Gradient phase transformation temperature design in nitinol tubes for enhanced elastocaloric cooling performance

Elastocaloric cooling is an emerging solid-state cooling technology that employs shape memory alloys (SMAs) as the solid refrigerants. Unlike conventional vapor-compression systems, elastocaloric cooling operates without harmful gaseous refrigerants, offering a new pathway to a more environmentally friendly cooling system. However, one key challenge is when the temperature lift develops between the hot end and cold end of the regenerator, an inhomogeneous phase transformation along the length of the active material will be created. This inhomogeneous phase accelerates the mechanical fatigue and shortens the lifetime of the regenerator, thereby limiting the system efficiency. To address this issue, the phase transformation temperature of the SMA must be engineered to align with the thermal gradient inside the regenerator. Here, we propose a simple heat-treatment method to fabricate nitinol tubes with a controlled gradient phase transformation temperature. Numerical simulation is used to optimize the gradient profile, demonstrating that such gradient tube achieved higher temperature lift and reduced strain maldistribution compare to uniform phase transformation tube. Overall, the gradient SMAs design provides a practical solution to enhance both the performance and the durability of the elastocaloric cooling systems.