A review of elastocaloric cooling: Materials, cycles and system integrations

Highlights

• Reviewed elastocaloric cooling materials and quantitatively assessed half of them.

• Discussed thermodynamic cycle options for elastocaloric cooling.

• Reviewed four key aspects of developing a successful elastocaloric cooling system.

• Summarized the up-to-date development status of elastocaloric cooling prototypes.

Abstract

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.

Introduction

Elastocaloric cooling, also known as thermoelastic cooling, has been recognized as the most promising alternative to the state-of-the-art vapor compression cooling systems (Goetzler et al., 2014). It is based on the latent heat associated with the martensitic phase transformation process, which has been found in shape memory alloys (SMAs) when they are subjected to cyclic uniaxial loading and unloading stresses (Manosa et al., 2009). Fig. 1 visualizes the stress-induced phase transformation process. The SMA is initially fully austenite under the stress-free state at the application temperature. When the stress on the SMA exceeds the “saturation stress” of phase change, the austenite starts to transform into the martensite phase. During the phase transformation, entropy reduces and latent heat is released to the ambient. The reverse phase change from martensite to austenite takes place once the loading stress decreases below the “saturation stress” of phase change. The “saturation stress” of unloading may not be identical to that of loading due to hysteresis, as will be discussed in more details later. The entropy increases and the SMA absorbs heat from the ambient, which could be used for producing the cooling effect. This principle determines the cyclic operation nature of elastocaloric cooling system, which is the most important difference from vapor compression cooling. Another major difference from the vapor compression system is the requirement of a heat transfer fluid (HTF), since solid-state SMAs cannot flow like a conventional liquid or gaseous refrigerant.

Fig. 2 shows the typical features of an elastocaloric cooling or heat pump system. The most important part is the elastocaloric materials, and therefore, an entire section of this paper focuses on the materials' research and development. Materials with different viable cycles are also discussed. The second feature is the loading/driving system requiring work input. Unlike those compressible gaseous refrigerants used in vapor compression systems undergoing more than 200% specific volume change, the solid-state phase change in elastocaloric material is usually corresponding to less than 10% strain, and as a result, requires much higher stress (pressure). To achieve the compression of solid, commercial linear drivers may be used and they are summarized in this paper. Heat transfer between the elastocaloric material and the HTF is important as well. The discussion of the third feature is based on a few proposed performance assessment indices. The system layout in Fig. 2 consists of two beds that are filled with elastocaloric materials and has the heat recovery (HR) and work recovery (WR) features, as proposed by Qian et al. (2015b) and Schmidt et al. (2015). The HR precools the hot bed and preheats the cold bed simultaneously before unloading/loading process to reduce the parasitic heat loss caused by cyclic variation of temperatures of the elastocaloric materials and other parasitic supporting parts (Qian et al., 2015a). The active regenerator design (Tusek et al., 2015b) is another viable option to achieve simultaneous heat transfer and HR. To recover the mechanical work, WR requires a conjugated elastocaloric material bed configuration in the system, as demonstrated by Fig. 2. The two beds are both halfway pre-loaded in the beginning so that when one bed is 100% loaded reaching the maximum strain, the other one is fully unloaded back to zero strain. In this way, the unloading energy from one bed is automatically recovered by the loading process of another one. Auxiliary parts, such as pumps, fans and even cooling towers consume power as well. This auxiliary power consumption is not trivial in the system performance evaluation and should not be neglected.