Hybrid pressure retarded osmosis-membrane distillation system for power generation from low-grade heat: thermodynamic analysis and energy efficiency

Integration of Membrane-Based Pretreatment Methods with Pressure-Retarded Osmosis for Performance Enhancement: A Review

Osmotic energy provides an emerging renewable alternative by leveraging the salinity gradient between two solutions. Among these technologies, pressure-retarded osmosis (PRO) has attracted attention; however, its deployment is hindered by obstacles resulting from impurities in feed and draw solutions and lack of suitable membranes. This review explores the integration of membrane-based pretreatments with PRO, highlighting their influence on resolving the technical drawbacks of standalone PRO systems. Membrane-based pretreatments have shown considerable potential to overcome these challenges by improving the quality of water, reducing membrane fouling and enhancing its performance, and ultimately contributing to recovery of energy, resulting in higher power density. Additionally, the use of different nanomaterials has been proposed for membrane modification to optimize PRO performance. Moreover, the study investigates recent advancements in hybrid configurations for harnessing existing infrastructure and to enhance energy efficiency. Offering a comprehensive review on this integrated approach contributes to valuable insights for advancing membrane-based hybrid systems toward commercial viability. Consequently, investment in developing advanced computational modeling and experimental validation, utilization of advanced membrane materials with higher fouling resistance, and optimization of system configurations by using dual-stage and multi-stage designs are required to overcome these limitations.

Hydroelectric energy conversion of waste flows through hydroelectronic drag

Hydraulic energy is a key component of the global energy mix, yet there exists no practical way of harvesting it at small scales, from flows with low Reynolds number. This has triggered a search for alternative hydroelectric conversion methodologies, leading to unconventional proposals based on droplet triboelectricity, water evaporation, osmotic energy, or flow-induced ionic Coulomb drag. Yet, these approaches systematically rely on ions as intermediate charge carriers, limiting the achievable power density. Here, we predict that the kinetic energy of small-scale "waste" flows can be directly and efficiently converted into electricity thanks to the hydroelectronic drag effect, by which an ion-free liquid induces an electronic current in the solid wall along which it flows. This effect originates in the fluctuation-induced coupling between fluid motion and electron transport. We develop a nonequilibrium thermodynamic formalism to assess the efficiency of such hydroelectric energy conversion, dubbed hydronic energy. We find that hydronic energy conversion is analogous to thermoelectricity, with the efficiency being controlled by a dimensionless figure of merit. However, in contrast to its thermoelectric analogue, this figure of merit combines independently tunable parameters of the solid and the liquid, and can thus significantly exceed unity. Our findings suggest strategies for blue energy harvesting without electrochemistry, and for waste flow mitigation in membrane-based filtration processes.

Exploiting the potential of a novel "in-situ latent heat recovery" in hollow-fiber vacuum membrane distillation process for simultaneously improved water production and energy efficiency

Thermal driven membrane distillation (MD) technology is a promising method for purifying & recovering various salty (especially high salty) or contaminated wastewaters with low-grade heat sources. However, the drawbacks of "high energy consumption" and "high cooling water consumption" pose special challenges for the future development of this technology. In this article, we report an innovative strategy called "in-situ heat transfer", which is based on the jacketed structure composed of hollow fiber membranes and capillary heat exchange tubes, to simplify the migration steps of condensation latent heat in MD heat recovery process. The results indicate that the novel heat recovery strategy exhibits higher growth rates both in the flux and gained output ratio (47.4 % and 173.1 %, respectively), and further reduces the system's dependence on cooling water. In sum, under the control of the "in-situ heat transfer" mechanism, the functional coupling of "vapor condensation (exothermic)" and "feed evaporation (endothermic)" in limited-domain space is an attractive alternative solution, because it eliminates the disadvantages of the imbalance between heat supply and demand in traditional heat recovery methods. Our research may facilitate the development of MD heat recovery modules for industrial applications, which will help to further achieve the goal of energy saving and emission reduction.

