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 the 2018 results mean for you?
2018 wind speed trends
In our last blog, we reviewed the quiet first half of 2018 and how this would have impacted the wind industry across the UK. The first semester of 2018 had been approximately 5.6% less windy than the seasonally adjusted long-term average of the entire UK Wind Index for all first semesters. This indicates that the average wind speed of the first semester of 2018 was below expectations, mainly due to the very low wind conditions in Q2 (89% seasonal index). This was highlighted further in May and June 2018, where the UK experienced very low wind conditions. There was a record of several days with almost zero wind power generated. This was largely caused by the jet stream moving north of the UK, which blocked out more windy, wet and cloudy conditions from the Atlantic Ocean. This meant that the UK missed the higher winds from the west which provide stronger and more stable winds.
Although the low trend continued into July 2018, wind resource started to return to normal expectations during the following months, resulting in an expected 99% seasonal index for Q3. A notable event was observed on 30th November when the UK reached a new installation record of 14.9 GW of onshore and offshore wind generating capacity. National Grid advised that wind farms had generated almost 32% of Britain’s electricity demand which can also been validated with the higher than expected November index of 111%.
Overall, DNV GL’s UK Wind Index demonstrates that 2018 was 2% less windy than the long‑term average. A full breakdown of the monthly and quarterly wind indices for 2018 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 2017 is also shown.
Interpreting the data to establish an answer
But what does the drop in mean wind speed during 2018 represent for owners, investors and developers?
Wind farm owners will have seen lower production at UK sites during 2018, compared to the long term, because of the reduced windiness. Using typical project wind speed to energy sensitivity ratios, it is noted that a 2% 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 site performance. Investors have seen slightly lower energy productions in wind projects when compared to 2017.
Wind farm developers who have been conducting wind measurements during 2018 would also have noticed that average wind speed was lower than the same period in 2017 and also lower than 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 2018 deviation from the long-term mean wind speed is noted to fall within the expected range of wind speed inter-annual variability. Therefore, while 2018 has been a normal 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 developers, 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 using Reanalysis validated data from NASA. 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. Therefore, a value exceeding 100% indicates that a period was windier than the long-term average, while a value below 100% suggests that a period was less windy.
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
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