In order to improve the performance of Darrieus turbine, the pitch angles of the blades are varied and tested in a low-speed wind tunnel. First, the pitch angles of the blades are varied with an interval of 15 degrees, to evaluate the possibility of the improvement. The tested tip-speed ratio is ranged from 0.5 to 2.8, which covers the peak of the power, and the generated torque is measured by a torque meter. However, none of the blades exceed the performance of zero-pitched blade. Then, the pitch angles of the blades are varied but with an interval of 2 degrees. As a result, it is found that the blades with pitch angle of 4 and 6 degrees give the highest torque, especially in a tip-speed ratio over 1. By changing the pitch angle of the blade from 0 to 6 degrees, the peak value of the power coefficients is improved by more than 30 percent.
An ocean current is an attractive renewable energy source for a country surrounded by sea, like Japan. In the ocean currents, a tidal current can reach to a speed above five meters per second, which has an equivalent energy with a wind of typhoon, nearly 47 meters per second. Moreover, the occurrence of a tidal current is highly periodical and easily predicted. A tidal current changes its flow direction by 180 degrees with regular intervals. The Darrieus-type water turbine is suitable for such situation because of its independency to a current direction, as well as simplicity. Actually, the Darrieus turbine is adopted in the demonstration of power generation from a tidal current at Kanmon Strait (Hiraki, et. al. 2010).
There are some drawbacks for the Darrieus-type turbine. One of them is the lower efficiency of the power generation, as compared to the horizontal-axis turbines. This is one of the reason why the Darrieus turbine is not commonly used in real situations. The attachment of a device that collects a current into the turbine is an option to enhance the efficiency (Hiraki, et. al. 2013). The incapability of self-starting is also a known problem for a turbine that uses a lift force, which is also true for Darriues turbine.
