Tailpipe power

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Sep 4th 2008
From The Economist print edition

Thermoelectric devices: Generating electricity directly from wasted heat is becoming more practical, in both vehicles and buildings


THERE is something unusual about the exhaust pipe on the test car driven by Rainer Richter, leader of a research group at BMW, a German carmaker, in Munich. The vehicle is fitted with a device to capture some of the heat normally lost via the exhaust and convert it into electricity.

AP

Time to put that heat to good useCar engines waste a lot of heat. According to some estimates as much as 60% of the potential energy contained in petrol can be expelled or radiated as heat by an engine. Although some of this may be blown into the car to warm the occupants on a cold day, much of it is lost. This is why BMW has fitted a thermoelectric generator in the exhaust system of the test car.

The generator converts heat into electricity using the Seebeck effect, named after Thomas Johann Seebeck, a physicist who discovered in 1821 that applying a temperature difference across two adjoined metals gave rise to a small voltage. This gave rise to thermocouples—devices that are now widely used to measure temperature differences. But the effect can also 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.

But although thermogenerators are relatively simple and require little maintenance, they are not very productive. At the temperatures found in a car engine, they convert just 6-8% of heat into electricity, says Dr Richter. Nevertheless, he is convinced that new materials will make thermoelectric generators much more effective. And to be prepared, he has started developing systems that can be built into an exhaust without affecting the performance of the engine.

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The system on his test car contains a box where the hot gas is split up and fed through a series of pipes covered on the inside with lead-telluride, a thermoelectric semiconductor. The outsides of the pipes are cooled by some of the coolant circulated by the engine. It works best while driving at around 130kph (80mph), says Dr Richter. At this speed a gauge on the dashboard shows the generator is producing 150 watts—enough to light a few domestic light bulbs, but much less than the 600-700 watts of electrical power needed by a modern car.

With better thermoelectric materials that produced more electricity, however, it might become feasible to turn off a car’s alternator (the device, driven by the engine, that generates electricity) at certain times. Dr Richter thinks using thermoelectric generation in this way could reduce fuel consumption by around 5%. He hopes to see production cars using the technology by 2013.

The new materials that could make this possible are already starting to show up in research labs. For example, Joseph Heremans and his colleagues at Ohio State University recently completed a study in which they doubled the energy conversion of lead-telluride by doping it with thallium. The new material was particularly effective within the typical temperature range of a car engine.

It is not only cars that could benefit from such developments. Dr Heremans thinks there will be other applications for thermoelectric generators, anywhere heat is lost—such as in the chimneys of homes, offices or factories. They could also be used to generate electricity from stoves or open fires in the developing world, where high-performance cars are still a rarity.