Wind Matters

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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 number and variety of wind turbine designs available has never been greater. With so many choices, it seems like we should be seeing all manner of wind turbines busily spinning and generating electricity. Yet too many sit idle—generating little, if any, electricity. That’s because a wind-electric system must encompass much more than just the turbine.

Besides a turbine (aka wind generator), you’ll need an adequate wind resource, a properly sized tower, a suitable site, and the wherewithal to maintain and watch over the system.

Swept Area Matters

A wind turbine’s blades are fixed to a hub. Together, they make up the rotor. The rotor turns in the wind, converting the kinetic energy in moving air (wind) into rotational momentum to spin an electrical generator. The rotor is the collector for a wind turbine, harvesting wind energy, which is then converted into electricity by the generator.

Hundreds of different rotor designs have been invented, tried, and discarded over the past eight decades. Regardless, there are still all sorts of claims about unprecedented efficiencies of various new turbine designs. So how can you sort out fact from fiction?

Let’s use a simple analogy to explain the concept of the rotor as a collector, using a solar hot water collector. A  4- by 8-foot collector is capable of collecting a certain amount of sunlight and converting that sunshine into a certain amount of hot water. If we double the size of the collector, it makes sense that the system would now be able to collect twice the sunlight and generate twice as much hot water.

The bigger the renewable energy collector, the more energy it is exposed to that can be collected—and the more output the system will generate. The area of the wind that the rotor intercepts is called the swept area. Just as with solar collectors, increasing the swept area of the rotor increases the amount of wind the turbine can intercept and convert (to electricity). There is no circumventing this concept; it’s just simple mathematics. Doubling the diameter of the rotor results in a four-fold increase in the swept area—and potentially four times the electricity for any given wind speed.

Fuel Matters

Wind is the fuel for the wind turbine. The more fuel the wind tur­bine has available to it, the more electricity it will generate. Wind fuel has two components, both equally important. One is the quantity of wind available. The other is the quality of the wind passing through the rotor. Let’s look at these separately.

Wind Quantity Matters

The equation that defines how much power is available to any wind turbine rotor is P = 1/2dA V3, where P is the power in the wind, d is the density of the air (affected by both elevation above sea level and air temperature), A is the swept area of the rotor, and V is the wind velocity (speed). For a given wind turbine and site, swept area and density are constants. As such, the power available in the wind is approximately the cube of the wind speed.

So, the power that is in the wind (P) is proportional to the cube of the wind speed, or V3. The interesting thing about this relationship is the effect of increasing wind speed. For example, a 2.5 mph difference in wind speed—say, from 10 to 12.5 mph, is an increase of only 25% in wind speed. However, since P ~ V3 (not just V), the numerical increase in the power equation attributable to wind speed is nearly 100%: 10 × 10 × 10 = 1,000, while 12.5 × 12.5 × 12.5 = 1,953. So even small increases in wind speed result in very large increases in power available in the wind that can be converted to electricity.

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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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