Dual optimization of a stepped solar desalination system using porous black wool absorber under semi-arid climatic conditions

Global freshwater scarcity, especially across arid regions such as Afghanistan, underscores the urgent need for sustainable and low-cost desalination technologies. Conventional solar distillation systems suffer from intrinsically low thermal efficiency and limited productivity. This study aimed to overcome these limitations by experimentally and analytically evaluating the performance enhancement achieved through integrating low-cost porous black wool insulation into the steps of a cascade solar still. A series of laboratory experiments were conducted under controlled clear-sky conditions, supported by quasi-dynamic thermal modeling and exergy analysis. The energy and exergy efficiencies were computed using validated thermodynamic relations, while an innovative cost-effectiveness index (CEI) was introduced to quantitatively assess the trade-off between thermal performance and economic feasibility. The results reveal a peak energy efficiency of 24.7% and an exergy efficiency of 4.35% for a thermodynamically optimal configuration at a thickness of 20 mm, representing a 7.08% improvement over an uninsulated baseline. However, the highest economic productivity, expressed by the maximum CEI of 0.49% /mm, was achieved at a thickness of 5 mm, identifying a critical divergence between thermal and economic optima. The novelty of this work lies in establishing a dual optimization pathway—thermal efficiency and cost-effectiveness—based on a material-thickness-dependent function validated by both experimentally and theoretically. These results provide a new, quantitative framework for sustainable design and scale-up of economically viable solar desalination systems.