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v t e

Tidal Power, sometimes called tidal energy, is energy derived by exploiting the rise and fall in sea levels due to the tides. There are two types of energy systems that can be used to extract energy: kinetic energy, the moving water of rivers, tides as face ocean currents; and potential energy from the difference in height (or head) between high and low tides. The former method - generating energy from tidal currents uses turbines in a similar way to underwater windmills - is gaining in popularity because of the lower ecological impact compared to potential systems that are similar to dams sometimes called barrages or tidal fences. Many coastal sites worldwide are being examined for the suitability to produce tidal (kinetic) energy. Sites that are suitable exhibit high water speeds which typically occur in channels such as the entrances to bays, rivers of between islands where water currents are concentrated.

Tidal power is classified as a renewable energy source, because tides are caused by the orbital mechanics of the solar system (ocean currents are caused by the surface effect of winds) and are considered inexhaustible. The root source of the energy is the orbital kinetic energy of the earth-moon system, and also the earth-sun system. Tidal power has great potential for future power and electricity generation because of the essentially inexhaustible amount of energy contained in these rotational systems. Tidal power is reliably predictable (unlike wind energy and solar power). In Europe, tide mills have been used for nearly a thousand years, mainly for grinding grains.

The efficiency of tidal power generation in ocean dams largely depends on the amplitude (height of the rise and fall) of the tidal swell, which can be up to 10 m (33 ft) where the periodic tidal waves funnel into rivers and fjords and extreme water velocities can be 16 knots (Vancouver Island Canada). 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 power, selection of location is critical for a tidal power generator. The potential energy contained in a volume of water is :

${\displaystyle E=xMg}$

where x is the height of the tide, M is the mass of water and g is the acceleration due to gravity at the Earth's surface. Therefore, a tidal energy generator must be placed in a location with very high-amplitude tides. Suitable locations are found in Russia, USA, Canada, Australia, Korea, the UK and other countries (see below).

Several smaller tidal power plants have recently started generating electricity in Canada and Norway. They all exploit the strong periodic tidal currents in narrow fjords using sub-surface water turbines.

Tidal Stream

The largest amount of renewable energy available today is in the form of moving water in tides, rivers and open ocean currents. It is no surprise therefore that there is commercial interest in "tidal energy" systems that harvest the kinetic energy in moving water using sub-surface ("underwater wind-mill") arrays.

These systems harness the flow in a naturally occurring tidal streams, river or open ocean currents that occurs at the entrances to bays and rivers, around rocky points, headlands, between islands, and the mainland to name a few.

Moving water is a largest renewable energy resource. It is 832 times more dense than air and more predictable than wind-turbines or solar panels. It is impossible to accurately predict if the wind will blow in the next 5 minutes or if the sun will shine tomorrow, but the predictability of moving water in tides and ocean currents makes water energy turbines systems attractive to commercialization.

The energy available from these kinetic systems can be expressed as,

P = Cp x 0.5 x ρ x A x V

Where Cp is the turbine coefficient of performance,

P = the power generated (in kW)

ρ = the density of the water (seawater is 1025 kg per cubic meter)

A = the sweep area of the turbine (in m)

V = the velocity of the flow cubed (i.e. V x V x V)

Modern advances in turbine technology will eventually see large amounts of power generated from the oceans using this method. Arrayed in high velocity areas where natural flows are concentrated such as the west coast of Canada, the Strait of Gibraltar, the Bosporus, and numerous sites in south east Asia and Australia. Such flows occur almost anywhere where there are entrances to bays and rivers, or between land masses where water currents are concentrated.

A factor in human settlement geography is water. Human settlements have often started around bays rivers and lakes. Future settlement may be concentrated around moving water, allowing communities to power themselves with non-polluting energy from moving water.

Norway was one of the first countries behind Itally in 2001 (Messina Kobold Turbine) and Australia in 2002 (Tidal Energy Pty Ltd) to generate electricity commercially using sea-bed tidal power. An underwater turbine park in the Kvalsund, south of Hammerfest, started generation November 13, 2003

Barrage

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. Head (a height of water pressure) 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 largest such installation has been working on the Rance river (France) since 1967 with an installed (peak) power of 240 MW, and an annual production of 600 GWh (about 68 MW average power)

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 power are the flap gate, vertical rising gate, radial gate and rising sector.

