Estimation of the upper limit of distillate flux through nanoparticle-coated hydrophobic membranes for a direct contact solar membrane desalination system

Nanophotonics-enabled solar membrane desalination (NESMD) systems utilize nanoparticle-coated hydrophobic membranes to efficiently harness solar energy for desalination purpose. These nanoparticles, on absorbing sunlight, locally heat seawater-feed flowing over the membrane, creating a temperature gradient across the membrane generating a vapor-pressure difference. It drives water vapor from hot feed side to cooler permeate side, where it condenses to produce distilled water. Such systems are getting wide attention in recent times due to high photothermal efficiency. However, reported values of distillate flux and photothermal efficiency vary considerably across the literature. In this study, an attempt is made to establish the theoretical upper limit of direct contact NESMD using an experimentally validated numerical model. The analysis assumes ideal conditions, including perfect solar absorption (100 %), zero heat loss, and an air-only membrane that minimizes heat conduction while maximizing vapor diffusion. Beyond these assumptions, other system parameters - such as membrane thickness, channel length, and feed or permeate velocities - are optimized through parametric studies. Results show that the maximum achievable distillate flux is 1.31 kg/m²hr at a channel length of 80 cm, corresponding to a photothermal efficiency of 89 %. A sensitivity analysis further illustrates pathways for approaching this theoretical limit in real systems. These findings provide a benchmark for evaluating future NESMD designs and guide ongoing research toward achieving higher efficiency.