ENHANCING SOLAR DESALINATION EFFICIENCY THROUGH CAPILLARY-STRUCTURED POROUS MEDIA: a THREE-DIMENSIONAL COUPLED NUMERICAL ANALYSIS and EXPERIMENTAL VALIDATION

Abstract

This study presents a comprehensive three-dimensional coupled numerical model for a solar-driven desalination device incorporating a capillary-structured porous membrane (CPM). The governing equations - encompassing conjugate heat transfer, capillary-driven mass transport, Darcy - Brinkman momentum, and interfacial evaporation via the Hertz - Knudsen formulation - are solved simultaneously using an adaptive finite-volume method (FVM) on a mesh of 6.4 × 10 6 control volumes, achieving a second-order L² error of 0.28%. Parametric optimisation over porosity (φ = 0.20 - 0.90) and capillary diameter (d=10 - 200 μm) identifies the global optimum at φ=0.65 and d=35 μm, yielding a peak thermal efficiency of η=83.7 ± 2.2% and a daily freshwater productivity of 14.8 ± 0.8 kg/m²·day under 1000 W/m² irradiance a 97.9% improvement over conventional flat-plate absorbers. A solar desalination system based on a capillary-structured porous membrane (CPM) was investigated using a three-dimensional mathematical model based on the Darcy – Brinkman – Forchheimer framework. Heat transfer, mass transport, and capillary-driven momentum equations were solved in a fully coupled manner. The adaptive finite volume method (FVM) was implemented with second-order accuracy on a computational mesh consisting of 6.4×10 6 control volumes. Parametric optimization was performed over a porosity range of φ =0.20 - 0.90 and capillary diameter range of d=10 - 200μm. The model results were validated against laboratory experiments. Evaporation kinetics were described using the Hertz – Knudsen equation, while salt transport was modeled using a reactive advection – diffusion equation. At the optimal configuration of φ=0.65 and d=35μm, the thermal efficiency reached η=83.7±2.2%, while the daily water yield achieved 14.8±0.8 kg/m²·day. These values are higher than those of a conventional flat-plate absorber by 97.9 % and 185%, respectively. The numerical model showed excellent agreement with experimental data, with a mean absolute error (MAE) of 1.9% and a coefficient of determination R² =0.9974. Capillary-driven lateral flow reduced concentration polarization by 44.3 % and decreased the effluent total dissolved solids (TDS) to 62±12 mg/L, which is eight times lower than the WHO standard limit. The optimal porosity peak (φ ≈0.645) remained stable regardless of variations in solar irradiance. Compared to the conventional absorber, the capillary-based system reduced salt concentration from 480 mg/L to 62 mg/L, significantly improving water purity.

Keywords

References

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