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Salinity gradient energy has gained attention in recent years as a renewable energy source, especially employing reverse electrodialysis technology (RED), which is based on the role of ion exchange membranes. In this context, many efforts have been developed by researchers from all over the world to advance the knowledge of this green source of energy. However, the influence of divalent ions on the performance of the technology has not been deeply studied. Basically, divalent ions are responsible for an increased membrane resistance and, therefore, for a decrease in voltage. This work focuses on the estimation of the resistance of the RED membrane working with water flows containing divalent ions, both theoretically by combining the one-thread model with the Donnan exclusion theory for the gel phase, as well as the experimental evaluation with Fumatech membranes FAS-50, FKS-50, FAS-PET-75, and FKS-PET-75. Furthermore, simulated results have been compared to data recently reported with different membranes. Besides, the influence of membrane resistance on the overall performance of reverse electrodialysis technology is evaluated to understand the impact of divalent ions in energy generation. Results reflect a minor effect of sulfate on the gross power in comparison to the effect of calcium and magnesium ions. Thus, this work takes a step forward in the knowledge of reverse electrodialysis technology and the extraction of salinity gradient energy by advancing the influence of divalent ions on energy recovery.

Divalent ions have a negative effect on the obtained power and efficiency of the reverse electrodialysis (RED) process when using natural waters. These effects can largely be attributed to the interaction between the various ions and the membranes, resulting in a decreased membrane voltage, an increased membrane resistance, and uphill transport of divalent ions. The aim of this study was to investigate the causes of these differences and, if possible, to find underlying causes. The approach mainly followed that in literature articles that specifically focused on the effect of divalent ions on RED. It transpired that seven publications were useful because the methodology was well described and sufficient data was published. I found two widely shared misconceptions. The first concerns the role of the stack voltage in uphill transport of divalent ions; itis often thought that the open circuit voltage (OCV) must be taken into account, but it is plausible that the voltage under working conditions is the critical factor. The second debatable point concerns the methodology used to make a series of solutions to study the effect of divalent ions. Typically, solutions with a constant number of moles of salt are used; however, it is better to make a series with a constant ratio of equivalents of those salts. Moreover, it is plausible that the decreased voltage can be explained by the inherently lower Donnan potential of multi-charged ions and that increased resistance is caused by the fact that divalent ions-with a lower mobility there than the monovalent ions-occupy relatively much of the available space in the gel phase of the membrane. While both resistance and voltage play a decisive role in RED and probably also in other membrane processes like electrodialysis (ED), it is remarkable that there are so few publications that focus on measurements on individual membranes. The implications of these results is that research on the effect of divalent ions in RED, ED and similar processes needs to be more structured in the future. Relatively simple procedures can be developed for the determination of membrane resistance in solutions of mixtures of mono- and divalent salts. The same applies to determining the membrane potential. The challenge is to arrive at a standard method for equipment, methodology, and the composition of the test solutions.

Reverse electrodialysis (RED) power generation using seawater (SW) and river water is expected to be a promising environmentally friendly power generation system. Experiments with large RED stacks are needed for the practical application of RED power generation, but only a few experimental results exist because of the need for large facilities and a large area of ion-exchange membranes (IEMs). In this study, to predict the power output of a large RED stack, the power generation performances of a lab-scale RED stack (40 membrane pairs and 7040 cm 2 total effective membrane area) with several IEMs were evaluated. The results were converted to the power output of a pilot-scale RED stack (299 membrane pairs and 179.4 m 2 total effective membrane area) via the reference IEMs. The use of low-area-resistance IEMs resulted in lower internal resistance and higher power density. The power density was 2.3 times higher than that of the reference IEMs when natural SW was used. The net power output was expected to be approximately 230 W with a pilot-scale RED stack using low-area-resistance IEMs and natural SW. This value is one of the indicators of the output of a large RED stack and is a target to be exceeded with further improvements in the RED system.

A membrane-based technique for production of pressure-retarded osmosis (PRO) is salinity gradient energy. This sustainable energy is formed by combining salt and fresh waters. The membrane of the PRO process has a significant effect on controlling the salinity gradient energy or osmotic energy generation. Membrane fouling and operating conditions such as temperature have an extreme influence on the efficiency of the PRO processes because of their roles in salt and water transportation through the PRO membranes. In this study, the temperature impact on the power density and the fouling of two industrial semi-permeable membranes in the PRO system was investigated using river and synthetic sea water. Based on the findings, the power densities were 17.1 and 14.2 W/m 2 at 5 °C for flat sheet and hollow fiber membranes, respectively. This is the first time that research indicates that power density at low temperature is feasible for generating electricity using PRO processes. These results can be promising for regions with high PRO potential that experience low temperatures most of the year.

Reverse electrodialysis (RED) is one of the techniques able to harvest energy from the salinity gradient between different salt solutions. There is a tradeoff between efficiency and generated power in a RED stack. This paper focuses on efficiency. A simple model is presented to calculate the efficiency in a co-flow or counterflow operated stack. Moreover, the efficiency can be improved by applying multistaging; the stacks in such a system can also be interconnected externally in co- and counterflow. The four combinations of internally and externally flow modes are the base of further considerations concerning procedures for optimization of these configurations. Three methods for optimization the energy efficiency in a multistage system are discussed: (A) successively maximizing the power of each individual stage, (B) maximizing the power of the whole system by adjusting the electrical current in all stages simultaneously, and (C) maximizing the power of the whole system by adjusting the same current through each stage. Method C is the most attractive because it only requires one converter (cheaper and easier to control) while the results are hardly inferior to B and much better than A. An alternative to multistaging is electrode segmentation and the advantages and disadvantages of both systems are briefly discussed.