Reverse electrodialysis (RED) is a nonpolluting sustainable technology that converts the free energy of mixing of two solutions with different salinity directly into electrical energy. Although the theoretical potential is high, the practical power output obtained is limited yet due to concentration polarization phenomena and spacer shadow effects. In this work we combine theoretical calculations with direct current and alternating current experimental stack characterization methods to quantify the contribution of concentration polarization phenomena, spacer shadow effects and stack resistance in RED under different hydrodynamic conditions in a temperature range from 10 to 40 °C to show the practical potential of RED. Concentration polarization phenomena play an important role and their influence can be minimized by optimal stack hydrodynamics. Improved spacer designs and new spacer concepts offer extensive room to reduce the spacer shadow effect and to further increase the practical power output. Improvement of hydrodynamics and reduction of the spacer shadow effect directly result in a significant increase in power output of the RED process, and values almost double the values currently obtained can be realized, which brings RED close to economical viability.