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
How it works: Tidal Barrages
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
- 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.
- A barrage across an estuary is very expensive to build, and affects a very wide area - the environment is changed for many miles upstream and downstream. Many birds rely on the tide uncovering the mud flats so that they can feed.
- There are few suitable sites for tidal barrages.
- Only provides power for around 10 hours each day, when the tide is actually moving in or out.
Is it renewable?
Tidal energy is renewable. The tides will continue to ebb and flow, and the energy is there for the taking.
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