Innovative circular uses in the renewable energy field and sustainable development: The case of a geothermal-hydrogen system

The global energy transition, driven by the Paris Agreement and Sustainable Development Goals, demands renewable-based systems that are both sustainable and flexible. In Italy, the rapid expansion of variable renewables such as solar PV and wind challenges grid stability and necessitates innovative storage and conversion solutions. Geothermal energy, traditionally valued as a reliable baseload source, can be re-envisioned as a circular and flexible platform when hybridised with hydrogen technologies. This thesis investigates and assesses the integration of geothermal power with hydrogen production, storage, and utilisation, transforming a conventional baseload plant into a bidirectional energy hub. A Modified Geothermal Power Plant (MGPP) concept was developed, coupling a single-flash geothermal unit (Bagnore 3, 20 Mwe) with a Hydrogen Energy Storage System (HESS). The system operates in two modes: Accumulation Mode, producing and storing hydrogen and oxygen via electrolysis using surplus renewable electricity, and Booster Mode, converting the stored gases into electricity through either oxy-combustion or PEM fuel cells. A multidimensional methodology combining Sustainability Life Cycle Assessment (LCA, LCC, S-LCA), thermodynamic and exergo-economic analyses was applied to capture technical, environmental, economic, and social implications. Results confirmed the technical feasibility of the hybridisation, achieving a power modulation of 5 MW between modes and enhancing the plant’s ability to provide spinning reserve and ancillary services. Exergo-economic analysis favoured the oxy-combustion configuration, with major irreversibilities attributed to base geothermal components (wells and the low-pressure turbine). Environmental impacts rose only marginally (+3.7%) with HESS integration, while recycling of critical materials such as nickel and platinum proved essential in reducing impacts. Social hotspots were identified in the upstream supply of these materials, particularly in high-risk regions. A regional case study showed that 33% of geothermal CO2 emissions (95 kton/year) can be valorised into methanol (22 kton/year) and heating/cooling services, achieving significant avoided emissions. This research demonstrates that geothermal–hydrogen hybrid systems can not only enhance flexibility and decarbonisation, but also open pathways for circular resource use, positioning them as transformative solutions for a sustainable and resilient energy future.