Group seminar on 29. June, 14:15 CET

 Compound Atmospheric Events Leading to Low Renewable Energy Supply

Jonas Pfau

In order to effectively combat climate change, a transition towards a carbon free energy system is essential. To achieve such a goal, the shares of renewable energies in the electricity mix must be increased, specifically wind and solar power. However, certain weather conditions can cause lulls leading to a lack of wind and solar energy generation over large areas. Since these incidents are scarcely researched in published literature, we use the ERA5 reanalysis data set to investigate the frequency of occurrence and duration of such lull events for Germany and Europe. We then compare our results with the prevailing atmospheric conditions. Additionally, we compare our results to the German Weather Service’s objective weather type classification and the North Atlantic Oscillation (NAO) index to test the predictability of dark lull events.

We find that solar lull and wind lull events do not occur often. Further, simultaneous wind and solar lulls (dark lulls) occur even less frequently and exclusively during winter. Additionally, we observe that local lull events are more frequent and last longer than large-scale events. These local lull events occur in areas where renewable energy plant deployment is difficult, i.e., mountain ranges. We also see that the prevailing atmospheric conditions during dark lull events are characterised by a strong high-pressure system near the surface and a strong ridge in mid-troposphere over Northern Europe. This results in reduced wind speeds and, during winter, reduced solar radiation. While we cannot detect dark lull events over offshore areas, the prevailing atmospheric conditions show a strongly reduced wind resource over these areas. For predictability, our results show that most events occur during weather type classes 1, 2, 11, and 31 as well as during a neutral NAO index. However, forecasting such events using these two atmospheric classification schemes is not reliable due to the rare occurrence of dark lulls.

Based on our results, the effect of large-scale and prolonged local dark lull events is not as severe as some experts have feared. Energy trading can be an effective tool to minimize the impact of local lull events. However, for the rare occasion of dark lulls, energy storage or balancing the grid using dispatchable renewable energy sources may be necessary. Additionally, while we do not find a reliable method for forecasting dark lulls, the occurrence of such events is more likely for periods of atmospheric blocking. With further work on the predictability of atmospheric blocking events, this result can be useful in reducing the impact of dark lulls