Your article - "How to Buy a Wind Generator" (HP131) was full of good advice to the consumer, but contained one glaring oversight. Your use of manufacturers’ power curves to calculate annual energy output (AEO) led to a very misleading and even unfair comparison of different manufacturers’ technologies.

It is well-established that some manufacturers offer promises of power output that turbines cannot live up to; some even defy the laws of physics! For Home Power to blindly use unconfirmed power curve data to determine energy production to guide consumer choice does not serve the consumer well. At the very least, a strong caveat to that effect should have been included in the article.

It is unfortunate that some of the AEO numbers were published—since once in writing they are taken as "the truth." This makes it very difficult for manufacturers trying to sell "the honest truth" in the marketplace. The promise of certification to a standard in the near future does not help consumers today. As test engineers, we are aware how easy it is to manipulate data and produce unrealistic and vastly overstated numbers even from actual test data. But in today’s unregulated arena, manufacturers don’t even need to use test data, they are free to use any power curve they wish—even a hypothetical one.

So what is a consumer to do? First, realize that all of the turbines presented in the article are suitable candidates for a personal renewable energy system, and the main difference in terms of energy capture is the size of the rotor area. There are simple tools to calculate energy capture for a small wind turbine given only rotor area and annual average wind speed. One such model calculates the total energy in the wind passing through the rotor area, then factors that by the overall turbine efficiency (OTE):

kWh per year = [rotor area (sq. ft)] x [annual average wind speed (mph)]3 x 0.085 x OTE

The 0.085 is a combination of constants that include 1/2 times air density times unit conversion factors. Empirical data suggests that the OTE will fall between 15% and 25% for small wind turbines—we suggest 20% be used as a generic guide. Readers can plug the rotor area and annual average wind speed numbers from your article into this equation and compare the result to the number your article presents. If the equation result is not within 20% of what the manufacturer claims, be very skeptical of that particular claim. Using a tool like this, consumers can separate out a wind turbine’s true renewable energy potential from empty promises.

David Laino, Dean Davis • Endurance Wind Power, Inc.

Thanks for weighing in on a really contentious issue in the small wind industry—the accuracy of the data supplied by manufacturers about their products.

I am no longer willing to use the swept area method you suggested to compare turbines—I have found that there are simply too many variables to use such a simplified equation. For example, I know of two turbines with identical swept areas and generator sizes, one of which outperforms the other on annual energy output (AEO) by more than 33%. There are clearly other design aspects of a wind turbine, other than swept area, that influence the amount of energy a wind turbine will generate.

I chose to use the manufacturers’ power curves to calculate AEO, and run that data through a spreadsheet program that actually calculates the AEO, rather than using the AEO numbers supplied by the manufacturers. Incidentally, this is the same way manufacturers determine AEO— with a spreadsheet calculator. They do not determine their AEOs based on extensive field testing for a year at average annual wind speeds from 8 to 14 mph—this simply takes too long and is too involved. However, feedback from the field is sometimes taken into consideration by manufacturers after the fact, as a reality check on their advertised numbers.

Using a standard AEO calculator, I was able to apply exactly the same mathematical assumptions to all the turbines rather than accept the many assumptions of manufacturers. The only variable in this process is the veracity of the manufacturers’ power curves. If manufacturers are honest with their power curves, then the resulting AEO may be accurate. If not, well…garbage in, garbage out.

At this point, we simply do not have independent verification of AEOs. I hope that will change in the near future once the AWEA Small Wind Turbine Performance and Safety Standard is adopted, and the independent review agency, the Small Wind Certification Council, certifies the results. Then, consumers will have applesto- apples numbers to compare turbine performance. Until such time, my advice is still: "Your mileage may vary." Sometimes considerably.

Mick Sagrillo • Lake Michigan Wind & Sun

Experts disagree, and that's a good thing. In this case, Mick decided to use a spreadsheet calculator to produce the numbers for the article. I do wish the information about the nature of the numbers had been more prominent and had emphasized that they are based on manufacturers' power curves.

I question whether using the calculator is a step forward for this article and the industry, though it's certainly one piece of information that might be useful. I would have preferred to use manufacturers' AEO projections in the article, and clearly state that we had nothing to do with the calculations. I think it's unfortunate if readers conclude that Home Power endorses or validates these numbers. Mick is correct that if we put garbage in, we get garbage out. And if we were not concerned with the possibility that the manufacturers' numbers might be garbage, we would not be having this discussion, or trying to apply other methods to get AEOs.

I think it's good to look at all available data, but overall, I have more faith in a generalized formula based on swept area and average wind speed. Variations of this method have been proposed and used by many well-known people in the industry, including Hugh Piggott, Jim Green of the National Renewable Energy Laboratory, Paul Gipe, Mike Klemen, and of course, David and Dean from Endurance and Windward Engineering (their wind consulting firm). All apparently have at least some mistrust of the manufacturers' power curves and AEO numbers.

But a generalized formula is a generalization—it cannot take into account all factors for an individual turbine. It does, however, take into account the two most important factors—swept area and wind resource. Beyond that, we can apply factors that tell us that a specific estimate is possible, impossible, optimistic, etc. I agree with Mick that a standard may help, depending on how it's structured. In the end, wind generator purchasers should get all the information they can, from as many sources as possible, before making a buying decision.

Ian Woofenden • Home Power Senior Editor