Understanding the phenomena of negative vapor flux in Nanophotonics-Enabled solar membrane distillation

•High feed water salinity leads to local negative flux in MD systems powered by sunlight concentrated using a Fresnel lens array;•A minimum transmembrane temperature difference is required to avoid negative flux;•A critical solar concentration ratio exists due to optical losses by the solar concentrating device;•Membrane module design must consider the configuration of the solar concentrating device to minimize the area with negative flux. Direct solar membrane distillation (MD) enabled by photothermally active membranes provides a low-cost solution to desalination. The design and optimization of direct solar MD systems, however, is hindered by the complex interaction among the optical, photothermal, and coupled heat and mass transfer processes involved. This study deals with the opto-thermo-fluidic modeling of the Nanophotonics-Enabled Solar Membrane Distillation (NESMD) process. A COMSOL Multiphysics model coupling the mass, momentum, and heat transfer processes is developed and used to study the impact of environmental and operating conditions on the performance of NESMD powered by a Fresnel lens array solar concentrator. The simulation results reveal the occurrence of negative flux as a result of this solar concentration method, especially at high feed water salinity. Consequently, a critical solar concentration ratio exists, below which solar concentration compromises instead of enhancing system performance. A simple change in membrane reactor design is demonstrated to greatly mitigate the impact of negative flux while reducing the membrane area needed. This is the first study that addresses the spatial variation of membrane flux due to variation in solar irradiation intensity in a concentrated solar scheme.