proportion of the input solar energy that causes evaporation. Figure 6.9
shows the results of calculations allowing for the transfer of heat and solute
(water vapour) by (air) convection inside the still, and for the relatively high
reflectance of the glass top (caused by condensed water droplets). We see
that the fraction of heat going into evaporation is almost independent of
Tw
−Tg but increases strongly with the water temperature Tw. This is to be
expected, since the vapour concentration #Tw increases non-linearly with
Tw (see Figure 6.6). The results also show that the maximum production
achievable with this type of still with basin water at ∼50 C is 60% of that
calculated in Example 6.4, i.e. 5 kg day−1m−2= 5litre/daym2
An alternative approach is the multiple effect still in which the heat given
off by the condensation of the distilled fresh water is used to evaporate a