A thermally efficient micro-reactor for thermophotovoltaic power generation

Abstract

Hydrocarbon fuels exhibit very high energy densities, and micro-generators converting the stored chemical energy into electrical power are interesting alternatives to batteries in certain applications. The increasing demands in power and total energy delivered to portable consumer electronics such as laptops, and to military equipment that most soldiers carry with them today, open up opportunities for the miniaturization of different energy conversion technologies. A thermally efficient MEMS (micro electro mechanical systems) suspended micro-reactor (SpRE) for thermophotovoltaic (TPV) power generation has been designed and fabricated. In TPV systems, photocells convert radiation from a combustion-heated emitter, into electrical power. TPV is an indirect conversion scheme that goes through the thermal domain and therefore does not exhibit very high efficiencies. However. because of its simple structure and because the combustor and photocell fabrication processes do not need to be integrated, the system is simpler to micro-fabricate than other generator types, such as thermoelectric systems and fuel cells. It is also a mechanically passive device that is virtually noiseless and less subject to wear than engines and turbines.

(cont.) The SptRE serves as a catalytic combustor-emitter, heated by combustion of propane in air, and the radiation emitted is converted into electrical energy by low-bandgap, gallium-antimonide (GaSb) photocells. The SpRE combines thermal isolation provided by long, thin-walled silicon nitride tubes, with the high thermal conductivity of a silicon reaction chamber and heat exchangers to obtain a thermally efficient device. The latest generation of reactors, SpRE III, has been designed, fabricated and thermally characterized, using a three-wafer bonding process to achieve circular tubes. The packaged device measures 11.5x15.5x3 mm, with a reaction chamber of dimensions 5x5x2 mm. Net TPV power generation of up to 1 mW during autothermal propane combustion has been achieved in an earlier and smaller version of the reactor, SpRE I, constituting a promising proof of concept and the first reported net power generation in a MEMS based TPV system.