A single-slope solar distiller, or solar still, is a simple yet effective technology that uses sun energy to filter drinking water from contaminated or saline sources. Simple: the sun heats a shallow basin of water, causing it to evaporate and ascend, leaving impurities. The vapor condenses on a tilted transparent cover, commonly glass or plastic, and collects as freshwater in a collection container. This clever device purifies water using evaporation and condensation, making it useful in locations without clean drinking water. Its simplicity, low cost, and sustainability appeal to populations facing water scarcity worldwide.

Now we know how important Solar still (distiller) is to engineers, especially in deprived areas. This made us start validating a numerical paper entitled “Productivity estimation of a single-slope solar still: Theoretical and numerical analysis [1]” published in Energy journal of Elsevier.

• Reference [1]: Rahbar, Nader, and Javad Abolfazli Esfahani. “Productivity estimation of a single-slope solar still: Theoretical and numerical analysis.” Energy49 (2013): 289-297.

Figure 1: Sketch of the geometry and coordinate system of the solar still [1]

Simulation Process

The configuration is simply designed. A combination of a triangle and a rectangle. So there wouldn`t be any challenge during the modeling of the geometry and creating the mesh. However, the solver settings are sensitive when it comes to paper validation. Literally, the domain is filled with air and water vapor, so Species Transport model is activated. Based on the solar still mechanism, parameters such as density and mass diffusivity play a prominent role in the simulation.

Post-processing

When compared to published numerical data, the post-processing study of the single-slope solar still CFD simulation shows strong validation results. The calculated Nusselt number, 14.48, is 6.5% off from the standard value of 15.5, which is well within the range of error that engineers can handle. Also, there is only a 4.7% difference between the benchmark value of 5.29e-5 kg/m³·s and the expected value of 5.04e-5 kg/m²·s. The velocity contour plot shows typical natural convection patterns, with speeds ranging from 0 to 0.294 m/s. This clearly shows the buoyancy-driven flow that is important for the distillation process. To summarize:

The implementation of the species transport model and the thermal-fluid coupling are confirmed by more analysis of the CFD data. In the contour plot, the velocity distribution shows clear circulation patterns. Higher speeds (0.228–0.294 m/s) are seen near the sloped surface where condensation happens, while slower speeds (0–0.065 m/s) are seen in the water basin area. This pattern of flow matches the expected thermosyphon effect in solar stills, where changes in density caused by temperature move water vapor. The computer model is a good way to predict how well a solar still will work because it agrees closely with reference data for both heat transfer (Nusselt number) and mass transfer (productivity).

Figure 3: a) Velocity b) water field inside the solar still system