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Thermoelectric Power Harvesting from Small Engines
Keywords: Energy Harvesting, Thermoelectric, Modeling
Small, single-cylinder internal combustion engines are widely used because of their attractive power-to-weight ratios and ready availability of energy-dense fuel sources. However, the attractiveness of these engines is diminished because of their rather poor thermal efficiency. The impact of this limited thermal efficiency is heightened when the generation of electric power is required in addition to the engine’s major purpose as a power source for other needs. Thus, a small, two-stroke engine that may have a thermal efficiency of 20% when used to drive an alternator with a total conversion efficiency of 50% will provide only 10% of the energy in the fuel when converted to electrical power. This paper, then, explores exploitation and optimization of the coupling to “waste energy” in the form of heat in the exhaust stream and mechanical vibration from a small engine. While projects to harvest waste heat from internal combustion engine exhaust have been undertaken over the past several years, the multi-source harvesting concept described here is new. Ambient Micro has been evolving the concept of multi-source energy harvesting by combining power from the mechanical vibration of the engine in conjunction with thermoelectric power generation from the exhaust. Previous work has aimed at milliwatt power levels, while the work described here targets much higher power levels. In contrast with previous work in this area, the thermodynamics of coupling to the exhaust stream is carefully analyzed. A CFD-based tool is described that enables parametric modeling of the thermoelectric power generation process including coupling to the exhaust stream on the “hot” side to the thermoelectric modules and heat rejection on the cold side. Modeled and measured data are presented demonstrating the efficacy of the CFD as a design tool for future applications. This work was funded under Contract No: FA 8650-07-C-2723 from AFRL.
John Langley, CEO
Ambient Micro
Half Moon Bay, CA

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