Wind Matters: Page 4 of 4

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Wind Matters
Wind Matters
Increasing swept area
Increasing swept area increases the captured wind energy proportionally.
The power in the wind is a function of the cube of its velocity.
The power in the wind is a function of the cube of its velocity.
Wind speed increases with height
Note how wind speed increases with the height above the ground.
An obstruction can create turbulence zones
An obstruction can create turbulence zones that are, above it, twice its height (2H); upwind, twice its height (2H) horizontally; and downwind, 20 times its height (20H) horizontally.
30-Foot Rule
The bottom of turbine’s swept area should be at least 30 ft. higher than any obstruction within 500 ft.
Trees have grown tall enough to render the wind turbine useless.
This wind turbine was installed in 1982 using the 30-foot rule for fixed obstacles—tree growth was not accounted for. In the last three decades, the trees have grown tall enough to render the wind turbine useless.
Wind turbine siting
The site for the first turbine (1) turned out to be highly compromised—downwind from almost all obstacles on the property relative to the prevailing wind direction. Subsequent turbines were placed at sites 2 and 3.
Wind rose
The wind rose for this site shows that the strongest winds come predominately from a southwest and south-southwest direction.
Wind Matters
Increasing swept area
The power in the wind is a function of the cube of its velocity.
Wind speed increases with height
An obstruction can create turbulence zones
30-Foot Rule
Trees have grown tall enough to render the wind turbine useless.
Wind turbine siting
Wind rose

The compromise I had to make, especially with the third site, is a longer wire run. However, wire cost constitutes a small percentage of the cost of an entire wind system installation. More important was getting upwind of the major sources of turbulence at our site—a strategy that will help optimize energy generation and reduces wear and tear on the equipment.

You should be able to access a wind rose for your location from your state energy office or wind map, or local agricultural office. The U.S. Department of Agriculture’s Natural Resources Conservation Service has downloadable wind roses from various climate stations (see Access). Other good sources of prevailing wind directions include the National Oceanic and Atmospheric Association or Weather Underground, both of which keep local climatological data.

Lessons That Matter

So, what can we take from all of this? The lessons are pretty straightforward:

  • Swept area determines how much of the wind’s power your turbine can extract. There is no way around the mathematics of your collector size.
  • Wind turbines exposed to laminar winds generate more useful amounts of electricity. Wind turbines sited in turbulent locations (on buildings, or at or below tree level) cannot—and will not—generate much, if any, electricity. There is no way around the physics of fluid dynamics.
  • The 30-foot tower height rule helps determine whether the wind turbine will be exposed to quality winds over the life of the system.
  • Siting your wind turbine upwind in the direction of the prevailing wind will minimize turbulence.

The economic payback in a wind turbine is directly proportional to the electricity it generates over its life. If a wind turbine is sited in turbulent winds, it simply will not generate much electricity, making it a questionable investment. In addition, the turbulence will cause increased wear and tear on the turbine, shortening its useful life. But a wind system includes more than just the turbine—it includes a tower that’s properly sized for the site, foundation, wire run, balance-of-system components, all labor and materials for installation, and various other costs.

People take care of investments when they make sense. Wind system owners invest maintenance and repair dollars in things that work, like a properly sized and sited wind turbine. Owners quickly abandon ideas that don’t work. The history of small wind tells us that rooftop wind turbines and wind turbines installed on towers too short for the site are quickly abandoned and become derelict once they need repair. Simply put, they were bad investments. A $20,000 wind-electric system that only lasts for two years is a poor investment compared to an $80,000 system that lasts 20 years.

Which Wind Turbine?

If you are shopping for a wind turbine, where do you go for help? Three organizations host websites with recommendations based on equipment that is certified to an American National Standards Institute standard, actual performance test results, and industry feedback.

The Small Wind Certification Council (SWCC) is a certifying body that confirms that published turbine test results conform to the American Wind Energy Association’s 9.1-2009 Small Wind Turbine Performance and Safety Standard. Turbines that are certified to have met the AWEA 9.1 criteria are listed at smallwindcertification.org. Make sure you peruse the list of SWCC-certified turbines, not the applicant turbine status.

The Interstate Turbine Advisory Council (ITAC) is a consortium of state public-benefits programs that fund the installation of renewable energy systems. They publish the Unified List of Wind Turbines (at bit.ly/ITACturbines) that participating state programs may be willing to fund.

Intertek is another organization that certifies wind turbine test results to the AWEA 9.1-2009 standard. However, this website (bit.ly/IntertekDirectory) is a bit more confusing since Intertek certifies components as well as entire wind turbines. 

If the wind generator you are considering is not on one of these three lists, move on. Or at least understand that you are making a risky purchase of an untested, unproven design, and be ready to accept the outcome of your speculative investment.

Access

Mick Sagrillo consults, teaches, and writes about wind power. He and his wife have powered their house with wind for 32 years, and Mick has flown dozens of models during that time.

Downloadable wind roses • bit.ly/NRCSWindRose

Comments (2)

Tommy Taylor's picture

Mick: I have an installation that I'm considering that you didn't address. I live on a bluff that's rises 160' in elevation at a 2/1 slope on three sides. The tower could be located right out on the end of the ridge. What's the design critera for this installation. My site is higher than anything within 1/2 mile...

Mick Sagrillo's picture

Tommy, great question, and the answer is specific to your site. It sounds like your site may have real potential. The rules for siting on a bluff can get rather involved, which is why this was not included in the article. First of all, you never want to get closer to the edge of the bluff than 25% of the height of the tower. Wind turbines don't do very well in updrafts, and that's a concern when you get too close to the edge of a bluff. This means that if you decide to install a 100' tower, you need to be back at least 25' from the edge of the bluff. Next, you need to consider the ground cover below the bluff ,as the surface friction will influence the airflow up and over the bluff. If the ground cover is dense trees--in other words, a very high alpha--25% of the height of the tower will do. For a 100' tower this means 25'. However, if the ground cover is very smooth--open water having a very low alpha--then you need to site the tower back 2.5x the height. For a 100' tower, this means 250'. This has to do with the amount of turbulence generated at the top of the bluff as the wind rolls up and over the edge of the bluff. The next obvious question is how tall of a tower do you need. If you're following the principles of "taller will always generate more electricity", then put up the tallest tower that the manufacturer offers, typically at least 100'. But if money is a consideration--and when is it not--and you wish to install a shorter tower, then you need to so some "experimenting". This is going to involve a kite and maybe the neighborhood kids, as it's fun. Stand at the tower site as determined above, and get the kite flying as best you can. As the kite gets off the ground, back up towards the direction of the wind at the edge of the bluff so as to keep the kite above the tower location as best you can. Kites don't like turbulence, and they zig and zag around a lot to show their displeasure. But once they break above the zone of turbulence and get into the laminar flow of air--where you want the wind turbine to be--they get boring because they just fly. That's the minimum height--where they stop zigging and just fly--that you need for your tower height. This experiment is going to vary with the wind speed, but you can't continuously readjust your tower height. So you need to pick a wind speed that occurs most of the time at your site to optimize your energy production. Oh, and the kids? They the ones that will chase the kite around and pick it up so you can get it flying again. Let us know how you "site assessment" turns out.--Mick

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