DEVELOPMENT OF COOLING SYSTEMS WITH ACTIVE ELASTOCALORIC REGENERATORS

Abstract

The vapor compression cycle (VCC) has been developed and optimized over a century to provide cooling in residential and commercial buildings, and transport systems. However, its usage has resulted in unpredicted environmental damage such as depleting the ozone layer and promoting global warming when its refrigerant leaks into the atmosphere. Because of this, it is important to develop a superior technological alternative without the environmental costs. One way to tackle this problem is to develop heat pumping cycles using solid-state refrigerant since a solid is incapable of leaking into the atmosphere. However, a solid-refrigerant cannot flow to deliver cooling the same way a fluid-refrigerant does. This requires a system conceptual redesign, which started with near-room temperature cooling with magnetocaloric materials in 1976 and elastocaloric materials in 2012. In this work, four different system configurations were studied with the following objectives: 1) maximizing the system’s temperature lift and 2) measuring the cooling capacity as a function of the useful temperature lift of the system when operating as a water chiller. The first configuration was based on the thermowave heat recovery strategy, while the other three were based on a single stage, two-stage and reciprocating variants of the active regeneration cycle. From the studied configurations the thermowave-based cycle achieved a system’s temperature lift of 8 K, at large average strain of 4.5%. It produced a maximum useful temperature lift of 5 K and a maximum cooling capacity of 125 W. All active regeneration-based cycles achieved similar final results while the best results was a system’s temperature lift of 21.3 K at a low average strain of 3.5% and a maximum useful temperature lift of 6.5 K and a maximum cooling capacity between 16 W and 25 W. The advantage of the reciprocating system integration is that it can achieve these results at lower strain than the one- stage and two-stage configurations. This dissertation identified a fundamental limitation of the active regeneration cycles using single composition elastocaloric materials. It is due to the fact that the local strain is larger than the average strain where the temperature is lower, which limits the maximum applicable average strain to prevent premature failure. This directly affects both the temperature lift and cooling capacity of the system. Different alternatives to address this issue, as well as how to improve the overall thermal and structural performance of the system within the constraints of the materials commercially available are suggested.