LONDON — Nokia has started demonstrating a prototype mobile phone that uses energy harvesting technology to recharge itself using only ambient radio waves - emitted from mobile antennas, TV masts and other sources.

The prototype has been developed at the Finnish group's Cambridge, England research laboratory.

Current prototypes can scavenge between 3 to 5 mW.

The short-term goal is to get in excess of 20 mW, enough power to keep a phone in standby mode indefinitely without having to recharge it. But this would not be enough to actually use the phone to make or receive a call. So ultimately the hope is to be able to harvest as much as 50 mW which would be sufficient to slowly recharge the battery.

The antenna and the receiver circuit are designed to pick up a wide range of frequencies — from 500MHz to 10GHz — and convert the electromagnetic waves into an electrical current, while the second circuit is designed to feed this current to the battery to recharge it.

The trick is to ensure that these circuits use less power than is being received.

Markku Rouvala, who heads the team developing the device and the energy harvesting technology, said commercial products featuring this approach to charging could reach the market within three to four years.

Ultimately, though, he says that Nokia plans to use the technology in conjunction with other energy-harvesting approaches, such as solar cells embedded into the outer casing of the handset.

The Nokia device will work on the same principles as a crystal radio set or radio frequency identification (RFID) tag: by converting electromagnetic waves into an electrical signal. This requires two passive circuits. "Even if you are only getting microwatts, you can still harvest energy, provided your circuit is not using more power than it's receiving," Rouvala says.

In order to achieve the 50mW threshold, scientists believe that a device would have to collect about 1,000 strong signals from a wide range of frequencies.

Last year, Nokia highlighted another exciting project being undertaken at Cambridge when it unveiled a shape changing mobile device concept based on nanotechnology.

Dubbed Morph, the joint nanotechnology concept was designed to demonstrate the possible future benefits of nanotechnology for mobile devices. Morph is both stretchable and flexible, but Nokia suggests nanotechnology could also allow future mobile phones to include self-cleaning surfaces and see-through electronics.

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