Schematic of the solar evaporator technology in action and XRD pattern (far left) of the CuS nanoflowers, which are embedded in an SCM membrane (middle), before being floated on water (right) to generate vapor.Researchers have created a novel composite membrane that floats on top of the surface of water, absorbs sunlight, and produces steam [Tao et al., Materials Today Energy 9 (2018) 285]. The steam or hot water vapor produced can be collected to produce clean, distilled water from saline, polluted, or dirty sources.
Dwindling freshwater supplies and increasing demand are driving the development of simple technologies such as solar vapor generation that can extract drinkable water from unusable sources. This is particularly important in locations or situations where clean water is not readily accessible.
“The efficiency of traditional photothermal devices is limited because the absorbed solar energy is used to heat up the entire water body, so only a small fraction is converted into vapor and distilled,” explains Xiaobo Chen of the University of Missouri, who led the research effort with Yuliang Zhang and his colleagues from Shanghai Maritime University.
Instead, the new solar absorber material works more efficiently because it only heats up and evaporates the layer of water at the interface with the membrane. The absorber is a composite of copper chalcogenide CuS nanoflowers embedded in a semipermeable nitrocellulose collodion membrane (SCM). The CuS nanoflowers absorb sunlight via a localized surface plasmon resonance mechanism and convert the light to thermal energy, which vaporizes the water in the immediate vicinity. The collodion membrane, which is extracted from cotton, provides a flexible, floating support.
“The thermal energy converted by the CuS nanoflowers is localized to the water near the surface, which is effectively converted into vapor, while the water body below the surface is only slightly heated by heat diffusion between water molecules,” describes Chen.
The three-dimensional nanoflowers have multiple ‘petals’ – or very thin sheets of CuS – that both maximize the surface area available for solar absorption and help with binding to the collodion membrane. The robust membrane also has good wettability, which means that hot water is transported very effectively from the bottom to the top surface.
The CuS nanoflower/collodion membrane is cost effective, easy to fabricate, and environmentally friendly, point out the researchers. Even though this is the first iteration of the membrane for solar water evaporation, it shows superior performance to many existing materials.
“This material is readily applicable for solar vapor generation,” says Chen. “Large-scale fabrication seems straightforward using roll-to-roll printing.”
Since the CuS nanoflower/collodion membrane produces hot water vapor, it can also be used to provide hot water or drive motors to produce electricity. The researchers are now hoping to join forces with industrial partners to push the solar evaporator technology toward practical applications.