Enhancement of automotive exhaust heat recovery by thermoelectric devices

Abstract

In an effort to improve automobile fuel economy, an experimental study was undertaken to explore the practical aspects of implementing thermoelectric devices for exhaust gas energy recovery. An experimental apparatus consisting of a hot-side (exhaust gas) rectangular duct and a cold-side (coolant liquid) rectangular duct enclosing six thermoelectric elements has been built and instrumented. Measurements of the thermoelectric voltage output, fluid and surface temperatures, and flowrates were acquired and analysed to investigate the power generation and heat transfer properties of the apparatus. The effects of inserting aluminium wool packing material inside the hot-side duct on augmentation of the heat transfer from the gas stream to duct walls were studied. Data were collected for both the unpacked and the packed cases to allow for deduction of the packing influence on the flow and surface temperatures. The effects of variations in the gas inlet temperature (300°C, 320°C, 340°C, 360°C, 380°C, and 400°C), coolant inlet temperature (40°C, 50°C, 70°C, and 90°C), and gas flowrate (40sl/min, 60sl/min, and 80sl/min) on the thermoelectric power output were examined. The results indicate that thermoelectric power production is increased at higher gas inlet temperatures or flowrates at a fixed coolant temperature. However, thermoelectric power generation decreases with a higher coolant temperature as a consequence of the reduced temperature differential between the hot side and the cold side. For the unpacked hot-side duct, a large temperature difference of up to 140°C existed between the gas and solid surface temperatures owing to poor heat transfer through the gaseous medium. Adding the packing material inside the exhaust duct enhanced heat transfer and hence raised the hot-side duct surface temperature by as much as 30°C and thermoelectric power by up to twofold, compared with the unpacked duct, particularly where the gas-to-surface temperature differential is highest. Therefore, it is recommended that packing of the exhaust duct becomes common practice in thermoelectric waste-energy-harvesting applications.