Wind-power systems can be cost effective if the average wind speed is 9 mph or more at the location of the wind generator. Adding a secondary charging source, like wind power, to PV can make an off-grid power system more stable by increasing the amount of time that energy is being produced, reducing dependence on energy stored in the batteries. Using off-grid wind to supplement solar photovoltaic power can be cost effective even if good wind is only partially available throughout the year, especially if the solar potential is low at that time.
The amount of power generated by a wind turbine is dependent on wind turbulence, wind speed, and tower height. Like water, air is a fluid, and is subject to the same fluid dynamics principles, such as turbulence created by obstructions in the flow. A stream flowing over boulders becomes turbulent, creating wakes and eddies, and is robbed of much of its energy by friction.
Similarly, wind blowing over a landscape with trees and buildings obstructing its flow also becomes turbulent and loses energy to friction. Turbulence degrades the wind resource, both upwind and downwind of obstructions. Wind turbines placed in turbulent air wear out prematurely and produce little usable power.
To avoid air turbulence, wind turbines should be placed on a tower high enough that the bottom of the turbine rotor’s swept area is at least 20’ to 30’ higher than any buildings, trees, or other obstructions within a 300’ to 500’ radius. If the wind at the site primarily comes from a particular direction, and the obstructions are not in the wind path, then less clearance may be allowable as long as the flowing air is laminar. In the illustration below, a kite with long streamers tied to the line at 10’ intervals can be used to find the height above ground level where the air flow smooths out. Look for the first streamer to be fully furled out.
The power available in the wind increases with the cube of the wind speed. This means that there is nearly twice as much power available in a 10 mph wind as there is at an 8 mph wind. Wind speed increases as you get higher above the ground due to the loss of friction between the air and the ground. You can expect that the wind speed at 30’ above the ground will be about 25% greater than at eye level (at 60’, it’s about 37% greater; at 90’ about 45% greater; and at 120’, about 50% greater). And since power output increases exponentially with increases in wind speed, a turbine mounted on a 60’ tower can produce about 40% more power than the same turbine would on a 30’ tower (75% more power at 90’ and 100% more power at 120’ compared to 30’). Most wind generators are designed to deliver maximum power at a wind speed of 30 mph. At 15 mph, they will deliver about 1/8 their rated power. Therefore, increasing tower height is a cost effective way to get more power out of a wind turbine.
The power output of a wind generator decreases roughly 3% for every 1,000’ of elevation above sea level due to lower air pressure.