Open-Cell Metal Foams as Heat Exchanger Core Design for Thermoelectric Generators: A Proof of Principle

Abstract

At the automotive development a major challenge is the achievement of the required reduction of CO2 emissions and the reduction of the fuel consumption for internal combustion engines (ICE). Therefore it is necessary to investigate all potential technologies to improve efficiency. Regardless of whether a passenger car or heavy duty truck concept with a conventional ICE is considered, approximately 2/3 of the fuels chemical energy is lost as waste heat. Specifically, the waste heat of the exhaust gas provides through its high temperature level the highest potential as secondary energy source. Therefore the technology of thermoelectric generators (TEG) is very promising for automotive application and under investigation for several years in the research field of alternative energy converter at the DLR Institute of Vehicle Concepts (DLR-FK). From the actual investigations for automotive application of TEG, an alternative heat exchanger design will be presented and evaluated. Metal foams have been under fundamental research but in heat transfer applications the practical feasibility and the correct implementation has not yet been concluded. Open-cell metal foams as heat exchanger structures provide a good heat conductivity and heat transfer by relatively low pressure drop. The main advantage for the TEG device is its comparatively low density by a high heat transfer surface per foam volume. This combination can offer a high TEG efficiency and help reducing the weight of the TEG device. Particularly when high exhaust gas enthalpies are available like in truck applications, large TEG systems could be integrated to increase the vehicle efficiency. The additional weight of the state of the art TEGs for truck application minimizes its benefits. The proposed open-cell metal foams as heat exchanger structure can be a solution for this problem. In this work the feasibility of open-cell metal foams as core of heat exchangers for TEG devices will be studied. By considering typical exhaust gas enthalpies of vehicle applications metal foam heat exchangers are simulated using a CFD-Simulation and evaluated by experiments on a hot gas test bench. The results will be discussed and compared to a conventional compact heat exchanger for TEGs with fin design. In different experiments and configuration the thermal, mechanical and thermoelectrically utilization will be investigated. The usage as TEG in a research functional mockup will be presented and an approach for further optimization in the future will be given.