High Performance PRO Membrane Elements and the Future of PRO Development

Pressure retarded osmosis (PRO) is rapidly gaining interest from both academic research groups and the renewable power generation industry. According to ACS Scifinder, the number of publications involving the harvesting of energy from salinity gradients has increased four-fold in the last five years. Majority of these publications focus on power generation by harvesting the entropy of mixing that occurs when seawater and fresh water sources (i.e., rivers) combine. More than paper and bench-scale research studies, Statkraft, a Norwegian power utility company, recently announced plans to develop a 2 MW PRO power generation facility. Yet, PRO membranes with sufficient performance efficiency (often described by one metric: the watts of power produced per m² of membrane) to enable this industry are still under development. Generally, membranes that can achieve power density values in excess of 10 W/m² under conditions of ~30 g/L seawater and <500 mg/L TDS feed, both at ambient temperature are preferred to improve the economic viability of commercial scale PRO systems.

Many groups are actively working to develop high efficiency PRO membranes that are enabling for this emerging market. Several studies investigated small coupons of novel membrane materials that are specifically optimized for power production in PRO applications; however, majority of these studies calculate peak power density by testing the membrane in the absence of applied hydraulic pressure and extrapolating the power density curve based on membrane material parameters derived outside of PRO testing conditions. In our efforts to develop highly effective forward osmosis (FO) membrane and membrane elements for unconventional oil and gas produced water purification applications, we have also developed highly effective PRO membrane elements. These commercial scale membrane elements are able to achieve a measured peak power density of 8 W/m² under the conditions described above without any optimization of the membrane element for PRO applications. Exciting – yet, the methods employed to calculate power density are not standardized within this emerging industry and should be presented with greater scrutiny, transparency, and consistency. Here we will provide an industry perspective on commercial membrane elements for power generation with salinity gradients.