Solar desalination is a tantalizing cure-all to Earth's fresh-water woes. After all, this is where fresh water comes from naturally as part of the planet's water cycle—the Sun. Heat yields water vapor, water vapor yields water rain. Fresh-water rain.
Surely we can harness that.
Well, we do actually harness solar energy for desalination purposes through a variety of different schemes, at least one of which has been commercialized. Yet the method remains inefficient relative to other desalination methods, costing between $1.52 and $2.05 per cubic meter of water produced, according to the World Bank. To truly scale, solar desalination will have to be in line with other, dirtier (read: fossil fuel-dependent) desalination methods, which currently cost about half that.
Engineers from Georgia Institute of Technology and Nanjing University have developed a new solar desalination process based on self-assembling nanoparticle membranes. Crucially, the technology is based on low-cost, abundant materials (aluminum, mainly) that remain stable after many uses and can be fabricated with very low overhead. The group's work is described in this week's edition of Nature Photonics.
"Our plasmon-enhanced solar desalination device can significantly increase the energy transfer efficiency with not only enhanced light absorption, but also more localized heating," the researchers explain. "To enable efficient solar desalination, broadband and efficient light absorption is the critical first step."
A key term above is "plasmon-enhanced." This is a big thing in solar energy, generally. The idea is that as incoming light hits certain materials (metals, crystals), the result is a mass excitation of free electrons within those materials, which together form a big unified wave. This is a plasmon, which is a sort of quasiparticle. Plasmons are potentially useful for applications ranging from molecular sensors to on-chip information transmission in computing to solar energy.
Plasmonically speaking, certain metals have responded very enthusiastically to radiation in the solar spectrum. Aluminum is one of these. As the paper explains, aluminum has already been explored as a plasmonic material for sensing and photocatalytic applications, but across relatively limited bandwidths of light. To work for solar desalination, this range needs to be expanded.
That's what the authors were able to come up with: "a broadband and efficient aluminum-based plasmonic absorber by self-assembly of the aluminum nanoparticles (NPs) into a 3D porous membrane." The self-assembly comes as heat is applied to aluminium oxide sheets that have been engineered with nanoscale pores. Some aluminium ions within the material vaporize and then collect on the very top layer of the sheet. The result is a plasmonic structure enabling efficient solar absorption through several mechanisms.
The end result is a cheaply manufactured plasmonic aluminium membrane that can be laid across the surface of a body of water, efficiently converting incoming solar energy into heat that in turn converts liquid water into steam. The steam rises and cools, condensing into now-fresh water. Based on a variety of salt water samples, the method was able to achieve a decrease in salinity of four orders of magnitude.
"Different from most existing desalination strategies, our plasmon-enhanced solar desalination device is highly portable, and thus ideal for personal or miniaturized applications," the authors conclude. "These aluminium-based plasmonic structures—with low-cost materials and scalable fabrication processes—could therefore provide a portable solution for solar desalination with a minimal carbon footprint."