpwater heating and integrated heat pump systems and theirrespective impact on energy consumption in households. Inaddition, the paper presents assessments of potential bene-fits from alternative technologies and a brief summary ofthe R&D opportunities that could develop such technolo-gies further. Potential barriers to implement these technol-ogies in the marketplace are discussed along with optionsfor each technology to achieve significant improvementsin energy efficiency or other environmental benefits fortheir application in space conditioning, water heating andrefrigeration in households.

Fig. 1. (A) Schematic of a thermoacoustic refrigerator. (B) Working pri

2. Thermoacoustic refrigeration

Thermoacoustic cooling is a technology that uses high-amplitude sound waves in a pressurized gas to generate atemperature gradient across a stationary element calledthe stack (Newman et al., 2006). A thermoacoustic deviceis placed inside a sealed pressure vessel consisting of anacoustic driver (e.g. a loudspeaker) that generates a high-amplitude sound wave, and hence large temperature andpressure oscillations into a resonator containing a regener-ator or stack. The sound wave may be generated usingeither thermal or mechanical energy. This cycle is shownin Fig. 1(A), and consists of four principal components:

1. A “stack” of porous material, parallel plates, orspiral rolls of thin sheets,

2. Hot and cold heat exchangers with large area tovolume ratio,

3. A rigid and sealed tube that may incorporate aHelmholtz resonator to shorten the device and mini-mize losses, and

4. An acoustic energy source.

nciple of a thermoacoustic refrigerator from Largrangian viewpoint.


The sound wave causes the gas to compress and expandadiabatically, which results in the gas to heat up and cooldown respectively. Heat is transferred from the workingfluid (i.e. gas) to the stack near the phase of greatest com-pression and from the stack to the gas parcel near the phaseof greatest expansion. The heat is then respectively dissi-pated to and received from an external fluid through a heatexchanger placed at each end of the stack. The standing-wave device, such as shown in Fig. 1, generates useful cool-ing by pumping heat from the cold heat exchanger to thehot heat exchanger. Fig. 1(B) shows an idealized thermoa-coustic refrigeration cycle, consisting of four processes:

� 1–2: gas parcel is compressed adiabatically while beingdisplaced toward the velocity node� 2–3: gas parcel is further compressed while heat is trans-

ferred to the stack� 3–4: gas parcel is expanded adiabatically while being dis-

placed toward the pressure node� 4–1: gas parcel is further expanded while heat is

absorbed from the stack

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

The complete cycle described above resembles a series ofBrayton cycles grouped together. Thermoacoustic refriger-ators employ environmentally friendly refrigerants, usuallya mixture of perfect gases, such as xenon and helium. Thestack is typically fairly short, on the order of few centime-ters, and is made of a material that does not conduct heatwell but has high heat capacity (e.g. ceramic).

Although the concept of thermoacoustic refrigerationhas been around for a while, there is still no commercial sys-tem available except for few examples of advanced develop-ments (Wollan and Swift, 2001; Naluai, 2002; Poese et al.,2004; Hotta et al., 2009). Tijani et al. (2002) achieved a tem-perature of �65 �C (�85 �F) from an optimized thermoa-coustic refrigerator. An early prototype thermoacousticrefrigerator (Swift, 1988) achieved 3 W of cooling at a tem-perature of �29 �C (�20 �F) and a sink temperature of25 �C (77 �F). Another prototype thermoacoustic refrigera-tion 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 belowvapor compression system performance. Other early proto-types achieved cooling capacities from 20 W (Garrett et al.,1993; Berhow, 1994) to as high as 10 kW (Garrett, 2002) in aunit designed for air-conditioning applications. A recentstudy by Nsofor and Ali (2009) found that, for a given fre-quency, there exists an optimum pressure that results inthe maximum temperature difference, which in turn yieldsin the maximum possible cooling load. The simulation/opti-mization study of standing-wave thermoacoustic coolers byPaek et al. (2007) suggests that maximum second law effi-ciency increases with temperature span and reaches a maxi-mum for temperature lifts of around 80 �C (144 �F). Zinket al. (2010) presented a study showing the environmentalmotivation for thermoacoustic refrigeration with other ben-efits being low cost and high reliability.

Some continuing major difficulties in achieving higherefficiencies with acoustic refrigerators have been the rela-tively low power density (Brown et al., 2010), low coolingcapacities, large physical size, heat conduction betweenthe heat exchangers and hence poor performance of the heatexchangers (Wetzel and Herman, 1997). Design and controlof compact heat exchangers in oscillating flow presents aunique challenge for thermoacoustic refrigeration unitswith large capacities. Due to these deficiencies, thermoacou-stic refrigeration will continue to be a non-competitive tech-nology for domestic applications in the foreseeable future.

3. Thermoelectric refrigeration

Thermoelectric refrigeration is based on the observationfirst made by Peltier (1834) that a direct electric current, i,passing through a circuit formed by two dissimilar conduc-tors or semiconductors, A and B, will cause a temperaturedifference to develop at the junctions of the two conduc-tors. A refrigeration effect develops at the cold junction,and heat is rejected at the hot junction. The heat producedor absorbed at each junction can be given by:

Q ¼ ðaA � aBÞ � i � T ð1Þ

where a is known as the Seebeck coefficient and is the prop-erty (positive or negative) of the material, i the electricalcurrent supplied to the thermoelectric device and T is theabsolute temperature of the junction.

