Electricity drawn from body heat

Do you dream of a future where your phone battery never goes flat? You may not have much longer to wait now that researchers have created new circuits that are making it possible to harness body heat for generating electricity.

According to the research, when your phone battery has run out of charge you just hold it in your hand (or against your skin) and you can recharge it.

It works on the principle of thermoelectric generators (TEG) made from semiconductor elements. The TEGs extract electrical energy from the temperature difference between a hot and a cold environment.

A difference of several tens of degrees is usually required to generate enough power to be useful for electronic devices. However, the difference between the body's surface temperature and that of its environment is usually only a few degrees.

Traditionally, only low voltages could be produced from the temperature differences generally found between the body's surface and the environment. A conventional TEG delivers about 200 millivolts, whereas many electronic devices require at least 1 or 2 volts.

Peter Spies, manager of this sub-project at the Fraunhofer Institute for Integrated Circuits IIS, revealed that the engineers have come up with a solution to that problem by combining a number of components in a completely new way to create circuits that can operate on 200 millivolts.

"This has enabled us to build entire electronic systems that do not require an internal battery, but draw their energy from body heat alone," Spies said.

"The temperature difference can be very low; for the moment we work with about 2-3°C, but we will improve our systems to work with even lower temperature gradients. Of course, the electrical energy is proportional to this temperature gradient."

Circuits that are 'excited' at 50 millivolts already exist.

Spies believes that when further improvements have been made to the switching systems, a temperature difference of only 0.5°C will be enough to generate electricity.

It also has applications in medicine. Numerous items of medical equipment are attached to the body of a patient in the intensive care ward. These require a jumble of cables, because each device requires its own power supply.

In future, medical sensors may be able to function without power from a wall socket, instead drawing all the power it needs from the warmth of the body it's attached to. The data can then be sent by radio to the central monitoring station.

This technology could be available to the mass market in five years.