How to use tidal energy?
Tidal range may vary over a wide range (4.5-12.4 m) from site to site. A tidal range of at least 7 m is required for economical operation and for sufficient head of water for the turbines. Traditional tidal electricity generation involves the construction of a barrage across an estuary to block the incoming and outgoing tide. The dam includes a sluice that is opened to allow the tide to flow into the basin; the sluice is then closed, and as the sea level drops, the head of water (elevated water in the basin) using traditional hydropower technology, drives turbines to generate electricity. Barrages can be designed to generate electricity on the ebb side, or flood side, or both. Tidal range may vary over a wide range (4.5-12.4 m) from site to site.
There are many ways to generate electricity from the huge power of the tides: building tidal barrages, eploiting naturally occuring tidal streams, and tidal fences (a scaled down version of the tidal barrage). However, all of these techniques require huge financial investment and have very long pay back periods. In addition there are well documented negative environmental effects which have to be considered.
The technology required to convert tidal energy into electricity is very similar to the technology used in traditional hydroelectric power plants. The first requirement is a dam or “barrage” across a tidal bay or estuary. The best tidal sites are those where a bay has a narrow opening, thus reducing the length of dam which is required. At certain points along the dam, gates and turbines are installed.. When there is an adequate difference in the elevation of the water on the different sides of the barrage, the gates are opened. This “hydrostatic head” that is created, causes water to flow through the turbines, turning an electric generator to produce electricity. Electricity can be generated by water flowing both into and out of a bay. As there are two high and two low tides each day, electrical generation from tidal power plants is characterized by periods of maximum generation every twelve hours, with no electricity generation at the six hour mark in between. Alternatively, the turbines can be used as pumps to pump extra water into the basin behind the barrage during periods of low electricity demand. This water can then be released when demand on the system its greatest, thus allowing the tidal plant to function with some of the characteristics of a “pumped storage” hydroelectric facility.
This leads us to look at tidal turbines. A tidal turbine acts underwater in a very similar way to how wind turbines operate in the air. Water is some 800 times denser than air, and so even slow moving tides can exert much greater forces than the wind on a turbine. Therefore a working tidal turbine can have much smaller diameter rotors than an equivalent power output wind turbine keeping costs of manufacture and transportation down.
The tides are much more predictable than the wind and so can be depended upon to supply power at certain times of the day. Wind turbines on the other hand depend on the vaguaries of the weather.
Tidal turbines are relatively cheap to manufacture. Any number of turbines can be built and installed at any time and so an underwater tidal turbine farm can be constructed and extended as and when funding is available. Underwater turbines are by definition out of sight and it is believed that their environmental impact is negligable thanks to their slow rotation rates of just 10-30 revolutions per minute (10 times slower than that of ships propellers). In addition, tidal turbines do not affect navigation or shipping and so there are no hidden extra costs to consider.
A tidal turbine is a method of generating electricity from tidal flows. It uses a turbine strategically placed in a tidal area to change the mechanical energy of the moving water into electrical energy which is then distributed to consumers.
There are three major type of tidal turbines: bulb turbines, rim turbines and tubular turbines.
A bulb turbine is a turbine that has the generator inside it. This means that it is very efficient in terms of space and size–by putting the generator inside the turbine, the need for a separate housing unit is eliminated, which means the turbine is less of an eyesore.
On the other hand, these turbines cannot be maintained without stopping the flow of water to them. This means that when the inevitable maintenance needs to be carried out the turbine will have to stop producing power for the duration of the maintenance.
A rim turbine’s generator is separate from the turbine itself, and is connected through a shaft that moves with the turbine. This means that the generator can be maintained easier. However, the more-delicate nature of the rim turbine means that it is harder to pump water through it and regulate the amount of power it produces. Where the bulb turbine is hardy, yet difficult to maintain, the rim turbine is precisely the opposite.
Tubular turbines are the turbines already commonly used in hydroelectric power generation. Rather than have the generator mounted directly on top of the turbine, it is mounted at a 45-degree angle from it.
However, the real advantage of a tubular turbine is that the blades can be adjusted. This means that they can be changed to meet electricity demand; smaller blades will generate less power while larger blades will generate more power. This allows the turbine to run more efficiently, generating only the amount of power it needs to without excessive wear and tear.