Passive Permeate-Side-Heated Solar Thermal Membrane Distillation: Single-Stage Tubular and Multistage Planar Systems

Scarcity of drinking water is a global problem and especially serious in rural areas of developing countries. Passive permeate-side-heated interfacial-heating solar membrane distillation has been shown recently as a promising system for off-grid distributed water and wastewater treatment. In this study, single-stage tubular systems were developed by giving the poly(vinylidene fluoride) (PVDF) membrane (pore size: 0.45 µm) a tubular structure with the outer surface (i.e., the permeate side) coated with carbon black NPs which absorbed solar thermal energy and evaporated the feedwater inside the membrane tube. Under natural sunlight, the vertical tubular system had a production rate of distilled water per footprint as 0.67‒2.06 kg/(m2·day) throughout the year, 71% higher than the single-stage planar system on average. The three-stage planar systems were developed by overlapping three single-stage modules. Two adjacent modules shared a common copper sheet that serves as both the top of condensation chamber of the preceding stage and the bottom of the feedwater chamber of the following stage. The copper sheet can partially transfer the condensation heat of the preceding stage to the feedwater of the following stage, thus enhancing the system energy efficiency (ƞsys) in producing distilled water. The ƞsys and distillate flux of the three-stage systems were 62% and 5.01 kg/(m2·day) at the average daytime irradiance of 422 W/m2, 34% higher than the single-stage systems.