Contact
Yi Wang
Department of Chemical Engineering
Introduction
Freshwater scarcity is a concern in many parts of the world necessitating the development of effective water collection and purification technologies. Interfacial solar-driven evaporation has emerged as a sustainable method for producing clean water using solar energy. Recent advancements in solar evaporators include the development of materials with high photothermal efficiency and system improvements such as the rational design of water channels, effective water supply control and the implementation of multistage evaporation processes.
Scaling solar evaporators for seawater treatment has encountered challenges, primarily due to salt fouling. Current systems are also hindered by rigid structural designs and passive operational cycles, impeding their capacity for sustained, autonomous operation. This paper discusses the development and testing of a bilayer-structured solar evaporator (SDWE) with a dynamic water-thermal controlling system that autonomously shifts between efficient thin water evaporation and salt washing.
Methodology
The SDWEs听in this study were fabricated using a nickel foam substrate and two key components 鈥� an upper interfacial polydopamine (PDA) nanosphere-assembled layer and a bottom thermo-responsive sporopollenin-engineered layer (PNm-g-SEC) 鈥� with the upper听layer acting as a photothermal interface and the lower layer acting as a switchable gating layer. The bilayer structure was designed to synergistically enhance solar water generation by balancing trade-offs between the water transport rate and light-to-heat energy consumption during the evaporation process. Importantly, the switch-like mechanism added the ability to autonomously shift to bulk water self-washing when surface temperatures were influenced by pollutants or the natural day-night cycle (Figures 1 and 2).
Figure 1: Bilayer-structured solar evaporator principles
Water transport occurs along the larger pores within the nickel foam at low temperature (right), while thin water transport takes place along the intine听PDA听layer at high temperature (left), driven by the thermodynamic modulation induced by the water gating layer of听PNm-g-SEC.
Figure 2: Bilayer-structured solar evaporator design
a听Schematic of the fabrication procedure.听b听The porous structure of foam skeleton.听c听p-PDA听nanospheres, SEM images of water transport gating layer听d听The skeleton decorated with听PNm-g-SEC听microparticles.听e听The interaction between听PNm-g-SEC听microparticles and听PDA听layer.
Outcomes
Using confocal laser microscopy and micro-computed tomography (micro-CT), experiments were conducted with the fabricated SDWEs to test solar water evaporation rates and efficiency and to characterize particle morphology, surface topography, thermodynamics, surface wettability and liquid transport. Figure 3 shows that SDWE solar water evaporation was more efficient than pure water evaporation without听SDWEs under identical illumination conditions as the structure produced a steady supply of thin water layers that enhanced evaporation by minimizing latent heat at high temperatures.
Figure 3: Solar evaporation performance and thermal localization of听SDWEs
a听Mass change of water over time of the听SDWEs听under one sun solar illumination.听b听UV-vis-NIR diffuse reflection spectra of the听PDA-coated听NiF听c听Temperature variations in听p-SDWE听compared to the reference sample听p-DF听with听z听denoting distance from the top water surface.听d听Temperature gradient of听SDWEs characterized by infrared thermal images under one sun solar irradiation.
Microscale flow dynamics were used to investigate the temperature-responsive, switchable water transport and demonstrated the ability of SDWEs to regulate the continuous water supply rate essential for maintaining efficient vapor generation despite factors such as the topography or water affinity of the channel.
To assess water transport on a macroscale, real-world conditions were simulated by characterizing the supply of a thin water layer during vapor generation and the bulk water backflow during salt washing. Observations confirmed that the thin water layer was capable of being transported and continuously supplied along the intine听PDA听water transport microchannel under solar illumination. The autonomous salt washing mechanism was investigated by simulating operational conditions when light was obstructed by salt accumulation or during periods of darkness. Salt accumulation triggered a self-washing phase where the collection rate significantly decreased during the bulk water backflow process before resuming high-efficiency evaporation using solar energy throughout the day. These observations confirmed the performance of the SDWE鈥檚 dynamic water and thermal controlling system in听enhancing water evaporation rates and ensuring stable, long-term operation (Figure 4).
Figure 4: Salt dissolving and backflow of听p-SDWE
a听Experimental setup, Temperature distribution traced by IR Camera and top view digital images of the evaporation surface:听b1听and听b2听Salt crystallization.听c1听and听c2听Salt dissolution after simulator light deactivation,听d1听and听d2听Salt dissolution after 20鈥塵in.听e1听and听e2听Salt dissolution after 30鈥塵in.听f1听and听f2听Salt dissolution after 40鈥塵in.听g听Operational regime varying the collection rate during one cycle of听p-SDWE.听h听Long-term cycling performance using 10鈥墂t% simulated seawater.
听
To further evaluate long-term water generation performance, the continuous solar evaporation performance of the听SDWE听was tested in a standard testing chamber for 6鈥塰 under one sun irradiation. The evaporation rate remained constant, and the water generation rate increased and remained constant with similar deposition and drainage characteristics to short-term tests. Continuous solar desalination performance was evaluated using simulated seawater. Observations showed the average evaporation rate for seawater remained high and after a single cycle of self-adaptive salt washing, evaporation rates were restored to original levels. After the solar desalination, the salinities of different simulated seawater samples decreased to below WHO and EPA drinking water standards and concentrations of Na+, Mg2+, Ca2+ and K+听were reduced by orders of magnitude. To demonstrate the possibility of continuous water generation, an outdoor experiment using the homemade solar evaporator system was conducted at the University of 蓝莓视频 campus where comparable water evaporation and purification rates were achieved (Figure 5).
听
Figure 5: Solar evaporation performance in an outdoor environment
a听Schematic of solar-driven water evaporation.听b听Evaporation rate and efficiency generated by听SDWEs.听c听Mass change of听p-SDWE听as a function of time for the evaporation of DI water under one sun illumination.听d听Prototype solar water purification.听e听Amount of purified water during 12鈥塰 of outdoor solar desalination.听f听Illustration of condenser applied in听SDWE听system with omniphobic liquid-like coating.听g听Droplet movement during evaporation process observed on the condenser鈥檚 surface by microscope.
Conclusions
The study successfully fabricated听SDWEs with a bilayer structure that enabled continuous thin water supply and efficient thermal energy management. Taking advantage of the thermo-responsive layer, the water transport channel was switchable to allow for the passage of thin water within the inner microchannel. This water gating mechanism gave the evaporator high solar-vapor conversion performance due to the thin water requiring low latent heat during evaporation compared to bulk water. Additionally, the system initiated a self-cleaning process through bulk water convection when temperature drops due to salt accumulation, thus maintaining increased evaporation efficiency.听The听SDWE听also demonstrated its potential in solar-driven seawater desalination, contaminated water purification and heavy metal ions removal.
Study findings provide fundamental understanding to the water transport and phase transition at the evaporated interface, offering opportunities for the advancement of solar evaporator design. The study suggests that this dynamic water transport mechanism surpasses traditional day-night cycles, offering inherent thermal adaptability for continuous, high-efficiency evaporation and may offer a cost-effective solution in enhancing the performance of solar-driven water generation on a practical and large scale.
听
Wang, Y., Zhao, W., Lee, Y., Li, Y., Wang, Z., Tam, K. C. Thermo-adaptive interfacial solar evaporation enhanced by dynamic water gating. Nature Communications. July 2024.
For more information about听WaterResearch, contact Julie Grant.
