Osmotic power generation by inner selective hollow fiber membranes: An investigation of thermodynamics, mass transfer, and module scale modelling

Abstract

A comprehensive analysis of fluid motion, mass transport, thermodynamics and power generation during pressure retarded osmotic (PRO) processes was conducted. This work aims to (1) elucidate the fundamental relationship among various membrane properties and operation parameters and (2) analyse their individual and combined impacts on PRO module performance. A state-of-the-art inner-selective thin-film composite (TFC) hollow fiber membrane was employed in the modelling. The analyses of mass transfer and Gibbs free energy of mixing indicate that the asymmetric nature of hollow fibers results in more significant external concentration polarization (ECP) in the lumen side of the inner-selective hollow fiber membranes. In addition, a trade-off relationship exists between the power density (PD) and the specific energy (SE). The PD vs. SE trade-off upper bound may provide a useful guidance whether the flowrates of the feed and draw solutions should be further optimized in order to (1) minimize the boundary thickness and (2) maximize the osmotic power generation. Two new terms, mass transfer efficiency and power harvesting efficiency 2 for osmotic power generation, have been proposed. This work may provide useful insights to design and operate PRO modules with enhanced performance so that the PRO process becomes more promising in real applications in the near future.

Ocean thermal energy conversion by deliberate seawater salinization

“…However, as well as controlling the mixing and desalination of the volume of water coming from the surface at the mass exchanger, forward osmosis FO can also be harnessed as an additional pushing pressure source term. This kind of energy is generally referred to as pressure retarded osmosis (PRO) . In the PRO process, the osmotic pressure difference across the semipermeable membrane (which increases the flow rate and dilutes the salinized water) can be converted into an increase in pressure by means of bifurcating the increased flow rate using a proper pressure exchanger (PEX) …”