Technology

Thermoelectrics

Thermoelectrics: an energy source with a future

The waste heat that was emitted into the surroundings can now be used as an energy source to power thermoelectric generators.

Thermoelectric effects were already observed in the early nineteenth century by Johann Seebeck and Jean Peltier, but it took until the 1950s for them to be well understood. Since then efforts have been underway to develop practice-oriented applications. Thermoelectric coolers have now been in use for quite some time; thermoelectric generators, however, have so far only been successful in very particular niche markets. But thanks to the latest developments in nanotechnology they can now be produced cheaper and more effectively. Thermoelectric generators and coolers are based on the Seebeck and Peltier effects. The Seebeck effect explains the relation between voltage and temperature difference: A difference in temperature generates electric voltage. The Peltier effect comes into play at the junction of two materials: It allows converting heat into current and vice versa when voltage drives current through a circuit.

Different materials have different electronic properties. For thermoelectric applications, carriers' energetic levels are decisive. At the junction of two materials, charge carriers of different energetic levels meet. Charge carriers (electrons) migrating to the other material have to either absorb energy to reach a higher energy level or release energy to reach a lower level. This energy is thermal energy, which means that on one side of the thermoelectric generator or cooler thermal energy is absorbed and the temperature there falls, whereas on the other side thermal energy is released and the temperature on that side rises.

Thermoelectric coolers need a supply of external voltage to drive a current through the device. Thermoelectric generators supply their own current through the Seebeck voltage when there is a temperature difference.

Efficiency

Unfortunately not all heat is directly converted into current, since we are dealing with thermodynamic systems and heat conduction causes heat to flow from the hot to the cold side, without being used for any purpose.

From this follows that the materials used have to be good electrical but poor heat conductors. Nature offers such materials, for example Bi2Te3. To achieve better efficiency, the material and the generator component properties have to be improved.