Experimental evaluation of the effect of boulders and fines in biodegradable organic materials on the improvement of solar stills

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In this research, for the first time, the effect of primary particles (boulders) and secondary particles (fines) in organic mixtures of coffee, black walnut hull, madder, and tea (which are cheap, abundant, and biodegradable) on the improvement of solar stills' daily efficiency is evaluated as an alternative to metal-based nanofluids. A laboratory still simulator is utilised under laboratory conditions to measure the organic mixture's behaviour accurately. Furthermore, the effect of the concentration of organic mixtures and the particle size of organic materials are investigated, as well as the effect of boulders and fines, independently. In addition, two identical solar still systems are used under environmental conditions. The results show that these mixtures increase solar radiation absorption and water production. In most experiments, the maximum amount of produced freshwater belongs to the organic mixtures of the black walnut hull. The highest daily efficiencies of the solar still containing saline water and organic mixtures of black walnut hull and coffee are 44.2% and 40.3%, respectively. Finally, an economic analysis of organic mixtures of black walnut hull and coffee shows that freshwater costs are 0.0107$/L and 0.0112$/L, respectively. Therefore, these organic mixtures seem feasible for solar irradiance absorption and freshwater production.