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 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, ther- moelectric 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 mate- rials 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. Introduction ...... 86 2. Thermoacoustic refrigeration ...... 87 3. Thermoelectric refrigeration ...... 88 4. Thermotunneling (thermionic) refrigeration ...... 90 5. Magnetic refrigeration ...... 90
⇑ 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 86 P. Bansal et al. / International Journal of Sustainable Built Environment 1 (2012) 85–101
Nomenclature
AMRR active magnetic regenerative refrigeration VS variable speed CCHP combined cooling heating and power Z figure of merit CD-MHHP compressor driven metal hydride heat pumps Greek symbols COP coefficient of performance a seebeck coefficient [VK 1] HX heat exchanger D difference HPWH heat pump water heater q electrical resistivity [X-m] i electric current [A] IHPS integrated heat pump system Subscripts K thermal conductivity [Wm 1 K 1] C cold MCE magneto caloric effect H hot NIK not-in-kind adiabatic adiabatic process PTR pulse tube refrigerator L low temperature Q heat transfer rate [W] R room temperature T temperature [K]
6. Stirling cycle refrigeration ...... 92 7. Pulse tube refrigerator (PTR) ...... 94 8. Malone refrigeration...... 94 9. Absorption refrigeration ...... 95 10. Adsorption refrigeration ...... 95 11. Compressor-driven metal hydride heat pump ...... 96 12. Developments in water heating and space conditioining...... 96 12.1. Heat pump water heater (HPWH) using transcritical CO2 ...... 96 12.2. Integrated heat pump systems (IHPS) ...... 96 13. Overall assessment of NIK technologies ...... 97 14. Conclusions ...... 99 References...... 99
1. Introduction for significant energy savings and environmental benefits compared to existing technologies. In addition, the status The vapor compression refrigeration has remained prac- of emerging technologies that are useful in a household, tically a predominant technology for well over 100 years. including space conditioning, water heating and refrigera- The fundamental principle is to use liquid–vapor and tion, are discussed. vapor–liquid phase transitions to transfer heat from a There have been a few integrated reviews of alternative low temperature state to a higher temperature state. It is technologies in the open literature. Fischer et al. (1994) pre- desirable to have these phase transitions occur at room sented one of the earliest and most comprehensive summa- temperature. The ideal refrigerant for the vapor compres- ries of not-in-kind technologies. This was then updated by sion systems should be non-toxic, noncorrosive, efficient, Fischer and Labinov (2000) with emphasis on economic cost effective and more importantly environmentally impact and potential commercialization. Lately there has benign. There is a general trend of increasing demand for been a flurry of activity (Radermacher et al., 2007; Dieck- heating, cooling and refrigeration services world-wide. This mann et al., 2007) in this area, where Navigant Consulting will eventually lead to the increase in related CO2 emis- Inc. (2009) provided an overview of some of the alternative sions. This trend could be alleviated by the performance technologies targeting energy savings for commercial enhancement of current heat pumping technologies and/ refrigeration applications. This was followed by a report or the development of new energy efficient technologies. from Brown et al. (2010) that assessed the prospects of In this context, the current paper reviews emerging thermoelectric, thermionic, thermotunneling, thermoacou- not-in-kind technologies (NIK) that offer the potential 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. 2. Thermoacoustic refrigeration (2010) provided a broader view of emerging technologies for food refrigeration applications. Thermoacoustic cooling is a technology that uses high- This paper evaluates the status of ten not-in-kind heat amplitude sound waves in a pressurized gas to generate a pump technologies relevant to domestic applications, temperature gradient across a stationary element called namely thermoacoustic refrigeration, thermoelectric refrig- the stack (Newman et al., 2006). A thermoacoustic device eration, thermotunneling, magnetic refrigeration, Stirling is placed inside a sealed pressure vessel consisting of an cycle refrigeration, pulse tube refrigeration, Malone cycle acoustic driver (e.g. a loudspeaker) that generates a high- refrigeration, absorption refrigeration, adsorption refriger- amplitude sound wave, and hence large temperature and ation, and compressor driven metal hydride heat pumps. pressure oscillations into a resonator containing a regener- The paper also discusses the development of heat pump ator or stack. The sound wave may be generated using water heating and integrated heat pump systems and their either thermal or mechanical energy. This cycle is shown respective impact on energy consumption in households. In in Fig. 1(A), and consists of four principal components: addition, the paper presents assessments of potential bene- fits from alternative technologies and a brief summary of 1. A “stack” of porous material, parallel plates, or the R&D opportunities that could develop such technolo- spiral rolls of thin sheets, gies further. Potential barriers to implement these technol- 2. Hot and cold heat exchangers with large area to ogies in the marketplace are discussed along with options volume ratio, for each technology to achieve significant improvements 3. A rigid and sealed tube that may incorporate a in energy efficiency or other environmental benefits for Helmholtz resonator to shorten the device and mini- their application in space conditioning, water heating and mize losses, and refrigeration in households. 4. An acoustic energy source.
