A simple and highly productive solar still: a vertical multiple-effect diffusion-type solar still coupled with a flat-plate mirror

This paper presents a newly designed simple solar still: a vertical multiple-effect diffusion-type solar still consisting of a flat-plate mirror, a number of vertical parallel partitions in contact with saline-soaked wicks with narrow air gaps between partitions, and casters for azimuth tracking by human power. The proposed still is designed so that non-skilled persons can construct it with common materials in developing countries. We theoretically analyzed the dependence of the solar absorption on the first (or heated) partition on the angle of the flat-plate mirror and the azimuth angle of the still, and found that the solar absorption on the first partition can be significantly increased by rotating the still at southing of the sun just once a day, and the daily solar absorption would be about 85% or 99% of the daily solar radiation on a horizontal surface on the spring equinox or winter solstice, respectively. We also theoretically analyzed the heat and mass transfer in the proposed still, and found that a proposed still of 10 partitions with 10 mm diffusion gaps between partitions was predicted to produce 29.2 and 34.5 kg/m2d on sunny spring equinox and winter solstice days respectively at the equator.

We proposed a newly designed, vertical multipleeffect diffusion-type solar still coupled with a flatplate mirror, and theoretically predicted the dependence of the solar absorption on the first partition on the angle of the flat-plate mirror and the azimuth angle of the still, and also the distillate productivity of the still. The results of this work are summarized as follows: 1) The optimum angle of the flat-plate mirror is 15° and 8° on a spring equinox and winter solstice day, respectively, at the equator. 2) The optimum azimuth angle of the still is –90° during the morning and 90° during the afternoon on a spring equinox day, and –45° and 45° on a winter solstice day, if the still is rotated once a day at southing of the sun. 3) The solar absorption of the first partition is significantly increased by rotating the still just once a day at southing of the sun, and daily solar absorption on the first partition with the optimum angle of the flat-plate mirror is about 85% and 99% of the daily solar radiation on a horizontal surface on a spring equinox and winter solstice day, respectively. 4) The proposed still of 10 partitions with 10 mm diffusion gaps between partitions is theoretically predicted to produce 29.2 and 34.5 kg/m2d on the spring equinox and winter solstice days respectively, on the equator. 5) Even if the actual distillate productivity is 50% of the prediction at the lowest estimate due to poor construction and operating skills, the distillate productivity would still be much larger than that of the single-effect stills, and only a little less than that of the conventional multiple-effect diffusion-type stills. 6) The proposed still has a very simple structure, and may be constructed by non-skilled persons with common materials in the developing countries.