Applying the solute fluxes given for every single transport layer, namely external boundary layers, selective-, and the support layer, and even the cake layer, new expressions were developed to define the overall mass transfer coefficient and the interface solute concentrations. These equations make possible much deeper investigation of the mass transport process and process efficiency in pressure retarded osmosis system. It was stated that the effect of the feed side boundary layer on the energy generation must not be neglected without checking its effect. The extractable energy is dominated mainly by the membrane selectivity, structural parameter and also by the solute concentrations. Essential improvement of the membrane selectivity and/or decrease of the value of the structural parameter is needed to get more efficient pressure retarded osmosis process for energy extraction. Furthermore the increase of the draw solution concentration and/or decrease of the feed concentration should be regarded as an alternative process instead of seawater-river water pair.