Spring-based SMA Heat Engines and Elastocaloric Heat Pumps: Analyses and Experiments

Shape memory devices have enormous potential for ground- and space-based thermal management applications. These include make and break type thermal switches where heat transfer is enabled by the SMA element (e.g., through a heat pipe) or a continuously operating heat engine where the SMA element can be the sensor, actuator and heat transferring conductor, all in one. Such an application would entail the SMA element sensing temperature, absorbing and transferring heat as a result of its actuation, and then subsequently rejecting heat, all in repeatable cycles. The phase transformation can also be caused by mechanical loading and the associated exothermic and endothermic changes facilitate use in elastocaloric heat pumps. We extend a material, mechanical and thermal framework for heat engines to include elastocaloric heat pumps. This is done by relying on the thermoelastic nature of the phase transformation and drawing parallels between enthalpy vs. temperature space and stress-strain space through the thermodynamics. Emphasis is also placed on systems-level analyses examining various configurations. A spring-based configuration with an off-set axis of rotation was selected for experimental validation, linking various aspects of SMA behavior with the corresponding system response. The use of springs enabled extended fatigue life, evaluating stress (by monitoring the pitch) with temperature and producing adequate starting torque to automatically activate engine operation. Multi-objective optimization offered guidelines for maximizing torque, speed, power or a coefficient of performance. This work has immediate implications for scavenging waste heat or heat from the ambient environment, and in environmentally friendly solid-state refrigeration.