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You must be familiar with the fact that turbines are used to convert the kinetic energy of flowing water (actually the word gushing would be more appropriate). The conventional turbines are useful for structures involving dams, but utilizing the energy of free unconstrained tidal currents requires a different design concept. If you ask why it is so, the reason is pretty simple. In dams, the water is stored and then passed on through the turbines in a controlled fashion and in a particular direction and angle of motion, but inside the ocean surface where a turbine simply lies bare to the tides and ocean currents, the turbine has to be versatile to handle all sorts of flow conditions, hence the reason for special design considerations. Helical turbines are one of these special types of turbines used in tidal power plants which do not utilize conventional dam structure to store water but make use of the random tidal currents to generate electricity.
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The Helical Turbines
The word helical refers to something which is in the shape of a helix, which means to say that it is in the form of a wound spiral. You can see the figure 1 below which depicts the mathematical description of a helix. So you must have understood by now that the helical turbine is having some similar type of shape, and you are right in guessing so. Helical turbines come in different makes and designs but the basic principle of operation is the same. We will talk about one such turbine namely the Gorlov Helical turbine, which was designed and patented by Professor Gorlov of Boston.
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Design of the Gorlov Turbine
The design of the blades of the Gorlov helical turbine can be seen in figure 2 in this article. You can easily compare this shape to that of the shape shown in figure 1, and figure out the helical shape. The blades are shaped like the threads of a screw which run over a cylinder and it is a cross flow turbine. Due to the shape of the blades a reaction thrust is exerted on the turbine which means that the turbine can rotate faster than what the normal flowing speed of water would make it to spin at, had there been no reaction thrust.
The requisite feature of this turbine is that the axis of rotation should be at right angles to the direction of flow of current. This means that for a given setup the turbine will rotate in the same direction even if the flow of water is reversed in direction. Since this is exactly what happens in case of tides and tidal currents, it is ideal for use in such situations.
Although two blades are shown in the figure, normally there could be more and these turbines that have been approved for use in Uldolmol Strait tidal power plant project in Korea consist of three such helical blades.