Posters

Abstract

Dunkelflaute is a term used to describe periods of calm wind conditions combined with low solar irradiation, especially in Germany and Northern European countries. As build out of offshore wind in north west Europe advances the potential impact of dunkelflaute could also increase.  This discussion examines this. We have used the ERA5 climate reanalysis model to isolate periods of low hub height (137m) windspeeds in the German offshore, in the area of the N-11.x  and N-12.x  offshore wind leases across a long term timescale (1980-2024), and have compared that to floating lidar observations at the same height acquired during 2024. From ERA5, the occurrence of 24hr-duration very low windspeed events (continuous hourly-averaged windspeeds beneath 5m/s) happen here on average 11 times every year.  The number of occurrences per year exhibits a downward trend,   suggesting calm periods offshore will occur increasingly less frequently in the future.  Longer period (48hr) calm events are rare occurring 1 or 2 times per year. In observed windspeeds between 23 April 2024 and 31 December 2024, the total number of 24hr calm events observed shows good but not identical correlation between model predictions; with the model simulating two events that were not observed as continuously calm.  This can be explained to the different (10min) sampling exhibited by the observation timeseries; and overall indicates that the model is adequately simulating very calm conditions at this location. It is important that dunkelflaute conditions experienced onshore Germany over 2024/12/11 –2024/12/12 and 2024/12/25-2024/12/28 defined by the above are not present for a full 24hr period at this offshore location.  Here, during 2024/12/11 –2024/12/13, where spot electricity prices in Germany crossed over €1,000/MWh, windspeed conditions fluctuated at approximately (above and below) 5m/s for 8hrs between 07:30 and 15:20; corresponding with peak supply prices, where onshore wind and solar production was low. This period of borderline calm conditions offshore varies markedly with conditions onshore (using reference site north of Münster), where windspeed conditions at hub heights were below 5m/s continuously for 55hrs.  Further, conditions onshore during 2024/12/25-2024/12/28 (where industrial demand was lower) exhibited 75hrs of continuous calm conditions, the longest period in 2024 at this location where windspeeds were below 5m/s.  Offshore at this time windspeeds again fluctuate below and above 5m/s during this period, but rarely cross 5.5m/s , suggesting that whilst the 5m/s threshold is crossed, a threshold of 5.5m/s during daylight periods might be a better absolute indicator of dunkelflaute inducing conditions in offshore locations.   To understand the correlation between instances of 24hr periods below 5.5m/s, the analysis was re-run at this threshold, with similar results.  In 2024, there were 10 instances of these conditions, significantly below the long term average of 14 per year.  Of these 10, 7 occurred during May, June and July, where insolation is typically strong.  ERA5 reflects this over the long term, where 70% of 24hr calm periods occur April-September.  It’s notable that December exhibits significantly more calm periods than other months between October-November, perhaps relating to the notoriety of midwinter “dunklflaute” conditions.