Feasibility of Thermoelectrics for Waste Heat Recovery in Conventional Vehicles

Abstract

Executive SummaryThermoelectric (TE) generators convert heat directly into electricity when a temperature gradient is applied across the junctions of two dissimilar metals. These devices have the potential to increase the fuel economy of conventional vehicles by recapturing a portion of the waste heat from the engine exhaust and generating electricity to power a vehicle's accessory loads.At present, device efficiencies are low (~5%); however, thin-film and quantum well technologies offer the possibility of higher efficiency in the future (~10 % to 15%). Four vehicle platforms are considered: a midsize car, a midsize sport utility vehicle, a Class 4 truck, and a Class 8 truck. A simple vehicle and engine waste heat model shows that the Class 8 truck presents the least challenging requirements for TE system efficiency, mass, and cost. This is because Class 8 trucks have a relatively large amount of exhaust waste heat, have low mass sensitivity, and travel a high number of miles per year, all of which help to maximize fuel savings and economic benefits.A driving and duty cycle analysis for the Class 8 truck elucidates trade-offs in system sizing and shows the strong sensitivity of waste heat, and thus TE system electrical output, to vehicle speed and driving cycle. It is not feasible for a TE system to replace the alternator, as too little waste heat is available during city driving and/or idling.Together with a typical alternator, a TE system could enable the electrification of 8% to 15% of a Class 8 truck's accessories, providing 2% to 3% fuel savings. Additional electrification would require a larger alternator and battery to augment the TE system so that adequate electrical power is available during low-speed driving and idling. Achieving an economic payback in three years dictates that the TE system cost less than roughly $450/kW, requiring an almost tenfold reduction from today's costs. Such a cost reduction might be enabled in the future by thin-film devices that use expensive TE junction materials more efficiently.

Thermal Management as a Force Multiplier within the Research, Development, and Engineering Command (RDECOM)

“…Similarly, inefficiencies in a turbine engine for an Abrams tank means that exhaust gasses can contain megawatts of thermal energy. Thus, the auto industry (5), Department of Energy (6)(7)(8), TARDEC (9), and others have all investigated waste-heat recovery as a way to improve overall system efficiency. Depending on the vehicle type and driving conditions, fuel mileage improvements on the order of 5% during highway cruising are expected (8, 10) Army vehicle modernization programs, which desire more onboard electrical generation, could potentially meet their targets by capturing and converting waste-heat energy to avoid increased fuel usage.…”

Integrated Vehicle Thermal Management for Advanced Vehicle Propulsion Technologies

“…Significant effort through the years has also focused on recovering waste heat from exhaust and engine coolant to provide additional power using various waste heat recovery techniques. Examples include Rankine cycle heat recovery, turbocompounding, thermoelectric devices, and thermoacoustic waste heat recovery [2,[10][11][12][13][14]. Waste heat utilization also includes storing waste heat for later use.…”

Enhancement of Thermoelectric Device Performance Through Integrated Flow Channels

“…Thus, the carbon foot-print and waste heat released into the atmosphere are proliferated from both energy consuming and producing equipment. Based on the DOE report by Smith and Thornton (2009), it is approximated that two-thirds of the supplied input energy to these systems is rejected as a waste heat to the atmosphere. Scavenging even a small percentage of waste energy (5-10%) can reduce negative environmental impacts and make current equipment more economical.…”