Wind energy
Wind energy from air in motion. Air in motion arises from pressure gradient.
Solar radiation heats the air near the equator, and this low density heated air is buoyed up.
At the surface it is displaced by cooler more dense higher pressure air flowing from the poles.
In the upper atmosphere near the equator the air thus tends to flow back toward the poles and away from the equator.
The net result is a global convective circulation with surface winds from north to south in the northern hemisphere.
During day light, the land heats up rapidly compared to the nearby sea or water bodies and there tends to be surface wind flowing from the water to the land.
At night
wind reverses, because the land surfaces cool faster than the water.
Local winds are
caused by two mechanisms.
The air above slopes heats up during the day and cools down at night, more rapidly than the air above the lands.
This causes heated air of the day to rise along the slopes
and relatively cool heavy air to flow at night.
So wind energy is the kinetic energy associated with movement of large masses of air.
It is estimated than 1% of all solar radiation falling on the face of the earth is converted into kinetic energy of the atmosphere.
30% of it occurs in the lowest 1000 m of elevation. It is thus an indirect form of solar energy.
Wind energy is harnessed as mechanical energy with the help of wind turbine.
The mechanical energy thus obtained can either be used as such to operate farm appliances, water pumping or converted into electric power and used locally or fed to a grid.
Very slow winds are useless, having no possibility of power generation.
On the other hand very strong stormy winds cannot be utilized due to safety of turbine.
Moderate to high speeds winds, typically from 5 m/s to
about 25 m/s are considered for most wind turbines.
Basic principles of wind energy conversion
Nature of wind
The circulation of air in the atmosphere is caused the non uniform heating of the earth by the sun.
In general, during the day the air above the land mass tends to heat up more rapidly than the air over water.
In the coastal regions this manifests itself in a strong onshore wind.
At night the process is reversed because the air cools down more rapidly over the land and the breeze therefore flows off the shore.
Cool surface air sweeps down from the poles forcing the warm air over the tropics to rise.
But the direction of the massive air movements is affected by the rotation of the earth and the net effect is a counter clockwise circulation of air in the southern hemisphere. The strength and direction of the planetary winds change with the seasons as the solar input varies.
Despite the wind’s intermittent nature, wind patterns at any particular site remain remarkably constant year by year.
Average speeds are greeter in hilly and coastal areas than they are in land. The winds also tend to blow more consistently and with greater strength over the surface of the water where is less surface drag.
Winds speeds increase with height. They have traditionally been measured at a standard height of ten meter where they are found to be 20-25% greater than close to the surface.
At a height of 60 m they may be 30-60%
higher because if the reduction in the drag effect of the earth surface.
Power in the wind
Wind possesses energy by virtue of its motion. Three factors
determine the output from a wind energy converter.
They are,
1) 1)The wind speed
2) 2)The cross section of wind swept by rotor, and
3) 3)The overall conversion efficiency of the rotor,
transmission system and generator or pump.
No device however well designed can extract all the wind
energy. A 100% efficient Aero Generator would be able to convert up to maximum
of around 60% of the available energy in the wind into mechanical energy. Well
Designed blades will typically 70% of the theoretical maximum. But the losses
incurred in the gearbox, transmission system and generator or pump could
decrease overall wind turbine efficiency to 35%.
The power in the wind can be computed by using the concept
of kinetics. The wind mill works on the principle of converting kinetic energy
of the wind to mechanical energy.
We know that power is equal to energy per unit time. The
energy available is the kinetic energy of the wind.
Wind energy conversion
The term Wind mill is widely used to describe wind energy
conversion system. Modern wind energy conversion system are more correctly
referred to as “WECS” Aero Generators, wind turbine generators or simply wind
turbines.
Operators
or users of wind turbines must ensure that there is some form of back up to
cover periods when there is insufficient wind available.
Small producers
Private Citizens in several countries have won the right to
operate wind generators, and to export power to the grid.
The output of most small wind generators is “Condition” so
that it confirms to the frequency and phase of the mains supply.
Only a few small units are designed to maintain a constant
rotational rate so that can be synchronized to the mains frequency and feed
electricity directly into the grid.
Power conditioning is readily achieved using an electronic
black box called a “synchronous” inverter.
Large producers
Large and
medium sized wind generators are designed to give a stable and constant
electrical output over a wind range of winds speeds and to feed current
directly into the grid.
The choice of generating type depends on the size of the
local distribution grid and it associated generating capacity.
Normally an induction generator would be used. Induction
generators are robust and reliable and require minimal control equipment.
For high degree of autonomous control, a synchronous
generator is used.
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