Development of Thin Film Composite (TFC) Flat-Sheet and Hollow Fiber Pressure Retarded Osmosis (PRO) Membranes for Osmotic Power Generation

Development of thin film composite (TFC) flat-sheet and hollow fiber pressure retarded osmosis (PRO) membranes for osmotic power generation

Gang Han¹, Sui Zhang², Xue Li², Peng Wang¹, Tai-Shung Chung^1,2*

 

¹Department of Chemical and Biomolecular Engineering

National University of Singapore, Singapore 117576

²NUS Graduate School for Integrative Science and Engineering

National University of Singapore, Singapore 117576

*Corresponding author

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

Abstract

 

Worldwide energy crisis has become an inevitable issue due to the explosive increase in energy demand and the shrinking reserves of fossil fuels. The global trend toward environmental sustainability has shifted the future power production from conventional fuels and internal combustion engines to renewable green energy. Salinity-gradient energy (or osmotic power) from the mixing of solutions with different salinities represents a high potential source of renewable energy. Pressure retarded osmosis (PRO), pioneered by Loeb and co-workers in 1970s, has been demonstrated to be a promising membrane based technology to harvest osmotic energy. Statkraft, the Norwegian power company, conducted serious research on osmotic power and built the first prototype plant in 2009 by mixing river water and seawater across a semi-permeable membrane under the PRO mode. The latest estimation of osmotic power generated from the mixing of river water and seawater is approximately 1600 TWh per year. More energy can be generated when high salinity RO retentate is purposely mixed with recycled water. Not only can this osmotic energy recover lower the overall energy consumption for the RO process, but also solve the disposal problem of RO retentate.

However, the lack of effective membranes which are the hearts of the system is still hindering the development of PRO technology. Statkraft has specified that in order to be competitive to other new, renewable sources of energy, a power output of 5 W/m² for flat-sheet membranes is required, whereas due to the higher packing densities obtainable, a target in the range of 3 W/m² should be sufficient for hollow fiber membranes. Comparing to flat sheet membranes, membranes with a hollow fiber configuration are of great interest because of their high packing density and ease of module fabrication. Most importantly, hollow fiber modules may not require spacers between the membranes. Not only could this eliminate membrane deformation and structure parameter enhancement owing to spacer-membrane interactions under high-pressure PRO operations, but also eliminate extra energy consumption for water to transport through woven or non-woven spacers. Since most conventional forward osmosis (FO) membranes are originally designed for no- or low-pressure operation environments, they are not suitable for power generation applications due to their thin support layers and relatively weak mechanical strengths. Under high pressure, conventional FO membranes are likely to be damaged, and only generate low power densities. Therefore, technology breakthroughs on high flux and mechanically strong forward osmosis membranes must be developed specifically tailored for the PRO process.

The purpose of this presentation is to illustrate the science and engineering of fabricating flat sheet and/or hollow fiber thin film composite membranes with desirable robustness and power density for osmotic power generation. Now, one of 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 deionized water. In addition, our hollow fiber membranes can withstand trans-membrane pressures up to 15-20 bar and exhibit a peak power density as high as 14-24 W/m² using seawater brine (1.0 M NaCl) as the draw solution and deionized water as the feed. We believe that the newly developed TFC membranes have great potential to be applied in PRO processes for osmotic power harvesting.

Acknowledgments

This research was funded by the Singapore National Research Foundation and Environment and Water Industry Programme Office (EWI) under the projects entitled (1) “Membrane development for osmotic power generation, Part 1. Materials development and membrane fabrication” (1102-IRIS-11-01, R-279-000-381-279) and (2) “Advanced FO membranes and membrane systems for wastewater treatment, water reuse and seawater desalination” (R-279-000-336-281). Special thanks are due to Dr. Panu Sukitpaneenit and Miss Xiuzhu Fu for their valuable suggestions and kind help.