Pressure Retarded Osmosis System Research Articles

Energy comparison and cost estimation of pressure-retarded osmosis using spiral wound membrane

Advancements in Pressure Retarded Osmosis (PRO) technology are enhancing the feasibility of evaluating its economic viability against other renewable energy production methods. This is done using the Levelized Cost of Energy (LCOE) as a metric. The study focuses on three PRO scenarios designed to minimize environmental impact and promote sustainable energy. These scenarios utilize a spiral-wound membrane module combined with hyper-saline solutions from Reverse Osmosis (RO) and wastewater from demineralization processes. Experimental results using a commercial spiral-wound membrane in the PRO system yielded LCOE values of USD 0.0702/kWh for a draw solution (DS) concentration of 36.2 g/l, USD 0.0563/kWh for 44.2 g/l, and USD 0.0721/kWh for 51.8 g/l. The study also evaluated environmental viability by considering the cost of CO2 emissions. This comprehensive comparison highlighted PRO's competitiveness with fossil fuels, showing it to be a reasonable alternative to coal and oil but less practical than natural gas. Specifically, the environmental analysis revealed that PRO is approximately 25.2 % more competitive than coal and 9.76 % more competitive than oil but 27.16 % less competitive compared to natural gas in terms of CO2 emission costs. This underscores the importance of considering carbon emission mitigation in energy generation.

Power feasibility of single-staged full-scale PRO systems with hypersaline draw solutions

Pressure retarded osmosis (PRO) is a process that allow to generate energy from osmotic gradient. This process uses selective membranes in order to produce electrical energy through a hydraulic turbine. PRO can be used as a renewable energy technology where water resources are inexhaustible. PRO has the advantage of knowing when and how much energy will be produced. Unfortunately at the moment there are certain limiting factors concerning membrane and module characteristics that have prevented PRO to be fully exploited at full-scale. This study aims to assess the impact of hypersaline draw solutions (60–180 g L−1), membrane characteristics such as structural parameter, module membrane surface and permeability coefficients on the net energy generated by single-staged full-scale PRO system with up to 8 spiral wound membrane modules (SWMMs) in series in a pressure vessel. To carry out this study, characteristics of existing PRO membranes at lab-scale were scaled up to 8 inches SWMM. The results showed the change in the optimal operating parameters with the change of membrane characteristics and draw solution concentration. This study concluded that single-staged full-scale PRO process would be viable from an energy point of view if membranes were manufactured on an industrial scale and with the characteristics of existing membranes on a laboratory scale.

Experimental study of the potential of concentrated NaCl solutions for use in pressure-retarded osmosis process

Pressure retarded osmosis is a process that enables useful work generation from the salinity difference of solutions. The literature most often considers using pressure retarded osmosis with natural sodium chloride (NaCl) solutions, such as seawater, dedicated for open systems. To explore the full potential of this process, however, optimized, highly concentrated solutions of various compounds can be used. The presented research is focused on evaluating the impact of increasing draw solution temperature and concentration on the permeate flow in the osmotic process. The permeate flow is directly related to achievable work in this process, therefore, it is important to find feed and draw solution parameters that maximize it. An experimental setup developed in this study provides full control over the process parameters. Furthermore, the performance characteristics of the membrane over process time were investigated, as it became evident during preliminary experiments that the membrane impact is significant. The studies were conducted without back-pressure, in a configuration typical of the forward osmosis process, with solution circulation on both sides of the membrane. The obtained results show a clear positive impact of both the temperature and concentration increase on the potential output of a pressure retarded osmosis system. The membrane behaviour study allowed for correct interpretation of the results, by establishing the dynamics of the membrane degradation process.

