Walnut Shell Bio‐Waste as Sustainable Thermal Storage in Solar Stills: Performance and Economic Evaluation

ABSTRACT Conical solar stills (CoSSs), with low heat utilization and low productivity, limit their widespread use for sustainable freshwater production. The current study aims to improve the thermal and exergy performance of a CoSS by incorporating walnut shells as an affordable bio‐waste‐based sensible heat storage medium. Three variants were investigated experimentally under the same climate: a traditional CoSS, a walnut‐shell‐integrated CoSS with shells facing down, and a facing‐up design. Freshwater yield, energy efficiency, and exergy efficiency were assessed. Research was conducted in El‐Oued, Algeria, in July 2024. The daily yields for CoSS, CoSS with half walnut shells facing down, and CoSS with half walnut shells facing up are 4.5, 6, and 6.7 kg, respectively. The rate of enhancement in the daily yield was 33.33% and 48.88% for CoSS using half walnut shells facing down and half walnut shells facing up, respectively. The daily energy and exergy efficiencies for the CoSS, CoSS with half walnut shells facing down, and CoSS with half walnut shells facing up are 31.03% and 1.02%, 44.89% and 1.4%, and 50.88% and 1.78%, respectively. The higher efficiency observed when the walnut shells are facing upwards indicates a greater rate of thermal energy utilization within the conical solar still and better heat transfer characteristics. These results highlight the potential of walnut shell biomass waste as an economical and efficient material to improve the energy and exergy performance of solar stills.