Many suggestions have been made in recent years for producing energy from physical activity. The idea of the human body functioning as a power plant, or even as a battery, has lead to some useful and some very strange concepts. Each person generates about 60 watts of power while walking, and the Swedish company Jernhusen AB uses this to heat the 13-storey 'Kungsbrohuset' office building.

The neighbouring Stockholm railway station, through which about 250,000 people pass every day, acts as a power plant. Using a special ventilation system, the accumulated body heat passes through a heat exchanger and heats water for central heating. The developers estimate that it reduces the energy demand for office heating by about 15 percent.

Electricity from typing
Computer keyboards that can charge laptop batteries using the energy input from typing are conceivable, although still far from ready for commercial exploitation. The computer manufacturer Compaq has already developed a prototype in which each key has a tiny magnet, weighing a fraction of a gram, attached to it.

This is moved through a coil when the key is pressed and creates a small, induced current that is temporarily stored in a capacitor. When this capacitor is charged after repeated keystrokes, it transfers its power to the battery and can extend the battery life by several hours. This patented method (U.S. patent 5 911 529) is, however, certain to be complex and expensive.

Knee dynamo delivers over 50 watts
Not only cyclists, but also runners could soon be generating electricity with a dynamo. Similar to orthopaedic support stockings, the mini power plant - developed at the Simon Fraser University in Burnaby, Canada - fits around the knee. With each step, forces act on a gear mechanism, so each leg movement turns a generator.

With the 1.6-kilogram prototype, fast runners generated up to 54 watts of power. Strolling pedestrians were able to generate at least five watts.

In Rotterdam, a sustainable nightclub is already a reality. At Club Watt, each dancer produces about ten watts by moving on the elastic dance floor. The floor moves and this kinetic energy is converted into electric power. Overall, the power conversion efficiency in this eco-disco is so low that it is more likely to be a publicity stunt for the club.

Small powerplants for implants
On the other hand, methods by which sensors or tiny pumps can be operated in the body by self-generated electricity are more meaningful. For example, glucose in the bloodstream can be used for electricity generation. Chemists at the University of Texas at Austin have built a battery from two thin carbon fibres coated with different catalysts.

Using the enzyme glucose oxidase, electrons can be extracted from blood sugar at one fibre, while, at the other fibre, they are given up to oxygen dissolved in the blood.

A prototype employing this technique is already producing nearly two microwatts. This is similar to a button cell and is sufficient for the operation of implanted sensors, but not yet enough for pacemakers. But scientists in this field regularly report on new approaches that have higher current yields.

Because long-term high prices are paid for medical products, it is to be expected that tiny, mass-producible power plants fuelled by the body itself will become available.

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