The absolute Seebeck coefficient, a, for metals does notexceed 50 lV per K (ASHRAE, 1981). However, a can bemuch 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 “n-type” semiconductor and with excess Te to make a “p-type”

semiconductor. In the cooling mode, direct current passesfrom the n- to p-type semiconductor materials. The temper-ature TC of the conductor decreases and the heat is absorbedfrom the space to be cooled. This occurs when electrons pass


from a low energy level in the p-type material through aninterconnecting conductor to a higher energy level in then-type material. This heat is then rejected to the surround-ings at TH. This phenomenon is illustrated in Fig. 2.

The advantages of thermoelectric refrigeration are thatit has no moving parts, no CFCs or other fluids that arehazardous to the environment (Riffat and Ma, 2003), highreliability, reduced weight, and flexible operation. In orderto achieve the maximum COP of the cycle, given by Eq. (2),TH and TC (being respectively the absolute temperatures atthe hot and cold junctions), should respectively be as lowand as high as possible, while Z (called the ‘figure of merit’defined by Eq. (3) – a temperature dependent property ofeach material) should be as high as possible

COPmax ¼T C

T H � T C

� ��

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1þ Z T HþT C

2

q� T H

T Cffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1þ Z T HþT C

2

qþ 1

ð2Þ

Z ¼ ðap � anÞ2ffiffiffiffiffiffiffiffiffiffiffiKpqp

p� ffiffiffiffiffiffiffiffiffiffiffi

Knqn

p� �2ð3Þ

Higher performance requires materials with high differ-ence in a’s, low thermal conductivity K, and high electricalconductivity (or low q). However, this is intrinsically con-tradictory. Thermoelectric modules, based on commer-cially available materials that have a ZT [T is the averageof TH and TC] of about 1, cannot compete in efficiency withtraditional 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

Fig. 3. COP of thermoelectric modules for different materials at TH = 300 K

thermoelectric modules increases rapidly with decreasingtemperature lift, where it may have some advantage overtraditional vapor compression systems.

A ZT with a value of 9 and above is required to produceenergy efficient cooling units. At an absolute temperatureof 300 K (27 �C or 80 �F), ZT = 1 would correspond to adisappointing 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 reportedto have ZT of �2.5 around room temperature. A signifi-cant ZT increase has been reported in bulk materials madefrom nano crystalline powders of p-type BiSbTe, with a ZT

peak of 1.4 at 100 �C (212 �F) (Yang et al., 2008). Signifi-cant advancements are taking place in the developmentof 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 applicationsare still limited. To date, reported thermoelectric systemefficiency could not compete with conventional vapor com-pression technology. Fig. 3 depicts the theoretical COP ofdifferent thermoelectric materials as well as the CarnotCOP and the COP for a conventional vapor compressionsystem using R134a as a working fluid. All thermoelectricmaterials are less efficient than vapor compression systemexcept for the single molecule devices (Finch et al., 2009;Alexandrov and Bratkovsky, 2010). Fig. 3 shows that theefficiency of a thermoelectric device exceeds the efficiencyof the vapor compression only when the temperature liftis less than 5 �C (9 �F).

Vian and Astrain (2009) built a thermoelectric domesticrefrigerator with a single food compartment (of 0.225 m3)

compared to Carnot and vapor compression system (using R134a) COPs.

Fig. 4. Advantage of thermionic phenomenon.


maintained at 5 �C (41 �F). A COP of 0.45 was demon-strated at a temperature lift of 19 �C (34.2 �F). Such perfor-mance is much below that of conventional vaporcompression technology. However, in order to achieve bet-ter COP, Yang et al. (2008) proposed a hybrid system byusing a low temperature lift thermoelectric subcooler toincrease the subcooling temperature in a vapor compressionsystem. This capitalizes on the fact that the thermoelectricCOP is higher for small temperature lifts such as 5 �C(9 �F). Other niche applications for thermoelectric refriger-ation include mobile coolers that are quiet and vibrationfree. They are also widely used as replacements for winecabinets, mini-refrigerators, and water coolers (NavigantConsulting Inc., 2009; Bansal and Martin, 2000).

Despite numerous advantages of thermoelectric refriger-ation, low figure of merit hinders its wide scale deployment.An order of magnitude increase in the ‘figure of merit’ isrequired for thermoelectric refrigeration to compete withthe energy efficiency of the ‘state-of-the-art’ vapor com-pression technologies. Molecular thermoelectric deviceshave great potential energy efficiency; however these can-not be produced economically at large scale with currentfabrication technologies. Furthermore, current fabricationand assembly technologies result in a high thermal contactresistance that causes the temperature lift to increase,thereby dramatically reducing the energy efficiency. Effortsare needed to integrate thermoelectric devices with heatexchangers to eliminate the contact resistance. It is unlikelyfor thermoelectric refrigeration to compete with vaporcompression technology for household applications in theforeseeable future.

