Maximizing Energy Recovery in Thermoelectric Generator Waste Heat Recovery Systems for Automotive Applications

Abstract

Thermoelectric generators (TEGs) are solid state devices with the ability to convert heat directly to electrical energy. According to the Seebeck effect, if the junctions of a thermoelectric couple are held at a temperature difference, an electromotive force is induced which translates to power generation when an electrical load is connected. Due to their direct energy conversion, small volume, no use of working fluids, and reliability, thermoelectric generators have been investigated for various waste heat recovery applications. Additionally, TEGs can operate in transient environments without drawbacks such as turbo-lag (experienced by other waste heat recovery technologies), which has made TEGs an exceptional candidate for recovery in dynamic waste heat sources such as the exhaust system of a vehicle. Greenhouse gas (GHG) emissions generated by the transportation sector account for 29% of global emissions, therefore exploring solutions to mitigate this problem is of most importance. Since light duty vehicles and freight trucks make up 78% of the transportation sector, it is crucial to investigate methods by which the efficiency of vehicles can be increased, thereby reducing losses. Although vehicle electrification will make an impact in reducing emissions, it is expected that over 40% of the future vehicle fleet will continue to use an internal combustion engine (ICE). Approximately 40-60% of the total fuel energy is lost through the exhaust system of an ICE vehicle, therefore, waste heat recovery methods should be investigated, with TEGs having been proven to be a viable technology. Waste heat recovery (WHR) through the use of TEGs in automotive applications has been investigated since the late 90’s. However, the focus of the research has been on optimizing the TEG module for fixed temperature differences to maximize the power output of the module. This steady-state optimization approach has been applied when investigating the power output from a dynamic waste heat environment such as the vehicle exhaust system. When comparing the performance of TEG WHR systems in vehicles, the current metric is maximum power output. TEG WHR systems both modeled and experimentally tested, have been evaluated for the most part at maximum engine load which is not indicative of real driving scenarios. The few studies which have tested the system during real operating conditions such as a vehicle drive cycle, have experimentally achieved lower power than that produced at steady-state. To further investigate the cause of this degradation in power during transient operation, this thesis developed a transient TEG WHR system model that considers heat capacity in all components. The model was validated at both steady-state and transient operating conditions. The transient model, coupled with a vehicle model that can predict the exhaust temperatures and mass flow rates for various drive cycles, was used to investigate the total energy produced by the system. By investigating the effect that system size has on power generation, it was found that an optimal system size exists since the additional TEG modules in the system degrade the power performance due to heat transfer through the heat exchanger bases. Therefore, the maximum power point does not coincide with the maximum energy design point. In addition to the development of a TEG WHR system design methodology through the use of the developed models, a maximum power point tracking (MPPT) method was proposed that can achieve a tracking efficiency of 98%, validated through transient experiments. The proposed method can be implemented in current MPPT configurations without the need of additional sensors.