ABSTRACT
Pressure-retarded osmosis (PRO) is a very promising technology to harvest renewable osmotic energy using a semipermeable membrane. However, in spite of having advantages over other traditional energy sources, harvesting osmotic energy from PRO processes is still in an infant stage because power density and water flux are severely reduced by many limiting factors, such as internal concentration polarization, external concentration polarization, reverse salt permeation, and membrane fouling. Therefore, it is of great importance to investigate the underlying mechanisms responsible for the significant reduction of water flux in order to overcome the limitations and to design innovative PRO membranes for real practices. This chapter focuses on the elucidation of various underlying mechanisms responsible for the flux reduction behaviors in PRO. Three cases are examined in the chapter: first, the flux reduction behaviors under a fixed bulk salinity gradient will be discussed. Second, the flux reduction behaviors under a growing bulk salinity gradient will be investigated, since in real operations, the bulk feed salinity is always increasing due to the water permeation to the draw solution side and the accumulated reverse salt flux along PRO modules. Third, the flux reduction behaviors due to scaling will be elaborated. An advanced nucleation theory will be employed to elucidate the dynamic scaling process. Finally, the flux reduction behaviors in PRO modules will be investigated. With an in-depth understanding of various underlying mechanisms responsible for the flux reduction behaviors, this study will surely provide useful insights to design more suitable PRO membranes and to operate them with enhanced water flux so that the PRO process can become more promising in the near future.
