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Tidal power, sometimes called ''tidal energy'', is a form of Hydropower that exploits the rise and fall in sea levels due to the Tide s, or the movement of water caused by the tidal flow. Although not yet widely used, tidal power has potential for future Electricity Generation and is more predictable than Wind Energy and Solar Power . In Europe, Tide Mill s have been used for over a thousand years, mainly for grinding grains. Tidal power can be classified into two types:
Modern advances in turbine technology may eventually see large amounts of power generated from the ocean and tidal currents using the tidal stream designs. 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 one day be concentrated around moving water, allowing communities to power themselves with non-polluting energy from moving water. BARRAGE TIDAL POWER ]] and road link proposed in 1989. The scheme would have generated 6% of the UK's Electricity Supply ]] 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 1966 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 Caisson s, embankments, Sluice s, 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 estuarine systems, and the environmental problems associated with changing a large ecosystem. 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 Another form of tidal barrage configuration is that of the dual basin type. 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 ecosystem. Many governments have been reluctant in recent times to grant approval for tidal barrages. 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 Ladder s, 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 : : where: ''h'' is the height of the tide ''M'' is the mass of water = 1025 kg per cubic meter (seawater varies between 1021 and 1030 kg per cubic meter) ''g'' is the Acceleration Due To Gravity = 9.81 Meters Per Second Squared 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 . Amplitudes of up to 17 m (56 ft) occur for example in the Bay Of Fundy , where Tidal Resonance amplifies the tidal range.
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 many years, and investors may be 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 {Link without Title} (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 these goals forward. 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:
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 Plant s. 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 Gas es 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. Operating tidal power schemes
Tidal power schemes being considered In the table, "-" indicates missing information, "?" indicates information which has not been decided
TIDAL STREAM POWER A relatively new technology tidal stream generators draw energy from currents in much the same way as Wind Turbine s. The higher density of water, some 832 times the density of air, means that a single generator can provide significant power. Even more so than with Wind Power , selection of location is critical for a tidal stream power Generator . 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 following potential sites have been suggested:
Prototypes Several commercial prototypes have shown promise. Trials in the in 2002. Tidal Energy Pty Ltd has commenced a rollout of shrouded turbines for remote communities in Canada, Vietnam, Torres Strait in Australia and following up with joint ventures in the EU. , Belfast, before installation in Strangford Lough ]] During 2003 a 300 kW Periodflow marine current propeller type turbine was tested off the coast of Devon , England , and a 150 kW oscillating hydroplane device, the Stingray, was tested off the Scottish coast. Another British device, the Hydro Venturi, is to be tested in San Francisco Bay. {Link without Title} Although still a prototype, the world's first Grid-connected turbine, generating 300 KW , started generation on November 13 , 2003 , in the Kvalsund , south of Hammerfest , Norway , with plans to install a further 19 turbines. {Link without Title} {Link without Title} SeaGen, a commercial prototype "open turbine" design will be installed by Marine Current Turbines Ltd in Strangford Lough in Northern Ireland in September 2007. The turbine could generate up to 1.2 MW and will be connected to the grid.http://www.seageneration.co.uk/ Verdant Power is runnng a prototype project in the East River between Queens and Roosevelt Island in New York City .http://www.verdantpower.com/what-initiative Shrouded turbines A shrouded turbine is enclosed in a Venturi shaped shroud or duct producing a sub atmosphere of low pressure behind the turbine, allowing the turbine to operate at higher efficiencies (typically 3 times higher then the Betz Limit of 59.3%) (See Kirke on the discussion page) than a turbine of the same size in free stream. In this way a shrouded turbine can generate the same amount of energy as a turbine of three to four times the size of the turbine in the shroud. If you measure the size of the rotors between shrouded and 'open flow' then the shrouded ones will generate more electricity. However, if you measure the area swept by the open flow rotors with the area of the mouth of the shroud then the energy is similar. The question of efficiency is then a matter of whether to increase the size of the rotors, or to encase them in a shroud. While the shroud may not be practical in wind, as a water turbine it is gaining more popularity and commercial use. A shrouded turbine is mono directional. It can be floated under a pontoon, fixed to the seabed on a mono pile and yawed like a wind sock to continually face upstream. A shroud can also be built into a tidal fence increasing the performance of the turbines. Cabled to the mainland they can be grid connected or can be scaled down to provide energy to remote communities where large civil infrastructures are not viable. Similarly to tidal stream open turbines they have little if any environmental or visual amenity impact. Energy calculations The energy available from these kinetic systems can be expressed as:
Where: Cp is the turbine coefficient of performance P = the power generated (in kW) ρ = the density of the water (seawater is 1025 kg/m³) A = the sweep area of the turbine (in m&2) V³ = the velocity of the flow cubed (i.e. V x V x V)
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. However, the tides are relatively reliable and more predictable than other alternative energy sources, such as wind. Tidal stream turbines deployed in run of rivers location are not subject to tidal cyclesand can produce energy 24 hour a day. Deployed from the banks of rivers close to the end user/s they avoid many complex issues of a salt water marine environment. SOURCE OF THE ENERGY Because the Tidal Force s 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. In fact though, the ultimate energy source is the rotational energy of the Earth, which will not Run Out in the next 4 billion years, although the Earth's oceans may boil away in 2 billion years. SEE ALSO PATENTS
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