Green machine: Cheaper home power from sunlight

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Solar cells are becoming an increasingly familiar sight on roofs, but soon even the walls and windows of your home could generate electricity and capture heat from the sun.

That's because researchers are borrowing a trick or two from solar power plants. Some of these use large parabolic mirrors or lenses to concentrate sunlight onto a photovoltaic (PV) device, allowing them to generate electricity efficiently while minimising their use of expensive PV materials. The concentrators typically track the sun across the sky, to ensure they direct as much light as possible onto the solar cells.

The same idea is being used by Soliant Energy, based in Monrovia, California, which has developed solar concentrating "buckets" for the roofs of commercial buildings. Arranged in clusters of six, the buckets tilt and swivel to track the sun, with lenses at the front to concentrate sunlight onto small cells at the rear.

But such systems are too complex and unwieldy to be fitted to homes, says Tapas Mallick at Heriot-Watt University in Edinburgh, UK. So instead he is developing grids of low-cost concentrators that would sit on walls, roofs or between the panes of double-glazed windows. Each concentrator is shaped like a funnel, with an egg-shaped opening facing the light and a PV device at the other end. It would trap and reflect light from different directions onto the solar cell – without moving.

No escape

Mallick is experimenting with making the concentrators from various different polymers. A leading contender is perspex, also known as plexiglas. Both because of the shape of the concentrators and because perspex has a higher refractive index than air, the light inside the concentrator cannot escape and simply bounces around until it hits the PV cell – a process known as total internal reflection.

By using a cheap polymer, Mallick hopes to reduce the cost of PV-based solar cells by 40 per cent. With an electrical efficiency of 20 per cent, he calculates the cells will produce up to 200 watts per square metre. This would make the technology much more efficient than previous attempts to concentrate sunlight onto window-mounted solar cells using a light-sensitive dye, which have an efficiency of around 5 to 6 per cent, says Mallick.

When fitted onto windows, the grid would allow 25 per cent of the available light through windows to illuminate the room and use the remaining 75 per cent to generate electricity. This would make rooms a lot darker, but it would be possible to alter the ratio to allow more light through in exchange for less electricity generation, Mallick says.

Solar panels currently waste a large amount of the solar energy that hits them as heat, so Mallick also plans to try fitting a panel made of a heat-recovering material to the back of the device. This would not be suitable for windows but could be used in roof-mounted systems to heat the home or provide hot water. (4.27.2010, Helen Knight)

Read previous Green machine columns: Power from the people, Rethinking internal combustion engines

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