[Tidal Power Case Studies]
La Rance Tidal Barrage, Future Systems
The construction of this barrage began in 1960. The system used consists of a dam 330m long and a 22km2 basin with a tidal range of 8m, it incorporates a lock to allow passage for small craft. During construction, two temporary dams were built on either side of the barrage to ensure that it would be dry, this was for safety and convenience. The work was completed in 1967 when 24, 5.4m diameter Bulb turbines, rated at 10MW were connected to the 225kV French Transmission network.
This barrage uses Bulb Turbines, which were developed by Electricite de France. This allows generation on both ebbs of the tide. These axial flow turbines were also designed to pump water into the basin for the purposes described earlier. This makes it easier to anticipate generation levels. This type of turbine is popular with Hydropower and has been used on mainland Europe in dams on the Rhine and Rhone rivers.
Estimation on the number of homes this would supply electricity for if a similar system was set up in Scotland can be found from some simple calculations.
The turbines are rated at 10MW, therefore a total capacity of 240MW.
Maximum Electricity generated per annum (kWh) = 240000 * 8760 (hours in a year)
= 2102400000 kWh
Wave energy is like most other forms of renewable energy in that it cannot be relied upon 100% of the time so the value quoted above will almost certainly never be generated in a year. A value of Capacity factor (CF) is used to estimate the percentage of the maximum that will actually be generated in a year. A capacity factor of approximately 40% is assumed for Scottish waters.
Electricity Generated per annum (kWh) = 2102400000 * 40%
= 840960000 kWh
To estimate the number of homes that this quantity of electricity will provide for in a year, the average annual household consumption is used. The average annual household consumption is assumed to be 4377 kWh/year.
Number of Homes = 840960000 / 4377
There are other options for how to extract energy from tidal streams. One example of a new technology that is stimulating interest at the present time is the Stingray. This technology consists of a parallel linkage that holds large hydroplanes. The angle of these hydroplanes to the flow of the tide is varied causing them to move up and down. This motion is used to extend and detract a cylinder, which produces high-pressure oil that drives a hydraulic motor that in turn drives an electric generator.
This concept has received recognition of its potential from the Department of Trade and Industry. Firstly, funding for a three-month fast track feasibility study into the technology was awarded. This lead to the DTI Water and Power Technologies Advisory Panel supporting the proposal to design, build and install a demonstrator system in 2002. This system will last for one year and the results will be invaluable to assessing the Stingrays potential. The support of the DTI is very important, as it is so difficult to raise money for a technology that has not yet been demonstrated and proved.
The Engineering Business (EB) who are developing this technology, did a feasibility study of potential sites for the deployment of the demonstrator system. They settled on a site at Yell Sound in Shetland. They are performing an all-encompassing environmental analysis of the deployment and take into consideration social considerations and local feelings. The project will be deployed during the summer of 2002; this will be an important project for tidal stream enthusiasts and the industry as a whole. If successful, this will be an example to the world on how to utilise energy from the tides.
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