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The fins of a windmill rotate
in a vertical plane which is
kept perpendicular to the

wind by means of a tail fin.
As wind flow crosses the
blades of the windmill, it is

forced to rotate and can be
used to generate electricity.
Another type of wind power
generator is the two hollow

half-drum-type wind
collectors. This wind

Figure 5.13 collector rotates in a single
vertical axis, making this
device independent of the wind direction, which may generate more
electricity.

A wind farm is a group of wind turbines in the same location
used to produce electric power. A large wind farm (fig.5.13) may
consist of several hundred individual wind turbines, and cover an

extended area of hundreds of square miles, but the land between the
turbines may be used for agricultural or other purposes. A wind farm
may also be located offshore.
Many of the largest operational onshore wind farms are located

in the USA and China. The Gansu Wind Farm in China has over 5,000
MW installed with a goal of 20,000 MW. The Alta Wind Energy
Center in California is the largest onshore wind farm with power of

1020 MW of power
Tidal power is extracted from the Earth's oceanic tides; tidal
forces are periodic variations in gravitational attraction exerted by
celestial bodies. These forces create corresponding motions or currents

in the world's oceans. Due to the strong attraction to the oceans, a bulge
in the water level is created, causing a temporary increase in sea level.
When the sea level is raised, water from the middle of the ocean is

forced to move toward the shorelines, creating a tide. This occurrence
takes place in an unfailing manner, due to the consistent pattern of the
moon’s orbit around the Earth. The magnitude and character of this

motion reflects the changing positions of the Moon and the Sun relative
to the Earth, the effects of Earth's rotation, and local geography of the

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