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UK windiness 2017: Lower than long-term averages – what are the implications 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. Do you know what do the 2017 results mean to you?

2017 wind speed trends

In our last blog, it had become apparent that the first semester of 2017 had been approximately 1% 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 2017 was reasonably representative of the expected long-term wind speed. The highlight was June, where the UK experienced the windiest June for 15 years, with the average wind speed 14% above the long-term average for the last month of the first semester.

Looking at the second semester, most of the months were equal to or less windy than the long-term average. However, October was approximately 12% windier than all previous October months recorded due to the effects of Hurricane Ophelia, which influenced windiness conditions over the UK.

Overall, DNV GL’s UK Wind Index demonstrates that 2017 was 1.9% less windy than the long‑term average. A full breakdown of the monthly and quarterly wind indices for 2017 is given in the tables below, along with the ‘windiness’ for each complete year in the index. For ease of referencing, the monthly index for 2016 is also shown.

Interpreting the data to establish an answer

But what does the drop in mean wind speed during 2017 represent for owners, investors and developers?

Wind farm owners can expect to have seen lower production at UK projects during 2017, compared to the long term, because of the reduced windiness. Using typical project wind speed to energy sensitivity ratios, it is noted that a 1.9% reduction in wind speed corresponds to a reduction of approximately 3% to 4% in terms of energy production. This should be taken into consideration during reviews of project performance. On the other hand, investors have seen higher energy productions in wind projects when compared to 2016.

Wind farm developers who have been conducting wind measurements during 2017 can also expect the average wind speed to be higher than the same period in 2016, but lower in the long-term. This trend should be taken into consideration when adjusting measurements to be representative of a long-term period.

However, the observed 2017 deviation from the long-term mean wind speed is noted to fall within the expected range of wind speed inter-annual variability. Thus, whilst 2016 has been a low wind speed year, this event is expected to be covered by “industry standard” assumptions for future wind speed uncertainty, as modelled in energy production assessments.

The UK Wind Index           

DNV GL maintains a UK Wind Index, which enables investors, 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 normalized 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 115%. If a specific (individual) January has an index value of 92%, the seasonally adjusted value for that January would be 92%/115% = 80%.

About DNV GL

DNV GL offers a new service for your Portfolio generating bespoken Windiness Reports on a monthly basis delivered to you on the 1st day of each month. This includes an average of your entire portfolio and a breakdown of each one of your assets. For more information please contact Ioannis Agiol.

Learn more about our full range of services and products.

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