Walnut shells as an eco-friendly material for enhancing conical solar still output

Abstract

This study explores the effectiveness of utilizing walnut shells as a natural thermal energy storage material to enhance the productivity of conical solar stills (CSS). The investigation focuses on two different walnut shell sizes (4 mm and 8 mm) distributed uniformly at the basin of a CSS with a surface area of 0.1 m². The still features a 3 mm-thick plastic conical cover and is filled with 1 liter of water. The study evaluates two modified stills—one with 4 mm walnut shells (CSS-WS4) and another with 8 mm walnut shells (CSS-WS8)—against a conventional conical solar still (TCSS) serving as the control. The experimental results reveal that the cumulative distillate yield for the TCSS, CSS-WS4, and CSS-WS8 are 4.80, 6.90, and 6.00 L/m²/day, respectively. The presence of walnut shells significantly improves the water output, with CSS-WS4 achieving a 43.75% increase and CSS-WS8 a 25.00% increase compared to the TCSS. Furthermore, the economic analysis indicates a reduction in the recovery period, with CSS-WS4 and CSS-WS8 requiring 22 and 25 days, respectively, compared to 31 days for the TCSS. The results highlight that the performance enhancement is primarily attributed to the superior heat retention properties of walnut shells, which improve evaporation rates within the still. This study underscores the potential of incorporating walnut shells as a cost-effective and eco-friendly solution for optimizing solar distillation efficiency.