Considering the tidal-current power generation in a narrow strait, a water turbine of vertical axis is preferred, since the multiple-stage system is applicable in the depth direction. The straight bladed vertical axis water turbine (S-VAWT) is appropriate in such system because of its simplicity in manufacture. On purpose to improve the performance of the S-VAWT a Gurney flap was attached on the trailing edge of the rotor blades. The modified turbine was tested in both the wind and water tunnels. Consequently, the attachment of the Gurney flap on the lower surface is effective to enhance the dynamic characteristics of the S-VAWT.

Tidal Current

Considering to stabilize energy supply and to protect the earth environment, the importance of the technologies of natural energies is getting higher on these days. Among them, ocean energy has received greater attention in natural energies because Japan is surrounded by the sea. The methods of power generation using ocean energy include tidal current, ocean current, tidal, wave and ocean thermal power generation. A tidal current is a flow of seawater characterized by strong regularity. The flow velocity curve in Kanmon Strait was shown in Fig.1, for example (Japan Coast Guard hydrographic office, 1974). The tidal current reverses its direction every six hours, and the current velocity varies like a sine wave. The tidal current energy is regarded as stable source of energy because the velocity changes are predictable to some extent from the positional relation of the earth, moon and sun. In addition, as a density of seawater is 800 times greater than that of air, the current velocity of 1 m/s corresponds to the wind speed of 9 m/s in air because fluid energy is proportional to the density and the cube of the current velocity.

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