Organizing Desalination Membrane Design Using Thermodynamic Cycles

David Rogers
Sandia National Laboratories

We present a multi-scale view of the materials design process suitable for optimizing the basic physics underlying energy conversion devices.
An inherently nonequilibrium formalism is developed to solve thermodynamic problems faced in reverse-osmosis desalination applications. Desalination is becoming an increasingly viable solution to the lack of potable water in many parts of the world, causing death, disease, and international tension. While the thermodynamic equilibrium cost of seawater desalination is fixed at RT=0.7 kW-h/m^3 / (Osm/L), carrying out the process at exactly the osmotic pressure is impossible due to back-flow from leaky membranes. Therefore, a theory capable of addressing the physical mechanisms responsible for irreversible entropy production has been developed to gain an understanding of energy loss in real devices. We show how nonequilibrium transport relations from local equilibrium theories connect to molecular simulations and are able to address the additional, Darcy's law fluid flow resistance. We present a novel, standardized flow resistance measure based on this theory and show biomimetic, self-assembled nanoporous materials recently developed at Sandia that achieve a three-fold decrease in excess energy consumption (compared to commercially available membranes). In this work, thermodynamic cycles emerge as an organizing principle for computing the properties of transient and steady-state systems using free energy sampling techniques well-known in equilibrium problems.

Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.

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