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Sunday, September 23, 2007
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 tidal currents. 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.Tidal power can be classified into two types:
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. These suffer from the dual problems of very high civil infrastructure costs and environmental issues.
Tidal stream systems make use of the kinetic energy from the moving water currents to power turbines, in a similar way to wind mills use moving air. This method is gaining in popularity because of the lower cost and lower ecological impact.
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 powerThe 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 esturary 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 turbine 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]
[edit] 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.
[edit] 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.
[edit] 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. If water is raised 2 feet by pumping on a high tide of 10 feet, this will have been raised by 12 feet at low tide. The cost of a 2 foot rise is returned by the benefits of a 12-foot rise.
[edit] Two-basin schemes
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.
[edit] 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.
[edit] 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.
[edit] 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.
[edit] 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.
[edit] 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.
[edit] 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% [citation needed] (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.
[edit] Energy calculations
The energy available from barrage is dependant on the volume of water. The potential energy contained in a volume of water is :
E = hMg
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.
Simple Approximation: P=hrk, where P is power in watts, h is height in meters, r is rate in cubic meters per second, and k is 7,500 watts (assuming an efficiency factor of about 75 percent).
[edit] 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[1] 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.
[edit] 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
[edit] Energy efficiency
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.
[edit] 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.
[edit] 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.[2] 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.[3] It has 18MW 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.[4]
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.[5]
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.[6]
Tidal stream power
A relatively new technology tidal stream generators draw energy from currents in much the same way as wind turbines. The higher density of water, some 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:
The Pentland Firth in Scotland
The Channel Islands in the United Kingdom
The Cook Strait in New Zealand
The Strait of Gibraltar
The Bosporus in Turkey
The Bass Strait in Australia
The Torres Strait in Australia
The Strait of Malacca between Indonesia and Singapore
The Bay of Fundy in Canada.
East River in New York City
Vancouver Island Canada
The Strait of Magellan south of mainland Chile
The Strait of Malacca between Indonesia and Singapore
[edit] Prototypes
Several commercial prototypes have shown promise. Trials in the Strait of Messina, Italy, started in 2001[9] and Australian company Tidal Energy Pty Ltd[10] 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 LoughDuring 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.[11]
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.[12][13]
SeaGen, a commercial prototype 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.[14]
Verdant Power .[15] is running a prototype project in the East River between Queens and Roosevelt Island in New York City
[edit] Shrouded Tidal turbines
Asymetric AirfoilAn 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 [16] of 59.3%) and typically 3 times higher power output [17] than a turbine of the same size in free stream.
As shown in the CFD (Computational Fluid Dynamics) generated figure here it can be seen that a down stream low pressure (shown by the gradient lines) draws upstream flow into the inlet of the shroud from well outside the inlet of the shroud. This flow is drawn into the shroud and concentrated (as seen by the red coloured zone). This augmentation of flow velocity corresponds to a 3-4 times increase in energy available to the turbine. Therefore a turbine located in the throat of the shroud is then able to achieve higher efficiency, and an output 3-4 times the energy the turbine would be capable of if it were in open or free stream. For this reason shrouded turbines are not subject to the properties of the Betz Limit.
Considerable commercial interest has been shown in recent times in shrouded tidal turbines as it allows a smaller turbine to be used at sites where large turbines are restricted. Arrayed across a seaway or in fast flowing rivers shrouded tidal turbines are easily cabled to a terrestial 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 utilised for commercial energy production.
While the shroud may not be practical in wind, as a tidal turbine it is gaining more popularity and commercial use. A shrouded tidal turbine is mono directional and constantly needs to face upstream in order to operate. It can be floated 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. 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.
SHROUDED TURBINE ADVANTAGES.
A shroud of suitable geometry can increase the flow velocity across the turbine by 3-4 times the open or free stream velocity allowing the turbine to produce 3-4 times the power then the same turbine minus the shroud.
More power generated means greater returns on investment for investors.
The number of suitable sites is increased as sites formally to slow for commercial development become viable.
Where large cumbersome turbines are not suitable smaller shrouded turbines can be sea bed mounted in shallow rivers and esturaries allowing safe navigation of the water ways.[18]
Hidden in a shroud a turbine is less likely to to be damaged by floating debris.
Bio-fouling is also reduced as the turbine is shaded from natural light in shallow water also,
The increased velocities through the turbine effectively water blast the shroud throat and turbine clean as bio-organisms are unable to attached at increased velocities. [19]
SHROUDED TURBINE DISADVANTAGES.
All shrouded turbines are mono-directional, in order to use both flood and ebb tide they need to be yawled like a windmill on a pivot or turbtable, or suspended under a pontoon on a marine swing mooring allowing the turbine to always face upstream like a wind sock.
Shrouded turbines need to be below the mean low water level. This can be accomplished by marine mono piles to to the sea/riverbed or suspended under a pontoon where inclement surface events don't buffet the turbine.
Shrouded turbine loads are 3-4 times those of the open or free stream turbine, so a robust mounting system is necessary. However this mounting system needs to be designed in such a way as to prevent turbulence being spilled onto the turbine or high pressure waves occuring near the turbine and detuning performance. Streamling the mounts and or including structural mounts in the shroud geometry performs two functions, that of supporting the turbine and providing a net benefit of 3-4 times the power output.
[edit] Energy calculations
The energy available from these kinetic systems can be expressed as:
P = Cp x 0.5 x ρ x A x V³
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 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. [20]
[edit] 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 cycles and 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.
[edit] Source of the Energy
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. 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.[citation needed]
[edit] See also
Category:Energy by country
Run-of-the-river hydroelectricity
Wave power
World energy resources and consumption
Aquanator
[edit] 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
[edit] References
^ [1] (see for example key principles 4 and 6 within Planning Policy Statement 22)
^ [2]
^ [3]
^ [4]
^ [5]
^ Independent Online Article
^ Potential Power Source: The Ocean?
^ http://www.elektropages.ru/article/4_2006_ELEKTRO.html
^ A.D.A.Group
^ Tidal Energy
^ [6]
^ [7]
^ [8]
^ http://www.seageneration.co.uk/
^ http://www.verdantpower.com/what-initiative
^ Betz Limit
^ Brian Kirke's published article Developments in Ducted Water Turbines
^ http://www.verdantpower.com/what-initiative
^ Brian Kirke's PhD Thesis
^ http://www.cyberiad.net/library/pdf/bk_tidal_paper25apr06.pdf tidal paper on cyberiad.net
[edit] 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, [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.
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