4. Thermotunneling (thermionic) refrigeration

There is a fine distinction between thermoelectric andthermionic cooling, where the former uses a flow of elec-trons through a pair of semiconductors in close physicalcontact, while the latter uses the flow of electrons betweentwo electrodes (i.e. cathode and anode) that are separatedby an extremely small gap (of the order of microns). Theo-

retically, under an applied electrical potential, hot electronsemitted by the cathode are at a higher energy level thanthose that are left behind, which reduces the average energylevel (temperature) of the cathode. Since the electrons beingabsorbed on the other side of the gap are at a higher energylevel than those in the electrode, the average energy level isincreased, and the electrode (i.e. anode) is heated. The newtype of materials called electrides that require only smallamount of energy to emit electrons at lower temperatures,make this technology attractive. The major advantage ofthermionic over thermoelectric refrigeration is the elimina-tion of the conduction heat transfer mode as shown in Fig. 4.

A number of studies have been carried out on thermo-tunneling (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 prom-ising way to develop room temperature vacuum thermionicrefrigerators is to combine new low work function emittermaterials with the nanometer gap techniques. Dillner(2010) calculated an upper limit for the dimensionless ther-moelectric figure of merit attainable by thermotunneling asp2/12, which suggests that thermotunneling cannot outper-form the state-of-the-art thermoelectric materials.

It is unlikely for thermotunneling to be an energy savingtechnology for household applications in the near future.Considerable R&D would be required including the devel-opment of cost effective low work function surfaces, withtypically less than 0.3 eV (O’Dwyer et al., 2006). In addi-tion, the requirement for extremely small inter-electrodespacing (nanometer-sized gaps) presents a unique challengefor large-scale manufacturing.

5. Magnetic refrigeration

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

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


(Dettmer, 2006. The temperature or point, at which a fer-romagnetic material loses its permanent magnetism andbecomes paramagnetic, and exhibits its greatest MCE, iscalled the ‘Curie temperature or Curie point’. The MCEeffect varies for different materials and can be intensifiedby increasing the magnetic field. MCE effect causes certainmaterials to warm adiabatically upon application of a mag-netic field and cool when the field is removed, and is cou-pled to an external heat transfer fluid to accomplish theheat pumping effect. The MCE behavior depends on thetype of material: ferromagnetic with magnetic domainsbelow the Curie point, or paramagnetic without magneticdomains. Fig. 5 depicts a theoretical magnetocaloric refrig-eration cycle using Gadolinium with a magnetic field of 7Tesla (T). In the magnetic refrigeration cycle, randomlyoriented magnetic spins in a paramagnetic material canbe aligned via a magnetic field, resulting in an adiabatic risein temperature and decrease in entropy. This phenomenoncan be used in heat pumping applications to reject heat athigher temperatures (Hull and Uherka, 1989). This processis highly reversible since, upon removal of the magneticfield, the magnetic spins return to their randomized state,resulting in an adiabatic decrease in temperature butincrease in entropy. The processes involved in magnetoca-loric refrigeration are summarized below:

� (A–B) Randomly oriented magnetic spins align afterapplying a magnetic field (H) along an isentropic processincreasing the magnetocaloric material temperature byDTadiabatic, 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 reducedby DTadiabatic, CD following an isentropic process.� (D–A) The magnetocaloric material absorbs heat from

the refrigerated volume. This raises its temperatureand the cycle continues.

The magnetic refrigeration technology, using activemagnetic regenerator (AMR) cycle, is claimed to have thepotential of higher energy efficiency than the current vaporcompression technology (Russek and Zimm, 2006), how-ever, no competitive system is commercially available to-date for room temperature applications. An AMR cycleuses magnetic material (or refrigerant) both as a thermalstorage medium as well as a means to convert magneticwork to net heat transfer. The solid material is cycledthrough a low and high magnetic field, while exchangingenergy with a heat transfer fluid (e.g. glycol water) oscillat-ing through the void space of the AMR. An effective regen-erator has high surface area per unit volume, highconductivity and low pressure drop. A prototype rotarymagnetic refrigerator built by Astronautics Corporationof America Inc., Milwaukee, USA is shown in Fig. 6.

There has been a vigorous research activity related tothis technology in the last decade where an exponentialincrease in publications has been seen; exceeding 250 in2007 (Gschneidner and Pecharsky, 2008). As a result, anumber of prototypes have emerged (Hiraro et al., 2010;Muller et al., 2010; Zimm et al., 1998, 2006, 2007; Pechar-sky and Gschneidner, 2006; Hirano, 2003; Zimm, 2003).Subsequently, new designs for magnetic refrigeration com-ponents and systems have evolved that use compact devicesand water-based heat transfer fluids. Yu et al. (2010)reviewed near room temperature magnetic refrigerationprototypes showing 41 working prototypes, 11 of whichwere demonstrated in 2009. Yu et al. (2010) reviewed nearroom temperature magnetic refrigeration prototypes andpatents and noted that there were 41 prototypes and almost135 patents were issued during 1997–2009. At Thermagconference in Grenoble during September 17–20, 2012, 29prototypes were presented in varying sizes from a fewWatts to 2 kW that employed rare earth alloys such asLaFeCoSi, LaFeMnSiH, LaFeSiH, MnFePas andMnFePGe (Bruck et al., 2012). The recent invigoration inpatents applications and prototypes of magnetic refrigeration

