Wind energy
Noteworthy Keyword
【Wind energy】

Takuya Homma
Professor Emeritus, The University of Tsukuba
Advisor, New Energy Research Institute

1. Wind energy resources

Wind is a phenomenon of air in motion or flow relative to the earth’s surface, especially in a horizontal direction. The energy contained in the movement of air masses is called wind energy. This movement or flow is naturally driven by solar radiative energy, therefore wind energy is regarded as a renewable energy resource. A human being has traditionally captured wind energy by means of the sails of a ship or the vane of a windmill, and currently by mechanical blades similar to airplane propellers. Wind mills have been typically used since early historic times for grinding grains such as wheat and corn.

Wind power is the electricity that can be produced by a wind turbine installed at the specified site, measured in watts. Wind power increases proportionally to both to the third power of wind speed (wind speed cubed) and to the mass density of air. Wind power density means the average quantity of wind energy per unit area passing  through a vertical plane perpendicular to the wind direction, and expressed as watts per square meter. A wind electric potential means an estimated forecast of the amount of energy that the winds at a given site or in a given area could provide if a wind energy facility were established there.

In order to decide on the site for installing a wind power facility, it is necessary to grasp the wind status around the candidate areas. A wind power class and wind rose are available for assessing the potential power output of a wind turbine. The wind power class categorizes of the quality of the wind resource based on wind power density at a given height (typically 10 meters). The wind rose is a map or diagram summarizing the frequency and strength of the wind from different compass directions, as represented by a line drawn in each direction from a common point of origin. In that diagram, the length and the thickness of the line indicate respectively the frequency and velocity of the wind in each respective direction. Similar diagrams have been used by sailors since ancient times.

2. Wind power generators

A wind turbine means any device that converts the energy of wind to electricity or mechanical power. Its structure typically consists of blades, or sails, or curved rotors  which are set radially around a horizontal or vertical revolving shaft. For the case of power generation, the revolving shaft of the wind turbine is connected to the shaft of an electric power generator so that wind or air pressure against the blades turns the  shaft and then derives a dynamo to produce electric power. There are several types of wind turbines such as propeller, darrieus, savonius, sail wing, and so on.

Propeller turbine is a collective term for wind turbines where the blades are analogous structures to an airplane propeller, and its common types have two or three blades. The blades may be fixed or self-adjusting in the pitch angle responding to the wind velocity and the load. This type of wind turbine is most commonly used especially in power generation, because of its highest power generation efficiency. Darrieus turbines were first designed by the French engineer Georges Darrieus. They are a type of vertical axis wind turbine that have a characteristic “eggbeater” shape with two or three thin C-shaped blades meeting at the top and bottom of the axis. This type of wind turbines is considered to be suitable for meteorological situations with frequent variations in wind direction.

3. Wind farm

A wind farm(or wind park) is a cluster of wind turbines installed at a given site, and are  used for harvesting wind energy to produce and deliver bulk electric power into a grid. The term “wind farm” originates from the sense of “harvesting” wind as if it were a farm crop. A cluster of wind power generators should be connected to the power grid system as a single electric power station, of which generation capacity may be in the order of hundreds of MW. Hence, modern wind farms are required to be responsible for control, stability, and power balance in the power system. Wind farms occasionally need batteries for electrical storage of electricity within the local grid, and farms are installed offshore as well as on land.

Reference

“Dictionary of Energy” by Cutler J. Cleveland and Christopher Morris


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