Turning body heat into electricity

The idea of converting the human body's energy into electricity has tantalized scientists for years. A resting male can put out between 100 and 120 watts of energy, in theory enough to power many of the electronics you use, such as your Nintendo Wii (14 watts), your cellphone (aboutone watt) and your laptop (45 watts). Eighty per cent of body power is given off as excess heat. But only in sci-fi fantasies such as the Matrix film series do you see complete capture of this reliable power source.

Current technology for converting body heat into electricity is capable of producing only a few milliwatts (one thousandth of a Watt), which is enough for small things such as heart rate monitors and watches. Some people fondly remember Seiko's Thermic watch, which runs continuously off body heat on 1 microwatt (one-millionth of a watt). It debuted in 1998 to rave reviews, but Seiko produced only 500 units before discontinuing it. If you own a Seiko Thermic, you never have to worry about changing batteries as long as your environment is cooler than your body.

Recent developments in nanotechnology engineering promise to usher in lots more body-powered devices. The basic technology behind the concept of turning body heat into electricity is a thermoelectric device. It is usually a thin conductive material that exploits the temperature difference between its two sides to generate electricity, known as the Seebeck effect. Such devices can work in reverse, meaning if you were to apply electricity to the device, one side would get extremely cold and the other extremely hot. If you own a USB-powered drink chiller, you probably own a thermoelectric generator only working in reverse. The same idea is also used in cooling some computers.

A thermoelectric device placed on skin will generate power as long as the ambient air is at a lower temperature than the body. A patch of material one square centimeter in area can produce up to 30 microwatts. Place these generators side by side to multiply the amount of power being harvested.

In 2006 Vladimir Leonov and Ruud J.M. Vullers from Belgium built a working prototype of a blood oxygen sensor, or pulse oxymeter, powered with body heat. It was about the size of a watch and was successfully tested on patients. It generated about 100 microwatts while the patient was asleep and up to 600 microwatts when awake and active. The group had to design the device so it could work with a record low power of 62 microwatts vs. commercially available 10-milliwatt pulse oxymeters.

In 2007 the duo built a body-heat powered electroencephalograph device that monitored brain activity. Leonov and Vullers started by redesigning the EEG device so it consumed less power. The EEG had to wirelessly transmit real-time data to a computer, too, so it had to consume a lot more power than their first prototype.

The 50-square-centimeter prototype was placed on the forehead of a person and harvested 3,500 microwatts, which was great, but came with a side effect: With so much area covered with thermoelectric devices sucking the heat, the sensation of cold became overwhelming to the patient.

The following year, the duo added photovoltaic cells to the top of the device to harvest solar power to offset some of the thermoelectric generation and make the device less chilly for the patient.

Next, they built a body-heat powered electrocardiograph device (EKG) that monitored heart activity. This time, they built the system as a washable shirt. In previous prototypes they used a super capacitor, which worked well. But when the capacitor was charged, it would waste any extra energy available from the thermoelectric device. In the shirt prototype they used a secondary battery as a storage device that constantly recharged using body heat. That cut out the waste and enabled them to shrink the device even more. Combining other forms of generation with smart storage systems will likely be the ways that body-heat-powered devices become practical.

At MIT, researchers are working on improving the efficiency of the circuitry that harnesses the minute amounts of power generated by standard thermoelectric generators. Scientists Anantha Chandrakasan, director of MIT's Microsystems Technology Laboratories, and his colleague Yogesh Ramadass have created extremely efficient circuitry in an EKG sensor with a built-in processor and wireless radio.

There's even greater potential in improving the efficiency of thermoelectric generators. Currently, a thermoelectric generator currently can only convert 0.4 per centof the heat energy into usable electrical power. With this efficiency, if you were to cover all of your body with thermoelectric generators you could produce 0.5 Watts of energy. This would feel extremely cold and would hardly be enough to power a cellphone. There is research being done by the U.S. Department of Energy and the University of California-Berkeley on developing more efficient thermoelectric generators.

MIT Professor Peter Hagelstein published a paper in November that showed a way to improve the efficiency of thermoelectric generators by up to 4 times in practice and up to 9 times in theory. Devices with that kind of efficiency could be used anywhere there is wasted heat on the walls of power plants or lining the hoods of automobiles. A company that is closely related to MIT, MTPV, is starting to work on commercializing Hagelstein's ideas.

In the not-so-distant future, we will very likely see cordless electronic devices in hospitals that sense and report vital signs of patients. Cellphones and laptops powered with body heat, however, are still many years away.