Comparison of 2D and 3D materials on membrane modification for improved pressure retarded osmosis (PRO) process

Pressure retarded osmosis (PRO) is a sustainable process that convert Gibbs free energy to osmotic energy by mixing two solutions of different salinities. The main challenges in the design of PRO membranes are obtaining a membrane with high water permeability and low salt permeability but also very high mechanical strength because the PRO process involves high pressure on the draw solution. Commercially available RO membranes with potential utility in a PRO system exhibit a high salt rejection rate but low water permeability and mechanical stability. Surface modification is a promising strategy for tuning the fundamental properties of the membranes (e.g. hydrophilicity, surface charge and thickness) that can improve the filtration performance of the membranes. The coating layer can also improve the mechanical stability of the membranes. Therefore, in this work, various types of modification materials were applied to the commercial available RO membranes to enhance their performance.With the assistance of hydrophilic materials (e.g. polydopamine – PDA), filtration performance of the membranes can be increased through membrane modification by 2D materials with high charge intensities (e.g. polyelectrolytes and graphene oxides) and by 3D mesoporous materials (e.g. zeolites), which increases the thickness of the membrane that can be beneficial in mechanically reinforcing the membrane. In this work, we modified commercial RO membrane with PDA, polyelectrolytes, graphene oxide and zeolites (ZSM-5). Improved filtration performance (increased water permeability and maintained salt permeability) of the modified membrane was observed. Tensile tests showed enhanced mechanical strength of the modified membranes, especially following 3D zeolites modification (up to 35 % of higher tensile strain was reported). Interestingly, a lower concentration of PDA (2 mg/mL) and zeolites resulted in higher mechanical strength of the modified membranes. Such results were likely due to a more homogenous coating layer when a low modifier concentration was applied. The thin and uniform layer can better absorb energy when membranes are under high pressure.

Role of permeability coefficients in salinity gradient energy generation by PRO systems with spiral wound membrane modules

Processes that can transform a salinity gradient into electrical energy have gained attention in recent years. One such process, which uses semipermeable membranes to generate electrical energy through a turbine, is pressure retarded osmosis (PRO). As a potential renewable energy technology, this process could also be integrated in desalination plants to reduce the energy consumption. However, principally because of certain drawbacks concerning membrane and module characteristics, PRO technology has not yet been fully exploited at commercial scale. This study aims to assess the impact of membrane permeability coefficients on the energy generated by full-scale single-stage PRO systems. This allow an evaluation of the performance of PRO modules in series considering variation of the permeability coefficients that may be due to the impact of fouling. An evaluation was made of the HTI OsMem™ 2521 spiral wound membrane module considering a diameter of 8 inches (high up-scaled active area) and different permeability coefficient ranges. The results showed that a 50% water permeability coefficient decrease would produce an approximately 25% decrease in the amount of energy that could be generated, while a 50% increase in the solute permeability coefficient would have virtually no effect when considering optimal operating points. Variation of the water permeability coefficient had more impact on the potential amount of generated energy than variation of the solute permeability coefficient.

The Potential of Multi-stage Pressure Retarded Osmosis for Blue Energy Harvesting

Objective: This study aims to investigate the potential benefits of using different multi-stage and multi-pass flow configurations for improving the efficiency of the pressure-retarded osmosis (PRO) process, which generates energy by utilizing the difference in osmotic pressure between two salinity solutions. Methods: To achieve the objective, the study used a numerical simulation based on the solution diffusion model. A flat sheet forward osmosis membrane was simulated to investigate the potential of the proposed multi-stage PRO configurations. Results: The results of the simulation showed that the multi-stage PRO arrangement generated 18% more power than the single-stage PRO system. Additionally, the multi-pass arrangement resulted in a 58% increase in power output. Conclusion: This study provides important insights into the development of optimized PRO membrane and module configurations to improve the efficiency of energy generation. The findings highlight the potential benefits of multi-stage and multi-pass flow configurations, which can help to advance the field of renewable energy generation using the PRO process.

