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Wind Energy

Wind Design

Although wind generators may cost less per watt than a comparable PV array, finding the right location is critical. The determining factor is the amount of wind available at the chosen site.

Measuring the Wind
There are three primary ways of determining how much wind is available.
1. Installing an anemometer.
2. Using existing data.
3. Visual observations.

An anemometer is the most accurate way to determine wind speed. For a large installation it usually takes several years of readings, an anemometer, a data logger and a computer to compile accurate data. Unless you plan on spending $15 000 or more on a wind energy system, this is usually not a necessary step.

There are inexpensive anemometers that can be read manually on a daily or weekly basis. For a small residential wind generator a minimum of three months of data should be recorded and correlated with another source of wind data. Some people install a small wind generator and use it as an anemometer if they are planning for a much larger installation.

Existing Data
Existing data can often be obtained from the local airport or meteorological stations. Other places to contact would be universities, colleges and radio stations. The Canadian government compiles wind data in catalogue form, and information is available on the internet. Wind speeds can vary a great deal within a small area, so this information should be correlated with another method like the Griggs-Putnam Index.

Griggs-Putnam Index
The Griggs-Putnam Index is the simplest and quickest method of determining average wind speed. Flagging of foliage (deformation of branches away from the prevailing wind) is correlated with average annual wind speed. If you have a flag at your site and the flag is stiff in the wind for a few hours a day then the site is a good candidate for wind power.

How much wind is enough?
As a general rule, an average annual wind speed of at least 10 mph (16 km/h) is necessary for a cost effective wind power installation. If wind power is being used as seasonal back up for a solar electric system, or if less power is required during calm periods of the year this rule can be modified. Use the average wind speed for the season in which you need the power.

How much power do I get?
Average annual wind speed is used to predict annual energy output. There is some variation in annual energy output because some locations have gusty, intermittent winds and other locations have steady trade winds. Different locations may have the same average wind speed yet produce very different amounts of energy.

Annual Energy Output (kWh)

Average Annual Wind Speed (mph)

wind turbine
AIR 303 60 140 250 400 n/a
WS 503 340 580 810 1040 1240
BWC 850 310 930 1530 2120 2620
BWC 1500 700 1500 2500 3500 4300

To determine how much power is available on a daily basis. The Annual Energy Output must be divided by 365 days to give the Daily Energy Output. For the purposes of battery based system design this Daily Energy Output must be divided by the System Voltage to give the Daily Amp-Hours.

Example In a 12 volt system, where the average annual wind speed is 10 mph, the AIR 303 will produce 140 kWh.

Daily Energy
= 140 kWh /365 days

= 383 watt hours/day

Daily Amp
= 383/12 volts
  = 32 amp_hours per day

Therefore, if you have an energy budget of 32 amp-hours per day the Air 303 will supply the power you require at an average annual wind speed of 10 mph.

Recommended Website
The Danish Wind Turbine Manufacturers Association

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