Step Solar Still: Solar Ray Tracing and Species Transport

Description

This project investigates the performance of a step solar desalination unit (solar still) using ANSYS Fluent, studied through CFD analysis. Producing potable water from saline sources is a central goal of clean water engineering, and the solar still achieves this passively — using nothing but solar energy to convert brackish or seawater into fresh water.

The model consists of a small chamber with a sloping glass surface on each side and a series of steps inside, over which saline water flows. Solar radiation passes through the glass to the water surface on the steps, evaporating it; the resulting vapor then meets the cold glass surface and condenses in a distillation process. The freshwater produced by condensation runs down the slope of the glass plate and is discharged as pure water.

The model was built in 3D using Design Modeler. Because the geometry is symmetric, only one-quarter of it is modeled. This quarter-geometry comprises two sloping glass surfaces and the steps that carry the water flow. Meshing was performed in ANSYS Meshing, producing 809,037 elements.

Methodology

To convert saline water into fresh water, the water must first be turned into vapor and then condensed back into liquid. A solar still typically consists of a sloped glass cover placed over the device and a stepped platform that holds the saline water to be distilled. The glass cover acts as both a rigid substrate and a transparent layer that admits the sun's rays, while the heat needed to generate vapor comes from the solar energy absorbed inside the device. The vapor then rises, strikes the cool glass cover, and condenses into fresh water.

To capture this, the Solar Ray Tracing model is enabled to represent the absorbed solar heat, and the Species Transport model is activated to model the two components — water and vapor — inside the chamber. This approach treats the chamber interior as a water-vapor mixture and does not explicitly simulate the flow of the distilled water. At the start of the simulation, the interior is filled entirely with vapor; the bottom plane of the chamber represents the water surface, and since evaporation occurs there, this surface is treated as vapor.

Constant values of 1.00314 × 10⁻⁵ m²/s and 0.0002 kg/m·s are assigned to the mass diffusion coefficient and the thermal diffusion coefficient, respectively, governing the conversion between water and vapor. The sloping plate where the vapor condenses is assumed to consist only of vapor. The ambient air temperature is taken as 311.75 K with a heat transfer coefficient of 25 W/m²K. The stepped platform holding the saline water is assumed to absorb heat with no transmissivity, while the glass cover is assumed to have maximum transmissivity and no absorbance. Gravity is enabled in the Y direction, and the incompressible ideal-gas model is used to account for the density difference between water and vapor.

Conclusion

On completion of the solution, three-dimensional contours of velocity, temperature, velocity vectors, and species mass fraction inside the still were obtained.

The velocity contours and vectors show how the generated vapor rises within the device to strike the upper sloped glass surface. The 3D temperature contour makes clear that the saline water on the stepped platform has heated up enough to evaporate, while the glass cover remains cold enough to condense the vapor, which then slides down to the lower part of the device where the fresh water is collected. Finally, the species mass fraction contour shows a water mass fraction of one on the stepped platform and the glass substrate, while the water fraction decreases within the interior space — confirming that vapor is forming there. Together, these results demonstrate the complete evaporation-condensation cycle that enables the solar still to deliver clean, desalinated water using only solar energy.