Fig. 1. (A) Schematic of a thermoacoustic refrigerator. (B) Working principle of a thermoacoustic refrigerator from Largrangian viewpoint. 88 P. Bansal et al. / International Journal of Sustainable Built Environment 1 (2012) 85–101
The sound wave causes the gas to compress and expand Some continuing major difficulties in achieving higher adiabatically, which results in the gas to heat up and cool efficiencies with acoustic refrigerators have been the rela- down respectively. Heat is transferred from the working tively low power density (Brown et al., 2010), low cooling fluid (i.e. gas) to the stack near the phase of greatest com- capacities, large physical size, heat conduction between pression and from the stack to the gas parcel near the phase the heat exchangers and hence poor performance of the heat of greatest expansion. The heat is then respectively dissi- exchangers (Wetzel and Herman, 1997). Design and control pated to and received from an external fluid through a heat of compact heat exchangers in oscillating flow presents a exchanger placed at each end of the stack. The standing- unique challenge for thermoacoustic refrigeration units wave device, such as shown in Fig. 1, generates useful cool- with large capacities. Due to these deficiencies, thermoacou- ing by pumping heat from the cold heat exchanger to the stic refrigeration will continue to be a non-competitive tech- hot heat exchanger. Fig. 1(B) shows an idealized thermoa- nology for domestic applications in the foreseeable future. coustic refrigeration cycle, consisting of four processes: 3. Thermoelectric refrigeration 1–2: gas parcel is compressed adiabatically while being displaced toward the velocity node Thermoelectric refrigeration is based on the observation 2–3: gas parcel is further compressed while heat is trans- first made by Peltier (1834) that a direct electric current, i, ferred to the stack passing through a circuit formed by two dissimilar conduc- 3–4: gas parcel is expanded adiabatically while being dis- tors or semiconductors, A and B, will cause a temperature placed toward the pressure node difference to develop at the junctions of the two conduc- 4–1: gas parcel is further expanded while heat is tors. A refrigeration effect develops at the cold junction, absorbed from the stack and heat is rejected at the hot junction. The heat produced or absorbed at each junction can be given by: The complete cycle described above resembles a series of Brayton cycles grouped together. Thermoacoustic refriger- Q ¼ðaA aBÞ i T ð1Þ ators employ environmentally friendly refrigerants, usually a mixture of perfect gases, such as xenon and helium. The where a is known as the Seebeck coefficient and is the prop- stack is typically fairly short, on the order of few centime- erty (positive or negative) of the material, i the electrical ters, and is made of a material that does not conduct heat current supplied to the thermoelectric device and T is the well but has high heat capacity (e.g. ceramic). absolute temperature of the junction. Although the concept of thermoacoustic refrigeration The absolute Seebeck coefficient, a, for metals does not has been around for a while, there is still no commercial sys- exceed 50 lVperK(ASHRAE, 1981). However, a can be tem available except for few examples of advanced develop- much higher for semiconductor materials. The highest a ments (Wollan and Swift, 2001; Naluai, 2002; Poese et al., (250 lV per K) is achieved from alloys of Tellurium (Te) 2004; Hotta et al., 2009). Tijani et al. (2002) achieved a tem- doped with antimony tri-iodide (SbI3) to produce an “n- type” semiconductor and with excess Te to make a “p-type” perature of 65 °C( 85 °F) from an optimized thermoa- coustic refrigerator. An early prototype thermoacoustic semiconductor. In the cooling mode, direct current passes refrigerator (Swift, 1988) achieved 3 W of cooling at a tem- from the n- to p-type semiconductor materials. The temper- ature T of the conductor decreases and the heat is absorbed perature of 29 °C( 20 °F) and a sink temperature of C 25 °C (77 °F). Another prototype thermoacoustic refrigera- from the space to be cooled. This occurs when electrons pass 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 below vapor 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)ina unit designed for air-conditioning applications. A recent study by Nsofor and Ali (2009) found that, for a given fre- quency, there exists an optimum pressure that results in the maximum temperature difference, which in turn yields in the maximum possible cooling load. The simulation/opti- mization study of standing-wave thermoacoustic coolers by Paek 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). Zink et al. (2010) presented a study showing the environmental motivation for thermoacoustic refrigeration with other ben- efits being low cost and high reliability. Fig. 2. Schematics of thermoelectric refrigeration cycle in cooling mode. P. Bansal et al. / International Journal of Sustainable Built Environment 1 (2012) 85–101 89 from a low energy level in the p-type material through an thermoelectric modules increases rapidly with decreasing interconnecting conductor to a higher energy level in the temperature lift, where it may have some advantage over n-type material. This heat is then rejected to the surround- traditional vapor compression systems. ings at TH. This phenomenon is illustrated in Fig. 2. A ZT with a value of 9 and above is required to produce The advantages of thermoelectric refrigeration are that energy efficient cooling units. At an absolute temperature it has no moving parts, no CFCs or other fluids that are of 300 K (27 °Cor80°F), ZT = 1 would correspond to a hazardous to the environment (Riffat and Ma, 2003), high disappointing figure of merit Z = 0.0033. The best ZT reliability, reduced weight, and flexible operation. In order materials are found in heavily doped semi-conductors. to achieve the maximum COP of the cycle, given by Eq. (2), BiTe3 (p-type)/Sb2Te3 (n-type) super lattices are reported TH and TC (being respectively the absolute temperatures at to have ZT of 2.5 around room temperature. A signifi- the hot and cold junctions), should respectively be as low cant ZT increase has been reported in bulk materials made and as high as possible, while Z (called the ‘figure of merit’ from nano crystalline powders of p-type BiSbTe, with a ZT defined by Eq. (3) – a temperature dependent property of peak of 1.4 at 100 °C (212 °F) (Yang et al., 2008). Signifi- each material) should be as high as possible cant advancements are taking place in the development qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi of thermoelectric nano composites, resulting in higher ZT T HþT C T H values (Lan et al., 2010). T 1 þ Z 2 T C qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi C COPmax ¼ ð2Þ Although ZT of thermoelectric modules has increased T H T C T HþT C 1 þ Z 2 þ 1 significantly in recent years, their practical applications are still limited. To date, reported thermoelectric system ða a Þ2 efficiency could not compete with conventional vapor com- Z ¼ p n ð3Þ pffiffiffiffiffiffiffiffiffiffiffi pffiffiffiffiffiffiffiffiffiffiffi 2 pression technology. Fig. 3 depicts the theoretical COP of Kpqp Knqn different thermoelectric materials as well as the Carnot COP and the COP for a conventional vapor compression Higher performance requires materials with high differ- system using R134a as a working fluid. All thermoelectric ence in a’s, low thermal conductivity K, and high electrical materials are less efficient than vapor compression system conductivity (or low q). However, this is intrinsically con- except for the single molecule devices (Finch et al., 2009; tradictory. Thermoelectric modules, based on commer- Alexandrov and Bratkovsky, 2010). Fig. 3 shows that the cially available materials that have a ZT [T is the average efficiency of a thermoelectric device exceeds the efficiency of TH and TC] of about 1, cannot compete in efficiency with of the vapor compression only when the temperature lift traditional vapor compression systems (Yang et al., 2008) is less than 5 °C(9°F). when operating at a relatively large temperature lift Vian and Astrain (2009) built a thermoelectric domestic 3 (TH TC), e.g., 30 °C (54 °F). However, the efficiency of refrigerator with a single food compartment (of 0.225 m )
Fig. 3. COP of thermoelectric modules for different materials at TH = 300 K compared to Carnot and vapor compression system (using R134a) COPs. 90 P. Bansal et al. / International Journal of Sustainable Built Environment 1 (2012) 85–101
Fig. 4. Advantage of thermionic phenomenon. maintained at 5 °C (41 °F). A COP of 0.45 was demon- retically, under an applied electrical potential, hot electrons strated at a temperature lift of 19 °C (34.2 °F). Such perfor- emitted by the cathode are at a higher energy level than mance is much below that of conventional vapor those that are left behind, which reduces the average energy compression technology. However, in order to achieve bet- level (temperature) of the cathode. Since the electrons being ter COP, Yang et al. (2008) proposed a hybrid system by absorbed on the other side of the gap are at a higher energy using a low temperature lift thermoelectric subcooler to level than those in the electrode, the average energy level is increase the subcooling temperature in a vapor compression increased, and the electrode (i.e. anode) is heated. The new system. This capitalizes on the fact that the thermoelectric type of materials called electrides that require only small COP is higher for small temperature lifts such as 5 °C amount of energy to emit electrons at lower temperatures, (9 °F). Other niche applications for thermoelectric refriger- make this technology attractive. The major advantage of ation include mobile coolers that are quiet and vibration thermionic over thermoelectric refrigeration is the elimina- free. They are also widely used as replacements for wine tion of the conduction heat transfer mode as shown in Fig. 4. cabinets, mini-refrigerators, and water coolers (Navigant A number of studies have been carried out on thermo- Consulting Inc., 2009; Bansal and Martin, 2000). tunneling (e.g. Dillner, 2008, 2010; O’Dwyer et al., 2009; Despite numerous advantages of thermoelectric refriger- Weaver et al., 2007; Shakouri, 2006; Hishinuma et al., ation, low figure of merit hinders its wide scale deployment. 2001; Ulrich et al., 2001; Kenny et al., 1996; Mahan, An order of magnitude increase in the ‘figure of merit’ is 1994). O’Dwyer et al. (2009) suggested that the most prom- required for thermoelectric refrigeration to compete with ising way to develop room temperature vacuum thermionic the energy efficiency of the ‘state-of-the-art’ vapor com- refrigerators is to combine new low work function emitter pression technologies. Molecular thermoelectric devices materials with the nanometer gap techniques. Dillner have great potential energy efficiency; however these can- (2010) calculated an upper limit for the dimensionless ther- not be produced economically at large scale with current moelectric figure of merit attainable by thermotunneling as fabrication technologies. Furthermore, current fabrication p2/12, which suggests that thermotunneling cannot outper- and assembly technologies result in a high thermal contact form the state-of-the-art thermoelectric materials. resistance that causes the temperature lift to increase, It is unlikely for thermotunneling to be an energy saving thereby dramatically reducing the energy efficiency. Efforts technology for household applications in the near future. are needed to integrate thermoelectric devices with heat Considerable R&D would be required including the devel- exchangers to eliminate the contact resistance. It is unlikely opment of cost effective low work function surfaces, with for thermoelectric refrigeration to compete with vapor typically less than 0.3 eV (O’Dwyer et al., 2006). In addi- compression technology for household applications in the tion, the requirement for extremely small inter-electrode foreseeable future. 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 elec- Magnetic refrigeration at room temperature is an emerg- trons through a pair of semiconductors in close physical ing technology that exploits the magnetocaloric effect contact, while the latter uses the flow of electrons between (MCE) found in solid-state refrigerants. These refrigerants two electrodes (i.e. cathode and anode) that are separated are environmentally friendly since they have zero ozone by an extremely small gap (of the order of microns). Theo- depletion potential and zero global warming potential P. Bansal et al. / International Journal of Sustainable Built Environment 1 (2012) 85–101 91
