## 5 Results and Discussion  
Experiments are conducted on clear sky days (17-19 May 2022) with close solar radiation values to provide a fuller understanding of solar still performance with water depths ranging from 1 to 3 cm. The daily variations in the solar radiation and ambient, glass, and water basin temperatures for the three considered cases are plotted in Figures 3-5. As time progressed, the solar radiation and all recorded temperatures increased until they achieved the maximum values at 13:00 p.m. Then, the behaviour is reversed, and they gradually decrease until sunset. For 1 cm water depth, the highest temperatures recorded were 61.73, 48.71, and 27.74 \({}^{\circ}\)C (at 13:00 p.m.) for water basin, glass, and ambient temperature, respectively. While it is 59.99, 47.38, and 26.69 \({}^{\circ}\)C for 2 cm, then 55.6, 45.42, and 27 \({}^{\circ}\)C for 3 cm water depth. Figure 3: Daily variations of temperature and solar intensity of the still with 1 cm water depth. The daily variations of the basin's water temperature of the still with a water depth ranging from 1 to 3 cm are plotted in Figure 6. From 8:00 a.m. to 14:00 p.m., the water temperature in the basin is higher at 1 cm than it is at 2 and 3 cm, and then it gradually drops. Rising the water depth from 1 to 2 cm caused a decrease in the basin's water temperature of up to 4% (from 8:00 a.m. to 14:00 p.m.), and an increase of up to 2% (from 15:00 p.m. to 18:00 p.m.), which has a direct impact on the energy, exergy, and water productivity. Whereas raising the water depth to 3 cm caused a decrease in the basin's water temperature of up to 9% (from 8:00 a.m. to 14:00 p.m.), and an increase of up to 3.5% (from 15:00 p.m. to 18:00 p.m.). This increase in the basin's water temperature is due to the energy-storing ability of the water held in the basin. Figure 4: Daily variations of temperature and solar intensity of the still with 2 cm water depth. Figure 5: Daily variations of temperature and solar intensity of the still with 3 cm water depth. Figure 6: Daily variations of water temperature with respect to time. The heat and mass transfer (inside the still) are largely affected by the water depth and basin water temperature depending on Equations (3)–(8). The daily variations in the heat transfer coefficients by convection and evaporation inside the still are depicted in Figures 7 and 8. As seen from the figures, at various water depths the rate of heat transfer by evaporation is higher than the rate of heat transfer by convection under the same operating conditions. It is revealed that the still with 1 cm water depth gives the highest evaporation heat transfer coefficient (32 W/m\({}^{2}\cdot\)k) at 13:00 p.m. and then decreases to (7.7 W/m\({}^{2}\cdot\)k) at 18:00 p.m. While at 2 and 3 cm gives the highest evaporation heat transfer coefficient (between 29.3 and 24.5 W/m\({}^{2}\cdot\)k) at 13:00 p.m. and then decreases to (between 7.76 and 8.18 W/m\({}^{2}\cdot\)k) at 18:00 p.m. As a result, the evaporation process is the dominant heat transfer mechanism, and it is greatly affected by temperature differences between the basin water and the glass cover. Figure 7: Influence of water depth on heat transfer coefficient by evaporative. Figure 9 shows the energy efficiency of the still with time for the three considered cases. It is observed that energy efficiency follows the same trend as water basin temperatures (from 8:00 p.m. to 14:30 p.m.), and then the curve's behaviour is reversed at about 15:00 p.m. Energy efficiency strictly depends on water productivity and solar radiation. As the basin's water temperature reduces, the evaporation rate declines, and therefore the energy efficiency decreases. The decrease in the energy efficiency with 2 cm water depth is about 25% and about 54.3% with 3 cm, compared to the still with 1 cm water depth.