Ocean Tidal Fences Harnessing the Power in the Tides
A Tidal Fence is another form of tidal stream technology, which directly exploits fast flowing underwater ocean currents for energy generation. In many ways, a tidal fence installation is a cross between a tidal barrage and a tidal turbine stream system. Unlike submerged tidal turbines which are individually positioned around the oceans floor, tidal fences are composed of individual vertical-axis turbines that are mounted together within a single fence like structure, (hence its name). The purpose of a tidal fence, also known as a “caisson”, is to harness the kinetic energy of the underwater tides.
These tidal fences act like a submerged tidal barrage across an inlet or estuary, with the tidal currents being forced to flow past the turbine blades, causing them to rotate, which in turn powers generators creating free electricity. But unlike a tidal barrage, tidal fences do not block the flow of the tidal water allowing the water to continuously ebb and flow through it, making them cheaper to install than a solid concrete tidal barrage.
As its name suggests, a “tidal fence” is just that, resembling a long open steel or concrete structure. Tidal fences are used in fast flowing areas such as the channels between two land masses where it directs the sea water to the turbines when it passes through the fence. As their structure is open, tidal fences have less impact on the environment than a solid tidal barrage type wall or dam, but they can still disrupt the movement of fish or large marine animals.
To overcome this problem, wide openings between the caisson wall and the rotating turbines allow fish to swim by unlike a tidal barrage which often prevents fish from swimming in and out of a basin when the sluice gates are closed. Also, being fully open it has no effect on the movement of the waves above or on the tide or corresponding tidal sea levels either side of the tidal fence.
A Tidal Fence Example
Generally, a tidal fence uses individual vertical axis cross flow turbines, similar to Savonius Rotor or Darrieus Helicoil wind turbines, lined up below the surface of the water in a single row. As the fence is fully open, there are only slight differences in water pressure between the incoming and outgoing sides of the fence which results in a relatively slow water current speed flowing through the fence, but this is offset by a large volume of water. One way to overcome this problem is to decrease the cross-section of the opening, forcing the water to flow through a smaller opening which significantly increases the tidal current velocity through the turbines.
One advantage of a tidal fence is that all the electrical generators, machinery and cabling can be kept high and dry above the water line where they can be easily accessed for maintenance and repair. Also, unlike a tidal barrage scheme which requires a closed estuary or basin, a tidal fence can be used in open tidal channels between the mainland and a nearby island, or between two islands with sufficient tidal stream. They are also able to generate electricity as individual modules connected together, rather than just one complete installation as in the case of tidal barrage schemes.
A Tidal Fence has much less impact on the environment than other forms of oceanic tidal generation, as they do not require flooding of a basin and are significantly cheaper to install using less concrete and steel. But tidal fences are not free from all environmental or social concerns, as a caisson structure is still required.
This fence like structure can disrupt fish migration and block both local navigation and shipping. In order to reduce the dangers to fish, the tidal fence could be engineered so that the spaces between the caisson wall and the rotor foil are large enough to allow fish to pass unaffected.
There are many advantages in using tidal energy. The first advantage is that the ocean is a reliable energy source to replace the burning of fossil fuels. In addition, it is a renewable source of energy that provides no waste product such as greenhouse gases. It needs no fossil fuel to power it and it generates electricity more efficient than wind and solar power using the predictable power of the tides.
Tidal stream generation whether by using tidal turbines on the ocean floor or a tidal fence across an open channel has little environmental impact and can be built in many different places in the ocean as there is a lot of renewable tidal energy around the worlds coastlines which could be harvested to provide a source of free alternative energy.
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