History of Tidal Power
The use of tidal power is not new. Tidal mills were used to provide the power for milling corn for centuries. Proposals to harness the very large tides in the Severn Estuary were made in the 19th century and in the 20th century there were several major studies.
Tidal Range is a form of hydro-power. Energy is generated from the difference in water level between the enclosure and the sea. It is different to Tidal Stream which uses the speed of the water to drive turbines.
There are two large Tidal Range schemes in operation today, at La Rance in France and Siwha in South Korea.
Most Tidal Range schemes constructed to date consist of a barrage across a river. A Tidal Lagoon is a scheme that does not enclose an estuary or large river to generate hydro-power.
Tidal Barrages and Lagoons
The first comprehensive studies were carried out for The Severn Barrage Committee (the Bondi committee) and were published as Energy Paper 46 in 1981. This was followed by the Severn Tidal Power Group’s detailed studies for a Cardiff to Weston barrage, published as Energy Paper 57 in 1989, which remain the most detailed engineering study to date. The most recent studies for the Severn were carried out in 2007-09 for the Department of Energy and Climate Change (DECC) and were published in 2010. These studies considered three options for a barrage and two lagoon options and included wide ranging environmental studies and assessments. Studies for barrages in other UK locations carried out in the 1980’s and 1990’s included the Mersey barrage and several small schemes on the West coast.
The first major scheme to be constructed was the La Rance Tidal Power Barrage in France which was completed in 1967 and has operated ever since. It has a capacity of 240MW and has now operated successfully for almost 60 years, with some upgrading of electrical systems carried out in the 1990’s. Another large scheme was constructed more recently, the Siwha tidal power plant in South Korea which has a capacity of 254MW. In addition, there are some small schemes operating in Canada, Russia and China.
Proven Technology
La Rance Tidal Power Station in Brittany, France was completed in 1967 and has now operated for nearly 60 years. The turbines are still in good condition and seem likely to last for another 60 years, thus providing a 120 year life, comparable to many hydropower installations. Some electrical equipment and controls have been replaced.
The equipment consists of:
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24 turbines of 5m diameter and 10 MW capacity, providing a total capacity of 240MW.
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6 sluice gates
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A lock for shipping
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A road crossing
Annual generation is 540MWh.
The power station operates mainly on ebb (outgoing) tide with pumping into the basin on the flood (incoming) tide. The benefit of pumping is that it increases the head of water for generating. The water is pumped in against a head of only about 1m but this water then generates through a head of 5m or more, thus providing a net gain in energy.
It is reported that passage of fish occurs without major problems.
Sihwa Lake Tidal Power Station in Gyeonggi Province, South Korea was Completed in 2011. It was designed to alleviate pollution in an existing flood protection lagoon. It operates on flood (incoming) tide only.
The equipment consists of:
•10 Turbines of 8m diameter and 25.4MW capacity, proving a total capacity of 254MW
•8 sluice gates
Annual generation is 552 GWh
How does it work?
An enclosure is formed to impound the tides. The tides, which are powered by the moon, flow in and out of the lagoon’s 125 bulb turbines to generate electrical energy, producing four periods of energy every 24 hours. At the end of each generation cycle, large sluice gates are opened to allow more water into the lagoon to increase water level and energy in the next generation cycle.
It will have a capacity of 2500 MW and will produce 6.5 TWh of electricity - enough to power 2 million UK ‘median usage’ homes. (Ofgem)
This is proven technology similar to the almost 60yr old La Rance scheme in France, which is still operating and should continue for at least another 60 years. In addition, floating Solar panels are planned to provide a further 500MW of renewable energy.
Construction and Operation The lagoon enclosure will be formed from large concrete caissons (cellular reinforced concrete boxes), prefabricated in existing port facilities in the Bristol Channel and Severn Estuary. The caissons will then be towed to site and installed on the seabed by filling with water, sand and gravel. The use of caissons is proven technology from offshore oil and gas and from harbour construction, and has the major benefit that it minimises offshore construction with the attendant risk of weather delays. The proposed layout consists of 125 bulb turbines of 7.2 m diameter and capacity 20 MW, thus providing a total generating capacity of 2500 MW. The turbines are in 5 groups of 25 machines and there are 160 sluiceways with vertical lift gates in 8 groups. Both turbines and sluice gates will be housed in the concrete caissons. The turbines and sluices have been spaced out around the lagoon perimeter to provide well dispersed water flows in both generating and sluice modes of operation with the intention of minimising silt build up within the lagoon, minimising tidal level change during construction, and encouraging safe fish movement. The semi-circular shape maximises lagoon area whilst minimising the wall perimeter, resulting in maximum energy generation per unit cost. The type of turbine proposed is a bulb turbine, similar to those used at La Rance and Siwha, in which the generator is housed in the bulb. A significant improvement in the design now proposed is to use variable speed and 3 blades (rather than 4). This will produce more energy and also reduce the risk of damage to any fish that pass through the turbine. There will be AFD (Acoustic Fish Deterrent) installed to minimise fish passage through the turbines. The operation of the lagoon has been modelled in detail to predict the water flows and energy output. This model also provides water levels throughout the Bristol Channel and Estuary and shows that the effect on water levels elsewhere in the Estuary is small, typically less than 200mm. Much more detailed modelling of both water flows and sediments will be carried out as part of the Environmental Assessment. Floating Solar panels in the lagoon are proposed in addition to the tidal turbines. This form of solar power installation is now proven for inland water areas, such as in the water reservoir near Heathrow airport, and has also being installed recently in various marine location worldwide. The economics are encouraging and likely to improve.
UK Energy Policy and Challenges
Government Policy is for
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UK energy to be carbon neutral by 2050
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Need to double electrical output by 2050
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Large increases in both offshore wind and solar
The challenges
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Most existing nuclear power stations will be closed by 2030 and there is uncertainty over the cost of new ones
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The current policy for a very large increase in offshore wind installations and in solar will result in a need for very large storage facilities in the form of batteries and conversion to hydrogen.
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There are risks of reliance on new technologies
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Carbon Capture and Storage (CCS) to allow continued use of gas power stations and Hydrogen for storage.
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Tidal Range Hydro-Power will add to diversity of supply sources and is totally predictable, unlike wind.
UK Energy Policies in more detail The major influence on future electrical demand will be the government’s commitment to net zero carbon emissions by the year 2050 combined with the move towards electric powered cars and home heating. Under most scenarios the required electrical generation capacity is expected to increase by 50% by 2030 and by 100% or more by 2050. Given the closure of existing fossil fuel power stations and the older nuclear power stations and the recent expansion of offshore windfarms and solar PV, it seems likely that offshore wind and solar will make up a substantial proportion of the new capacity. However, diversity of supply is now considered essential for future security of supply. New nuclear capacity seems likely to provide some of the new capacity. Power from Tidal Range generation in the UK can provide a further important component of a future diversified electrical supply. Although intermittent, unlike wind power and solar, tidal power is totally predictable for years ahead. Also, studies show that a combination of tidal range schemes in the Severn and the North West could provide near to continuous power during spring tide periods with only a 2 hour gap during neap tides. Alternatively, because the no generation periods are totally reliable and predictable, tidal power linked to some storage could provide continuous power.