Exploiting waste heat with thermoelectrics

Waste heat recovery

Exploiting waste heat with thermoelectrics

Thermoelectric technology lends itself to the utilization of waste heat by converting heat directly into electrical energy. Although waste heat is generated in all areas of daily life – e.g. in industrial processes, private households, and in traffic – it forms an energy resource that has been largely untapped up to now.

During combustion processes more than 60 percent of the chemical energy employed is lost, primarily as waste heat. This accounts for automobiles as one prominent example, where merely one third of the chemically stored energy is utilized for propulsion. In industrial applications such as incinerators, melting furnaces (metal and glass industries) and in drying plants (e.g. cement production) large quantities of waste heat remain unused.

Efficient materials for »Energy Harvesting«

Researchers around the world, and at Fraunhofer IPM in particular, are working flat out on the prerequisites for thermoelectric energy harvesting, namely the conversion of waste heat from combustion processes into electricity. These efforts primarily concern efficient TE materials and optimized production processes.

The actual carriers for the conversion are various semiconductor materials, some of which are well known and can be processed industrially. Connecting two different thermoelectrically active materials and producing a temperature difference at the junction points creates an electric field. Conversely, applying electricity at the junction points results in a temperature gradient. The amount of electricity produced and the size of the cooling effect depend essentially on the quality of the thermoelectric materials. The ZT value, which describes the efficiency of thermoelectric materials, has stagnated at ZT 1 for decades – which is too low for waste heat recovery. Thanks to new classes of materials, Fraunhofer IPM is now achieving laboratory values that are significantly higher. Values of 1.5 to 2 are regarded as the profitability threshold for the cost-effective exploitation of thermoelectric generators. The extent to which TEGs can be used in the future for waste heat recovery will succeed or fail according to the material quality.

Durable, maintenance-free generators

As thermoelectric generators function without any moving parts, they are durable and maintenance free. In addition, they are easily scalable, making it possible to use waste heat from small, middle and even large sources.

With its extensive scientific expertise, Fraunhofer IPM is at the cutting-edge of the field of thermoelectric measurement technology. Our measuring services are also available to customers. New developments include a hot gas test stand which is used to test thermoelectric generators at temperatures of up to 700°C, and which can also be cycled extremely quickly. This is an essential prerequisite for testing both modules and system prototypes under realistic circumstances before they are fitted into equipment. The measurement results can additionally be used to test or adjust the values used in simulations.