Systematic analysis and optimization of power generation in pressure retarded osmosis: Effect of multistage design

Abstract

This work presents a systematic method for analysis and optimization of specific energy production (SEP) of pressure retarded osmosis (PRO) systems employing single-stage configuration as well as multistage design with interstage hydro-turbines. It is shown that the SEP normalized by the draw solution feed osmotic pressure increases with the number of stages as well as a dimensionless parameter c tot 5A tot L p p 0 =Q 0 . As compared to the single-stage PRO, the multistage arrangement not only increases flux and volume gain, but also allows a stage-dependent, progressively decreasing hydraulic pressure, both of which contribute to enhanced SEP and power density. At the thermodynamic limit where c tot goes to infinity, the theoretical maximum SEP by an N-stage PRO system is Nð12q 21=N totÞp 0 , where q tot is the ratio of the draw solution flow rate at the outlet to the inlet on the system level. For single-stage PRO, it is no more than p 0 . For infinite number of stages, the theoretical limit becomes ðln q tot Þp 0 . SEP under realistic conditions and practical constraints on multistage design are discussed.

Multistage pressure-retarded osmosis configurations: A unifying framework and thermodynamic analysis

“…Research into multistaging of PRO for improved performance is still in its infancy, with no clear multistage configurations identified for maximizing PRO performance. This is because researchers [13][14][15][16] have used different performance metrics and analyzed contrasting multistage configurations without sufficient theoretical justification or a common design philosophy. This makes it difficult to compare different multistage designs available in the literature in a coherent manner.…”

“…Hence, it is difficult to compare the disparate multistage configurations against one another. Different systems even have opposing design philosophies: interstage pressurization in [20] as opposed to interstage depressurization in [13,16], as well as parallel [14,16] vs. countercurrent [13,20] flow of feed and draw streams.…”

“…8a there is an unspecified pressure control device; this can be either a turbine or a pump to achieve pressurization or depressurization. This general pressure control device allows us to directly compare systems such as those in [20] with [13,16]. Pressure of the draw stream, after leaving stage 1, would be controlled such that the resulting net power density of the dual-stage system is increased.…”

Dynamic Operation of Batch Reverse Osmosis and Batch Pressure Retarded Osmosis

“…3 It is shown that multistage design with interstage turbines improves both power generation and power density (PD) on the basis of the same membrane area. 4 Recently, batch or semibatch operation of RO has caught attention because it can potentially reduce SEC by recycling the retentate stream and applying a time-dependent pressure. The early concept was originated by Efraty in late 2000s when he named it "closed-circuit desalination".…”

Optimization of module pressure retarded osmosis membrane for maximum energy extraction

“…Salinity gradient processes experiences dynamic changes in the full-scale PRO element due to the continuous dilution and concentration of the draw and feed solutions, respectively [3,9,23]. M. Li [12,13] provided a dimensionless design model to evaluate the impact of operating parameters on the performance of the PRO process. The study concluded that the concentration and dilution of feed and draw solutions, respectively, reduces the power density and the optimal ∆𝑃𝑃 shifts away from ∆𝜋𝜋/2 value.…”