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Part VIII: Wind, Water, and BiofuelsChapter 12. Small Wind and Micro-Hydro

Is Your Site Windy Enough? Honest Wind Assessment

Why average wind speed is a harsh test, and how to measure it before you buy a turbine.

Difficulty
Intermediate
Cost
Free to a few hundred dollars
published

Wind power follows the cube of wind speed. A site that is a little slower is a lot less energy, and a rated wattage printed next to a high speed is not the power you will see on an ordinary afternoon. Most houses sit in the rough air behind trees and other houses. The honest test is whether your site, at the height you will actually build, is windy enough that a small turbine beats using less electricity. Often it is not.

Maps come before a purchase. The U.S. Department of Energy WINDExchange collection is the public starting set: wind resource maps and data. Those maps are model estimates at a stated height. They say so. They are not the wind in your yard. Tower safety and the machine itself are the next page. Projects linked below are other people's work. Read them there.

Difficulty

Level
Intermediate
Time
A map hour now, and a season of measurement if the purchase is large
Skills
Refusing a rated wattage that assumes a wind you do not have

Cost estimate

Free to a few hundred dollars

Reading the maps is free. A small anemometer and a logger, rented or bought, can use a few hundred dollars if you want a season at hub height. That money is cheaper than a turbine on a calm lot. The turbine is not in this estimate.

Materials

ItemQtyNotes
A wind map for your region at a stated height1Note the height. A 30 meter map is not a 10 meter eave.
A look at obstacles1Trees, houses, and the ridge. Height above the obstacles matters more than a brochure.
An anemometer at the height you mean, if you are still seriousOptionalA week of a windy month is not a year. Log long enough to include calm weeks.

Why the average misleads

If two sites have similarly shaped weather and one averages a fifth more wind speed, the power ratio is about the cube of that, near 1.7. That is the comparison the cube is good for. It is not a way to turn a rated kilowatt into annual energy. Rated power is measured at a high speed, often far above what a residential lot holds for many hours. Cubing the ratio of your real speed to that rated speed is a rough way to see the disappointment before you buy. A year of energy still needs the distribution of speeds, because the fast hours produce most of the energy and the calm hours produce almost none. Multiplying rated watts by the hours in a year is the wrong product.

Sea-level air is denser than mountain air. The sketch below uses a sea-level density. A high site makes less than the sketch at the same speed.

Open wind and a sheltered yardAn open field with a tall rotor, and a house among trees with a short rotor in rough air.Open fetchTrees and a roofA regional map does not measure the wind at your eaves.Power follows the cube of speed. The brochure speed is rarely your speed.
Open fetch versus a yard of trees and roofs. The map does not know about your eaves.

Power at one steady wind speed

Watts are one-half times an air density of 1.225, times rotor area, times speed cubed, times a power coefficient. Miles per hour are converted at 0.447. The 1.5 meter, 12 mile-per-hour, 0.25 coefficient starters are a small-rotor picture, not a product. This is power while the wind holds still at that speed. It is not a year.

42 W

Type the speed your site actually sees, then type the speed printed next to a rated wattage. The cube is why those two answers do not resemble each other. High elevation air is thinner than 1.225 and makes less power than this sketch.

Type 12, then type a brochure speed such as the high twenties. Leave the rotor and the coefficient alone, and compare the two wattages. A power coefficient of 0.25 is a stand-in for a small real rotor. The Betz limit near 0.59 is a physics ceiling, not a machine you can buy.

What to measure

Read the map at the height closest to a tower you would actually permit, not at the height that looks green. Then stand on the site. If the rotor would live among crowns and gables, the map is already optimistic. Turbulence behind obstacles is slower and harder on bearings. The tower page is where that becomes a structure.

If the map and the site both still look promising, measure. Hub height is the honest sensor height. A logger on the porch measures the porch. Keep it up through dull weeks. Write down icing. A winter average that counts iced, stopped cups as calm wind is wrong, and so is one that ignores the ice. You want to know both the speed and the hours the machine would be stopped.

Composite: the rated kilowatt

This composite is not a product and not a town. A household buys a turbine whose large number is the power at a speed they have felt once, during a storm. Their sheltered lot, in ordinary weeks, is far below that speed. The cube turns "a bit less wind" into a small fraction of the sticker. They have purchased a sculpture that sometimes makes a light bulb's worth of average power, plus a tower.

Type your speed into the sketch before you type the sticker speed into a budget. The composite names no street or brand and does not claim that a particular machine did this. Real turbines people have built are in the further-reading links.

  1. Read a map at a real height

    Use the Department of Energy maps. Record the height in the legend. Do not borrow a color from an 80 meter map for a backyard pole.

  2. Subtract the obstacles

    Trees and roofs steal speed and add turbulence. If you cannot put the rotor above them, believe the worse number.

  3. Cube a speed you have, not a speed you admired

    Compare your likely speed with the rated speed. If the wattage collapses, stop. The purchase will not fix the air.

  4. Measure for a season before a large buy

    Hub height, including calm weeks. A windy weekend is how people order machines they regret.

Further reading

These links go to outside sources. This book cites them by link and does not copy their words or pictures.

General education, not a permit or a stamped design. Follow the safety note in the guide, read the safety disclaimer, and hire a licensed trade when the work requires one.