Impact of temperature and forward osmosis membrane properties on the concentration polarization and specific energy consumption of hybrid desalination system

This study investigates how temperature and forward osmosis (FO) membrane properties, such as water permeability (A), solute permeability (B), and structural parameter (S), affect the specific energy consumption (SEC) of forward osmosis-reverse osmosis system. The results show that further SEC reduction beyond the water permeability of 3 LMH bar-1 is limited owing to high concentration polarization (CP). Increasing S by 10-fold increases FO recovery by 177.6%, causing SEC decreases by 33.6%. However, membrane with smaller S also increases external CP. To reduce SEC, future work should emphasize mixing strategies to reduce external CP. Furthermore, increasing the temperature from 10 to 40 °C can reduce SEC by 14.3%, highlighting the energy-saving potential of temperature-elevated systems. The factorial design indicates that at a lower temperature, increasing A and decreasing S have a more significant impact on reducing SEC. This underlines the importance of developing advanced FO membranes, particularly for lower-temperature processes.

Review of Hybrid Membrane Distillation Systems

Membrane distillation (MD) is an attractive separation process that can work with heat sources with low temperature differences and is less sensitive to concentration polarization and membrane fouling than other pressure-driven membrane separation processes, thus allowing it to use low-grade thermal energy, which is helpful to decrease the consumption of energy, treat concentrated solutions, and improve water recovery rate. This paper provides a review of the integration of MD with waste heat and renewable energy, such as solar radiation, salt-gradient solar ponds, and geothermal energy, for desalination. In addition, MD hybrids with pressure-retarded osmosis (PRO), multi-effect distillation (MED), reverse osmosis (RO), crystallization, forward osmosis (FO), and bioreactors to dispose of concentrated solutions are also comprehensively summarized. A critical analysis of the hybrid MD systems will be helpful for the research and development of MD technology and will promote its application. Eventually, a possible research direction for MD is suggested.

Thermo-electrochemical redox flow cycle for continuous conversion of low-grade waste heat to power

Here we assess the route to convert low grade waste heat (< 100 °C) into electricity by leveraging the temperature dependency of redox potentials, similar to the Seebeck effect in semiconductor physics. We use fluid-based redox-active species, which can be easily heated and cooled using heat exchangers. By using a first principles approach, we designed a redox flow battery system with Fe(CN)₆³⁻/Fe(CN)₆⁴⁻ and I⁻/I₃⁻ chemistry. We evaluate the continuous operation with one flow cell at high temperature and one at low temperature. We show that the most sensitive parameter, the temperature coefficient of the redox reaction, can be controlled via the redox chemistry, the reaction quotient and solvent additives, and we present the highest temperature coefficient for this RFB chemistry. A power density of 0.6 W/m² and stable operation for 2 h are achieved experimentally. We predict high (close to Carnot) heat-to-power efficiencies if challenges in the heat recuperation and Ohmic resistance are overcome, and the temperature coefficient is further increased.

Seasonal shift of water quality in China Yangtze River and its impacts on membrane fouling development during the drinking water supply by membrane distillation system

Membrane distillation (MD) technique is increasingly regarded as a promising process for drinking water supply and wastewater treatment owing to its great water purification and usage of renewable energy. Like other membrane separation processes, the membrane fouling issue is widely considered as the main obstacle for real applications of large-scale MD systems. Feedwater characteristics, as the predominant factors for membrane fouling layer formation, mostly determined the membrane fouling trend of MD. Thus the impacts of seasonal shifts of initial feedwater quality on the MD membrane fouling were detailedly researched in this study, and the biofilm development mechanism was especially explored. The bacterial community structure of membrane biofilms was clearly clarified in MD runs of Yangtze River waters that collected in four seasons. The results revealed that the winter run posed a quite sharp flux drop, while a relatively milder flux decline behaviour was seen for other groups despite of the higher bacteria concentration of initial feedwaters. The poorer water quality in winter induced the establishment of a rather thick biofilm on the MD membrane, in which the biofilm-forming bacteria (Gammaproteobacteria and Alphaproteobacteria) and organic matters (EPS) were remarkably observed. Comparatively, a relatively thin biofilm containing abundant live cells and fewer organics finally formed in summer and autumn runs, causing a mitigated flux decline trend. Hence, it can be inferred that the membrane flux decline of MD was likely to be more sensitive to the organic attachment on the membrane in comparison with the bacteria adhesion. Finally, a three-phase pretreatment method was suggested for MD fouling control, including heating course, sterilization course, and filtration course.

