Tidal power is a means of electricity generation achieved by capturing the energy contained in moving water mass due to tides. Two types of tidal energy can be extracted: kinetic energy of currents due the tides and potential energy from the difference in height (or head) between high and low tides. Itaipu Dam is a hydroelectric generating station Electricity generation is the first process in the delivery of electricity to consumers. ...

The extraction of potential energy involves building a barrage. The barrage traps a water level inside a basin. Head is created when the water level outside of the basin changes relative to the water level inside. The head is used to drive turbines. In any design this leads to a decrease of tidal range inside the basin, implying a reduced transfer of water between the basin and the sea. This reduced transfer of water accounts for the energy produced by the scheme.

Tidal power 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 orbit of the moon around the Earth, and the deceleration of the Earth's rotation. Tidal power has great potential for future power and electricity generation because of the total amount of energy contained in this rotation. Tidal power is reliably predictable (unlike wind energy and solar power). Renewable energy (sources) or RES capture their energy from existing flows of energy, from on-going natural processes, such as sunshine, wind, flowing water, biological processes, and geothermal heat flows. ... This article is about tides in the ocean. ... Mosaic of the planets of the solar system, excluding Pluto, and including Earths Moon. ... Transmission lines in Lund, Sweden Electric power, often known as power or electricity, involves the production and delivery of electrical energy in sufficient quantities to operate domestic appliances, office equipment, industrial machinery and provide sufficient energy for both domestic and commercial lighting, heating, cooking and industrial processes. ... Itaipu Dam is a hydroelectric generating station Electricity generation is the first process in the delivery of electricity to consumers. ... Categories: Stub ... Solar power describes a number of methods of harnessing energy from the light of the sun. ...

The efficiency of tidal power generation 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 fiords. Amplitudes of up to 17 m (56 ft) occur for example in the Bay of Fundy, where tidal resonance amplifies the tidal waves. Efficiency is the capability of acting or producing effectively with a minimum of waste, expense, or unnecessary effort. ... The Bay of Fundy is a bay located on the Atlantic coast of North America, on the northeast end of the Gulf of Maine between the provinces of New Brunswick and Nova Scotia. ... In oceanography, tidal resonance is a phenomenon perhaps best exemplified in the Bay of Fundy. ...

Like with Wind Power, selection of location is critical for a tidal power 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 earth's gravitational force. 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). Wind power is the kinetic energy of wind, or the extraction of this energy by wind turbines. ... Potential energy (U, or Ep), a kind of scalar potential, is energy by virtue of matter being able to move to a lower-energy state, releasing energy in some form. ...

Several smaller tidal power plants have recently started generating electricity in Norway. They all exploit the strong periodic tidal currents in narrow fiords using sub-surface water turbines. A Francis turbine and generator cut-away view A water turbine is a rotary engine that takes energy from moving water. ...

Barrages

An artistic impression of a tidal barrage, including embankments, a ship lock and caissons housing a sluice and two turbines.

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. I created this image myself using Blender. ... I created this image myself using Blender. ... A caisson is: In engineering, a retaining structure used, for example, to work on the foundations of a bridge pier. ... A sluice is a water channel that is controlled at its head by a gate. ... A Francis turbine and generator cut-away view A water turbine is a rotary engine that takes energy from moving water. ...

The sluice gates applicable to tidal power are the flap gate, vertical rising gate, radial gate and rising sector.

Turbines

The turbines used in this application are variations of the Kaplan turbine, which have an ultra fast specific speed that allows them to operate with very low heads. They may also have variable pitched blades to further improve efficiency for varying tides and flow rates. A Bonneville Dam Kaplan turbine after 61 years of service The Kaplan turbine is a propeller-type water turbine that has adjustable blades. ... A Francis turbine and generator cut-away view A water turbine is a rotary engine that takes energy from moving water. ... In fluid dynamics, head refers to the constant right hand side in the incompressible steady version of Bernoullis equation. ...

Installed capacity

The total rated capacity (in MW) of the turbines is optimised for each barrage. At a low relative installed capacity, a small amount of energy will be produced, but production will last for a long period of time. At maximum installed capacity, the basin will drain very quickly in every cycle, producing the maximum possible energy, at very high rate, but only for a short period of time, making it difficult to absorb into the grid. A fast decrease in water level could also have negative effects on the environment.

The addition of capacity to a turbine follows the law of diminishing returns: for each additional MW, less extra energy will be produced. In economics, diminishing returns is the short form of diminishing marginal returns, the concept that, as more of an input is applied, each additional unit produces less and less additional output. ...

Modes of operation

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).

Two-way generation

Generation occurs both as the tide ebbs and floods. This mode is only comparable to ebb generation at spring tides, and in general is less efficient. Turbines designed to operate in both directions are less efficient.

