A device that has the potential to provide electrical power to wireless sensors is investigated in this thesis. The device uses a ferromagnetic material to exploit the temperature difference between a heat source and a heat sink to produce oscillating motions and temperature polarizations, which can then be converted into electrical energy by piezoelectric materials and pyroelectric materials, respectively. One advantage of the proposed device is that it can exploit the characteristics of both piezoelectric materials and pyroelectric materials to harvest waste heat energy; therefore, it is expected to have potential high power output and conversion efficiency. Furthermore, the advantages of the reversible liquid interface can be exploited in this device to improve its performance.
The work presented in this thesis uses a coupled thermal and mechanical mathematical model to optimize the design of the proposed device. Three important parameters used in the mathematical model, the spring constant, the capillary force and the magnetic force, are calculated and then validated with experimental results to ensure that the modeling predictions match the actual behaviors of the device.
The mathematical model is then solved, and the modeling results are validated with experimental results to confirm that the model is able to correctly predict the behaviors of the device with reasonable levels of accuracy. The oscillation frequency is an important parameter for the device operation because a higher oscillation frequency means that more electrical energy can be collected in a given amount of time. Thus, the oscillation frequency is used as an index to evaluate the performance of the device. A parametric study is conducted for some design parameters, including the liquid volume of the reversible liquid interface, the total gap distance and the cold surface temperature, to attempt to increase the oscillation frequency.
The mathematical model predicts that the device can achieve a relatively high oscillation frequency when the optimized design parameters are used.
A thermal energy harvester using a ferromagnetic material with a reversible liquid interface is then physically built. Experiments are performed to study the effect of the reversible liquid interface on the performance of the device, and the results indicate that the reversible liquid interface can effectively increase the oscillation frequency in the low hot surface temperature region. Finally, when the optimized design parameters predicted by the mathematical model are applied, the device achieves a maximum oscillation frequency of 8.3Hz at a hot surface temperature of 41.8oC.