Desalination: From Ancient to Present and Future

Water is life, and without water, there would be no civilizations and a vacant Earth. Water is considered an abundant natural resource on the earth. Water covers 3/4 of the surface. However, 97% of the available water on the earth is salty oceanic water, and only a tiny fraction (3%) is freshwater. This small portion of the available water supplies the needs of humans and animals. However, freshwater exists in underground, rivers, and lakes and is insufficient to cover all the world’s water demands. Thus, water saving, water reuse, rainwater harvesting, stormwater utilization, and desalination are critical for maintaining water supplies for the future of humanity. Desalination has a long history spanning centuries from ancient times to the present. In the last two decades, desalination has been rapidly expanding to meet water needs in stressed water regions of the world. Yet, there are still some problems with its implementation in several areas of the world. This review provides a comprehensive assessment of the history of desalination for wiser and smarter water extraction and uses to sustain and support the water needs of the earth’s inhabitants.

Thermal associated pressure-retarded osmosis processes for energy production: A review

Climate change is an existential threat to global environments and human life. To achieve global mean temperature rise of below 1.5 °C, increasing utilization of renewable energy and minimizing CO₂ emission from fossil fuel industries have been emphasized by the United Nations. Pressure-retarded osmosis (PRO) has displayed its technical feasibility in capturing renewable energy from the salinity gradient of two streams through a semipermeable membrane. Towards achieving economic feasible PRO, process optimization, waste stream/heat utilization, and hybrid PRO processes have been attempted by theoretically modelling and experimental examination. Among these efforts, the thermal associated PRO processes have received great attention due to their improved power generation. In this paper, we aim to provide a comprehensive review on thermal associated PRO processes, focusing on the role of thermal behaviour in both stand-alone PRO and hybrid PRO processes (e.g. PRO-membrane distillation, PRO-thermosiphon, PRO-solar pond). Meanwhile, thermal associated draw solution development has been highlighted. Finally, a combination of PRO with high temperature/high pressure geothermal waste gas as draw solution is proposed and its technical and economic feasibility is discussed, especially under Icelandic scenario.

Nanoparticle-free and self-healing amphiphobic membrane for anti-surfactant-wetting membrane distillation

In membrane distillation (MD), complicated feed water with amphiphilic contaminants induces fouling/wetting of the MD membrane and can even lead to process failure. This study reports a facile approach to fabricate robust and self-healing hybrid amphiphobic membranes for anti-surfactant-wetting MD based on the ultra-low surface energy of fluorinated polyhedral oligomeric silsesquioxanes (F-POSS) and its thermal induced motivation and rotation. The thermal treatment makes the membranes achieving amphiphobicity at a very low cost of F-POSS (13.04 wt.%), which is about 1/3 of without thermal treatment. The prepared membrane exhibits excellent amphiphobicity, i.e. ethanol contact angle of 120.3°, without using environmentally toxic fluorinated nanoparticles. Robust MD performance was observed for the amphiphobic membrane in concentrated sodium dodecyl sulfate (SDS) feed solutions. Furthermore, the fabricated membrane exhibited stable amphiphobicity even in extreme environments, including strong acid or alkaline solutions. In the event of a damaged or abraded membrane surface where the F-POSS can be removed, the amphiphobic membrane exhibits self-healing ability with additional thermal treatment. This simple approach without the use of nanoparticles provides an environmentally friendly way for fabrication of amphiphobic membranes for anti-surfactant-wetting membrane distillation.

Dye synthetic solution treatment by direct contact membrane distillation using commercial membranes

The reuse of treated dyeing wastewater has become a viable option to minimizing water scarcity problems and environmental impacts in the textile industry. The potentiality of commercial flat sheet membranes of polytetrafluoroethylene (PTFE) and polypropylene (PP) in direct contact membrane distillation (DCMD) for dye synthetic solution treatment has been explored in this work. DCMD is interesting for the textile industry since a recovery of heat by hot dyeing wastewater for thermal energy is possible. Moreover, DCMD enables water and dye reclamation with possible reuse in the textile process. The commercial availability of membranes may expedite the DCMD commercialization in the textile industry. Experiments were conducted in a laboratory-scale circulating unit with synthetic solutions containing reactive or disperse dye. High mean permeate flux up to 18.8 kg m^-2 h^-1 with complete colour rejection was obtained. The dyes tested in this study are not able to completely wet the membranes and the increase of the permeate flux when compared to distilled water is attributed to electrostatic interactions between the dyes and the membranes. Moreover, a partial wetting reduced vapour diffusion path and the permeate flux was increased. PP membrane showed higher performance due to higher porosity when compared to the PTFE membrane. In addition, an influence of dye class on permeability was observed. The results were promising when compared to other studies, which used commercial or lab-scale membranes.

