UK COASTAL POWER GENERATION
The world has, at last, woken up to the need to generate clean
energy, and investigations are still continuing into the full effects on the
atmosphere of the burning of fossil fuels. In every country, government and
industry must now ensure that the maximum advantage is taken of all forms of renewable
energy resources that can be found within its national boundaries.
The UK has the world's greatest tidal resources - mainly
because its position on the European continental shelf provides workable depths
of water. In addition, its proximity to the European mainland and Ireland
causes concentrated flows at useable velocities during its lunar tidal cycles.
Some members of the public and the government do
understand the basic idea behind tidal flow generators, but the devices are
generally regarded as nothing more than waterproof versions of a wind turbine.
Consequently, that is the direction taken by most designers, and several
R&D programmes have involved millions of pounds being spent on the
development of large tidal flow generators of that style. However, these
high-precision LIFT-type propeller-style turbines are not ideal for meeting the
challenges posed by the harsh underwater environment and such devices can be
deployed at only a limited number of locations for several simple reasons.
(a) To operate effectively, they need very smooth
waterflows.
(b) The water needs to flow faster than 2.0 m/s (fast walking
speed).
(c) The surfaces of the blades need to remain smooth and
clean.
Smooth waterflows can be found in only a narrow band of
the water column - above the undulations of the seabed but also well below the
action of the waves at the surface. Whereas the blades of a wind turbine can
bend and flex in the air to adjust to the turbulence caused by gusts of wind,
water - being incompressible - can actually shear the blades off a turbine in
similar conditions underwater.
From the very first day of deployment, baby barnacle
spores and other tiny marine organisms can attach themselves to the surfaces of
the blades and begin to reduce the efficiency of the rotor, in much the same
way as ice affects an aircraft's wing.
A ship’s propeller does not suffer from such a
disadvantage, as its rotation is driven by the thrust of an engine and results
in the ship’s movement through the water. A propeller-style tidal stream
generator is based on the reverse of that principle, as the blades have to
absorb the energy from the water and the shape of the blade helps to provide
the lift that causes the rotation - just as the shape of the aircraft's wing
gives the required amount of lift. For that reason, the waterflows and the
surfaces of the blades need to be smooth if a good turbine efficiency and power
output are to be achieved.
The UK and the rest of the world need to provide enough
usable energy to continue advancing technologically, in order to cope with the
steady increase in population and also improve living standards. Therefore, in
addition to installing offshore wind turbines and wave energy devices, the
governments of coastal nations should combine modern technology with
engineering lessons from the past in order to bring the best and the most
wide-ranging energy harvesting methods to the world.
The human race has been using the energy contained in
flowing water for thousands of years. Even today, in many parts of the world,
various styles of waterwheel are used to grind corn or raise water etc. The
designs are simple, effective and inexpensive.
Let
us consider a simple example of the potential of this resource.
The Severn Estuary is approximately 40 miles (64 kms)
long. Its width at Cardiff is 9 miles (14 kms), and more than 100 sq kms of its
area carries fast-flowing tidal currents in water having a depth of between 20
and 60 metres. Many of those tidal flows move at speeds well above 2.0 m/s.
Both the basic modular version and the Venturi version of
the patented Hales Tidal Stream Turbine can be used in a standard Tidal Fence
configuration. Both versions have been fully designed and tested. With the
Venturi version installed, a Tidal Fence spread over one square kilometre of
the estuary will produce 286MW of power.
That
figure is based on the following reasoning.
(a) Tidal Flow - At a speed of 2.0 metres per
second, one square metre of the flow’s cross-section area contains kinetic
energy that produces 4.0 kilowatts of power.
(b) Venturi effect - The simple Venturi system
used on the Hales Tidal Stream Turbine increases the speed of the flow by a
factor of 1.6, so a speed of 2.0 m/s would become 3.20 m/s acting on the
turbine. At that speed, one square metre produces 16.38 kilowatts of power. Adding
the Venturi system to the turbine actually doubles the width of the device, so
the turbines themselves occupy only half of the width of the Tidal Fence.
(c) Height of the Tidal Fence - As the tidal range
is so great in the Severn Estuary, the height of the Tidal Fence should not
exceed 20 metres, with the generators contained in the ballasted base.
(d) Spacing of the Tidal Fence - The Hales Tidal
Stream Turbines are reaction-type devices and not significantly affected by
turbulence, so the Tidal Fence lines can safely be installed only 200 metres
apart. That distance allows ample time and space for the water’s energy to be
replenished by the flows from beside and above the Tidal Fence. Furthermore, the
non-rotational form of the turbine's wake enables the turbines to extend all
the way down to the estuary floor. Such an arrangement enables the installation
of five complete Tidal Fence lines per square kilometre of the estuary.
(e) Hales Turbine Efficiency - In its simplest
form, the Hales Tidal Stream Turbine is 35% efficient at speeds down to 1.0
m/s.
Calculation
of the Power Output
Per Tidal Fence line 1km wide, the turbine capture-area
width = 500m
[That is explained in part (b) Venturi effect.]
The height of the Tidal Fence line is 20m, so:
Total turbine capture-area per line = 500 x 20 = 10,000
sq metres, so:
Total turbine capture-area for 5 Tidal Fence lines = 50,000 sq metres
1 square metre of the flow produces 16.38 kilowatts of power.
[That is explained in part (b) Venturi effect.]
Owing to the efficiency figure, the turbine captures 35% of that power.
So, the total power produced by a Tidal Fence spread over
1 sq km
= (50,000 x 16.38 x 0.35) kilowatts = (50 x 16.38 x 0.35)
megawatts
= 286.65 megawatts, so:
286MW is
the power produced per sq km of Tidal Fence block.
No comments:
Post a Comment