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PowerPedia:Tidal Energy

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Tidal energy is a means of electricity generation achieved by capturing the energy contained in moving water mass due to tides.


There are two types of tidal energy - kinetic energy that harneses the currents between ebbing and surging tides and potential energy from the difference in height (or head) between high and low tides. The former method - generating energy from tidal currents - is considered far less costly and much more feasible today than building expensive ocean-based dams or barrages, and many coastal sites worldwide are being examined for their suitability to produce tidal (current) energy. Since 1958, man has been harnessing the energy of tides efficiently to produce electricity. But harnessing tide energy has been ongoing since prehistoric times.

The barrage method of extracting tidal energy involves building a barrage and creating a tidal lagoon. The barrage traps a water level inside a basin. A height of water called head is created when the water level outside of the basin or lagoon changes relative to the water level inside. The head is used to drive turbines. The barrage design needs to have a very large rise and fall of tide to be practical. Otherwise it leads to a decrease of height of tidal range inside the basin or lagoon. This reduces transfer of water between the basin and the sea. This transfer of water accounts for the energy produced by the scheme.

Tidal energy is classified as a renewable energy source, because tides are caused by the orbital mechanics of the solar system and are considered inexhaustible within a human timeframe. The root source of the energy comes from the slow deceleration of the Earth's rotation. The Moon gains energy from this interaction and is slowly receding from the Earth. Tidal energy has great potential for future energy and electricity generation because of the total amount of energy contained in this rotation.

Tidal energy is reliably predictable (unlike wind energy and solar energy) years in advance.

In Europe, Tide Mills have been used for nearly 1,000 years, mainly for grinding corn.

The efficiency of tidal energy generation in ocean dams largely depends on the amplitude of the tidal swell, which can be up to 10 m (33 ft) where the periodic tidal waves funnel into rivers and fjords. Amplitudes of up to 17 m (56 ft) occur for example in the Bay of Fundy, where tidal resonance amplifies the tidal waves. As with wind energy, selection of location is critical for a tidal energy generator. The potential energy contained in a volume of water is,

:E = xMg \,

where x is the height of the tide,

M is the mass of water and g is the acceleration due to gravity.

Therefore, a tidal energy generator must be placed in a location with very high-amplitude tides. Suitable locations are found in the former USSR, USA, Canada, Australia, Korea, the UK and other countries (see below). Several smaller tidal energy plants have recently started generating electricity in Norway. They all exploit the strong periodic tidal currents in narrow fjords using sub-surface water turbines.


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Barrages are used to close off a basin for trapping a water level inside them. The basic elements of a barrage are caissons, embankments, sluices, turbines and ship locks. Sluices, turbines and ship locks are housed in caisson (very large concrete blocks). Embankments seal a basin where it is not sealed by caissons. The sluice gates applicable to tidal energy are the flap gate, vertical rising gate, radial gate and rising sector.

Ebb generation: The basin is filled through the sluices and freewheeling turbines until high tide. Then the sluice gates and turbine gates are closed. They are kept closed until the sea level falls to create sufficient head across the barrage and the turbines generate until the head is again low. Then the sluices are opened, turbines disconnected and the basin is filled again. The cycle repeats itself. Ebb generation (also known as outflow generation) takes its name because generation occurs as the tide ebbs.

Flood generation: The basin is emptied through the sluices and turbines generate at tide flood. This is generally much less efficient than ebb generation, because the volume contained in the upper half of the basin (which is where ebb generation operates) is greater than the volume of the lower half (the domain of flood generation). This is compounded by the fact that there is usually a river flowing into the basin, filling the basin as the tide rises and making the difference in levels between the basin side and the sea side of the barrage (and therefore the available potential energy) less than it would otherwise be. This is not a problem with the lagoon model: the reason being that there is no current from a river to slow the flooding current from the sea.

Pumping:Turbines can be powered in reverse by excess energy in the grid to increase the water level in the basin at high tide (for ebb generation and two-way generation). This energy is returned during generation.

