Abstract
Hydrogen is becoming a key energy carrier in the transition toward decarbonization, as electrolysis creates strong interdependencies between electricity and hydrogen markets. Accurately representing strategic behaviour in these coupled markets is essential, yet current models fail to capture price-responsive bidding. To address this, a joint hybrid Cournot-Linear Supply Function Equilibria (CLSFE) model is developed and reformulated as an equivalent optimization problem, enabling tractable large-scale analysis. The model is applied to the Iberian system for 2030 and compared with perfect competition and Cournot benchmarks. Results show that hydrogen prices are lowest under CLSFE, with a reduction of about 44% relative to perfect competition and 10% to Cournot, while hydrogen demand increases by up to 58%. Electrolytic hydrogen production rises up to 92%, displacing grey hydrogen and reducing hydrogen-sector emissions. However, renewable self-curtailment reaches 82 TWh, indicating increased market power. These results highlight cross-sector trade-offs and support market design and policy analysis.

Abstract
Renewable Energy Communities (RECs) are a promising driver for energy system decarbonization, engaging end consumers in energy markets, promoting local renewable generation and affordability, and enhancing community resilience. This paper presents a methodology for planning REC by optimally sizing distributed energy resources (DERs), including conventional resources such as photovoltaic (PV) systems and battery energy storage systems (BESSs), but also power-to-hydrogen (P2H) systems composed of an electrolyzer (EC), hydrogen tank, and fuel cell (FC). A mixed-integer linear programming (MILP) model is used to minimize DER investments and REC operational costs, using a reduced input data set based on clustering techniques to decrease complexity and improve computational efficiency. Then, a detailed MILP operation model is used to assess the REC performance with the newly sized DERs under different scenarios using conventional indicators such as the net present value (NPV) and the payback period. A sensitivity analysis is also conducted on electricity costs, EC costs and the hydrogen selling price for an external combined heat and power (CHP) client. Results show that integrating P2H systems increases NPV and provides new revenue opportunities, thereby reinforcing their role as a complementary technology to PV and BESS within RECs.