Ionogels at the Water-Energy Nexus for Desalination Powered by Ultralow-Grade Heat

Industrial processes emit enormous amounts of waste heat below 40 °C into the environment as it is cannot be used in other processes. Adsorption desalination can be driven by low-grade heat but has never been proven at temperatures below 40 °C as current adsorption materials require heat sources of 50-150 °C. Here, we present the first experimental study on adsorption desalination using a novel class of ionogel adsorption materials, which can be regenerated at 25 °C or a driving temperature difference of 5 °C. This outstanding property contrasts with the benchmarking silica gel, which requires heat sources of at least 50 °C. Ionogels are solid-state ionic materials retaining the sorption properties of the constituent ionic liquid. Thermodynamic vapor-liquid equilibrium data of water sorption on commercial ionic liquids reveal 1-ethyl-3-methylimidazolium acetate as the best fluid for this specific application. A full experimental characterization of the material is performed from imaging at the nanoscale to testing on a real adsorption desalinator. At 25 °C, the material achieves a specific daily water production of 6.7 kg_water/(kg_ionogel d), increasing to 17.5 kg_water/(kg_ionogeld) at 45 °C, outperforming silica gel by a factor of 2.

Energy capture from thermolytic solutions and simulated sunlight coupled with hydrogen peroxide production and wastewater remediation

A novel solar salinity energy nexus was developed by combining a photocatalytic fuel cell (PFC) and a reverse electrodialysis (RED). The hybrid cell (called a PRC) can effectively transfer the solar energy and salinity gradient energy (which is regenerated by low grade industrial waste heat) to electrical energy coupled with enhanced pollutant (Rhodamine B, RhB) degradation and H₂O₂ production. Energy-Environment win-win will be realized. The open circuit voltage of the PRC was the sum of those of the PFC and the RED, and the RED stack made the larger contribution to the electricity production of PRC. The bias voltage generated from the RED stack accelerated the separation of photo-induced holes and electrons on the three-dimensional TiO₂ array photoanode, which enhanced RhB degradation and H₂O₂ production. The flow rate and concentration of the working fluids (ammonium bicarbonate) and the reaction conditions in the electrode chambers had substantial effects on the PRC performance. Under the optimal condition, the peak power density and energy efficiency of PRC reached 1500 mW m^-2 and 4.21% respectively. The performance of PRC on electricity production is better than photocatalytic electrolytic cell driven by desalination(PFCD), but not good as microbial reverse electrodialysis electrolysis cell (MREC).

Mechanism of biofilm formation on a hydrophobic polytetrafluoroethylene membrane during the purification of surface water using direct contact membrane distillation (DCMD), with especial interest in the feed properties

The impact of feed water quality on biofilm formation during membrane distillation (MD) was investigated in this study, particularly emphasizing the interrelationship between organics, salts, and microbes. Two types of typical natural surface waters in Nanjing, China, were chosen as feed solutions for long-term MD operation, including the Qinhuai River and Xuanwu Lake. The biofilms that developed under different feed water qualities exhibited distinct Foulant compositions and structures, causing different flux decline trends for the MD system. Accordingly, two typical patterns of biofilm formation were suggested for the MD operation of the two different kinds of surface waters in this study. Organics from a primal feed solution and dead bacteria were the key to the establishment of a biofilm on the membrane, and this needs to be effectively removed from the MD system through pre-treatment and process control strategies. Finally, a feasible strategy for MD biofouling control was suggested.

