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This study presents a novel approach to analysing the Japanese energy system transition from a mostly fossil fuels‐based system as of today, to a sustainable renewable energy‐based system by 2050. This research uses a novel technology‐rich, multi‐regional, multi‐sectoral, and cost‐optimal energy system model for the analysis, describing pathways to achieving the Japanese climate neutrality vision by 2050 based on renewables in the most cost‐effective manner. The energy transition of Japan is analysed for both rapid and delayed defossilisation pathways, impact of demand sensitivity, and benefit of sustainable energy imports in the form of electricity from Russia and China or e‐fuels from countries like Australia. The results show that a self‐sufficient energy system is achievable; however, some sustainable energy imports in the form of electricity and e‐fuels can be beneficial in further reducing overall investments, relieve pressure to develop local renewable resources and improving energy system flexibility. In addition, this research highlights the critical advantage of a fully renewable pathway, sector coupling, and high electrification rates as the most cost‐efficient way of achieving the Japanese climate neutrality vision by 2050. The renewable pathway also delivers an energy system with high levels of efficiency gains through direct and indirect renewable electricity usage. [ABSTRACT FROM AUTHOR]

The energy transition plays a crucial role in the ongoing process of defossilisation and carbon emissions reduction. Considering the substantial cost of the transition and importance of the energy system for the global economy, the transition must be accurately modelled to allow future system development analyses and discussions to avoid stranded investments and guarantee a reliable energy supply at every transition step. Energy system models must be operated with a technology-rich portfolio in high temporal and spatial resolution to properly estimate the impact of renewable energy variability and the role of storage and sector coupling technologies. Consequently, modern energy system models comprise tens of millions of variables and constraints, and demand vast computational resources for optimisation. This study proposes a hierarchical approach to run simulations based on partial regional disaggregation as a solution to decrease computational time without significant change in the modelling results. The discussed method is tested on the case of Japan and shows that the hierarchical approach allows for a reduction in computational time by a factor of 2.3–3.3 compared to the full spatial resolution simulation approach, while the installed capacities and costs of the energy system stay within a ±3% range for all steps of the transition through 2020 to 2050. • Hierarchical approach enables an ample reduction in spatial resolution and model size. • Simulation time for complex transition tasks reduces by a factor of 2.3–3.3 • Capacities and costs of the energy system stay within a ±3% range. • Option for further increase of spatial resolution without compromising complexity. [ABSTRACT FROM AUTHOR]