Multi-level optimisation of the integrated renewable energy system within a water-exergy-hydrogen nexus concept

An integrated solar-driven renewable energy system for the simultaneous production of electricity, freshwater, and green hydrogen is proposed and systematically analysed within a water–exergy–hydrogen (WExH) nexus framework. The configuration couples a parabolic trough solar collector (PTSC) with a high-pressure steam turbine–assisted multi-effect desalination (MED) unit, an organic Rankine cycle (ORC) for bottoming power recovery, and a proton exchange membrane electrolyzer (PEME) for hydrogen generation. A comprehensive thermodynamic, exergy, and techno-economic model is developed and implemented in a quasi-steady framework, evaluated at monthly-representative operating points, to capture seasonal solar variability and the operational interactions among subsystems. To address the complexity of competing performance objectives, a hierarchical multi-level optimisation strategy is employed, targeting system-level exergy performance, WExH nexus indicators, and economic metrics, including levelized costs of electricity, water, and hydrogen. The results reveal that the parabolic trough solar collector is the dominant source of overall exergy destruction (50.2%), owing to the large temperature disequilibrium between the solar source and the receiver, followed by the steam generator (16.7%) and the MED unit (10.1%). A system-level exergy balance yields an overall exergetic efficiency of 38.1% at the design point, under hierarchical multi-objective optimisation this rises to 41.2%, while the levelized cost of hydrogen is reduced by 32% (to 6.76 €/kg) and the levelized cost of water by 40% (to 0.56 €/m3). A case study based on Dublin’s solar conditions demonstrates that the proposed system remains technically feasible as a multi-product configuration even under a low and strongly seasonal solar resource, delivering 3,872 MWh of electricity, 14.6 t of hydrogen, and 56,313 m3 of freshwater annually, supporting its relevance for integrated renewable energy planning. The novelty of this work lies in the combined implementation of an explicit WExH nexus formulation with a hierarchical multi-level optimisation framework under time-varying solar conditions, enabling a structured quantification of thermodynamic, resource, and economic trade-offs that cannot be captured using conventional steady-state or single-level optimisation approaches.