Solar water purifiers for small rural African homesteads: Evaluation of alternative designs

This paper presents five different designs of single slope solar water stills. These still designs would probably be useful in small homes wishing to purify water for their daily drinking needs. Models of the stills ranging between 1 m² and 1.5 m² glazing area are manufactured and tested in real outdoor conditions. Two of the stills are double glazed and have external condensers, with one of them using forced steam extraction. A third still is mounted on a sun tracking pillar. The remaining two are simple collapsible and portable units but of different slopes. A method of evaluating the designs is illustrated. It relies on a focus on the purpose of a solar still and what scarce resources are required to get outputs from it. Thus, the stills are evaluated based on function, quality, productivity and cost. It is found that the tracking unit out performs the others on function and quality criteria but it is outcompeted on efficiencies and cost criteria. Condensate drip-back was found to be an issue that necessitates use of slopes that do not necessarily yield maximum energy incidence on the stills. It was also found that double glazing and external condensation did not help improve performance on any of the criteria. It is concluded and recommended that an ideal single slope solar water purifier operating in weather similar to Cape Town's summer season, would need to have a thin single glazing, sloped towards 45°, portable and sun tracking.