Whether you are contemplating buying a wind-electric system or trying to analyze the system you own, understanding wind speed is crucial. Wind is the fuel for the system, and measuring it will help you verify present performance and predict future performance.
Differentiating between instantaneous and average wind speed is very important. These are two very different measures—as different as your average speed while driving between Philadelphia and Boston, and the (instantaneous) speed the cop caught you driving at on the New Jersey Turnpike. If you were going 78 mph when the radar hit your car, no amount of explaining to the cop that your average was 57 mph will let you escape that ticket.
With wind energy, instantaneous wind speed is the less important measure, and average wind speed steps into the primary position. For residential systems, the average wind speed at the tower top is the primary measure used to quantify the wind resource. Let’s look at these two measures and the real-world ranges you will find.
Recorded instantaneous wind speeds range from 0 mph (of course) to about 231 mph—the latter has been experienced at Mt. Washington, New Hampshire. Higher wind speeds likely have occurred elsewhere, but have not been measured. At typical home sites, winds might run up to the 70 mph to 100 mph range a few times a year. More normal instantaneous wind speeds top out in the 40 mph to 50 mph range. But for wind energy production, the important range of instantaneous speeds is between 10 mph and 25 mph. Why?
Below about 10 mph, there isn’t a huge amount of energy available in the wind. Wind power is cubic—so assuming 1 mph produces 1 unit of power, a 10 mph resource will yield 1,000 units (10 × 10 × 10). At 5 mph—50% of that speed—you’ll wind up with 125 units (5 × 5 × 5)—12.5% of the power in 10 mph winds. At the other end of the scale, 25 cubed— which is 15,625—represents a lot of potential energy, and a lot of force on your turbine and tower. Between 25 and 30 mph, most turbines worth buying start shedding wind in one way or another, to protect the machine. A machine built to generate reliably in higher wind speeds would have to be awfully beefy, and it would probably not perform well at the lower wind speeds experienced most of the time at most sites.
It is useful to know the typical high wind speeds at your site, since this will impact the strength and durability required of your turbine and tower. If your site rarely has 50 mph winds, you might be able to use a less-expensive, medium-duty turbine. A heavy-duty turbine is needed for sites that regularly experience 80 mph winds.
Instantaneous wind speeds are interesting to monitor, but beyond determining the stoutness of your turbine and tower, that data isn’t very useful. The goal is not peak power (watts), or power at any particular wind speed, but energy (watt-hours). And to predict that, we need to use average wind speed.
Average wind speed is calculated by measuring instantaneous wind speeds over time on a site, and averaging them (see Mike Klemen’s nerdy article, “Wind Speed Data & Its Application to Wind-Generated Power,” in HP62). Most sites suitable for home-scale wind systems lie within the 6 to 12 mph average range. For an on-grid household, a 6 mph site will not typically be cost-effective, but, if you’re off-grid, it might be better than running your generator a lot in the winter. In most cases, a 12 mph site will make economic sense.
Utility-scale wind farmers are looking for even higher average wind speeds, in the 12 to 18 mph range. The highest recorded annual average wind speed is also on Mt. Washington, about 35 mph. But this is an extreme site, and most wind farms don’t exceed the low 20 mph average. Those predicting or reporting higher averages are either looking at shorter-term averages (like monthly, not annual, averages), or they are blowing smoke.
We use the average wind speed to predict wind generator production by looking at manufacturers’ energy curves (see example), looking at certification data, or using formulae (see “Estimating Wind Energy” in HP102 for one example). Average wind speed is to wind generator production what peak sun-hours are to solar-electric production.
To help avoid unrealistic expectations, I suggest that you:
• Clearly understand the difference between instantaneous and average wind speeds, and realize that the latter is most important.
• Understand that instantaneous wind speeds have lesser value, since the wind varies and the power available varies cubically with wind speed. Knowing a specific instantaneous wind speed only gives us one point of data, which is just as useless as one point in the power (kW) curve.
• Understand the realistic range of average wind speeds on residential sites, and try to ascertain the average wind speed at turbine height on your own site. This number is the most useful in predicting energy (kWh) output.
Ian Woofenden (ian.woofenden at homepower.com) lives off-grid with an average wind speed of 7 mph. While not remarkable, it saves lots of generator run time, fuel, and racket. His wind turbines see 60 to 70 mph a few times a year, and have experienced 100 mph at least once in 25 years of providing wind energy for his home.