Techno-Economic Analysis towards Full-Scale Pressure Retarded Osmosis Plants

Pressure retarded osmosis (PRO) is a power generation process that harnesses the salinity gradient between two water bodies of different salinities. Using high salinity water as a draw solution, this work assesses the techno-economic feasibility of the technology to generate electricity using single and multistage systems. This work utilizes a simulator built on the rigorous Q-Electrolattice equation of state and a mass transfer model that accounts for concentration polarization, combined with the Dakota optimization tool to perform sensitivity analysis and optimization studies. The economic indicator of interest is the Levelized Cost of Electricity (LCOE), which serves to compare PRO with other sources of renewable energy. An LCOE value of USD 0.1255/kWh was obtained from the use of commercial membranes at an efficiency of 100% for the mechanical components of the PRO system. This LCOE drops to USD 0.0704/kWh when an ideal membrane is used—thus showing the improvements to economics possible with improved membrane properties. With currently obtainable membrane properties and mechanical equipment, the LCOE of a single-stage process increases to USD 0.352/kWh, which is not cost-competitive with other renewable energy sources. Setting up multistage PRO systems towards minimizing the LCOE was found to be detrimental to the net power production by the plant.

Towards cost-effective osmotic power harnessing: Mass exchanger network synthesis for multi-stream pressure-retarded osmosis systems

Pressure-retarded osmosis (PRO) is a promising technique for osmotic power generation by recovering salinity gradient from high concentration effluents. Prior research has shown that the PRO systems integrating multiple draw/feed streams can achieve a 20+% increase in energy recovery compared with standalone design. However, to date few approaches have been developed to fully utilize this potential. Herein, we propose a novel method for the multi-stream PRO system that optimizes its modules (mass exchangers) layout as a mass exchanger network in three steps. In the first step, the maximum energy conversion is determined and used as an energy target for subsequent mass exchanger network synthesis. In accordance with the energy target, the second step establishes the optimal match of streams by introducing three criteria of “number of streams”, “KP coefficient”, and “maximum mass exchange”. The third step balances the system complexity and energy recovery by relaxing the energy target. An economic model is formulated to evaluate the performance of PRO systems. Several multi-stream PRO system design examples are presented to illustrate the proposed method. Results indicate that the integrated designs generally outperforms the standalone designs by 11.75–21.65 % for energy recovery, and achieves better economic performance and higher power density for large osmotic pressure difference cases with minimum net osmotic pressure difference greater than 0.8 MPa. A minimum levelized cost of energy of 0.1364 $/kWh can be achieved by the proposed design method, constituting a 6.95 % reduction compared with the standalone design.

Challenges Facing Pressure Retarded Osmosis Commercialization: A Short Review

Pressure-retarded osmosis (PRO) is a promising technology that harvests salinity gradient energy. Even though PRO has great power-generating potential, its commercialization is currently facing many challenges. In this regard, this review highlights the discrepancies between the reported power density obtained by lab-scale PRO systems, as well as numerical investigations, and the significantly low power density values obtained by PRO pilot plants. This difference in performance is mainly due to the effect of a pressure drop and the draw pressure effect on the feed channel hydrodynamics, which have significant impacts on large-scale modules; however, it has a minor or no effect on small-scale ones. Therefore, this review outlines the underlying causes of the high power density values obtained by lab-scale PRO systems and numerical studies. Moreover, other challenges impeding PRO commercialization are discussed, including the effect of concentration polarization, the solution temperature, the pressure drop, and the draw pressure effect on the feed channel hydrodynamics. In conclusion, this review sheds valuable insights on the issues facing PRO commercialization and suggests recommendations that can facilitate the successful development of PRO power plants.

