Optimal design of a solar-driven multistage direct contact membrane distillation system under series and parallel arrangements

Solar-driven direct contact membrane distillation is promising for decentralized desalination, but limited attention has been given to the techno-economic comparison of series and parallel multistage arrangements within the same solar system. This study develops an hourly time-dependent model of a standalone system integrating photovoltaic/thermal and flat-plate collectors, thermal storage, an auxiliary heater, and multistage membrane distillation under NEOM climatic conditions. The novelty lies in the direct optimization and comparison of both arrangements under identical membrane capacity and common energy constraints. Following a parametric analysis, a single-objective optimization is performed to minimize the levelized cost of water for a fixed capacity of 10 membrane modules by optimizing the collector areas and feed flow rate. The optimized series arrangement achieves a water cost of 1.16 $/m 3 and annual production of 1484 m 3 /yr, compared with 1.61 $/m 3 and 625 m 3 /yr for the parallel arrangement, representing approximately 28% lower cost and 137% higher production. Solar radiation is identified as the most influential parameter, while membrane distillation and flat-plate collectors dominate the total cost. Overall, the series arrangement provides superior techno-economic performance under the investigated capacity and climatic conditions.