Electricity from waste heat

Heat is also released in many industrial processes and in combined heat and power plants. However, thanks to the so-called Seebeck effect, this heat can potentially be utilised. If the contact points between two different electrical conductors or semiconductors are kept at different temperatures, an electric voltage is produced that increases with the temperature differential. The problem is that thermoelectric generators (TEGs) are currently only able to convert a small amount of the waste heat into electricity – usually less than 10 percent. In order to improve this level of efficiency, we need materials that conduct electric current well and heat poorly. The difficulty is that good electrical conductors tend to be good heat conductors as both properties are influenced by the mobility of electrons. “However, thermal conductivity is not determined by electrical charge carriers alone; it can also be influenced by lattice vibrations. By reducing these vibrations, we can create more efficient thermoelectric materials,” says Dr. Raphaël Hermann. Together with his team at the Jülich Centre for Neutron Science, he uses neutron scattering and synchrotron radiation to investigate the mechanisms of thermal conductivity. The energy quanta of lattice vibrations are called phonons. Their speed, free path lengths and heat-storing capacity determine thermal conductivity. “It’s like the transport of goods by road: if a truck is faster, spends less time in traffic jams and has a larger cargo space, more goods will be transported,” says Hermann. Examining thermoelectric materials such as Zintl phases, skutterudites and clathrates, the researchers identified the factors that play a key role in limiting the thermal conductivity of the phonons. In this way, the scientists have gained insight into the parameters that can be tweaked in order to further increase the efficiency of these components.

Forschungszentrum Jülich/Red.