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 fer- The magnetic refrigeration technology, using active romagnetic material loses its permanent magnetism and magnetic regenerator (AMR) cycle, is claimed to have the becomes paramagnetic, and exhibits its greatest MCE, is potential of higher energy efficiency than the current vapor called the ‘Curie temperature or Curie point’. The MCE compression technology (Russek and Zimm, 2006), how- effect varies for different materials and can be intensified ever, no competitive system is commercially available to- by increasing the magnetic field. MCE effect causes certain date for room temperature applications. An AMR cycle materials to warm adiabatically upon application of a mag- uses magnetic material (or refrigerant) both as a thermal netic field and cool when the field is removed, and is cou- storage medium as well as a means to convert magnetic pled to an external heat transfer fluid to accomplish the work to net heat transfer. The solid material is cycled heat pumping effect. The MCE behavior depends on the through a low and high magnetic field, while exchanging type of material: ferromagnetic with magnetic domains energy with a heat transfer fluid (e.g. glycol water) oscillat- below the Curie point, or paramagnetic without magnetic ing through the void space of the AMR. An effective regen- domains. Fig. 5 depicts a theoretical magnetocaloric refrig- erator has high surface area per unit volume, high eration cycle using Gadolinium with a magnetic field of 7 conductivity and low pressure drop. A prototype rotary Tesla (T). In the magnetic refrigeration cycle, randomly magnetic refrigerator built by Astronautics Corporation oriented magnetic spins in a paramagnetic material can of America Inc., Milwaukee, USA is shown in Fig. 6. be aligned via a magnetic field, resulting in an adiabatic rise There has been a vigorous research activity related to in temperature and decrease in entropy. This phenomenon this technology in the last decade where an exponential can be used in heat pumping applications to reject heat at increase in publications has been seen; exceeding 250 in higher temperatures (Hull and Uherka, 1989). This process 2007 (Gschneidner and Pecharsky, 2008). As a result, a is highly reversible since, upon removal of the magnetic number of prototypes have emerged (Hiraro et al., 2010; field, the magnetic spins return to their randomized state, Muller et al., 2010; Zimm et al., 1998, 2006, 2007; Pechar- resulting in an adiabatic decrease in temperature but sky and Gschneidner, 2006; Hirano, 2003; Zimm, 2003). increase in entropy. The processes involved in magnetoca- Subsequently, new designs for magnetic refrigeration com- loric refrigeration are summarized below: ponents and systems have evolved that use compact devices and water-based heat transfer fluids. Yu et al. (2010) (A–B) Randomly oriented magnetic spins align after reviewed near room temperature magnetic refrigeration applying a magnetic field (H) along an isentropic process prototypes showing 41 working prototypes, 11 of which increasing the magnetocaloric material temperature by were demonstrated in 2009. Yu et al. (2010) reviewed near DTadiabatic, AB. room temperature magnetic refrigeration prototypes and (B–C) Excess heat is rejected to ambient maintaining patents and noted that there were 41 prototypes and almost constant magnetic field H. 135 patents were issued during 1997–2009. At Thermag (C–D) When the magnetic field is turned off, the spin conference in Grenoble during September 17–20, 2012, 29 moments re-randomize and the temperature is reduced prototypes were presented in varying sizes from a few by DTadiabatic, CD following an isentropic process. Watts to 2 kW that employed rare earth alloys such as (D–A) The magnetocaloric material absorbs heat from LaFeCoSi, LaFeMnSiH, LaFeSiH, MnFePas and the refrigerated volume. This raises its temperature MnFePGe (Bruck et al., 2012). The recent invigoration in and the cycle continues. patents applications and prototypes of magnetic refrigeration 92 P. Bansal et al. / International Journal of Sustainable Built Environment 1 (2012) 85–101
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 under- Japan has launched a national project of developing a lying demand. The research theme in recent studies is room temperature magnetic refrigerator with a COP focused on developing better magnetocaloric materials, exceeding 10 by using new materials and other innovations cycles, magnets and working prototypes, as highlighted in (Hiraro et al., 2010). The group fabricated a sample of some of the exemplary references (Bruck et al., 2012; Bahl Mn1+dAs1 xSbx that has a magneto-caloric effect several et al., 2010; Bjork et al., 2010; Russek et al., 2010; Kim times higher than Gd, while developing another magnetic et al., 2007; Phan and Yu, 2007; Allab et al., 2006; Basso material, Pr2Fe17 that has the same relative cooling power et al., 2006; Bingfeng et al., 2006; Gao et al., 2006; Tagliafico as that of Gd at 10% of the cost. et al., 2006; Zimm et al., 2006). Smaller temperature A record COP of 4.6 was claimed to have been achieved differences are more feasible for magnetic refrigeration by the Cooltech magnetic refrigeration prototype [Muller technology due to the limited temperature difference of et al., 2010]. The prototype measures 230 300 magnetocaloric materials, while cascade systems are more 250 mm3, weighs 34 kg, and uses 0.6 mm thick and desirable for higher temperature differences (Kitanovski 100 mm long magnetocaloric material strips. The device and Egolf, 2009). A layered regenerator bed from several achieved minimum and maximum temperatures of 17 °C magnetic refrigeration materials (Rowe and Tura, 2006) (1.4 °F) and +45 °C (113 °F) respectively. The system that have Curie temperatures tailored to the local employs a permanent magnet with 1.6 T magnetic fields. regenerator temperature in active magnetic regenerative The prototype achieved a 110 W cooling capacity between refrigeration (AMRR) can result in maximizing the MCE 13 °C (55.4 °F) and 43 °C (109.4 °F) (DT of 30 K). (Engelbrecht et al., 2006, 