Maximizing Power Density Generation from Seawater Via Pressure Retarded Osmosis (Pro) Using Commercially Available Membranes

Abstract

Pressure retarded osmosis (PRO) is a green technology for harvesting Gibbs free energy from mixing solutions with different salinity gradients. Although lab-synthesized membranes showed high PRO performance, there is no available flat-sheet industrial-scale PRO membrane production. Most of the previous studies have focused on enhancing the power density of the PRO process by using a hypersaline draw solution that potentially causes severe internal concentration polarization (ICP) and limits the enhancement of power density. Using the most accessible resources – seawater and commercially available membranes – can be a more practical way to develop a large-scale PRO plant. However, there were very few studies using such resources that reached the threshold power density – 5 W/m2 – for  a PRO process to be commercially viable. In our study, we compared the PRO performance of some commercial FO and RO membranes. We observed that, at an elevated feed velocity and temperature, the RO membrane had a significant enhanced water flux and power density. Due to turbulent flow at a high feed velocity and low viscosity at a high feed temperature (30 °C), the RO membrane was able to perform at low ICP, hence maximum power density (6.3 W/m2) could be obtained at half the osmotic pressure (15 bar).

Keywords: Salinity gradient energy, pressure retarded osmosis, commercial membrane, power density, process optimization