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SUN: An emission-free and renewable energy source that can be harnessed when photovoltaic cells (PVs) convert sunlight directly into electricity. PV panels can be spotted on house and business roofs in many countries. And on a larger scale, systems supplying whole communities have also been built or are planned in a number of countries including Germany and the US.

Solar is possibly the most expensive form of renewable energy, and is rarely economical except for localised generation. But costs are coming down and once installed the energy source is free. An alternative approach uses solar radiation to heat water directly.

WAVE AND TIDAL POWER: The oceans contain large amounts of untapped energy. Wave and tidal power technologies are comparatively new and largely unexplored compared with efforts to harness solar and wind energy. Development costs are high, meaning they are unlikely to be able to compete economically with conventional power sources in the near future.

The total power of waves breaking on the world's coastlines is put at 2 to 3 million megawatts, according to the US energy department. Areas considered rich in tidal and wave power include: the western coasts of Scotland, northern Canada, southern Africa, Australia, and the north-east and north-west coasts of the US.

Large solar array at Chambery, France
Solar power is a big growth area
graph showing projected world primary energy demand for next 25 years


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