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We think nothing of being able to watch TV, talk on the phone and use the internet wirelessly, but the need for electrical power means we never get away from wires for long. Imagine how much easier life would be if we could power and recharge our gadgets without plugging them in. We could place lighting, speakers and even our PCs anywhere in a room with no messy cabling of any kind. Universal wireless power sources could charge all our portable gadgets without us even thinking about it. We could charge our notebooks, MP3 players and phones simply by placing them on a desk, or even by walking into a suitably equipped room. Wireless electricity could also make electrical devices more reliable, as power adaptors and sockets are common points of failure.

This may sound wildly futuristic, but wireless power transfer was first demonstrated by the Serbian-American scientist and inventor Nikola Tesla at the beginning of the 20th century. He was ahead of his time, but a century later his discoveries are helping to shape the race to make everyday wireless power a reality.

Catching waves

Electricity doesn't travel well through the air. It does happen - lightning bolts and spark plugs are two examples - but as Doc Brown from Back to the Future will confirm, it's a problematic way to transmit energy. However, energy travels through the air much more efficiently and safely in the form of electromagnetic waves. Electricity can be converted to visible light, microwaves or radio waves, which can then

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be directed at a receiver and converted back to electricity.

In the case of visible light, power can be transmitted by directing a laser at solar panels (more correctly known as photovoltaic or PV cells), but there are several drawbacks. The most powerful lasers produce many kilowatts of light, but even the best are typically less than 50 per cent efficient. PV cells are similarly inefficient, so at least 75 per cent of the power is lost between the source and destination. Laser light is absorbed by the atmosphere and high-powered lasers can be extremely dangerous, so they are rarely used to transmit power. However, in 2003, NASA demonstrated a model aeroplane powered by laser light.

Lower-frequency microwave energy is more efficient and straightforward. Microwave transmitters can achieve efficiencies of greater than 70 per cent, and the conversion back to electricity is up to 90 per cent efficient. In 1997, the University of La Réunion, a French overseas territory in the Indian Ocean, proposed an arrangement that could deliver 10 kilowatts of power to an unspoilt mountain village with a predicted overall efficiency of 57 per cent. Physicist and science fiction writer Geoffrey Landis has suggested that microwaves could be used to transmit energy to the earth from orbiting solar power stations, or from the earth to departing spacecraft.

Aerial view

Microwaves have been used experimentally to transmit tens of kilowatts, but anyone with a wireless network already has a microwave transmitter with a power in the region of 100 milliwatts. Wireless networks use microwaves with a frequency of 2.4GHz or 5GHz as carrier waves, the frequency or phase of which is subtly modified to transmit information. Although these systems are designed for sending data, the receiving electronics obtain a tiny amount of power from the carrier wave when the antenna converts it to electricity. This is insignificant compared with the amount of energy a wireless device actually consumes, but one company is using the same phenomenon to broadcast milliwatts of electricity to low-power devices.