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Reverse electrodialysis (RED) is an emerging electro-membrane technology that generates electricity out of salinity differences between two solutions, a renewable source known as salinity gradient energy. Realizing full-scale RED would require more techno-economic and environmental assessments that consider full process design and operational decision space from the RED stack to the entire system. This work presents an optimization model formulated as a Generalized Disjunctive Programming (GDP) problem that incorporates a finite difference RED stack model from our research group to define the cost-optimal process design. The solution to the GDP problem provides the plant topology and the RED units´ working conditions that maximize the net present value of the RED process for given RED stack parameters and site-specific conditions. Our results show that, compared with simulation-based approaches, mathematical programming techniques are efficient and systematic to assist early-stage research and to extract optimal design and operation guidelines for large-scale RED implementation. This work was supported by the LIFE Programme of the European Union (LIFE19 ENV/ES/000143); the MCIN/AEI/10.13039/501100011033 and “European Union NextGenerationEU/PRTR” (PDC2021–120786-I00); and by the MCIN/AEI/10.13039/501100011033 and “ESF Investing in your future” (PRE2018–086454).

Developing technical alternatives to increase the volume of remediated waters is a promising way to alleviate pressure on natural water basins. However, the extra energy consumed in Wastewater Treatment Plants (WWTPs), mainly powered by fossil fuels, hampers this strategy. This work focuses on promoting efficient upgrading alternatives of treated waters by recovering energy within the treatment process. The approach consists on the recovery and integration of Salinity Gradient Energy (SGE) generated from the contact between SWRO brines and reclaimed wastewaters in reverse electrodialysis modules. The analysis of opportunity of implementing integrated processes in Spanish WWTPs near SWRO desalination plants is tackled. 16 SWRO-WWTP pairs have been inventoried, 10 of them located in Jucar and Segura river basins, hot spot areas for water reclamation. A gross power density of 0.46 W/m2 (71 Wh/m3 reclaimed water) of SGE has been generated in the contact between SWRO brines and reclaimed wastewater, increased up to 0.23 kWh/m3 in the most favourable scenario. Estimates of freshwater withdrawals savings up to 434,387 m3/day within the selected installations are obtained. Decrease in water abstraction and integration of renewable source of energy in the remediation process will contribute to water sources protection and water industry decarbonisation. This work was supported by the European LIFE programme [LIFE19 ENV/ES/000143] and the Spanish Ministry of Science and Innovation [PDC2021-120786-I00 and PLEC2021-007718].

Salinity gradients are a non-conventional source of renewable energy based on the recovery of the Gibbs free energy related to the mixing of solutions at different concentrations. Reverse Electrodialysis is a promising and innovative technology able to convert this energy directly into electric current. The worldwide availability of salinity gradients is limited to those locations where water bodies at different salinity levels are present. The present work analyses a number of different scenarios worldwide, in locations where salinity gradients are naturally available or generated by anthropogenic activities. A techno-economic model of the Reverse Electrodialysis process is presented. The model is used to evaluate the energy that can be harvested in each real scenario using a reverse electrodialysis plant and relevant results are reported in terms of power densities and energy yields. Finally, an economic analysis based on the estimation of the Levelized Cost Of Electricity (LCOE) for each scenario is presented, and perspective considerations are reported. Results suggest that competitive values of LCOE may be achieved in some scenarios.

The integration of membrane distillation with reverse electrodialysis has been investigated as a sustainable sanitation solution to provide clean water and electrical power from urine and waste heat. Reverse electrodialysis was integrated to provide the partial remixing of the concentrate (urine) and diluate (permeate) produced from the membrane distillation of urine. Broadly comparable power densities to those of a model salt solution (sodium chloride) were determined during evaluation of the individual and combined contribution of the various monovalent and multivalent inorganic and organic salt constituents in urine. Power densities were improved through raising feed-side temperature and increasing concentration in the concentrate, without observation of limiting behaviour imposed by non-ideal salt and water transport. A further unique contribution of this application is the limited volume of salt concentrate available, which demanded brine recycling to maximise energy recovery analogous to a battery, operating in a ‘state of charge’. During recycle, around 47% of the Gibbs free energy was recoverable with up to 80% of the energy extractable before the concentration difference between the two solutions was halfway towards equilibrium which implies that energy recovery can be optimised with limited effect on permeate quality. This study has provided the first successful demonstration of an integrated MD-RED system for energy recovery from a limited resource, and evidences that the recovered power is sufficient to operate a range of low current fluid pumping technologies that could help deliver off-grid sanitation and clean water recovery at single household scale., Graphical abstract Image 1, Highlights • Membrane distillation with reverse electrodialysis allows off-grid sanitation. • Heat and concentrate development with MD enhance RED power density. • The complex salt matrix within the feed does not limit power density. • Good energy recovery from small salt concentrate volumes with recycle.

