ASK THE EXPERTS: Multiple Wind Turbines

Under what conditions, if any, would it be desirable to install multiple smaller turbines instead of one larger turbine? In particular, I was thinking about two 10 kW units versus one 20 kW unit; and two 50 kW units versus one 100 kW. How far apart should multiple turbines be spaced?

Mark Malcolm • via e-mail

While it is certainly more cost-effective to install one larger turbine instead of several smaller turbines, there are a variety of reasons why someone might want to install multiple turbines. My company has a number of multiple turbine installations in Wisconsin, as well as a variety of reasons why they were installed:

• Different uses on the same property. At a home and business that occupied one property, the installer put up two Bergey Excels on 120-foot towers. One powers the home and the other powers the owner’s office.
• Usually, there is an economy of scale with wind where the larger you go, the more cost-effective it is. But this is not always the case. The Prehn Cranberry Farm installed two Endurance E3120s because they were more cost-effective than one Northern Power 100. The Endurance turbines were better suited to the site’s lower wind speed.
• No larger turbines are available that fit the load. Two Northwind 100s power the Village of Cascade Waste Water Treatment Plant.
• Often, it is easier to get permits for multiple small turbines rather than one large one. Wisconsin turbine manufacturer Renewegy pioneered this very clever business model. There are at least five such installations in Wisconsin, including five turbines at the Menasha Corp., four turbines at SCA Tissue, three turbines at JJ Keller, and two turbines at Kaukauna High School. You can see these at www.renewegy.com.
• As a teaching tool, Lakeshore Technical College has a Vestas V-15, Endurance E3120, Entegrity 15-50, and a Proven 7. 
• For the fun of it and to gain experience. Lake Michigan Wind & Sun has four turbines and towers. I have three. The reason? John Hippensteel and I are both wind weenies—we like to play with multiple turbines, learning more for our businesses. In addition, we both installed these incrementally, as time and money allowed.

For horizontal spacing, 10 times the rotor diameter is a reasonable distance. The utility wind industry uses 7 to 10 times if the turbines are downwind of each other in the prevailing wind direction, or 3 to 5 times if perpendicular to the prevailing wind direction. These are minimums that allow as many rotors as possible to be located on a piece of land. There may still be significant disruption of wind with these minimum distances, as has been noted recently in several papers on array layout and performance.

Another thing to consider is that wind farms are typically on ridges or locations were there is a predominant prevailing wind direction. It’s not the same with small turbines, which tend to be sited in locales where the wind comes from all directions over the course of the year, even with a prevailing wind direction. Because of this, a minimum spacing of 10 times the rotor diameter is a prudent distance to separate small turbines.

Mick Sagrillo • Sagrillo Power & Light