featuring a horizontal turbine
Tidal energy is virtually a blank new field, scarcely utilised
at all. Yet the energy potential is huge; and unlike wind
and solar power, the tide times are known. So why has tidal
energy not been utilised before now? Has it been tried and
found to be a non-starter?
The answer is, no.
Historically we have managed quite well enough using fire,
air, water, animals, fossil fuels and nuclear energy. But
now we are being challenged to find sustainable sources of
energy that are more powerful than the pre-industrial ones.
A fantastic amount of energy is involved in the ebb and flow
of the tides around the UK, in particular the Severn estuary
with up to 40 feet tidal range. However, a conventional barrage
there would have so many problems that it makes any barrage
proposal a non-starter. The
worst problems would be the low intermittency of delivered
power (around 4 hours in 24), an increased risk of flooding,
the silting, the difficulties presented to shipping, substantial
changes in rare and protected habitats - and local disaffection.
So we are discounting Severn barrages. But there are many
other options. Harnessing tidal power is a new challenge,
calling for new technology, new ideas, new forms of turbines
- and support for research and development.
We see five main possibilities for harnessing tidal power
(1) The Tidal Reef proposal for
the Severn estuary (Rupert Armstrong-Evans): a comparatively
low barrier that can rise and fall with the tide, using turbines
for energy generation on both ebb and flow with only 2 metres
difference in sea levels. Atkins Engineering have given it
a basic thumbs-up on all counts, and estimates that it would
generate considerably more electricity than a barrage; moreover
it would operate most of the time rather than lying idle as
would a barrage.
(2) Double lagoons or "coffer dams". These can
be built completely offshore so that neither shipping nor
habitats nor beaches are disturbed. They collect water at
high tide, and "space without water" at low tide,
and can be set to drive turbines by emptying or by filling,
at chosen times. In themselves, lagoons are uncompetitive,
but if they ran at the turns of the tide which are the idle
times of the Reef, this could possibly be used to ensure a
continuous electricity supply.
(3) As yet undeveloped: Why not harvest tidal energy by the
direct power of its rise and fall? This would require a very
large-area flotation tank driving a generator and would be
particularly applicable to the Severn Estuary which has the
2nd highest rise and fall in the world at 40 feet mean difference.
(4) Tidal Stream Turbines harness the horizontal flow of
the tide where there is a reasonably fast tidal flow eg Strangford
Lough, Ireland. These turbines are not suitable for the
Severn estuary except in a few isolated places. They do not
dam the water at all, so there is no change of surface level,
which means a free channel for ships and fish; silting is
scarcely changed; beaches and habitats remain undisturbed.
At Strangford Lough, Lynmouth, and elsewhere, such turbines
are already in use and generating electricity.
(5) One very interesting option is illustrated below, and
our thinking on this was prompted by a visit to Aquascientific
in Exeter. This is a horizontal-axis turbine that turns at
a slower speed than the prevailing current flow, and has a
variable movement of its blades. The turbine blade axles are
geared to the main axle to present the most efficient configuration
to the flow, thus maximizing the capture of energy. According
to our calculations, this happens if each blade is geared
to rotate 180º counter-clockwise about its own axle,
for every 360º clockwise turn of the main axle, so that
it is parallel to the flow when turning against it, and perpendicular
to the flow when turning with it. Click
to see the animation of the blades turning.
Range of blade positions in the
water and effects of the flow. This is only a line diagram
to indicate blade positions;
there are actually only 3 blades.
Diagram of turning blades
(click for animation)
This design has a whole host of extremely positive features.
It would be suspended from floats, working in the optimum
flow which is just under the surface, requires very little
depth, and causes virtually no siltation, no change to tide
levels or tidal habitats, and no interference to shipping.
Its slow speed means that marine life is not at risk. Its
neatest asset is its unobtrusiveness. Only the floats would
be visible; the areas where it could function efficiently
in the Severn estuary are neither close to the shore nor in
the shipping lane; it functions best in the mid-ranges. Its
flexible size and simple design means it has worldwide potential
for both offshore and rivers.
Individual slow-action turbines could be arranged in "farms"
over a wide area, and this way it could provide, in the Severn
estuary alone, a total power output three times as great as
the conventional barrage, and at a fraction of the cost -
because of the lack of solid core construction. Around the
UK, other "farms" could be built in places with
different tide times, so that electricity would be delivered
overall at a steady rate. Best of all, because these small
units would start producing electricity almost immediately,
the cost would be amortized in around two years, unlike the
excessively long time needed to amortize barrage costs. Design
can be improved continually. The slow horizontal turbine would
work equally well in rivers. Maintenance and replacement of
parts can be achieved easily. The export potential is enormous.
A tidal fence simply consist of a string of turbines in a
line, with gaps between them. It can utilize many different
types of turbine, including the horizontal turbine illustrated,
but for best effect there may be other types more suitable
for the purpose.
See also the excellent 12-page pdf from "Save
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