A more general mass transport model of pressure retarded osmosis has been developed and will be presented in this lecture. Essential of this model is that it does not have any simplifications and/or neglects for description of the mass transport process. It takes into account the effect of the external boundary layers on both sides of the membrane, the dense and the sponge layers of an asymmetric membrane used, applying the diffusive-convective mass transport equation for every sub-layer except of the skin/dense layer of the membrane. A widely applied, "diffusive" transport equation was used for the dense layer, for the salt transport through it. Accordingly this model enables the user to calculate the membrane performance under all possible operating conditions. Thus, it can be used to optimize the operating conditions in order to get efficient energy generation unit. The energy density obtained by means of the presented and the literature model have been compared in the paper and showed the process performance under different conditions.