Elastocaloric cooling is a new alternative solid-state cooling technology undergoing early stage research and development. This study presents a comprehensive review of key issues related to achieving a successful elastocaloric cooling system. Fundamentals in elastocaloric materials are reviewed. The basic and advanced thermodynamic cycles are presented based on analogy from other solid-state cooling technologies. System integration issues are discussed to characterize the next generation elastocaloric cooling prototype. Knowledge acquired from the elastocaloric heat engines is provided as the basis for the design of cooling system configuration. Commercially available drivers enabling proper compression and tension are also presented. A few performance assessment indices are proposed and discussed as guidelines for design and evaluation of future elastocaloric cooling system. A brief summary of the up-to-date elastocaloric cooling prototypes is presented as well.

Elastocaloric cooling, which exploits the latent heat released and absorbed as stress-induced phase transformations are reversibly cycled in shape memory alloys, has recently emerged as a frontrunner in non-vapor-compression cooling technologies. The intrinsically high thermodynamic efficiency of elastocaloric materials is limited only by work hysteresis. Here, we report on creating high-performance low-hysteresis elastocaloric cooling materials via additive manufacturing of Titanium-Nickel (Ti-Ni) alloys. Contrary to established knowledge of the physical metallurgy of Ti-Ni alloys, intermetallic phases are found to be beneficial to elastocaloric performances when they are combined with the binary Ti-Ni compound in nanocomposite configurations. The resulting microstructure gives rise to quasi-linear stressstrain behaviors with extremely small hysteresis, leading to enhancement in the materials efficiency by a factor of five. Furthermore, despite being composed of more than 50% intermetallic phases, the reversible, repeatable elastocaloric performance of this material is shown to be stable over one million cycles. This result opens the door for direct implementation of additive manufacturing to elastocaloric cooling systems where versatile design strategy enables both topology optimization of heat exchangers as well as unique microstructural control of metallic refrigerants.One Sentence Summary: 3D printing produces highly efficient solid-state cooling nanocomposites with long fatigue life.

With advances in solid-state cooling materials in the past decade, non-vapor compression technologies, or not-in-kind (NIK) cooling technologies have garnered great attention. Therefore, a universal performance index is urgently needed to compare these NIK technologies with each other and vapor compression cooling as well. In this study, a systematic method is developed to visualize the contributions to the coefficient of performance (COP) from materials (working fluids) level to the system level as a function of temperature lifts. Since the materials level COP depends solely on the materials properties under the specified cycle, it can be used for comparing refrigerants for all NIK technologies. We chose the water-cooled water chiller operating under identical conditions as the basis for the system performance comparison of all NIK cooling

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To avoid global warming potential gases emission from vapor compression air-conditioners and water chillers, alternative cooling technologies have recently garnered more and more attentions. Thermoelastic cooling is among one of the alternative candidates, and have demonstrated promising performance improvement potential on the material level. However, a thermoelastic cooling system integrated with heat transfer fluid loops have not been studied yet. This paper intends to bridge such a gap by introducing the single-stage cycle design options at the beginning. An analytical coefficient of performance (COP) equation was then derived for one of the options using reverse Brayton cycle design. The equation provides physical insights on how the system performance behaves under different conditions. The performance of the same thermoelastic cooling cycle using NiTi alloy was then evaluated based on a dynamic model developed in this study. It was found that the system COP was 1.7 for a baseline case considering both driving motor and parasitic pump power consumptions, while COP ranged from 5.2 to 7.7 when estimated with future improvements.

Thermoelastic cooling is a recently proposed, novel solid-state cooling technology. It has the benefit of not using high global warming potential (GWP) refrigerants which are used in vapor compression cycles (VCCs). Performance enhancements on a thermoelastic cooling prototype were investigated. A few novel design options aiming to reduce the cyclic loss were proposed. It was found that the maximum temperature lift increased from 6.6 K to 27.8 K when applying the proposed novel designs, corresponding to 0 to 152 W cooling capacity enhancement evaluated under 10 K water-water system temperature lift. In addition, a multi-objective optimization problem was formulated and solved using the genetic algorithm to maximize the system capacityACCEPTED MANUSCRIPT 2 and coefficient of performance (COP). With all the novel designs, the optimization could further enhance 31% COP, or 21% cooling capacity, corresponding to COP of 4.1 or 184 W maximum cooling capacity. Nomenclature Symbols A area [m 2 ] COP coefficient of performance [-] c p specific heat [J·g -1 ·K -1 ] D diameter [m] GWP global warming potential HR heat recovery HTF heat transfer fluid h heat transfer coefficient [W·m -2 ·K -1 ] ID internal diameter [m] k thermal conductivity [W·m -1 ·K -1 ] L length [m] N quantity [-] OD outside diameter [m] PEEK polyether-ether-ketone ra nitinol heat transfer area to volume ratio [m -1 ] SMAs shape memory alloys sec second T temperature [K] M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT 3 t time, or duration [sec] t* heat recovery duration coefficient [-] u fluid mean velocity [m·s -1 ] VCC vapor compression cycle α thermal diffusivity [m 2 ·s -1 ] β tubes holder contact area ratio [-] δ equivalent thickness [m] κ thermal mass factor ρ density [kg·m -3 ]

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This paper reports the elastocaloric effect of two Cu-based shape memory alloys: Cu 68 Al 16 Zn 16 (CuAlZn) and Cu 73 Al 15 Mn 12 (CuAlMn), under compression at ambient temperature. The compression tests were conducted at two different rates to approach isothermal and adiabatic conditions. Upon unloading at a strain rate of 0.1 s −1 (adiabatic condition) from 4% strain, the highest adiabatic temperature changes (Δ T ad ) of 4.0 K for CuAlZn and 3.9 K for CuAlMn were obtained. The maximum stress and hysteresis at each strain were compared. The stress at the maximum recoverable strain of 4.0% for CuAlMn was 120 MPa, which is 70% smaller than that of CuAlZn. A smaller hysteresis for the CuAlMn alloy was also obtained, about 70% less compared with the CuAlZn alloy. The latent heat, determined by differential scanning calorimetry, was 4.3 J g −1 for the CuAlZn alloy and 5.0 J g −1 for the CuAlMn alloy. Potential coefficients of performance (COP mat ) for these two alloys were calculated based on their physical properties of measured latent heat and hysteresis, and a COP mat of approximately 13.3 for CuAlMn was obtained. This article is part of the themed issue ‘Taking the temperature of phase transitions in cool materials’.