The power of thermoelastic harvesting of low-grade waste heat: A question of timing the heat exchange

Waste heat harvesting of industrial processes is necessary to improve energy efficiency, reduce environmental impact, and, in the best cases, also pay off economically. Consequently, many different harvesting systems have been developed, but for temperatures below 100 °C, a mature technology that achieves sufficient efficiency and output power is not yet available. Here, we focus on thermoelastic harvesting, which uses shape-memory alloy wires as active materials. We demonstrate that the cycle frequency and, thus, the output power of a thermoelastic harvesting system can be increased by replacing the common fluid flow axial to the wire with a transverse flow. This results in simultaneous heat exchange along the length of the wire. This improvement in timing allows one to increase the wire length and cycle frequency, both of which increase output power, effectively enabling the upscaling of thermoelastic devices. We validate our approach through in-operando experiments for a 1 mm thick wire and demonstrate that transverse flow allows us to increase the switching frequency from 1 to 11 Hz, which increases output power by a factor of 4.3. As slow frequency is also a shortcoming of other ferroic harvesting and cooling devices, we discuss the feasibility of our timing approach for these emerging technologies as well.