A Thermal-First Solar Spectral Beam-Splitting PV–DCMD System for Enhanced Energy–Water Nexus Performance

Efficient use of the solar spectrum for simultaneous freshwater and electricity production remains challenging because conventional solar membrane-distillation and PV/T systems involve intermediate heat-transfer stages or recover only residual photovoltaic heat. This study proposes a thermal-first spectral beam-splitting PV-DCMD system in which a thin flowing water layer simultaneously acts as a spectral filter, volumetric solar absorber, and thermal-energy carrier. Infrared-dominated radiation is absorbed directly by the water to drive DCMD, while part of the visible spectrum is transmitted to the underlying photovoltaic module. A coupled Multiphysics model combining non-grey multi-band radiative transfer, computational fluid dynamics, photovoltaic calculations, and DCMD transport is developed to investigate Reynolds numbers of 30–150 and concentration ratios of 3–100. Energy, economic, and Response Surface Methodology analyses are also performed. The concentration ratio is the dominant parameter controlling thermal generation, permeate flux, electrical output, and levelized cost of water (LCOW). The highest temperature-compliant permeate flux reaches approximately 47 kg/m 2 .h at CR-100 and Re-75; lower Reynolds numbers at this concentration are excluded because the predicted temperatures exceed the adopted 90°C DCMD limit. Increasing the Reynolds number reduces outlet temperature and permeate flux but increases useful thermal-energy recovery by up to 48%. Multi-objective optimisation identifies Re-138 and CR-61.2 as the optimum condition, yielding an outlet temperature of 61.72°C, a permeate flux of 24.59 kg/m 2 .h, and an LCOW of 7.66 $/m 3 . The novelty lies in harvesting the thermal portion of the solar spectrum before photovoltaic conversion and integrating spectral absorption, electricity generation, desalination, economic assessment, and constrained optimisation within a unified framework. The results demonstrate the potential of thermal-first spectral management to improve solar energy–water utilisation.