Using shape memory alloys for low temperature waste heat conversion - material properties, requirements, and challenges.

Close to 60% of the primary energy loss in conversion processes comes from domestic systems, which operate in a temperature range below 100 °C. This waste heat cannot be utilized in technical applications, and it also contributes to global warming when emitted into the environment. Recovering it offers a path to reduce energy loss and to improve overall system efficiency. While industrial waste heat recovery is well-established for high-temperature processes, there is a major technical gap in the utilization of low-grade (temperature) thermal energy. Shape memory alloys (SMAs) represent a promising solution to tackle this challenge. Specifically, shape memory driven thermoelastic generators (TEGs) can be used to convert thermal energy into a mechanical work output. Different thermal engine designs have been proposed since the 1970’s, most rely on commercial NiTi wires which are not optimized for TEG applications.

Our research aims to identify and develop SMA materials tailored for optimal TEG performance. Using the Ashby material selection framework, we establish key property targets such as work output, latent heat, specific heat, hysteresis width, transformation temperatures, and fatigue resistance (etc.) to screen potential alloys. We consider SMAs based on NiTi, Cu, Ti, and other less-common systems. Property maps were generated from literature data, materials databases and thermomechanical cycling experiments. The data provide a good overview on the potential performance of different types of SMAs. We discuss material cost effectiveness in comparison with other techniques and strategies related to multi-parameter optimization, which help to identify compositions with good compromise property profiles.