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Reverse electrodialysis (RED) is a renewable energy technology used to recover dissipated chemical energy in river estuaries globally. This technology has recently attracted significant attention owing to its great reliability and scalability. In this study, we propose the use of a spacer-less RED (i.e., a system in which a woven mesh is excluded from the flow channel). The performance of spacer-less RED, including its gross power density, internal resistance, and hydraulic loss, is compared with that of the spacer-filled RED, in relation to the variation in the inlet flow rate. The mixing enhancement is more important than the spacer shadow effect when considering power generation. The spacer-filled RED has uniform internal resistance over the whole range of flow rates, while the spacer-less RED shows a dramatic decrease in resistance with the increasing flow rate. The hydraulic loss is much lower in the spacer-less RED. The maximal net power, accordingly, is generated at the flow rate of 3 ml/min (for spacer-filled RED) and 12.5 ml/min (for spacer-less RED). In the end, a maximal net power density of 0.62 W/m2 was obtained in both structures.

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Closed-loop reverse electrodialysis (RED) systems that use a thermolytic solution for lowgrade waste heat recovery have attracted significant attention. They have several cost benefits, e.g., the absence of repetitive pretreatment and removal of locational constraints, when compared with open-loop RED systems using seawater and river water. This study presents a model of RED that uses ammonium bicarbonate, and this is a promising solution for closed-loop systems. The modified Planck-Henderson equation is used to calculate the ion exchange membrane potential. The calculation is based on the conductivity measurements as ionization carbonate electrochemical information has not been reported before this study. The solution resistance is experimentally determined. The experimentally obtained permselectivity is implemented into the model to predict the membrane potential more accurately. The results of the improved model are well matched with experimental results under results under various operating conditions of the RED system. In addition, in the model of this study, the net power density was characterized with the consideration of the pumping loss. The improved model predicts a maximum net power density of 0.84 W/m 2 with an intermembrane distance of 0.1 mm, a flow rate of 3 mL/min, and a concentration ratio of 200 as optimum conditions. The results of the study are expected to improve our understanding of the ammonium bicarbonate-RED system and contribute to modeling studies using ammonium bicarbonate or certain other compounds for novel technologies of waste heat recovery.

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