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 to 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
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
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,
2. Vertical axis turbines. The Gorlov turbine[12] is an improved helical design which is being prototyped on a large scale in S. Korea. Neptune Renewable Energy has developed Proteus[14] 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[15]
During 2003 a 150kW oscillating hydroplane device, the Stingray, was tested off the Scottish coast.[16]
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,
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
Another device, the Hydro Venturi, is to be tested in
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
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
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
British Columbia Tidal Energy Corp. plans to deploy at least three 1.2 MW turbines in the
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
Barrage tidal power
An artistic impression of a tidal barrage, including embankments, a ship lock and caissons housing a sluice and two turbines.
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).[citation needed]
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.
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 changes tidal direction.
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 (filled first during flood generation). Therefore the available level difference — important for the turbine power produced — between the basin side and the sea side of the barrage, reduces more quickly than it would in ebb generation. Rivers flowing into the basin may further reduce the energy potential, instead of enhancing it as in ebb generation. Which of course is not a problem with the "lagoon" model, without river inflow.
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 ft (61 cm) by pumping on a high tide of 10 ft (3 m), this will have been raised by 12 ft (3.7 m) at low tide. The cost of a 2 ft rise is returned by the benefits of a 12 ft rise.
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 favorable geographies, however, which are well suited to this type of scheme.
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.
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.
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.
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.
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%. 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. The Open-Centre turbine reduces this problem allowing fish to pass through the open centre of the turbine.
Recently a run of the river type turbine has been developed in
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
Global environmental impact
A tidal power scheme is a long-term source of electricity. A proposal for the
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.
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. It has 240 MW installed capacity.
• The first 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.[35] It has 18 MW installed capacity.
• The first in- stream tidal current generator in North America (Race Rocks Tidal Power Demonstration Project) was installed at on Southern Vancouver Island in September 2006.
• A small project was built by the Soviet Union at Kislaya Guba on the
• Another 12MW project at Kislaya Guba in
•
• China is also developing a tidal lagoon near the mouth of the Yalu.
• 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 fuelled power station.
• South African energy parastatal Eskom is investigating using the
