We don't often think about our access to clean water, but water scarcity is an everyday problem for many parts of the world. Technologies that can cheaply deliver clean water hold tremendous potential, especially in light of our growing population and changing climate.
Solar steam generation and desalination represent a water-purification strategy that has the added benefit of a minimal carbon footprint. Unfortunately, generating steam with solar power is challenging, and it works most efficiently in very large systems. That's in part because water's handled in bulk, and a lot of the energy is lost into heating water that isn't converted to steam. Two other concerns with such systems are the need for thermal insulation and the limited quantities of water that can be generated with smaller systems.
Recently, a team of scientists and engineers designed a solar desalination device with a constrained, effectively two-dimensional water path that provides both an efficient water supply and minimizes heat loss.
The device design
The device itself has several layers that produce a two-dimensional water-flow pathway. The bulk of the device is composed of a polystyrene foam with low thermal conductivity, which functions as a thermal insulator to reduce heat loss. The foam is wrapped in cellulose (a 50µm film), a hydrophilic, bio-based film. A separate graphene oxide film functions as the solar absorber at the surface of the device.
The foam insulator can float on top of water, allowing only the bottom side of the cellulose wrapper to be in direct contact with the bulk of the water. Capillary forces draw water through the cellulose in a two-dimensional pathway from the bottom of the insulator, around the sides, and to the top. There, it comes into contact with the graphene oxide sunlight absorber. Development of this water-flow path using capillary forces minimizes heat dissipation and energy use.
graphene oxide was selected as the absorber for several reasons. graphene oxide films function as efficient broadband light absorbers with low thermal conductivity. They can also be produced at low cost using a scalable process. In addition, chemical modification of the graphene oxide film allows it to be dispersed in water, making it well-suited for scalable spray coating or spin coating processes.
graphene oxide will naturally attach to cellulose, ensuring efficient water transport. Moreover, the porosity of graphene oxide also helps enable vapor to leave the surface. Finally, the flexible film is also foldable—it can be folded over 50 times, which might be used for portability and large-scale deployment. Thus, the graphene brings a lot of useful properties to the table.
Additionally, since the heat loss is minimized, the high efficiency of the solar desalination does not depend on the water quantity. Nor does it require thermal insulators, which typically limits the scalability of hardware. The one thing that's missing is a way to gather the steam and condense it for use.
The simplicity of this system coupled to the improved efficiency could mean we are getting closer to the development of personalized water desalination devices. Additional studies on how the material holds up over time could help us figure out if this is a practical solution. Coupled to further analysis of cost, we should be able to understand whether this device is a viable a solution for developing nations where water scarcity already makes daily living harder.
PNAS, 2016. DOI: 10.1073/pnas.1613031113 (About DOIs).
Source : https://arstechnica.com/science/2016/12/a-personalized-water-purification-system-that-floats/