Tidal power - energy from the sea

Introduction    How it works    More details    Advantages    Disadvantages    Is it renewable?   

Introduction

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 power works 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!

More details

The largest tidal power station in the world (and the only one in Europe) is in the Rance estuary in northern France. 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.

There have been plans for a "Severn Barrage" from Brean Down in Somerset to Lavernock Point in Wales. It may have over 200 large turbines, and provide over 8,000 MegaWatts of power (that's over 12 nuclear power station's worth). It would take 7 years to build, and could provide 7% of the energy needs for England and Wales.

There may be a number of benefits, including protecting a large stretch of coastline against damage from high storm tides, and providing a ready-made road bridge. However, the drastic changes to the currents in the estuary could have huge effects on the ecosystem.

Advantages

• Once you've built the dam, 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.

Disadvantages

• Very expensive to build.
• 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.
• Only provides power for around 10 hours each day, when the tide is actually moving in or out.
• There are very few suitable sites for tidal power stations.

Is it renewable?

Tidal energy is renewable. The tides will continue to ebb and flow, and the energy is there for the taking.