Tower Height Considerations

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Once you’ve identified the best site, you must determine optimum tower height. To produce as much electrical energy as possible at a site, the rule of thumb is that the entire rotor should be at least 30 feet above the tallest obstacle within a radius of 500 feet.

When calculating minimum tower height, don’t forget to take tree growth into account. If the trees on your property will grow 20 feet in the next 20 to 30 years, the life expectancy of a wind system, add that to the tower height for the best long-term performance. (To learn two ways to estimate the height of trees and buildings, see the accompanying box.)

How High?

Determining the height of a tower seems pretty straightforward until you have to do it. The first challenge you’ll
face is determining the actual height of nearby objects, such as trees or barns. How do wind site assessors determine
the height of ground clutter?

One way, shown in Figure 6.15, is to place a stake (a metal fence post, for instance) next to the object you want to measure. On a sunny day, measure the height of the stake and then measure its shadow. Then measure the shadow of the object under question.

The height of the object can be determined by ratios using the equation: _H__1 H2 = S__L_1 SL2. H1 is the unknown height and H2 is the height of the fence post. SL1 is the length of the shadow of the object you are trying to measure. SL2 is the length of the shadow of the fence post. To solve for H1, you just need to rearrange the equation: H1 = (_S__L_1_)_(_H_2_) SL2. Note that the ground around both the tree and the fence post must be level for this method to be accurate.

Consider an example. Let’s assume that the fence post is four feet high and the shadow it casts is two feet long.
The shadow cast by the tree or building is 14 feet. How high is the tree? As illustrated, in Figure 6.15a, you begin
by setting up ratios: __x 4 = _1_4_ 2 , then solve for x: x = (4 x _1_4_ 2 ,) = 28 feet. Another simple method is explained in Figure 6.15b.

If your site is within a quarter of a mile from a forest or goodsized wooded lot, the top of the nearby tree line is the height you want to exceed. Mount the wind turbine using the tree line as the height you must exceed. Don’t forget to factor in tree growth.

If you are installing a wind turbine in an area with more than 50 percent deciduous tree cover, the effective ground level is two thirds of the tree height. If trees are 60-feet high, for instance, the

Fig. 6.15a and 6.15b: Measuring Height. (a) Driving a fence post or some other object of known length into the ground next to an object of unknown height and comparing the length of the shadows allows one to calculate the height of an object. (b) Another method for determining height is shown here. In this method, you’ll be solving for C, the height of the tree. Measure the distance from the tree (D). Measure the distance from your eye to the ruler in your hand. This is B. Measure the height of the object in inches on the ruler. This is A. Then set up a ratio equivalence as follows: __AB= __C___ B + D . The rest of the math is shown in the figure.

effective ground level is 40 feet. A 100-foot equivalent tower would, therefore, need to be 140 feet high to take into account the trees.

Bear in mind that the height recommendation is the minimum acceptable tower height. Savvy wind energy installers exceed the rule and see increased performance because of it. It usually costs very little to increase tower height by another 20 to 40 feet and the return on this small investment is quite impressive. We don’t know anyone who has installed a wind turbine who says, “I wish I’d bought a shorter tower.” However, we know lots of people who wish they had purchased a taller one.

Tall Tower Economics: Overcoming the Small-Turbines-on-Short-Tower Myth

When you talk to professional wind system installers, you may hear statements to the effect that it doesn’t make sense to mount a smaller turbine, for example, one with a seven- or eight-foot diameter rotor, on a tall tower. This is flawed reasoning. Tower height should be determined by the height of obstructions in the area, not the size of the wind turbine or the towers a manufacturer or dealer sells. A 50-foot tower slightly downwind from a 65-foot-high tree
line isn’t going to produce much electricity. Moreover, the turbine will produce even less electricity as the trees grow over the 20- to 30-year life of the wind system. Remember: energy output and the economics of the wind system are both proportional to V3 (the cube of the wind speed).

Although it is sometimes hard to justify a tall tower for a small turbine, that doesn’t mean that the right decision is a short tower. The right decision is to invest enough in your tower to make the most of your turbine’s potential — or choose another renewable energy system.

If you are thinking about installing a smaller wind generator, but are nervous about the cost of a taller tower, we recommend that you calculate how much more the tower will cost and how much more electricity the turbine will produce on a taller tower. In our experience, installing a taller tower always results in the production of substantially more electricity. Even though it will always cost more money, the important question to ask is whether the increased tower height is justified economically by the increase in electrical production. In most cases, it is.