You might have heard of the Seebeck effect. It's named after the German physicist, Thomas Johann Seebeck, who in the 1820s found that a circuit made from two dissimilar metals with junctions at different temperatures would deflect a compass magnet (due to an electrical current induced in the circuit). So how's that useful? Converting waste heat into electricity, of course. Researchers from KTH Royal Institute of Technology are working with the automotive manufacturer Scania to test their semi-trucks equipped with a system that converts exhaust heat into power through thermoelectric generation (TEG). Researcher Arash Risseh says the voltage produced by the system can power the truck and reduce the strain on the engine. "Most fuel energy is not used to drive a truck forward," Risseh said. "Some 30 percent of this unused energy is lost as heat from the exhaust pipe. A truck that generates 440kW would see about 132kW of energy disappear in the form of heat coming out of the exhaust pipes, according to Risseh, which is enough to power a typical passenger vehicle. Capturing excess energy takes a load off the truck's generator, and in turn, the engine, which means better fuel efficiency and lower emissions.

Not just trucks
The Seebeck effect requires a temperature differential--cool on one end of the circuit and hot on the other, which means a truck must rely on a coolant in order to stimulate the voltage. Other applications, like data centres, might lend themselves more easily to TEG. Near the Arctic circle in northern Sweden, a data centre that uses 1TW-hour per year could potentially recover 1GW-hour per year, which in that country would translate to saving of about €100,000 (\$113,570), according to Risseh. TEG can even be used in ships since seawater can be used to cool the circuit to deliver a temperature differential against a ship's engines. Moreover, the constraints of weight and volume are much more relaxed by buoyancy in this application than in automobiles. For those curious about the Seebeck effect, here's the equation that governs the conversion of temperature differential to electricity.

V = a(Th - Tc)

where:

V is the voltage difference generated between two dissimilar metals a is the Seebeck coefficient Th - Tc is the temperature difference between the hot and cold junctions