A precise understanding of your wind resource is the cornerstone of any wind project. While some of the best resources are found on agricultural lands, the power in the wind varies greatly from one location to another. There are two initial steps to determine the amount of wind available at your site.
First, you need to decide how you want to use wind power. There are many kinds of wind turbines, many kinds of projects, and many uses for wind energy. Some homeowners produce a small amount of electricity just for use on their farms. Others produce excess electricity and sell it back to a utility company. Still others lease their land to developers who build large wind facilities and market the power. The size of your project will help determine the scope of your wind analysis.
Second, you need to determine how much wind blows across your site, and when. Analyzing your wind resource is similar to evaluating a traditional crop to be harvested. Talking to experts and other landowners who have invested in wind energy can help you use "wind prospecting" techniques for estimating your resource. If your site looks promising after a first-pass assessment, you may want to install monitoring devices and take measurements over a period of time.
The extent of your wind resource may help determine what type of a turbine you want to install, and your lifestyle and profit goals may tell you whether a wind project is worth your time. Many landowners install wind turbines simply because they believe strongly in using non-polluting, inexhaustible forms of energy.
Speed is the most important factor to consider. Wind speed varies from year to year, season to season, day to day, and with the height above ground. You may determine your site's average annual wind speed, or compare detailed data showing fluctuations in wind speed to your electric usage. Because the power in the wind has a cubic relationship to wind speed, a site with an average 15 mph wind speed contains nearly 100 percent more energy than a site with an average wind speed of 12 mph. For those who find rules of thumb useful, remember the "4-5 rule": observe that 4 is 20% less than 5 and that 5 is 25% more than 4. A wind speed that is 25% greater will have almost 100% more energy, and so a wind whose speed is 20% lower will have only about 50% of the energy.
One place to start is reviewing existing wind maps. Wind Powering America's State Wind Resource Maps depict the wind resource potential of states throughout the country. Many states have also developed more detailed maps. Keep in mind, a wind map will not show you everything you need to know about your wind resource, as they are only computer models of the wind speed at any one spot. If you are in Minnesota and considering a home or farm scale system, an excellent resource is this interactive tool provided by the state.
The best method of assessing wind speed is to measure it with an anemometer placed on a tower at the location and height you plan to install your wind turbine(s). You may even use several anemometers mounted at different heights or at several locations to provide more detailed data. You can learn how to collect and analyze the data yourself, or hire a professional. A good study of wind resources will include data from at least one year and preferably two years.
If you decide not to monitor the wind at your own site, you may be able to use data from nearby sites such as airports or state-administered meteorological stations. Computer programs are available to help you extrapolate your own wind resource from other sites. Correlations to long-term local weather data can also help provide insight into the magnitude of annual variations you may experience.
Wind Speed Distribution
Wind doesn't blow at the same speed all the time. The ideal wind resource has relatively stable high speeds. If your trees and vegetation are permanently deformed due to constant wind exposure, also known as “flagging,” you may have a good wind resource to generate electricity.
Flagging graphic from the Department of Energy
Daily and Seasonal Wind Cycles
We tend to need electricity most during the daytime and either during the coldest or hottest months. If your wind blows strongly when you need electricity most or year-round, then you have a good match between the wind energy and your load.
To ensure the most effective use of a wind turbine, it should be exposed to the most energetic wind. Though the wind may blow more frequently from the west, more wind energy may come from a different direction if those winds are stronger. It is very important to find out which directions have the best winds for electricity production. A wind rose chart, which is generated from your wind resource assessment, is a helpful tool to determine wind direction and distribution.
Wind Rose graphic from Oregon Water Science Center
Wind shear is the increase in wind speed at greater heights above ground. You may be able to increase your energy production by installing a turbine on a taller tower.
Air Pressure and Temperature
Air pressure and temperature affect the amount of energy in the wind to a minor degree. Regional data is adequately accurate to estimate pressure and temperature influences.
When wind flows around buildings and other structures in the landscape, it slows down or becomes turbulent. A wind turbine should be placed in a location where the influence of obstacles is minimized.
"Roughness" refers to the terrain and density of vegetation on the landscape. Your turbine will be affected by roughness - the smoother the better - within a 19-mile (30-kilometer) radius, so you want to determine how rough your land really is.
All of the above are factors that impact your wind power. Wind power determines the amount of electricity available to be produced on an annual basis. The more information you gather about your wind resource, the better your decision about investing in wind energy will be.