Harvest Salinity Gradient Energy Research Articles

Enhanced Salinity Gradient Energy Conversion by Photodegradable MXene/TiO2 Membrane Utilizing Saline Dye Wastewater

AbstractThe utilization of nanochannel membranes for harvesting salinity gradient energy has shown promising potential in addressing the energy crisis and environmental pollution issues. While previous studies have mainly focused on extracting salinity energy from mixing artificial seawater with river water, the research introduces a creative approach involving dye wastewater treatment using MXene/TiO2 membranes to achieve simultaneous photocatalytic degradation and power generation via salinity gradients. The built‐in electric field generated by MXene/TiO2 heterojunctions enhances ionic transport and electron‐hole separation. After photocatalytic treatment of saline dye wastewater, the membrane is defined as a photodegradable membrane. This membrane exhibits increased surface charge and expanded layer spacing to optimize salinity gradient energy conversion efficiency, yielding an impressive power density of up to 11.78 W m−2, which is 20% higher than that of the original MXene/TiO2 membrane. This work offers valuable insights into the development of multifunctional materials for sustainable power generation that comprehensively utilize industrial wastewater and domestic sewage.

In Situ Growth of MOF-303 Membranes onto Porous Anodic Aluminum Oxide Substrates for Harvesting Salinity-Gradient Energy.

As an emerging metal-organic framework (MOF) material in recent years, the MOF-303 membrane has shown great potential applications in seawater desalination, dehydration, and atmospheric water harvesting. Herein, we report on a dense and uniform MOF-303 membrane fabricated by a facile in situ hydrothermal synthesis approach in the presence of an anodized aluminum oxide (AAO) channel membrane acting as the only Al source and substrate. Interestingly, the MOF-303 isomer can be obtained due to an insufficient amount of organic ligand caused by the less hydrophilic and larger pore size of the AAO substrate. The MOF-based composite membranes possessed surface-charge-governed ionic transport behavior. Moreover, the MOF-303/AAO membrane yielded an output power density of 1.87 W/m2 under a 50-fold KCl concentration gradient. Under a 50-fold gradient of artificial seawater and river water, a maximum power density of 1.46 W/m2 can be obtained. After 30 days of stability testing, the composite membrane still maintained the power output, and the power density was higher than 1.20 W/m2. This work provides a facile and effective strategy for constructing Al-based MOF composite membranes and boosts their applications in harvesting salinity-gradient energy.

A novel spiral wound module design for harvesting salinity gradient energy using pressure retarded osmosis

Pressure retarded osmosis (PRO) is an evolving form of a renewable energy process which utilizes the salinity gradient energy from two solutions of different concentrations. One of the main problems limiting the application of PRO is the low performance of the commercially available spiral-wound modules which have poor flow distribution and high pressure drop. The present paper proposes a new spiral-wound module design for PRO application. The performance of the new module design was investigated numerically and compared with the available spiral-wound module. Compared to the available module, the power density of the new design was higher by 25% and 15% at draw concentrations of 35 g/kg and 60 g/kg respectively, while there was a 35% decrease in the pressure drop. The new spiral-wound module design presents an enhanced overall performance due to the better flow distribution and lower pressure drop, yielding a higher water flux and power density.

Surfactant-Assisted Sulfonated Covalent Organic Nanosheets: Extrinsic Charge for Improved Ion Transport and Salinity-Gradient Energy Harvesting.

Charge-governed ion transport is the vital property of nanofluidic channels for salinity-gradient energy harvesting and other electrochemical energy conversion technologies. 2D nanofluidic channels constructed by nanosheets exhibit great superiority in ion selectivity, but a high ion transport rate remains challenging due to the insufficiency of intrinsic surface charge density in nanoconfinement. Herein, extrinsic surface charge into nanofluidic channels composed of surfactant-assisted sulfonated covalent organic nanosheets (SCONs), which enable tunable ion transport behaviors, is demonstrated. The polar moiety of surfactant is embedded in SCONs to adjust in-plane surface charges, and the aggregation of nonpolar moiety results in the sol-to-gel transformation of SCON solution for membrane fabrication. The combination endows SCON/surfactant membranes with considerable water-resistance, and the designable extrinsic charges promise fast ion transport and high ion selectivity. Additionally, the SCON/surfactant membrane, serving as a power generator, exhibits huge potential in harvesting salinity-gradient energy where corresponding output power density can reach up to 9.08W m-2 under a 50-fold salinity gradient (0.5m NaCl|0.01m NaCl). The approach to extrinsic surface charge provides new and promising insight into regulating ion transport behaviors.

