The instantaneous thermal efficiency is expressed asequation(331)ηi=he,w−ght,g−asecθht,w−g+ht,g−asecθ[(ατ)effUt+Ub(1−e−at)+Tw0−TaI(t)se−at]whereequation(332)(ατ)eff=[(ατ)w+(ατ)bhwhw+hb]secθequation(333)Ut=[1ht,w−g+1ht,g−asecθ]−1
Based on the numerical computations, it BIM 187 was found that the solar still gives the maximum yield for an east–west orientation for θ<55°, during the winter period. For lower depth of water, the hourly variation of yield is similar to those of solar intensity due to negligible thermal capacity. The instantaneous thermal efficiency increases with an increase of inclination due to the increase of solar radiation on the inclined surface for the winter condition. But the instantaneous thermal efficiency decreases with an increase of water depth because most of the thermal energy is not utilized for evaporation and stored in the water mass itself. Also the cumulative efficiency becomes constant after reaching a steady state condition.