• Energy Research

Reverse electrodialysis (RED) is an electro-membrane process to harvest renewable energy from salinity gradients. RED process models have been developed in the past, but they mostly assume that only NaCl is present in the feedwaters, which results in unrealistically high predictions. In the present work, an existing simple model is extended to accommodate the presence of magnesium ions and sulfate in the feedwaters, and potentially even more complex mixtures. All power loss mechanisms deriving from the presence of multivalent ions are included in the new model: increased membrane electrical resistance, uphill transport of multivalent ions from the river to the seawater compartment, and membrane permselectivity loss. This new model is validated with experimental and literature data of membrane electrical resistance (at 10 mol. % MgCl2 for the CEMs and 25 mol. % Na2SO4 for the AEMs), RED stack performance (up to 50 mol. % MgCl2 or Na2SO4 in the feedwaters), and ion transport (at 10 mol. % MgCl2 or Na2SO4 in the feedwaters) showing very good agreement between model predictions and experimental data. Finally, we showed that the developed model not only describes experimental data but can also predict RED performances under a variety of conditions and cross-flow configurations (single-stage with and without electrode segmentation, multi-stage in co-current and counter-current mode) and feedwater compositions (only NaCl, with Na2SO4, with MgCl2, and with MgSO4). It thus provides a very valuable tool to design and evaluate RED process systems.

A multistage reverse electrodialysis system was studied at the REDstack research facility (the Afsluitdijk, the Netherlands) for over 30 days to describe the performance of such configuration under natural water conditions. The experiments were done with two 0.22 × 0.22 m2 stacks in series comprising 32 cell pairs (3.1 m2 of membrane area) for stage 1 and 64 cell pairs (6.2 m2 membrane area) for stage 2. The total gross power density at the available salinity gradient was stable at around 0.35 W•m􀀀 2. The total net power density, corrected for the initial pressure drop of the stacks, was 0.25 W•m􀀀 2 at an energy efficiency of 37 %. Throughout the operation, due to increased stack pressure drop, the actual total net power density lowered to 0.1 W•m􀀀 2. A distinct behaviour was found for multivalent ions in each stage. For stage 1, Ca2+ and SO42􀀀 were transported from the river water to the seawater side, so-called uphill transport. For stage 2, uphill transport was not found, in line with Donnan potential calculations. Stack autopsy revealed microorganisms with sizes ten times larger than the cartridge filter nominal pore size (5 μm) and biofilm covering part of the spacer open area, both contributing to the increasing pressure drop in the stacks. This study showed that stable gross power densities and high energy efficiencies were obtained from feeding natural waters to a multistage reverse electrodialysis system, independent of fouling. In addition, it emphasized the importance of maintaining pumping power losses low for a viable deployment of the technology.

Reverse electrodialysis has been established as a promising method to harvest salinity gradient energy. To achieve market viability, an optimum process configuration is needed, in addition to material and stack development, to increase energy efficiency without compromising power density. Multistage reverse electrodialysis is a practical strategy providing several degrees of freedom, such as independent electrical control of the stages, asymmetric staging, and different configurations. This study tests a two-stage configuration experimentally, using seawater and river water (NaCl only), at several residence times and changing the electrical control. Furthermore, the results are compared with a numerical model that is subsequently used to predict the behavior of alternative multistage configurations. The results show that multistage reverse electrodialysis yields higher gross power density and energy efficiency than a single-stage configuration fed with the same salinity gradient. A new strategy named “saving the gradient” (i.e., lowering the discharge current in the first stage) increased the gross overall performance of the two stages up to 17% relative to single-stage and up to 6% relative to a sequentially optimized two-stage system. Modeling different configurations revealed that only two stages are needed when feeding seawater and river water. When retrieving 40% net energy efficiency, the net power density for a single stage is 0.86 W∙m−2 and 0.94 W∙m−2 for a two-stage system, representing an improvement of 9%. Multistage reverse electrodialysis is therefore a viable concept to enhance power and energy efficiency, and benefits from optimization through electrical control.

Reverse electrodialysis harvests energy from salinity gradients establishing a renewable energy source. High energy efficiencies are fundamental to up-scale the process and to minimize feedwater pre-treatment and pumping costs. The present work investigates electrode segmentation to strategically optimize the output power density and energy efficiency. Electrode segmentation allows the current density to be tuned per electrode segment. Segmentation experiments were performed with a dedicated electrode configuration in a cross-flow stack using a wide range of residence times. Moreover, an experimentally validated model was extended and used to further compare single and segmented electrode configurations. While operating the electrode segments, the highest efficiencies were obtained when considering the overall power, i.e. not maximized by segment. Results show that at a given net power density (0.92 W·m−2), electrode segmentation increases the net energy efficiency from 17% to 25%, which is a relative increase of 43%. Plus, at 40% net energy efficiency the net power output for a segmented electrode configuration (0.67 W·m−2) is 39% higher than in a single electrode configuration. Higher power density reduces capital investment and higher energy efficiency reduces operating costs. Electrode segmentation increases these parameters compared to a single electrode and can be potentially applied for up-scaling.