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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 combinetheoretical 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 degrees 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 spacerdesigns and newspacerconceptsofferextensive 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.

Combining two solutions of different composition releases the Gibbs free energy of mixing. By using engineered processes to control the mixing, chemical energy stored in salinity gradients can be harnessed for useful work. In this critical review, we present an overview of the current progress in salinity gradient power generation, discuss the prospects and challenges of the foremost technologies - pressure retarded osmosis (PRO), reverse electrodialysis (RED), and capacitive mixing (CapMix) and provide perspectives on the outlook of salinity gradient power generation. Momentous strides have been made in technical development of salinity gradient technologies and field demonstrations with natural and anthropogenic salinity gradients (for example, seawater-river water and desalination brine-wastewater, respectively), but fouling persists to be a pivotal operational challenge that can significantly ebb away cost-competitiveness. Natural hypersaline sources (e.g., hypersaline lakes and salt domes) can achieve greater concentration difference and, thus, offer opportunities to overcome some of the limitations inherent to seawater-river water. Technological advances needed to fully exploit the larger salinity gradients are identified. While seawater desalination brine is a seemingly attractive high salinity anthropogenic stream that is otherwise wasted, actual feasibility hinges on the appropriate pairing with a suitable low salinity stream. Engineered solutions are foulant-free and can be thermally regenerative for application in low-temperature heat utilization. Alternatively, PRO, RED, and CapMix can be coupled with their analog separation process (reverse osmosis, electrodialysis, and capacitive deionization, respectively) in salinity gradient flow batteries for energy storage in chemical potential of the engineered solutions. Rigorous techno-economic assessments can more clearly identify the prospects of low-grade heat conversion and large-scale energy storage. While research attention is squarely focused on efficiency and power improvements, efforts to mitigate fouling and lower membrane and electrode cost will be equally important to reduce levelized cost of salinity gradient energy production and, thus, boost PRO, RED, and CapMix power generation to be competitive with other renewable technologies. Cognizance of the recent key developments and technical progress on the different technological fronts can help steer the strategic advancement of salinity gradient as a sustainable energy source.

Reverse electrodialysis (RED) or blue energy is a non-polluting, sustainable technology for generating power from the mixing of solutions with different salinity, that is, seawater and river water. A concentrated salt solution (e.g., seawater) and a diluted salt solution (e.g., river water) are brought into contact through an alternating series of polymeric anion-exchange membranes (AEMs) and cation-exchange membranes (CEMs), which are either selective for anions or cations. Currently available ion-exchange membranes are not optimized for RED, whereas successful RED operation notably depends on the used ion-exchange membranes. We designed such ion-exchange membranes and for the first time we show the performance of tailor-made membranes in RED. More specifically, we focus on the development of AEMs because these are much more complex to prepare. Herein we propose a safe and more environmentally friendly method and use halogenated polyethers, such as polyepichlorohydrin (PECH) as the starting material. A tertiary diamine (1,4-diazabicyclo[2.2.2]octane, DABCO) was used to introduce the ion-exchange groups by amination and for simultaneous cross-linking of the polymer membrane. Area resistances of the series of membranes ranged from 0.82 to 2.05 Ω cm² and permselectivities from 87 to 90 %. For the first time we showed that tailor-made ion-exchange membranes can be applied in RED. Depending on the properties and especially membrane thickness, application of these membranes in RED resulted in a high power density of 1.27 W m⁻², which exceeds the power output obtained with the commercially available AMX membranes. This shows the potential of the design of ion-exchange membranes for a viable blue energy process.

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Salinity gradient energy is currently attracting growing attention among the scientific community as a renewable energy source. In particular, Reverse Electrodialysis (RED) is emerging as one of the most promising membrane-based technologies for renewable energy generation by mixing two solutions of different salinity. This work presents a critical review of the most significant achievements in RED, focusing on membrane development, stack design, fluid dynamics, process optimization, fouling and potential applications. Although RED technology is mainly investigated for energy generation from river water/seawater, the opportunities for the use of concentrated brine are considered as well, driven by benefits in terms of higher power density and mitigation of adverse environmental effects related to brine disposal. Interesting extensions of the applicability of RED for sustainable production of water and hydrogen when complemented by reverse osmosis, membrane distillation, bioelectrochemical systems and water electrolysis technologies are also discussed, along with the possibility to use it as an energy storage device. The main hurdles to market implementation, predominantly related to unavailability of high performance, stable and low-cost membrane materials, are outlined. A techno-economic analysis based on the available literature data is also performed and critical research directions to facilitate commercialization of RED are identified.

Renewable energy can be generated when mixing seawater and river water. This energy is captured in reverse electrodialysis (RED) using ion exchange membranes. Although natural sources of seawater and river water are composed of a mixture of monovalent and multivalent ions, laboratory research on RED is generally performed with artificial solutions of sodium chloride. This research demonstrates that the presence of magnesium-and sulphate ions in feed solutions with NaCl has a major effect on the obtained open circuit voltage and power density for three different membrane types. When using a mixture with a molar fraction of 10% MgSO 4 and 90% of NaCl in both feed waters, the experimentally obtained power density in steady state decreases from 29% to 50% compared to the case where the feed solutions contain only NaCl as a salt. This effect is among others explained by the transport of Mg 2+ and SO 4 2À against their concentration gradient, as is elaborated in a theoretical framework and which is justified by experimental data. Non-stationary cases, where feed water is switched from a NaCl solution to a mixture of NaCl and MgSO 4 , show that the voltage response time is in the order of tens of minutes up to several hours, due to ion exchange between the membranes and the feed water. The knowledge gained from electrochemical measurements under stationary and non-stationary conditions and a novel technique to monitor the ion transport inside cation exchange membranes can be used to improve the obtained power density in practical applications of RED using natural feed water. Broader context This paper describes the energy generation from waters that contain a mixture of monovalent and multivalent ions in different concentrations, which resembles the practical application of renewable energy generation from mixing natural seawater and river water, or brine and seawater or fresh water. Although the fraction of multivalent ions (such as Mg 2+ and SO 4 2À) is relatively small compared to that of monovalent ions (such as Na + and Cl À) under environmental conditions, their impact on the power production is signicant, as demonstrated in this paper. The theoretically available energy has been determined, which is relevant to all technologies that extract energy from salinity gradients, and the decrease in voltage and power density has been evaluated theoretically and experimentally for reverse electrodialysis. Furthermore, the remarkable large sensitivity of the power density to the fraction of multivalent ions has been unravelled from a (theoretical and experimental) analysis of the mass transport of monovalent and multivalent ions. These results imply new insights into the potential of salinity gradient power and strategies to maintain high power densities in reverse electrodialysis.