Sub-national implications of Greece's power-sector transition: The HERMES energy-system model

Greece has set ambitious energy and climate targets, aiming to decarbonise its power sector by 2050, despite the sector's geographical complexity and historical coal reliance. Delivering on this goal entails multiple challenges, including temporal and geographical stochasticity of renewable electricity generation, integration of energy storage technologies, variable renewables curtailment, and management of electricity flows at cross-regional and cross-border scales. Navigating this complex landscape requires the use of sophisticated energy-system modelling tools. We introduce the Hellenic Regional Model for the Electricity System (HERMES) for Greece's power sector, based on the OSeMOSYS open-source modelling framework and drawing on state-of-the-art developments in its community. The HERMES model features high temporal and technological granularity, while critically offering insights disaggregated at the NUTS-2 and island level. The paper documents all methodological novelties and extensions, evaluates the model's robustness and accuracy against recent historical data as well as official trajectories of the country's National Energy and Climate Plan (NECP), and illustrates its analytical added value through insightful sub-national implications of the NECP—including RES deployment patterns and unit-dispatch outcomes in regions with increased policy-relevant insights. Our results emphasise the central role of strengthening cross-regional transmission capacity and insular grid interconnections for energy security alongside decarbonisation as well as the significance of cross-regional and cross-border electricity exchanges in the country's ambition to emerge as an intercontinental energy hub. The analysis also sheds light on certain distributional implications of the energy transition and the vital contribution of storage technologies to achieving a near-zero emissions power sector by 2050. • We introduce the OSeMOSYS-based Hellenic Regional Model for the Electricity System. • HERMES disaggregates Greece into ten mainland NUTS-2 regions and eleven major islands. • Our results are compared to official NECP trajectories and regional results discussed. • We emphasise the role of various dimensions in Greece's power-sector decarbonisation. • These include transmission capacity, island interconnections, and energy storage.