Energy recovery from concentrated seawater brine by thin-film nanofiber composite pressure retarded osmosis membranes with high power density

Abstract

A significant amount of energy stored in the form of salinity in seawater reverse osmosis (SWRO) brine can be harvested by a pressure retarded osmosis (PRO) process for power generation. The crucial performance-determining factor of the PRO process is the semi-permeable PRO membrane, which separates the SWRO brine from a lower salinity solution and sustains the salinity difference between the two solutions. However, accumulation of solutes in the support membrane, namely internal concentration polarization (ICP), significantly reduces the effective salinity difference and thus severely limits the efficiency of conventional PRO membranes. In this paper, we report the fabrication and optimization of thin-film nanofiber composite PRO (TNC-PRO) membranes with a unique support membrane structure (inter-connected, low tortuousness and highly porous properties), aiming to facilitate mixing of accumulated solutes with a dilute feed stream and overcome the ICP problem. With such low structure parameter (S) value (150 μm) nanofiber support membranes (NSMs), the optimum water permeability (A) and solute permeability (B) of the TNC-PRO membrane for power generation were determined to be 4.1 L m⁻² h⁻¹ bar⁻¹ and 1.74 L m⁻² h⁻¹ respectively. This highly efficient TNC-PRO membrane can achieve a power density of 15.2 W m⁻² and maximum energy recovery of 0.86 kW h m⁻³, using synthetic brackish water (80 mM NaCl, π = 3.92 bar) and seawater brine (1.06 M NaCl, π = 51.8 bar) as feed and draw solution, respectively. For a more dilute synthetic river water (0.9 mM NaCl, π = 0.045 bar) feed solution, the same membrane can achieve a higher power density of 21.3 W m⁻². The main performance limiting factors in PRO application such as ICP, External Concentration Polarization (ECP) and Reverse Solute Permeation (RSP) are quantified and their values are related to A, B and S values of TNC-PRO membranes.