Experimental study on the effects of salt solution pH on the performance of reverse electrodialysis stack

Reverse electrodialysis (RED) is a promising way of harvesting salinity gradient energy (SGE). The seawater or industrial wastewater may have various pHs. Here the RED performance involving sodium salt solutions with different ion valence ratios including anions of Cl-, SO42-, and PO43- is experimentally investigated in symmetric and asymmetric pH configurations. In the symmetrical pH configuration, increasing the solution pH significantly weakens the energy conversion performance for the 1:1 and 1:3 salts; for the 1:2 salt, the power density and energy conversion efficiency increase and then decrease with increasing pH due to the coupling effects of OH- on the ion transportation through AEMs and CEMs. In the asymmetric pH configuration, increasing the pH of the low concentration solution decreases the power density and energy conversion efficiency. As the pH of the high concentration solution increases, the output power and energy conversion efficiency decrease and then increase for the 1:1 salt due to the coupling effects of the ion transmembrane concentration difference and OH- on the ion migration of IEMs; for the 1:2 salt, the output power and energy conversion efficiency increase and then decrease; for the 1:3 salt, OH- inhibits the hydrolysis of Na3PO4 and anion migration, leading to the lowered output power density and energy efficiency.

Industrial scale thin-film composite membrane modules for salinity-gradient energy harvesting through pressure retarded osmosis

The current work highlights development of novel thin-film composite hollow fiber membranes for pressure retarded osmosis applications to harvest salinity gradient energy. The membranes were developed with a specific target of harnessing the salinity gradient energy between wastewater reverse osmosis retentate and seawater reverse osmosis brine via osmotic mixing. The hollow fiber membranes were prepared by coating a polyethersulfone substrate with a thin-film composite polyamide layer via interfacial polymerization, which were assembled into modules of different diameters for lab scale and pilot scale evaluation. As the module size increased from 1-in. to 2-in., and 4-in., the water permeability, tested against a 1000 mg/L sodium chloride solution at 15 bar, decreased from 2.6 L m−2 h−1 bar−1 to 2.0 L m−2 h−1 bar−1, and 1.2 L m−2 h−1 bar−1, respectively. The power density, measured in the lab-scale unit using 1 M sodium chloride draw solution, and DI water feed solution, also decreased from 9.1 W/m2 to 5.3 W/m2 with increasing module size. Pilot scale evaluation of the 4-and 8-in. modules on a 24 m3/day unit resulted in lower power densities of 2.5 W/m2 and 1.5 W/m2 which translated to 0.024 kWh/m3 and 0.05 kWh/m3 of salinity-gradient energy harvested, respectively.

Principles and Materials of Mixing Entropy Battery and Capacitor for Future Harvesting Salinity Gradient Energy

Principles and Materials of Mixing Entropy Battery and Capacitor for Future Harvesting Salinity Gradient Energy

Comparison of Physicochemical Properties of Two Types of Polyepichlorohydrin-Based Anion Exchange Membranes for Reverse Electrodialysis

The development of the most effective, suitable and economic ion-exchange membranes is crucial for reverse electrodialysis (RED)—the most widely studied process to harvest salinity gradient energy from mixing seawater and river water. RED utilizes two types of membranes as core elements, namely cation exchange membranes (CEM) and anion exchange membranes (AEM). Since the preparation of AEMs is more complex compared to CEMs, the design and development of anion exchange membranes have been the focus in this study. Homogeneous AEMs based on two types of polyepichlorohydrin (PECH) with different chlorine amounts (PECH-H, 37 wt% and PECH-C, 25 wt%) were synthesized, and first-time benchmarking of the membrane properties was conducted. In addition to physicochemical membrane properties, some instrumental analyses such as SEM, FTIR and DSC were investigated to characterize these anion-exchange membranes. Based on the results, although the PECH-H-type membrane had enhanced ion-exchange properties, PECH-C-based anion-exchange membranes exhibited a higher power density of 0.316 W/m2 in a lab-scale RED system. Evidently, there is room for the development of new types of PECH-C-based AEMs with great potential for energy generation in the RED process.

