Continuous Solar Vapour Generation and Salt Harvesting from Seawater

Fresh water on our planet is locked in the ocean as seawater, which has to be desalinated for domestic, industrial, and agricultural use. Solar vapour generation using photothermal materials to efficiently convert solar energy into heat for water evaporation is an emerging technology for seawater desalination. It is promising to mitigate the worldwide water scarcity problem in a green and sustainable way. However, developing high-efficient continuous solar evaporators for practical desalination applications still remains challenging due to the salt accumulation issue on the photothermal materials, the difficulties in scaling up the evaporators due to the limitation of water supply driven by capillary force, and the insufficient understanding of multiphase transport and salt harvesting mechanisms inside porous structures. To address these limitations, this work aims to develop high-efficient solar evaporators that can achieve continuous solar vapour generation and salt harvesting concurrently from seawater, with improved salt harvesting mechanisms understanding. For this, the thesis primarily focuses on the following four parts: i) light-trapping photothermal material development, ii) evaporator structure improvement for continuous concurrent solar vapour generation and salt harvesting, iii) salt harvesting mechanism exploration, and iv) scale-up of the evaporator for large area applications.
In this work, a light-trapping nanofiber photothermal coating was proposed by copolymerization pyrrole with dopamine, which can be rapidly synthesized at room temperature by ultrasonic spray coating. The highest solar absorptance reached 97.73%. This nanoscale coating significantly improved solar absorption at different incident angles across the full solar spectrum, achieving the highest solar evaporation rate of 1.385 kg•m-2•h-1 under 1 sun. Then an umbrella evaporator was developed, which achieved high efficiencies of continuous fresh water production and salt harvesting concurrently via double-sided evaporation. The evaporation process was simulated to reveal that better mass transfer condition of the umbrella evaporator by double-sided evaporation greatly improves the evaporation rate, and the influence of environmental conditions, surface height and opening angles on the evaporation performance were also analysed by this simulation. The salt nucleation, growth and falling mechanisms were also revealed. The salt mobility on the evaporation surface determines if the salt accumulates at the edge to fall, affected by water supply, salinity, and Mg2+ and Ca2+ contents. The salt on the edge grows from the front by wicking the water through its porous structures inside and falls due to the dissolution of its connecting parts. Salt creeping behaviours on a glass slide can indicate if the pre-treated seawater fits for salt harvesting. To address the water supply and distribution problems of the umbrella evaporator for large area application, a double-sided suspending evaporator with top water supply and surface water distribution systems was developed. Top central water supply gets away from the limitation of capillary force and allows larger area application. Its evaporation rate achieved 1.40 kg•m-2•h-1 with deionized water under 1 sun (95.7% energy efficiency), with a remarkable low surface average temperature (28.2 ºC). Then the salt distribution process on the surface was simulated to develop a novel floriform evaporation surface with a radial arterial water distributor to forcedly expose salts at the edge for harvesting and efficiently distribute the water on a larger evaporation surface.
This work shows great potential of the polypyrrole-dopamine nanofiber light-trapping coating as photothermal materials for continuous vapour generation, provides new ideas for the structure design of the evaporators that achieve solar vapour generation and salt harvesting concurrently, and allows us to better understand and control the salt harvesting process.