Electrocaloric cooling over high device temperature span

Abstract

Conventional cooling method consumes large amount of electricity and heavily relies on hydrofluorocarbon coolants, which are considered “super-greenhouse effect” gases. Electrocaloric (EC) cooling demonstrates high energy efficiency, involves no evaporative coolant, and is considered a promising next-generation cooling technology. A safe operational temperature change of the EC material is currently limited to well below 10 K, and EC devices for practical application scenarios need to be able to pump heat at over 20 K. Since the 90s, the EC device community has mostly focused on fluid-based prototypes that employ an oscillatory moving liquid to actively transfer and regenerate heat; this approach has proved to be successful in building a temperature gradient of over 10 K. Recently, a few solid-state EC heat pumps have emerged featuring direct heat transfer in a cascade style. Discussion over these two EC device architectures, material, and the actual device manufacture challenges should be of broad interest to the design of efficient and compact cooling systems. Electrocaloric cooling demonstrates direct electricity utilization and high energy efficiency, and is often proposed as an environmentally benign, solid-state alternative to the conventional vapor compression cooling technology. As a result, the fabrication of effective electrocaloric materials and engineering of electrocaloric cooling devices have attracted increasing attention in the cooling community. However, the limited material adiabatic temperature change under safe operation field poses a major challenge to practical electrocaloric cooling device development, since real-world applications usually require device temperature span over 20 K. This perspective presents recent efforts devoted to design electrocaloric cooling devices based on active heat regeneration and cascading approaches; both strategies have achieved significant device temperature lift ~10 K with adiabatic temperature change of 2~3 K. The strategies discussed in this work are broadly applicable to the design of efficient and compact cooling systems. Electrocaloric cooling demonstrates direct electricity utilization and high energy efficiency, and is often proposed as an environmentally benign, solid-state alternative to the conventional vapor compression cooling technology. As a result, the fabrication of effective electrocaloric materials and engineering of electrocaloric cooling devices have attracted increasing attention in the cooling community. However, the limited material adiabatic temperature change under safe operation field poses a major challenge to practical electrocaloric cooling device development, since real-world applications usually require device temperature span over 20 K. This perspective presents recent efforts devoted to design electrocaloric cooling devices based on active heat regeneration and cascading approaches; both strategies have achieved significant device temperature lift ~10 K with adiabatic temperature change of 2~3 K. The strategies discussed in this work are broadly applicable to the design of efficient and compact cooling systems. The ubiquitous vapor compression cooling (VCC) technology has been long criticized for its limited energy efficiency, release of hydrofluorocarbon gases that aggravates the disastrous global warming, and difficulty to scale down for individual personal or device-level cooling.1Shi J.Y. Han D.L. Li Z.C. Yang L. Lu S.G. Zhong Z.F. Chen J.P. Zhang Q.M. Qian X.S. 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While a liquid refrigerant is forced to go through repeated evaporation and condensation cycles in the VCC system to pump heat, these caloric effects make use of the latent heat of a solid-state phase transition regulated by an external field, such as electrical, magnetic, mechanical stress, and pressure. In particular, the electrocaloric (EC) cooling has attracted wide attention, thanks to its direct utilization of electricity, high energy efficiency, and a relatively simple and compact cooler architecture. EC devices are often considered an ideal solution for compact, chip-scale active thermal management.1Shi J.Y. Han D.L. Li Z.C. Yang L. Lu S.G. Zhong Z.F. Chen J.P. Zhang Q.M. Qian X.S. Electrocaloric cooling materials and devices for zero-global-warming-potential, high-efficiency refrigeration.Joule. 