Although not yet widely used, tidal power has potential for future electricity generation. Tides are more predictable than wind energy and solar power. Historically, tide mills have been used, both in Europe and on the Atlantic coast of the USA. The earliest occurrences date from the Middle Ages, or even from Roman times.
Tidal energy is generated by the relative motion of the Earth, Sun and the Moon, which interact via gravitational forces. Periodic changes of water levels, and associated tidal currents, are due to the gravitational attraction by the Sun and Moon. The magnitude of the tide at a 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 and coastlines.
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.
A tidal energy generator uses this phenomenon to generate energy. The stronger the tide, either in water level height or tidal current velocities, the greater the potential for tidal energy generation.
Tidal movement causes a continual loss of mechanical energy in the Earth-Moon system due to pumping of water through the natural restrictions around coastlines, and due to viscous dissipation at the seabed and in turbulence. This loss of energy has caused the rotation of the Earth to slow in the 4.5 billion years since formation. During the last 620 million years the period of rotation has increased from 21.9 hours to the 24 hours we see now; in this period the Earth has lost 17% of its rotational energy. Tidal power may take additional energy from the system, increasing the rate of slowing over the next millions of years.
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 but also from the major thermal current systems such as the Gulf Stream, which is covered by the more general term marine current power. 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.
Since tidal stream generators are an immature technology (no commercial scale production facilities are yet routinely supplying power), no standard technology has yet emerged as the clear winner, but a large variety of designs are being experimented with, some very close to large scale deployment. Several prototypes have shown promise with many companies making bold claims, some of which are yet to be independently verified, or operated commercially for extended periods to establish performances and rates of return on investments.
1. Horizontal axis turbines. These are close in concept to traditional windmills operating under the sea and have the most prototypes currently operating. These include:
Kvalsund, south of Hammerfest, Norway. Although still a prototype, a turbine, generating 300 kW, started supplying power to the community on November 13, 2003.
A 300 kW Periodflow marine current propeller type turbine was tested off the coast of Devon, England in 2003.
Since April 2007 Verdant Power has been running a prototype project in the East River between Queens and Roosevelt Island in New York City; it is the first major tidal-power project in the United States. The strong currents pose challenges to the design: the blades of the 2006 and 2007 prototypes broke off, and new reinforced turbines were installed in September 2008.
A fullsize prototype, called SeaGen, has been installed by Marine Current Turbines Ltd in Strangford Lough in Northern Ireland in April 2008. The turbine is expected to generate 1.2 MW and was reported to have fed 150kW into the grid for the first time on July 17, 2008. It is currently the only commercial scale device to have been installed anywhere in the world.
OpenHydro an Irish based company, exploiting the Open-Centre Turbine turbine developed in the US, has a prototype being tested at the European Marine Energy Centre (EMEC), in Orkney, Scotland.
2. Vertical axis turbines. The Gorlov turbine is an improved helical design which is being prototyped on a large scale in S. Korea. Neptune Renewable Energy has developed Proteus which uses a barrage of vertical axis crossflow turbines for use mainly in estuaries.
3. Oscillating devices. These don't use rotary devices at all but rather aerofoil sections which are pushed sideways by the flow.
Oscillating stream power extraction was proven with the omni or bi-directional Wing'd Pump windmill
During 2003 a 150kW oscillating hydroplane device, the Stingray, was tested off the Scottish coast.
4. Venturi effect. This uses a shroud to increase the flow rate through the turbine. These can be mounted horizontally or vertically.
Australian company Tidal Energy Pty Ltd undertook successful commercial trials of highly efficient shrouded tidal turbines on the Gold Coast, Queensland in 2002.
Tidal Energy Pty Ltd has commenced a rollout of their efficient shrouded turbine for a remote Australian community in northern Australia where there exist some of the fastest flows ever recorded (11 m/s, 21 knots) – two small turbines will provided 3.5 MW.
Another larger 5 meter diameter turbine, capable of 800 kW in 4 m/s of flow, is planned for deployment as a tidal powered desalination showcase near Brisbane Australia in October 2008.
Another device, the Hydro Venturi, is to be tested in San Francisco Bay.
A number of other approaches are being tried.
In late April 2008, Ocean Renewable Power Company, LLC (ORPC) successfully completed the testing of its proprietary turbine-generator unit (TGU) prototype at ORPC’s Cobscook Bay and Western Passage tidal sites near Eastport, Maine . The TGU is the core of the OCGen™ technology and utilizes advanced design cross-flow (ADCF) turbines to drive a permanent magnet generator located between the turbines and mounted on the same shaft. ORPC has developed TGU designs that can be used for generating power from river, tidal and deep water ocean currents.
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.
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.
Nova Scotia Power has selected OpenHydro's turbine for a tidal energy demonstration project in the Bay of Fundy, Nova Scotia, Canada and Alderney Renewable Energy Ltd for the supply of tidal turbines in the Channel Islands. Open Hydro
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:
Relative to an open turbine in free stream, shrouded turbines are capable of efficiencies as much as 3 to 4 times the power of the same turbine in open flow.
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:
With only three operating plants globally (a large 240 MW plant on the Rance River, and two small plants, one on the Bay of Fundy and the other across a tiny inlet in Kislaya Guba Russia), the barrage method of extracting tidal energy involves building a barrage across a bay or river as in the case of the Rance tidal power plant in France. Turbines installed in the barrage wall generate power as water flows in and out of the estuary basin, 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).
The basic elements of a barrage are caissons, embankments, sluices, turbines, and ship locks. Sluices, turbines, and ship locks are housed in caissons (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.
Recently a run of the river type turbine has been developed in France. This basically is a very large slow rotating Kaplan type turbine mounted on an angle. Testing for fish mortality has indicated much lower mortality figures, less than 5%. This concept seems very suitable for adaption to marine current/tidal turbines also.
The energy available from barrage is dependent on the volume of water. The potential energy contained in a volume of water is:
The factor half is due to the fact, that as the basin flows empty through the turbines, the hydraulic head over the dam reduces. The maximum head is only available at the moment of low water, assuming the high water level is still present in the basin.
Mass of the water = volume of water × specific gravity
A barrage is 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.
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 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.
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:
If fossil fuel resources decline during the 21st century, as predicted by Hubbert peak theory, tidal power is one of the alternative sources of energy that will need to be developed to satisfy the human demand for energy.
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)|
|Passamaquoddy Bay, Maine||5.5||-||?|
|Knik Arm, Alaska||7.5||-||2900|
|Turnagain Arm, Alaska||7.5||-||6501|
|Golden Gate, California||?||-||?|