The Wind Index, as calculated by 3E, is a useful tool to show the wind potential of a certain period with respect to the long term . The analysis is based on the MERRA2 reanalysis dataset from NASA and focuses on the entire year 2017. The results demonstrate that the past year was better than 2016, however still below average.
3E’s approach to wind Index
3E has two distinct approaches to assess the Wind Index: a first approach is called the “Cumulative” Wind Index, comparing the wind performance of a certain month with respect to the average monthly wind performance between 2000 and 2017 (the reference period). This method is typically used for long term extrapolation based on the Wind Index.
The “Short Term” Wind Index shows the wind performance of a given month with respect to the average performance of the same month between 2000 and 2017. The main difference between the 2 approaches is that the second one eliminates the effects of seasonality, and is used to look at the performance of a specific month.
Promising rise brings hope for 2018
In the present analysis, the “Cumulative” Wind Index for Belgium was calculated based on 16 reference points spread over the country. Results demonstrate that the wind index for 2017 represents 95.9% of the average over the past 18 years (2000-2017). This implies that the energy production potential of wind in Belgium during 2017 was 4.1% lower compared to the previous 18-year reference period. There are only 3 other years since 2007 which have lower averages (2010, 2013, 2016). However, it still implies a relevant increase compared to 2016 which had a wind index of 93.1%.
In the following graph, the 100 line (Benchmark) represents the average wind potential of the considered reference period.
The “Cumulative” Wind Index for France was based on several reference points spread over the country. The figures for France show a similar trend for 2017, with an average wind index of 94% for the considered points. It is again 6% lower than the last 18 years average, but shows an increase comparing with 2016 (91%), which was the worst of the last 10 years.
Inconsistency throughout the year
A “Short term” Wind Index analysis for the same period and data points, shows that there is a high irregularity in monthly performances. As it can be followed in the figure, 2017 is full of rises and falls. There is no consecutive 3 months period over or below benchmark (100) line.
High hopes for 2018
A closer look on the graph demonstrates that the number of overachieving months (over 100) is very low, especially for France. This confirms again that 2017 was not a very good wind year.
On the other hand, last months of the year were a lot better and partly made up for the previous months. . Average “Short Term Wind Index” performances for the first 8 months of 2017 are respectively 89.4% and 91.3% for France and Belgium. Whereas, last 4 months of the year averaged respectively 102.9% and 104.6% for France and Belgium.
How does this translate in production values?
A decrease in wind index values also corresponds to a decrease of the wind farm production for the considered period with respect to the expected long-term average. However, such variation over time is a very well-known phenomenon and can be modelled relatively easily. 3E’s uncertainty analysis, which is part of a standard P90 calculation, typically accounts for this concept through the future wind variability. Any P90 figure would therefore sufficiently cover for these lower wind periods.