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


depicts advancing nature of this technology and the under-lying demand. The research theme in recent studies isfocused on developing better magnetocaloric materials,cycles, magnets and working prototypes, as highlighted insome of the exemplary references (Bruck et al., 2012; Bahlet al., 2010; Bjork et al., 2010; Russek et al., 2010; Kimet al., 2007; Phan and Yu, 2007; Allab et al., 2006; Bassoet al., 2006; Bingfeng et al., 2006; Gao et al., 2006; Tagliaficoet al., 2006; Zimm et al., 2006). Smaller temperaturedifferences are more feasible for magnetic refrigerationtechnology due to the limited temperature difference ofmagnetocaloric materials, while cascade systems are moredesirable for higher temperature differences (Kitanovskiand Egolf, 2009). A layered regenerator bed from severalmagnetic refrigeration materials (Rowe and Tura, 2006)that have Curie temperatures tailored to the localregenerator temperature in active magnetic regenerativerefrigeration (AMRR) can result in maximizing the MCE(Engelbrecht et al., 2006, 2007).

Room temperature applications require materials with aCurie temperature around 22 �C (71.6 �F). Gadoliniumand Gadolinium alloys exhibit large MCE around this tem-perature. They are, therefore, among the most widely usedmaterials for room temperature refrigeration and spacecooling applications. These materials undergo second-order phase transitions and do not exhibit magnetic orthermal hysteresis, the physics of which is discussed byBasso et al. (2006). By using such materials and applyinga 2 T magnetic field, researchers have demonstrated tem-perature lifts of 5 �C (9 �F). Higher magnetic fields resultin larger temperature lifts, but at higher cost and lowerefficiency. Most prototypes rely on the use of the activemagnetic regenerative cycle to provide high temperaturelift for air-conditioning and refrigeration applications.Recent research on materials that exhibit a large entropychange, such as Gd5(SixGe1�x)4, La(FexSi1�x)13Hx andMnFeP1�xAsx alloys, provide acceptable performancefor near room temperature applications. These materialsare called giant magnetocaloric effect materials (Pecharskyand Gschneidner, 2006).

Japan has launched a national project of developing aroom temperature magnetic refrigerator with a COPexceeding 10 by using new materials and other innovations(Hiraro et al., 2010). The group fabricated a sample ofMn1 + dAs1�xSbx that has a magneto-caloric effect severaltimes higher than Gd, while developing another magneticmaterial, Pr2Fe17 that has the same relative cooling poweras that of Gd at 10% of the cost.

A record COP of 4.6 was claimed to have been achievedby the Cooltech magnetic refrigeration prototype [Mulleret al., 2010]. The prototype measures 230 � 300 �250 mm3, weighs 34 kg, and uses 0.6 mm thick and100 mm long magnetocaloric material strips. The deviceachieved minimum and maximum temperatures of �17 �C(1.4 �F) and +45 �C (113 �F) respectively. The systememploys a permanent magnet with 1.6 T magnetic fields.The prototype achieved a 110 W cooling capacity between13 �C (55.4 �F) and 43 �C (109.4 �F) (DT of 30 K).

Despite all the above advancements, there is still noexperimental data available in the open literature to com-pare magnetic refrigeration with vapor compression refrig-eration technology. Various studies, including Kitanovskiand Egolf (2010), have outlined major challenges facingthe magnetocaloric technology, which include scarcity ofmagnetocaloric materials, high cost of materials and mag-nets, limitations of physical properties of materials, andtime 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 devicesare still not able to compete with vapor compression sys-tems. Some of the recent research efforts are devoted tosynthesizing and characterizing properties of MCEs, andmodeling and testing of AMRs including designing, build-ing and testing of prototypes.

6. Stirling cycle refrigeration

An ideal Stirling cooler is a reversed Stirling engine. Itconsists of a closed-cycle regenerative heat engine with a

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 con-sists of two isothermal reversible processes and two con-stant volume reversible processes, as shown in Fig. 7.Like the Brayton cycle, the working fluid is moved betweenthe hot and cold spaces through a regenerator by a systemof displacers. The power piston is driven by an electricmotor for a refrigeration device. Large flow rates arerequired to produce large capacities. Although Stirlingcycle cryocoolers are commercially available for infraredsensors and high temperature superconducting devices,their application at room temperature is practically non-existent due to a relatively low COP and high first cost.

Otaka et al. (2002) designed, simulated, and tested a dis-placer-type or b-type Stirling cycle prototype refrigerator

with a 100 W capacity, as shown in Fig. 8, to operatebetween �40 �C (�40 �F) and 30 �C (86 �F) at an operatingfrequency of 16.7 Hz and a sealing pressure within therefrigerator of less than 1.0 MPa. At a cooling temperatureof �20 �C (�4 �F), radiator temperature of 30 �C (86 �F),and mean pressure of 0.4 MPa, the cooling capacity ofthe refrigerator increased by 20% when hydrogen was usedas a working fluid instead of helium.

