A laboratory-scale prototype of thermomagnetic generator for harvesting of waste heat

The harvesting of low-grade waste heat from sources at temperatures below 100°C, which represents more than 60% of civil and industrial waste heat, and its conversion in mechanical or electrical energy constitute a key goal for the transition to a green economy based on renewable energies and with a low human impact on the environment. In the last decade, many materials and different prototypes have been proposed for this type of applications. In this scenario, there is an emerging need for techniques able to characterize the materials properties under realistic working conditions. However, materials performance is strictly connected to the thermomagnetic generators design, therefore a continuous process of device optimization is required along with material properties engineering. In this work we present a laboratory-scale prototype of thermomagnetic generator designed for the rapid evaluation of thermomagnetic materials for energy-harvesting applications in different operating conditions. The core of the device is a thermomagnetic motor, based on the Curie wheel principle, that converts thermal energy into mechanical energy and works between two temperature reservoirs, allowing the evaluation of materials performance on an adjustable temperature range. The motor shaft is connected to a custom-built, two-phase, miniaturized, permanent magnet electric generator specifically optimized for low-speed operation, which generates electric power and enables the simultaneous measurement of torque, rotation speed and mechanical power output. The device can be operated with as low as 500 mg of active thermomagnetic material, allowing the use and characterization of candidate compounds even when only gram-scale batches can be obtained. The rotors are made, starting from powder of the thermomagnetic material and a plastic support, by means of a repeatable and easy-to-perform preparation method. Thanks to the modular design of the prototype, different operating parameters (e.g. magnets arrangement and positioning, rotor size and shape, heat exchange mechanisms) can be modified and tested to evaluate their impact on the overall power output. We report, as an example, the complete mechanical and electric power output evaluation of the prototype working with different rotors based on Ni-Mn-In Heusler alloy in the 20-60 °C temperature range. Thermal imaging technologies were employed to investigate thermal gradients in TM rotors during operation.