An important source of data on wind power performance in Denmark is the wind index which is available at http://vindstat.dk/. I noticed that the wind index had practically the same variation as the average production of onshore wind power (ref. 1). Therefore I suggested the "wind index" to be renamed to "wind energy index".

Inspired by the work of Professor Gordon Hughes (ref. 2) Per Nielsen from EMD International in Denmark has made some additional analyses which seem to confirm Professor Hughes' results. Per Nielsen has published his new results at http://vindstat.dk/ (ref. 3).

Based on this new recognition Per Nielsen says: "Conclusion is that the "wrong trend" or bias on index is 0.4% per year." The wind index has been adjusted correspondingly.

It is rather difficult to identify a systematic variation in the magnitude of 0.4% per year because other circumstances such as wind variations, design problems and maintenance policies are causing much larger variations.

In the Danish version of the paper Per Nielsen suggests the following possible reasons for the decreasing capacity factors of wind turbines:

1. Technical degradation, for instance rugged wing surface.

2. Forced outage, for instance due to gear problems.

3. Changing landscape

4. Metering errors

0.4% degradation per year will amount to nearly 10% in 20 years. This could be significant information for planners and investors.

Much larger degradations have been mentioned in the discussions. Therefore it is important to follow the development henceforward, to improve the statistical methods and to adjust the maintenance policy in accordance with the results.

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