The Islay wave plant uses a shoreline compression chamber

As for waves, various different technologies have been designed to harness their power.

The UK's sole commercial wave energy plant uses a shoreline chamber, in which seawater enters and leaves naturally.

As it enters, the air in the chamber is compressed and forced through a hole into a turbine, making it move. As the water recedes, the air is sucked back, keeping the turbine moving.

The turbine drives a generator which converts the energy into electricity.

How much UK electricity currently comes from wind and wave power?

There are currently about 60 operating wind farms in the UK, supplying enough power for 250,000 homes each year, or about 0.3% of total UK electricity consumption.

Offshore wind farms are set to make a significant contribution to UK energy supply

There is also one commercially operating offshore wind farm, Blyth Offshore (about 1km from the coast in Blyth, Northumberland), which supplies enough energy to power up to 3,000 households.

The one commercially operating wave farm - Wavegen's plant on the island of Islay, on the west coast of Scotland - supplies power to about 400 local households.

What projects are coming up?

The UK Government has a target that 10% of energy must come from renewable resources by 2010.

The new Cabinet Office Performance and Innovation Unit (PIU) report recommends this rise to 20% by 2020.

Eighteen new offshore wind farms were given the green light in April 2001 - when they are built, they should generate enough energy to supply more than one million homes.

Various other onshore projects - such as a 140-turbine wind farm south of Glasgow, which could power 150,000 homes - are under way.

Wavegen is also planning an offshore wave farm on the Orkney islands, on the north coast of Scotland, which should generate enough electricity for 1,400 homes.

How much electricity could potentially come from wind and waves?

The UK's resources are huge. The British Wind Energy Association (BWEA) says offshore wind could supply current UK demand three times over, and onshore wind five times.

As for wave power, Wavegen says the recoverable wave energy resource also exceeds total UK electricity demand.

So what practical difficulties are there?

Various issues such as planning, infrastructure and capital costs mean that realistically, much less power than this will come from wind and wave energy.

Friends of the Earth says a conservative estimate is that by 2050, wind power could meet about 20% of current energy demand.

Wavegen says that realistically, about 20-25% of UK electricity demand could come from wave power by 2050.

Can I buy wind or wave energy now?

Most regional electricity suppliers often a "green" option, in line with a government target that they buy 10% of their energy from renewable resources.

Green tariffs currently involve a typical premium of �3-�10 every three months on top of the normal electricity bill.

Are there any drawbacks?

Wind and wave energy are widely considered environmentally clean options.

And the UK would not necessarily have to be smothered with windmills or wave plants - Greenpeace says 80 square miles of offshore space would be enough to supply all of the UK's electricity needs.

Most objections are more concerned with local issues - such as noise, the visual effect on the landscape, or the impact of roads leading to the sites.

Plans for a �30m wind farm in Denbighshire were recently scrapped after rare birds were found on the site.