Manufacturers of small wind turbines are enjoying a boom. Fascinated by the idea of generating their own electricity, many rural homeowners have invested thousands of dollars — sometimes tens of thousands of dollars — in a backyard wind generator.
Devotees of wind energy face several hurdles, however. A good site for a wind turbine — generally a large, rural lot with an average wind speed of at least 10 miles per hour — is rare. Moreover, many communities are reluctant to grant a permit for a wind tower, which may need to be from 80 to 120 feet tall. It’s not unusual for wind tower plans to run afoul of zoning regulations or neighbors’ aesthetic judgments.
Although the market for small wind turbines is growing, many energy experts remain skeptical of the devices’ usefulness. If you are lucky enough to own a great wind site, and if you don’t mind troubleshooting occasional glitches with mechanical and electrical equipment, a backyard wind turbine might make sense for you. Many purchasers of small wind turbines end up disappointed, however, so it’s important to do your homework before you shop for a wind machine.
Decades of wind turbine history
While photovoltaic (PV) systems are now the dominant technology for on-site renewable electricity generation, wind came first. From the late 1920s through the 1940s, off-grid homeowners installed thousands of wind turbines to power DC lights and radios.
In the late 1940s, however, as the wires strung by the Rural Electrification Administration reached more and more farms, the market for small wind turbines began to dry up. When offered the chance, most rural residents jumped at the opportunity to hook up to the grid.
Three decades later, the Arab oil embargo sparked a small-wind revival. During the 1970s, hundreds of back-to-the-land hippies began restoring abandoned Jacobs wind turbines and hooking them up to 24-volt or 36-volt batteries. This renewed demand for small-scale wind equipment encouraged several new wind turbine manufacturers to enter the market.
Eventually, however, dropping prices for PV modules siphoned away some of the turbine manufacturers’ market for off-grid battery-charging equipment. By 1980, when the price of PV modules reached $9 per watt, PV systems began to become attractive for those living beyond the reach of powerlines. Since PV systems have no moving parts, they are almost always more dependable than wind systems. As the 1980s progressed, the price of both PV modules and oil continued to drop, dealing a double blow to manufacturers of small wind turbines.
These manufacturers are now enjoying their most recent growth cycle, sparked by three factors: the energy price surge of 2008; a kind-of “warm fuzzy glow” effect attributable to the growing acceptance of utility-scale wind turbines; and the available federal tax credit for 30% of wind turbine costs.
Only time will tell whether the current wind-turbine mini-boom will fade as quickly as the earlier booms of the 1930s and 1970s.
Understanding wind equipment
The typical small-scale wind system consists of a tower-mounted turbine with a rotor connected to a permanent magnet alternator that produces three-phase AC. The electrical output of the turbine is usually rectified to DC and then converted to grid-synchronized AC power by an inverter.
Because the wind industry has not yet agreed to a standard procedure for rating wind turbines, there’s no easy way to compare the performance of competing models. Reputable manufacturers should be able to provide monthly energy production estimates (in kWh) for a variety of average wind speeds; although these numbers need to be taken with a grain of salt, they can be compared more fruitfully than peak-power ratings.
Rotor size is usually reported as the area of the apparent circle created by the spinning rotor. Known as the “swept area,” this number is an indicator of a turbine’s potential energy production. A turbine with a larger swept area almost always produces more energy than a turbine with a smaller swept area.
Wind power increases with height. According to Wisconsin wind expert Mick Sagrillo, the three most common mistakes made by installers of small wind turbines are “too short a tower, too short a tower, and too short a tower.” According to Sagrillo’s calculations, increasing the height of a wind turbine tower from 60 feet to 100 feet can result in a power production increase of 344%.
A wind tower should be tall enough to locate the bottom of the rotor at least 30 feet higher than any object, including trees, within 300 feet. For small wind installations, the price of the necessary tower is often equal to the price of the turbine.
There are three common tower types: freestanding, fixed guyed, and tilt-up. To perform maintenance — typically consisting of tightening bolts — on a turbine attached to a freestanding or fixed guyed tower, the tower must be climbed. That’s why many owners of small wind systems prefer to install a tilt-up tower.
Tilt-up towers are hinged at the bottom so that the entire tower can be lowered to the ground. Tilt-up towers take up a lot of room, because of the need for guy wire anchors and the need for a clear area where the tower can be lowered for maintenance.
Assessing your site’s wind speed
Available wind power is proportional to the cube of wind speed. That’s why a site with an average wind speed of 14 mph can produce eight times more power than a site with an average wind speed of 7 mph. When it comes to wind power production, sites with low wind speeds are useless.
Before you purchase a wind turbine, it would be great to know your site’s average wind speed. Unfortunately, wind site assessment is expensive.
According to Anne Bijur, a customer communications representative for Earth Turbines in Hinesburg, Vermont, small wind turbine installations are usually installed without any prior wind monitoring. Most installers of small wind turbines look for clues to high wind speeds. “Look at wind maps and flagging on trees,” Bijur advises. “To get an anemometer up high enough, you need to spend about $7,000 for a tower, so you might as well put the money into a turbine.”
One rule of thumb: a site isn’t windy enough to justify a turbine unless the wind is frequently irritating to residents.
Steven Strong, president of Solar Design Associates in Harvard, Mass., has been involved with several successful wind installations. “We are advocates of wind when it makes sense, but it doesn’t make sense that often,” said Strong. “People thinking about wind ask, ‘Can we afford it? Will the neighbors hate us? Will it make noise and keep us awake?’ But the last thing they get to is the most important: ‘Is there any wind here?’ ”
Any calculation of the cost-effectiveness of a small wind turbine needs to account for maintenance. Most manufacturers recommend that bolts (including bolts at the top of the tower) be checked for tightness annually or every two years. Some turbines will require oiling or greasing on the same schedule. Additional maintenance tasks include adjusting and tightening tower guy wires.
Component failures are always possible; many small turbine owners have had inverter problems. Eventually, turbine bearings will wear out and need to be replaced.
Finally, since wind turbines are located at the top of a tall metal tower, they are particularly vulnerable to lightning strikes. Some installers of small wind turbines have reported that lightning strikes from a single storm have damaged half a dozen of their customers’ turbines.
If you still want to take the plunge…
On the rare site where wind speeds are high enough — in certain areas of the Great Plains, on a mountain ridge, or on the Atlantic coast — a wind system may be a wise investment. But there are good reasons why small wind turbines are rare.
Even in locations where a $40,000 wind turbine will produce more electricity than a $40,000 PV system, it’s important to remember that a wind turbine will always require more maintenance, and will have more downtime, than solar equipment.