Body Heat Used to Power Electronics
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originally posted at:
http://dsc.discovery.com/news/2007/08/17/bodyheat_tec_print.html


Body Heat Used to Power Electronics
Tracy Staedter, Discovery News


Aug. 17, 2007 — It used to be that human-generated electricity meant riding a stationary bicycle, or some such thing, to power a generator. But couch potatoes take note: simply sitting around could one day generate enough electricity to power electronic devices.

Scientists have developed new circuits that are able to harness electricity from body heat that would otherwise be wasted to the air. The advance could lead to battery-less cell phones and medical monitors that draw energy from their users.

"The idea behind it is that you can replace the batteries or at least you can enlarge the operation times of batteries," said Peter Spies, electrical engineer and group manager at Fraunhofer Institute for Integrated Circuits in Erlangen, Germany.

Spies and his team improved upon semiconductors called thermoelectric generators that produce electrical energy in the face of temperature differences.

Normally, a difference of several tens of degrees would be required in order to generate enough power, but the differences between the body's surface temperature and that of its environment are only a few degrees. That produces about 250 millivolts, while electronic devices require at least one or two volts.



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Spies and his team devised a solution. They incorporated a component into the circuit called a charge pump. The pump temporarily stores the incoming millivolts until they reach 1.8 volts. At that threshold, an internal transistor turns on and delivers the higher voltage to a component that can transfer the electricity to a device.

"The idea of of generating some electric power from the heat of the body is a great idea, " said Matthias Ueltzen, an application engineer at Freiburg, Germany-based Micropelt, a developer and manufacturer of thermoelectrics. But, he says, the difficulties lie in the low temperature differences between the skin and the outside air.

"Only a very small part of the thermal heat flow can be converted into electrical power," said Ueltzen. And for that reason, the technology may only work for applications that don't require a lot of energy.

The technology has already been shown to work on a wireless sensor that could be used to constantly monitor a patient's temperature and send the information to a nurse's station. It could also be used to power a hearing aid or to supplement the battery power on larger electronic devices, such as a sports watch or a mobile phone.

And because the circuit essentially converts waste heat into energy, it could have applications outside the body. For example, it could be used to convert the heat from radiators, refrigerators, or air conditioning systems into energy that can be reused by a building.

Spies and his team plan to have an optimized prototype by the end of the this year and think they can get the 3/4 inch by 3/4 inch device down to 1/5 inch on its side.