2007). Despite all the above advancements, there is still no Room temperature applications require materials with a experimental data available in the open literature to com- Curie temperature around 22 °C (71.6 °F). Gadolinium pare magnetic refrigeration with vapor compression refrig- and Gadolinium alloys exhibit large MCE around this tem- eration technology. Various studies, including Kitanovski perature. They are, therefore, among the most widely used and Egolf (2010), have outlined major challenges facing materials for room temperature refrigeration and space the magnetocaloric technology, which include scarcity of cooling applications. These materials undergo second- magnetocaloric materials, high cost of materials and mag- order phase transitions and do not exhibit magnetic or nets, limitations of physical properties of materials, and thermal hysteresis, the physics of which is discussed by time delay required to reach the required temperature lift. Basso et al. (2006). By using such materials and applying Although significant developments (Jung et al., 2012; a 2 T magnetic field, researchers have demonstrated tem- Rowe, 2011; Tura and Rowe, 2011; Arnold et al., 2011) perature lifts of 5 °C(9°F). Higher magnetic fields result have occurred lately in the AMR devices, magnetic devices in larger temperature lifts, but at higher cost and lower are still not able to compete with vapor compression sys- efficiency. Most prototypes rely on the use of the active tems. Some of the recent research efforts are devoted to magnetic regenerative cycle to provide high temperature synthesizing and characterizing properties of MCEs, and lift for air-conditioning and refrigeration applications. modeling and testing of AMRs including designing, build- Recent research on materials that exhibit a large entropy ing and testing of prototypes. change, such as Gd5(SixGe1 x)4, La(FexSi1 x)13Hx and MnFeP1 xAsx alloys, provide acceptable performance 6. Stirling cycle refrigeration for near room temperature applications. These materials are called giant magnetocaloric effect materials (Pecharsky An ideal Stirling cooler is a reversed Stirling engine. It and Gschneidner, 2006). consists of a closed-cycle regenerative heat engine with a P. Bansal et al. / International Journal of Sustainable Built Environment 1 (2012) 85–101 93
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- with a 100 W capacity, as shown in Fig. 8, to operate sists of two isothermal reversible processes and two con- between 40 °C( 40 °F) and 30 °C (86 °F) at an operating stant volume reversible processes, as shown in Fig. 7. frequency of 16.7 Hz and a sealing pressure within the Like the Brayton cycle, the working fluid is moved between refrigerator of less than 1.0 MPa. At a cooling temperature the hot and cold spaces through a regenerator by a system of 20 °C( 4 °F), radiator temperature of 30 °C (86 °F), of displacers. The power piston is driven by an electric and mean pressure of 0.4 MPa, the cooling capacity of motor for a refrigeration device. Large flow rates are the refrigerator increased by 20% when hydrogen was used required to produce large capacities. Although Stirling as a working fluid instead of helium. cycle cryocoolers are commercially available for infrared A free piston Stirling cooler prototype with a closed sensors and high temperature superconducting devices, thermosyphon system and R134a refrigerant was inte- their application at room temperature is practically non- grated into a domestic refrigerator by Oguz and Ozkadi existent due to a relatively low COP and high first cost. (2002). The prototype was tested at different refrigerant Otaka et al. (2002) designed, simulated, and tested a dis- charges and voltage inputs to the cooler. It was found to placer-type or b-type Stirling cycle prototype refrigerator consume approximately 30.5 W to maintain a cabinet 94 P. Bansal et al. / International Journal of Sustainable Built Environment 1 (2012) 85–101
Fig. 9. Schematics of a pulse tube refrigerator. temperature of 5 °C (41 °F) at 25 °C (77 °F) ambient tem- The PTR works on following four adiabatic compression perature. Luo et al. (2006) tested a thermoacoustic-Stirling and expansion processes in the pulse tube (de Waele, 2000): heat engine driven refrigerator, which achieved a no-load temperature of 65 °C( 85 °F) with a cooling capacity (i) Gas is compressed to high temperature, of 270 W at 20 °C( 4 °F) and 405 W at 0 °C (32 °F). (ii) Gas at high temperature and pressure flows through These results were quite encouraging for the application the orifice to the reservoir, and rejects heat to the of thermoacoustic-Stirling refrigerator for food refrigera- ambient through a heat exchanger (at room tempera- tion and air-conditioning. Recently, Sun et al. (2009) tested ture TH), a V-type Stirling cycle for domestic refrigeration with He (iii) The piston moves up and expands the gas adiabati- and N2 as the working fluids and a cold end temperature cally in the pulse tube, of 35 °C( 31 °F). Using He, cooling capacities up to (iv) This cold gas at low pressure in the pulse tube travels 70 W were achieved with a COP of 0.8. back through the cold heat exchanger at the low tem- Stirling cycle refrigeration may provide incremental perature TL (providing cooling capacity QL). energy savings in domestic applications. However, for Stirling cycle refrigeration to compete with vapor compres- The flow in either direction stops when the pressure in sion technologies, regenerator performance needs to the tube is either lower (when moving forward) or higher improve substantially to achieve higher effectiveness, lower (when moving backwards) than the average pressure in pressure drop, lower void volume and lower cost. Further- the tube. The PTR has a regenerator (made of a porous more, cold and hot heat exchangers need to be designed matrix) that precools the incoming high pressure gas before with a higher heat transfer density and lower log mean it reaches the cold end (and vice-versa) and a hot-end heat temperature differences. exchanger that rejects heat to room temperature. PTRs are commercially available for temperature appli- cations between 196 °C( 320.8 °F) down to 269 °C 7. Pulse tube refrigerator (PTR) ( 466.7 °F), where their relative Carnot efficiency is stea- dily improving (Swift, 