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Tidal power, sometimes called tidal energy, is a form of hydropower that exploits the movement of water caused by tidal currents or the rise and fall in sea levels due to the tides.
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 mills have been used for over a thousand years, mainly for grinding grains.
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This is the only form of energy which comes from the motion of the Earth-Moon system, though some of it comes from the solar tides as well. Most other sources of energy come directly or indirectly from the Sun, including fossil fuels, conventional hydroelectric, wind, biofuels, and solar. The remainder, nuclear and geothermal for instance, have radioactive material on Earth as their source.
Tidal energy is generated by the relative motion of the Earth, Sun and the Moon, which interact via gravitational forces. Due to these gravitational forces, water levels follow periodic highs and lows. Associated with these water level changes, there are tidal currents. The specific tidal motion produced at a certain location is the result of the changing positions of the Moon and Sun relative to the Earth, the effects of Earth rotation, and the local shape of the sea floor.
The tidal energy generator uses this phenomenon to generate energy. The stronger the tide, either in water level height or tidal current velocities, the more promising it is to harness tidal energy.
Tidal power can be classified into two main types:
Modern advances in turbine technology may eventually see large amounts of power generated from the ocean especially tidal currents using the tidal stream designs. Tidal stream turbines may be arrayed in high velocity areas where natural tidal current flows are concentrated such as the west and east coasts 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.
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A relatively new technology, tidal stream generators draw energy from currents in much the same way as wind turbines. The higher density of water, 832 times the density of air, means that a single generator can provide significant power.
Similar to wind power, selection of location is important for the tidal turbine. 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:
Several commercial prototypes have shown promise. Trials in the Strait of Messina, Italy, started in 2001A.D.A.Group and Australian company Tidal Energy Pty Ltd undertook successful commercial trials of highly efficient shrouded turbines on the Gold Coast, Queensland in 2002. Tidal Energy Pty Ltd has commenced a rollout of shrouded turbines for remote communities in Canada, Vietnam and Torres Strait in Australia and following up with joint ventures in the EU.
The SeaGen rotors in Harland and Wolff, 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.[1]
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.[2][3]
SeaGen, a commercial prototype design will be installed by Marine Current Turbines Ltd in Strangford Lough in Northern Ireland in March 2008. The turbine could generate up to 1.2 MW and will be connected to the grid.http://www.seageneration.co.uk/
RWE\'s NPower announced that it is in partnership with Marine Current Turbines to build a tidal farm off the coast of Anglesey in Wales, though strictly speaking this is not a prototype, but a commercial farm.http://www.forbes.com/markets/feeds/afx/2008/02/07/afx4626015.html
British Columbia Tidal Energy Corp. plans to deploy at least three 1.2-MW turbines in the Campbell River or in the surrounding coastline of British Columbia by 2009. http://www.alternative-energy-news.info/press/tidal-power-west-coast-canada/
In November 2007, British company Lunar Energy announced that, in conjunction with E.On, they would be building the world\'s first tidal energy farm off the coast of Pembrokshire in Wales. It will be the world\'s first deep-sea tidal-energy farm and will provide electricity for 5,000 homes. Eight underwater turbines, each 25 metres long and 15 metres high, are to be installed on the sea bottom off St David\'s peninsula. Construction is due to start in the summer of 2008 and the proposed tidal energy turbines, described as "a wind farm under the sea", should be operational by 2010.
Verdant Powerhttp://www.verdantpower.com/what-initiative is running a prototype project in the East River between Queens and Roosevelt Island in New York City.
An emerging tidal stream technology is the shrouded tidal turbine 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 efficiency (than the Betz Limit Betz Limit of 59.3%) and typically 3–4 times higher power output Brian Kirke\'s published article Developments in Ducted Water Turbines than a turbine of the same size in free stream.
This working example of a shrouded turbine in the photo was deployed by Clean Current Power at Race Rocks in southern British Columbia in 2006. It operates bi-directionally and has proven to be efficient in contributing to the integrated power system of Race Rocks.
