Evaluation of metal foam based thermoelectric generators for automobile waste heat recovery

This paper focuses on harvesting heat emitted by exhaust systems efficiently using thermoelectric generators (TEGs) and converting it to electricity. In a TEG that employs gaseous working fluid, up to 80% of the thermal resistance is due to the gas side. Maximizing the energy transferred from the hot exhaust gas to the hot side of the thermoelectric modules by suitable enhancement techniques can result in an efficiency gain for the TEG. To this end, we have investigated the performance of metal foam-based heat exchangers for reducing thermal resistance of the hot side in TEGs. A computational model of the metal foam-enhanced TEG, solving for the coupled thermal and electrical energy transfer processes, was developed to investigate the enhancement in system performance for a range of metal foam porosities and pore densities, and mass flow rates of the exhaust gas. Skutterudites with multiple cofillers were selected as thermoelectric materials. The primary performance metrics that were analyzed include the electrical power output and the associated pressure drop for various inlet conditions of the exhaust gas. Based on the trade-off between the increased pumping power required to offset the increase in pressure drop, and the gain in heat transfer coefficient with increase in mass flow rate of the exhaust gas, an optimal mass flow rate that maximizes the net electric power produced by the metal foam-enhanced TEG was obtained. The results show a critical exhaust flow rate for different pore densities of metal foam beyond which the net electric power produced by the TEG is less than of the TEG with no metal foam. At this critical flow rate, the maximum net electric power produced from exhaust waste heat by metal foam enhanced TEG is 5.7 (20 PPI) to 7.8 (5 PPI) times higher than that generated by the configuration without metal foam.