Technical Information


Wind Turbine Site Selection

Since wind is the fuel that drives a wind turbine, a wind turbine needs to be placed where the wind is. Installing it on too short a tower is like installing solar photovoltaic panels in the shade or under a tree or in your basement. Neither will work very well. Nor will any type of wind or breeze work appropriately. A wind turbine needs air that moves uniformly in a linear direction.  Eddies and swirls, “turbulence”, do not make good “fuel” for a wind turbine.  The rotor cannot extract energy from turbulent wind.  A constantly changing, turbulent wind direction causes excessive wear and premature failure of a wind turbine.  The wind turbine needs to be placed high enough to catch strong winds and be above any turbulent air.  Tower prices increase quickly with height, so there is a limit as to what is practical and affordable.

The industry rule of thumb for turbine height is to be a minimum of 30’, plus the length of the turbine rotor (blade) above the tallest obstacle (trees, house etc.) within a 500’ radius, having a minimum tower height of at least 60’.  This should be regarded as an absolute minimum hub height (tower height) for a wind turbine.  At 30’ above any obstacle there will still be some amount of turbulence.  Additional clearance is highly desirable.  For example, if there are trees that will grow to 60’ feet high, it is advisable to use a 100’ tower.  Likewise, a 60’ tower should only be used when the terrain is very, very flat with no obstacles in a wide area around it, such as the edge of the ocean, a large body of water or in the middle of a very large area of cleared land.

Site Selection

To go beyond the rule of thumb, the airflow over any obstruction, including trees, tends to create a “bubble” of turbulent air of twice the height of the obstacle, extending 20 times the height of the obstacle behind it. A 30’ tall building disturbs the air up to 600’ away! The figure above illustrates this.  A wind turbine needs to be sited either upwind of the obstructions or far enough downwind so there is not an impact on production. Notice from the figure above that preference should be given to a site upwind of obstructions, but keep in mind that tall structures and/or obstructions downwind of the turbine can also influence the wind going through the rotors.

For proper wind turbine placement, the bottom of a hill, valley, or ravine makes a poor choice to site a wind turbine. The wind tends to drop in speed at the bottom of a hill, then speeds up as it goes up the hill reaching approximately twice the wind speed at the top of the hill. The figure below illustrates this. You can use this effect to your advantage if you have hills on your property.

Site Selection 2

For obstructions that are not smooth, such as a cliff or a sudden rise in the landscape, it is a bit more complex.  As illustrated in the figure below, sharp edges create turbulence. The airflow at the top of the cliff can be stronger than the average wind speed in the area, but close to the cliff’s edge it may also be very turbulent. This makes it a poor site for a wind turbine. If you have a cliff edge or sudden rise in the landscape on your property and want to use it for siting your turbine, you should still use a minimum 60’ tower to get above the turbulent air, even if it appears that the wind is always blowing at or around ground level.

The lee side of a bluff or hilltop (downwind of the prevailing winds) also makes for a very poor wind turbine site. The bluff or hilltop will create turbulence on its downwind side and the average wind speed will drop off rapidly as well, leaving no energy for the wind turbine to harvest.

Site Selection

An inexpensive way to visually observe the height at which turbulent air ends and smooth, laminar airflow begins is to fly a kite at your proposed wind turbine location on a windy day, preferably when the wind is coming from the prevailing wind direction.  To visualize airflow, use tape or streamers tied to the kite’s string every 15 feet or so.  (Home improvement stores sell plastic marking tape in fluorescent colors for very little money).  Wildly fluttering tape indicates turbulent wind, while smoothly extended tape means a clean, non turbulent and linear air flow.  Consider the angle of the kite’s string when calculating overall height.

Two crucial facts to understand are that wind speed increases with height and the power available in the wind is related to the cube of the wind speed, i.e. the energy in the wind increases with the cube of the wind speed (P ~ V3).  Doubling the wind speed gives eight times the power.  For example, a 20 mph wind has eight times the energy (20 x 20 x 20 = 8,000) of a 10 mph wind (10 x 10 x 10 = 1,000).  If a wind generator produces 3,000 watts at 24 mph, it will produce approximately 375 watts in a 12 mph wind.  Understanding V3 will help you understand wind energy more appropriately.  A small, additional investment in tower height may be well worth it, due to the increased energy production that will be realized from the additional height. 

Despite the current marketing pitch of many small wind turbine manufacturers, vertical axis units in particular, it is generally a poor idea to mount a wind turbine, vertical or horizontal axis, directly onto a building.  The building was not designed to support the horizontal load (moment) or vertical load (weight) of a wind turbine.  The airflow that close to the building is generally very turbulent, leading to premature equipment failure and poor power production.  There is also the issue of sound and vibration transmitted to the building structure.  Every wind turbine has some amount of vibration and sound associated with it and this will most likely be noticeable inside the building.

 

Illustrated by Rob Beckers @ Solacity, Inc.