Most solar still models simplify the system by neglecting wall-specific heat losses and ignoring the solar energy incident on the front and side walls. They also rely on heat and mass transfer correlations that do not distinguish between the water–humid air and humid air–glass regions or account for the influence of still geometry. To address these gaps, this study presents a new thermal model that applies individual energy balance equations to every component of the still, including the basin liner, brackish water, humid air, glass cover, and all four walls. A radiation model is used to evaluate the solar energy input to all surfaces, and new geometry-dependent heat and mass transfer coefficients are established using Nusselt and Sherwood numbers. Moreover, an experimental study for single-slope solar stills with different glass-cover angles was conducted to generate the data required for model calibration and validation. Using these data, the dimensionless constants of the new transfer coefficients were optimized via a genetic algorithm. Furthermore, the developed model shows excellent agreement with experimental measurements. Additionally, heat loss analysis reveals that the basin liner contributes 42.5% of total heat loss, followed by the back wall (25%), each of side walls (14.5%), and front wall (3.5%). Increasing wall height from 0.1 to 0.3 m raises daily heat loss by 20.6% and decreases thermal efficiency from 32.3% to 24.7%. The results demonstrate that incorporating detailed component-level energy balances and experimentally optimized transfer coefficients enables more accurate prediction of thermal behavior of stills.
Kochaki et al. (Tue,) studied this question.