Molecular Design of Membranes for Osmotic Power Generation

Molecular design of membranes for osmotic power generation

Xue Li¹, Sui Zhang¹, Gang Han², Shipeng Sun²,

Panu Sukitpaneenit², Tai-Shung Chung^1,2*

 

¹NUS Graduate School for Integrative Science and Engineering

National University of Singapore, Singapore 117576

²Department of Chemical and Biomolecular Engineering

National University of Singapore, Singapore 117576

*Corresponding author

Tel: +65-65166645; fax: +65-67791936; Email: chencts@nus.edu.sg

Abstract

The concept of generating energy by mixing of seawater and fresh water was proposed 60 years ago. In the 1970s, Prof. Loeb pioneered studies on pressure retarded osmosis (PRO) across a semi-permeable membrane to exploit the mixing energy by depressurizing the permeate through a hydroturbine. In 2009, Statkraft of Norway built the first prototype plant by mixing river water and seawater under the PRO mode. However, the lack of effective membranes which are the hearts of the system is still hindering the development of osmotic power. Nowadays, researchers are focusing on the design of membranes for power generation under the PRO mode. The operation pressure of the seawater compartment may increase up to 13.5 bar if the membrane is mechanically strong. Since most forward osmosis (FO) membranes are originally designed for no- or low-pressure operation environments, they are not suitable for power generation due to their thin support layers and relatively weak mechanical strengths. Therefore, technology breakthroughs on high flux and mechanically strong FO membranes must be developed.

Conventionally, seawater was used as the draw solution in osmotic power generation. If RO retentate is to be used instead of seawater, the salinity gradient between RO retentate and river water is much greater than that between seawater and river water (about 7.9-8.5 vs. 3.5 wt%). The former can result in a much higher osmotic pressure (about 70-77 vs. 28 bar at 22.5 °C) and osmotic energy than the latter. Consequently, the operation pressure may become 30-38 bar. Furthermore, if osmotic power generation and RO plants can be successfully integrated, not only can it make seawater desalination less energy dependent and more sustainable, but also significantly alleviate the disposal and environmental issues of waste RO retentate. In addition, since the RO retentate has been well pre-treated in its previous processes, it can significantly reduce the membrane fouling in the high pressure compartment. As a result, the integration may save some of expensive pre-treatment costs originally required for seawater before PRO.

In our work, we have systematically investigated the effects of membrane characteristics on PRO performance as well as the performance limitations. The investigation provides perspectives on the design criteria of a desirable osmosis membrane by revealing the influence of dense and support layer properties. After one year of study, the membrane power density has improved from less than 0.1 W/m² up to our newly developed flat sheet asymmetric membranes producing more than 6 W/m². Today, our flat sheet membranes not only show an osmotic density of about 6-10 W/m² but also withstand up to 15 bar using model seawater (0.59 M NaCl) and DI water. The withstanding pressure is the highest ever reported in the literature.