Micro generators pick up power

High-speed generators the size of pound coins may one day replace batteries in portable equipment

A tiny generator a centimetre across and three millimetres high has produced more than a watt of electricity, enough to power a cellphone.

The experimental device, created by researchers at the Georgia Institute of Technology, is part of a battery replacement project funded by the US Army Research Laboratory, and is expected to have applications for a wide variety of electronic equipment. "We can now get macro-scale power from a micro-scale device", researcher David Arnold said.

The complete finished system, called a microengine, has the potential to deliver more energy and last 10 times longer than a conventional battery, claim the Georgia researchers.

A compressed air system, similar to a dentist's drill, powered the prototype. When research is concluded, the designers hope to get between 20 and 50 watts -- enough to power a laptop -- with the generator powered by a micro-machined turbine or jet of similar size.

Many of these are in development elsewhere, with the University of Birmingham in the UK last year demonstrating three designs of comparable size to the Georgia Tech generator. The three types -- geared single-piston, free-piston, and micro-Wankel rotary engines -- used liquid CO2 for fuel, but will eventually use petrol, propane or other hydrocarbons.

The generator works by spinning a small coin-shaped magnet above a mesh of coils fabricated on a chip. High power is achieved by high speeds -- in the prototype the magnet revolves at 100,000 revolutions a minute (RPM), more than five times faster than the top speed of a car's alternator. As high performance magnets are brittle, it was shaped to minimise the centrifugal effect and encased in titanium.

"This is an important step in the development of MEMS-based micro-power systems," said Professor Mark Allen, also involved in the project.

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