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UK windiness: First semester in 2016 lower than long-term expectations – what does this mean for you?

Monitoring and recording wind speed trends can be beneficial to assess project performance. But interpreting and truly understanding those trends are where the real value lies.

The UK Wind Index

DNV GL maintains a UK Wind Index, which enables owners and investors to assess the performance of potential or operating projects. Likewise, the UK Wind Index is a robust tool for wind farm developers, empowering them to understand the ‘windiness’ of their wind monitoring campaigns compared to a long-term period.

The UK Wind Index is continually maintained with a wealth of monthly average wind speed data from over 60 Met Office stations geographically spread across the UK mainland. Our tool covers the long-term period, from January 1996 to the present day.

The UK Wind Index is normalised so that the average wind speed over the period January 1996 to the present day is 100%, for a period representing all complete years in the index. The windiness of any given period is expressed as a percentage of the long-term average wind speed. Thus, a value exceeding 100% indicates that a period was windier than the long-term average, whilst a value below 100% suggests that a period was less windy.

Seasonal effects

Wind speeds in the UK exhibit strong seasonality, with a tendency for higher wind speeds during the winter months and lower wind speeds during the summer months. As a result, we in DNV GL also derive a seasonally adjusted UK Wind Index, which has been corrected for seasonal bias.

To be more precise, the windiness of any given period is expressed as a percentage of the long-term average wind speed for that specific period. For example, the long-term windiness of the month of January is 116%. If a specific (individual) January has an index value of 105%, the seasonally adjusted value for that January would be 105%/116% = 91%. In this case, this January is less windy than the long‑term average for January, despite being windier than the long-term average of the index.

Recent wind speed trends

Using our UK Wind Index, we have reviewed wind speed trends across the UK over the first six months of 2016 and these are presented in the below figures. For reference, the monthly index for 2015 is also shown.

The data shows that whilst January and February 2016 were windier than the long-term expectations for those months, the period from March to June has been less windy than the long-term trend. Most notably, March 2016 was more than 10% below the long-term average for March. The six-month period as a whole has been approximately 4% less windy than the long-term average for the first semester.

If seasonal effects are not considered, the first six-month period of 2016 has been approximately 0.5% less windy than the long-term average of the entire UK Wind Index. This indicates that the average wind speed of the first semester of 2016 is reasonably representative of the expected long-term wind speed.

A full breakdown of the monthly and quarterly wind indices for 2016 is given in the tables below, along with the ‘windiness’ for each complete year in the index.

The answer is in the data

But what does this drop in mean wind speed represent for owners, investors and developers?

Wind farm owners can expect to have seen lower production at their projects during the first semester of 2016, compared to the same period in 2015, as a result of the reduced windiness. This should be taken into consideration in reviews of the performance of a project.

Wind farm developers who have been conducting wind measurements during the first semester of 2016 can expect the average wind speed to be lower than the same period in 2015. However, the average wind speed recorded during this six-month period can provide a reasonable indication of the expected long‑term average wind speed at the measurement location.

And what does this mean for the future?

Will the wind speeds recover for the rest of 2016? Will they exceed the 20 year average wind speed record set in 1998? We will publish our 2016 wind speed trends review in February 2017. Stay tuned for more!

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