Transient Analysis and Optimization of Thermoelectric Generators for Waste Heat Recovery From Automotive Engine Exhaust

Abstract The overall efficiency of automobiles can be enhanced by recovering some of the waste heat from the engine, where more than 30% is lost from exhaust. Thermoelectric generators (TEGs) have gained a lot of popularity for this purpose because of their reliability and scalability. In this study, a three-dimensional model of a TEG assembly was built, and its performance was simulated using computational fluid dynamics. The focus has been on carrying out the simulations in the transient mode. Experimentally measured exhaust data from a vehicle running on a standard Indian drive cycle have been used as the boundary condition for the simulations. The TEG assembly was designed to make use of the flowing air onto the radiator to cool the cold side of the TEG. Initially, the model was validated under pseudo-transient conditions using experimental tests carried out under engine operating conditions. The effect of transience on the performance was evaluated and compared to the results based on steady-state simulations. Later, various geometric parameters of the TEG assembly were varied to optimize the overall performance of the assembly. The results showed that the steady-state analysis under-predicts the performance of the TEG assembly. The nonlinear temperature dependence of the TEGs and manifold walls acting as a heat storage medium leads to the enhanced performance of the TEG assembly under transient conditions. The optimization studies established a method to design the TEG assembly to maximize waste-heat recovery without creating excessive back pressure on the engine. The results have shown that up to 355 kJ of energy with an overall efficiency of 9.9% can be recovered from the exhaust heat. The optimization studies have shown that adding more TEGs in-line increases the power generated, but also increases the back pressure on the engine. Increasing the number of parallel paths for the exhaust increases the power generated and limits the back pressure on the engine to a substantially lower value. The wall thickness of the manifold plays an important role in the performance of the TEG assembly operating in a transient mode.