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.