Energy harvesting is a concept that involves extracting energy from light, heat or motion. The German Institute for Physical Measurement Techniques, for example, uses thermoelectricity, whereby electricity is generated by a flow of heat – at the wrist, for instance.

It sounds like a modern outdoor enthusiast’s dream. While the coffee is brewing on the camping stove, the cellphone is recharging too. It could be done by placing a thermoelectric module between the flame and the kettle – a thermo- electric generator that is able to produce electricity from a thermal difference. The structural design has already been worked out and tested, though only in a Japanese laboratory so far.

To produce electricity this way, the Japanese made use of an effect that had been described even before the electric light bulb was invented. In 1821, Thomas J Seebeck made the observation that a compass needle would be deflected in the presence of two different, interconnected conductors when there was a temperature difference at the junctions. In his experiment, Seebeck converted thermal energy into electrical energy. Jean CA Peltier described the reverse procedure in 1834.

The US National Aeronautics and Space Administration uses the Seebeck effect for its space probes. In space, thermoelectric generators convert the decay heat of radio- active materials into electricity. On earth, heat can be a by-product of numerous processes, and usually ‘fades away’ unused into the atmosphere – though such energy would be first-class fodder for the “energy-producing domestic pig”, as Dr Harlad B�ttner, of the German Institute of Physical Measurement Techniques, in Freiburg, occasionally calls thermo- electricity with a wink.

Sus scrofa domestica, one of the order of ungulate mammals, is not choosy about its food. Nor is thermoelectricity. “It can eat just about anything, and make good use of almost everything, as long as it is warm,” B�ttner says.

Even human body heat can be harnessed to power a wrist watch, for instance.

The “energy-producing domestic pig” does not make optimum use of its fodder, however. This is due to the material, which needs to have a high Seebeck coefficient while acting as a good electrical conductor and a poor thermal conductor.

“It is not normally possible to optimise all those properties at the same time,” B�ttner remarks. For thermoelectricity to achieve a widespread breakthrough, the efficiency of the materials would have to be at least doubled.

Micropower engineering can achieve this with nanomaterials and special manufacturing pro-cesses. The key to success lies in composite materials that must not be more than a few micrometres thick – even though they are made up of between one and two thousand nanometre-scale layers. Such materials can already be produced in the laboratory, while developers are still working to scale up the processes to industrial level.

Scientists at the German Institute for Physical Measurement Techniques have discovered a promising approach. In collaboration with researchers at Infineon, they have developed micro-Peltier elements along with industrial-scale manufacturing processes. The elements can be used for the highly accurate cooling of microprocessors and for the rapid tempering of very small samples in a laboratory.

Manufacturing and sales have, meanwhile, been entrusted to an Infineon subsidiary, Micropelt, of Freiburg.

In addition to the cooling elements, thermogenerators – both small and large – will play a bigger part on the institute’s agenda in future microthermal energy generators for operating tiny self-sufficient sensors in sensor networks and, likewise, their big brothers, which, for instance, convert the waste heat from garbage incineration plants into electricity.

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