This article describes the development of a numerical model that can consider variations in the concentration profile along the flow direction; not doing so can result in overestimation of the power generation capability. The developed model was used for studying the transport phenomena in a cell pair of a reverse electrodialysis system for system design and optimization. Furthermore, the effects of solution velocity, compartment width, and membrane resistance on the power generation capability were investigated. A net power output density of 4.4 W/m2 per cell pair was achieved for a membrane resistance per unit area of 3 Ω cm2. The applicability and accuracy of the suggested model are limited owing to the assumptions made for problem simplification. However, the proposed model is expected to be useful in the early stages of reverse electrodialysis system design.