Enhancing solar still productivity in the Agadir region using phase change materials

• Transient energy balance model for a PCM coupled single slope solar still (Agadir climate). • Combined influence of basin water (10–30 kg) and PCM mass (8–30 kg) analyzed for four PCMs. • Higher water mass reduces yield; whereas higher PCM mass enhances nocturnal production. • The highest nocturnal yield (∼0.5787 kg. m⁻². night⁻¹) was achieved with 30 kg of STSPH. Solar distillation using solar stills offers a sustainable route for seawater desalination in arid and semi-arid regions affected by freshwater scarcity. This study presents a comparative numerical assessment of a single slope solar still integrated with phase change materials to enhance thermal storage and distillate production. A transient model based on energy balance equations for the main components was developed and solved, and the predictions were validated against published experimental and theoretical results. Simulations were carried out for a representative partly cloudy day in Agadir, Morocco, to examine the combined influence of basin water thickness and PCM mass. Four PCMs were investigated: paraffin wax, stearic acid, sodium thiosulfate pentahydrate (STSPH), and paraffin RT27. The results show that PCM integration enhances nocturnal productivity by releasing stored latent heat after sunset; for paraffin wax, stearic acid, and STSPH, overnight yield increased with PCM mass, confirming the role of storage capacity, while increasing water thickness reduced the thermal response of the still due to higher thermal inertia, delaying temperature rise and distillate generation. Among the tested materials, stearic acid at 8 kg achieved the highest cumulative daily yield of 5.35 kg.m⁻².day⁻¹, outperforming paraffin wax, STSPH, and paraffin RT27 at the same mass. While, STSPH at 30 kg provides the highest nocturnal yield, reaching 0.5787 kg.m⁻².night⁻¹, higher than the conventional solar still by approximately 399.3%. The energy efficiency ranged from 39.17% to 46.59%, with the highest performance obtained using 8 kg of stearic acid and the lowest recorded for 18 kg of RT27.