1. Field of the Invention
The present invention relates to a horizontal axis wind turbine and a method for measuring an upflow angle.
2. Description of Related Art
In recent years, horizontal axis wind turbines have been in practical use in order to gain electric power from natural wind. The performance of the horizontal axis wind turbines is shown by a power curve representing relationship between the wind speed at hub (the wind speed at the rotational central portion of a rotor of a horizontal axis wind turbine) and power (production of electricity) in general. The production of electricity leading directly to profitability is predicted based on the power curve and the wind speed at hub estimated by simulation or observed in advance.
Wind-generated electricity systems are often installed in complex topography. In such topography, the wind speed varies with not only the height from ground but also the horizontal position, and in addition, an upflow wind is often generated. The power generation and structure damage are affected by not only horizontal component of the wind but also vertical. Therefore it is meaningful to measure 3-D wind speed with consideration of “upflow angle” for improvement of predictability of the production of electricity.
3-D ultrasonic anemometers and Pitot tubes are proposed as earlier developments for measuring 3-D wind speed with consideration of a “upflow angle”. (For example, see Kaijo Corp. “measurement and control system business—atmospheric apparatus” [online] 1997, Kaijo Corp.)
The above-described 3-D ultrasonic anemometers, however, have a problem that it is expensive and large. An anemometer for measuring the wind speed at hub of a horizontal axis wind turbine requires durability to endure even in a relative harsh environment for long periods without maintenance, whereas 3-D ultrasonic anemometers and Pitot tubes are not produced on an assumption of such an operating environment. Therefore they have reservations about durability and lack reliability.