A Comprehensive 4E Study on the Performance of Hemispherical Solar Stills Utilizing Walnut Shells

ABSTRACT Freshwater scarcity is a critical challenge in arid and semi‐arid regions, requiring cost‐effective and sustainable desalination solutions. Hemispherical solar stills (HSS) are promising due to their high efficiency and simple design, but further improvements are needed to maximize output. This study investigates the performance enhancement of HSS through the integration of walnut shells as a thermal storage medium. Experiments were conducted on August 13, 2024, at the University of El Oued in Algeria, with testing carried out from 7:00 a.m. to 7:00 p.m. under local climatic conditions. A comprehensive 4E analysis (Energy, Exergy, Economic, Environmental) evaluated the effect of walnut shell sizes (3 and 6 mm) on productivity and sustainability. The results show that the 3 mm walnut shell configuration (HSS‐WS3) achieved the highest daily yield of 7.10 L/m²/day, a 51.06% improvement compared to the traditional HSS (THSS), while the 6 mm configuration (HSS‐WS6) yielded 6.40 L/m²/day (+36.17%). Energy efficiency increased by 50.50% for HSS‐WS3% and 35.90% for HSS‐WS6, while exergy efficiency improved by 118.30% and 70.60%, respectively. Economically, the cost per liter of distilled water decreased to $0.0051 (HSS‐WS3) and $0.0056 (HSS‐WS6) compared to $0.0077 (THSS), with payback periods reduced to 20 and 23 days. Environmentally, HSS‐WS3 achieved the highest CO₂ mitigation of 2.049 tons over a 10‐year lifespan. These findings demonstrate that walnut shells, particularly at smaller particle sizes, are effective, low‐cost, and sustainable materials for enhancing solar still performance, providing a promising desalination strategy for water‐scarce regions.