A fine load of hot air
THERE is something unusual about the exhaust pipe on the test car driven by Rainer Richter, leader of a research group at BMW in Munich. The vehicle is fitted with a device to capture some of the heat normally lost through an exhaust and convert it into electricity.
Car engines waste a lot of heat. According to some estimates around 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 in 1821 discovered that a voltage could be created in the presence of a temperature difference on a conductor. It can be achieved with the coupling of two metals—one that gets hot while the other stays cooler. This gave rise to thermocouples, devices now widely used to measure temperature differences.
Using the effect as a generator goes back a long time: in the 1950s 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 Voyager 1 and 2. Some plutonium-powered thermogenerators have also been used to run equipment like lighthouses and radio beacons in isolated areas.
The problem with thermogenerators (even if they are not powered by plutonium) is that although they can be relatively simple and require little maintenance, they are not very productive. At the temperatures produced by 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.
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 outside of the pipes is cooled by some of the coolant circulated by the engine. It works best while driving between 75 to 80 miles per hour, says Dr Richter. At 80mph a gauge on the dashboard shows the generator is producing 150 watts—enough to light a domestic bulb, but not to supply all the electrical power a modern car needs. That requires about 600-700 watts, adds Dr Richter.
With new thermoelectric materials more electricity could be produced, which would then make it more feasible to start turning off the alternator on a car at certain times. Alternators take power from the engine when they produce electricity. Dr Richter thinks it will be possible to reduce fuel consumption by around 5% using a thermoelectric generator in the exhaust. He hopes to see production cars using the technology by 2013.
The new materials that could make that a reality 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 altering its properties by doping it with thallium. The new combination was particularly effective at the typical temperature range of a car engine.
Nor is it 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 or factories. They might also generate electricity from open fires in undeveloped parts of the world where cars of any sort are a rarity.