www.turbotricity.com
Estimating Wind Speed and Turbine Output on Your Site
There are many optimistic figures being thrown around for wind turbine production. We don’t believe in
misleading the public – payback times for turbines are not short, and they are a long term investment. During the
life of your turbine, energy prices will almost certainly rise, and payback times will come down as a result.
In the days when people actually bought houses, they ended up with mortgages way above current rent, because
rents would go up and sooner or later pass out their mortgages. So even if doing these sums comes up with a 16
year payback time, that is a lot better than putting money in the bank at 4%.
That said, there is a need to evaluate your site and work out the wind speed, and other factors that influence output
(turbulence caused by buildings and shade from certain directions.
This work is broken down as follows;
1) Look at wind maps and assess the wind speed at your site. This will be given in metres per second (m/sec) at
an assumed height of 50m, 75m or 100m.
2) Examine the “roughness class” of the land. This is friction which reduces the wind speed closer to the
ground. We use an online calculator to adjust the wind speed from the 50m height to the 11.5m height we
use, based on the effect of the roughness class.
3) Using this wind speed as a mean wind speed and the turbine power curve, we can use another online
calculator to estimate the annual output of the turbine.
4) We then look to a wind rose to see what direction the wind normally comes in. If there are obstacles
interfering with wind coming from the NE for example, we make an adjustment to the output based on
losing some or all of the output from this direction.
5) We can then put that information into a spreadsheet to estimate the financial savings.
If all of this is Greek to you, please let us know and we can carry out an assessment for you. We like to give
people the information so they can do this for themselves, but please contact us if you would like us to carry
out a site assessment.
Wind Maps
In the Republic of Ireland, we use the wind map at
http://esb2.net.weblink.ie/SEI/MapPage.asp - you should be able to click on this
link. This give you a window like this;
Click on the area of your site and keep clicking until you zoom in to your site.
If you make a mistake you can click on the Zoom Out button and then click on
the screen. Then click on the zoom in button and continue to find your
location. When you are satisfied that you have identified your site from the
map, you can click the Info button.
You then click on a part of the map that is closest to your
site. At this stage a new information box will appear in the
left side of the screen, with a range of wind speeds ad
different heights. The higher you go, the stronger the wind is
this chart marked “layer info”. .
top
on
In our case here, the info is showing a wind speed of 8.75 to 9 m/sec at 100m, 8.5 to 8.75m/sec at 75m height and
between 8 and 8.25m/sec at 50m. For our purposes, we will use the lower figure as the mean wind speed at a height
of 50m. This is the unconstrained on-shore wind speed average over the year.
Roughness Class
Vegetation, buildings and other factors slow the wind close to the gound, and to estimate the effect of this, we assess the roughness class of the land. A
roughness class of zero would be the sea (which is why offshore turbines work so well). Roughness class 0.5 would be an airport runway or very flat expanse
of land with short grass.
The following is a table of the various roughness clases.
Roughness
Class
0
0.5
1
1.5
2
2.5
3
3.5
4
Landscape Type
Water surface
Completely open terrain with a smooth surface, e.g.concrete runways in airports, mowed grass, etc.
Open agricultural area without fences and hedgerows and very scattered buildings. Only softly rounded hills
Agricultural land with some houses and 8 metre tall sheltering hedgerows with a distance of approx. 1250 metres
Agricultural land with some houses and 8 metre tall sheltering hedgerows with a distance of approx. 500 metres
Agricultural land with many houses, shrubs and plants, or 8 metre tall sheltering hedgerows with a distance of approx. 250 metres
Villages, small towns, agricultural land with many or tall sheltering hedgerows, forests and very rough and uneven terrain
Larger cities with tall buildings
Very large cities with tall buildings and skyscrapers
Lets assume that our site has a roughness class of 2, with tall hedgerows (8m high) about 500 m from
the turbine. We now use an online calculator to estimate the wind speed at our proposed hub height of
11.5m by clicking on http://www.talentfactory.dk/en/tour/wres/calculat.htm
We enter our 8.25m/sec wind speed into the box corresponding to our roughness class and height of
50m
And press the calculate button
This gives us a wind speed of 6.11 m/sec
at a height of 10m. We could take that as a rough
figure, or press the PLOT button below this to get a
wind shear graph.
This wind shear graph
can be read off
against the height of
our mast, which is
11.5m giving a mean
wind speed at this
height of about
6.3m/sec.
This is the mean wind
speed at your site at a
hub height of 11.5m.
Turbulence caused by local topography or buildings
We now need to consider whether there are obstacles from specific wind directions that affect the output of the
turbine. You may have a site that is wide open to winds from south-east via
south and west to north-west, but a house or barn to the north and east of the
turbine blocking winds from those directions.
If you suspect that there is an issue with this, you can either get a wind rose for
your local met office station, or put an estimate on the overall loss from that
wind direction.
A wind rose for wind site assessment in the UK looks like this, but the Irish Met
Office doesn’t use this format. This format shows the frequency of direction,
but more importantly in red, the usefulness of the wind to turbines from each
direction (this is based on wind frequency X wind speed cubed.
Irish Met office wind roses look like this and are less useful
because they only indicate frequency of direction, but without
correlating the useful wind speed from that direction. For most
purposes, people are happy to put an estimate of the loss from a
particular direction.
You can take an approximation from the map here which appears on
the Met office website at http://www.met.ie/climate/wind.asp . This
shows the frequency of winds from different directions, but again
without any information on wind strength.
If you believe your site has turbulence or loss of output from a specific
wind direction, you can either assess the site over a period of time using
an anemometer and note the effect on wind speed from specific
directions, or simply make an estimate of the lost output.
Generally, winds from the East are at lower wind speeds, so their
effect on a turbine’s performance is quite low. However,
obstacles upwind of the turbine to the West should be avoided.
What Next?
When you know your mean wind speed at the hub height, you get a power curve and enter all your details into a
calculator at http://www.talentfactory.dk/en/tour/wres/pow/index.htm to get an estimate of annual electricity
production. There is a guide to this calculator at www.talentfactory.dk/en/tour/wres/guidep.htm
We can do this for you in relation to our own turbine – email your mean wind speed to [email protected]