Performance enhancement of solar still desalination systems using revolving tubes: CFD simulation and experimental investigation

Performance enhancement of solar still desalination systems using revolving tubes: CFD simulation and experimental investigation

Water and energy are indispensable resources for life and civilisation. The scarcity of both water and energy have emerged as amongst the most serious concerns of our time due to the dramatic growth in population, enhancement in our standards of living, and the rapid development of the agricultural and industrial sectors in many countries. Desalination seems to be one of the most promising solutions to solve the problem of water scarcity; however, it is not without costs, as the desalination process is energy-intensive. Conventional sources of energy, such as fossil fuels, are limited, depleted and pollute the environment. Therefore, the use of renewable sources of energy such as solar energy is essential and represents a better option. Solar desalination systems are environmentally friendly and offer a win-win solution to solve shortages of both water and energy. The simplest and the most straightforward solar desalination process is the natural evaporation-condensation process of the Solar Still Desalination (SD) system. An SD system simply consists of a water basin and a tilted transparent cover that is exposed to solar radiation. SD systems have a low capital cost; however, the low productivity of these systems make the cost of the water that they produce higher than that produced via other traditional desalination systems. On balance, the selection of SD systems for remote areas that have relatively low demand for water makes those systems a feasible option, due to the elimination of the high costs of water transfer if such systems are deployed locally. SD systems can work powered by solar energy, which makes them environmentally friendly and suitable for areas that have no access to electricity, such as remote villages and less developed regions of the world.

The main purpose of this study is to find a method to increase the productivity of SD systems to provide people in remote and less developed regions of the world with freshwater. The proposed technique is a simple amendment to the regular double-sloped SD system. The suggested modification was to add three parallel and symmetrical PVC tubes into the water basin of the SD system to be rotated by small DC motors. These tubes were wrapped in an absorbent black mat and were placed horizontally along the basin to be semi immersed in the water. The purpose of this modification is to stir the water in the basin and to generate a thin water layer around the tubes’ circumference, which leads to an increase in the surface area for water evaporation. This modification enhanced the water evaporation rate within the SD system, and thereby increased the productivity of this system.

In this study, two SD systems: the Normal SD (NSD) system and the Modified SD (MSD) system were designed and manufactured, simulated numerically and tested experimentally. The CFD simulation was done using ANSYS-Fluent software. The experimental investigations were carried out during spring and summer in Toowoomba, Australia. The effective operation and design parameters such as water depth, the tubes’ diameter and the tubes’ rotation speed were analysed and optimised using the sensitivity analysis. The dimensional analysis, uncertainty analysis, and the cost analysis for the present experimental setup of both the SD systems were conducted as well.

The daily productivity of the SD systems is equal to the distillate yield within a day and the daily efficiency is equal to this productivity divided by the daily insolation. The results show that the daily productivity and the daily efficiency of the MSD system were always higher than that of the NSD system. According to the experimental results, the maximum daily productivity and the maximum daily efficiency of the MSD system were 2.908