Simulation tool for full-scale PRO systems using SWMMs

Pressure retarded osmosis (PRO) is a process that is able to convert a salinity gradient into electrical energy through a turbine. This process has gained attention as a possible renewable energy technology for integration into desalination plants to improve their energy efficiency. Despite recent efforts, PRO is not yet commercially available due to drawbacks related to, among others, PRO membrane and module development. The aim of this study is to provide a simulation tool for full-scale PRO systems that allows accurate estimates of PRO-related energy generation to be made. The proposed tool enables analysis of single-stage systems with PRO modules in series and the setting of boundary conditions per module in terms of maximum flux recovery, and maximum and minimum feed/draw flow. The HTI OsMem™ 2521 spiral wound membrane module (SWMM) was evaluated considering an 8 in. diameter (high active area). Increasing the number of SWMMs in series was found to increase permeate flow and the energy that can be generated, even when considering the pressure drop on both draw and feed side and the effect of the dilution and concentration of the draw and feed solutions. The proposed tool allows to determine the safe operating windows and operating points for maximization of energy generation for fixed and variable operating conditions.

An environmental and economic sustainability assessment of a pressure retarded osmosis system

Desalination can meet increasing water demands, but results in high energy consumption and the discharge of highly concentrated brine. Pressure retarded osmosis (PRO) can serve as an efficient energy recovery technology for desalination with the potential to address these challenges. This study provides an environmental and economic sustainability assessment on a hypothetical full-scale PRO system by performing a life cycle assessment (LCA) and a life cycle cost analysis (LCCA). In this hypothetical case study (in Tampa, Florida, USA), the amount of recovered energy impacted most LCA categories, indicating that the overall environmental impact would be lowered when integrated with a desalination plant. The levelized cost of energy (LCOE) ranged from 0.62 $(kWh)−1 to 0.69 $(kWh)−1 depending on the feed water source. Compared to wind power and photovoltaics, the PRO system was highly environmentally competitive due to low GHG emissions, but the baseline design was not found to be economically competitive. Lowering the LCOE would be possible when the feed solution comes from a wastewater treatment plant within 1 km and when high-quality treated wastewater is used to avoid pretreatment; this design achieved an LCOE of 0.14 $(kWh)−1. Future research is recommended on membrane advancements and maintenance improvements for fouling mitigation.

A differential evolution based henry gas solubility optimizer for dynamic performance optimization problems of PRO system

As a promising renewable energy resource, pressure retarded osmosis (PRO) is developing rapidly. Under the fluctuating environmental condition, fewer oscillations and higher convergence speeds are necessary for a stable operation of the PRO system and higher energy extraction. Metaheuristic algorithms are potential techniques for PRO at an accelerating rate, but the balance between the exploitation and exploration process is an inherent challenge in real-time efficiency and accuracy. In this work, a differential evolution (DE) based henry gas solubility optimization (EHO) is proposed for the scaled-up PRO module based on experimental data with respect to varying operational situations. In EHO, the DE mechanism and levy flight technique are applied to enhance the reliability and effectiveness of the classic HGSO strategy. The most advanced intelligent algorithms, including DFOA, GWO and WOA, are conducted for competitive research for verification purposes. Moreover, the superiority of the proposed algorithm has been evaluated and validated in complex operational environments under variations in temperature, draw concentrations and flow rates levels. The modelling results indicate that compared with the classic HGSO method, the proposed method leads to an improvement of the extracted specific energy of the PRO system by an astonishing 84.21%, 111.11% and 175.03%, respectively.

Optimizing pressure retarded osmosis spacer geometries: An experimental and CFD modeling study

