Modeling and simulation of the dual stage pressure retarded osmosis systems

Abstract

Utilization of renewable energy sources, as an approach to reduce greenhouse gas (GHG) emissions, have been globally popular in the last few decades. Among renewable energy sources, pressure retarded osmosis (PRO) has been scrutinized by scientists since the mid 70's. However, even today, the existing river-sea PRO systems can only marginally meet the generally approved criterion of 5 W/m 2 power density, a threshold for an economically feasible PRO system. As an approach to increase the performance of PRO systems, multi-staging of PRO modules are investigated. A mathematical model of the scaled up PRO process is proposed with consideration for internal and external concentration polarization, reverse salt flux, and spatial variations along the membrane. A thermodynamic model is also developed with consideration for entropy generation and losses in the process. It predicts the percentile of each work loss source compared to the net work in the system. Several configurations of dual stage PRO system are presented and compared to single stage PRO. The comparison is based on three proposed target functions of power density (PD), specific energy (SE), and work per drawn freshwater (W drawn). Applied hydraulic pressures and flow rates of draw and feed solutions are optimized for maximizing the target functions. The results indicate that overall performance of the system could be improved by up to 8 % with a dual stage PRO in the case of SE. The system iv performance is not improved by depressurizing the draw solution before the second module in cases of SE and W drawn. The thermodynamic analysis demonstrate the contribution of each work loss and justify the reason of diminishing the net work over the losses. The effect of membrane area and membrane characteristics on the SE target function is also investigated. The distribution of membrane area in each module depends on the selected configuration and inlet draw solution. In the dual stage systems, the SE value increases up to 14% by improving the membrane characteristics. Reducing the salt rejection coefficient (B) is the most effective membrane characteristic in our configurations. Replacing seawater with RO brine in draw solution results in a significant improvement in SE values.

A Review of CFD Modelling and Performance Metrics for Osmotic Membrane Processes

“…Nevertheless, if the river water-seawater pairing is replaced with wastewater-desalination brine, a 2-fold increase in the extractable specific energy could be expected because of the much higher osmotic gradient between two solutions. In another module-scale analysis carried out by Soltani and Struchtrup [ 121 ], it was found that a dual-stage system showed improvement in comparison to a single-stage system in terms of specific energy generation.…”

“…Table 3 summarises the literature on modelling studies for PRO systems. Although some module-scale analyses are available [ 120 , 121 ], detailed 3D CFD modelling for the PRO process has yet to be investigated. The insight of 3D modelling is important for scaling up the PRO processes; thus, further modelling work for PRO is recommended.…”

“…A study conducted by Soltani and Struchtrup [ 121 ] used these same definitions (for and SE), but they replaced the term with W net , which is the system output that considers both power consumption (to pressurise the stream in a pump) and generation (obtained from depressurising in the turbine).…”

Potential and analysis of an osmotic power plant in the Magdalena River using experimental field-data

“…A lot of effort is being made in that direction: Wan et al developed membranes with one of the highest power densities [19], Nagy et al modelled fouling mechanisms and the effects on PRO [20], Long et al gave indications about energetically optimal operation strategies [21]. Control strategies have also been proposed [22], as well as different process configurations [23], even integration with other processes, such as desalination [9], [17]. Taking into account these achievements and the expected solution to the current challenges, PRO is deemed to be a viable option [14] to exploit SGE at the location presented in this research.…”