If you have the space and can deal with the bureaucracy, a next step is to find out what sort of wind resource you have. “It’s always windy here” is not a scientific observation, and it’s also rarely true. Wind is the fuel, and finding out its strength and quality will be crucial to your wind-electric system’s performance.
What you want to find is an average wind speed. This is not a guess, not a peak gust, not an observation, but a measured or extrapolated average of the winds on your site over the years. At home sites where the turbine sits on a tall tower, a realistic average is between 6 and 14 mph. However, a 6 mph average wind speed site is not ideal for on-grid systems, and is marginal at best for off-grid ones. A site with an average wind speed of 14 mph will give high production (and will require robust equipment), but is rare where most people live. While there are certainly exceptions, average wind speeds of 10 to 12 mph at tower-top height are typical at rural home sites.
Determining the average wind speed on your site is not always easy. You may be able to do short-term monitoring with anemometry and correlate with nearby data, but this is difficult to do without proper training and experience. More likely, you’ll lean on a professional site assessment or wind mapping data. Don’t grab map data and take it at face value, though. Most trained site assessors have learned what data they can trust, and know how to derate or adjust the mapping data to determine real-world conditions in your area.
With an estimate of your average annual wind speed at hub height (the wind generator’s rotor center), you can look at production estimates from manufacturers (see the wind turbines table on pages 56–59). Then you’ll have a rough idea of what you can expect in annual energy production (kilowatt-hours; kWh) from your system, and compare it to your energy use. In this way, you’ll be realistic about what portion of your energy consumption and utility bill the system will offset.
Once it has been determined that a good resource exists, the most common mistake for small wind systems is putting a wind generator on too short of a tower. This is akin to putting a solar collector in the shade. Wind power is a cubic resource—doubling its speed increases the power available to the system eight times. So going a little higher to get into faster wind speeds can reap large rewards.
Since most people don’t live in a perpetual “tornado alley,” where winds are constant and high, another way to think of this is in reverse—what happens when wind speeds are cut in half, or even more? The “Power Available & Wind Speed” table gives you an idea of how much power is available in winds below the rated wind speed (often about 24 mph) of most well-designed wind turbines.
Choosing your wind turbine is, first of all, a match between your electricity loads—how many kWh per year you use—and the prediction of what your system’s production will be—also in kWh per year—based on the average wind speed at your site. Scan the table across the row for average wind speed, to see which of the turbines listed can meet your needs.
In addition to predicted energy output, you should consider the turbine’s durability, which is harder to gauge. Heavier-built turbines will tend to stand up better in rough storm conditions. Within a particular size range, more expensive turbines may also fare better. Turbines with longer warranties perhaps will last longer, or at least you’ll have recourse if there are problems.
This year, the first “certified” turbines will be available in the United States. As of mid-April, the Small Wind Certification Council (SWCC; www.smallwindcertification.org) had applications from 21 companies for 25 different turbine models. By the end of the year, some turbines will receive their certification with verified testing of energy production. This is an exciting development in the industry, which has been troubled for years by misleading marketing claims.