February 9, 2009 Thermoelectrics - the phenomena in which a temperature difference creates an electric potential - have been known about for almost 200 years, but practical applications have not been widespread due to their low energy efficiency. That may all now be about to change as Germany automakers Volkswagen and BMW have developed thermoelectric generators (TEG) that recover waste heat from a combustion engine.
According to a report by Prof. Rowe of the University of Wales in the International Thermoelectric Society, Volkswagen claims 600W output from the TEG under highway driving condition. The TEG-produced electricity meets around 30% of the car’s electrical requirements, resulting in a reduced mechanical load (alternator) and a reduction in fuel consumption of more than 5%.
BMW and DLR (German Aerospace) have also developed an exhaust powered thermoelectric generator that achieves 200 W maximum and has been used successfully for more than 12,000-km road use.
Thermoelectric have been used for refrigeration utilizing the Peltier effect originally discovered in 1834. An electrical current at the junction of two different metals results in heat being absorbed by one metal and expelled by the other metal. Thermoelectrics can also be used to generate electricity using the Seebeck effect that dates back to 1770. Thermoelectric power generators convert heat energy to electricity. When a temperature gradient is created across the thermoelectric device, a DC voltage develops across the terminals.
Typical applications for this technology include providing power for remote telecommunications and navigation beacons. A more familiar application is a thermocouple that is a type of temperature sensor that can generate a current proportional to the amount of heat it is exposed to. Thermocouples were used in remote parts of Russian in the 1920s to power radios from a wood fireplace and they also form the basis of radioisotope thermoelectric generators (RTG) that use heat from a radioactive material to power deep space satellites. The drawback to all thermocouple based electric generation is that they are very inefficient at between 3-7%.
Automotive thermoelectric generators (ATEG) have been developed intermittently since 1988 when Porsche made a exhaust ATEG capable of 20-30 watts out of a 944 exhaust system but they have never made it past the prototype stage of development.