GENERATING ELECTRICITY FOR HOME FROM WIND POWER

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Environmental concerns and desire for energy independence cause increasing interest in renewable energy sources. There is plenty information on this subject scattered throughout the web. It is just not always easy to quickly find what you are looking for among the numerous web sites. This site is designed as an online reference guide for anyone looking for information on utilizing the wind's energy at home.

WIND POWER FACTS

Uneven heating of the atmosphere by solar radiation coupled with the Earth rotation and different rates of heating of land and water, create unequal air pressure distributions, which contribute to the flows of air.

Note that any moving object has kinetic energy, which is proportional to the mass of the object and the square of its speed. The energy in the wind is due to the moving air molecules. Thus a portion of the sun's radiated energy (about 1-2%) is converted to the energy of the moving air. Power derived from the moving air is casually called wind power and is often referred to as a converted form of sun's radiated power. It is one the five main alternative energy sources. A device that converts air movement into electricity is often called a wind generator. Such generators come in various sizes, from those with rotors a few feet in diameter for sailboat battery charges to utility-grade systems with rotors measuring hundreds of feet across. Depending on the rotor's size, their peak ratings range anywhere from 20 W to 2.5 megawatts.

HOW WIND POWER GENERATORS WORK

The main components of a wind generator are rotor, alternator, and structural support. Small residential grade systems are usually sold with a rectifier, a DC-AC inverter and an optional battery.
The rotor assembly includes the aerodynamically designed blades for converting kinetic air energy to rotational energy of the shaft. Most modern turbines have three blades. The rotor is connected to the main shaft of the alternator. The alternator in turn converts the mechanical power of the spinning shaft into electricity. It contains one or several permanent magnets or electromagnets that spin inside or outside stationary coils of wire. Small windpower systems normally use permanent magnets. When the wind turns the blades, they spin the shaft with the magnets, which creates an alternating magnetic field in the stationary coils. The alternating magnetic field induces voltage in the coils according to Faraday's law (for more information see How a generator works). Depending on the design, the alternator can operate either at variable speed or at constant speed. In the first case, the speed of the rotor varies with the wind; in the second, the speed of the rotor remains relatively constant.

The voltage produced by an alternator is determined by the number of turns in its coils, the strength of the magnetic field, and the rotation speed. In small wind generators for home use the blade rotor drives the magnet rotor directly. Larger systems may have a gearbox that converts the low speed high torque incoming rotation of the blade rotor to high speed low torque rotation of the alternator rotor.

The available power obtainable from the moving air is proportional to the cube of its speed. For example, when air speed doubles, the available power increases by a factor of 8. Because wind speeds increase with height, the turbines are normally mounted on toll towers to capture the most of air movement. In general, the higher the tower, the more electricity the system can generate. However, only a portion of available wind power can be converted to electricity. A typical overall efficiency of a home system is about 20%. Available wind resources vary with the location. Different geographical areas have different average wind speeds and power densities. For more information see Calculation of Wind System Output Capacity.

REFERENCES

Guided Tour on wind power: how it works, energy output, turbine design.
Wind Energy Systems: a complete free online textbook on the characteristics of large and small turbines.

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