Barrage systems are sometimes affected by problems of high civil infrastructure costs associated with what is in effect a dam being placed across two estuarine systems, and the environmental problems associated with changing a large ecosystem.

Modes of operation

Ebb generation

The basin is filled through the sluices until high tide. Then the sluice gates are closed. (At this stage there may be "Pumping" to raise the level further). The turbine gates are kept closed until the sea level falls to create sufficient head across the barrage, and then are opened so that 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 filled through the turbines, which 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 (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 are able to be powered in reverse by excess energy in the grid to increase the water level in the basin at high tide (for ebb generation). This energy is more than returned during generation, because power output is strongly related to the head.

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.

Variable nature of power output

Tidal power schemes do not produce energy all day. A conventional design, in any mode of operation, would produce power for 6 to 12 hours in every 24 and will not produce power at other times. As the tidal cycle is based on the rotation of the Earth with respect to the moon (24.8 hours), and the demand for electricity is based on the period of rotation of the earth (24 hours), the energy production cycle will not always be in phase with the demand cycle.

Mathematical modelling of tidal schemes

In mathematical modelling 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
• Sediment movements

Environmental impact

Tidal Energy Efficiency

Tidal energy has an efficiency of 80% in converting the potential energy of the water into electricity, which is efficient compared to other energy resources such as solar power. There are not many effects on the environment, but it can damage some fish (reduce the population).

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. A tidal current turbine will have a much lower impact.

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

As a result of less water exchange with the sea, the average salinity inside the basin decreases, also affecting the ecosystem. "Tidal 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.

With turbine generation, taking its power from the flow of the tidal stream, there will likely be a swirl of water down stream of the turbine. If this horizontal vortex touches the bottom, it will cause erosion. While the amount of sediment added to the tidal stream will likely be insignificant, this could, over time, erode the foundation of the turbine. Turbines held down with pilings would be largely immune to this problem but turbines held by heavy weights sitting on the bottom could eventually tip over.

Pollutants

Again, as a result of reduced volume, the pollutants accumulating in the basin may be less efficiently dispersed, so their concentrations may 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.

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.

Using tidal stream systems that do not close off rivers and streams would allow fish migration at times of spawning. These water current turbines typically turn very slowly at around 20-30 r.p.m., allowing fish to safely navigate either past or through the turning impellor drastically reducing or eliminating fish kills.

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 considerations

Tidal barrage 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 barrage 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.

Resource around the world

Operating tidal power schemes

• 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 Generating Station, Annapolis Royal, Nova Scotia, 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 apparently 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 coast 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

Tidal power schemes being considered

In the table, '-' indicates missing information, '?' indicates information which has not been decided

See also

• World energy resources and consumption
• Aquanator

External links

• Climate Change Chronicles -- Article about new tidal power technology
• University of Strathclyde ESRU -- Summary of tidal and marine current generators
• University of Strathclyde ESRU-- Detailed analysis of marine energy resource, current energy capture technology appraisal and environmental impact outline
• Renewable Energy UK Tidal -- Articles about tidal power generation.
• The British Library - finding information on the renewable energy industry
• Independent Online - information about South African ventures into coastal current power
• State explores renewable energy powered by tides
• Sustainable Development Commission - Project looking at 'Tidal Power in the UK'

Sources

• 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.
• Hammons, T. J. 1993, 'Tidal power', Proceedings of the IEEE, , v81, n3, pp 419-433. Available from: IEEE/IEEE Xplore. .
• 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.

Patents

• U.S. patent 6,982,498, Tharp, January 3, 2006, Hydro-electric farms
• U.S. patent 6,995,479, Tharp, February 7, 2006, Hydro-electric farms
• U.S. patent 6,998,730, Tharp, February 14, 2006, Hydro-electric farms

Template:Energy related development

• Landscape
• Tidal power stations
• Renewable energy
• Coastal construction
• Energy conversion
• Tide
• Sustainable technologies