Wind power in the form of windmills has been around for a long time. Relatively short towers once hosted large rotors with many blades to produce the power and torque needed for tasks such as grinding grains and running machinery. During the 1930s, wind-electric generators made their way into rural areas lacking electric power lines. These low-voltage machines were primarily used to charge batteries that powered low-voltage direct current (DC) home appliances. Some were dedicated to pumping water.
Today, a handful of manufacturers produce wind-electric turbines for both grid-tied systems and off-grid battery-charging applications. “Turbine” generally refers to the combination of blade set and generator assembly, while “generator” refers specifically to the electricity-producing unit.
Modern wind machines use high-efficiency generators or alternators and highly refined blade designs and materials for maximum efficiency. Other wind-electric devices range from small rooftop wind machines to vertical axis wind turbine (VAWT) designs. While there is encouraging research on these new designs, current best practices for harvesting wind energy highlight the traditional horizontal axis wind turbine (HAWT). I’ll follow suit and focus on how wind fundamentals apply to HAWT.
Wind turbines generate electricity by capturing the wind’s energy as it rotates two or three propeller-like blades attached to a generator that produces electricity when it spins. The turbines are mounted on towers 100 feet tall or taller to take advantage of the faster, stronger, and less turbulent wind at that height.
Many locations have some potential for capturing wind energy, but the resource varies widely with location, season, and time of day. Elevation, exposure, terrain, and trees or other obstructions all affect available wind energy.
Most small wind turbines employ an “upwind” configuration, meaning the rotor spins in front of the tower, oriented into the wind by a tail vane downwind of the rotor. Kingspan wind products are the notable exception; they’re downwind and don’t have a tail. Downwind turbines look unconventional, but they can perform just as well as their upwind counterparts.
Large utility-scale generators and residential grid-tied systems produce alternating current (AC), but a number of DC wind generators can be used as battery chargers for off-grid applications. In either case, the power must be managed and manipulated before it can be used. Residential wind generators range in peak power generation from 50 watts to 10 kilowatts or more, and may cost between $3 and $5 per peak “rated” watt.
However, while the peak power rating of a wind generator may help you understand the relative size and capacity of the unit, it’s not the best measure for comparing different machines, because it has little bearing on how much energy will be delivered over time.
Before buying and installing any wind-electric system, you must assess your site for wind power potential, determine how much energy you hope the wind will produce, and research which models will deliver what you need based on your site and the models’ specifications. You’ll likely have a few options, and you’ll need to compare differences in cost, quality, durability, sound level, and production level at your site. Also, be sure to know what’s covered by each manufacturer’s warranty, as this is a good indicator of a company’s trust in its own products.
The amount of energy that can be captured from moving air is a function of wind speed, wind collector area (called the swept area), and air density. You can calculate the wind energy available at a particular site with a particular machine yourself, but be sure to stick to one set of units — metric or imperial — when working with formulas.
Swept Area Trumps All
In terms of evaluating the potential performance of a wind turbine, swept area is the most important factor. Swept area is the circular area described by the spinning rotor. It can be expressed in square feet or square meters. It’s reported on manufacturer specifications, but you can calculate it yourself using the rotor diameter and applying the formula for the area of a circle:
Swept area = π x radius 2
For example, if your wind turbine has a rotor diameter of 10 units (the units may be any measure of length), the radius is 5 units. Therefore, the swept area is 3.14 x 5 2 = 78.5 square units.
Swept area increases exponentially with rotor diameter. Compare our 10-unit rotor with the swept area of a 12-unit-diameter rotor:
3.14 x 6 2 = 113 square units
As these examples show, a 20 percent increase in rotor diameter results in a swept area increase of 44 percent. Doubling the length of the rotor blades quadruples the swept area, and so on. Simply put, the greater the swept area, the more energy a wind generator will produce, given the same wind resource. The chart in the slideshow shows that doubling the swept area doubles the potential power output.
Air density, expressed in pounds per cubic foot or kilograms per cubic meter, decreases with increasing altitude, temperature, and humidity. Manufacturers rate the output of their machines at a standard temperature of 59 degrees Fahrenheit and sea level elevation.
Air density has a relatively smalleffect on available energy when compared with wind speed, but basically, there’s more energy available in a flow of cold, dry air at low altitude than from warm, humid air high in the mountains. All other things being equal, the change in air density is roughly 3 percent for every 1,000 feet in elevation change.
Wind speed is expressed in miles per hour (mph) or meters per second (mps); 1 mph is equivalent to 0.447 mps, conversely, 1 mps is equivalent to 2.24 mph.
Power increases cubically with velocity, so doubling the wind speed increases the available energy eightfold. Small changes in wind speed yield dramatic changes in energy produced (see "Power Relative to Wind Speed," slideshow). Keep in mind that the actual power output of a turbine varies with the swept area and generator capacity, but wind velocity and power always maintain the same relationship.
Note: Many wind turbines don’t start turning until the wind speed overcomes the inertia of the system, often between 7 and 10 mph. This is called the cut-in speed. It’s not worthwhile to try to capture winds below about 8 mph; in addition, winds above 30 mph are likely to damage a turbine that’s still trying to generate electricity with them. Most turbines have mechanisms to limit speed and protect themselves in high winds. Be wary of advertising that claims energy performance values in winds below 6 mph — it just isn’t going to happen! Likewise, performance claims above about 30 mph indicate that someone wants to sell you a machine that may end up tearing itself apart in high winds.
