Enhanced energy generation and membrane performance by two-stage pressure retarded osmosis (PRO)

A systematic model of a salinity power plant involving two PRO (TwoPRO) modules is presented in this paper. The results clearly indicate that the performance of a PRO plant can be improved with a two-stage configuration. For comparison purposes, a single-stage PRO (SinglePRO) is first analyzed and optimized with respect to three key concepts, power density, extractable energy and average power density. The two-stage configuration of the TwoPRO is then studied using two schemes: continuous scheme in which the stream is treated as a continuous flow between the two stages and divided scheme in which the flow is divided first and separately treated in each stage. Accordingly, four different configurations are proposed, namely continuous draw and continuous feed solution (CDCF), divided draw and divided feed solution (DDDF), continuous draw and divided feed solution (CDDF), and divided draw and continuous feed solution (DDCF). The results indicate that the configurations CDCF, CDDF and DDCF have advantageous capacity in salinity energy harvesting. Configurations CDCF and CDDF also perform better than the single-stage PRO in terms of average power density with a higher feed solution flow ratio. Additionally, the membrane performance of the TwoPRO strengthens with higher level of membrane usage.