Home wind turbines consist of a rotor, a generator mounted on a frame, and (usually) a tail. With the spinning blades, the rotor captures the kinetic energy of the wind and converts it into rotary motion to drive the generator. Rotors can have two or three blades, with three being more common. The best indication of how much energy a turbine will produce is the diameter of the rotor, which determines its “swept area,” or the quantity of wind intercepted by the turbine. The tail keeps the turbine facing into the wind.
A 1.5kW wind turbine can meet the needs of a home requiring 300 kWh per month, for a location with a 14-mile-per-hour (6.26-meters-per-second) annual average wind speed. The manufacturer will provide you with the expected annual energy output of the turbine as a function of annual average wind speed and elevation at the site. The manufacturer will also provide information on the maximum wind speed in which the turbine is designed to operate safely.
Most turbines have automatic speed-governing systems to keep the rotor from spinning out of control in very high winds. This information, along with your local wind speed distribution and your energy budget, is sufficient to allow you to select the turbine size.
Because wind speeds increase with height in flat terrain, the turbine is mounted on a tower. Generally speaking, the higher the tower, the more energy the wind system can produce. The tower also raises the turbine above the air turbulence that can exist close to the ground. A general rule of thumb is to install a wind turbine on a tower with the bottom of the rotor blades at least 30 feet (9 meters) above any obstacle that is within 300 feet (90 meters) of the tower. An important consideration about tower height is zoning. Many local governments place limitations on the installation of structures taller than 30 to 35 feet. Because taller towers are almost always preferred, a review by the local zoning authority may be required. This is an especially important issue in urban and suburban areas.
There are two basic types of towers:
selfsupporting (free standing) and guyed. Most home wind power systems use a guyed tower. Guyed lattice towers are the least expensive option. They consist of a simple, inexpensive framework of metal strips supported by guy cables and earth anchors.
However, because the guy radius must be one-half to three-quarters of the tower height, guyed-lattice towers require enough space to accommodate them. Most towers have just three earth anchors. Guyed towers with four earth anchors can be hinged at the base so that they can be lowered to the ground for maintenance, repairs, or during hazardous weather such as hurricanes. Aluminum towers are prone to cracking and should be avoided.
Wind turbine warranties vary between one and five years depending on the manufacturer.
Wind can be a difficult resource to estimate. For one thing, wind resources are extremely site dependent. The U.S. Department of Energy has compiled wind resource maps that are available from the American Wind Energy Association and the National Technical Information Service. These maps are excellent sources for regional information and can show whether wind speeds in your area are generally strong enough to justify further investigations. Checking with your local airport or weather bureau will help give you an idea of the wind speeds in your area, but your site may experience higher or lower average wind speeds. Your site should have average wind speeds of at least nine (9) miles per hour or more.
If you do not have on-site data and want to obtain a clearer, more predictable picture of your wind resource, you may wish to measure wind speeds at your site for a year. You can do this with a recording anemometer. The most accurate readings are taken at “hub height” (the elevation at the top of the tower where you will install the wind turbine). The standard wind sensor height used to obtain data for the DOE maps is 33 feet (ten meters). Because a small wind turbine can be purchased for about the same price as an anemometer, another option is to install a small turbine and measure its performance for a year. A dealer can then predict the performance of a larger machine sized to meet your loads.
You can have varied wind resources within the same property. If you live in hilly terrain, take care in selecting the installation site. If you site your wind turbine on the top or on the windy side of a hill, for example, you will have more access to prevailing winds than in a gully or on the leeward (sheltered) side of a hill on the same property. Consider existing obstacles and plan for future obstructions, including trees and buildings, which could block the wind.