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 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.
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)
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.
|= 140 kWh /365 days
= 383 watt hours/day
|= 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.
The Danish Wind
Turbine Manufacturers Association http://www.windpower.dk/tour