Design guidelines for efficient thermoelastic harvesting of low-grade waste heat

Today, a substantial fraction of the energy used for industrial and domestic processes is wasted as heat, which contributes to global warming. Although recuperation of waste heat at high temperatures is already feasible and applied, this remains a challenge for low-grade waste heat below 100 °C. For low-grade waste heat, most approaches are either inefficient or use harmful materials. Here, we address this gap by examining thermoelastic harvesting, which utilizes shape memory alloys. By coupled finite element simulations of a reciprocating heat engine, we analyze the intimate connection between the functional properties of shape memory alloys and a thermodynamic cycle, which converts thermal to mechanical energy. Our approach allows for a systematic optimization of efficiency by varying design and operation parameters, which include fluid pressure, tube diameter, temperature, prestrain, spring constant and damping. We obtain an efficiency of 8.6%, which is equivalent to 51% with respect to Carnot — an outstanding value for low-grade waste heat of just 55°C. We derive design guidelines, which connect both, system engineering of thermoelastic harvesters and material science of thermoelastic shape memory alloys.