Vertically-oriented paper sheets with fibers coated with carbon black as heat absorbers in solar stills for water harvesting and desalination

Abstract

Solar stills are economical and efficient small-scale water-purification devices. Owing to their portability, they are easy to use in remote locations where clean, drinkable water is scarce. Solar stills absorb solar radiation to vaporize saline or contaminated water, which is condensed in contact with walls cooled by the surrounding atmosphere and collected aside as drinkable water. Hence, the drinkable-water production rate of solar stills depends upon their ability to absorb solar energy and on the cooling environment. Here, vertically oriented fibrous cellulose substrates coated with carbon black (CB) particles via supersonic spraying efficiently absorb solar energy (similarly to a Helmholtz jar) to evaporate saline or contaminated water. When saline or contaminated water is supplied to cellulose fibers in the vertical substrate (10 cm × 10 cm) using a capillary pump for 5 h, the CB-coated (the number of coatings Ncoat = 15) and uncoated (Ncoat = 0) systems exhibit the maximum temperature of soaked cellulose Ts = 78.8 ℃ and Ts = 71.5 ℃, respectively, in the steady state. The coated case would yield a water production rate of 0.79 kg⋅h−1⋅m−2, which is equivalent to 7.9 L⋅day−1⋅m−2 while assuming an absorber size of 1 m × 1 m exposed for 10 h of solar light per day. Moreover, the cellulose-based solar still desalination process produces water that meets the salinity standards (i.e., a salinity level of 0.02 % or 20 mg⋅L−1) recommended by the World Health Organization.