Trapping water at high tide and releasing it when there is an adequate head is an ancient technology. A medieval tide mill is still in working order in Woodbridge, Suffolk. In the first quarter of the last century this principle was applied to electricity generation in the feasibility studies for a barrage across the River Severn.
Tidal power works on the principle that water is held back on the ebb tide to provide a sufficient head of water to rotate a turbine. Dual generation is possible if the flow tide is also exploited. However, this has the disadvantage that the turbines are less efficient, having to cater for two-directional flow. Also the changing from flow to ebb generation means that the turbines cannot exploit the full tidal range.
The only operational barrage in Europe is at La Rance, Normandy. It was designed in the 1960s with a capacity of 240 MW, giving an annual production of about 610 gigawatt hours (GWh). Despite some mechanical problems in 1975 the scheme has been a success. Even so, the French government elected to concentrate its generation policy on nuclear power which accounts for about 75% of its capacity (see Figs 11.8 and 11.9).
The technology of barrages was transformed by the caisson techniques employed in the construction of the Mulberry Harbour floated into place after D-Day in World War II. It is a
Figure 11.9 Tidal barrage and turbine hall, La Rance, Normandy modular technique with turbine caissons constructed on slipways or temporary sand islands. The Energy Technology Support Unit (ETSU) stated in 1992: 'The UK has, probably the most favourable conditions in Europe for generating electricity from the tides'. In fact, it has about half all the European Union's tidal generating potential of about 105 terawatt
hours per year (TWh/y). The report concludes:
There are several advantages arising from the construction of tidal barrages in addition to providing a clean, non-polluting source of energy. Tidal barrages can assist with the local infrastructure of the region, create regional development opportunities and provide protection against local flooding within the basin during storm surge.1
Around the world the estimated potential for capturing tidal energy is about 1000 GW. However, constraints like access to construction sites and centres of population or a national grid reduce the realistic potential to around 100 GW2
One of the best tidal opportunities exists in Canada in the Bay of Fundy where there is a proposal to generate 6400 MW China has 500 possible sites with a total capacity of 110.0 GW. In the UK the longest running debate about tidal energy has centred on the River Severn estuary. It is still no nearer realization despite the fact that it has a potential capacity of 8.6 GW. However, its load factor of 23% (the proportion of rated power actually delivered to the grid) means that its realistic capacity is about 2 GW.
The most pressing case for a hard barrage in the UK is located in the Thames estuary. It is a situation identified by the Met Office as being the most vulnerable in northern Europe to severe storm surges up to and beyond 2080. Especially threatened would be the proposed Thames Gateway housing development. A tidal barrage would also be able to generate considerable quantities of electricity via underwater turbines and 5 MW wind turbines along its apex (see Fig. 11.10).
In the 1970s and 1980s one of the arguments against tidal barrages was that they would trap pollution upstream. Since rivers are now appreciably cleaner in Europe than in the 1970s thanks largely to EU directives, this should not now be a factor. The Thames is claimed to be the cleanest river in Europe, playing host to salmon and other desirable fish species. A group of engineering companies has renewed the argument in favour of the River Severn barrage, indicating that it would meet 6% of Britain's electricity needs whilst protecting the estuary's coastline from flooding.3
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Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.