What Are Tidal Stream Generators?
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 at low tidal flow velocities (compared with 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 (1m/s) even close to neap tides. The SeaGen rotors in Harland and Wolff, Belfast, before installation in Strangford Lough
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, but they have not operated commercially for extended periods to establish performances and rates of return on investments.
Engineering Approaches
The European Marine Energy Centre categorises them under four heads: The world's first commercial tidal stream generator — SeaGen — in Strangford Lough. The strong wake shows the power in the tidal current. A number of other approaches are being tried.
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 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 a variant of the Darrieus design featuring a helical design which is being commercially piloted 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.
The 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 shrouded turbine for a remote Australian community in northern Australia where there are some of the fastest flows ever recorded (11 m/s, 21 knots) – two small turbines will provide 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.
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.
Trials in the Strait of Messina, Italy, started in 2001 of the Kobold concept.
Commercial Plans
RWE's npower announced that it is in partnership with Marine Current Turbines to build a tidal farm of SeaGen turbines off the coast of Anglesey in Wales.
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
Energy calculations
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:
- P = Cp x 0.5 x ρ x A x V³
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 efficiencies as much as 3 to 4 times the power of the same turbine in open flow.
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