Wind Energy - A short descripion

All renewable energy (except tidal and geothermal power), ultimately comes from the sun. The earth receives 1.74 x 1017 watts of power (per hour) from the sun About one or 2 percent of this energy is converted to wind energy which is about 50-100 times more than the energy converted to biomass by all plants on earth Differential heating of the earth’s surface and atmosphere induces vertical and horizontal air currents that are affected by the earth’s rotation and contours of the land  WIND. Winds are influenced by the ground surface at altitudes up to 100 meters Wind is slowed by the surface roughness and obstacles. When dealing with wind energy, we are concerned with surface winds. A wind turbine obtains its power input by converting the force of the wind into a torque (turning force) acting on the rotor blades. The amount of energy which the wind transfers to the rotor depends on the density of the air, the rotor area, and he wind speed. The kinetic energy of a moving body is proportional to its mass (or weight). The kinetic energy in the wind thus depends on the density of the air, i.e. its mass per unit of volume. In other words, the "heavier" the air, the more energy is received by the turbine .At 15° Celsius air weighs about 1.225 kg per cubic meter, but the density decreases slightly with increasing humidity. A typical 600 kW wind turbine has a rotor diameter of 43-44 meters, i.e. a rotor area of some 1,500 square meters. The rotor area determines how much energy a wind turbine is able to harvest from the wind. Since the rotor area increases with the square of the rotor diameter, a turbine which is twice as large will receive 22 = 2 x 2 = four times as much energy. To be considered a good location for wind energy, an area needs to have average annual wind speeds of at least 12 miles per hour.

DESIGN

A Windmill captures wind energy and then uses a generator to convert it to electrical energy. The design of a windmill is an integral part of how efficient it will be when designing a windmill, one must decide on the size of the turbine, and the size of the generator.

Wind Turbine Generators

Wind power generators convert wind energy (mechanical energy) to electrical energy. The generator is attached at one end to the wind turbine, which provides the mechanical energy. At the other end, the generator is connected to the electrical grid the generator needs to have a cooling system to make sure there is no overheating.

SMALL GENERATORS

Require less force to turn than larger ones, but give much lower power output. Less efficient i.e if you fit a large wind turbine rotor with a small generator it will be producing electricity during many hours of the year, but it will capture only a small part of the energy content of the wind at high wind speeds.

LARGE GENERATORS

Very efficient at high wind speeds, but unable to turn at low wind speeds. i.e.. If the generator has larger coils, and/or a stronger internal magnet, it will require more force (mechanical) to start in motion. A windmill built so that it too severely interrupts the airflow through its cross section will reduce the effective wind velocity at its location and divert much of the airflow around itself, thus not extracting the maximum power from the wind. At the other extreme, a windmill that intercepts a small fraction of the wind passing through its cross section will reduce the wind’s velocity by only a small amount, thus extracting only a small fraction of the power from the wind traversing the windmill disk. Modern Windmills can attain an efficiency of about 60 % of the theoretical maximum.

Importance of Wind Speed for wind turbine 
No other factor is more important to the amount of power available in the wind than the speed of the wind

·         The power in wind is proportional to the cubic wind speed v^3

·         0% increase in wind speed means 73% more power

·         Doubling wind speed means 8 times more power

WHY?

·         Kinetic energy of an air mass is proportional to v^2

·         Amount of air mass moving past a given point is proportional to wind velocity (v)

Calculation of Wind Power

This is the equation for the power in the wind. Each of the terms in this equation can tell us a lot about wind turbines and how they work. Let’s look at wind speed (V), swept area (A), and density (Greek letter “rho,” r) one at a time. First, let’s look at wind speed, V.  Because V is cubed in the equation, a small increase in V makes for a increase in power. 

·         Power in the Wind = ½ρAV³

·         Effect of air density, r

·         Effect of swept area, A

·         Effect of wind speed, V

·         Swept Area: A = πR2 Area of the circle swept by the rotor (m2).

Advantages of Wind Power

The wind blows day and night, which allows windmills to produce electricity throughout the day. Energy output from a wind turbine will vary as the wind varies, although the most rapid variations will to some extent be compensated for by the inertia of the wind turbine rotor. Wind energy is a domestic, renewable source of energy that generates no pollution and has little environmental impact. Up to 95 percent of land used for wind farms can also be used for other profitable activities including ranching, farming and forestry. The decreasing cost of wind power and the growing interest in renewable energy sources should ensure that wind power will become a viable energy source in renewable energy power generation.
For more please visit at  www.kidwind.org