Evaluation of metal foam based thermoelectric generators for automobile waste heat recovery K. Nithyanandam 1 , R.L. Mahajan ⇑ Department of Mechanical Engineering, Virginia Tech, Blacksburg, VA 24060, United States article info Article history: Received 20 November 2017 Received in revised form 6 February 2018 Accepted 7 February 2018 Keywords: Automotive waste heat recovery Thermoelectric generators Metal foams abstract This paper focuses on harvesting heat emitted by exhaust systems efficiently using thermoelectric gen- erators (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 effi- ciency 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 devel- oped 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 crit- ical 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. Ó 2018 Published by Elsevier Ltd. 1. Introduction According to a recent report [1], it is estimated that 20–50% of the energy input in transportation, power generation, industrial processes, residential sectors, and many other industries is wasted as heat. Transportation accounts for approximately one quarter of global energy use and is a major consumer of fossil fuels. Nearly two-thirds of the energy produced by a typical gasoline engine is lost through waste heat in the engine’s exhaust and coolant [2]. Harvesting the waste heat energy using a thermoelectric generator (TEG) can improve fuel economy by decreasing the electric gener- ator load on the engine. The attendant benefits are reduced green- house gas emissions and sustainable development. A TEG usually consists of four elements: a hot-side heat exchan- ger, a cold-side heat exchanger, thermoelectric modules (TEM) and a power-conversion system. Thermoelectric modules contain pairs of doped n- and p- type thermoelectric (TE) elements wired electri- cally in series and thermally in parallel [3]. Thermoelectric gener- ators work based on Seebeck effect and can convert a given temperature differential to voltage differential. Fig. 1 shows a sche- matic of the TEG design for automobile waste heat recovery. This design consists of thermoelectric modules sandwiched between the exhaust pipe (hot loop) and the coolant pipe (cold loop). The space constraint in installing thermoelectric generators in automobiles dictates that the TEGs provide excellent power density. The efficiency of a TEG in converting the waste heat into maximum electrical power output, g TE — which is achieved when the electrical load is matched with the TEG electrical resistance — is governed by the following expression [3]: g TE ¼ DT T h 1 2 þ 4=ZT h þ T c =T h ð1Þ where DT is the temperature difference between the hot and cold side of the thermoelectric couples, T h is the temperature of the https://doi.org/10.1016/j.ijheatmasstransfer.2018.02.029 0017-9310/Ó 2018 Published by Elsevier Ltd. ⇑ Corresponding author. E-mail address: mahajanr@vt.edu (R.L. Mahajan). 1 Presently with Axiom Exergy Inc. International Journal of Heat and Mass Transfer 122 (2018) 877–883 Contents lists available at ScienceDirect International Journal of Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ijhmt