The giant and sustainable salinity gradient energy broadly occurs when mixing solution sources with different concentrations can be potentially harvested through the intensively studied membrane-based indirect power generation technologies. This kind of technology commonly has two functional submodules, namely the osmosis submodule to induce driven forces and the electric submodule to produce electricity. However, almost all relevant reviews only concentrate on the traditional pressure retarded osmosis (PRO) technology without involving newly emerged ones such as the forward osmosis-electrokinetic (FO-EK) technology, leading to outdated and incomplete knowledge in this field. Therefore, this study is going to provide a comprehensive and up-to-date review of the membrane-based indirect power generation technologies through adequately outlining most related research. The authors not only provide a systematic overview of the theoretical background and the development of the state-of-the-art membrane-based indirect power generation technologies but also highlight their essential characterizations. Meanwhile, the challenges and the optimizing strategies in aspects of efficient semipermeable membranes, available fluid materials, and operation conditions, as well as future promising applications in different scenarios are also elaborated in detail. This work was supported by the National Natural Science Foundation of China [grant numbers 52106246, 52106046, 51976157]; the National Defense Pre-Research Foundation of China [grant number 50910020100]; and the Natural Science Foundation of Jiangsu Province [grant numbers BK20200687, BK20200680].

Reverse electrodialysis (RED) is a sustainable method to harvest energy using the salinity gradient between fresh and seawater. RED technology is developing but efficiencies are still limited when using natural feed water sources. One significant constraint is induced by the presence of multivalent ions in sea and river water (i.e. Mg2+, Ca2+, SO4 2-). Uphill transport and an increase in membrane resistance in the presence of magnesium ions significantly reduce the power density output obtainable. The choice of cation exchange membrane determines the magnesium transport and as such the power density. Here we investigate four cation exchange membrane types and relate their properties to the stack performance using three different magnesium concentrations on either river and/or seawater side: 1) a highly cross-linked styrene-divinyl benzene monovalent selective cation exchange membrane (Neosepta CMS); 2) a monovalent selective cation exchange membrane that contains a thin polyethyleneimine (PEI) anion exchange layer (Selemion CSO); 3) a multivalent ion (e.g. magnesium) permeable cation exchange membrane with an engineered molecularly open structure facilitating the transport of multivalent ions as recently developed (T1 Fujifilm); 4) a standard cation exchange membrane (Type I Fujifilm (reference)). The first two membranes both retain magnesium ions, while the other two membranes are considered permeable for magnesium ions. The results show that power density strongly depends on the composition of both river and seawater. Power density decreases in the presence of magnesium, an effect being strongest with magnesium at both river and seawater side, followed by the river water side and the seawater side. The negative effect of multivalent ion transport against the concentration gradient, so called uphill transport, in RED can be significantly minimized when monovalent selective membranes such as the highly cross-linked Neosepta CMS membrane or the AEM coated Selemion CSO membrane are used. However, the use of such membranes directly results in a strong increase in membrane resistance due to the lower ion mobility of magnesium ions inside these membranes. As a consequence, power densities in RED are not improved. Especially at high magnesium concentrations, this effect is very strong at higher concentrations, the membranes are no longer able to retain magnesium ions effectively. More beneficial is the application of multivalent permeable membranes with a more ‘open’ structure that allow the free movement of both sodium and magnesium ions through the membrane. Maybe somewhat counter intuitively, such membranes (especially the Fujifilm multivalent permeable T1 membrane) have low resistance values combined with reasonable OCV values leading to high power densities under almost all magnesium concentrations, especially at long term applications. Highest power densities well exceeding 0.3 W/m2 are still obtained when only sodium is present. However, when magnesium ions are present power densities in the order of 0.2–0.25 W/m2 can still be obtained for these membranes.

