Experimental and Theoretical Investigations of Performance of Multi-Stage Solar Still Water Desalination Unit Coupled With an Evacuated Tube Solar Collector

Abstract

At present scarcity of potable and drinking water is a pressing issue in certain parts of the Middle East region. Important advances have been made in desalination technologies but relatively high capital and running costs restrict their wide application even in cases when solar energy is used. Flat-plate solar collectors mainly have been employed in the past to distill water in compact desalination systems. Currently, it is possible to replace the above collectors by more advanced evacuated tube ones, which are available on the market at a similar price. This paper describes results of experimental and theoretical investigations of the operation of a solar still desalination system coupled with a heat pipe evacuated tube collector with the aperture area of about 1.7 m2. A multi-stage solar still water desalination system was designed to recover latent heat from evaporation and condensation processes in four stages. The variation in the solar radiation (insolation) during a typical mid-summer day in the Middle East region was simulated using an array of 110 halogen flood lights covering the area of the solar collector. The synthetic brackish lab water solution was used for experiments and its total dissolved solids (TDS), electrical conductivity and pH were measured prior to and after the distillation process. The system’s operation was numerically simulated using a mathematical model based on the system of ordinary differential equations describing mass and energy conservation in each stage of the system. The experimental and theoretical values for the total daily distillate output were found to be in good agreement. The results of tests demonstrate that the system produces about 6.5 kg of clean water per day and have the distillation efficiency equal to 76%. However, the overall efficiency of the laboratory test rig at this stage of investigations was found to be low at the level of 26% and this is due to excessive heat losses in the system. The analysis of the distilled water shows that its quality is within the World Health Organization guidelines. Further research is being performed to improve the performance of the installation.