Researchers from the KTH Royal Institute of Technology, working with automotive manufacturer Scania, have tested semi-trucks equipped with a system that converts exhaust heat into power through a process called thermoelectric generation (TEG). The voltage produced by the system can help power the truck and reduce the strain on the engine.

"Most fuel energy is not used to drive a truck forward," says KTH researcher Arash Risseh. "Some 30% of this unused energy is lost as heat from the exhaust pipes."

A truck that generates 440kW would see about 132kW of energy disappear in the form of heat coming out of the exhaust pipes, Risseh says. "That's enough to power a typical passenger vehicle."

A truck that generates 440kW would see about 132kW of energy disappear in the form of heat coming out of the exhaust pipes. Image credit: KTH Royal Institute of Technology.Capturing this excess energy takes a load off the truck's generator and, in turn, the engine, Risseh notes. According to KTH, the system could save vehicles hundreds of liters of fuel and reduce their carbon emissions by as much as 2 to 3 metric tons per year.

The TEG system operates on the principle of the thermoelectric effect, by which differences in temperature are converted into voltage—a phenomenon discovered in 1821 by German physicist Thomas Johann Seebeck and often referred to as the "Seebeck effect." 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.

Cooling the circuit is easier with natural alternatives, such as seawater for a ship's engines. Ships also make good candidates for TEG because their buoyancy offsets the constraints of weight and volume that road vehicles face, Risseh says.

According to KTH, TEG is also regarded as a potential energy saver in data centers that are located in cold climates. Near the Arctic Circle in northern Sweden, a data center that uses 1 terawatt hour per year could potentially recover 1 gigawatt per year—a savings of some €100,000, according to Risseh.