annual mean wind speed

[Gustaaf Vandeborre]

Hello. I'm new here, and I hope to find lots of useful info.

As for this thread, how does one obtain data to calculate average annual wind speed for a specific location ? I'd like to install a windmill on my property, but first, I need to find out if there will be sufficient wind to make that a realistic and preferably cost-effective project.

I know there are wind data loggers available commercially, but how does that work in practise ? Should the data logger be on the same height as the mill ? At what intervals should data be logged ? How can I export the data to my computer ? And how can I calculate mean annual wind speed from the collected data ?

Looking forward to your input.

[Rob Beckers]

Hi Gustaaf,

Easiest would be if there is a wind atlas for your area. Generally those let you look up the annual average wind speed, and monthly averages, for various heights above ground. Putting up a data logger is even better of course, but expensive! That is usually not done for household size wind turbines. It doesn't have to be at hub-height, though the lower you get the more uncertainty that introduces in the calculated values for hub height (and there's a point where the effect of the ground and objects completely overwhelms the actual, regular winds).

-RoB-

[Gustaaf Vandeborre]

Hello Rob,

thanks for you reply. On the European windatlas, I'm in a region with 6.5 to 7.5 m/sec average windspeed at 50m hub height. Also, there's a weather station at 20 km from where I live that quotes an average windspeed of 5.2 m/sec at 10m hub height.

That might sound promising, but the thing is, the weather station is at a few hundred meters from the sea, whereas I am 20 km's land-inward. I live in Belgium, and we have lots of scattered villages, industrial buildings and farmhouses, with few open spaces.

There is an Enair Pro 70 (a 5kw machine) like 30 kms away, which so far has had an annual production in the 3000-3500 kwh range. That suggests an average wind speed of 4 m/s at 16m hub height. I'm afraid that's far more realistic. And to be honest, a return of 3500 kwh/year on an investment of approx. 20.000 euros doesn't sound like a great deal to me.

On the other hand, to complicate matters, I'm on top of a small gently sloped "hill" something like 50 meters above sea level, with the surrounding land at or slightly below sea level. I'm told that might make a considerable difference as to wind speed.

All in all, to make an informed decision, I think I'll have to obtain reliable data. So I'm trying to find out how to do that.

[Rob Beckers]

Hi Gustaaf,

One problem is with the measurement height of 50m: While not exactly linear, a first approximation would be, and if the usual hub height in your area is more like 16 - 20m then that would bring 6.5 m/s down to around 3 m/s or so.

The other one is about hub height. I know it's fashionable to use very short towers in Belgium / Netherlands for small turbines (I'm from The Netherlands), that also flies in the face of common sense rules about turbine siting (https://www.solacity.com/small-wind-turbine-site-selection/). Even in very flat terrain a number of 20m is used as the absolute minimum, anything less puts your more and more at the mercy of the effects of the ground and obstacles (and turbulence, a long-term turbine wrecker).

Being on top of a gentle hill would help (possibly quite a bit, as air speeds up going over the hump).

Ultimately the gold standard is an anemometer at the planned hub-height to log data for a year or so. That pricey though, in comparison with installing a relatively small turbine. Shorter towers for an anemometer can work, if you are certain you're outside of the effects of the ground or obstacles, or you'll just get very skewed data.

Realistically, unless you like DIY and can do this on a shoe-string budget I doubt a logging anemometer is a realistic option, and it's down to the wind atlas (hopefully an online one that allows for calculating at various heights), and as you noted, the results of others in the area.

I wish I had any great ideas for you, but unfortunately don't...

-RoB-