Solar-driven membrane distillation with direct ambient-temperature feed heating: design, modeling, and experimental validation

Freshwater scarcity in off-grid regions necessitates the development of simple and sustainable desalination systems. Conventional membrane distillation (MD), while promising, typically relies on external preheating and complex solar collectors, limiting its compactness and practical applicability. This study presents the design, numerical modeling, and experimental validation of a compact direct contact MD prototype powered exclusively by distributed solar energy. The system operates without external feedwater preheating or centralized solar collectors. Instead, the custom-built module integrates a solar absorber directly into the top surface of the feed channel, enabling direct solar heating of the ambient-temperature feed stream. The flat-sheet module employs reduced channel thickness to enhance thermal coupling and minimize temperature polarization. An extensive experimental campaign was conducted under realistic conditions, covering a broad range of feed flow rates. A numerical model was developed to simulate coupled heat and mass transfer processes, and validated against experimental results, showing strong agreement. The model was subsequently used to explore alternative configurations and guide system optimization through internal heat recovery. A promising thermal management strategy is thus introduced, enabling simultaneous feed preheating and permeate cooling, thus increasing effective transmembrane temperature difference without additional energy input. A maximum distillate flux of 0.51 +/- 0.02 kg m-2 h-1 was achieved under 1000 W m-2 solar irradiation, representing 59% improvement over comparable tested systems lacking internal heat recovery. Results demonstrate the feasibility of a modular and scalable system suitable for off-grid or decentralized water purification. Future work will investigate multistage implementations and integration with alternative low-grade heat sources.