The tide moves a huge amount of water twice each day, and harnessing it could provide a great deal of energy - around 20% of Britain's needs.

Although the energy supply is reliable and plentiful, converting it into useful electrical power is not easy.

There are eight main sites around Britain where tidal power stations could usefully be built, including the Severn, Dee, Solway and Humber estuaries. Only around 20 sites in the world have been identified as possible tidal power stations.

A few years ago, "tidal power" meant "tidal barrage", but these days there are other options as well.

These work rather like a hydro-electric scheme, except that the dam is much bigger.

A huge dam (called a "barrage") is built across a river estuary. When the tide goes in and out, the water flows through tunnels in the dam.

The ebb and flow of the tides can be used to turn a turbine, or it can be used to push air through a pipe, which then turns a turbine. Large lock gates, like the ones used on canals, allow ships to pass.

If one was built across the Severn Estuary, the tides at Weston-super-Mare would not go out nearly as far - there'd be water to play in for most of the time.

But the Severn Estuary carries sewage and other wastes from many places (e.g. Bristol & Gloucester) out to sea. A tidal barrage would mean that this stuff would hang around Weston-super-Mare an awful lot longer!

Also, if you're one of the 80,000+ birds that feeds on the exposed mud flats when the tide goes out, then you have a problem, because the tide won't be going out properly any more.

More:

The largest tidal power station in the world (and the only one in Europe) is in the Rance estuary in northern France, near St. Malo. It was built in 1966.

A major drawback of tidal power stations is that they can only generate when the tide is flowing in or out - in other words, only for 10 hours each day. However, tides are totally predictable, so we can plan to have other power stations generating at those times when the tidal station is out of action.

Video clip:

Tidal barrage, Rance Estuary, France

Another option is to use
offshore turbines, rather like an underwater wind farm.

This has the advantage of being much cheaper to build, and does not have the environmental problems that a tidal barrage would bring.

There are also many more suitable sites.

Find out more at www.marineturbines.com

The University of Wales Swansea and partners are also researching techniques to extract electrical energy from flowing water.

The "Swanturbines" design is different to other devices in a number of ways. The most significant is that it is direct drive, where the blades are connected directly to the electrical generator without a gearbox between. This is more efficient and there is no gearbox to go wrong. Another difference is that it uses a "gravity base", a large concrete block to hold it to the seabed, rather than drilling into the seabed. Finally, the blades are fixed pitch, rather than actively controlled, this is again to design out components that could be unreliable.

Find out more at www.swanturbines.co.uk

At first glance this looks like a tidal barrage, but this design does not block the water movement as much, so it wouldn't affect the tides as severely and the environmental consequences would be much less. It could be built in sections, so power could start being generated sooner.

Migratory fish could get through, mud flats could still be exposed at low tide, and it would be able to generate power for more hours in the tidal cycle. Sections of it would open to allow shipping through, and it could be used to control tidal levels further upstream, for example preventing storm surges from flooding low-lying land. (Author's note - I live on low-lying land near the Severn Estuary, so I'm quite keen on this!)

Tidal barrages have been built before, whereas this idea is untested - so it'll be interesting to see if it gets approved.

Find out more at www.severntidal.com.

Yet another option:
vertical-axis turbines

Find out more from the Canadian company Blue Energy at www.bluenergy.com

• Once you've built it, tidal power is free.
  
  
• It produces no greenhouse gases or other waste.
  
  
• It needs no fuel.
  
  
• It produces electricity reliably.
  
  
• Not expensive to maintain.
  
  
• Tides are totally predictable.
  
  
• Offshore turbines and vertical-axis turbines are not ruinously expensive to build and do not have a large environmental impact.