A free piston Stirling cooler prototype with a closedthermosyphon system and R134a refrigerant was inte-grated into a domestic refrigerator by Oguz and Ozkadi(2002). The prototype was tested at different refrigerantcharges and voltage inputs to the cooler. It was found toconsume approximately 30.5 W to maintain a cabinet

Fig. 9. Schematics of a pulse tube refrigerator.


temperature of 5 �C (41 �F) at 25 �C (77 �F) ambient tem-perature. Luo et al. (2006) tested a thermoacoustic-Stirlingheat engine driven refrigerator, which achieved a no-loadtemperature of �65 �C (�85 �F) with a cooling capacityof 270 W at �20 �C (�4 �F) and 405 W at 0 �C (32 �F).These results were quite encouraging for the applicationof thermoacoustic-Stirling refrigerator for food refrigera-tion and air-conditioning. Recently, Sun et al. (2009) testeda V-type Stirling cycle for domestic refrigeration with Heand N2 as the working fluids and a cold end temperatureof �35 �C (�31 �F). Using He, cooling capacities up to70 W were achieved with a COP of 0.8.

Stirling cycle refrigeration may provide incrementalenergy savings in domestic applications. However, forStirling cycle refrigeration to compete with vapor compres-sion technologies, regenerator performance needs toimprove substantially to achieve higher effectiveness, lowerpressure drop, lower void volume and lower cost. Further-more, cold and hot heat exchangers need to be designedwith a higher heat transfer density and lower log meantemperature differences.

7. Pulse tube refrigerator (PTR)

The pulse tube refrigerator (PTR) or pulse tube cryoco-oler is a developing technology that pumps heat throughthe compression and expansion of a gas. It offers severaladvantages over Stirling refrigerators, including no displac-er and no mechanical vibrations. It can be made withoutany moving parts in the low temperature section of thedevice. The gas in-phase motion is achieved by the use ofan orifice and a reservoir volume to store gas (Radebaugh,2000). A PTR, as shown in Fig. 9, consists of eight maincomponents – (i) a compressor that compresses the gas,typically He, to higher temperatures, (ii) a heat exchanger(HXH1) that rejects heat at room temperature cooling thegas, (iii) a porous regenerator (to absorb and dischargeheat, from and to the gas, when it flows to the right andto the left respectively), (iv) another heat exchanger(HXL) absorbing the useful cooling power (QL) at low tem-perature TL, (v) a tube in which gas moves back and forth,(vi) a hot end heat exchanger (HXH2) rejecting heat toroom temperature, (vii) an orifice as a flow resistancedevice, and (viii) a large buffer volume containing He gas.

The PTR works on following four adiabatic compressionand 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 theambient through a heat exchanger (at room tempera-ture TH),

(iii) The piston moves up and expands the gas adiabati-cally in the pulse tube,

(iv) This cold gas at low pressure in the pulse tube travelsback through the cold heat exchanger at the low tem-perature TL (providing cooling capacity QL).

The flow in either direction stops when the pressure inthe tube is either lower (when moving forward) or higher(when moving backwards) than the average pressure inthe tube. The PTR has a regenerator (made of a porousmatrix) that precools the incoming high pressure gas beforeit reaches the cold end (and vice-versa) and a hot-end heatexchanger that rejects heat to room temperature.

PTRs are commercially available for temperature appli-cations between �196 �C (�320.8 �F) down to �269 �C(�466.7 �F), where their relative Carnot efficiency is stea-dily improving (Swift, 1997; Hu et al., 2010)]. However,the COP of PTR at room temperature is quite low and isunlikely 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 nearits critical point, instead of the customary gas, in a regen-erative or recuperative refrigeration cycle such as Stirlingor Brayton cycles. Due to the inherent incompressible nat-ure of liquid, this cycle has the advantage over gas cycle forachieving higher pressure change per unit volume (Malone,1931). The machine can be driven externally to produce arefrigerating effect. In a refrigeration system, the Malonecycle uses the cooling associated with the expansion of aliquid, but without a phase change. One of the earliestpapers studying the physics of the liquids working in heatengines was published in 1980 (Allen et al., 1980). Mostof the preliminary research was conducted at the Los


Alamos National Laboratory (USA) in the early 1990s(Swift, 1989, 1993; Swift and Brown, 1994), where one ofthe first Malone refrigerators (Swift, 1989) used liquid pro-pylene (C3H6) in a double acting 4 cylinder Stirling config-uration, followed by liquid CO2 (Swift and Brown, 1994).

Although CO2 being an environmentally friendly refrig-erant could make a good candidate for the Malone cycle,its critical temperature (31 �C, 88 �F) is too low for efficientoperation in many HVAC/R applications. Other possiblecandidate fluids include methyl alcohol, ethyl alcohol, ace-tone or sulfur dioxide, which have higher critical tempera-tures, but present safety issues. Other fluids include sulfurhexafluoride and various fluorocarbons such as HFC-134a. In general, these compounds have critical tempera-tures more suited to HVAC applications and critical pres-sures lower than CO2. Although high heat capacity liquidsoffer the advantage of reduced mass flow rate for good heattransfer, the main drawback of Malone refrigeration is thatthese liquids are unable to achieve the desired (or required)temperature change for efficient refrigeration. There hasbeen practically no activity in the recent past on Malonerefrigeration and it is unlikely for this technology to pene-trate in the household applications in the near future.