Considerable commercial interest has been shown in shrouded tidal stream turbines as they can produce 3-4 times the power output of similar sized open turbine. They can operate in slower moving water and allow a smaller turbine to be used at sites where large turbines are restricted. Arrayed across a seaway or in fast flowing rivers shrouded tidal stream turbines are easily cabled to a terrestrial base and connected to a grid or remote community. Alternatively the property of the shroud that produces an accelerated flow velocity across the turbine allows tidal flows formerly too slow for commercial use to be utilized for commercial energy production.
While the shroud may not be practical in wind, as the next generation of tidal stream turbine design it is gaining more popularity and commercial use. Tidal Energy Pty LtdTidal Energyin Australia make use of the design and Lunar Energy use a double ended shroud. The Tidal Energy Pty Ltd tidal turbine is mono directional and the Lunar Energy turbine bi directional. Both constantly need to face upstream in order to operate. In the Tidal Energy Pty Ltd case this can be achieved by floating the turbine under a pontoon on a swing mooring, fixed to the seabed on a mono pile and yawed like a wind sock to continually face upstream. Lunar Energy use a wide angle diffuser to capture incoming flow that may not be inline with the long axis of the turbine. A shroud can also be built into a tidal fence or barrage increasing the performance of turbines.
Cabled to the mainland they can be grid connected or can provide energy to remote communities where large civil infrastructures are not viable. Described as eco benign the slow R.P.M. of tidal stream turbines does not interfere with marine life or the environment and have little if any visual amenity impact. They are ideal for remote communities that are far from grid connected infrastructure such as islands and rivers.
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Various turbine designs have varying efficiencies and therefore varying power output. If the efficiency of the turbine "Cp" is known the equation below can be used to determine the power output.
The energy available from these kinetic systems can be expressed as:
Where:
Cp is the turbine coefficient of performance
P = the power generated (in Watts)
ρ = the density of the water (seawater is 1025 kg/m³)
A = the sweep area of the turbine (in m²)
V³ = the velocity of the flow cubed (i.e. V x V x V)
Relative to an open turbine in free stream. Shrouded turbines are capable of higher efficiencies as much as 3–4 times the power of the same turbine in open flow. http://www.cyberiad.net/library/pdf/bk_tidal_paper25apr06.pdf tidal paper on cyberiad.net
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Prices paid for electricity varies around the globe. The kilowatt price can be 10-15 British Pence in the UK, or 30-40 US cents. In remote areas, electricity can cost 50-60 US cents or more.[citation needed]
The following equation can be used to calculate the revenue from a tidal stream turbine.[citation needed] By substituting variables such as size of the turbine, flow velocity and price into the equation it is possible to accurately predict an annual return.
Keeping in mind this equation does not include the cost of civil infrastructure which would vary with manufacturer and from site to site.
In order to calculate the revenue that a tidal stream generator would return the following equation can be used as a guide. Assuming 1000 meters of cabling then the following would be a close approximation.
Annual Revenue = Cp x 0.5 x ρ x A x V³ x Hr x LL x GGL x $ x Y (x 3 for shrouded turbines)
Where:
Cp = the turbine coefficient of performance (say 20% for free stream or 60% for shrouded)
ρ = the density of the water (seawater is 1025 kg/m³ or 998 kg/m³ for fresh water)
A = the sweep area of the turbine (in m²)
V³ = the velocity of the flow cubed (i.e. V x V x V)
Hr = the number of hours per day that the turbine would operate at maximum efficiency (12-22 hours for tidal and 24 for run of river)
LL* = x .95 line losses (multiply by .95 )assuming a 5% loss in a cable run of 1000 meters. This may vary by manufacturer.
