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Created by Andy Darvill,
Science teacher at
Broadoak Community School,
Weston-super-Mare, England
Web site www.darvill.clara.net
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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.
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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 a wading bird 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.
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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.
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There
have been plans for a "Severn Barrage" from Brean Down in
Somerset to Lavernock Point in Wales. Every now and again the idea gets
proposed, but nothing has been built yet.
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
would 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.
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
about the world's first offshore tidal power station at www.marineturbines.com/technical.htm
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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
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Yet
another option:
vertical-axis turbines
Find out more
from the Canadian company Blue Energy at www.bluenergy.com
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Advantages
- 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.
Disadvantages
- 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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