Performance of Multi-Stage Tubular Solar Still Operating Under Vacuum

Multi-stage solar desalination running under vacuum is a low-cost, effective and sustainable way to mitigate fresh water shortage. However, questions remain on the operation pressure (Pop) and stage number of the multi-stage solar still. To address these questions, a predictive model was developed for the yield of multi-stage tubular solar still under vacuum (MSSV) and validated against field experiments. A 3-stage tubular still under typical weather (i.e. 19.75 MJ/m2 cumulative solar radiation) produced maximum water yield (7.15 kg/m2) with optimal Pop (48 kPa), much higher than under normal (5.81 kg/m2) and lower pressure (1.58 kg/m2 with Pop=20kPa). At optimal Pop, a delicate balance exists between the energy adsorbed and dissipated. Stronger solar intensity leads to more water yield and higher optimal Pop, while increasing the stage number improved the fresh water yield with diminishing marginal effect and slightly shifted the optimal Pop to a lower value. Using water immersion cooling other than air cooling significantly increased the water yield from 7.15 kg/m2 to 10.51 kg/m2. The MSSV model enriched the fundamental understanding of energy utilization for solar desalination and could serve as a design tool to maximize the performance of multi-stage solar still by optimizing still parameters and geometry.