I’ve been researching wind turbines — the backyard size, not the multi-megawatt off-shore size — off and on for a few years now. What I have found is somewhat disheartening. Some companies offer too little information on their turbine. They describe the product in prose and pictures, but with few specifications. The other problem is that the specs that they provide are, it seems to me, skewed to make their turbine seem more effective than it really is. I should be clear, though, that this article is only my opinion. You will have to decide for yourself if you agree.

As a point of reference on what constitutes fair specifications, I use the American Wind Energy Association (AWEA) Small Wind Turbine Performance and Safety Standard (2009) found here: PDF file. There are many different parts to this set of standards. Perhaps the most important point is the rated power.

“Rated Power: The wind turbine’s power output at 11 m/s (24.6 mph)”

This standard of 11 m/s for the wind speed, used in determining the rated power, is very important. The amount of power in the wind is determined by a formula in which the wind velocity is cubed. So a modest increase in the rated wind speed greatly increases the power in the wind. If the wind speed increases by a factor of 2 (twice the wind speed), the power in the wind increases by a factor of 8 (8 times the power). And even a 20% increase in wind velocity will increase the power in the wind by 72.8% Use 1.2 to represent the 20% increase, then cube that number to get 1.728, which is a 72.8% increase over the original 1.0 wind speed.

Using a higher wind speed in the rated power will greatly increase the rating of the turbine, but also make it less likely that you will actually get that much power. In other words, it artificially inflates the power rating of the turbine.

“The theoretically available power in the wind can be expressed as

P = 1/2 ρ A v3

where

P = power (W)

ρ = density of air (kg/m3)

A = area wind passing through perpendicular to the wind (m2)

v = wind velocity (m/s)” Source

The density of air at sea level is about 1.2 kg/m3. Let’s use 1.0 square meter as the area, so that we can easily scale up the power for larger turbines. This area is called the swept area of the turbine. So the formula becomes:

P = 1/2 * 1.2 * 1 * v3

or

P = 0.6 times the velocity of the wind cubed.

At 11 m/s, the power is:

P = 0.6 * 1331 (which is 11 cubed) = 798.6 watts

Keep in mind, this is only the power in the wind. A giant off-shore wind turbine might have an efficiency of 30 to 40% (if manufacturer specs are to be believed). But a small backyard turbine, even one that is well designed and well made, will only be about 20% efficient (or less). So 20% of 800 watts is 160 watts. You might expect, therefore, that a small turbine would give you 160 watts per square meter of swept area. You would need about 7.5 square meters of swept area to be in the range, as a good approximation, of 1200 watts of rated power.

Four Examples

1. The Windspire turbine from Windspire Energy has a swept area of 7.43 square meters, and is rated at 1200 watts (1.2 kW), with a rated wind speed of 11 m/s (per the AWEA standard). That all makes sense.

Windspire Energy uses the AWEA standards for rating their wind turbine. They also give more information about their turbines than many other manufacturers, including an estimated annual energy production, in kW-hrs, depending on average wind speed. Nice.

2. Another wind turbine manufacturer, one that I won’t name, doesn’t even list a power rating for its small wind turbine. But the specs do give you the “peak” power rating of 2000 watts. What is the swept area? Only 3.2 square meters. At that size, and assuming an efficiency that might actually be too good for this particular design, the rated power at 11 m/s would be about 512 watts, not 2000 watts. But they did say “peak”, so they are not lying. I guess.

3. I’ll name this next turbine company, Zephyr Corporation, even though I’m critical of the way they present their specs, because I think overall it is a good product. The Air Dolphin has a diameter of 1.8 meters, giving it a swept area of about 2.54 square meters (the radius squared * pi). At that size and using a wind speed of 11 m/s, I would expect a rated power of about 400 watts. However, the company rates their turbine at 12.5 m/s wind speed, for a rated power of 1000 watts.

Using the aforementioned power in the wind formula: 0.6 * 12.5 cubed (1953) is 1171 watts per square meter, times 2.54 square meters, which gives us 2976 watts power in the wind. A rating of 1000 watts suggests an efficiency rating of about 33.6%. That number seems high, even given a turbine with an excellent design. If we give their turbine a 33.6% efficiency but at 11 m/s, we end up with about 680 watts as a power rating. However, their power chart seems to indicate about 750 watts at 11 m/s.

I like the Air Dolphin, even though I consider the 1000 watt rating to be exaggerated, for several reasons: low weight (18 kg), a spin-up feature that makes it more efficient, the blades and ‘dolphin tail’ show excellent design work went into this product, and they provide more detailed information than most other companies.

Whether or not an air dolphin turbine is a good purchase for you depends on the price, and how much power you need.

4. As one final example, I’ll use one more unnamed company. Their 1.2 kW turbine has a swept area of 3.56 square meters. If it were rated at 11 m/s wind speed, I’d expect a rating of about 550 to 600 watts — half of what they are claiming. The wind speed they use for rating is 13 m/s. That 18% higher wind speed results in 65% more power in the wind (because velocity is cubed), thereby inflating the rated power of their turbine.

Using the aforementioned power in the wind formula: 0.6 times 13 cubed (2197) is 1318 watts per square meter, times 3.56 square meters, which gives us 4693 watts power in the wind. A rating of 1200 watts suggests an efficiency rating of about 25.6%. That seems a little high. But the biggest problem is using 13 m/s as the wind speed for the power rating. At 11 m/s (the AWEA standard), the power in the wind for that swept area is only 2843 watts, not 4693.

Notice how using only a modestly higher wind speed gives you much more power in the wind, resulting in a much higher power rating. But how often does the wind blow at 13 m/s? The answer is: less often than at 11 m/s. Does this difference really matter? It matters a great deal when you are comparing different turbines. Suppose you look at two different wind turbines from two different companies, and both are rated at 1.2 kW. But in the real world, one of them will produce substantially more power because the other one uses an inflated power rating to reach that 1.2 kW rating.

If I were to rate the Windspire turbine at 13 m/s, instead of the 11 m/s that the company wisely uses, their 1.2 kW turbine would then be rated at about 2000 watts. On the other hand rating the unnamed competitor at 11 m/s instead of 13 m/s causes their rated power to drop from 1.2 kW to about 600 watts. Both are called 1.2 kW wind turbines.

Unfortunately, there are far too few companies who follow the AWEA standards for rating their turbines. And there are far too many companies who play fast and loose with the specs for their products. Let the buyer beware.

– Thoreau

I’ve read that many wind farms achieve a real-world output of about 15% of their theoretical capacity. The wind power advocates do their cause a disservice by overstating the output of their systems. CFLs are overrated in the same way. The CFLs that say they replace a 100 watt incandescent lamp will at best be close to a 75 watt incansescent. Another issue I have seen is in a dock light application where the lamps are used to illuminate box trailers for loading. They don’t “project” (for lack of a better term) light as far.

1. My accountant has stated that “wind farms” are “tax farms.” Without

government tax incentives for manufacturers, purchasers, and energy

buyers; they seldom pencil out.

2. History has demonstrated that the long term reliability of many

turbines that have been produced is poor. Repair and replacement

costs are seldom factored in.

3. They may be justified in areas of energy shortages.

4. Solar is looking better. The cost of PV is going down and power

yield is going up.