Higher efficiency thermoelectric devices have the potential to make more than a little electricity from waste heat

In a prior post, it was noted that work has progressed on the use of infra-red energy to generate electricity, and that the major stumbling block at present is the lack of an appropriate rectifier.  One hypothesized application was the placement of such a device next to a waste heat source to generate power.  One approach that is already in the practical test phase is the use of thermoelectric devices [see http://en.wikipedia.org/wiki/Thermoelectric_effect] to generate electricity.  One area in which such is being tried currently is in automobiles.

Car engines waste a lot of heat.  According to some estimates as much as 60% of the potential energy contained in gasolinel is expelled or radiated as heat by an engine.  Although some of this may be used to heat the interior of a car on a cold day, much of it is lost.  To try and harvest some of this waste heat BMW has fitted a thermoelectric generator in the exhaust system of the test car.  [See http://www.economist.com/science/tq/displaystory.cfm?story_id=11999209.]  The generator converts heat into electricity using the Seebeck effect [see http://en.wikipedia.org/wiki/Seebeck_effect#Seebeck_effect], which is the direct conversion of temperature differences into electricity.  The effect can be used as a generator, an idea that goes back to the 1950s, when thermogenerators were placed over kerosene lamps to power radios in remote areas that had no electricity.  More recently, thermogenerators that obtain their heat from the radioactive decay of plutonium have been used in spacecraft, such as the Voyager probes.  Plutonium-powered thermogenerators have also been used to power lighthouses and radio beacons in isolated areas.

The good news.  Thermogenerators are relatively simple and require little maintenance.  The bad news.  They are not very productive.  At the temperatures found in a car engine, they convert just 6-8% of heat into electricity.  More research needs to be done.  However, the strategy, like that of infra-red "solar cells", has the potential for widespread application, including in the "third world".  Thermogenerators could produce incidental electricity from stoves and even open fires.  For more information on the potential of thermoelectric devices, see http://spectrum.mit.edu/issue/2007-spring/energy-saver/.