2019; 3: 1200-1225Abstract Full Text Full Text PDF Scopus (109) Google Scholar,14Ožbolt M. Kitanovski A. Tušek J. Poredoš A. 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Li Q. Gu H.M. Jiang S.L. Han K. Gadinski M.R. Haque M.A. Zhang Q.M. Wang Q. Ferroelectric polymer nanocomposites for room-temperature electrocaloric refrigeration.Adv. Mater. 2015; 27: 1450-1454Crossref PubMed Scopus (157) Google Scholar In order to utilize the ECE for active cooling, the active EC material (referred to as EC element hereafter) needs to go through cooling cycles (Figure 1A), and the simplest cycling behavior can be described using a Brayton cycle process in 4 stages (Figure 1B).16Zhang Q.M. Zhang T. The refrigerant is also the pump.Science. 2017; 357: 1094-1095Crossref PubMed Scopus (16) Google Scholar Starting at stage 1, with no electric field applied (E = 0), the electric dipoles within the EC element are randomly oriented. When a field E0 is applied (stage 1→2), the dipoles align along the electric field. The entropy holds constant in the initial instance and the temperature of the EC element rapidly increases by ΔTECE under an adiabatic condition. Next, (stage 2→3), while the electric field is maintained, the EC element makes contact with a heat sink, so that heat is ejected from the EC element to the heat sink toward a thermal equilibrium; both temperature and entropy of the EC element decrease. In the process (from stage 3→4), the electric field is removed and the dipoles return to a state of disorder, no heat exchange occurs in this adiabatic process, and the temperature of EC element rapidly drops by ΔTECE. Finally, from stage 4→1, the EC element makes contact with a heat source and absorbs heat from the heat source. The temperature and entropy of the EC element both increase and the system returns to its original stage 1. This scheme represents the most straightforward design strategy for an EC heat pump, and so long as the cycle is repeated, heat can be continuously transported from the heat source to the heat sink (in the opposite direction of spontaneous heat transfer). Recent developments in the EC material have seen a largely enhanced peak adiabatic temperature change of almost 50 K (via DSC measurement),30Zhang G.Z. Li Q. Gu H.M. Jiang S.L. Han K. Gadinski M.R. Haque M.A. Zhang Q.M. Wang Q. Ferroelectric polymer nanocomposites for room-temperature electrocaloric refrigeration.Adv. Mater. 2015; 27: 1450-1454Crossref PubMed Scopus (157) Google Scholar however, such an impressive ΔTECE is obtained under very high field intensity (>250 MV/m) approaching the material breakdown limit. Cooling applications require an active EC working body with a significant size, and the ECE cooling cycles can be operated repeatedly. Analogous to thin film dielectric capacitor applications, the safe operation of such high voltage devices normally has the electric field set to ~30% of the breakdown field. Such safely operable electric fields for the ECE materials lead to adiabatic temperature changes of just 1~10 K1 in current EC materials, whereas most practical cooling applications require temperature spans of 20 K or higher. There is a general need to produce a large EC device temperature span (ΔTdevice) from EC materials of limited ΔTECE. Active heat recovery with an oscillatory heat transfer fluid is a conventional solution; recently, there are reports on active solid-state regeneration systems as well as emerging device prototypes employing a cascade structure. This work reviews these two unique but thermodynamically relevant approaches, a.k.a. active heat recovery (AHR) and cascade cooling process, respectively. Performance-limiting factors that are vital in further expansion of the cooling device temperature lift and cooling power are also discussed. AHR with fluidic media, is a mature practice in multiple caloric-based cooling technologies, including magnetocaloric, elastocaloric, and thermoelectric device systems.6Kitanovski A. Energy applications of magnetocaloric materials.Adv. Energy Mater. 2020; 10Crossref PubMed Scopus (101) Google Scholar,8Tušek J. Engelbrecht K. Eriksen D. Dall'Olio S. Tušek J. Pryds N. A regenerative elastocaloric heat pump.Nat. Energy. 