Energy retrieval: waste heat converted into electricity

The natural resources for our energy supply are becoming scarce and the costs to exploit them are increasing. The energy industry must thus work ever more efficiently. Energy retrieval thereby plays an important role, because large amounts of energy are lost in the form of waste heat when technical devices and facilities are operated, whereas this thermal waste energy could be used secondarily.

For this reason, thermoelectric materials are probably going to gain in importance in the future. With these materials, the effectiveness of the energy conversion depends on their transport properties, i.e. on the Seebeck coefficient S (a temperature- dependent material parameter) as well as on the electric conductivity σ and on the thermal conductivity κ, which are summarized in the thermoelectric figure of merit S²σT/κ. The exact determination of the figure of merit is the pre-condition required to assess and compare newly developed thermoelectric materials. To this end, measurements of the transport properties that are traceable to the SI units are indispensable. With the aid of the measuring system which is installed at PTB, the Seebeck coefficients and the electric conductivities of thermoelectric bulk materials and of thin layers can be measured with low measurement uncertainties.

The Seebeck coefficient describes the voltage drop caused by a temperature gradient over a sample and decisively determines the figure of merit of thermoelectric materials. Within the scope of the EMRP project “Metrology for Energy Harvesting”, reference materials have been metrologically investigated and characterized for the first time – at PTB and worldwide – with regard to their Seebeck coefficients in the higher temperature range between 300 K and 900 K. This temperature range is important for applications, e.g., in the automotive area. The measurement uncertainties attained for the Seebeck coefficients of the two reference materials ISOTAN^® and bismuthdoped lead telluride lie between 2.5 % and 8 %, depending on the material and on the temperature. Both materials can be obtained from PTB.