The transition towards carbon-neutral energy systems requires solutions capable of integrating large shares of variable renewable energy while simultaneously supplying multiple energy carriers. This thesis analyses sector coupling as an approach to increase system flexibility by linking the electricity, \ce{H2}, and heat sectors through energy conversion and storage technologies. The study investigates how sector coupling can enable a flexible, cost-optimal, closed-grid renewable energy system for the Rotterdam area.

A techno-economic optimisation model is developed to determine the optimal sizing of renewable generation, power-to-X technologies, and energy storage under a renewable-only energy supply. The system comprises onshore wind and photovoltaic generation, an electrolyser, heat pump, fuel cell, battery storage, H2 salt cavern storage, and thermal storage, and is evaluated using hourly supply and demand profiles. Besides the base optimisation and respective techno-economics, a sensitivity analysis is presented, which includes CAPEX sensitivity, component-removal scenarios, and a carbon break-even analysis to assess competitiveness against conventional energy pathways.

The results show that the cost-optimal system is characterised by oversizing of renewable generation relative to conversion capacity by a factor of approximately 5.2 Consequently, storage plays a complementary role by providing flexibility across different timescales. Wind generation is identified as the essential supply technology, while photovoltaic generation primarily complements battery operation. Moreover, the fuel cell serves mainly as a backup pathway, and battery and H2 storage provide complementary short- and long-term flexibility. Although the system achieves a net present value over its lifetime of 2\,995 M€, corresponding to a payback time of approximately 12 years, it is not yet competitive with conventional alternatives under current market conditions. The system-level carbon break-even analysis indicates that cost parity would require a carbon price approximately 5.4 times higher than the current EU Emissions Trading System level.

Overall, the results demonstrate that sector coupling can enable reliable and economically viable operation of closed-grid renewable energy systems while reducing dependence on large-scale energy storage through the strategic combination of renewable oversizing and energy conversion pathways. The findings provide insights into the role of individual technologies in integrated energy systems and identify key technological and policy developments required to improve their future competitiveness.
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