A tall tower is the single most important factor in the economic viability of a small wind system. Tall towers enable turbines to access faster in better quality winds, and even small increases in wind speed translate to exponentially more energy the turbine can generate. In other words, a taller tower means far more – and cheaper – energy.
The best sites for turbines are those where the wind is least obstructed, which is often the highest point on a property. The bottom of the turbine rotor should clear the highest wind obstacle (rooftop, mature tree, etc.) within a 500 foot radius by at least 30 feet. Doing so ensures the turbine reaches consistent, fast wind speeds and prolongs the life of the turbine by avoiding stressful air turbulence.
Overly conservative zoning height restrictions therefore cost the owner money – and a lot of it. They can also mean more sound, since taller towers raise the generator high above the ground, diluting sound considerably. Sound decreases four-fold with every doubling of distance from the turbine (including distance above the ground) so taller towers are better for their owners as well as neighbors.
For zoning officials, the importance of strong winds also means that tower height cannot be compromised as a gesture to neighbors concerned about the visibility of the turbine. “Hiding” a turbine from neighbors using a shorter tower almost always means hiding it from the wind, too.
Nor do two shorter installations make an acceptable substitute for a single, taller one. A tower alone can comprise 50% or more of a system’s total cost, so multiple, shorter turbines (on multiple towers) cost the owner far more than a single, taller system.
It is also important to keep in mind that a turbine’s generator size (generating capacity, measured in kilowatts or kW) has little, if anything, to do with its tower height. Sometimes zoning regulations mistakenly limit tower heights based on the size of the turbine’s capacity, thinking that a 2kW turbine, for example, always corresponds to a 40 foot tower. This is not the case. Appropriate tower height is matched to a turbine depending on surrounding terrain, trees and buildings, and wind resource. Therefore, tower height restrictions, if any, should only reflect sound and safety concerns rather than be designed to correspond to a system’s generating capacity. Most often, in fact, established sound and setback requirements negate the need even to mention height in regulations for small wind systems.