Heat-concentrating solar steam generation and salt extraction based on water-repellent germanium nanoparticles-coated oxidized copper foams

Abstract

We demonstrate solar steam generation with high evaporation rates and extraction of dry salts from solutions by using water-repellent germanium nanoparticles (GeNPs)-coated oxidized copper foams (CFs) as light absorbers and heat concentrators. The CF surface was first oxidized into black CuO, then dip-coated with colloidal GeNPs for enhancing infrared absorption, and lastly treated with perfluoroalkyl silane to render hydrophobicity. The CF can absorb more than 95% of the AM1.5G solar irradiance spectrum, for heating up a water-permeated cellulose paper underneath to generate steam which is then evacuated through the CF's interconnected pores. Furthermore, due to the CF's high thermal conductivity, the heat generated in the peripheral region can be efficiently concentrated to the center where the water evaporates. With the light-absorbing CF area about 7 times larger than the water-evaporating cellulose paper and the convective heat loss mitigated by an acrylic cover, under one sun solar irradiance, the CF temperature can reach 70 °C, resulting in an evaporation rate as high as 3.2 kg m−2 h−1. Moreover, when a NaCl solution is used for the heat-concentrating evaporation inside an enclosed system in which the water condensate is recycled, readily useful dry salts are formed and can be harvested from the hydrophobic CF surface, while the bulk solution is gradually desalinized.