9. Absorption refrigeration

Absorption and adsorption refrigeration are thermallydriven technologies that respectively use liquid and solidsorbents. These systems are popular in applications wheredemand side management is important and/or waste heatis readily available. An absorption cycle, shown inFig. 10, utilizes a binary mixture of refrigerants such asammonia–water or water–LiBr. The single effect cycle con-sists of an absorber, a generator or desorber, a condenser,an evaporator, and an electric solution pump, with the pos-sibility of additional components, such as internal heatexchangers, to enhance efficiency. An external heat source,such as a gas burner in a direct fired system, steam or hotwater in an indirect fired system, or waste heat, is used inthe generator (or desorber). Heat absorbed in the generatorallows the refrigerant to desorb from the absorbent, creat-ing a high pressure vapor. In cases where a volatile absor-bent is used (e.g. ammonia–water), a rectifier is needed toreduce the concentration of the volatile absorbent (e.g.water) in the vapor to the condenser. A number ofadvanced cycles have been proposed in the literature inorder to improve the COP starting from single effect toGAX (Altenkirch and Tenckhoff, 1911), double-effect (Vli-et et al., 1982), cycle with two absorbers (CMostofizadehand Kulick, 1998), compression-absorption (Hulten andBerntsson, 1999), auto cascade (Chen, 2002), two stageabsorption (Fan et al., 2007) and more recently an expan-der-compressor cycle (Hong et al., 2010) and several wasteheat/renewable energy operated absorption systems (Wanget al., 2012). Single- and double-effect absorption chillersare commercially available for large scale applications,while absorption refrigerators are available for small

capacity applications, such as mini-bar fridges, recreationalvehicles, hotel rooms, and boats. Some of the advantagesof small refrigerators include quiet operation and flexibleuse of any energy source such as gas, battery and electricity(Bansal and Martin, 2000). Although absorption coolersoffer many advantages over vapor compression systems(e.g. environmentally friendly absorbent/refrigerant pairs,fewer moving parts), they are mainly limited to large scaleapplications and are not competitive for small scale appli-cations due to system complexity, high cost and lower effi-ciency compared to vapor compression systems.

10. Adsorption refrigeration

An adsorption system uses multiple beds of adsorbentssuch as silica-gel in a silica-gel water system, to providecontinuous capacity, and does not use any mechanicalenergy but only thermal energy. An adsorption refrigera-tion system usually consists of four main components: asolid adsorbent bed, a condenser, an expansion valve andan evaporator. The solid adsorbent bed is linked to theevaporator. It desorbs refrigerant when heated and adsorbsrefrigerant vapor when cooled such that the bed works likea thermal compressor to drive the refrigerant around thesystem to heat or cool a heat transfer fluid or to providespace heating or cooling. When the bed becomes saturatedwith refrigerant, it is isolated from the evaporator and con-nected to the condenser. The refrigerant vapor is con-densed to a liquid, followed by expansion to a lowerpressure in the evaporator where the low pressure refriger-ant is vaporized producing the refrigeration effect (i.e. cool-ing the refrigerator air). When further heating no longerproduces desorbed refrigerant from the adsorbent bed,the refrigerant vapor from the evaporator is reintroducedto the bed to complete the cycle. To obtain a continuousand stable cooling effect, generally two (or multiple) adsor-bent beds are used, where one bed is heated during desorp-tion while the other bed is cooled during adsorption. Inorder to achieve high efficiency, heat of adsorption needsto be recovered to provide part of the heat needed to regen-erate the adsorbent. A recent literature review of conven-tional adsorption cycle was presented by Wang et al.(2010).

Adsorptive beds of the chillers can be regenerated bylow-grade temperatures using waste heat or solar energyas heat source. These chillers can also be employed inCCHP systems. The overall thermal and electrical effi-ciency in these systems can be above 70% (Wang et al.,2005). Some of the recent adsorption system performanceenhancement technologies include heat pipes (Yang et al.,2006) and consolidated compound adsorbents (Tamainot-Telto and Critoph, 2003). Lu et al. (2006) designed a pro-totype icemaker with specific cooling power of 770 Wkg�1

and a COP of 0.39, at �20 �C evaporation temperature.Adsorption systems are known to suffer from low coef-

ficient of performance (COP) and low specific coolingpower (Wang et al., 2010; Wang and Oliveira, 2006).

Fig. 10. Schematics of single-effect absorption cycle.


Although commercial adsorption systems for air-condi-tioning applications between 35 and 350 kW (Tassouet 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 strongresearch need for advanced refrigerant/adsorbent pairs,advanced cycles and design to overcome the system tran-sient effects. Their review acknowledged the lower systemefficiency and lower specific capacity compared to absorp-tion and vapor compression refrigeration systems. It isunlikely for adsorption refrigeration to be considered as areplacement for vapor compression refrigeration systemin the near future.