An electrochemical system for salinity gradient energy harvesting

Salinity gradient energy, which is abundant along the coastline in ocean, is a kind of chemical energy existing the solutions of different concentrations mixing. The pressure retarded osmosis, reverse electrodialysis and accumulator mixing technology are currently the most common approaches to harvest salinity gradient energy. Here we report an electrochemical device using active carbon as anode and manganese phosphate as cathode for salinity gradient energy harvesting, which shows an energy density of 5.31 J g−1, indicating a potential application in the field of ocean energy utilization.

Development of Electrodes for Harvesting Salinity Gradient Energy

Development of Electrodes for Harvesting Salinity Gradient Energy

Use of Reverse Electrodialysis to Harvest Salinity Gradient Energy from Oilfield Produced Water

Two lab-scale electrodialysis (RED) stacks with different intermembrane spacing were used in this study. Each stack consists of two membrane pairs. Thick intermembrane spacing stack was made of four identical plexiglass sections, with outer dimensions 5 cm * 5 cm * 1.5 cm and an inner cross-section of 3 cm diameter each to construct two diluted solution compartments and two concentrated solution compartments. For the thin intermembrane spacing configuration, four rubber spacers with a thickness of 1 mm and an inner opening of 3 cm each were used instead of these sections. Two copper sheets were used as anode and cathode electrodes. Different solutions with NaCl concentrations of 15,000, 30,000 and 200,000 mg/l were used as a concentrated solution and different solutions with relatively low NaCl concentrations of 25, 1000 and 3600 mg/l were used as a diluted solution. A 30,000 mg/l NaCl solution was used as a diluted solution when the concentrated stream was with NaCl concentration of 200,000 mg/l. The electrode solution was of 15,000 mg/l (~0.25 M) NaCl and 8,000 mg/l (~0.05 M) CuSO4.5H2O. The results of this study confirmed the validity of using RED technology to harvest energy from salinity gradient using saline and freshwater available abundantly particularly in Iraq. An experiment on a synthetic hypersaline oil field co-produced water as a concentrated stream and seawater as a diluted stream showed that the system performance is reproducible and stable. A maximum power density of 0.029 W/m2 could be harvested.

Miniaturized Salinity Gradient Energy Harvesting Devices.

Harvesting salinity gradient energy, also known as “osmotic energy” or “blue energy”, generated from the free energy mixing of seawater and fresh river water provides a renewable and sustainable alternative for circumventing the recent upsurge in global energy consumption. The osmotic pressure resulting from mixing water streams with different salinities can be converted into electrical energy driven by a potential difference or ionic gradients. Reversed-electrodialysis (RED) has become more prominent among the conventional membrane-based separation methodologies due to its higher energy efficiency and lesser susceptibility to membrane fouling than pressure-retarded osmosis (PRO). However, the ion-exchange membranes used for RED systems often encounter limitations while adapting to a real-world system due to their limited pore sizes and internal resistance. The worldwide demand for clean energy production has reinvigorated the interest in salinity gradient energy conversion. In addition to the large energy conversion devices, the miniaturized devices used for powering a portable or wearable micro-device have attracted much attention. This review provides insights into developing miniaturized salinity gradient energy harvesting devices and recent advances in the membranes designed for optimized osmotic power extraction. Furthermore, we present various applications utilizing the salinity gradient energy conversion.

