Assessment of Pyrolytic Biochar as a Solar Absorber Material for Cost-Effective Water Evaporation Enhancement

Abstract

Enhancement of water evaporation using carbon-based solar absorbers has been gaining acceptance for its potential use in solar-driven water desalination. The evaporation rate can be increased by using these materials as either a floating suspension or as particles dispersed in water. Although several materials have been proposed for the purpose of increasing the rate of water evaporation, there is still a need to develop low-cost materials that exhibit strong solar absorbance with the purpose of providing freshwater to rural and low-income communities. Biochar is a carbon-rich material, obtained from the thermochemical conversion of biomass. Although it is mainly used as an organic amendment for soil, in this study, its potential utilization as a low-cost solar absorber is analyzed. Two pyrolytic biochars are investigated, that is, Ponderosa pine from forest waste, and walnut shell from the agricultural sector. Measured solar absorptivity values were >92% at 1 g/L dispersion concentration, whereas the transmittance of light was <1% for a 10 mm path length. Controlled indoor experiments with a solar simulator resulted in evaporation rates between 1 and 1.1 kg/m2-h, which are significantly higher than for pure water. Biochar was found to be more economical than some of the other carbon materials analyzed in the literature, thus, opening an opportunity for feasible use in low-cost solar stills.