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Tidal stream generators draw energy from water currents in much the same way as wind turbines draw energy from air currents. However, the potential for power generation by an individual tidal turbine can be greater than that of similarly rated wind energy turbine. The higher density of water relative to air (water is about 800 times the density of air) means that a single generator can provide significant power at low tidal flow velocities compared with similar wind speed. Given that power varies with the density of medium and the cube of velocity, it is simple to see that water speeds of nearly one-tenth of the speed of wind provide the same power for the same size of turbine system; however this limits the application in practice to places where the tide moves at speeds of at least 2 knots (1 m/s) even close to neap tides. Furthermore, at higher speeds in a flow between 2 to 3 metres per second in seawater a tidal turbine can typically access four times as much energy per rotor swept area as a similarly rated power wind turbine.
Types of tidal stream generators
Since tidal stream generators are an immature technology, 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, but they have not operated commercially for extended periods to establish performances and rates of return on investments.
The European Marine Energy Centre categorises them under four heads although a number of other approaches are also being tried.
These are close in concept to traditional windmills operating under the sea and have the most prototypes currently operating. These include:
The AR-1000, a 1MW tidal turbine developed by Atlantis Resources Corporation which was successfully deployed and commissioned at the EMEC facility during the summer of 2011. The AR series turbines are commercial scale Horizontal Axis Turbines designed for open ocean deployment in the harshest environments on the planet. AR turbines feature a single rotor set with highly efficient fixed pitch blades. The AR turbine is rotated as required with each tidal exchange. This is done in the slack period between tides and fixed in place for the optimal heading for the next tide. AR turbines are rated at 1MW @ 2.65m/s of water flow velocity.
Kvalsund, south of Hammerfest, Norway. Although still a prototype, a turbine with a reported capacity of 300 kW was connected to the grid on 13 November 2003.
A 300 kW Periodflow marine current propeller type turbine — Seaflow — was installed by Marine Current Turbines off the coast of Lynmouth, Devon, England, in 2003. The 11m diameter turbine generator was fitted to a steel pile which was driven into the seabed. As a prototype, it was connected to a dump load, not to the grid.
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 was 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.
Following the Seaflow trial, a full-size prototype, called SeaGen, was installed by Marine Current Turbines in Strangford Lough in Northern Ireland in April 2008. The turbine began to generate at full power of just over 1.2 MW in December 2008 and is reported to have fed 150 kW into the grid for the first time on 17 July 2008, and has now contributed more than a gigawatt hour to consumers in Northern Ireland. It is currently the only commercial scale device to have been installed anywhere in the world. SeaGen is made up of two axial flow rotors, each of which drive a generator. The turbines are capable of generating electricity on both the ebb and flood tides because the rotor blades can pitch through 180˚.
OpenHydro, an Irish company exploiting the Open-Centre Turbine developed in the U.S., has a prototype being tested at the European Marine Energy Centre (EMEC), in Orkney, Scotland.
A prototype semi-submerged floating tethered tidal turbine called Evopod has been tested since June 2008[ in Strangford Lough, Northern Ireland at 1/10 scale. The company developing it is called Ocean Flow Energy Ltd, and they are based in the UK. The advanced hull form maintains optimum heading into the tidal stream and it is designed to operate in the peak flow of the water column.
Tenax Energy of Australia is proposing to put 450 turbines off the coast of the Australian city Darwin, in the Clarence Strait. The turbines feature a rotor section that is approximately 15 metres in diameter with a gravity base which is slighter larger than this to support the structure. The turbines will operate in deep water well below shipping channels. Each turbine is forecast to produce energy for between 300 and 400 homes.
Tidalstream, a UK-based company, has commissioned a scaled-down Triton 3 turbine in the Thames, see picture on the right, and photographs maintained on their website. It can be floated out to site, installed without cranes, jack-ups or divers, and then ballasted into operating position. At full scale the Triton 3 in 30-50m deep water has a 3MW capacity, and the Triton 6 in 60-80m water has a capacity of up to 10MW, depending on the flow. Both platforms have man-access capability both in the operating position and in the float-out maintenance position. http://en.wikipedia.org/wiki/Space-based_solar_power
As with wind power, selection of location is critical 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 are under serious consideration:
- Pembrokeshire in Wales
- River Severn between Wales and England
- Cook Strait in New Zealand
- Kaipara Harbour in New Zealand
- Bay of Fundy in Canada.
- East River in the USA
- Golden Gate in the San Francisco Bay
- Piscataqua River in New Hampshire
- The Race of Alderney and The Swinge in the Channel Islands
- The Sound of Islay, between Islay and Jura in Scotland
- Pentland Firth between Caithness and the Orkney Islands, Scotland
- Humboldt County, California in the United States
- Columbia River, Oregon in the United States
- Taiwan in the East Coast
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 Southeast 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.
Very little direct research or observation of tidal stream systems exists. Most direct observations consist of releasing tagged fish upstream of the device(s) and direct observation of mortality or impact on the fish.
One study of the Roosevelt Island Tidal Energy (RITE, Verdant Power) project in the East River (New York City), utilized 24 split beam hydroacoustic sensors (scientific echosounder) to detect and track the movement of fish both upstream and downstream of each of six turbines. The results suggested (1) very few fish using this portion of the river, (2) those fish which did use this area were not using the portion of the river which would subject them to blade strikes, and (3) no evidence of fish traveling through blade areas.Work is currently being conducted by the Northwest National Marine Renewable Energy Center (NNMREC )to explore and establish tools and protocols for assessment of physical and biological conditions and monitor environmental changes associated with tidal energy development. (Original.....http://en.wikipedia.org/wiki/Space-based_solar_power