11. Compressor-driven metal hydride heat pump

Compressor driven metal hydride heat pumps (CD-MHHP) are based on a modified adsorption heat pumpsystem and use environmentally friendly refrigerants. Themain difference is that the adsorption/desorption processis controlled via a low speed refrigerant compressor. Thecompressor imposes a pressure drop causing the refrigerant(hydrogen in this case) to desorb from the charged metalhydride bed and to be adsorbed in a second dischargedreactor. The refrigerant is desorbed from the adsorbent(e.g. lanthanum pentanickle, LaNi5) at low pressure andtemperature on the suction side and adsorbed by the LaNi5on the high pressure side. The refrigerant flow directionand cycling can be controlled via three or four-way valves.

Park et al. (2001) demonstrated the practical applicabil-ity of Zr0.9Ti0.1Cr0.55Fe1.45 hydride for air-conditioning sys-tems by using an oil free compressor between 1 and 18 atm,and achieved a specific cooling power of 410 Wkg�1 of alloywith a COP of 1.8. A schematic of this system is shown inFig. 11, where Magnetto et al. (2006) reported a COP above2.5 at ambient conditions between 21 �C (69.8 �F) and35 �C (95 �F).

Muthukumar and Groll (2010) concluded in their com-prehensive review that compressor-driven metal hydridesystems can compete with vapor compression technology;however, the major bottlenecks include the developmentof a low capacity hydrogen compressor and high cost ofhydride alloys. CD-MHHP mainly uses hydrogen as the

working fluid, which is flammable and hence not suitablefor domestic applications. Other challenges for using CD-MHHP in domestic applications include high cost of metalhydrides, non-availability of suitable materials with fastreaction kinetics, and the need for improved hydrogencompressor technology.

12. Developments in water heating and space conditioining

12.1. Heat pump water heater (HPWH) using transcritical

CO2

The use of a vapor compression heat pump in waterheating applications dates back to the early work of Wilkesand Reed (1937). Heat pump water heater technologies didnot receive the required attention until the 1973 oilembargo (Dunning et al., 1978a,b). While US researchfocused on developing fluorinated carbon refrigerant basedvapor compression HPWHs since early 2000, the Japaneseresearchers focused more on the development of a naturalrefrigerant HPWH (Hepbasli and Kalinci, 2009). Carbondioxide proved to be among the top performing fluidsdue to several reasons, including its large temperature glidein transcritical cycle.

State of the art Japanese HPWHs depend on the trans-critical CO2 vapor compression cycle. Several types of com-pressors can be used, including scroll compressor(Hashimoto, 2006), single rotary compressor with brushlessDC motor (Maeyama and Takahashi, 2007), or two-stagerotary compressor (Sanyo, 2010). In water heater applica-tions, transcritical CO2 exiting the compressor flows in agas cooler submerged inside the water heater storage unit,where water is heated up to 90 �C. An ejector or an expan-sion valve reduces the refrigerant pressure and tempera-ture. It then flows back to the evaporator, completing thecircuit. The CO2 HPWH is generally more expensive thanconventional technology; however, according to (Kawashi-ma, 2005) they offer 30% reduction in primary energy con-sumption and 50% reduction in CO2 emission comparedwith conventional combustion type boilers. The overallheat pump market is expected to grow by 8.1% in Japan(IIR, 2010).

CO2 HPWHs are appealing in Japan, particularly whenthe COPs range between 3 and 4.9 (as compared with effi-ciencies for electric water heating at 1.0 and gas heating at0.8 including pilot light losses). In order to achieve higheroverall energy efficiencies and to expand the use in house-holds, a multi-functional CO2 HPWH is being developed(KEPC, 2012) that combines a floor heater and bathroomheater/dryer. In addition, compact CO2 HPWH units thatcan be used on small lots in urban areas and in multi-household dwellings are also being developed.

12.2. Integrated heat pump systems (IHPS)

Low-energy and passive houses are designed with highlevels of insulation and air-tightness, resulting in low space

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

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


conditioning requirements. Domestic hot water (DHW)demand typically constitutes 50–85% of the total annualheating demand in Scandinavian residence (Stene, 2007),and up to 21% in US homes (Tomlinson et al., 2005). Anintegrated heat pump system (IHPS) for combined spaceheating and hot water heating can ideally meet both theserequirements (Baxter et al., 2005) with reduced overall costand better efficiency. IHPS can be designed to utilizedifferent heat sources, such as bedrock, ground, exhaust ven-tilation air, ambient air, or a combination of exhaust venti-lation air and ambient air. Although an IHPS will beinstrumental in the promotion of the zero energy buildingconcept, it faces the challenge of delivering efficient spaceheating and cooling, efficient water heating, and space dehu-midification, particularly at times when latent heat loads arelarge.