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.

Marine current turbines

Various designs are proposed to extract kinetic energy from tidal currents. A generic current turbine will be rotated by water currents in the same way as wind power generators are rotated by air currents. The scheme would convert the currents' energy into electricity, slowing the currents down.

An experimental tidal power station was installed in Kvalsundet, south of Hammerfest, Norway, and became operational in November 2003. The installed tidal power turbine generates a maximum of 300 kW at a maximum speed of the current of 2.5 m/s. More information can be found at http://www.e-tidevannsenergi.com/index.htm

Intermittent nature of power output

Tidal power schemes do not produce energy 24 hours a 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 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 power transmission grid, as capacity with short starting and stopping times (such as hydropower or gas fired power plants) will have to be available to alternate power production with the tidal power scheme. Transmission towers Transmission lines in Lund, Sweden Electric power transmission, or more accurately Electrical energy transmission, is the second process in the delivery of electricity to consumers. ...

Mathematical modelling

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

Turbidity standards of 10, 100, and 1000 NTU Turbidity is a cloudiness or haziness of water (or other liquid) caused by individual particles that are too small to be seen without magnification, thus being much like smoke in air. ... Salinity is the saltiness or dissolved salt content of a body of water. ...

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.

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.

Turbidity

Turbidity (the amount of matter in suspense 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. Phytoplankton shown highly magnified Phytoplankton refers to the plant component of the plankton that drifts in the water column. ... Food chains and food webs describe the feeding relationships between species in a biotic community. ... In ecology, an ecosystem is a naturally occurring assemblage of organisms (plant, animal and other living organisms—also referred to as a biotic community or biocoenosis) living together with their environment (or biotope), functioning as a unit of sorts. ...

Salinity

Again as a result of less water exchange with the sea, the average salinity inside the basin decreases, also affecting the ecosystem.

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 cubature, 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. Biodegradation is the decomposition of organic material by microorganisms. ...

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. Cavitation is the phenomenon where small cavities of partial vacuum form in fluid, then rapidly collapse, producing a sharp sound. ... This article is about biological spawning. ... Pool-and-weir fish ladder at Bonneville Dam on the Columbia River Fishways, most commonly referred to as fish ladders but also known as fish passes, are structures placed on or around man-made barriers (such as dams and weirs) to assist the natural migration of diadromous fishes. ...

Global environmental impact

A tidal power scheme is a long-term source of electricity. The Severn Barrage, when built, is projected to save a 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 power schemes have a very high capital cost and a very low running cost. As a result, a tidal power scheme, will not produce returns for decades after it is built. Clearly, investors will not participate in tidal power projects. Only governments may be able to finance tidal power, but many are unwilling to do so also due to the extremely long time before returns and the huge irreversible commitment.

Resource around the world

Operating tidal power schemes

The first tidal power station was built over a period of 6 years from 1960 to 1966 at La Rance (Fr (http://membres.lycos.fr/chezalex/projets/rance/sommaire_rance.htm|)). It has 240MW installed capacity. There was a tidal power scheme built at Annapolis Royal in the Bay of Fundy as a pilot project for further developments in the Bay. Another pilot project was built by the Soviet Union on Kislaya Guba, with 400kW installed capacity. China has developed a number of small tidal power projects and one large, in Jiangxia.

Tidal power schemes being considered

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

San José – or its anglicised form San Jose – is the Spanish for Saint Joseph. ... Cobequid Bay is an inlet of the Bay of Fundy and the easternmost part of the Minas Basin, located in the Canadian province of Nova Scotia. ... The word Cumberland may have a variety of possible meanings. ... Kutch (also Cutch or Kachh) is a district of Gujarat state in western India. ... Cambay is a town in Gujarat state, India. ... Korea is a formerly unified country, situated on the Korean Peninsula in northern East Asia, bordering on China to the west and Russia to the north. ... Colorado River can refer to one of the following rivers: The Colorado River in the western United States and Mexico that passes through the Grand Canyon. ... Tiburon is a town located in Marin County, California. ... The Severn is the name of a river in the United Kingdom. ... The River Mersey is a river in the north west of England. ... This article is about the district of Wyre in England. ... This article is about the town, Conwy. ... Passamaquoddy Bay is an inlet of the Bay of Fundy, between Maine and New Brunswick, at the mouth of the St. ... Cook Inlet, showing Knik and Turnagain Arms The Cook Inlet or Nuti Inlet is a large inlet of the Gulf of Alaska in south-central Alaska. ... Cook Inlet, showing Knik and Turnagain Arms The Cook Inlet or Nuti Inlet is a large inlet of the Gulf of Alaska in south-central Alaska. ... Mezen (Мезень) is a river in the European part of Russia, flowing into the White Sea. ...

References

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, [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.