Monitoring and recording wind speed trends can be beneficial to assess project performance. However, interpreting, understanding and acting on these trends are where the real value lies. Do you know what the 2019 results mean to you? How is 2020 looking so far?
2019 wind speed trends
The windiness index followed a rather abnormal trend across 2019 based on the seasonality of wind resource in the UK. Most months would be characterized as extreme either on the positive or negative side with indices deviating from -20% to +20% for various months. Windiness was impacted by a series of storms which hit the UK, including storms Erik (8-9 February), Freya (3-4 March), Gareth (12-13 March), Hannah (27 April) and Atiyah (8-9 December), but also by more settled spells of weather with low wind speeds.
Overall, DNV’s UK Wind Index demonstrates that 2019 was nearly 1.5% less windy than the long‑term average (98.4%). A full breakdown of the monthly and quarterly wind indices for 2019 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 2018 is also shown.
The year started slowly with the first two months exhibiting wind speeds lower than the seasonal expectations based on the long-term reference. But the wind did pick up in March, comprising a Q1 seasonal index of 98.6%. This downward trend persisted in Q2 (95.9%) with low wind speeds and generally high-pressure systems persisting in the country, resulting in pleasant weather and low wind speeds. However, in Q3 the country saw a shift in the pattern with wind speeds 5% higher than the seasonal mean, bringing a healthy mix of wind and solar-generated power to the grid. In the end however, weather conditions in the UK were not good enough to raise the index to higher levels as per the seasonal expectations, which finally recorded a value of 95%.
It is also important to consider the spatial distribution of the wind regime across the country during last year. Looking at the windiness maps below, we can observe that the traditionally high wind speed regions of the UK (Northern Ireland, Scotland, Offshore, etc) were almost always less windy that the long-term expectations (with the exception of Q3) which played a profound role in the final annual windiness index.
Interpreting the data to establish an answer
But what does the drop in mean wind speed during 2019 represent for owners, investors and developers?
Wind farm owners can expect to have seen lower production at UK projects during 2019, 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 3-4% in terms of energy production. This should be taken into consideration during reviews of project performance. Investors have seen slightly lower energy productions in wind projects when compared to 2018.
Wind farm developers who have been conducting wind measurements during 2019 can also expect the average wind speed to be lower than the same period in 2018, 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 2019 deviation from the long-term mean wind speed is noted to fall within the expected range of wind speed inter-annual variability. Therefore, whilst 2019 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.
UK Wind Index during the first quarter of 2020
How is 2020 looking so far and has the wind resource pattern changed? Q1 in 2020 has been a record quarter during the long-term period since 1996. February was a record month (seasonally adjusted 140%) while wind in both January and March were exceptionally high. This is largely due to the higher than normal wind speeds brought by storms Brendan (13-14 January), Ciara (8-9 February), Dennis (15-16 February) and Jorge (28 February – 1 March) during Q1.
The following map best illustrates this positive trend (notice the different range extended to accommodate the extreme values) with the entire country in the “red zone”.
It will be interesting to see how the wind resource pattern will continue to develop in Q2 and Q3, as these are generally the less windy months of the year.
The UK Wind Index
DNV 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 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 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 119%. If a specific (individual) January has an index value of 92%, the seasonally adjusted value for that January would be 99%/119% = 83%.
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