Turbulence is the enemy of small wind turbine productivity. By turbulence, we mean interruptions in the steady flow of wind to the turbine. What causes turbulence? Trees and buildings, mostly, but also anything else that stands close to the turbine and has the potential to block or redirect wind traveling toward the turbine.
The tower plays a big role in avoiding turbulence, primarily by getting the turbine well clear of all turbulence-causing elements. As a general rule, it’s best for a small turbine to be about 10 meters (~30 feet) above all trees, buildings and other structures. That’s where the “good wind” is going to be, and your wind turbine’s power production will be much higher as a result of getting into this altitude.
For small turbines, wind speed is generally best at 30 meters (about 100 feet). One reason this matters so much is that the power a small turbine generates is a cubed function of wind speed. In other words, an increase of wind speed from 10 to 20 mph results in 8 times the power production (2^3 = 8). That’s why it’s so beneficial to get to the good wind!
Therefore, it’s usually best for a small turbine to be mounted on a tower that’s anywhere from 40 feet to 100 feet tall. There are of course outlying situations: In some cases, a turbine could be at 150 feet atop a lattice tower, if the owner has access to such a tower.
Steel turbine towers are usually found in two main varieties: Guyed and monopole. Guyed towers are held in place by four sets of guy wires extending from the tower’s sections to anchors set in the ground away from the base of the tower. The math for where to anchor the guy wires is simple. The formula, where n is the distance from the base to the anchor, is:
n = h/2
…where “h” is the height of the tower. So, for a 100 foot tower, the distance from the base to the anchors should be 50 feet.
Wind turbine owners who don’t want guy wires can opt for a more expensive monopole solution. This tower stands independent of any wires for a cleaner look and more access to the ground surrounding it.
To make installation and servicing easier, many small wind turbines are installed on tilt-up towers. Both guyed and monopole towers can be set up as tilt-up towers. This means that the tower is affixed to a hingeplate at its base, which has a length of pipe called a gin pole protruding at a 90 degree angle from the tower. Installers attach a winch to the gin pole and crank the tower up and down when needed.
Simple? Yes, but it’s still a careful process requiring knowledgeable installers who understand tower physics. Counter-tension, a solid, level base and proper anchoring are all critical factors in a plumbed, happy tower.
When you’re planning for your small wind turbine, consider your tower options. Again, you’ll want to be 30 feet clear of obstacles, so height is a prime factor. If you’re opting for a guyed tower, make sure you have enough cleared land to allow for the radius of the tower anchors.
The next and final part of this series will relate what we’ve learned about small wind turbines to the concept of the smart home microgrid.
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