Using solar energy to evaporate and purify water

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Methodology

The SDWEs in this study were fabricated using a nickel foam substrate and two key components – an upper interfacial polydopamine (PDA) nanosphere-assembled layer and a bottom thermo-responsive sporopollenin-engineered layer (PNm-g-SEC) – with the upper 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 PDA layer at high temperature (left), driven by the thermodynamic modulation induced by the water gating layer of PNm-g-SEC.

Figure 2: Bilayer-structured solar evaporator design
a Schematic of the fabrication procedure. b The porous structure of foam skeleton. c p-PDA nanospheres, SEM images of water transport gating layer d The skeleton decorated with PNm-g-SEC microparticles. e The interaction between PNm-g-SEC microparticles and PDA 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 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 SDWEs
a Mass change of water over time of the SDWEs under one sun solar illumination. b UV-vis-NIR diffuse reflection spectra of the PDA-coated NiF c Temperature variations in p-SDWE compared to the reference sample p-DF with z denoting distance from the top water surface. d Temperature gradient of 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 PDA 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’s dynamic water and thermal controlling system in enhancing water evaporation rates and ensuring stable, long-term operation (Figure 4).

Figure 4: Salt dissolving and backflow of p-SDWE
a Experimental setup, Temperature distribution traced by IR Camera and top view digital images of the evaporation surface: b1 and b2 Salt crystallization. c1 and c2 Salt dissolution after simulator light deactivation, d1 and d2 Salt dissolution after 20 min. e1 and e2 Salt dissolution after 30 min. f1 and f2 Salt dissolution after 40 min. g Operational regime varying the collection rate during one cycle of p-SDWE. h Long-term cycling performance using 10 wt% simulated seawater.

To further evaluate long-term water generation performance, the continuous solar evaporation performance of the SDWE was tested in a standard testing chamber for 6 h 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⁺ 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 Waterloo campus where comparable water evaporation and purification rates were achieved (Figure 5).

Figure 5: Solar evaporation performance in an outdoor environment
a Schematic of solar-driven water evaporation. b Evaporation rate and efficiency generated by SDWEs. c Mass change of p-SDWE as a function of time for the evaporation of DI water under one sun illumination. d Prototype solar water purification. e Amount of purified water during 12 h of outdoor solar desalination. f Illustration of condenser applied in SDWE system with omniphobic liquid-like coating. g Droplet movement during evaporation process observed on the condenser’s surface by microscope.

Conclusions

The study successfully fabricated 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. The SDWE 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.

Read more in Nature Communications

For more information about WaterResearch, contact Julie Grant.