Error processing SSI file
Questions about Tidal Energy
Contents
- What is tidal energy?
- How does tidal power work?
- Is it possible to change the timing of electricity production to allow for peak demand times?
- Does there need to be a big difference between high and low tide heights?
- How much electricity is produced by tidal power?
- Is tidal power reliable?
- What is the visual impact of the technology?
- What is the environmental impact of the technology?
What is tidal energy?
Tidal energy uses the natural ebb and flow of coastal tidal waters. Coastal water levels change twice a day, filling and emptying natural basins along the shoreline. The currents flowing in and out of these basins can turn mechanical devices to produce electricity.
One kind of tidal energy can be harnessed by tidal stream (or marine current) technology. Tidal streams are fast sea currents created by the tides, often increased by features such as headlands, inlets and straits or by the shape of the seabed when water is forced through narrow channels.
How does tidal power work?
The technology needed to convert tidal energy into electricity is very similar to that used in traditional hydroelectric power plants. Tidal energy first needs a dam or ‘barrage’ across a tidal bay or estuary. Gates and turbines are placed along the dam. When the height of water on either side of the dam is different enough, the gates are opened and the ‘hydrostatic head’ that is created causes water to flow through the turbines, turning a generator to produce electricity.
Electricity is generated by water flowing both into and out of a bay. As there are two high and two low tides each day, tidal power generates most electricity every 12 hours. When demand for electricity is low, the turbines can also be used to pump extra water into the basin behind the barrage. This water can then be released when there is greatest demand on the system. This gives the tidal plant some of the characteristics of a ‘pumped storage’ hydroelectric facility. In order to produce practical amounts of electricity, a difference between high and low tides of at least five metres is required.
The technology used for tidal streams is slightly different from that used in tidal barrages and is still in its infancy. Tidal stream devices are similar to submerged wind turbines and are used to exploit the kinetic energy in tidal currents.
Is it possible to change the timing of electricity production to allow for peak demand times?
When demand for electricity is low, the turbines can be used to pump extra water into the basin behind the barrage. When demand is high, this water can be released, giving the tidal plant some characteristics of a ‘pumped storage’ hydroelectric facility.
Does there need to be a big difference between high and low tide heights?
Yes. In order to produce worthwhile amounts of electricity, there should be at least five metres difference between high and low tide. There are only about 40 sites in the world with this tidal range, including some on the west coast of the UK. The higher the tides, the more electricity can be generated from a given site, and the lower the cost of electricity produced.
How much electricity is produced by tidal power?
If all reasonably exploitable estuaries were used, the annual generation of electricity from UK tidal power plants would be 50 terawatt hours, equivalent to about 15 per cent of the current UK electricity consumption.
Is tidal power reliable?
Although tidal power varies, it is reliable, predictable and can make a valuable contribution to the diversity, and hence security, of an electrical system. The technology needed to harness tidal energy is well established. However, tidal power remains expensive and there are relatively few applications worldwide.
What is the visual impact of the technology?
The largest example of the technology is the 240-megawatt La Rance tidal barrage in Brittany, France. This has:
- a 330-metre-long dam, which also serves as a road bridge linking St Malo and Dinard;
- a 22-square kilometre basin with a tidal range of 8 metres
- a small lock which allows small craft to pass.
What is the environmental impact of the technology?
Tidal energy and tidal stream energy do not emit gases responsible for global warming or acid rain. Tidal energy reduces the level of greenhouse gases released into the atmosphere.
Changing tidal flows by damming a bay or estuary might harm aquatic and shoreline ecosystems, as well as shipping navigation and recreation. There are concerns that the extra mixing of water caused by tidal barrages could stimulate the growth of the red tide organism Gonyalaux excavata which paralyses shellfish. Estuaries are particularly important to migratory species of fish. Barrages could act as barriers to migration.