Wood surface treatment techniques for enhanced solar steam generation

Water vapor is vital both as an energy carrier and as an intermediary state for removing impurities from water. In nature, transpiration occurs when water is transported (against gravity) from the roots to the underside of leaves where it evaporates. Using this process, one large tree can pump and purify 400 L of water each day. Based on trunk cross-sectional area, this corresponds to a water flux range of ∼100–1000 kg/m2day, but based on evaporation area it only corresponds to a rate of ∼0.1 kg/m2day. Compared to industrial mechanisms of producing water vapor (i.e. typical thermal-driven systems have a flux of ∼4000 kg/m2day), natural wood has a relatively low flux. In an effort to boost the flux of sustainable, natural wood, we investigated wood surface modifications, laser carbonization and deposition of gold nanolayers, which achieved an instantaneous evaporation rate of ∼4 kg/m2h—under 3 kW/m2 light intensity, exceeding all previous studies of synthetic materials (including 3.8 kg/m2h reported by Zhou et al. in a 2016 Nature Photonics article) for solar steam generation applications. The cost analysis of different natural and synthetic material-based techniques for solar steam generation indicated that the carbonization and laser treatments are very cost-effective and even the gold coating was comparable to previously reported synthetic materials. Based on these results, we suggest that natural, surface-modified poplar wood could represent a viable alternative to synthetic materials for liquid/vapor separation.