Residential Wind Power - Some Basics
As you investigate the installation of a wind turbine on your property, you should consider a number of factors: the average wind speeds in your geographic region, the total costs of installing a wind turbine, the potential energy savings such an installation will give back over time, maintenance issues, the scope of combining wind generated energy with utility generated and (if part of your overall plan) solar or other types of generated energy. Unless you live on a piece of property that is at least one acre in size and you pay more than 10 cents per killowat-hour of electricity (check your latest electric bill), wind power generation may not make economic sense for you to implement. Focus instead on using solar energy and energy conservation measures. Let's look at the other factors below.
Average Wind Speeds
First, remember that two different wind turbines will behave - and therefore generate more, or less, power in comparison to each other - in exactly the same spot under identical wind conditions. This fact is only one variable that one needs to consider when searching for an appropriate wind turbine. (For example, a "small wind" residential turbine may require a higher wind to produce the same power output as a slower but much larger commercial-grade turbine). Second, the power of the wind in question is related to the cube of the wind's speed; i.e. a 25 mph gust will have twice the power of a 20 mph gust. This means that a turbine that produces X amount of power in a 25 mph wind is not the same (or as desirable) as a turbine that produces X amount of power in a 20 mph (or slower) wind. While some manufacturers will advertise a "cut in" wind speed of only 6 mph on their turbines, the fact is that these turbines may not generate reasonable power until wind speeds reach approximately 20 mph. The moral: do not look at "power output" only when determining which wind turbine to install. Look also at the average wind speed, over time, which occurs on your property. Winds in the 10-16 mph range are the most common; winds in excess of 25 mph are very uncommon. It is generally recommended that wind speeds reach 7-10 miles per hour before a wind turbine can generate power. Wind power is measured on a scale of 1 to 7 (1 being the weakest sustained winds) by the U.S. Department of Energy. Acreage which falls in the DoE's "2" classification (where average wind speeds are between 9.8 mph and 11.5 mph), is considered worthwhile for the installation of a wind turbine.
Wind Power Components
Wind turbines take the energy from a windmill's turning blades and use that energy to power an electrical generator, which in turn supplies the electric current. There are two major types of wind turbines: the "horizontal-axis" types (the most common); and the "eggbeater" style of wind turbines or "Darrieus" model turbines (so named for their inventor). The major horizontal components are:
- blade or rotor, which converts the energy in the wind to rotational shaft energy;
- a drive train, usually including a gearbox and a generator;
- a tower that supports the rotor and drive train;
- other equipment, including controls, electrical cables, ground support equipment, and interconnection equipment, and
- net metering equipment, which provides a way to measure the energy both produced and consumed at a residence or business which has wind power equipment. Any excess power produced will spin the electricity meter backwards, allowing the residence or business to not only reduce what it consumes from a connected "grid" utility, but sell back power should any excess power be generated beyond which the residence or business needs.
Wind Tower Height
For purposes of residential or farm wind power generation, a smaller wind power machine has rotors between 8 and 25 feet in diameter, and stands upwards of 30 feet, although some wind component manufacturers recommend towers of 80 feet or more (to get above treelines and capture stronger prevailing winds). Such a machine can supply the power needs of an all-electric home or small business. Single small turbines generally produce below 50 kilowatts.
Upwind vs. Downwind / Yaw or Tail or Neither?
Small residential wind turbines can be installed so that they are either upwind or downwind of the prevailing winds on a given property. The propeller blades on an "upwind" turbine will be hit by the winds before the tower is, allowing unobstructed access to prevailing winds - this means more efficient movement of the propeller blades. However, a tail or other type of "yaw" mechanism is required as part of the turbine construction, to keep the propeller facing the prevailing winds. Upwind turbines are more prone to instability, especially during high wind or gusty conditions. "Downwind" turbines, on the other hand, involve propellers which face away from prevailing winds. They require no tail or yaw mechanism, and tend to be more stable - even in strong wind situations. However, because downwind turbine props face away from the winds, these winds will "hit" the installation tower before hitting the blades. This causes a slight inefficiency in the wind propeller operation, known as "tower shadow". Tower shadow can increase the wear and tear on propeller blades, as well as create the slight thumping sound that can sometimes be heard around downwind turbines as each blade passes through the "shadow". Which is best? Upwind or downwind? There are pros and cons to each - upwind turbines allow for efficiency and quiet, but may require more elaborate design or mechanics (and therefore, expense); downwind turbines are simple and inexpensive, but may be a little noisier and prone to wear out.
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