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 A British company, in conjunction with Oxford University
researchers, believes it has devised a way to overcome this obstacle
by creating a new type of horizontal axis turbine that can be used
underwater at depths of up to 30 meters, at an economical cost.
Conventional propeller-type turbines are like underwater wind
turbines and the number of suitable sites for them are vastly
reduced by the size of their large blades, limiting their use to
waters at least 30 meters deep. The THAWT (Transverse Horizontal
Axis Water Turbine) technology, by contrast, is designed for
deployment in shallower, lower velocity, tidal waters.
Developed by Oxford University's Department of Engineering Science
in conjunction with Kepler Energy, THAWT uses a stressed truss
configuration with carbon composite hydrofoil blades.
Put simply, as the water flows past the fence a head of water is
produced that increases the turbine's efficiency. The phenomenon is
called a 'blockage' of the turbines and gets larger in proportion to
the length of the fence.
 Guy Houlsby, professor of civil engineering at Oxford University,
says their design is an improvement on the vertical Darrieus wind
turbine used in some turbine systems.
"The original Darrieus turbine has blades that are parallel to the
axis of rotation, and that means that the loads in the blades are
carried entirely by bending of the blades. That results in very high
stresses," said Houlsby. "The re-design that we've done changes the
blades so that they form this triangulated structure, and that's a
very stiff and very strong structural form. And that means that the
loads in the blades are principally carried by axial forces and that
means that the stresses are much lower."
Kepler says their design has minimal moving parts in the water,
while its generator and other electrical equipment are installed in
dry columns, increasing their reliability, efficiency, and shelf
life. The generating units consist of two sets of blades sitting on
three columns with a single generator in between.
"The water flows at right angles to the axis of the turbine so, as
the turbine turns, lift is generated by these blades," explained
Houlsby.
Peter Dixon, chairman of Kepler Energy, says the patented turbine is
the most efficient yet designed. According to Dixon, "the rotor is
suited to lower velocity, shallower waters, which are areas where
you can't put conventional axial flow turbines, because to make them
powerful enough you need to make them very big in diameter and if
you make them very big in diameter they're going to stick out of the
water. So this turbine goes places other turbines cannot and
generates electricity at an economical cost."
A one kilometer (0.6 mile) long tidal energy fence, capable of
creating 30 megawatts at peak performance, has been proposed for
installation in the Bristol Channel, a major inlet and river estuary
between England and Wales. The project would cost an estimated £143
million ($224 million) and could be operational by 2021. Some
experts believe that if the tides flowing in and out of the channel
are correctly harnessed, they could supply up to five percent of the
energy requirements of the UK.
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"The design we have at the moment and the proposition we have at the
moment is to put a tidal fence, which is a chain of these turbines
in the Bristol Channel, and if we can build up to say ten kilometers
worth, which is a very extended fence, you're looking at power
outputs of five or six hundred megawatts and just to visualize that
that's like one small nuclear reactor's worth of electricity being
generated from the tides in the Bristol Channel," said Dixon.
Dixon says that THAWT has a series of other advantages. It's hardy,
with each rotor having a 25 year design life and the columns and
electricity connectors 100 years. It could also have positive
knock-on effects for Britain's carbon fiber manufacturing industry.
He says that THAWT's electrical output would be equal to that of a
nuclear power station, without any of the risk, and because the
blades move at a relatively slow speed there is no danger to fish
swimming through the fence.
In addition, it could be used in conjunction with a separate, less
productive, tidal lagoon system, consisting of circular retaining
walls embedded with turbines which capture the tide's energy.
"A lagoon generates maximum at the turn of the tide when a tidal
turbine like ours is actually static, not turning, so together they
constructively interfere, as the scientists would say. Very
effective....it's very advantageous to have both," said Dixon.
Dixon says production costs will be between £100 and £130 ($157 USD
to $203 USD) per MWh for the 10 kilometer fence proposed for the
Bristol Channel in the future, markedly cheaper than lagoons.
A scale prototype of THAWT has been stress tested successfully twice
at Newcastle University.
The developers say the system could be used in waters off France and
many Asian countries, such as Japan, China, the Koreas, Indonesia,
India, and the Philippines.
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