Experimental study on the air velocity effect on the efficiency and fresh water production in a forced convective double slope solar still

The fresh water production in single and double slope solar stills (DSSS) depends on the rates of simultaneous processes of evaporation and condensation, where optical material properties, solar irradiance, temperature, velocity and air direction and the operating mode, natural or forced convection, are involved. Scarce experimental studies of the behavior of air velocity on water production on DSSS have been published, but mostly, the applied methodology is not clear. We analyzed the effect of the air velocity on the water production, temperature distributions and efficiency in a modified double slope solar still and compared the results with another with the same characteristics operating at natural convection. The influence of the other climatic parameters was minimized, placing a transparent wind tunnel on the solar still external cover. We tested the solar still at different average air velocities; 2.5, 3.5, 5.5 and 6.9 m/s. During the test period, the climatic parameters such as daily average solar irradiance (750–850 W m−2), and the maximal ambient temperature (33–37 °C), had small variations. We experimentally demonstrated that the thermal efficiency and production increment when the air velocity increases up to the value limit around 5.5 m/s and it then decreases at higher velocities and the velocity of 3.5 m/s is considered to be the optimum. The experimental uncertainty and error analysis was also presented. These results can be applied to any type of solar still; single or double slope, operating at natural or forced convection and improve both efficiency and production within the domain of the tested air velocities.