Existing desalination plants face the challenges of high energy costs and environmental impacts from brine disposal. Pressure retarded osmosis (PRO) is a membrane-based technology that could mitigate these issues by capturing the potential energy in the salinity gradient between brine and dilute (e.g., freshwater/wastewater) solutions. Currently, a major challenge facing PRO is concentration polarization (CP), which is the reduction of the salinity gradient caused by the buildup of solutes along both the brine side of the membrane surface (external) and within the membrane support structure (internal). CP can be reduced using membrane spacers, but they have the drawback of increasing the pressure drop along the length of the feed channel. Therefore, it is important to design a membrane spacer that can minimize the effects of both CP and the internal pressure drop. In this study, we simulated the effect that membrane spacer geometries have on CP and the internal pressure drop via computational fluid dynamics (CFD) using the open-source software, OpenFOAM. Simulations were validated using experimental results for five physical spacer geometries on a bench-scale PRO system. The highest performing physical spacer geometry was a 47 mil (1.41 mm) thick parallel oriented spacer (47P), which was then improved upon through integrated CFD-based optimization using the open-source optimization toolkit, DAKOTA. Single-objective genetic algorithms were used to iteratively modify the spacer geometry by increasing the linearity of the thinner cross-strand and decreasing the linearity of the thicker parallel strand, which created a more hexagonally shaped cross-section. This modification was able to increase overall system performance, which was represented by the ratio of the mass flux density of the feed permeate through the membrane to the pressure drop across the length of the feed channel, by 16.3%. This suggests that a membrane spacer with a parallel hexagonal configuration allows the fluid to flow with minimal internal pressure drop while still creating the necessary back-mixing from the membrane surface to the bulk fluid needed to promote mass transfer and reduce the impacts of CP. Implementing these improvements in spacer design would cause the feed pump to consume less energy, which would increase the net energy generation potential and overall sustainability of PRO systems.

Simulation of forward osmosis and pressure retarded osmosis membrane performance: Effect of TiO2 nanoparticles loading on the semi-permeable membrane

Forward osmosis (FO) has made significant progress in recent years as an emerging membrane technology for the treatment of water and wastewater. Differential osmotic pressure is used to extract pure water across a semi-permeable membrane in this process. CFD simulations were used to investigate the performance of flux changes in membrane systems in this study. This paper simulates the performance of a thin film composite (TFC) membrane with a titanium dioxide (TiO2) modified substrate in terms of FO and pressure retarded osmosis (PRO). The results showed how the simulation results (CFD) can be used to detect flux changes with additive particles. It was discovered that even a small amount of TiO2 nanoparticles can alter water flux. In the FO and PRO systems, adding 1 wt% TiO2 to the membrane matrix increased water flux by about 15% and 14%, respectively. Nevertheless, the increased solute flux was insignificant in simulation, which indicates the positive effect of adding TiO2 nanoparticles. Finally, a comparison of the results for different membranes indicated the best correlation between the simulation and experimental results, when 0.75 wt% TiO2 was used, and a relative error of 0.3% has been recorded in PRO process. Also, the maximum pure water flux rate (8·974×10−4molm2·s)obtained in the simulation of thin film nanocomposite (TFN) 1 and PRO process.

Feasibility of Pressure-Retarded Osmosis for Electricity Generation at Low Temperatures.

A membrane-based technique for production of pressure-retarded osmosis (PRO) is salinity gradient energy. This sustainable energy is formed by combining salt and fresh waters. The membrane of the PRO process has a significant effect on controlling the salinity gradient energy or osmotic energy generation. Membrane fouling and operating conditions such as temperature have an extreme influence on the efficiency of the PRO processes because of their roles in salt and water transportation through the PRO membranes. In this study, the temperature impact on the power density and the fouling of two industrial semi-permeable membranes in the PRO system was investigated using river and synthetic sea water. Based on the findings, the power densities were 17.1 and 14.2 W/m2 at 5 °C for flat sheet and hollow fiber membranes, respectively. This is the first time that research indicates that power density at low temperature is feasible for generating electricity using PRO processes. These results can be promising for regions with high PRO potential that experience low temperatures most of the year.

