So far, very little attention has been focused on forced circulation solar stills. In this study, the heat and mass transfer relationships in the forced circulation solar still with enhanced water recovery will be developed. Following this the simulation model for this still
will be incorporated into the SOLSTILL program. Finally, the SOLSTILL program will be validated by comparing its results with those from the experimental model.

The forced circulation solar still has been chosen in this study for several reasons. Compared with other types of solar powered distillation systems such as the solar multistage flash distillation, solar vapour compression, solar powered reverse osmosis, solar powered electrodyalisis, and solar membrane distillation systems, solar stills represent simple, yet mature technology. This is suitable for developing countries like Vietnam.

The low efficiencies of a conventional solar still may be overcome by changing the principle of operation as follows:

• Using air as an intermediate medium and substituting forced convection for natural convection to increase the heat coefficients in the still, resulting in increased evaporation of water.

• Replacing saturated air in the standard still by “drier” air to increase the potential for mass transfer in the still, leading to higher outputs.

• Circulating the air-vapour mixture from the standard still to external water cooled condensers to gain efficiency from a lower condensing temperature. The cooler the cooling water available, the more effective this condensing process will be.

• Recovering some of the heat extracted in the condensing process and using it to preheat the air-vapour mixture entering the still.

• Substituting the condensing area of the flat sheet covers in the standard still by the external condenser with much larger heat exchange areas to increase condensation efficiencies.