a b s t r a c tThe purpose of reverse electrodialysis (RED) is to produce electricity upon the mixing of two solutions. We studied the power density (W/m 2 ) and the energy efficiency (the amount of energy produced from specified volumes of river and sea water in relation to the thermodynamic maximum). With a stack of 50 cells (of 10 cm × 10 cm), a power density of 0.93 W/m 2 was obtained with artificial river water (1 g NaCl/L) and artificial sea water (30 g NaCl/L), which is the highest practical value reported for RED. This value is achieved due to an optimized cell design using a systematic measurement protocol.The main factor in the power density is the cell resistance. With the used membranes (Fumasep FAD and FKD) and a spacer thickness of 200 m, a cell resistance of 0.345 is measured under RED conditions. This is about one and a half times the value as expected from the contribution of the individual components. This high value is probably caused by the shielding effect of the spacers. The largest contribution to this resistance (about 45%) is from the river water compartment.The hydrodynamic loss resulted in a maximal net power density of about 0.8 W/m 2 at a flow rate of 400 mL/min. At this optimum the consumed power for pumping is 25% of the total generated energy. The majority of the pump power is lost in the manifolds.Multistage experiments were performed at maximal power conditions (a current density of about −30 A/m 2 and at a flow rate of 300 mL/min). At these conditions the theoretical energy efficiency is maximal 50%. In practice however, the energy efficiency of a single stack is 9%. The effluent concentrations of the so operated stack are used for a second experiment and so on, simulating a multistage operation. With 3 stages a cumulative energy efficiency of 18% is achieved. A fourth stage did not increase this value. The power density of the 3 stages together was 50% of the power density of the first stage, indicating that energy efficiency and power density are counteracting.Further increase of power density and energy efficiency can be obtained with a better spacer and manifold design. A more open spacer is beneficial for RED in two ways: less shielding and lower pressure drop. Less shielding decreases the electrical resistance of the cell. A lower pressure drop permits the use of thinner water compartments, resulting again in a decreased electrical resistance of the cell and an improvement of the power density.

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Reverse electrodialysis (RED) is a method for directly extracting electrical energy from salinity gradients, especially from sea and river water. For the commercial implementation of RED, the electrode system is a key component. In this paper, novel electrode systems for RED were compared with existing systems on safety, health, environment, technical feasibility and economics. Systems with inert DSA-type electrodes and a NaCl-HCl supporting electrolyte with the reversible Fe 2? /Fe 3? redox couple or the [Fe(CN) 6 ] 4-/Fe(CN) 6 ] 3-couple achieved the highest ranking. Improvements of the electrode system are also discussed like the use of special stable metal electrodes, graphite electrodes, other reversible redox couples, capacitive electrodes and electrolytes with carbon particles.

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Electricity can be produced directly with reverse electrodialysis (RED) from the reversible mixing of two solutions of different salinity, for example, sea and river water. The literature published so far on RED was based on experiments with relatively small stacks with cell dimensions less than 10 × 10 cm(2). For the implementation of the RED technique, it is necessary to know the challenges associated with a larger system. In the present study we show the performance of a scaled-up RED stack, equipped with 50 cells, each measuring 25 × 75 cm(2). A single cell consists of an AEM (anion exchange membrane) and a CEM (cation exchange membrane) and therefore, the total active membrane area in the stack is 18.75 m(2). This is the largest dimension of a reverse electrodialysis stack published so far. By comparing the performance of this stack with a small stack (10 × 10 cm(2), 50 cells) it was found that the key performance parameter to maximal power density is the hydrodynamic design of the stack. The power densities of the different stacks depend on the residence time of the fluids in the stack. For the large stack this was negatively affected by the increased hydrodynamic losses due to the longer flow path. It was also found that the large stack generated more power when the sea and river water were flowing in co-current operation. Co-current flow has other advantages, the local pressure differences between sea and river water compartments are low, hence preventing leakage around the internal manifolds and through pinholes in the membranes. Low pressure differences also enable the use of very thin membranes (with low electrical resistance) as well as very open spacers (with low hydrodynamic losses) in the future. Moreover, we showed that the use of segmented electrodes increase the power output by 11%.