Tidal energy is based on the surge of enormous amounts of water as ocean tides rise and fall. This energy is recovered through a dam built across an estuary.
The worldwide potential of tidal powerTidal power involves harnessing the energy of tidal currents or of the differences in sea level between high and low tide... is underexploited at present. With a current installed capacityThe power generation capacity of a particular plant. It is usually expressed in megawatts (or sometimes even gigawatts)... of just over 500 MW, it could in fact provide up to 380 TWh per year1 with a capacity of 160 GW. This potential remains, however, far below that of land-based hydropower, since worldwide hydroelectricity production from conventional dams stood at over 3,600 TWh in 2012 (See Close-Up: "Hydropower, the Leading Renewable Energy").
254 MW: installed capacity of the Sihwa plant in South Korea
The idea behind tidal power plants is simple. A dam built across a river estuary lets seawater through twice a day, at high tide and low tide, driving turbines that produce electricityForm of energy resulting from the movement of charged particles (electrons) through a conductor... using a generator. At low tide, the dam can also be closed to hold back river water, which is then used in the same way as in a conventional hydroelectric dam. The tidal rangeThe maximum vertical difference between high and low tide. Tidal ranges vary depending on the time of year and geographical location... must be over 5 meters, and ideally between 10 and 15 meters, to ensure profitable operations.
Tidal Power: Advantages and Disadvantages
Tidal energy has the advantage of being a renewable energyEnergy sources that are naturally replenished so quickly that they can be considered inexhaustible on a human time scale... that, once the dam has been built, produces no greenhouse gasGas with physical properties that cause the Earth's atmosphere to warm up. There are a number of naturally occurring greenhouse gases... emissions. However, powerIn physics, power is the amount of energy supplied by a system per unit time. In simpler terms, power can be viewed as energy output... generation is intermittent. There are (predictable) periods when there is not enough current or retained water for the turbines to operate. It has a significant environmental impactAny change to the environment, whether adverse or beneficial, wholly or partially resulting from human activity..., altering balances upstream and downstream of the dam. In addition, investment and maintenance costs are very high.
Environmental groups are concerned about the impact of tidal power plants on ecosystem balances.
Tidal energy could be used away from the coast. There are a number of tidal power plantA power-generating station that harnesses the energy of tidal currents... projects that use a new design: artificial tidal lagoons2, a model that would limit the environmental impact of the plant. These lagoons are like artificial lakes, filling with water at high tide, and then emptying though locks equipped with turbines. Surveys are currently being carried out at three sites: Swansea and Severn in the United Kingdom and Yalu in China.
France pioneered tidal energy in 1966 with the commissioning of the La Rance tidal power plant near St. Malo. The area around the plant is known for having some of the highest tides in the world. In the 12th century, watermills were built to capture energy from the tides. Set up 750 meters off the coast on the Rance estuary, the plant's 24 turbines have an installed capacity of 240 MW and operate during both high and low tides. Annual output is 540 GWh, equivalent to the energy use of a city with a population of 300,000.
For a long time it was the biggest such plant in the world, but was overtaken in August 2011 by the Sihwa tidal power plant in Gyeonggi Bay, South Korea. It has an installed capacity of 254 MW, just ahead of the La Rance plant (240 MW).
Other plants have been built, but with much lower capacities.
- In Canada, the Annapolis Royal Tidal Power Generating Station was built in 1968. It has a capacity of 20 MW and annual production of 50 million kWh, enough to supply 4,500 homes. Its average tidal range is 6.4 meters.
- The Jiangxia tidal plant in China, in operation since 1980, produces 3.2 MW with an average tidal range of 5 meters. There are seven other tidal power plants in the country, producing a total of 2.92 MW annually.
- In Russia, the Kislaya Guba tidal power station, which has been operational since 1968, produces just 0.4 MW for an average tidal range of 2 to 3 meters.
Against the backdrop of efforts to develop renewable energies and reduce greenhouse gas emission reductions, tidal power is attracting renewed interest. Two countries are particularly active in this area:
- South Korea
South Korea has a highly ambitious project underway: the Garolim Tidal Power Plant, with a planned installed capacity of 500 MW. Studies on the project, which is opposed by fishermen, were continuing in January 20143.
- The United Kingdom
Several ambitious projects are being studied for the Severn estuary, near Bristol. However, they are facing opposition from environmental groups. As of fall 2013 , no decision had yet been reached4.
(1) Inter-Mines (p.24, French only)
(2) Inter-Mines (p.30, French Only)