Energy Lost in a Hydrogel Osmotic Engine Due to a Pressure Drop

Utilizing hydrogels to harvest salinity gradient energy from solutions of different salinities has recently attracted interest. Polyelectrolyte hydrogels exhibit cyclic swelling/deswelling when alternately exposed to freshwater and seawater. This can be utilized to convert the mixing energy of the two solutions into mechanical energy. Hydrogels consisting of a semi-interpenetrating network (semi-IPN) of poly(4-styrene sulfonic acid-co-maleic acid) sodium salt and polyacrylic acid was prepared at various cross-linking densities. The energy lost due to a pressure drop in the system during the deswelling/swelling process of these hydrogels is examined, and the effects of tubing dimensions, hydrogel cylinder size, gel particle size, and the volume fraction within the hydrogel cylinder occupied by the flowing liquid (ε) are investigated. In addition, a small-scale osmotic engine was compared to a scaled-up system. ε was found to be the factor that had the largest effect on the energy loss. It was found that ε is strongly dependent on the degree of swelling of the hydrogels. When the hydrogels swell, they deform more easily under pressure. This markedly decreases ε, thereby inducing a high pressure drop in the system and a correspondingly large energy loss. Accordingly, the pressure drop when pumping through the hydrogel is the major contributor to the energy loss in the system. When the hydrogel particles deform too much, the energy needed to pump the flowing liquid through the hydrogels exceeds the energy produced by the system. Developing a hydrogel system that deforms less in its swollen state is therefore essential for improving the energy efficiencies of these osmotic engines.

High energy recovery from salinity gradients in a concentration flow cell enhanced by bioelectrochemical currents

An enormous source of clean energy, called salinity gradient (SG) energy, exists from mixing waters with different salinities. Harvesting SG energy has attracted lots of attentions to develop efficient technologies. However, the power output is still limited. In this study, a concentration flow cell (CFC) powered by a bioelectrochemical system, defined as a bio-CFC, was proposed to recover SG energy from synthetic seawater (30 g/L NaCl) and river water (1 g/L NaCl) efficiently. The maximum power density of the bio-CFC reached 42 ± 2 W/m2, which was three times higher than that of a single CFC (10.6 ± 0.1 W/m2). The significant improvement was attributed to the additionally developed capacitive potential, which was formed by the bioelectrochemical currents from degradation of the organics in wastewater. The capacitive potential enhanced the Na+ intercalation/deintercalation on the electrodes and accelerated the Cl− transfer across the anion exchange membrane. This new strategy provides a promising way to recover energy from seawater and wastewater.

Optimizing multistage reverse electrodialysis for enhanced energy recovery from river water and seawater: Experimental and modeling investigation

• Fair comparison between single-stage and multistage reverse electrodialysis. • New “saving the gradient” electrical control strategy improves process performance. • To harvest energy from river water and seawater, applying two stages is optimal. • Reducing stack pressure drops favors multistage application. 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.

Pressure retarded osmosis – a multi-functional osmotic energy membrane-based harvest technology

Pressure retarded osmosis (PRO) is a re-emerging membrane-based technology to harvest salinity gradient energy (SGE). Membrane is the key factor to further PRO development. Polymer blend is an effective method to enhance membrane properties. In this work, conventional polysulfone (PSf) and polyethersulfone (PES) were blended in 1:1 ratio to fabricate the support membrane. The intrinsic water and salt permeabilities of PSf:PES membrane were enhanced to 1.90 ± 0.42 LMH/bar and 0.46 ± 0.10 gMH, respectively compared to the pristine PSf and PES with only slight drop (1.2 %) in salt rejection of 97.7 %. PSf:PES membrane gave the highest water fluxes of 26.19 and 22.62 LMH at applied hydraulic pressure of 2 and 4 bar, respectively. The resultant power density obtained was 1.46 and 2.51 W/m 2 with the respective hydraulic pressure. The PSf:PES membrane substrate was found to bring a positive impact on the IP reaction with resultant fully cross-linked, leaf-like PA structure which ultimately gives a greater surface/volume ratio on its respective TFC PRO membrane performance.

Effect of current-induced ion transfer on the electrical resistance of reverse electrodialysis stack by chronopotentiometry

Reverse electrodialysis (RED) technology is a direct and effective technology for power generation by harvesting salinity gradient energy (SGE). The output current is one of the key operating parameters to control the power output of a RED stack. This research investigates the specific effect of the current-induced ion transfer on each electrical resistance component of the RED stack. The results showed that the internal resistance of the RED stack decreases with the increase of the current, because larger ion transfer induced by larger current would initiate the variation of concentrations of the feeding concentrated and diluted solutions along the flow path. It is worth highlighting that the ohmic resistance of the diluted solution component takes the dominant proportion in the total ohmic resistance of a RED stack, which decreases significantly with the increase of current. The non-ohmic resistance also decreases with the current increases. Therefore, the internal resistance of RED stack depends on the current. Moreover, it is found that the polarization curve and power density curve can be in good accordance with the experimental data if the influence of current on internal resistance is taken into consideration. The energy efficiency increases first and decreases with the current increase. The maximal energy efficiency of 3.4% was achieved when the RED stack operated in the maximum power density of 3.67 W/m2 at the current density of 44.9 A/m2. The coulombic efficiency increases with the increase of the current. The residual ratio of the SGE in effluent decreases with the increase of the current.