A conceptual design of IHPS, shown in Fig. 12, inte-grates space heating and cooling, water heating, ventilation,and humidity control (humidification and dehumidifica-tion) functions into a single unit. This concept consists ofa modulating compressor, two variable-speed (VS) fans,and heat exchangers, including two air-to-refrigerant, onewater-to-refrigerant, and one air-to-water to meet all theHVAC and water heating loads. The air-to-water HX usesexcess hot water generated during the cooling and dehumid-ification modes to temper the ventilation air, as needed, toprovide space-neutral conditions. The outdoor unit airsource heat exchanger could be replaced by a ground sourceheat exchanger that would result in higher energy efficiency,but at a higher initial cost. Simulation results for IHPS indi-cate an approximate 50% reduction in energy use for spaceheating & cooling, water heating, dehumidification, andventilation, compared to that of the base system (Riceet al., 2008). Research efforts are currently devoted to build-ing 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 GSIHPsystem was predicting to use up to 61% less energy thanthe baseline system while meeting total annual space condi-tioning and water heating loads (Rice et al., 2013).

IHPS with CO2 as a working fluid can achieve a highCOP due to the unique characteristics of the transcritical

cycle with heat rejection in a gas cooler at a gliding CO2

temperature. A counter-flow CO2 gas cooler in combina-tion with an external single-shell hot water tank and alow temperature heat distribution system, as shown inFig. 13, can deliver domestic hot water in the required tem-perature range from 60 to 85 �C with a COP up to 20% bet-ter than the baseline system (Stene, 2007).

13. Overall assessment of NIK technologies

It is clear that there is a growing interest in not-in-kindtechnologies for household applications in a quest for sus-tainable energy development. However, it is almost impos-sible to rank these technologies due to insufficientinformation being available in the open literature on theirperformance, size, reliability and cost compared to currentvapor compression technologies. Thermoacoustic, Stirling,absorption and compressor driven metal hydride heatpumps are developing technologies, and may be classifiedin the medium and long term range of developments, whilethermotunneling, Malone and adsorption refrigeration liein the long term development range. The two emerging

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


technologies that show promise are thermoelectric andmagnetic refrigeration, where the latter is ahead due tothe amount of international interest, research and develop-ment activity. The efficiency improvement for these twotechnologies is highly dependent on significant break-throughs in materials development. These technologies willhave to compete with conventional vapor compression athigher levels of efficiency. However, recent developments

Table 1Qualitative comparison of not-in-kind refrigeration technologies for househol

Technology Potential for energy efficiency R&D sta

Thermoacoustic Poor Limited aThermoelectric Promising Well esta

materialThermotunneling Poor No recenMagnetic Promising Strong acStirling cycle Poor ManufacPulse-tube refrigeration Poor DevelopeMalone cycle

refrigerationTheoretically good performance No repor

Absorption None – except when integrated withrenewable energy

Well esta

Adsorption None except for using renewableenergy integration

On-going

Compressor driven metalhydride

Poor No recen

Heat pump water heaterusing CO2

High Strong

Integrated heat pumps High Limited o

in the area of linear compressors (F&P, 2010) will makeit increasingly difficult for emerging technologies to dis-place evolving vapor compression technologies.

Due to the varying sizes, varying operating conditionsand varying methods for calculating performance of a spe-cific system, comparing these systems quantitatively witheach other and making recommendations is neither accuratenor can be justified. However, based on our review and

d applications.

tus Technical risks (low,med, high)

Time tocommercialize (years)

ctivity High Long termblished, on-goingresearch

Medium Medium term

t activity High Long termtivity Medium Long term

turing issues Medium Long termd Medium Long termted activities High Long term

blished Medium Short term

Medium Medium term

t activities Medium Long term

Low Short

ngoing Low Short to medium


compiled data, a qualitative and/or subjective analysis ofthese technologies is presented in Table 1, where ‘short term’,‘medium term’ and ‘long term’ are respectively defined aswithin 5 years, less than 15 years and beyond 15 years.

14. Conclusions

A review of NIK refrigeration technologies has beenpresented in this paper, where thermoelectric and magneticrefrigeration technologies show promise for energy effi-ciency improvements compared to vapor compression tech-nology. However, these technologies are still developingdue to current limitations posed by the state-of-the-art inmaterials research. A significant amount of research hasrecently been pursued in the area of magnetic refrigerationwhere fast developments are occurring both in new materi-als and system architecture. It is envisioned that magneticrefrigeration equipment may initially be costly, but thefuture of the technology may be promising.

Technologies such as thermoacoustic refrigeration,absorption, and adsorption refrigeration have lower energyefficiency compared to vapor compression refrigeration.However, these have the advantage of flexibility in energysources and can improve household energy efficiency whenwaste heat is available. Absorption is the most developedNIK, adsorption is currently available for large air-condi-tioning capacities, and thermoacoustic refrigeration is stilldeveloping. The thermotunneling refrigeration technologyhas advantage over thermoelectric refrigeration; however,materials and fabrication roadblocks limit its development.In a nutshell, significant breakthroughs are needed in mate-rials research, fabrication technologies, and systems inte-gration for both thermoelectric and magnetic refrigerationtechnologies to compete with conventional vapor compres-sion technology.

Substantial energy savings may be achieved throughimplementing heat pump technologies for water heating.Furthermore, domestic energy efficiency can be greatlyimproved through systems integration such as using anintegrated heat pump system serving both air-conditioningand water heating loads.

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