From left: the foam, cellulose wrapper, and graphene oxide.
We don’t often think about our access to clean water, but water scarcity is an everyday problem in many parts of the world. Technologies that can provide clean water cheaply have enormous potential, especially in light of our growing population and changing climate.
Solar steam generation and desalination represent a water treatment strategy that has the added benefit of a minimal carbon footprint. Unfortunately, solar steam generation is challenging and works most efficiently in very large systems. That’s partly because water is treated in bulk and a lot of energy is wasted heating water that isn’t converted to steam. Two other problems with such systems are the need for thermal insulation and the limited amounts of water that can be generated with smaller systems.
Recently, a team of scientists and engineers designed a solar desalination device with a constrained, effective two-dimensional water path that both provides an efficient water supply and minimizes heat loss.
The design of the device
The device itself has several layers that produce a two-dimensional water flow path. The main part of the device consists of low thermal conductivity polystyrene foam, which acts as a thermal insulator to reduce heat loss. The foam is wrapped in cellulose (a 50 µm film), a hydrophilic, bio-based film. A separate graphene oxide film acts as a solar absorber on the surface of the device.
The foam insulator can float on water, leaving only the underside of the cellulose wrap in direct contact with most of the water. Capillary forces pull water through the cellulose in a two-dimensional path from the bottom of the insulator, down the sides, and to the top. There it comes into contact with the sunlight absorber of graphene oxide. Developing this water flow path using capillary forces minimizes heat dissipation and energy consumption.
Graphene oxide was chosen 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 a 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 naturally bonds to cellulose, ensuring efficient water transport. In addition, the porosity of graphene oxide allows vapor to exit the surface. Finally, the flexible film is also foldable: it can be folded 50 times, which can be used for portability and large-scale deployment. The graphene therefore brings many useful properties.
Moreover, since the heat loss is kept to a minimum, the high efficiency of the solar desalination does not depend on the amount of water. It also does not require thermal isolators, which typically limits hardware scalability. The only thing missing is a way to collect the steam and condense it for use.
The simplicity of this system combined with the improved efficiency could mean that we are moving 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 with further cost analysis, we should be able to understand whether this device is a viable solution for developing countries where water scarcity is already making everyday life more difficult.
PNAS2016. DOI: 10.1073/pnas.1613031113 (About DOIs).