Entropy generation in single-slope solar stills: influence of glass cover angle and solar radiation intensity

This study investigates the entropy generation behavior in a single-slope solar still using a transient finite element analysis. The system incorporates five pipes filled with n-Eicosane as the phase change material (PCM), while Al2O3 nanoparticles are dispersed in the basin water to form a nanofluid. A two-phase model is employed to simulate the nanofluid flow over 13 h under variable solar heat flux. The glass cover angle is varied between 10° and 40°, and the external heat transfer coefficient ranges from 5 to 300 W/m2·K. Under constant conditions of h = 5, the average fluid entropy rises from 8.46 × 10−12 to 1.03 × 10−10, equivalent to a growth of more than 1117 %, while the average thermal entropy decreases from 0.0169 to 0.00697, showing a drop of about 59 %. The total entropy also follows this trend, decreasing from 0.0133 to 0.00503, representing a reduction of more than 62 %. A comparison of the maximum values shows a similar pattern; fluid entropy increases by more than eight times, while thermal and total entropy decrease by about 47 % and 56 %, respectively. Changing the glass heat transfer coefficient from 5 to 300 W/m2 K has a very minor effect on the results, producing only about 1–2 % variation. The molten fraction of PCM remains nearly constant at around 0.515, with its maximum always equal to 1. These findings indicate that the glass angle is the key parameter in reducing system irreversibility, while the effect of h is minimal, and the PCM primarily functions as a stable thermal buffer.