Energy, expressed in watt-hours for our purposes, is a quantity of power (wattage) produced over time. The energy produced by a wind generator is a function of four elements:
1. Average wind speed at the tower site.
2. Tower height (taller towers provide access to faster, more consistent winds than those available closer to the ground).
3. Wind speed frequency distribution, based on data showing how many hours during the year the wind blows within a certain speed range (this range of combined data points is called bin data).
4. Wind turbine power curve, indicating the power produced by the generator at various wind speeds.
The power available in the wind can be expressed by the following relationship:
Power = (air density ÷ 2) x swept area x (wind speed 3)
Manufacturers publish power curves showing their machines’ power output, in watts, at various wind speeds. A power curve is mostly useful for determining whether, when, and to what extent the machine will protect itself in high winds. If the power curve drops off steeply at wind speeds above 25 mph, the turbine’s over-speed protection mechanism has likely activated, and the rotor will furl, or turn itself out of the wind, and stop producing. A small drop or flattening of the curve may indicate at what speed the blades will pitch, limiting the rotor to a maximum speed.
More useful than a power curve is the energy curve. This is the real nugget of information you'll want to use when estimating the value of a wind turbine at your site. The energy curve indicates how many kilowatt-hours are produced over a specific period of time, given the average wind speed at the turbine’s location.
It’s tempting to judge wind generators by their maximum power output; however, this measurement is taken at a specific (but arbitrary) wind speed. One manufacturer might rate the output of its machines in 24 mph winds, while another might rate its machines’ output in 28 mph winds. The additional power produced at higher wind speeds might make the 28 mph machine look somehow better than the other. However, the rated power output isn’t an indication of energy delivered!
Until recently, there was no standard for small wind turbine performance ratings. A new American Wind Energy Association performance and safety standard specifies 24.6 mph as the speed at which output power is rated. The Small Wind Certification Council is working independently to verify test results and to certify and label wind machines to the AWEA standard so that consumers have a better understanding of performance ratings and comparisons.
Rather than being overly concerned with the maximum power rating (watts), you’ll want to know how much energy (kilowatt-hours) a generator will deliver at your site and in your wind conditions. For this reason, it’s best to compare wind machines by swept area and the test results of energy production at various average wind speeds, rather than by rated power output. When comparing machines using the SWCC ratings, the important value is the rated annual energy.
Wind-electric systems operating in lower average wind speeds will produce less energy than those in higher average speeds. Remember, the cubic relationship between wind speed and watts means that doubling the wind speed available to your wind turbine will increase the available power eightfold. Cutting the wind speed in half will result in one-eighth the performance.
Increasing your turbine’s aboveground height will offer access to faster, more consistent, less turbulent winds. In general, a minimum average annual wind speed of 10 to 12 mph is the point at which wind power generation makes economic sense, depending on the cost of electricity in your area. (There are, of course, other values you might want to place on wind power.) This doesn’t mean the wind blows 10 mph or more all the time; it’s an average of the wind velocities occurring throughout the year. Sometimes the wind is at 0 mph, while other times it may blow at gale force.
Before building an expensive addition to the list of things you need to maintain, obtain long-term (at least one year’s worth) wind speed data for the specific site you’re considering. There are several ways to get this information, and they vary in ease and accuracy.
Internet tools. The U.S. Department of Energy’s WINDExchange website offers a wealth of information about wind power. The National Centers for Environmental Information maintain long-term records of wind speeds in various locations around the United States. In addition to this historical data, there are other online resources that offer reasonable estimates of wind energy availability in many places around the country. The National Renewable Energy Laboratory has produced some excellent wind resource maps (estimated for turbines placed 80 meters aboveground) that others are using to build estimating tools. One such effort is the Distributed Wind Site Analysis Tool, which guides you through selecting your site and a wind machine. Most residential systems use towers about 30 meters tall, which will operate with somewhat lower wind speeds.
Subjective assessment. Aids such as the Beaufort scale and the Griggs-Putnam Index of Deformity can give you a general sense of the wind resource at your location, but an accurate assessment will require more rigorous testing. The Beaufort scale only works in the moment, and you’ll need long-term data; the Griggs-Putnam index can be useful if you have a lot of conifers around, as they’re especially susceptible to permanent deformation in consistent winds. However, even if you don’t see any growth deformation in local trees, you may have a decent wind resource. You’ll need concrete data to confirm the wind resource available to you.
You can assess the presence of buildings, trees, and other obstructions that may create wind turbulence subjectively. Turbulence is very hard on wind turbines, creating lots of stress on bearings and the tower without much power to show for it. Avoid subjecting your turbine to unnecessary turbulence by locating it well away from wind obstructions.
Recording anemometer. The most accurate way to measure site-specific wind resources is to use a recording anemometer. This device measures wind speed along with the duration of specific speed ranges and stores the data electronically, allowing you to quantify the “wind regime” at your site accurately and to estimate long-term energy production. For accurate measurements, place the anemometer at the same height as your proposed wind generator. Data-logging systems are available from professional wind equipment suppliers.
Before buying anything, assess not only wind-electric systems, but also the wind resource at your proposed site. With the right preparation, you can harvest energy from the air itself.
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