Harvesting waste heat with a rotary thermomagnetic generator

Low grade heat is a byproduct of many man-made and natural processes. Recently, there has been advancement in the development of new devices and materials that can harness such low-grade heat. Thermomagnetic materials exhibit significant changes in magnetization following a small change in temperature around its Curie temperature. This physical property of the material can be harnessed in a device to convert thermal energy into other useful forms of energy like mechanical work. The thesis activity is carried out at the Singapore Institute of Manufacturing Technology, a research institute of A*STAR Singapore. The aim of the research is to improve the performance of a rotating thermomagnetic motor composed of thermomagnetic material. The latter is included in a heat energy harvesting device that uses water to heat and air to cool. A numerical study is carried out, using COMSOL Multiphysics, to investigate the temperature response of a thermomagnetic material subjected to water and air jet impingement, in order to enhance heat transfer and identify the limits of jet impingement to control the temperature-dependent magnetic properties of thermomagnetic materials. Beyond the Curie temperature, in fact, the material loses its magnetic properties and becomes paramagnetic. The rapid change in magnetization around the Curie temperature can be used to design a device able to produce a useful form of energy. In particular, the study aims to identify the suitable conditions required to operate the device and optimize the time required for heating and cooling of the material. For this reason, both simulations and laboratory experiments are performed to understand how the temperature on the material varies over time for different operating conditions, analyzing the effect of jet impingement velocity, nozzle diameter and nozzle-to-surface distance on the heat transfer characteristics. Finally, once the optimal configuration is obtained, the model is validated by comparing the simulation results with the laboratory experimental results. The latter are obtained after processing the data acquired, through LabVIEW, by thermocouple sensors inserted on the thermomagnetic material during the operation of the device. The scientific discovery made in this research would lead to more widespread use of thermomagnetic material for thermal energy harvesting. The practical realization of a rotary thermomagnetic generator with higher power density and efficiency will make harvesting waste heat more economically viable.