2016; 1: 16134Crossref Scopus (185) Google Scholar,10Snodgrass R. Erickson D. A multistage elastocaloric refrigerator and heat pump with 28 k temperature span.Sci. Rep. 2019; 9: 18532Crossref PubMed Scopus (17) Google Scholar The caloric materials function as refrigerant, and at the same time as regenerator. The process is referred to as active magnetocaloric regeneration (AMR) in the field of magnetocaloric cooling and is termed as active electrocaloric regeneration (AER) for EC cooling. The active caloric regenerator requires a rather bulky system; in general, a porous structure of a material bed that periodically goes through caloric effect, with voids through which an oscillatory fluid can flow as the moving part. The porous EC regenerator bed is housed in a sealed tube-shaped chamber equipped with a fluid pumping apparatus, an electric field generator, and is connected to heat source and heat sink. Figure 2A depicts the schematic of the setup where four regeneration phases take place in turn.31Plaznik U. Kitanovski A. Rožič B. Malič B. Uršič H. Drnovšek S. Cilenšek J. Vrabelj M. Poredoš A. Kutnjak Z. Bulk relaxor ferroelectric ceramics as a working body for an electrocaloric cooling device.Appl. Phys. Lett. 2015; 106: 043903Crossref Scopus (116) Google Scholar, 32Tahavori M. Veje C. Lei T. Nielsen K.K. Engelbrecht K. 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Fourth, the working fluid, this time pushed to flow in the counter direction as in the second stage, deposits heat back into the porous EC material; as the fluid enters the cold heat exchanger, it absorbs heat from the heat source. With the cycle of the four stages goes on, heat is systematically pumped from the heat source to the heat sink through the working fluid. After certain cycles of operation, a steady state with temperature gradient along the fluid flow direction can be built up, within both the EC material and the fluidic media. Although the temperature change of working fluid on either the cold or hot side must not exceed ΔTECE, it is worth noting that the temperature span between the hot and cold sides at the two ends of the EC regenerator (and the working fluid as well) can be higher than the material adiabatic temperature change (see temperature profile in Figure 2A). The buildup of device temperature span is most effective if the EC regenerator (EC element) possesses anisotropic conductivity, a.k.a., high orthogonal heat exchange with the surrounding liquid media, but low bulk thermal conduction along the temperature gradient dimension (i.e., the liquid flow direction) along the regenerator.34Reinecke B.N. Shan J.W. Suabedissen K.K. Cherkasova A.S. On the anisotropic thermal conductivity of magnetorheological suspensions.J. Appl. Phys. 2008; 104: 023507Crossref Scopus (65) Google Scholar,35Fang X.P. Xuan Y.M. Li Q. Anisotropic thermal conductivity of magnetic fluids.Prog. Nat. Sci. 2009; 19: 205-211Crossref Scopus (32) Google Scholar EC material within AER system most conveniently goes through Brayton-like cycles (see Figure 2B). Potentially more efficient thermodynamic cycles such as Ericsson or the hybrid Brayton-Ericsson cycles are also achievable,36Plaznik U. Tušek J. Kitanovski A. Poredoš A. 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As a result, thermodynamic paths of the neighboring EC material largely overlap within iso-field heat transfer phases, but they differ in adiabatic processes. The dashed, dotted, and solid paths in the T-S graph shown in Figure 2B represent partially overlapping thermodynamic cycles performed by the coldest, the middle, and the hottest parts of the EC regenerator, respectively. The area enclosed in a specified thermodynamic cycle indicates specific electric work done to drive a certain infinitesimally small piece of EC material in the AER:w=∫s(Tc,E>0)sTh,E>0Tds−∫sTc,E=0sTh,E=0Tds(Equation 1) where s refers to specific entropy of the EC material under certain temperature and electric field. Integration of the specific work values across the entire EC working body mass results in the overall electric work done to the device:W=∫wdm(Equation 2) where dm represents the mass of an infinitesimally small cross-sectional slice of the EC material along the AER, whose specific work is