The energy balance for water
It was found that URMC-099 the enhancement of water productivity per day was more than 100% when using vacuum inside the solar still. It was inferred that the existence of non-condensable gas (such as air) adjacent to the condensate surface behaves as a thermal barrier to the heat transfer. The authors argued that the enhancement was due to the absence of convection heat transfer loss from the water and also the absence of non-condensable gases inside the still when complete vacuum was applied.
7.1.9. External condenser
Mohamad Abu-Qudais et al.  presented a microfilaments theoretical model for external condenser type solar still to predict the efficiency. The productivity and efficiency of the external condenser type still were compared with conventional still under different conditions. A low power, variable speed fan was used to exhaust the water–vapor from the solar still to an external condenser as shown in Fig. 12.
Fig. 12. Schematic of solar still with external condenser .Figure optionsDownload full-size imageDownload as PowerPoint slide