Water covers around 75% of the Earth’s surface. Freshwater, on the other hand, accounts for only about 2.7% of this vast natural resource. Oceans and seas contain almost 97% of the world’s water, which contains salts and minerals that cannot be consumed directly in everyday lives. This article will look at research conducted by Kunjaram, U. P. U et al. published in EcoMat.
Image Credit: VectorMine/Shutterstock.com
Freshwater shortages are one of humankind’s most important problems. As a result, desalination is the most common method for converting seawater to freshwater for drinking, irrigation, and daily use.
Solar-driven interfacial evaporation has recently attracted a lot of interest because of its potential in terms of water, energy, and ecological sustainability. This method uses solar thermal processes to evaporate water from brines or contaminated water, and it has the potential to achieve zero-liquid-discharge (ZLD) desalination.
To boost overall water production, researchers created better materials, enhanced thermal management strategies and procedures, and cooling technologies throughout the last decade.
When seawater is evaporated using an evaporator under the sun, a salinity gradient forms between the bulk water and the evaporator, enabling salts to diffuse and redistribute. Since solar absorbers are primarily black to allow maximum sunlight uptake, salt accumulation on them is generally regarded as unfavorable. The formation of salt on the absorber obstructs steady vapor generation and degrades the solar thermal efficiency over time.
Figure 1A depicts a typical architecture that uses a controlled water flow to guide salt movement through evaporative substances (e.g., so-called edge preferential salt crystallization).
Figure 1. The umbrella architecture for enhanced solar evaporation. (A) Schematic illustration of salt crystallization based on water delivery direction on a flat system. (B) Schematic illustration of the tapered architecture disclosed in reference. (C) Schematic illustration of the umbrella architecture. The water flow directions in these three architectures are indicated by the blue arrows. (D) Optical absorption spectrum of the black surface for water evaporation. The upper inset: the hydrophilic feature of the fabric materials used as the evaporation surfaces. The lower inset: SEM image of the fabric. Scale bar: 200 μm. (E) Photos and thermal images of the flat sample (upper panel) and the “umbrella” structure (lower panel) under one sun illumination. (F) Mass change of the three structures over 8 h using fresh water under one sun illumination. Image Credit: Kunjaram, et al., 2022
The tapered evaporation architecture described in the new paper is shown in Figure 1B. Due to the reduced solar energy density on the bigger surface, the surface temperatures of this architecture remain close to or lower than the ambient temperature under ordinary one-sun lighting. As a result, rather than heating the vapor, incoming solar energy was primarily designed to transform liquid water to gas-phase moisture.
Water is delivered from a central vertical channel to the upper surface, as shown in Figure 1C.
As a result, instead of the surface of the evaporative material in the tapered architecture depicted in Figure 1B, water inside the evaporative substance will flow toward the two dead endpoints, evaporate, and deposit salts at the surface of EPE foams.
Researchers created a flat (Figure 1A), a tapered edge to center triangle (Figure 1B), and an “umbrella” evaporation system employing carbon-coated fabric (CCF) as the evaporative interfaces to test this idea.
Results and Discussion
The flat structure and the suggested “umbrella” structures consisting of 1-layer and 12-layers in the two wings were exposed to saltwater to describe the solar-driven evaporation rates. The water’s saltiness was raised from 3.5 wt% to 7 wt% and 10 wt%.
As the salinity rises, the center clean area proportion decreases from 55.2% (Figure 2A) to 35.3% (Figure 2C).
Crystallized salts filled roughly 9% of the surface area in the 7 wt% salt water (as depicted in the second column in Figure 2B), which is significantly less than the salt occupancy rate of 57% on the flat surface (as shown in the first column in Figure 2B). Surprisingly, as seen in the third column of Figure 2B, the salt accumulation problem was eradicated in the 12-layer construction.
Due to no evident salt crystallization, the evaporation rate of the 12-layer umbrella structure is over 2.65 kg m−2 h−1, as represented by red spheres in Figure 2D.
Figure 2. Self-cleaning evaporative architecture. (A–C) Comparison of the three structures (i.e., the first column: Top-view of the flat structure, the second column: side-view of the 1-layer structure, and the third column: side-view of the 12-layer structure) after 10 hours of operation under one sun illumination with the salinity of (A) 3.5 wt%, (B) 7 wt%, and (C) 10 wt%, respectively. (D) Measured evaporation rates under 1 sun illumination. Insets: Architecture of the flat (the lower panel), 1-layer (the central panel), and 12-layer (the upper panel) evaporative structures. Image Credit: Kunjaram, et al., 2022
The water was transferred to the upper surface by the center fabric due to capillary force, as shown in Figure 3A. A large surface evaporation rate, QE, is always desirable for water evaporation purposes. To avoid salt accumulation, the basic goal is to increase water flow rates to reduce local salinity.
