Tidal power
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Tidal Energy sometimes called tidal power is the power achieved by capturing the energy contained in moving water currents tides and open ocean currents. There are two types of energy systems that can be used to extracted energy: kinetic energy The moving water of rivers tides and open ocean currents and the rise and fall of the tides that uses the height difference between ebbing and surging tides and potential energy from the difference in height (or head) between high and low tides. The former method - generating energy from tidal currents - is considered much more feasible today then building ocean-based dams or barrages that flood eco systems and are expensive to build. Many coastal sites worldwide are being examined for their suitability to produce tidal (current) energy.
Tidal power is classified as a renewable energy source, because tides are caused by the orbital mechanics of the solar system and to a lesser extent the surface effect of winds and are considered inexhaustible within a human timeframe. The root source of the energy comes from the slow deceleration of the Earth's rotation. The Moon gains energy from this interaction and is slowly receding from the Earth. 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). In Europe, Tide Mills have been used for nearly 1,000 years, mainly for grinding grains.
The efficiency of tidal power generation in ocean dams largely depends on the amplitude (height of the rise and fall) of the tidal swell, which can be up to 10 m (33 ft) where the periodic tidal waves funnel into rivers and fjords and extreme water velocities can be 16 knots (Vancouver Island Canada). Amplitudes of up to 17 m (56 ft) occur for example in the Bay of Fundy, where tidal resonance amplifies the tidal waves.
As 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 the acceleration due to gravity at the Earth's surface. 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).
Several smaller tidal power plants have recently started generating electricity in CanadaNorway. They all exploit the strong periodic tidal currents in narrow fjords using sub-surface water turbines.
[edit] Tidal energy jet engine type turbine
The emerging technology that holds the most commercial promise is the jet engine type turbine in Australia developed by inventors Davidson and Hill working with company Tidal Energy Pty Ltd. [[1]] This Patented new and innovative design has the capability of using any turbine design and improving the turbine efficiency or output power by 3 to 4 times. This done by placing the turbine in a shroud or duct that causes flow to be sucked across the turbine allowing it to harvest more energy then would otherwise be possible.
[edit] Commercial attention is shifting to the new breed of eco friendly turbines
The commercial focus is shifting toward far more cost effective systems that use the kinetic energy in moving water using sub-surface (underwater wind mills) arrays to harness the huge largely untapped resource of moving water.
Moving water is the greatest renewable energy resource on the planet. It is 832 times more dense than air and more reliable than wind mills or solar panels. It is impossible to accurately predict if the wind will blow in the next 5 minutes or if the sun will shine tomorrow, but the predictability of moving water in tides and ocean currents makes water energy turbines systems attractive to commercialisation.
The energy available from these kinetic systems can be expressed as,
Cp = 0.5.ρ.A.V3
Where Cp is the turbine coefficient of performance,
ρ = the density of the water (seawater is 1025 kg per cubic meter)
A = the sweep area of the turbine
V3 = the velocity of the flow cubed (i.e. V x V x V)
Modern advance in turbine technology may eventually see whole countries supplied with energy from the oceans using this method. 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 it occurs almost anywhere where there are entrances to bays and rivers or between islands where water currents are concentrated.
A factor in human settlement geography is water. Human settlements have always sprung up around bays rivers and lakes. Future settlement will be concentrated around moving water allowing communities to avail themselves of the clean green unpolluting energy from moving water.
[edit] Weir
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. In any design this leads to a decrease of tidal range inside the basin or lagoon, 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. 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 have been plagued with the dual problems of high civil infrastructure costs associated with what is in effect a dam being placed across two estuarine systems, one for the high water dam storage and the other a low water dam for the release of the storage, and, the environmental problems associated with the flooding of two ecosystems.
[edit] Modes of operation
[edit] 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.
[edit] Flood generation
The basin is filled 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 (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 and two-way generation). This energy is returned during generation
[edit] 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.
[edit] Intermittent nature of power output
Tidal power schemes don't 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 Moon's period of revolution around the Earth (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. 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.
[edit] 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
[edit] 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.
[edit] Environmental impact
[edit] Tidal 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.
[edit] 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. A tidal current turbine will have a much lower impact.
[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
Again as a result of less water exchange with the sea, the average salinity inside the basin decreases, also affecting the ecosystem. Again, 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.
With turbine generation, taking its power from the flow of the tidal stream, there will likely be a swirl of water down stream of the turbine. If this horizontal vortex touches the bottom, it will cause erosion. While the amount of sediment added to the tidal stream will likely be insignificant, this could, over time, erode the foundation of the turbine. Turbines held down with pilings would be largely immune to this problem but turbines held by heavy weights sitting on the bottom could eventually tip over.
[edit] Pollutants
Again, as a result of reduced volume, 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.
The concentrations of conservative pollutants will also increase.
[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% (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.
Using kinetic energy systems that do not close off rivers and streams to fish allow migration at times of spawning. These water current turbines typically turn very slowly at around 20-30 r.p.m., allowing fish to safely navigate either past or through the turning impellor drastically reducing or eliminating fish kills.
[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. 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.
[edit] Economic considerations
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[2] (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 its goals forward.
[edit] Resource around the world
[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 ([3]). 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[4]. 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 [5])
- 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. [6]
- 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.Independent Online Article
[edit] Tidal power schemes being considered
In the table, '-' indicates missing information, '?' indicates information which has not been decided
| Country | Place | Mean tidal range (m) | Area of basin (km²) | Maximum capacity (MW) |
| Argentina | San Jose | 5.9 | - | 6800 |
| Australia | Secure Bay | 10.9 | - | ? |
| Canada | Cobequid | 12.4 | 240 | 5338 |
| Cumberland | 10.9 | 90 | 1400 | |
| Shepody | 10.0 | 115 | 1800 | |
| India | Kutch | 5.3 | 170 | 900 |
| Cambay | 6.8 | 1970 | 7000 | |
| Korea | Garolim | 4.7 | 100 | 480 |
| Cheonsu | 4.5 | - | - | |
| Mexico | Rio Colorado | 6-7 | - | ? |
| Tiburon | - | - | ? | |
| United Kingdom | Severn | 7.8 | 450 | 8640 |
| Mersey | 6.5 | 61 | 700 | |
| Strangford Lough | - | - | - | |
| Conwy | 5.2 | 5.5 | 33 | |
| United States | Passamaquoddy Bay | 5.5 | - | ? |
| Knik Arm | 7.5 | - | 2900 | |
| Turnagain Arm | 7.5 | - | 6501 | |
| Russia[7] | Mezen | 9.1 | 2300 | 19200 |
| Tugur | - | - | 8000 | |
| Penzhinskaya Bay | 6.0 | - | 87000 | |
| South Africa | Mozambique Channel | ? | ? | ? |
[edit] See also
[edit] External links
- Directory of Tidal Power Resources
- 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
- Tidal Power at PESWiki
[edit] 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.
[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
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