Calcium alginate–bentonite evaporation module for façade integration and decentralized freshwater production using low-thermal-grade heat

Decentralized freshwater production from building greywater could reduce the energy demand and environmental impacts associated with centralized water treatment and distribution. However, passive solar technologies designed for façade-integrated greywater reclamation and freshwater production remain largely unexplored. This work develops and experimentally characterizes a calcium alginate–bentonite composite as an evaporative medium for a façade-integrated freshwater production system driven by low-grade thermal energy. The proposed concept employs tubular elements in which feedwater is transported through the porous composite and evaporated using low-temperature heat, while non-volatile contaminants concentrate in the liquid stream. The material and tubular elements are experimentally characterized in terms of water uptake, dimensional stability under repeated wetting–drying cycles, and evaporation performance. Bench-scale experiments at 20–40 °C demonstrated stable evaporation rates of 183–312 g h⁻¹ m⁻² of tube surface area with pure water. Tests with saline feedwater confirmed selective water vapor generation, although the evaporation rate decreased to 63 g h⁻¹ m⁻². Based on the experimental results, a preliminary scale-up analysis of a façade module was performed for comparison with solar still devices reported in the literature, yielding estimated freshwater productivities of 7.3–17.8 L m⁻² day⁻¹ , depending on material composition and operating temperature. These results indicate that the proposed tubular architecture can achieve competitive freshwater productivity below 40 °C, using low-grade thermal energy sources, while offering a modular geometry suitable for integration into building façades. The study provides an experimental basis for developing low-temperature, low-grade-heat-driven systems for decentralized greywater reclamation and freshwater production.