New Insight into the Coefficient of Performance and Related Material Indices of Merit for the Selection of Shape Memory Elastocaloric Materials

Abstract

We re-evaluate the thermodynamics of a generic elastocaloric (EC) device based on shape memory alloys (SMA) operating between a hot and a cold reservoir, acting either as a heat pump or as a cooling device. The work is motivated by the noticeable discrepancies in the literature regarding the key parameters used to evaluate the coefficient of performance (COP) of these devices. We show that the COP depends on the temperatures of both the cold and hot reservoirs, TC and TH , on the ratio of the SMA material specific heat to entropy of transformation, cp∕Δstr , and on the hysteresis, ΔThys , associated with the stress-induced martensitic transformation involved. The effect of heat recovery (regeneration) efficiency on COP is also evaluated. In addition, we have reviewed the concept of adiabatic temperature change, ΔTad , including its experimental determination and supporting the analysis with our own measurements. Material selection charts are proposed as a useful guide for the conceptual design of EC devices. They emphasize the importance of correctly considering the operating parameters of the system, in addition to the specific SMA propertiesmentioned above.