Heat-driven direct reverse osmosis for high-performance and robust ad hoc seawater desalination

Abstract

This work presents an approach to using a low-grade thermal energy source to power seawater desalination at high performance ratios and with minimal complexity - important requirements for a system that can generate potable water off the grid. In the proposed process - called heat-driven direct reverse osmosis (HDRO) - heat is transferred to a piston containing a saturated working fluid, which in turn directly applies pressure to a feed water reservoir connected in series with a standard reverse osmosis membrane assembly. Presented here are the underlying operating principles and thermodynamics for a generalized HDRO device, including criteria for selecting a working fluid and the definition of several key performance metrics. An idealized HDRO device is analyzed and a theoretical upper bound on performance is computed. At low recovery ratios, a lossless device using ethanol as a working fluid can achieve a performance ratio of up to 49, with a first-law efficiency 5.7%, indicating the significant potential for using this approach for small-scale, thermally driven desalination.