Year-round experimental analysis of the productivity of vapour-based multistage solar still: a developmental study

Abstract

The standalone vapour-based multistage solar still with stacked stages (MSS-SS) belongs to a pool of widely studied small-scale water desalination devices through solar thermal energy. This work contributes to the body of knowledge by presenting a system with new configurations. There is a need to develop small-scale systems to be reliable devices for freshwater provision, as brackish water is available for processing. The experimental study was conducted in a field under actual weather conditions, with the data logged and analysed to study the systems' behaviour under varying meteorological conditions. A maximum distillate yield of 7790ml, corresponded to a maximum daily average solar radiation at high range. There was a 21.8% decrease to 6090ml at moderate daily average range and a further decline of 80.5% to 1190ml in the low daily average range, representing a significant drop in the distillate yield caused by the insufficient heat collection at low range. The high, moderate, and low ranges corresponded to summer, spring and autumn, and winter, respectively. The lower values of the moderate range were the most optimum operationally. The impulsive modes were ideal for high rates of the heat inputs, while the continuous were for low rates. The assumption of a continuous mode and a further increase in the rate of thermal energy input caused thermal damage necessitating the augmentation of the thermal energy storage (TES) device due to a larger collector-to-basin area (CBA) ratio. The distillate yield trends from the stages were dynamic and were the inverse of the stage temperature, which was dictated by the mode and rate of the thermal energy input. These trends were such that stage 5 > 3 > 2 > 1 > 4 at moderate to high ranges and changed a low range. The summer season enhanced the cumulative saline water (SW) preheating and heat recovery to 66.8°C. The economic analysis found that at its most productive level, the cost of producing water per litre (CPL) from the vapour-based MSS-SS was R 4.05. The small-scale water purification systems are helpful, especially in remote areas.