Multistage solar-driven membrane distillation with direct ambient-temperature feed heating

Water scarcity is intensifying worldwide, driving the need for compact, energy-efficient desalination technologies suitable for decentralized and off-grid operation. This study aims to enhance freshwater productivity of solar-driven membrane distillation by developing and analyzing a multistage direct contact membrane distillation (DCMD) system powered solely by distributed solar energy under ambient-temperature feed conditions. The work extends a previously demonstrated single-stage concept to multistage operation without external feed preheating, active cooling, or solar concentration. Two system configurations are investigated: (i) a counter-current multistage system with external heat exchange for cooling, used as a benchmark, and (ii) a fully self-contained configuration relying exclusively on internal thermal management. A numerical model is developed and validated against experimental data, showing good agreement. An optimization algorithm is then employed to determine optimal feed and permeate mass flow rates and to assess the effects of stage number and solar irradiance. Results show that the externally cooled configuration exhibits an approximately linear increase in specific productivity with the number of stages, reaching 1.71 kg m −2 h −1 for four-stage configuration under 700 W m −2 . The configuration with internal heat recovery further enhances performance, achieving 2.33 kg m −2 h −1 and a gain output ratio of 1.58. Model extrapolation indicates potential productivity up to about 3 and 4 kg m −2 h −1 for seven-stage configuration at 700 and 1000 W m −2 , respectively, with emerging saturation effects. These results demonstrate that the proposed design enables higher freshwater production compared to state-of-the-art compact solar-powered MD systems, offering a scalable and energy-efficient solution for decentralized and off-grid desalination applications.