Low-Grade Waste Heat Recovery via an Osmotic Heat Engine by Using a Freestanding Graphene Oxide Membrane

The osmotic heat engine represents a new and promising technology for the harvesting of low-grade waste heat from various sources. However, the lack of an adequate semipermeable membrane hinders the technology's advancement. In this study, we investigated the application of a freestanding graphene oxide membrane (GOM) for energy generation in an osmotic heat engine. The synthesized GOM has a water permeability coefficient of 4.4 L m^-2 h^-1 bar^-1 (LMH-bar). The internal concentration polarization in the osmosis filtration system can be minimized because no membrane support layer is needed for the freestanding GOM. As a result, high water flux and high power density are obtained. For example, under an applied hydraulic pressure of 6.90 bar, with a 2 M draw solution of ammonium bicarbonate solution, a power density of 20.0 W/m² is achieved. This study shows that the freestanding GOM is promising for application in the osmotic heat engine. Future research regarding improving the mechanical properties and water stability of the GOM is beneficial for further advancing the technology.

Domestic wastewater treatment by forward osmosis-membrane distillation (FO-MD) integrated system

In this study, real domestic wastewater treatment by forward osmosis-membrane distillation (FO-MD) integrated system was investigated in laboratory scale. The integrated membrane system presented a good separation performance and the removal efficiency of most contaminants in the domestic wastewater was higher than 90%. High molecular weight contaminants were completely removed, while a few low molecular weight contaminants permeated through the membrane. The FO membrane fouling layer mainly consisted of organic substances like polysaccharides and proteins, and was very loose and could be effectively removed by rinsing the membrane surface with tap water. By comparison, the MD membrane fouling was mainly induced by inorganic salts and was not as severe as that of the FO membrane. During 120 h continuous operation, the FO-MD integrated system exhibited satisfying performance stability and maintained a high water yield and high product water quality. The results indicated the potential of the FO-MD integrated system for municipal wastewater treatment in coastal cities, water purification and desalination.

Hierarchical Composite Membranes with Robust Omniphobic Surface Using Layer-By-Layer Assembly Technique

In this study, composite membranes were fabricated via layer-by-layer (LBL) assembly of negatively charged silica aerogel (SiA) and 1H,1H,2H,2H-perfluorodecyltriethoxysilane (FTCS) on a polyvinylidene fluoride phase inversion membrane and interconnecting them with positively charged poly(diallyldimethylammonium chloride) (PDDA) via electrostatic interaction. The results showed that the PDDA-SiA-FTCS coated membrane had significantly enhanced the membrane structure and properties. New trifluoromethyl and tetrafluoroethylene bonds appeared at the surface of the coated membrane, which led to lower surface free energy of the composite membrane. Additionally, the LBL membrane showed increased surface roughness. The improved structure and property gave the LBL membrane an omniphobic property, as indicated by its good wetting resistance. The membrane performed a stable air gap membrane distillation (AGMD) flux of 11.22 L/m² h with very high salt rejection using reverse osmosis brine from coal seam gas produced water as feed with the addition of up to 0.5 mM SDS solution. This performance was much better compared to those of the neat membrane. The present study suggests that the enhanced membrane properties with good omniphobicity via LBL assembly make the porous membranes suitable for long-term AGMD operation with stable permeation flux when treating challenging saline wastewater containing low surface tension organic contaminants.

Energy Efficiency and Performance Limiting Effects in Thermo-Osmotic Energy Conversion from Low-Grade Heat

Low-grade heat energy from sources below 100 °C is available in massive quantities around the world, but cannot be converted to electricity effectively using existing technologies due to variability in the heat output and the small temperature difference between the source and environment. The recently developed thermo-osmotic energy conversion (TOEC) process has the potential to harvest energy from low-grade heat sources by using a temperature difference to create a pressurized liquid flux across a membrane, which can be converted to mechanical work via a turbine. In this study, we perform the first analysis of energy efficiency and the expected performance of the TOEC technology, focusing on systems utilizing hydrophobic porous vapor-gap membranes and water as a working fluid. We begin by developing a framework to analyze realistic mass and heat transport in the process, probing the impact of various membrane parameters and system operating conditions. Our analysis reveals that an optimized system can achieve heat-to-electricity energy conversion efficiencies up to 4.1% (34% of the Carnot efficiency) with hot and cold working temperatures of 60 and 20 °C, respectively, and an operating pressure of 5 MPa (50 bar). Lower energy efficiencies, however, will occur in systems operating with high power densities (>5 W/m²) and with finite-sized heat exchangers. We identify that the most important membrane properties for achieving high performance are an asymmetric pore structure, high pressure resistance, a high porosity, and a thickness of 30 to 100 μm. We also quantify the benefits in performance from utilizing deaerated water streams, strong hydrodynamic mixing in the membrane module, and high heat exchanger efficiencies. Overall, our study demonstrates the promise of full-scale TOEC systems to extract energy from low-grade heat and identifies key factors for performance optimization moving forward.

