Development of vapour-based multistage solar still desalination system

Abstract

The current study conceived, constructed, and experimentally tested the vapour-based multistage solar still with stacked stages (MSS-SS). The construction of the system was done at Cape Peninsula University of Technology (CPUT), Bellville campus. The experimental tests were conducted for a period of 10 months which covered the four seasons of the year, starting from Sept. 2020 to the end of June 2021. Moreover, there were key difference that can be noted from the waterbed-based MSS-SS and the current vapour-based MSS-SS. The vapour-based MSS- SS did not have waterbed in the stages, it employed a direct vapour input through the vapour make-up tubes to the stages, adopted a dynamic mode (impulse circulation) of waterbed in the entry stage (evaporator), it was a standalone system, brine solution was recirculated to aid in saline water (SW) pre-heating, direct SW heating in the evacuated tube solar collectors (ETSCs), vapour cooling SW in a zigzagged SW tube, minimal SW quantity in the entry stage to mention few. The vapour-based MSS-SS was integrated with a passive basin solar still (BSS) situated at the top of the stacked stages. The distillation unit operated at SW temperatures ranging from 90°C to 100°C which indicated increased vapour production in the evaporator. However, the experimental tests indicated the collector to basin area (CBA) ratio was larger and the distillation unit required a thermal energy storage (TES) as the system reached thermal damage conditions under increased thermal energy input. For a typical day, the thermal efficiency of the distillation unit was found to be 18.23%. The MSS- SS daily cumulative distillate yield ranged from non-measurable (NM) quantities to a maximum of 7790 ml (7.8 litres) under low to high average solar irradiance, respectively. Furthermore, the quality of the distillate suggested that food grade material should be used for construction to minimize contaminants. At its most productive days, the cost per litre (CPL) was R4.69 and R4.05 (R=South African Rands) for the MSS-SS stacked stages and the MSS-SS stages plus the BSS, respectively. For the thermally insulated condensing tower, the thermal efficiency was 18.23%. Under insulated conditions, the thermal efficiency decreased further. The experimental tests showed that the performance of the MSS-SS can be enhanced by an improved SW pre-heating and heat recovery processes. Therefore, it was concluded that the developed standalone MSS-SS with circulation waterbed, improved SW pre-heating and heat recovery processes, had a potential to be an alternative and supplement to the existing distillation.