1997; Hu et al., 2010)]. However, The pulse tube refrigerator (PTR) or pulse tube cryoco- the COP of PTR at room temperature is quite low and is oler is a developing technology that pumps heat through unlikely to play a role in domestic refrigeration. the compression and expansion of a gas. It offers several advantages over Stirling refrigerators, including no displac- er and no mechanical vibrations. It can be made without 8. Malone refrigeration any moving parts in the low temperature section of the device. The gas in-phase motion is achieved by the use of Malone refrigeration, invented in 1931 (Malone, 1931), an orifice and a reservoir volume to store gas (Radebaugh, uses a liquid without evaporation as the working fluid near 2000). A PTR, as shown in Fig. 9, consists of eight main its critical point, instead of the customary gas, in a regen- components – (i) a compressor that compresses the gas, erative or recuperative refrigeration cycle such as Stirling typically He, to higher temperatures, (ii) a heat exchanger or Brayton cycles. Due to the inherent incompressible nat- (HXH1) that rejects heat at room temperature cooling the ure of liquid, this cycle has the advantage over gas cycle for gas, (iii) a porous regenerator (to absorb and discharge achieving higher pressure change per unit volume (Malone, heat, from and to the gas, when it flows to the right and 1931). The machine can be driven externally to produce a to the left respectively), (iv) another heat exchanger refrigerating effect. In a refrigeration system, the Malone (HXL) absorbing the useful cooling power (QL) at low tem- cycle uses the cooling associated with the expansion of a perature TL, (v) a tube in which gas moves back and forth, liquid, but without a phase change. One of the earliest (vi) a hot end heat exchanger (HXH2) rejecting heat to papers studying the physics of the liquids working in heat room temperature, (vii) an orifice as a flow resistance engines was published in 1980 (Allen et al., 1980). Most device, and (viii) a large buffer volume containing He gas. of the preliminary research was conducted at the Los P. Bansal et al. / International Journal of Sustainable Built Environment 1 (2012) 85–101 95
Alamos National Laboratory (USA) in the early 1990s capacity applications, such as mini-bar fridges, recreational (Swift, 1989, 1993; Swift and Brown, 1994), where one of vehicles, hotel rooms, and boats. Some of the advantages the first Malone refrigerators (Swift, 1989) used liquid pro- of small refrigerators include quiet operation and flexible pylene (C3H6) in a double acting 4 cylinder Stirling config- use of any energy source such as gas, battery and electricity uration, followed by liquid CO2 (Swift and Brown, 1994). (Bansal and Martin, 2000). Although absorption coolers Although CO2 being an environmentally friendly refrig- offer many advantages over vapor compression systems erant could make a good candidate for the Malone cycle, (e.g. environmentally friendly absorbent/refrigerant pairs, its critical temperature (31 °C, 88 °F) is too low for efficient fewer moving parts), they are mainly limited to large scale operation in many HVAC/R applications. Other possible applications and are not competitive for small scale appli- candidate fluids include methyl alcohol, ethyl alcohol, ace- cations due to system complexity, high cost and lower effi- tone or sulfur dioxide, which have higher critical tempera- ciency compared to vapor compression systems. tures, but present safety issues. Other fluids include sulfur hexafluoride and various fluorocarbons such as HFC- 10. Adsorption refrigeration 134a. In general, these compounds have critical tempera- tures more suited to HVAC applications and critical pres- An adsorption system uses multiple beds of adsorbents sures lower than CO2. Although high heat capacity liquids such as silica-gel in a silica-gel water system, to provide offer the advantage of reduced mass flow rate for good heat continuous capacity, and does not use any mechanical transfer, the main drawback of Malone refrigeration is that energy but only thermal energy. An adsorption refrigera- these liquids are unable to achieve the desired (or required) tion system usually consists of four main components: a temperature change for efficient refrigeration. There has solid adsorbent bed, a condenser, an expansion valve and been practically no activity in the recent past on Malone an evaporator. The solid adsorbent bed is linked to the refrigeration and it is unlikely for this technology to pene- evaporator. It desorbs refrigerant when heated and adsorbs trate in the household applications in the near future. refrigerant vapor when cooled such that the bed works like a thermal compressor to drive the refrigerant around the 9. Absorption refrigeration system to heat or cool a heat transfer fluid or to provide space heating or cooling. When the bed becomes saturated Absorption and adsorption refrigeration are thermally with refrigerant, it is isolated from the evaporator and con- driven technologies that respectively use liquid and solid nected to the condenser. The refrigerant vapor is con- sorbents. These systems are popular in applications where densed to a liquid, followed by expansion to a lower demand side management is important and/or waste heat pressure in the evaporator where the low pressure refriger- is readily available. An absorption cycle, shown in ant is vaporized producing the refrigeration effect (i.e. cool- Fig. 10, utilizes a binary mixture of refrigerants such as ing the refrigerator air). When further heating no longer ammonia–water or water–LiBr. The single effect cycle con- produces desorbed refrigerant from the adsorbent bed, sists of an absorber, a generator or desorber, a condenser, the refrigerant vapor from the evaporator is reintroduced an evaporator, and an electric solution pump, with the pos- to the bed to complete the cycle. To obtain a continuous sibility of additional components, such as internal heat and stable cooling effect, generally two (or multiple) adsor- exchangers, to enhance efficiency. An external heat source, bent beds are used, where one bed is heated during desorp- such as a gas burner in a direct fired system, steam or hot tion while the other bed is cooled during adsorption. In water in an indirect