SOLAR TODAY - January/February 2009

ENERGY CENTER OF WISCONSIN

Rule of thumb: The bottom of the turbine rotor arc should be 30 feet above the highest obstruction within 500 feet.

For example, let’s say that you have done a load analysis on your home, and know the annual average wind speed for your location. Manufacturers’ specifications indicate that a wind generator with a 20-foot rotor will supply your electrical needs. You look around your rural property and note that the highest obstacle around is a silo, 60 feet high, attached to your barn. You are also surrounded by some trees, but at 40 to 55 feet, they are not quite as tall as the silo. What is the minimum height that you can get by with for your tower?

If you add the 60-foot height of the silo to the 30-foot minimum rule plus the blade length of 10 feet, you get a minimum hub height of 100 feet. That’s what your wind turbine needs for access to its fuel.

feet. If we take mature tree height into consideration, the numbers are now an 80-foot tree height plus the 30-foot rule plus the blade length of 10 feet for a total hub height of 120 feet. The second rule of tower sizing: Trees grow, but towers don’t, regardless of how much it rains. In case of a calculated tower height that falls between two standard heights available, good practice is to round up, not down.

While different manufacturers offer a variety of towers, what the manufacturer offers is not necessarily what will work on any given piece of property. The line of towers is a marketing decision, having little relationship to what may or may not be adequate for a given piece of property. Minimum tower height is determined by obstacles in the “neighborhood” surrounding the site that it is to be installed on, nothing else.

Note that the minimum tower height is just that — it’s the minimum tower height that you can get by with and still produce electricity without the wind’s power either being considerably diminished by ground drag or seriously compromised by turbulence. The decision to install a tower above the minimum height required by the site is impacted by a variety of inputs, including the additional electricity that will be generated due to a higher wind speed, the economics of a taller tower, and who will service the system.

Next issue we’ll look at rooftop wind turbines. ST

We need to
estimate the

However, we can’t ignore future tree height. We are not interested in today’s tree height, but need to esti-

height that the

mate the height that the trees will reach during the 20- to

trees will reach

30-year life of the wind system. I recently analyzed a site where the wind generator was installed on its 70-foot tow-

during the 20-

er 27 years ago, when it towered above the nearby trees.

Today, leaves brush the blades as they sit quietly, no longer

to 30-year life of

generating electricity.

I nnovIn the above ctalculaitiono, it wounld not be unusual for the the wind system. 40- to 55-foot trees to reach a mature height of nearly 80

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