Silicon-micromachined thermoelectric generators for power generation from hot gas streams

The demand for long-lasting wireless electronic devices with increasing functionality has spurred research of alternative power sources to traditional batteries. Additionally, there is great interest for energy harvesting devices that can locally generate power for wireless sensor networks from ambient environmental energy. To this end, thermoelectric generators are attractive for their capability to directly convert heat energy into electrical energy, with no moving parts. As a result, they are silent, reliable, and require no maintenance.;This dissertation focuses on the development and characterization of miniaturized radial thermoelectric generator systems to directly convert waste thermal energy from hot gas streams into electrical energy. For example, a hot gas line in an automobile or aircraft could be used for a self-powered wireless temperature sensor. Alternatively, a thermoelectric generator could be coupled with a small-scale heat engine or combustor for conversion of hydrocarbon fuels into electrical energy.;In this dissertation, a radial thermoelectric generator configuration is investigated. The structure consists of coin-sized silicon-micromachined chip modules that, when stacked, form a cylindrical heat exchanger with finned surfaces on both the inner and outer sides. Hot exhaust gas flows through the finned central channel heating the inner surface, and outer annular fins keep the outer surfaces cool. This design readily accommodates hot gas flow, which overcomes one of the primary inadequacies of the typical parallel-plate thermoelectric module design. Each module consists of two thermally isolated concentric silicon rings connected by a thin polyimide membrane that supports radially oriented thin-film thermoelements.;This radial device design was first modeled using analytic heat transfer and electrical models. Using PbTe semiconductor thermoelements, model predictions indicate reasonably high power outputs and power densities (e.g. 1.3 mW and 27 mW/cm3) could be expected from gas flows 400 °C. To demonstrate the concept, micromachined generators consisting of thin-film metals (Au and Ni) were fabricated and tested. The 13-mm-diameter, 0.36-mm-thick (48 mm3) modules demonstrated 0.8 microW of power generation (power density of 17 microW/cm3) using a 195 °C hot gas stream. While limited, these results provide model validation and serve as a stepping-stone toward higher-performing modules using semiconductor thermoelements.