Thermoelectric Energy Harvesting for Sensor Powering

The dissertation solved some critical issues in thermoelectric energy harvesting and tried to broaden the thermoelectric application for energy recovery and sensor powering. The scientific innovations of this dissertation were based on the new advance on thermoelectric material, device optimization, fabrication methods, and system integration to increase energy conversion efficiency and reliability of the thermoelectric energy harvester.

The dissertation reviewed the most promising materials that owned a high figure of merit (ZT) value or had the potential to increase ZT through compositional manipulation or nano-structuring. Some of the state-of-art methods to enhance the ZT value as well as the principles underneath were also reviewed. The nanostructured bulk thermoelectric materials were identified as the most promising candidate for future thermoelectric applications as they provided enormous opportunities for material manipulation.

The optimizations of the thermoelectric generator (TEG) depended on the accuracy of the mathematical model. In this dissertation, a general and comprehensive thermodynamic model for a commercial thermoelectric generator was established. Some of the unnecessary assumptions in the conventional models were removed to improve the accuracy of the model. This model can quantize the effects of the Thomson effect, contact thermal and electrical resistance, and heat leakage, on the performance of a thermoelectric generator. The heat sink can be another issue for the design of high-performance TEG. An innovative heat sink design integrated with self-oscillating impinging jet generated by the fluidic oscillator arrays were adopted to enhance the heat convection. The performance of the heat sink was characterized by large eddy simulation.

The compatibility mismatch had been a practical problem that hindered the further improvement of energy conversion efficiency of thermoelectrics. In this dissertation, a novel method to optimize the geometry of the thermo-elements was developed. By varying the thickness and cross-sectional area of each thermoelectric segment along the length of the thermo-element, the compatibility mismatch problem in the segmented TEG construction was eliminated. The optimized segmented TEG can make the best of the existing thermoelectric materials and achieve the maximum energy conversion efficiency in a wide temperature range. A segmented TEG with an unprecedented efficiency of 23.72% was established using this method. The complex geometry structure of the established thermo-elements would introduce extra difficulty in fabrication. Thus selective laser melting, a high-temperature additive manufacture method, was proposed for the fabrication. A model was built based on the continuous equations to guide the selective-laser-melting manufacturing of thermoelectric material with other nanoparticles mixed for high thermoelectric performance.

Thermoelectric energy harvesting played a critical role in the self-powered wireless sensors, as it was compact and quiet. In this dissertation, various thermoelectric energy harvesters were established for self-powered sensors to in-situ monitor the working condition in the gas turbine and the interior conditions in nuclear canisters. The sensors, taking advantage of the thermal energy existing in the local environment, can work continuously and provide tremendous data for system monitor and diagnosis without external energy supply.