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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 Iskra AT5-1 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.
The height of the tower can also have a significant
impact as described on our Which Tower FAQ page.
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
Iskra 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.
Further Reading
A more detailed guide to the energy generation from
wind generators can be found on the
Danish Wind
Energy Association's excellent web site, and this is well worth a
read when considering your turbine.
There is a good guide to
Wind Resource Assessment for Small Wind Turbines on the web site of
the Small Wind Industry Implementation Strategy's (SWIIS) web site.
The EU funded project
WINEUR
has its own web site containing many documents on urban wind for those
considering such an installation.
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