Is my site windy enough?
Wind speed varies with time, from nothing on calm days to occasional violent gusts. If the wind speed at a site is recorded over a year, it will be seen to vary about a mean wind speed value. This is the annual mean wind speed (AMWS) and is an indication of how much wind energy is available. In Europe, the AMWS could be as low as 4 m/s (9.0 mph) for an inland site to around 8 m/s (13 mph) or higher on the most exposed sites.
In general, you need an AMWS of at least 5 m/s to be able to generate a reasonable amount of energy, and ideally more than 5.5 m/s. The SENWEI turbine works well at low wind speeds, but because the energy available in the wind is a function of the cube of its speed, there is very little energy available to be harvested at wind speeds less than 4 m/s.
Seasonal Variations
Another thing to bear in mind is that there is normally more wind in the winter months than the summer ones. A wind turbine is therefore a very good match for heating and lighting requirements which are of course needed more in winter, but maybe not so well matched to air conditioning needs.
Wind Direction
In order for the turbine to be effective, it must have access to a good wind resource. This is one of the most difficult things to asses but in general the further away from buildings and trees, the better the average wind speed will be. This is especially true for any obstacles which are in the direction of the prevailing wind.
Hills and Valleys
An important consideration is that of valleys and hills. Wind speeds up as it nears the top of a slope and slows down considerably on the other side of hill as it descends into the valley. The exact impact of this effect is very dependant upon the exact local conditions but the diagram to the right gives a good indication of the impact of this. This can have a dramatic effect on the energy that the wind turbine will generate.
Urban Environments
SENWEI does not generally recommend a small wind generator in an urban environment as the AMWS in a town or city is likely to be substantially lower than in a rural environment and this will significantly reduce the annual energy output. This is due to the cumulative impact of the buildings that may surround the wind generator causing increased drag on the available wind and possibly severe turbulence.
Wind Speed Measurement
One thing you can do is actually measure your wind speed with an anemometer, which simply measures the wind speed. When this is connected however to a data logger, you can collect and analyse your average wind speed over a period of time. You would need to measure the wind speed for a period of at least 3 years to get a truly accurate average, as the wind speed not only varies during a year, but you can get “windy” and “calm” years, which can make a significant difference. You can however measure for a much shorter period and correlate your reading with a local weather station.
URL:http://www.windpowercn.com
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