DEVELOPMENT OF MULTI-STAGE ELASTOCALORIC COOLING DEVICES

Abstract

Elastocaloric solid-state refrigerants have lower environmental impact compared to conventional vapor compression refrigerants, but they require significant advancements to gain widespread implementation. Two barriers that prevent adoption are low temperature lift and poor fatigue life. This dissertation addresses those challenges through a single, scalable architecture with the objectives of 1) designing high-performing elastocaloric devices, and 2) maximizing temperature lift. The developed prototype consists of twenty-three 17 mm long, thermally insulated Ni-Ti tubes in a staggered pattern that exchange heat with the surrounding fluid medium through their external surface areas. They are contained inside a 3D-printed plastic that provides alignment and restricts heat transfer to other components. A top loader and fixed bottom plate transfer compressive loads to the tubes, and a 3D-printed housing encapsulates all components. Single, two, and three-stage configurations were experimentally investigated. A sensitivity analysis was conducted on the single-stage device and identified fluid-solid ratio, loading/unloading time, and strain as three parameters that could increase temperature span by over 1.5 K each. The combination of these findings resulted in a maximum steady-state temperature span of 16.6 K (9.7 K in heating and 6.8 K in cooling) at 4% strain and under zero load conditions. The temperature lift was increased in the two and three-stage configurations which achieved 20.2 K and 23.2 K, respectively, under similar operating conditions. Validated 1D numerical models developed for this work confirm that the multi-staging approach positively impacts thermal response, though with decaying significance as the number of banks increases. By further optimizing the operation condition which minimized the water volume in the fluid loop, the three-stage device was ultimately able to develop the largest lift of 27.4 K. The tubes used in the single and two-stage tests also withstood over 30,000 cycles without failure, showing promising fatigue life behavior and emphasizing the viability of this alternative cooling technology.