Development of a new solar desalination technology to facilitate early-stage growth of developing nations: Direct desalination of salt water using a sunlight-driven ion pump

W. White, C.D. Sanborn, E. Schwartz, L.A. Renna, S. Ardo
University of California Irvine,
United States

Keywords: desalination, solar, ion pump, dialysis, ion-exchange membrane

Summary:

Affordable, reliable, and efficient desalination of salt water for human consumption and agriculture is a global grand challenge that requires advances in energy and water technologies. In a step toward meeting this challenge, the Ardo Group recently invented Integrated Solar Photo-Dialysis (ISPD), a sustainable process that uses solar energy to directly drive desalination. Central to ISPD is an innovative mechanism for light-to-ionic energy conversion that mimics the electrostatics of a solar cell so that the energy in sunlight is directly converted into ionic power. Direct conversion bypasses generation of intermediate electronic power, which requires energy-intensive and wasteful water electrolysis to generate ionic power. Therefore, ISPD only requires ~15% of the energy used in comparable technologies consisting of a solar cell wired to a single electrodialysis desalination cell, and theoretically ISPD can generate potable water from sea water more than ~20 times faster than solar thermal distillation. The Ardo Group’s enabling demonstration consisted of an anion-selective membrane laminated to a rationally designed cation-selective membrane that was covalently functionalized with custom pyrene-based photoacid dye molecules. This led to the development of several first-of-their-kind dye-functionalized polymer membranes that were each observed to be ionically photovoltaic, i.e. absorption of sunlight generated power by pumping ions against an ion concentration gradient. Two polymers under intense investigation by the Ardo Group include poly(ethylene terephthalate) and poly(phenylene oxide), two plastics produced on enormous scales, e.g. the U.S. produces ~1 billion plastic poly(ethylene terephthalate) water bottles each week. We expect that ISPD’s major impact will be in drip irrigation or as a tubular flow reactor to deionize agricultural runoff to levels of salinity suitable for farming. Approximately one billion people lack access to clean potable water, a number the United Nations predicts will nearly quadruple in the next decade, mostly due to population growth in developing nations. The United Nations states that clean potable water is vital for sustained, long-term economic growth. However, developing nations cannot afford the capital costs required to construct a state-of-the-art reverse-osmosis desalination plant (~1 billion USD) and they do not have the grid infrastructure to attain the large amount of electricity required for plant operation (~20 times the thermodynamic minimum energy requirement). Thus, for economic development a sustainable clean-water technology that does not require grid integration or complex coupling of solar cell modules to desalination cell stacks could be world-changing; ISPD is this technology.