fired system, or waste heat, is used in order to achieve high efficiency, heat of adsorption needs the generator (or desorber). Heat absorbed in the generator to be recovered to provide part of the heat needed to regen- allows the refrigerant to desorb from the absorbent, creat- erate the adsorbent. A recent literature review of conven- ing a high pressure vapor. In cases where a volatile absor- tional adsorption cycle was presented by Wang et al. bent is used (e.g. ammonia–water), a rectifier is needed to (2010). reduce the concentration of the volatile absorbent (e.g. Adsorptive beds of the chillers can be regenerated by water) in the vapor to the condenser. A number of low-grade temperatures using waste heat or solar energy advanced cycles have been proposed in the literature in as heat source. These chillers can also be employed in order to improve the COP starting from single effect to CCHP systems. The overall thermal and electrical effi- GAX (Altenkirch and Tenckhoff, 1911), double-effect (Vli- ciency in these systems can be above 70% (Wang et al., et et al., 1982), cycle with two absorbers (CMostofizadeh 2005). Some of the recent adsorption system performance and Kulick, 1998), compression-absorption (Hulte´n and enhancement technologies include heat pipes (Yang et al., Berntsson, 1999), auto cascade (Chen, 2002), two stage 2006) and consolidated compound adsorbents (Tamainot- absorption (Fan et al., 2007) and more recently an expan- Telto and Critoph, 2003). Lu et al. (2006) designed a pro- der-compressor cycle (Hong et al., 2010) and several waste totype icemaker with specific cooling power of 770 Wkg 1 heat/renewable energy operated absorption systems (Wang and a COP of 0.39, at 20 °C evaporation temperature. et al., 2012). Single- and double-effect absorption chillers Adsorption systems are known to suffer from low coef- are commercially available for large scale applications, ficient of performance (COP) and low specific cooling while absorption refrigerators are available for small power (Wang et al., 2010; Wang and Oliveira, 2006). 96 P. Bansal et al. / International Journal of Sustainable Built Environment 1 (2012) 85–101
working fluid, which is flammable and hence not suitable for domestic applications. Other challenges for using CD- MHHP 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. The use of a vapor compression heat pump in water heating applications dates back to the early work of Wilkes Although commercial adsorption systems for air-condi- and Reed (1937). Heat pump water heater technologies did tioning applications between 35 and 350 kW (Tassou not receive the required attention until the 1973 oil et al., 2010: Mayekawa, 2012) are reported to be available, embargo (Dunning et al., 1978a,b). While US research household scale systems are not yet commercially available. focused on developing fluorinated carbon refrigerant based Wang et al. (2010) concluded that there is still a strong vapor compression HPWHs since early 2000, the Japanese research need for advanced refrigerant/adsorbent pairs, researchers focused more on the development of a natural advanced cycles and design to overcome the system tran- refrigerant HPWH (Hepbasli and Kalinci, 2009). Carbon sient effects. Their review acknowledged the lower system dioxide proved to be among the top performing fluids efficiency and lower specific capacity compared to absorp- due to several reasons, including its large temperature glide tion and vapor compression refrigeration systems. It is in transcritical cycle. unlikely for adsorption refrigeration to be considered as a State of the art Japanese HPWHs depend on the trans- replacement for vapor compression refrigeration system critical CO2 vapor compression cycle. Several types of com- in the near future. pressors can be used, including scroll compressor (Hashimoto, 2006), single rotary compressor with brushless 11. Compressor-driven metal hydride heat pump DC motor (Maeyama and Takahashi, 2007), or two-stage rotary compressor (Sanyo, 2010). In water heater applica- Compressor driven metal hydride heat pumps (CD- tions, transcritical CO2 exiting the compressor flows in a MHHP) are based on a modified adsorption heat pump gas cooler submerged inside the water heater storage unit, system and use environmentally friendly refrigerants. The where water is heated up to 90 °C. An ejector or an expan- main difference is that the adsorption/desorption process sion valve reduces the refrigerant pressure and tempera- is controlled via a low speed refrigerant compressor. The ture. It then flows back to the evaporator, completing the compressor imposes a pressure drop causing the refrigerant circuit. The CO2 HPWH is generally more expensive than (hydrogen in this case) to desorb from the charged metal conventional technology; however, according to (Kawashi- hydride bed and to be adsorbed in a second discharged ma, 2005) they offer 30% reduction in primary energy con- reactor. The refrigerant is desorbed from the adsorbent sumption and 50% reduction in CO2 emission compared (e.g. lanthanum pentanickle, LaNi5) at low pressure and with conventional combustion type boilers. The overall temperature on the suction side and adsorbed by the LaNi5 heat pump market is expected to grow by 8.1% in Japan on the high pressure side. The refrigerant flow direction (IIR, 2010). and cycling can be controlled via three or four-way valves. CO2 HPWHs are appealing in Japan, particularly when Park et al. (2001) demonstrated the practical applicabil- the COPs range between 3 and 4.9 (as compared with effi- ity of Zr0.9Ti0.1Cr0.55Fe1.45 hydride for air-conditioning sys- ciencies for electric water heating at 1.0 and gas heating at tems by using an oil free compressor between 1 and 18 atm, 0.8 including pilot light losses). In order to achieve higher and achieved a specific cooling power of 410 Wkg 1 of alloy overall energy efficiencies and to expand the use in house- with a COP of 1.8. A schematic of this system is shown in holds, a multi-functional CO2 HPWH is being developed Fig. 11, where Magnetto et al. (2006) reported a COP above (KEPC, 2012) that combines a floor heater and bathroom 2.5 at ambient conditions between 21 °C (69.8 °F) and heater/dryer. In addition, compact CO2 HPWH units that 35 °C (95 °F). can be used on small lots in urban areas and in multi- Muthukumar and Groll (2010) concluded in their com- household dwellings are also being developed. prehensive review that compressor-driven metal hydride systems can compete with vapor compression technology; 12.2. Integrated heat pump systems (IHPS) however, the major bottlenecks include the development of a low capacity hydrogen compressor and high cost of Low-energy and passive houses are designed with high hydride alloys. CD-MHHP mainly uses hydrogen as the levels of insulation and air-tightness, resulting in low space P. Bansal et al. / International Journal of Sustainable Built Environment 1 (2012) 85–101 97