Highlights • Reverse electrodialysis stack design studied for large salinity gradients in recycle. • Energy efficiency of a Reverse electrodialysis and Electrodialysis stack compared. • Thicker electrodialysis membranes exhibited lower water permeance. • Membrane properties and intermembrane distance critical for high concentrations., In this study, stack design for high concentration gradient reverse electrodialysis operating in recycle is addressed. High concentration gradients introduce complex transport phenomena, which are exacerbated when recycling feeds; a strategy employed to improve system level energy efficiency. This unique challenge indicates that membrane properties and spacer thickness requirements may differ considerably from reverse electrodialysis for lower concentration gradients (e.g. seawater/river water), drawing closer parallels to electrodialysis stack design. Consequently, commercially available electrodialysis and reverse electrodialysis stack design was first compared for power generation from high concentration gradients. Higher gross power densities were identified for the reverse electrodialysis stack, due to the use of thinner membranes characterised by a higher permselectivity, which improved current. However, energy efficiency of the electrodialysis stack was twice that recorded for the reverse electrodialysis stack at low current densities, which was attributed to: (i) an increased residence time provided by the larger intermembrane distance, and (ii) reduced exergy losses of the electrodialysis membranes, which provided comparatively lower water permeance. Further in-depth investigation into membrane properties and spacer thickness identified that membranes characterised by an intermediate water permeability and ohmic resistance provided the highest power density and energy efficiency (Neosepta ACS/CMS), while wider intermembrane distances up to 0.3 mm improved energy efficiency. This study confirms that reverse electrodialysis stacks for high concentration gradients in recycle therefore demand design more comparable to electrodialysis stacks to drive energy efficiency, but when selecting membrane properties, the trade-off with permselectivity must also be considered to ensure economic viability.

Pressure retarded osmosis (PRO) has gained attention due to its use as a salinity gradient energy-generating membrane process. This process can convert difference in salinity between two streams into energy as it allows water transport through a semi-permeable membrane against the application of hydraulic pressure. This review provides a comprehensive look at the history and latest developments in preparation of membranes and modules for the PRO process, as well as the various applications of PRO. This review also explored the influence of feed characteristics and pretreatment strategies on water permeation and power generation during PRO operation. The current status and technological advancements of PRO as a process were reviewed, revealing how PRO can be operated as a stand-alone process or in integration with other hybrid processes. Despite the recent advancements in material and process development for PRO, membrane performance, wide-scale implementation, and commercialization efforts still leave much to be desired. Recognizing the current challenges facing the PRO technology, the advancements in PRO membrane and module development, and the various applications of the process, this review also draws out the future direction of PRO research and generation of osmotic salinity gradient energy as a viable energy source.

Unlike traditional fossil fuel plants, the wind and the sun provide power only when the renewable resource is available. To accommodate large scale use of renewable energy sources for efficient power production and utilization, energy storage systems are necessary. Here, we introduce a scalable energy storage system which operates by performing cycles during which energy generated from renewable resource is first used to produce highly concentrated brine and diluate, followed up mixing these two solutions in order to generate power. In this work, we present theoretical results of the attainable energy density as function of salt type and concentration. A linearized Nernst-Planck model is used to describe water, salt and charge transport. We validate our model with experiments over wide range of sodium chloride concentrations (0.025–3 m) and current densities (−49 to +33 A m −2 ). We find that depending on current density, charge and discharge steps have significantly different thermodynamic efficiency. In addition, we show that at optimal current densities, mechanisms of energy dissipation change with salt concentration. We find the highest thermodynamic efficiency at low concentrate concentrations. When using salt concentrations above 1 m, water and co-ion transport contribute to high energy dissipation due to irreversible mixing.

Whilst the efficiency of reverse electrodialysis (RED) for thermal-to-electrical conversion has been theoretically demonstrated for low-grade waste heat, the specific configuration and salinity required to manage power generation has been less well described. This study demonstrates that operating RED by recycling feed solutions provides the most suitable configuration for energy recovery from a fixed solution volume, providing a minimum unitary cost for energy production. For a fixed membrane area, recycling feeds achieves energy efficiency seven times higher than single pass (conventional operation), and with an improved power density. However, ionic transport, water flux and concentration polarisation introduce complex temporal effects when concentrated brines are recirculated, that are not ordinarily encountered in single pass systems. Regeneration of the concentration gradient at around 80% energy dissipation was deemed most economically pragmatic, due to the increased resistance to mass transport beyond this threshold. However, this leads to significant exergy destruction that could be improved by interventions to better control ionic build up in the dilute feed. Further improvements to energy efficiency were fostered through optimising current density for each brine concentration independently. Whilst energy efficiency was greatest at lower brine concentrations, the work produced from a fixed volume of feed solution was greatest at higher saline concentrations. Since the thermal-to-electrical conversion proposed is governed by volumetric heat utilisation (distillation to reset the concentration gradient), higher brine concentrations are therefore recommended to improve total system efficiency. Importantly, this study provides new evidence for the configuration and boundary conditions required to realise RED as a practical solution for application to sources of low-grade waste heat in industry., Highlights • Power dissipation using reverse electrodialysis in recycle is described. • Recycle is more economically and energetically efficient than single-pass. • Water flux, ionic transport and concentration polarisation impactful in recycle. • Control of ionic concentration in dilute feed can improve system efficiency. • Optimum current density is specific to chemical gradient.