The umbrella architecture provided bigger surface areas for improved QE when compared to the flat structure shown in Figure 1A.
Figure 3. Water transfer capability. (A) Characterization of the salt accumulation on the umbrella structure. (B) Comparison of materials (ProCool Athletic Pique®, Texwipe 609®, CCF, and Highland® fabric) to determine maximum Qc. (C) Thermal images of the 1-layer evaporation surface (the left panel) and the dry surface (the right panel) under the same solar under four sun illumination. The color bar is tuned to better show the temperatures on the evaporators. (D) Measured ratio of Ls/m (red spheres) and m (blue spheres) as the function of the evaporative film thickness Ls. The blue dashed line is the fitting curve to show the trend of Qe with the increasing Ls. Image Credit: Kunjaram, et al., 2022
Researchers chose four samples attached with the lower ends immersed in fresh water and captured their thermal images to examine the water transportation height, as shown in Figure 3B. To adjust the surface temperature and evaporation rate of the umbrella structure, the incoming light strength was raised in Figure 3C.
It should be indicated that due to the Marangoni effect, the temperature difference could possibly produce directed water flow. It has the potential to help prevent salt accumulation, but more research is needed. The local surface evaporation, Qe(x), on the other hand, will lead to a salinity gradient, as seen in Figure 3A.
By adjusting Ls alone for given umbrella constructions in Figure 3D, researchers characterized the entire evaporation rate under one sun illumination.
The photographs in Figure 4A show the construction after a 10-hour operation under one solar illumination for four days. These salt crystals are firmly combined with the carbon powder on the fabric microstructure, as shown in the SEM image of the contaminated surface in Figure 4B, making mechanical removal challenging.
On this structured fabric, bigger salt crystals were seen, as shown in Figure 4C.
Figure 4. Experimental results for salinity of 20 wt%. (A) Measured evaporation rates of the four-day indoor experiment under one sun illumination. Inset: photos of the umbrella structure on each day. (B, C) SEM images of the accumulated salt on (B) the CCF (scale bar: 100 μm), and (C) the Highland fabric (scale bar: 200 μm), respectively. (D) A photo of the outdoor experiment on August 3rd, 2021, at the University at Buffalo. (E) Measured environmental conditions for the outdoor experiment (from top to bottom): the relative humidity (RH), the temperature, the solar intensity (SI), and the wind velocity. (F) Measured evaporation rates of the umbrella structure (red), the flat structure (blue), and the bulk water surface (black), respectively. (G) Photos of the salt accumulation after the 4-day operation on the umbrella structure (the upper panel), the flat structure (the central panel), and the bottom of the container (the lower panel). Image Credit: Kunjaram, et al., 2022
Lastly, to conduct the field test, researchers placed this umbrella structure, a flat structure, and a beaker containing 20 wt% saltwater in an outdoor setting in Buffalo, NY (Figure 4D).
Figure 4E depicts the weather conditions over four days, including ambient temperature, relative humidity, sun intensity, and wind speed. The evaporation rates of the three components were tested once an hour during the day to capture their weight losses. The greatest evaporation rate of the umbrella structure reaches 9.05 kg m−2 h−1 due to natural wind input, as shown in Figure 4F.
Surprisingly, salts only crystallized along the umbrella structure’s edges, as illustrated in Figure 4G’s upper panel.
This research highlights the ability of the suggested umbrella structures for enhanced salt mining applications formerly provided by solar and wind-driven evaporation procedures.
Finally, researchers described “umbrella” architecture for solar-driven interfacial evaporation that may generate cold vapor while preventing salt accumulation on the evaporative surface. The umbrella architecture has a substantially wider surface area than flat structures, which were frequently used in most solar evaporation studies, resulting in much reduced solar power densities on its wings.
It was discovered that maintaining a suitable amount of water transportability and salt resistance are important elements in reducing local salt saturation.
Surprisingly, most salts crystallized outside of the evaporative surfaces, making salt collection simpler. This type of low-cost “umbrella” architecture appears to have the potential to lay the groundwork for ZLD desalination and faster salt mining technologies.
Kunjaram, U. P. U., Song, H., Liu, Y., Booker, B. K., Cooke, T. J., Gan, Q. (2022) A self‐salt‐cleaning architecture in cold vapor generation system for hypersaline brines. EcoMat, 4(2), p. e12168. Available Online: https://onlinelibrary.wiley.com/doi/10.1002/eom2.12168.
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