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 ven- tilation air, ambient air, or a combination of exhaust venti- lation 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 dehu- midification, particularly at times when latent heat loads are large. A conceptual design of IHPS, shown in Fig. 12, inte- Fig. 12. Conceptual design of an integrated heat pump system (after grates space heating and cooling, water heating, ventilation, Tomlinson et al., 2005). and humidity control (humidification and dehumidifica- tion) functions into a single unit. This concept consists of cycle with heat rejection in a gas cooler at a gliding CO a modulating compressor, two variable-speed (VS) fans, 2 temperature. A counter-flow CO gas cooler in combina- and heat exchangers, including two air-to-refrigerant, one 2 tion with an external single-shell hot water tank and a water-to-refrigerant, and one air-to-water to meet all the low temperature heat distribution system, as shown in HVAC and water heating loads. The air-to-water HX uses Fig. 13, can deliver domestic hot water in the required tem- excess hot water generated during the cooling and dehumid- perature range from 60 to 85 °C with a COP up to 20% bet- ification modes to temper the ventilation air, as needed, to ter than the baseline system (Stene, 2007). 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, 13. Overall assessment of NIK technologies but at a higher initial cost. Simulation results for IHPS indi- cate an approximate 50% reduction in energy use for space It is clear that there is a growing interest in not-in-kind heating & cooling, water heating, dehumidification, and technologies for household applications in a quest for sus- ventilation, compared to that of the base system (Rice tainable energy development. However, it is almost impos- et al., 2008). Research efforts are currently devoted to build- sible to rank these technologies due to insufficient ing and testing a ground source integrated heat pump information being available in the open literature on their (GSIHP) prototype at the Oak Ridge National Laboratory performance, size, reliability and cost compared to current (USA) to convert this concept into a reality. Their GSIHP vapor compression technologies. Thermoacoustic, Stirling, system was predicting to use up to 61% less energy than absorption and compressor driven metal hydride heat the baseline system while meeting total annual space condi- pumps are developing technologies, and may be classified tioning and water heating loads (Rice et al., 2013). in the medium and long term range of developments, while IHPS with CO2 as a working fluid can achieve a high thermotunneling, Malone and adsorption refrigeration lie COP due to the unique characteristics of the transcritical in the long term development range. The two emerging 98 P. Bansal et al. / International Journal of Sustainable Built Environment 1 (2012) 85–101
Fig. 13. Schematics of an integrated CO2 heat pump water heating system. technologies that show promise are thermoelectric and in the area of linear compressors (F&P, 2010) will make magnetic refrigeration, where the latter is ahead due to it increasingly difficult for emerging technologies to dis- the amount of international interest, research and develop- place evolving vapor compression technologies. ment activity. The efficiency improvement for these two Due to the varying sizes, varying operating conditions technologies is highly dependent on significant break- and varying methods for calculating performance of a spe- throughs in materials development. These technologies will cific system, comparing these systems quantitatively with have to compete with conventional vapor compression at each other and making recommendations is neither accurate higher levels of efficiency. However, recent developments 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, Time to med, high) commercialize (years) Thermoacoustic Poor Limited activity High Long term Thermoelectric Promising Well established, on-going Medium Medium term material research Thermotunneling Poor No recent activity High Long term Magnetic Promising Strong activity Medium Long term Stirling cycle Poor Manufacturing issues Medium Long term Pulse-tube refrigeration Poor Developed Medium Long term Malone cycle Theoretically good performance No reported activities High Long term refrigeration Absorption None – except when integrated with Well established Medium Short term renewable energy Adsorption None except for using renewable On-going Medium Medium term energy integration Compressor driven metal Poor No recent activities Medium Long term hydride Heat pump water heater High Strong Low Short
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Construction and performance measurement of a als and system architecture. It is envisioned that magnetic portable thermoacoustic refrigerator demonstration apparatus. MS refrigeration equipment may initially be costly, but the Thesis, Physics Department, Naval Postgraduate School, Monterey, CA. future of the technology may be promising. Bingfeng, Y., Yan, Z., Qiang, G., Dexi, Y., 2006. Research on Technologies such as thermoacoustic refrigeration, performance of regenerative room temperature magnetic refrigeration absorption, and adsorption refrigeration have lower energy cycle. Int. J. Refrig. 29, 1348–1357. efficiency compared to vapor compression refrigeration. Bjork, R., Bahl, C. Smith, A., Pryds, N., 2010. Review and comparison of However, these have the advantage of flexibility in energy magnet designs for magnetic refrigeration. In: 4th IIF–IIR Int. Conf. on Magn. 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