Thermal buffering with medium-temperature PCM for enhanced solar still performance in hot Egyptian conditions

Solar-still desalination is a promising, sustainable option for decentralizing freshwater production in areas with useful solar irradiation. Nevertheless, the relatively low productivity of conventional solar stills is always an obstacle to their large-scale application. In response to this challenge, thermal energy storage has been a subject of extensive research as a potent method that can be employed in the solar desalination system via phase-change materials (PCMs). The current study integrated a medium-temperature PCM into a single-slope solar still to enhance thermal energy storage and prolong the effective evaporation period. Moreover, an experimental investigation of the hot domestic water "HDW heat exchange"-assisted hybrid configuration was performed to further improve the savings of stored thermal energy. Three identical solar still designs were investigated under the same outdoor conditions, specifically a conventional solar still (CSS), a PCM-assisted solar still (PCM-SS), and finally a hybrid solar still with PCM storage and HDW heating hybrid solar still (HSS). Experimental findings during the evaluated summer revealed that the implementation of PCM accordingly increased freshwater production over conventional solar stills. Under the experimental conditions, the average daily distillate yield was 1.428 L/day for CSS and 2.482 L/day for the PCM-assisted system, representing an improvement of about 73.8%. The maximum daily energy efficiency was 24.77%, and the exergy efficiency was 7.10%. From an economic viewpoint, the PCM-assisted configuration obtained a lower cost per liter of produced freshwater than the conventional design under the adopted economic assumptions. The environmental assessment also suggested a possible decrease in carbon dioxide emissions linked to freshwater production. Because the current experiments were performed over a limited summer measurement period, long-term economic and environmental indicators presented in this study should be viewed as scenario-based estimates rather than direct year-round measurements. More studies are suggested to confirm system behavior in other seasons and explore the PCM's long-term performance under different climate conditions.