Thermal switching to improve time-averaged efficiency of thermoelectric energy harvesting

Recent improvements in thermoelectric (TE) materials have expanded the potential to use this technology for the generation of electricity from waste heat in a variety of applications. The performance of a TE generator improves when the temperature difference across the generator is maximized. A "thermal switch," located between the heat source and the TE device, is posed to modulate the heat flow through the TE device permitting heat to flow from the source to the TE device only when the source temperature is near maximum, yielding a higher time-averaged temperature drop across the TE, and therefore a higher efficiency.;A numerical model is used to evaluate the benefits of a thermal switch in series with a TE generator for both time-varying and constant heat inputs. The results demonstrate that modulating the heat flow through the TE device and maintaining the source temperature near a constant maximum value improves the time-averaged efficiency of the TE device. For ideal conditions, improved efficiencies of more than five times are realized, though for more realistic conditions, efficiencies are improved up to 35%.;Using an experimental set-up with a variable thermal resistance air gap serving as a thermal switch, the time-averaged power output to power input ratio improved up to 15% and 30% respectively for constant and variable heat input in certain design space conditions. The experimental results, as supported by model predictions, suggest that the thermal capacitance of the heat source must be greater than the thermal capacitance of the TE device in order for thermal switching to improve the time-averaged power output to power input ratios of waste heat recovery systems.;A micro heat pipe design for an integrated thermal switch is proposed based on a steady-state model of the energy harvesting system. After fabrication, the micro heat pipe is characterized for thermal resistance performance as a function of source temperature. After testing two fluids at various fill ratios, the micro heat pipe conceptually performs as a thermal switch for both constant and variable heat sources.