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Template:EnergyPortal Tidal power, sometimes called tidal energy, is a form of hydropower that exploits the rise and fall in sea levels due to the tides, or the movement of water caused by the tidal flow. Because the tidal forces are caused by interaction between the gravity of the Earth, Moon and Sun, tidal power is essentially inexhaustible and classified as a renewable energy source.

Although not yet widely used, tidal power has great potential for future electricity generation and is more predictable than wind energy and solar power. In Europe, tide mills have been used for nearly a thousand years, mainly for grinding grains.

Tidal power can be classified into two types. Tidal stream systems make use of the kinetic energy from the moving water currents to power turbines, in a similar way to underwater wind turbines. This method is gaining in popularity because of the lower ecological impact compared to the second type of system, the barrage. Barrages make use of the potential energy from the difference in height (or head) between high and low tides, and their use is better established.

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Tidal stream power

The greatest quantum of energy available to an energy starved is available from the kenetic energy in moving water.

Relatively new as a resource, emerging technologies exploit water current harvesting tidal streams rivers and open ocean currents in much the same way that wind turbines do. The higher density of water, some 832 times the density of air, means that a single generator can provide vastly significant power then conventional windmills. This readily available energy holds a significant attaraction for business.

Even more so than with wind power, selection of location can be critical for a tidal stream power . Tidal stream systems need to be located in areas with fast currents where natural flows are concentrated between obstructions, for example at the entrances to bays and rivers, around rocky points, headlands, or between islands or other land masses. The Pentland Firth in Scotland, the Cook Straits in New Zealand, the Strait of Gibraltar and the Bosporus have been suggested as potential sites.

Various designs and Prototypes

Several prototypes have shown promise. Trials in the Strait of Messina, Italy, started in 2001 and an Australian company Cite error: A <ref> tag is missing the closing </ref> (see the help page).

Environmental impact

Tidal systems do not interfere with fish migration at times of spawning, since the water remains open. As water current turbines typically turn very slowly at around 20-30 r.p.m., fish are able to safely navigate either past or through the turbines, drastically reducing or eliminating fish kills compared to barrage systems.

Energy calculations

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)

• Relative to a turbine in open or free stream. Shrouds creating a vortex have been tested with varying success achieving a multiplier effect of up to 4 times the efficiency of an open turbine. (See http://tidalenergy.net.au/)

Barrage tidal power

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.

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.

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.

Energy calculations

The energy available from barrage is dependant on the volume of water. 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
g is the acceleration due to gravity at the Earth's surface.

A barrage is therefore best placed in a location with very high-amplitude tides. Suitable locations are found in Russia, USA, Canada, Australia, Korea, the UK and elsewhere. Amplitudes of up to 17 m (56 ft) occur for example in the Bay of Fundy, where tidal resonance amplifies the tidal waves.

Economics

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 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.

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

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 or fossil fuel power plants.

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.

If fossil fuel resource is likely to decline during the 21st, as predicted by Hubbert peak theory, tidal power is one of the alternative source of energy that will need to be developed to satisfy the human demand for energy.

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.

Tidal power schemes being considered

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

See also

Template:EnergyPortal

• Category:Energy by country
• Aquanator
• Run-of-the-river hydroelectricity
• Wave power
• World energy resources and consumption

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'

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

References

1. A.D.A.Group
3. (see for example key principles 4 and 6 within Planning Policy Statement 22)
8. Independent Online Article

Other 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.

Template:Energy related development

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