Development of a Shape Memory Alloy Heat Engine Through Experiment and Modeling

Few technologies can produce meaningful power from low temperature waste heat sources below 250°C, particularly on a per-mass basis. Since the 1970’s energy crisis, NiTi shape memory alloy (SMA) and associated thermal engines have been considered a viable heat-to-power transducer but were not adopted due to previously poor material quality, low supply, design complexity, and cost. Decades of subsequent material development, research, and commercialization have resulted in the availability of consistently high-quality, well-characterized, low cost alloys and a renewed interest in SMA as a waste heat energy recovery technology. The Lightweight Thermal Energy Recovery System (LighTERS) is an ongoing ARPA-E funded collaboration between General Motors Company, HRL Laboratories, Dynalloy, Inc., and the University of Michigan. In this paper we will present initial results from investigations of a closed loop SMA thermal engine (a refinement of the Dr. Johnson design) using a helical coil element and forced-air heat exchange. This engine generates mechanical power by continuously pulling itself through separate hot and cold air streams using the shape memory phase transformation to alternately expand and contract at frequencies between 0.25 and 2 Hz. This work cycle occurs continuously along the length of the coil loop and produces steady state power against an external moment. We present engine features and the thermal envelope that resulted in devices achieving between 0.1 and 0.5 W/g of shape memory alloy material using only forced air heat exchangers and room temperature cooling.