Digitalisation enables the clean energy transition but introduces fragmentation across platforms, protocols, and standards. The PARMENIDES project addresses these challenges by introducing EMS4HESS – an ontology-based Energy Management System for Hybrid Energy Storage Systems within Renewable Energy Communities. EMS4HESS leverages the PARMENI-DES Energy Community Ontology (PECO) to ensure semantic interoperability across data models and information layers. PECO describes community structures, asset properties, optimisation strategies, and flexibility signals, enabling automated reasoning and adaptive control. The system integrates diverse storage technologies and optimises their operation to improve resilience and sustainability. Real-world pilots in Austria and Sweden validate the approach under contrasting regulatory frameworks. Initial results show significant improvements in self-sufficiency and self-consumption compared to baseline scenarios without HESS, confirming the feasibility of ontology-driven, grid-aware energy management as a scalable solution for the clean energy transition.