calculated to be w. The maximum cooling capability Qc_max and the maximum heating capability Qh_max that can be obtained in the AER cycle can also be defined. In a no-load steady state where the cold end of the regenerator is cooler than the heat source (Tsource) by ΔTECE, and the hot end of the regenerator is warmer than the heat sink (Tsink) by ΔTECE, the theoretical cooling power (blue region in Figure 2B) is calculated using:Qc_max=∫∫s(Tc,E=0)s(Tsource,E=0)Tdsdm(Equation 3) where Tc denotes the lowest temperature within a specified Brayton cycle, and the integration range of dm is limited to those pieces whose Tc is lower than Tsource. Likewise, the theoretical heating power (red region in Figure 2B) is:Qh_max=∫∫s(Tsink,E>0)s(Th,E>0)Tdsdm(Equation 4) where Th denotes the highest temperature within a specified Brayton cycle, and the integration range of dm is limited to those pieces whose Th is higher than Tsink. The regenerated heat Qh_reg and Qc_reg (red and blue regions in Figure 2C) refer to that heat exchanged between the working fluid and the regenerator (EC working body) but cannot leave the system for cooling or heating purpose. The regenerated heat is essential to build up the high device temperature span. From a thermodynamic point of view, in the heating stage, the working fluid picks up a total heat of (Qh_reg + Qh_max) from the regenerator and rejects Qh_max to the heat sink; during the cooling stage, a total heat of (Qc_reg + Qc_max) is stripped off the counter fluid flow, and the cooled fluid is able to absorb Qc_max from the heat source; the entire cycle is driven by a work input of W, and the ideally regenerated heat Qh_reg is equal to Qc_reg. Hence, if we neglect hysteresis loss within the EC material and work done to propel the oscillatory fluid flow, the theoretical coefficient of performance (COP) of the AER cooling device, which signifies the cooling efficiency of the refrigerator, can be calculated as:COP=Qc_maxW(Equation 5) This ideal COP value equals to COPCarnot of Tc/(Th – Tc). The fluid-based AER requires relatively low degree of engineering and can conveniently build up device temperature span over 10 K, achieving a high regeneration factor (ΔTdevice/ΔTECE) over 10. 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Simulation of device performance predicts regeneration factor reaching almost 10 upon further optimization, only to be marginally affected by the magnitude of ΔTECE. Recent work by Torrelo et al. reports an achievement of 13.0 K—the highest ΔTdevice so far for an EC device, based on PST multilayer capacitor (MLC). The work employs finite element modeling as a strategy to guide the device configuration with better performance. The optimized AER chamber houses 128 PST-MLCs (16 columns by 8 rows, with individual MLC plates of 10.6 × 7.6 × 0.5 mm), with total mass of 38.4 g and regenerator length of 115 mm. The electric field employed was 15.8 MV/m, which gives rise to ΔTECE of 2.2 K, and the frequency was 0.125 Hz. Prediction of ΔTdevice of 47.5 K and a maximum cooling power of 16.3 W (850 W kg−1) are projected, based on a ΔTECE of 5.5 K.42Torelló A. Lheritier P. Usui T. Nouchokgwe Y. Gérard M. Bouton O. Hirose S. Defay E. Giant temperature span in electrocaloric regenerator.Science. 2020; 370: 125-129Crossref PubMed Scopus (38) Google Scholar The use of working fluid and the presence of external pump to drive the oscillatory liquid flow makes it difficult to scale down the AER device for chip-scale applications; besides, mixing of the fluid along the flow direction mitigates the regeneration-induced temperature gradient and lowers the cooling power of the device.43Brown G.V. Magnetic heat pumping near room temperature.J. Appl. Phys. 1976; 47: 3673-3680Crossref Scopus (767) Google Scholar Solid-state regeneration provides an alternative to achieve high ΔTdevice within a compact device size. Gu and coworkers showed that, with stainless-steel plates as heat transfer media (solid regenerator), the active regeneration cycle can be transformed into alternating displacement of EC material with respect to the regenerator corresponded with timely triggered EC heating and cooling effects (Figures 2D and 2F).26Gu H.M. Qian X.S. Li X.Y. Craven B. Zhu W.Y. Cheng A.L. Yao S.C. Zhang Q.M. A chip scale electrocaloric