Advanced Materials, Technologies, and Complex Systems Analyses: Emerging Opportunities to Enhance Urban Water Security

Innovation in urban water systems is required to address the increasing demand for clean water due to population growth and aggravated water stress caused by water pollution, aging infrastructure, and climate change. Advances in materials science, modular water treatment technologies, and complex systems analyses, coupled with the drive to minimize the energy and environmental footprints of cities, provide new opportunities to ensure a resilient and safe water supply. We present a vision for enhancing efficiency and resiliency of urban water systems and discuss approaches and research needs for overcoming associated implementation challenges.

Salinity Gradients for Sustainable Energy: Primer, Progress, and Prospects

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.

A Thermally-Regenerative Ammonia-Based Flow Battery for Electrical Energy Recovery from Waste Heat

Large amounts of low-grade waste heat (temperatures <130 °C) are released during many industrial, geothermal, and solar-based processes. Using thermally-regenerative ammonia solutions, low-grade thermal energy can be converted to electricity in battery systems. To improve reactor efficiency, a compact, ammonia-based flow battery (AFB) was developed and tested at different solution concentrations, flow rates, cell pairs, and circuit connections. The AFB achieved a maximum power density of 45 W m(-2) (15 kW m(-3) ) and an energy density of 1260 Wh manolyte (-3) , with a thermal energy efficiency of 0.7 % (5 % relative to the Carnot efficiency). The power and energy densities of the AFB were greater than those previously reported for thermoelectrochemical and salinity-gradient technologies, and the voltage or current could be increased using stacked cells. These results demonstrated that an ammonia-based flow battery is a promising technology to convert low-grade thermal energy to electricity.

Environmental Applications of Interfacial Materials with Special Wettability

Interfacial materials with special wettability have become a burgeoning research area in materials science in the past decade. The unique surface properties of materials and interfaces generated by biomimetic approaches can be leveraged to develop effective solutions to challenging environmental problems. This critical review presents the concept, mechanisms, and fabrication techniques of interfacial materials with special wettability, and assesses the environmental applications of these materials for oil-water separation, membrane-based water purification and desalination, biofouling control, high performance vapor condensation, and atmospheric water collection. We also highlight the most promising properties of interfacial materials with special wettability that enable innovative environmental applications and discuss the practical challenges for large-scale implementation of these novel materials.

Aspects of Mathematical Modelling of Pressure Retarded Osmosis

In power generating terms, a pressure retarded osmosis (PRO) energy generating plant, on a river entering a sea or ocean, is equivalent to a hydroelectric dam with a height of about 60 meters. Therefore, PRO can add significantly to existing renewable power generation capacity if economical constrains of the method are resolved. PRO energy generation relies on a semipermeable membrane that is permeable to water and impermeable to salt. Mathematical modelling plays an important part in understanding flows of water and salt near and across semipermeable membranes and helps to optimize PRO energy generation. Therefore, the modelling can help realizing PRO energy generation potential. In this work, a few aspects of mathematical modelling of the PRO process are reviewed and discussed.

