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Germany is under increasing pressure to rapidly decarbonize its electricity system, while ensuring a secure and affordable electricity supply. In this context, energy storage systems (ESSs) can play a crucial role in enabling a high share of variable renewable electricity generation. To investigate the complex interplay of ESSs in the electricity system, bottom-up energy system optimization models have been utilized to create strategies for the decarbonization of electricity systems at minimum cost. Previous studies have used models to investigate the role of storage technologies in decarbonizing electricity systems. However, in most of them, key aspects pertaining to the optimal storage technology mix (what), the daily and seasonal operation of storage technologies (what), the timing of the investment (when), and their optimal placement (where) are omitted. In this work, we present MANGOelec, a model for the long-term optimal design of electricity systems that allows us to study all the what-where-when dimensions for ESSs. We apply the MANGOelec model to the German electricity system considering the horizon until 2050, as well as various scenarios to investigate the effects of several external factors like building rates of renewable generation technologies or grid expansion limitations on the optimal transformation pathways. The study results indicate that a mix of short- and long-term storage is needed, independent of external factors. For instance, battery storage potentials are close to fully exploited across all regions and the technology is solely used for daily operation. Hydrogen storage, on the other hand, is primarily installed in a small set of regions and operated exclusively as a seasonal storage option. Here, the results indicate that the demand for hydrogen storage capacity is substantial, reaching 2.7–5.9 TWh of hydrogen-based storage capacity in 2050 depending on the scenario. The results also suggest that independent of the storage technology chosen, their deployment commences after 2030 and is completed by 2045. In conclusion, the study shows that already existing ESSs provide sufficient flexibility at first but need to be expanded substantially after 2030 to enable a cost-optimal transition of the German electricity system. Moreover, it illustrates the need for ESSs in order to achieve the decarbonization goals of the German electricity system, which can provide valuable insights to energy planners and policy makers. • Role of energy storage systems in the German electricity system is investigated. • Modeling of daily and seasonal storage investments and operation in 2021–2050. • Quantification of regional and temporal patterns in energy storage installations. • High hydrogen-based seasonal storage demand in selected federal states is shown. [ABSTRACT FROM AUTHOR]

• Probabilistic simulation of security of electricity supply in central Europe. • Quasi-absolute levels of security of supply are unlikely to be maintained in future scenarios. • Sensitivity of security of supply towards meteorological influences is increasing. • Dependency on contributions from neighboring countries is growing. • Relevance to coordinate capacity planning on an international level is demonstrated. Against the backdrop of expansions of intermittent renewable energy capacity, planned nuclear phase-outs, and current debates on the additional mothballing of coal-fired power plants in central Europe, there is a substantial increase in uncertainty regarding security of electricity supply. In this context, we present a probabilistic and scenario-based analysis of impacts on security of supply for Germany as a case-study in the medium-term perspective (years 2020, 2022, and 2023). For our analysis, we introduce the energy security assessment module of the JERICHO energy system model from RWTH Aachen University. Our model allows for an hourly probabilistic simulation of security of supply and accounts for stochastic characteristics of (non-)availability of conventional generators, renewable feed-in, electricity demand, and import potentials from central-European countries. To increase the robustness of results, we use a comprehensive dataset of temperature, wind speeds, solar radiation and water levels from weather years 1986 to 2015 as input for our simulations. Our results indicate that Germany is unlikely to maintain its quasi-absolute levels of security of supply in the near future with a mean Loss of Load Expectation (LoLE) of up to 2.6 h in scenario year 2023. However, this does not necessarily imply that future levels of security of supply are insufficient. There is need for further assessments on identifying threshold levels for security of supply, which are economically efficient and acceptable for society. Our results also indicate that the sensitivity of security of supply for meteorological changes increases with reductions of conventional power plants. Finally, we demonstrate that the dependency of security of supply on imports from neighboring countries increases substantially calling for increased international coordination of national energy policies. [ABSTRACT FROM AUTHOR]