Understanding solar thermal gradient to improve solar evaporation performance for water collection

Solar evaporation has attracted great interest in water collection, gaining considerable attention recently. While many efforts have been made to enhance solar thermal conversion performance from materials design aspects, little attention has been given to the fundamental solar thermal gradient concept, which significantly affects local heating during evaporation. In this work, the polymer sponge evaporator was designed to control the solar thermal gradient by adding copper–carbon core–shell (Cu@C) nanoparticles with similar solar absorptance to understand the effect of solar thermal gradient or local heating on evaporation performance. The optimized solar evaporation can be 2.0 kg·m⁻²·h⁻¹ under 1000 W·m⁻² (one sun) with a Cu@C mass fraction of 0.5 wt.%, which was higher than that observed in cases with either higher or smaller Cu@C mass fraction. A too-small or large Cu@C mass fraction would enhance heat loss from the bottom or top parts, which was also confirmed by simulation results. Further outdoor water yield experiment showed that the optimized Cu@C mass fraction of 0.5 wt.% achieved the highest water collection (6.67 kg·m⁻²·d⁻¹) compared with the other cases, such as 5.92 kg·m⁻²·d⁻¹ for 0.1 wt.%, 5.29 kg·m⁻²·d⁻¹ for 1 wt.%. These results highlighted the impact of local heating on evaporation performance under the solar thermal gradient during the solar evaporation process.