Solar still productivity improvement: active and passive modes of external condenser integration

Abstract

Solar stills are simple and sustainable distillation systems, but their widespread application is hindered by low freshwater productivity. This theoretical study investigates a condenser-integrated single-slope solar still oper ating under passive and active vapor extraction modes to enhance distillate yield and thermal performance. In the passive configuration, diffusion-purging-driven vapor transport mechanisms were analyzed, while the active configuration employed a photovoltaic-powered fan to enhance vapor extraction toward an external ground- cooled condenser. The condenser was in the soil to maintain lower operating temperatures and minimize direct solar heating. The theoretical analysis was conducted for a basin water depth of approximately 1 cm under representative climatic conditions in Birjand, Iran, with peak solar radiation reaching approximately 1150 W/m². The results showed that the fan-assisted configuration increased cumulative freshwater productivity by up to 24.76% compared with a conventional solar still, while the passive condenser-assisted system achieved only modest productivity enhancement. The thermal efficiencies of the conventional, passive, and active systems were 34.95%, 35.45%, and 44.24%, respectively. Although the active system demonstrated superior freshwater pro ductivity and thermal efficiency, the conventional system exhibited higher exergy efficiency because of its higher saline water temperature and associated exergy potential. The results further indicated that productivity enhancement strongly depended on the fan volumetric flow rate and the effectiveness of condenser cooling. The proposed ground-cooled condenser-assisted configuration demonstrates the potential of vapor extraction and condenser thermal management for improving solar still performance under off-grid operating conditions.