Hot prospect

Converting residual heat from car exhaust fumes into electricity could cut fuel consumption and reduce carbon emissions. Siobhan Wagner reports.

Researchers in Germany are developing a thermoelectric generator to convert the heat from car exhaust fumes into electricity. The device will feed energy into the car’s electronic systems and could cut fuel consumption and help reduce the vehicle’s CO2 emissions.

A thermoelectric generator creates voltage when there is a different temperature either side of it. So the greater the temperature difference, the more current the generator can produce.

A team from the Fraunhofer Institute for Physical Measurement Techniques is developing new thermoelectric materials, modules and systems to harness this kind of residual heat.

‘The temperature in the exhaust pipe can reach 700ºC or more,’ said Harald Böttner, head of the Thermoelectric Systems department. ‘The temperature difference between the exhaust and a pipe carrying engine cooling fluid can thus be several hundred degrees,’ he added.

The thermoelectric converter will make use of this huge differential. With the flow of heat from the exhaust fumes on one side and the cold side of a coolant pipe on the other, the charge carriers will pass through special semiconductors and produce a current similar to a battery.

Böttner said thermoelectric effects were first recorded in the 1800s, but thermoelectric materials have not significantly improved in efficiency since the 1950s. ‘During the last decade there have been many efforts to increase the quality of materials, using new developments in nanotechnology and additional knowledge about the physics behind thermoelectric material,’ he said.

Inadequate efficiency was one of the key factors preventing such materials from being used in cars, but new developments mean this is no longer the case.

Earlier this year, for example, researchers at Boston College and MIT in the US demonstrated a boost in the efficiency of bismuth antimony telluride — a semiconductor alloy commonly used for thermoelectric cooling. By breaking the bulk material into tiny chunks — from five to 50nm across — the researchers increased a key measure of thermoelectric conversion, called the ZT of the alloy, from one to 1.4.

Böttner said the thermoelectric generator technology will not be ready commercially for another five years. But with many people concerned about rising fuel costs and CO2 emissions, he added, it cannot come fast enough.

Böttner said most drivers do not realise that two-thirds of the fuel consumed in their car is emitted, unused, as heat. Approximately 30 per cent is lost through the engine block and a further 30 to 35 per cent as exhaust fumes. Any way to harness the unused waste heat could significantly lower the car’s fuel consumption. The technology could also be applied to other machines and even power stations.

The long-term objective for Böttner and his team is to reduce dependence on alternators — the generators currently used to provide electricity in cars — and to use thermoelectricity to supply energy to the ever-rising amount of electronic gadgetry.

Thermoelectric generators could step in and cover a significant proportion of these requirements, but in no way replace alternators entirely.

‘It would not be possible to get rid of the alternator,’ said Böttner. ‘A standard car consumes about 5kW of electrical energy while driving, while a small car will consume roughly about 1kW. We hope to produce a generator that could produce about 1kW.’

The most important feature of thermoelectric generator technology, however, could be the fuel savings. ‘This could make it possible to cut fuel consumption by between five and seven per cent,’ claimed Böttner.