Two-basin schemes: With two basins, one is filled at high tide and the other is emptied at low tide. Turbines are placed between the basins. Two-basin schemes offer advantages over normal schemes in that generation time can be adjusted with high flexibility and it is also possible to generate almost continuously. In normal estuarine situations, however, two-basin schemes are very expensive to construct due to the cost of the extra length of barrage. There are some favourable geographies, however, which are well suited to this type of scheme.

Tidal "wind farms": A new scheme plans to use turbines simailar to those found in wind farms to generate electricity via large current areas such as Cook Strait in New Zealand

Output Energy

Tidal energy plants do not produce energy 24 hours a day. A conventional design, in any mode of operation, would produce energy for 6 to 12 hours in every 24 and will not produce energy at other times. As the tidal cycle is based on the period of revolution of the Moon (24.8 hours) and the demand for electricity is based on the period of revolution of the Sun (24 hours), the energy production cycle will not always be in phase with the demand cycle. This causes problems for the electric energy transmission grid, as capacity with short starting and stopping times (such as hydropower or gas fired energy plants) will have to be available to alternate power production with the tidal power scheme. However, a new Scottish invention called GENTEC venturi solves the problem of intermittency and will generate, at full capacity, 24/7. It stores the tidal stream energy and generates continuous electricity from storage, not from the tide itself.

Models and theory

In mathematical theory of a scheme design, the basin is broken into segments, each maintaining its own set of variables. Time is advanced in steps. Every step, neighbouring segments influence each other and variables are updated. The simplest type of model is the flat estuary model, in which the whole basin is represented by one segment. The surface of the basin is assumed to be flat, hence the name. This model gives rough results and is used to compare many designs at the start of the design process. In these models, the basin is broken into large segments (1D), squares (2D) or cubes (3D). The complexity and accuracy increases with dimension. Mathematical modelling produces quantitative information for a range of parameters, including water levels (during operation, construction, extreme conditions, etc.), currents, waves, power output, turbidity, salinity, and sediment movements.

In physical modelling, small-scale physical representations of a tidal power scheme can be built. These have to be large to be accurate. Physical models are very expensive and are used only in critical projects.

Tidal plants

Plants operational

The first tidal power station was the Rance tidal power plant built over a period of 6 years from 1960 to 1966 at La Rance, France. It has 240MW installed capacity.

The first (and only) tidal power site in North America is the Annapolis Royal, Nova Scotia Generating Station, which opened in 1984 on an inlet of the Bay of Fundy. It has 20MW installed capacity.

A small project was built by the Soviet Union at Kislaya Guba on the Barents Sea. It has 0.5MW installed capacity.

China has developed several small tidal power projects and one large facility in Jiangxia.

China is also developing a tidal lagoon (near the mouth of the Yalu)

Scotland has committed to having 18% of its power from green sources by 2010, including 10% from a tidal generator. The British government says this will replace one huge fossil fueled power station.

South African energy parastatal Eskom is investigating using the Mozambique Current to generate power off the cost of KwaZulu Natal. Because the continental shelf is near to land it may be possible to generate electricity by tapping into the fast flowing Mozambique current.Independent Online Article

Plants planned

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|Mean tidal range (m)

|Area of basin (kmĀ²)

|Maximum capacity (MW)



|San Jose


| N/a




|Secure Bay






|Cobequid Bay






|Cumberland Basin






|Shepody Bay
















| rowspan="2|Korea













|Rio Colorado










|rowspan="4"|United Kingdom

|Severn Barrage




















| rowspan="3"|United States

|Passamaquoddy Bay





|Knik Arm





|Turnagain Arm














|10000 or 7000


|Penzhinskaya Bay





Key: "N/a" indicates missing information : "Unknown" indicates information which has not been decided

Environmental impact

Local environmental impact:The placement of a barrage into an estuary has a considerable effect on the water inside the basin and on the fish. Lagoons, on the other hand, could be used for fish or lobster farming, adding to their economic viability.

