Critical impact of permeate-to-feed ratio and feed flow rate fraction on performance of pressure-retarded osmosis process

In this study, we analyzed the influence of permeate-to-feed ratio (R = Q_p / Q_f) and feed flow rate fraction (ϕ = Q_f / (Q_d + Q_f)) on the net power of a pressure-retarded osmosis (PRO) process. A membrane power density (W_m) of 5 W/m² is known to be a target value for viable PRO osmotic power generation, but it is not a necessary and sufficient condition. According to our net power analysis, the PRO-based power generation was affected significantly by both the water permeation and the draw and feed flow rate in the PRO membrane module. The W_m and optimal hydraulic pressure difference (ΔP_opt) of the membrane module are the most important parameters to be improved for net power. Additionally, the feed-side pressure drop (P_{feed inlet}) and membrane area (A_m) of the module and the efficiency (η_ERD) of the energy recovery device (ERD) should be greatly improved for high positive net power. In addition to net power, the mechanical efficiency and specific energy of the PRO process were examined for the parameters mentioned above as well. As a result, our performance analysis shows that it is desirable to operate the PRO process at high R and high ϕ values (i.e., low Q_f and Q_d) to achieve reasonably high net power, mechanical efficiency, and specific energy.