UNDERSTANDING WIND POWER GENERATORS FOR HOME USE

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Concerns for the environment and the desire to be independent from utility cause increasing interest in various renewable energy sources. There is plenty of information on this subject scattered throughout the web. It is just not always easy to quickly find reliable practical data among a myriad of web sites. This site is designed as a concise online reference guide for anyone looking for practical facts on wind power generation for the home.

THE BASIC FACTS

We know that energy can't be created, but can only convert from one form to another. So, let's quickly review how winds are formed and where does their energy come from.

Solar electromagnetic radiation heats our atmosphere unevenly.

This process, coupled with the Earth rotation, different rates of heating of land and water, and irregularities of the Earth's surface creates unequal air pressure distributions, which in turn 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: E=mv²/2. The energy in the wind is due to the moving air molecules. Thus, when an air stream is formed, a portion of the sun's irradiance (about 1-2% of what reached the Earth) is converted to the energy of the moving air. That's why wind energy is often referred to as a converted form of solar energy. It has been in use for several millennia and it's one of the five main alternative power sources. Originally people utilized wind to power sailing ships. Later on they began using it in grinding or milling turbines. For more than a century it has also been used for generation of electricity. Electricity powered by the moving air is casually referred to as wind power, and the device that converts air movement into electricity is called wind generator. Not surprisingly, such machine comes in various sizes. For example, rotors in sailboat battery charges can be just a few feet in diameter. Utility-grade systems have rotors measuring hundreds of feet across. Likewise, depending on the size, the peak output ratings can range anywhere from several watt to megawatts. For a reference, the world largest turbine has 417-ft rotor and is rated for 7.5 MW.

HOW WIND GENERATORS WORK

In short, a wind generator operates like inverse fan. Its main components are rotor, alternator, structural support and control electronics.
The rotor assembly includes the aerodynamically designed blades for transferring kinetic air energy to the rotation of the shaft. Most modern wind turbines have two to five blades. The rotor is connected to the main shaft of the alternator. The alternator in turn converts the mechanical energy of the spinning shaft into electricity. It contains one or several permanent magnets or electromagnets that spin relative to stationary coils of wire. Small windpower systems, such as homemade designs normally use permanent magnets. When the air hits the blades, they spin the shaft with the magnets, which creates an alternating magnetic field in the stationary coils. This field induces voltage in the coils according to Faraday's law (for more information on how it works see operation of generators). Depending on the design, the alternator rotor can operate either at variable speed which varies with the wind or at relatively constant speed.

The voltage level 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 power generators for home use, the blades usually drive the rotor directly. Larger systems may have a gearbox that converts the low-speed high-torque incoming rotation of the rotor to high-speed low-torque rotation of the alternator.

The alternator in small systems usually generates 3-phase AC output with variable amplitude and frequency. To be usable it first has to be rectified. The rectified DC voltage can be used directly for battery charging. If its level is sufficiently high it can also power the loads that can operate on DC, such as light bulbs and space heaters. However, to energize conventional household appliances or an entire home, DC voltage has to be converted to a regulated fixed frequency AC. Small residential systems are usually sold with a rectifier and a charge controller. A DC-AC inverter and a battery may be optional.

The amount of wind power generation is proportional to the cube of the air speed. For example, when this speed doubles, the available wattage increases by a factor of 8. Because air speeds increase with height, the turbines are normally mounted on tall towers to capture the maximum amount of airflow. In general, the higher the tower, the more electricity the system can generate. Nevertheless, only a small portion of the air energy can be captured. The typical efficiency of the blades is 25-45%, alternators-- 65-80%. As the result, an average overall efficiency of a home wind turbine is about 20%. Available annual wind resources of course vary with the location. Different geographical areas have different average air speeds and power densities. For more information see our calculator of wind system's output capacity.

REFERENCES and ADDITIONAL INFORMATION

Wind power guide: how it works, turbines design, energy output.
Wind energy generators: a complete free online textbook about operation and characteristics of large and small turbines.

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