Stable freshwater production of multi-stage solar still unit by latent and sensible heat packed bed thermal energy storage

The scarcity of freshwater in arid regions necessitates reliable and sustainable desalination methods. While traditional solar-powered stills offer a viable solution, their performance is inherently limited by daily solar intensity, leading to inconsistent freshwater production. This study introduces an innovative integrated system that couples a solar still with a latent/sensible heat packed bed thermal energy storage unit. This novel design aims to achieve stable, continuous freshwater production throughout both day and night. This research addresses a research gap in the existing literature by providing a comprehensive analysis of this integrated system under realistic solar conditions. A numerical model has been developed, coupling differential equations for a solar still and solar collector with a dispersion concentric model for a packed bed storage system. The system's performance has been assessed under Sinai Desert solar conditions and validated against existing literature. Various phase change materials (PCMs), sensible heat storage materials, and geometric parameters were analyzed. Results showed that RT65, as a PCM, delivered the best performance with 40.94 % thermal efficiency and 26.17 kg/day freshwater output. Among sensible heat materials, quartzite rock performed optimally, achieving 34.31 % efficiency and 21.90 kg/day water production. Further analysis revealed that smaller storage capsules (20 mm diameter) and lower bed porosity (0.22) enhanced distilled water yield to 27.04 kg/day with 42.29 % system efficiency. The geometric optimization of the solar still resulted in maximum freshwater production of 29.51 kg per day when using a length of 6.75 m with a first effect water depth of 0.04 m and subsequent effect depths of 0.06 m, demonstrating how staged water depth variations significantly improve system performance. Additionally, various correlations have been developed to predict accumulated distillate output and system efficiency, aiding in system performance estimation. These findings provide critical insights for designing efficient integrated thermal storage and desalination systems in arid regions.