BARRAGE TIDAL POWER

The Severn Barrage

There are very few tidal barrages in the world, especially those which can generate large quantities of energy. However, a barrage across the Severn Estuary is planned (see Fig 9), which would increase the UK’s supply of renewable energy, in order to meet requirements set by the EU. If built, the Severn Barrage would be the “largest ever renewable energy” structure (REUK, 2009). It would have the potential to harness up to 8GW of energy, which is equivalent to over 5% of the energy usage in England and the power output of 400 wind turbines [14].

Fig 9 – Proposed site (in red) of the Severn Barrage (http://www.marinet.org.uk/refts/7estuarydebate1.jpg)

Tidal movements

The Severn Barrage would generate this enormous amount of energy through tidal movements. The energy that tides possess originates from the work done by the gravitational forces of the Moon and the Sun on the Seas. When water on the Earth is closest to the Moon, the gravitational force of the Moon has a greater effect on it. On the closest side to the Moon, the body of water bulges slightly, as it does on the furthest side. This occurs on the further side, because the Moon is attracting the Earth more than the body of water on that side. As the Earth is rotating, a body of water at a single point continually rises and falls over a set time, resulting in the formation of tides (see Fig 10). (The Sun also causes a similar effect, although it is not as effective.) [15] This change in potential energy can be exploited for energy production, so a bigger tidal range is beneficial. The tidal range of the Severn Estuary helps by being the second highest in the world, at 14m [14].

Fig 10 – High tides occur at points closest to and furthest from the Moon, whereas low tides occur in between these points. (http://static.howstuffworks.com/gif/tide-tables-a-2.jpg)

Power generation

As high tide approaches, water flows through the sluice gates in the barrage, to the landward side. At high tide, the sluice gates close and remain closed as low tide nears. The height difference of 14m between the water levels on either side of the barrage relates to a very high value of potential energy. As the gates open at low tide, this energy is translated into kinetic energy, resulting in a high flow speed for the water. The water is then able to turn the 214 turbines with a high rotational speed, generating over 8GW of power as a result (see Fig 11) [14] [16].

Fig 11 – Simple model showing how the Severn Barrage would produce electricity (http://news.bbc.co.uk/1/hi/england/7202413.stm)

Disadvantages

Although the idea seems highly beneficial to the future of the UK’s energy supply, there are also several negative points to consider. Many conservationists believe that the barrage will impede the future of marine wildlife there, including “65,000 birds and migrating fish” (Gray, 2009) [17]. Original estimates of £20 billion have also appeared to have been underestimated, as Atkins -well known engineering consultants - has calculated the costs of such a project to be well over £40 billion. This cost will significantly restrict the expenditure on other renewable energy projects. Several other issues include restricted access on ports in the estuary, increasing pressure on others; as well as carbon dioxide emissions through the manufacture, transportation and construction of the barrage [18].

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