Thermal, Exergetic, Economic and Environmental Analysis From Tubular Solar Still Using Walnut Shell as Thermal Energy Storage

ABSTRACT Across the arid and semi‐arid regions, the scarcity of fresh water is a critical problem. The demand for fresh water can be solved using conventional desalination plants; however, their high energy demand and initial capital cost remain their constraint. A solar still is such an alternative, as it is a sustainable and low‐cost solution and uses solar heat as a renewable form of energy. However, solar stills are less attractive than conventional desalination plants due to their low thermal efficiency and poor heat retention. To address these constraints, the present work reports the use of agro‐industrial waste (walnut shells) as thermal energy storage (TES) in a tubular solar still (TSS) in a flat absorber. Walnut shells have been used as a thermal energy storage in conical geometries, but still their influence on the mass of TES loading has not been studied so far. In the present experimental investigation, three identical solar stills, namely traditional TSS (TTSS) and solar stills with 5 and 10 kg mass of walnut shell, are tested under the identical outdoor conditions, and their thermal performance (energy and exergy), economic performance, and environmental performance (4E) analysis are studied. The experimental findings revealed that walnut shell with 10 kg mass as thermal energy storage produced a maximum cumulative yield of 8.01 kg/m 2 /day which is 31% higher compared to the TTSS. Similarly, the mean energy and exergy efficiencies are improved from 49.13% to 64.41% and 3.23% to 5.81%, respectively, compared to the TTSS. Compared to the baseline (TTSS), the solar still with 10 kg walnut shell extended the water temperature for an effective evaporation rate, especially during the off‐sunshine period due to their higher porosity and higher carbon content. Economic analysis revealed that the cost of fresh water produced from the modified solar still decreased to 0.0069 from 0.0085 $/L, with a reduction in the payback period to 33 days. Similarly, the CO 2 mitigation reached 21,413 kg. These findings establish walnut shells as a low‐cost, circular‐economy‐aligned TES that materially improves the technical and financial viability of solar‐driven desalination.