Knowledge Bank

Combustion processes are a series of chemical reactions between a fuel and an oxidant that produce heat at enormous energy density per unit weight of fuel consumed (40-50 MJ/kg). Thermoelectric generation is the direct conversion of heat to electricity. Thermoelectric devices are useful because there are no moving parts, and thus have long lifespans. The goal of this project was to use the heat from the combustion process in a millimeter scale combustor for thermoelectric generation in portable energy systems. The radial design with concentric spirals allows a heat-exchanger type configuration that permits a balance between heat conducted by the thermoelectric elements and convection between the gas streams due to the radial temperature gradient. The goal is for each unit to produce approximately 75 mV with Iron and Constantan thermoelectric metals integrated with an alumina microcombustor. The combustors are prepared by a multi-step fabrication process that begins with soft lithography of poly(dimethylsiloxane) or PDMS to generate a mold, followed by gel-casting of the alumina slurry in the mold to create a green body. This green body is then sintered in a high temperature furnace to yield a high density ceramic structure. Combustors have been fabricated, though some warping of the material occurs during the sintering. Research efforts focused on optimizing the fabrication procedure for repeatable construction of the combustors. First, a thermocouple was integrated with a simple microcombustor to produce a maximum voltage of 10.60 mV. In the next stage, the integration of the thermoelectric elements with the spiral geometry combustor was evaluated, though the radial gradient was not produced as expected. A maximum voltage of 1.06 mV was obtained. With less warping of the combustor during drying as well as more efficient sealing of the channel walls, this system could be capable of providing a high density power source by conversion of the high energy density of conventional fuel source for portable energy needs.