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Farm wind turbines: How to compare power production

February 03, 2012

When comparing models of farm wind turbines, many of the people I talk to are very keen on the published “power rating” of the generator. Very understandable... but we should look a little closer.  Does it actually do what it says on the tin? The answer may surprise you.

What exactly does the rating on each turbine tell us? The answer is that this is the optimum power generated under certain highly specific conditions:  principally optimum wind speed. So in itself, the rating is accurate.  And it’s a useful starting point. 

However, the devil is in the detail – that we do not use a standard wind speed for these ratings starts to undermine this kind of comparison.

Leading US commentator Mick Sagrillo recently looked at three farm wind turbines, all with similar rated power. *

Two of the farm wind turbines were rated at 10 kW while the third was 11 kW:  I merely mention that this is the rating of our Gaia-Wind 133 model.  His findings were that although the three had claimed pretty much equal power “ratings” when tested at the AWEA standard wind speed of 25 mph:

•    one generated 2.4kW while another -  our 133 model - generated  nearly 14kW
•    that’s a difference of over 400% in performance!

In addition, what’s important to our customers is comparing how much energy your proposed farm wind turbine will generate over time. On annual performance Mick had this to say:

“I picked an 11-mph average annual wind speed [approx. 5m/s], which makes a respectable small wind turbine site. Turbine No. 3 (the Gaia-Wind 133) produces more than EIGHT times the electricity annually as turbine No. 1 at this site, even though their generator power ratings are nearly identical.”

The study concluded that the swept area of your farm wind turbine is more important than the size of your generator. Not surprisingly the “low producer”, in this study had a significantly lower swept area than the Gaia-Wind model but the point is made: The findings of the 2010 Glasgow University research study into blade size support this position.

So why is all this important when comparing farm wind turbines?

•    over the course of this year the variety of turbines in the 10-20 kW range is set to grow
•    customers need practical and realistic ways of judging what’s best for them

In the UK we have the benefit of the MCS certification scheme. This means that don’t need to rely on marketing claims alone: if it’s MCS certified, the actual Annual Energy Production (AEP) figures will be available – ask the supplier. Make sure you have seen and compared these before making any decision.

Johnnie Andringa, CEO

*Mick Sagrillo - Shopping for a Wind Turbine: Power vs. Energy  

An article on this topic was published in The Scottish Farmer, 3rd February 2012 – “Comparing Apples With Apples”

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