Energy system planning under variable solar and wind conditions: Pyrolysis as a key to system robustness through flexibility and negative emissions

Interannual weather variability significantly influences the design and cost of climate-neutral electricity systems. Using forty historical weather years, this study quantifies how variations in solar and wind generation affect the optimal capacity mix, storage deployment, and total system costs for Germany’s future net-zero system. Total annualized system costs range from 63 billion € to 86 billion €, while required hydrogen storage varies by more than a factor of three. The linear correlation analysis between weather characteristics and model outcomes shows that this variability is driven primarily by the duration of dunkelflauten (extended periods of simultaneously low wind and solar generation), which exhibits by far the strongest relationship with the total installed capacity of hydrogen storage systems. Each additional day of dunkelflaute increases hydrogen storage requirements by about 0.5 TWh. Integrating pyrolysis, which simultaneously provides dispatchable electricity and negative emissions from residual biomass, fundamentally changes this relationship. In myopic capacity expansion scenarios, pyrolysis reduces hydrogen storage requirements by more than 80 % in extreme weather years and enables temporary gas power operation without violating net-zero constraints. The findings highlight that pyrolysis can strengthen the robustness of renewable electricity systems by coupling flexibility provision with durable carbon removal, thereby mitigating uncertainty from future weather extremes. • Weather variability strongly affects system cost and technology scale. • Dunkelflaute duration determines the scale of required hydrogen storage capacity. • Pyrolysis reduces hydrogen storage by over 80% in extreme years. • Negative emissions from biochar enable temporary gas power operation. • Pyrolysis enhances robustness of climate-neutral power systems.