Tailoring the Surface Chemistry of Anion Exchange Membranes with Zwitterions: Toward Antifouling RED Membranes.

Fouling is a pressing issue for harvesting salinity gradient energy with reverse electrodialysis (RED). In this work, antifouling membranes were fabricated by surface modification of a commercial anion exchange membrane with zwitterionic layers. Either zwitterionic monomers or zwitterionic brushes were applied on the surface. Zwitterionic monomers were grafted to the surface by deposition of a polydopamine layer followed by an aza-Michael reaction with sulfobetaine. Zwitterionic brushes were grafted on the surface by deposition of polydopamine modified with a surface initiator for subsequent atom transfer radical polymerization to obtain polysulfobetaine. As expected, the zwitterionic layers did increase the membrane hydrophilicity. The antifouling behavior of the membranes in RED was evaluated using artificial river and seawater and sodium dodecylbenzenesulfonate as the model foulant. The zwitterionic monomers are effective in delaying the fouling onset, but the further build-up of the fouling layer is hardly affected, resulting in similar power density losses as for the unmodified membranes. Membranes modified with zwitterionic brushes show a high potential for application in RED as they not only delay the onset of fouling but they also slow down the growth of the fouling layer, thus retaining higher power density outputs.

Influence of output current on decolorization efficiency of azo dye wastewater by a series system with multi-stage reverse electrodialysis reactors

A series system with multi-stage reverse electrodialysis reactor (REDR) can harvest salinity gradient energy (SGE) from working fluids as much as possible to treat organic wastewater. In this paper, the series system simulated with a single REDR system to degrade azo dye wastewater was investigated experimentally. The results showed that the series system could not only degrade azo dye wastewater effectively, but also output a certain of electric energy simultaneously. In the series system, the salinity gradient variations of working fluids which flowing through each stage REDR affected only on the output voltage of each stage REDR, but which almost did not affect on the decolorization efficiency of azo dye wastewater of each stage REDR. However, the output current variations of the series system effected significantly on the decolorization efficiency. Although the increase of output current was benefit to the decolorization efficiency for each reactor in the series system, the number of reactors in the series system will reduce quickly, which results in total decolorization rate of azo dye wastewater decrease. When the current was 0.1A, the overall performances of the series system would be relatively optimal under the experimental conditions. The number of reactors, total decolorization rate of azo dye wastewater, utilization efficiencies of SGE and membrane area, and net output power could reach 19, 1.91 mg·s−1, 6.92 mg·J−1, 31.21 mg·m−2s−1 and 0.68 W, respectively.

Toward scale-up of seawater reverse osmosis (SWRO) – pressure retarded osmosis (PRO) hybrid system: A case study of a 240 m3/day pilot plant

Pressure retarded osmosis (PRO) has been considered as a potential technique to harvest salinity gradient energy (or osmotic energy). The SWRO-PRO hybrid system is one of the viable ways of implementing PRO technology to resolve problems associated with seawater desalination. However, relatively few works have been attempted in pilot scales, which are inevitable for the scale-up and practical application of the SWRO-PRO technology. In this context, this paper reports the pilot study results of SWRO-PRO hybrid system (240 m3/day). To examine the long-term performance, the plant was operated for more than 2 years. The flux and energy consumption in the plant were analyzed to examine the potential of PRO as a secondary energy recovery device. The fouling propensity of the PRO membranes was examined. Results showed that the use of PRO can reduce the energy consumption of SWRO process by approximately 20%. Moreover, the SWRO brine was diluted up to 63%. This work will provide an insight into the researchers and scientific community by filling the gap between the bench-scale works and the commercial-scale application.