Waste Heat Recovery through Thermoelectrics
Along with high efficiency engine technologies and emission control, the Vehicle Technologies Office (VTO) is supporting research and development to increase vehicle fuel economy by recovering energy from engine waste heat. In current gasoline vehicles, only about 25 percent of the fuel's energy is used to drive the wheels; in contrast, more than 70 percent is lost as waste heat in the exhaust gases and to engine coolant. For diesel engine powered vehicles, about 64 percent of the fuel energy is lost to waste heat. Recovering energy from the engine exhaust could improve the overall vehicle fuel economy by more than 5 percent. To make use of this wasted energy, VTO is supporting research to develop thermoelectric generators (TEGs) that can directly convert energy from the hot engine exhaust into electricity that can power vehicle auxiliary loads and accessories.
Thermoelectric generators are comprised of an array of thermoelectric couples (pairs of n-type and p-type semiconductor elements connected electrically in series and thermally in parallel). When placed across a temperature gradient (between a heat source and a heat sink), these couples produce a voltage that drives a current through an electrical circuit. This set-up generates electricity without releasing additional carbon dioxide (CO2) or other harmful emissions into the atmosphere.
Current conventional vehicles power auxiliary loads and accessories with engine-driven alternator/ generators that decrease fuel economy. As electrical demand increases for accessories, such as communication systems, navigation systems, and stability controls, it increases the drag on the engine. This increased demand for electric power provides a perfect opportunity to improve efficiency through TEGs that produce electricity directly from engine waste heat.
In the future, thermoelectric units may also be used as air conditioners and heaters. Rather than converting waste heat to electricity, this application uses direct current electricity to move or "pump" heat from the cold to the hot junctions of the thermoelectric couple array. Whether the unit is heating or cooling an area depends on what direction the current is flowing.
When used for heating, thermoelectric units are almost three times more efficient than typical electrical resistance heaters. This application is particularly relevant for plug-in electric vehicles because they have little or no exhaust heat to warm occupants. Unlike current versions that draw on the battery to power electric resistance heaters, plug-in electric vehicles with thermoelectric heaters could have longer battery ranges in cold weather.
When used for air conditioning, these units can cool only the occupants instead of the entire cabin, allowing them to use less than a fifth of the power of a conventional air conditioner. In addition, air conditioning based on thermoelectric units would eliminate the use of the refrigerant gas R134a, which has 1,300 times the climate warming potential of carbon dioxide (CO2), the primary greenhouse gas.
VTO's Solid State Energy Conversion research focuses on developing system level vehicle applications for thermoelectric devices, adapting and improving their design for the vehicle architecture, and supporting their transfer to mass production. Currently, the research focuses on using bulk thermoelectric materials that are available and have the best efficiency, as well as integrating TEGs into vehicle systems. As higher efficiency thermoelectric materials are developed, VTO will assess these new materials' use in vehicle applications.
VTO's research also seeks to overcome a number of technical barriers, including:
- Lack of availability of cost-competitive automotive thermoelectric materials
- Improving manufacturing technology that can reduce cost and produce more than 100,000 units annually
- Scaling up materials developed in the laboratory
- Integrating TEGs with other vehicle components
- Maintaining electrical contact
- Developing materials that optimize electrical conductivity, but minimize thermal conductivity
Research supported by VTO is helping address these technical barriers and should enable TEGs to make significant contributions in transportation beginning later this decade. In fact, VTO has a goal to develop applications of advanced thermoelectric devices that will improve overall vehicle fuel economy by at least 7.5 percent by 2020. As research improves both the efficiency and cost of these devices, and fuel economy standards increase, manufacturers are likely to rapidly adopt thermoelectric generators over the next few decades.