Turbidity: Turbidity (the amount of matter in suspension in the water) decreases as a result of smaller volume of water being exchanged between the basin and the sea. This lets light from the Sun to penetrate the water further, improving conditions for the phytoplankton. The changes propagate up the food chain, causing a general change in the ecosystem.

Salinity: Again as a result of less water exchange with the sea, the average salinity inside the basin decreases, also affecting the ecosystem. Again, lagoons do not suffer from this problem.

Sediment movements: Estuaries often have high volume of sediments moving through them, from the rivers to the sea. The introduction of a barrage into an estuary may result in sediment accumulation within the barrage, affecting the ecosystem and also the operation of the barrage.

Pollutants: Once again, as a result of reduced volume, the pollutants accumulating in the basin will be less efficiently dispersed. Their concentrations will increase. For biodegradable pollutants, such as sewage, an increase in concentration is likely to lead to increased bacteria growth in the basin, having impacts on the health of the human community and the ecosystem. The concentrations of conservative pollutants will also increase.

Fish: Fish may move through sluices safely, but when these are closed, fish will seek out turbines and attempt to swim through them. Also, some fish will be unable to escape the water speed near a turbine and will be sucked through. Even with the most fish-friendly turbine design, fish mortality per pass is approximately 15% (from pressure drop, contact with blades, cavitation, etc.). This can be acceptable for a spawning run, but is devastating for local fish who pass in and out of the basin on a daily basis. Alternative passage technologies (fish ladders, fish lifts, etc.) have so far failed to solve this problem for tidal barrages, either offering extremely expensive solutions, or ones which are used by a small fraction of fish only. Research in sonic guidance of fish is ongoing.

Global environmental impact: A tidal power scheme is a long-term source of electricity. A proposal for the Severn Barrage, if built, has been projected to save 18 million tons of coal per year of operation. This decreases the output of greenhouse gases into the atmosphere. More importantly, as the fossil fuel resource is likely to be eliminated by the end of the twenty-first century, tidal power is one of the alternative source of energy that will need to be developed to satisfy the human demand for energy.

Economic factors

Tidal power schemes have a high capital cost and a very low running cost. As a result, a tidal power scheme may not produce returns for years, and investors are thus reluctant to participate in such projects. Governments may be able to finance tidal power, but many are unwilling to do so also due to the lag time before investment return and the high irreversible commitment. For example the energy policy of the United Kingdom (see for example key principles 4 and 6 within Planning Policy Statement 22) recognizes the role of tidal energy and expresses the need for local councils to understand the broader national goals of renewable energy in approving tidal projects. The UK government itself appreciates the technical viability and siting options available, but has failed to provide meaningful incentives to move its goals forward.

External articles and references

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Sources and citations

Baker, A. C. 1991, Tidal power, Peter Peregrinus Ltd., London.

Baker, G. C., Wilson E. M., Miller, H., Gibson, R. A. & Ball, M., 1980. 'The Annapolis tidal power pilot project', in Waterpower `79 Proceedings, ed. Anon, U.S. Government Printing Office, Washington, pp 550-559.

There was an error working with the wiki: Code[1], Wikipedia: The Free Encyclopedia. Wikimedia Foundation.

Hammons, T. J. 1993, 'Tidal power', Proceedings of the IEEE, [Online], v81, n3, pp 419-433. Available from: IEEE/IEEE Xplore. [26 July 2004].

Lecomber, R. 1979, 'The evaluation of tidal power projects', in Tidal Power and Estuary Management, eds. Severn, R. T., Dineley, D. L. & Hawker, L. E., Henry Ling Ltd., Dorchester, pp 31-39.

Climate Change Chronicles -- Article about new tidal power technology

University of Strathclyde ESRU -- Summary of tidal and marine current generators

Tidal Power News Consolidated news links

The British Library - finding information on the renewable energy industry

Think Tidal - News, Stocks, Tidal Power Information

Independent Online - information about South African ventures into coastal current power


There was an error working with the wiki: Code[6], Tharp, January 3, 2006, Hydro-electric farms

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There was an error working with the wiki: Code[8], Tharp, February 14, 2006, Hydro-electric farms

Energy Conversion

See also

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