Investigation of design configurations and effective parameters on productivity enhancement of vertical diffusion solar stills

Solar distillation is one of the most promising solutions to provide freshwater in limited-population communities with high availability of solar radiation and poor accessibility to safe and clean water. Until now, several types of solar stills have been developed including wick-type stills, stepped stills, inclined and double-slope stills, tubular stills, and pyramid stills. Among them, vertical diffusion solar stills (VDSSs) have been newly developed, offering several priorities over conventional types. This study presents a detailed review of VDSSs by investigating effective parameters and employed techniques to enhance the productivity of these solar stills. For this purpose, design factors along with operational parameters including heat energy input, type of carrier gas, basin water depth, and feedwater flow rate are studied and profoundly discussed. Additionally, enhancement techniques including feedwater preheating, shape modifications, waste heat utilization, and integration with reflectors are studied. According to the literature, the productivity of VDSSs is enhanced by a reduction of the diffusion gap and a decrease in the number of effects. In some studies, the use of waste heat and electricity as energy inputs has led to higher productivity in comparison with solar energy. Moreover, considering design factors, a modified configuration can increase productivity by 44%. In this review, the environmental and economic aspects of VDSSs are also discussed. Numerical analyses have predicted that a passive VDSS with a multiple-chamber configuration and 13.85% exergy efficiency could cut 67.93 tons (average) of CO2 emission during its 20-year lifetime, while its financial payback time is 0.63 years. The studies have also demonstrated that an optimized VDSS can reach the water price of 6.1 USD/m(3) which makes this technology more attractive than other low-capacity stills from economic aspects.