Fig xA Schematic of the multistage evacuated

The solar AT 56 absorbed on the glass cover and the evaporating wick can be determined as follows:
For a still with an external reflector:equation(423)Qsun,g=αgαw[(Qsun,dr+Qsun,re)/τg(βg)+Qsun,df/(τg)df]equation(424)Qsun,w=Qsun,dr+Qsun,re+Qsun,dfQsun,w=Qsun,dr+Qsun,re+Qsun,df
The value of τg(βg) can be calculated as follows:equation(425)τg(βg)=2.642cosβg−2.163cos2βg−0.320cos3βg+0.719cos4βg
For a still without the external reflector:equation(426)Qsun,g=αgαw[Qsun,dr/τg(βg)+Qsun,df/(τg)df]equation(427)Qsun,w=Qsun,dr+Qsun,dfQsun,w=Qsun,dr+Qsun,df
In the above expressions, Qsun,dr, Qsun,re, and Qsun,df are the direct solar radiation absorbed on the evaporating wick, the solar radiation reflected from the external reflector and absorbed on the wick, and the diffuse solar radiation absorbed on the wick, respectively.
It was observed that the average daily amount of distillate peaks when the angle of the still θ=20° for the still with reflector, and peaks at θ=30° for the still without reflector. Also the average daily amount of distillate of the still with reflector is predicted to be about 9% larger than that of the still without reflector, and the vertical flat plate external reflector would be less effective for the tilted wick still than for the basin still.