Gearbox and Generator Losses* = x .95 (multiply by .95) assuming 5% for gearbox and generator losses
$ = the price per watthour that would be paid (prices vary with location)
Year = 350 days (allowing 15 days per year for maintenance if necessary)
Shrouded turbine produce approximately 3 times as much revenue.[citation needed]
For example, a tidal stream turbine with a sweep area of 1m² at a site with a 3 m/s flow velocity, operating at maximum output for 12 hours, and earning 10 cents per kilowatthour would earn
Annual Revenue = Cp x 0.5 x ρ x A x V³ x Hr x LL x GGL x $ x Y
Annual Revenue = 0.20 x 0.5 x 1025 x V³ x 12 x 0.95 x 0.95 x 0.10/1000 x 350
Revenue Revenue = $1,049.02 (or $3,147.06 for a shrouded turbine)
Keeping in mind this is only a 1m² sized turbine, in 3m/s flow velocity for only 12 hours per day. Many commercial turbines are 20-30 times or greater in size, in faster flow velocity, at 20 or more hours per day. A run of river turbine would operate for as long as the river flows, which is obviously 24 hours per day.
From the above equation it can be demonstrated that the predictability of tidal power holds very great potential and interest for renewable investment dollars. Wind and solar are unpredictable by nature, but tidal stream can be predicted years in advance, allowing businesses to plan years in advance.
As the flow velocity doubles, the revenue increases by 8 times (as power is a function of the velocity cubed). The same turbine given in the example above, if installed in a 6 m/s velocity flow, would return $8,392 (or $25,176 for a shrouded turbine) for every square meter of sweep area of the turbine.
As mentioned above, "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."
Sites with high tidal stream velocities are highly sought after when tidal power station sites are under consideration. In some instances government entities in North America have begun legislating to prevent a "gold rush" mentality.[citation needed]
Because the Earth\'s tides are caused by the tidal forces due to gravitational interaction with the Moon and Sun, and the Earth\'s rotation, tidal power is practically inexhaustible and classified as a renewable energy source.
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Artist\'s impression of the Severn Barrage and road link proposed in 1989. The scheme would have generated 6% of the UK\'s electricity supply
With only three operating plants globally Rance River, Bay of Fundy and Kislaya Guba the barrage method of extracting tidal energy involves building a barrage as in the case of the Rance River in France. The barrage turbines generate as water flows in and out the estuary bay or river. These systems are similar to a hydro dam that produces Static Head or pressure head (a height of water pressure). When the water level outside of the basin or lagoon changes relative to the water level inside, the turbines are able to produce power. 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).[citation needed]
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 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.[citation needed]
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.
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.
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. If water is raised 2 ft (61 cm) by pumping on a high tide of 10 ft (3 m), this will have been raised by 12 ft (3.7 m) at low tide. The cost of a 2 ft rise is returned by the benefits of a 12 ft rise.
Another form of energy 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.
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 (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.
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.
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.
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%[citation needed] (from pressure drop, contact with blades, cavitation, etc.). 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[citation needed].
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.
Assumptions:
Mass of the water = volume of water * specific gravity
= (area * height) of water * specific gravity
= (9 * 106 m2 * 10 m) * 1025.18 kg/m3
= 92266 * 106 kg (approx)
Energy content of the water mass = Mass of water * g * height
= 92266 * 106 kg * 9.81 m/s2 * 10 m
= 9051 * 109 J (approx)
Now we have 2 high tides and 2 low tides every day.
Therefore the total energy generation potential per day = Energy for a single tide * 4
= 9051 * 109 J
= 36 * 1012 J
Therefore, the power generation potential = Energy generation potential / time in 1 day
= 36 * 1012 J / 86400 s
= 419 MW
Since we have assumed the power conversion efficiency to be 30%, The power generated = 419 MW * 30%
= 126 MW (approx)
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.
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[4] (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.
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:
Tidal energy has an efficiency of 80% in converting the potential energy of the water into electricity,[citation needed] which is efficient compared to other energy resources such as solar power or fossil fuel power plants.
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 tonnes 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 century, 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.
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In the table, "-" indicates missing information, "?" indicates information which has not been decided
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