Selectivity and Mass Transfer Limitations in Pressure-Retarded Osmosis at High Concentrations and Increased Operating Pressures

Pressure-retarded osmosis (PRO) is a promising source of renewable energy when hypersaline brines and other high concentration solutions are used. However, membrane performance under conditions suitable for these solutions is poorly understood. In this work, we use a new method to characterize membranes under a variety of pressures and concentrations, including hydraulic pressures up to 48.3 bar and concentrations of up to 3 M NaCl. We find membrane selectivity decreases as the draw solution concentration is increased, with the salt permeability coefficient increasing by a factor of 2 when the draw concentration is changed from 0.6 to 3 M NaCl, even when the applied hydraulic pressure is maintained constant. Additionally, we find that significant pumping energy is required to overcome frictional pressure losses in the spacer-filled feed channel and achieve suitable mass transfer on the feed side of the membrane, especially at high operating pressures. For a meter-long module operating at 41 bar, we estimate feedwater will have to be pumped in at a pressure of at least 3 bar. Both the reduced selectivity and increased pumping energy requirements we observe in PRO will significantly diminish the obtainable net energy, highlighting important new challenges for development of systems utilizing hypersaline draw solutions.

Membrane-based osmotic heat engine with organic solvent for enhanced power generation from low-grade heat

We present a hybrid osmotic heat engine (OHE) system that uses draw solutions with an organic solvent for enhanced thermal separation efficiency. The hybrid OHE system produces sustainable energy by combining pressure-retarded osmosis (PRO) as a power generation stage and membrane distillation (MD) utilizing low-grade heat as a separation stage. While previous OHE systems employed aqueous electrolyte draw solutions, using methanol as a solvent is advantageous because methanol is highly volatile and has a lower heat capacity and enthalpy of vaporization than water. Hence, the thermal separation efficiency of a draw solution with methanol would be higher than that of an aqueous draw solution. In this study, we evaluated the performance of LiCl-methanol as a potential draw solution for a PRO-MD hybrid OHE system. The membrane transport properties as well as performance with LiCl-methanol draw solution were evaluated using thin-film composite (TFC) PRO membranes and compared to the results obtained with a LiCl-water draw solution. Experimental PRO methanol flux and maximum projected power density of 47.1 L m(-2) h(-1) and 72.1 W m(-2), respectively, were achieved with a 3 M LiCl-methanol draw solution. The overall efficiency of the hybrid OHE system was modeled by coupling the mass and energy flows between the thermal separation (MD) and power generation (PRO) stages under conditions with and without heat recovery. The modeling results demonstrate higher OHE energy efficiency with the LiCl-methanol draw solution compared to that with the LiCl-water draw solution under practical operating conditions (i.e., heat recovery<90%). We discuss the implications of the results for converting low-grade heat to power.

Comparison of energy efficiency and power density in pressure retarded osmosis and reverse electrodialysis

Pressure retarded osmosis (PRO) and reverse electrodialysis (RED) are emerging membrane-based technologies that can convert chemical energy in salinity gradients to useful work. The two processes have intrinsically different working principles: controlled mixing in PRO is achieved by water permeation across salt-rejecting membranes, whereas RED is driven by ion flux across charged membranes. This study compares the energy efficiency and power density performance of PRO and RED with simulated technologically available membranes for natural, anthropogenic, and engineered salinity gradients (seawater-river water, desalination brine-wastewater, and synthetic hypersaline solutions, respectively). The analysis shows that PRO can achieve both greater efficiencies (54-56%) and higher power densities (2.4-38 W/m(2)) than RED (18-38% and 0.77-1.2 W/m(2)). The superior efficiency is attributed to the ability of PRO membranes to more effectively utilize the salinity difference to drive water permeation and better suppress the detrimental leakage of salts. On the other hand, the low conductivity of currently available ion exchange membranes impedes RED ion flux and, thus, constrains the power density. Both technologies exhibit a trade-off between efficiency and power density: employing more permeable but less selective membranes can enhance the power density, but undesired entropy production due to uncontrolled mixing increases and some efficiency is sacrificed. When the concentration difference is increased (i.e., natural → anthropogenic → engineered salinity gradients), PRO osmotic pressure difference rises proportionally but not so for RED Nernst potential, which has logarithmic dependence on the solution concentration. Because of this inherently different characteristic, RED is unable to take advantage of larger salinity gradients, whereas PRO power density is considerably enhanced. Additionally, high solution concentrations suppress the Donnan exclusion effect of the charged RED membranes, severely reducing the permselectivity and diminishing the energy conversion efficiency. This study indicates that PRO is more suitable to extract energy from a range of salinity gradients, while significant advancements in ion exchange membranes are likely necessary for RED to be competitive with PRO.