200 watts of power output through exhaust heat

The waste heat from car exhausts currently escapes alternator car battery. “Hidden reserves lie here. We could use the waste heat and convert it into electrical power,” explains Dr. Wolf Eckhard Müller, Head of Department at the Institute of Materials Research at the German Aerospace Center. Because a temperature difference between two different metals or semiconductor materials can generate a voltage via the socalled Seebeck effect. Such thermoelectric generators, called TEGs in short, generate power in space probes, for example, from the decay heat of a radioactive isotope.

However, the technology used there is expensive and unsuitable for road traffic. “Rather, we are interested in taking this technology out of space and bringing it back to Earth. To achieve this, we are working on affordable and safe thermoelectric generators that use the waste heat that is produced anyway,” says Müller. The first step has already been completed. Working in cooperation with DLR experts, car maker BMW Group has already equipped a test vehicle with a TEG that can withstand up to around 200 watts of power for the on-board electrics from the waste heat. This TEG, which was directly integrated into the exhaust gas system, contains the compound bismuth telluride, which can generate temperatures of around 250° Celsius.

However, most of the waste heat from engines and energy plants – whether in traffic or in industry – arrives at much higher temperatures of 400° to 500° Celsius. This is why the DLR materials experts are already researching completely new material classes, such as chalcogenides, skutterides, silicides and clathrates, mostly in the form of solid solutions and nanomaterials. In the lab, they have already set their sights on later mass production and are also working on new manufacturing and bonding methods. “We expect to achieve fuel savings of up to 5 per cent with such improved thermoelectric generators,” explains Müller.