By MICK SAGRILLO May 21, 2013
Awhile back, SOLAR TODAY received the following comment about a column I did on tower heights: “Tall towers can easily mean over 70 feet [21 m] — maybe even 100 or more. Very few turbine manufacturers make towers in this range, and it becomes difficult to obtain them, not to mention quite costly.”
Points taken, but there are a number of misconceptions in this thinking. Let’s analyze them one at a time.
Tall towers can easily mean over 70 feet…
Correct. However, the tower height is dictated by the site’s ground clutter, not by what the wind turbine manufacturer offers. The rule of thumb is that the entire rotor must be at least 30 feet (9 m) above any obstruction within 500 feet (152 m), or 30 feet above the local tree line, whichever is higher. This is simply to get the rotor into clear, steady wind. Turbulence from ground clutter reduces power output and increases mechanical wear on the turbine.
Unless you live in a mountain pass, wind can come from any direction, shifting as weather fronts pass. This means that even if you properly site your tower upwind from most obstructions, towards the prevailing wind, your tower will still be downwind of obstacles a significant part of the time. If you live with wide-open flat plains in all directions, with only a one-story house nearby, a 70-foot tower might work. However, most prospective wind- turbine owners live in more complicated landscapes, with trees, buildings and neighbors scattered around.
maybe even 100 or more.
For some locations, an 80-foot tower is an entry-level tower height. Most locations, however, will require towers in the range of 100 to 140 feet (30 to 43 m), perhaps taller.
Very few turbine manufacturers make towers in this range …
Actually, the few turbine manufacturers that do sell tall towers are the ones you want to consider seriously. Any turbine manufacturer that understands the physics of wind flow and turbulence sells towers in the 80- to 120- or 140-foot range. These result in wind installations that will match advertised power and meet owner expectations. Just about any- one can offer towers in the 10- to 30-foot range, maybe even all the way up to 70 feet. But just because that’s all they offer is no reason to purchase one of those systems. Due to the 30-foot rule, this stuff is just expensive kinetic yard art — something that spins but doesn’t generate much usable electricity.
The only manufacturers whose equipment I might recommend are on the Interstate Turbine Advisory Council’s Unified List of Wind Turbines (see www.cleanenergystates.org/projects/ITAC/itac-unified-list-of-wind-turbines). All but one of the manufacturers listed offer 80- to 140- foot towers — and that exception is now out of business.
and it becomes difficult to obtain them …
Not really. Manufacturers of small wind turbines with a reputation for quality offer towers in the higher range but do not, interestingly, offer short towers.
not to mention quite costly.
There is a difference between “cost” and “value.” A tall tower does cost more than a short tower. But the main reason for purchasing a wind system is not to own the equipment. It’s to generate electricity. So the question that we should ask is “What is the cost of electricity that the system will generate over its life-time?”— not “What is the cost of the tower?” Any way you slice it, given the fact that wind speed increases with height above surrounding obstacles, the value of electricity is invariably greater with taller towers, within reason. There is a point of diminishing returns. Towers above 140 or 160 feet probably are uneconomical. But a 70-foot tower just doesn’t make sense in an area with 70-foot trees.
If you want to find value in generating your own electricity, look to manufacturers that offer towers suitable for the site.
Mick Sagrillo(msagrillo@wizunwired.net) teaches and consults about wind power, and has powered his home with wind power since 1982.