Evaluations of adsorbents and salt-methanol solutions for low-grade heat driven osmotic heat engines

To recovery low temperature waste heat, a methanol-based adsorption-driven osmotic heat engine is investigated, which consists of an adsorption-based desalination system for producing concentrated and diluted solutions, and a pressure retarded osmosis system for extracting power from the Gibbs free energy of mixing. Salt-Methanol is used as working solution and the activated carbons and metal-organic frameworks are employed as the adsorbents. Impacts of desorption temperature, salt concentrations, salt types, and adsorbents on the system performance are numerically analyzed. Criteria for screening the adsorbents and salts are evaluated and discussed. Qualified adsorbents should have larger relative pressure where half of the maximum absorption uptake is achieved and moderate adsorption enthalpy. Salt-methanol solutions with larger solubility and osmotic coefficient are beneficial for improving the system energy efficiency. When operating between 60 °C and 20 °C, a maximum energy efficiency of 6.68% was achieved at 5 mol/kg LiBr-methanol solution for MAXSORB3 as the adsorbent with one-stage solid porous matrix regenerator employed. This study contributes to methanol-based adsorption-driven osmotic heat engines for efficient heat to electricity conversion, as well as providing criteria for selecting appropriate adsorbents.

The Economic Infeasibility of Salinity Gradient Energy via Pressure Retarded Osmosis

Pressure retarded osmosis (PRO) harvests the chemical potential difference between water sources of two different salinities and has been widely researched as a source of low carbon energy. This study identifies the cost-optimal design and operation using a nonlinear programming model to minimize the levelized cost of electricity (LCOE) of a PRO system for specified process and financial parameters. Our model includes a detailed finite difference module-scale model that considers the pressure drop along the module, nonconstant solution properties, local mass transfer coefficients, and equipment inefficiencies that previous analyses lacked. Under realistic case specifications and present-day costs, the expected median LCOE ranges from $2.37/kWh (seawater draw) to $0.11/kWh (Dead Sea draw). The optimistic case specifications, which assume significant improvements in process performance with no additional increase in cost, lower these median costs to $1.00/kWh (seawater draw) and $0.05/kWh (Dead Sea draw). These estimates exclude variable pretreatment costs and are likely to underestimate the actual LCOE. Membrane costs and maintenance are the primary cost components for all cases. Comparing the LCOE and capacity factor of PRO to other low carbon energy sources suggests that reductions in PRO component costs are unlikely to make PRO cost competitive with renewable energy technologies.

Evidence of solution-diffusion-with-defects in an engineering-scale pressure retarded osmosis system

An engineering-scale seawater reverse osmosis-pressure retarded osmosis (SWRO-PRO) system was designed and deployed to evaluate the energy recovery during seawater desalination through salinity gradient energy. Experimental data on energy recovery and consumption were collected from a PRO system composed of up to five 4040 spiral-wound membrane elements, utilizing freshwater and RO concentrate to drive permeation. During experimental testing, specific energy recoveries as high as 0.14 kW h/m3 were achieved; however, energy consumption due to pressure losses in the system reduced the net specific energy recovery to a maximum of −0.07 kW h/m3. A 100% increase in energy recovery or 50% decrease in energy consumption would be necessary to yield a positive net energy recovery. Also, a combined approach of simultaneously increasing energy recovery and decreasing energy consumption would be a more desirable path for SWRO-PRO to become an energy positive technology. Experimental data were then paired with modeling software utilizing a solution-diffusion-with-defects model to demonstrate the presence and extent of defects in the membrane structure that allow for pressure-driven pore flow. The solution-diffusion-with-defect model explains the lower than expected water permeability and salt rejection often seen in experimental PRO results. Integrating this model into future software will allow for more accurate simulation of larger systems and aid in investigations of PRO scale-up.

Commercial Pressure Retarded Osmosis Systems for Seawater Desalination Plants.

The development of renewable energy technologies is of global importance. To realize a sustainable society, fossil-resource-independent technologies, such as solar- and wind-power generation, should be widely adopted. Pressure retarded osmosis (PRO) is one such potential renewable energy technology. PRO requires salt water and fresh water, both of which can be found at seawater desalination plants. The total power generation capacity of PRO, using concentrated seawater and fresh water, is 3 GW. A large amount of energy is required for seawater desalination; therefore, the introduction of renewable energy should be prioritized. Kyowakiden Industry Co., Ltd., has been working on introducing PRO to seawater desalination plants since 2001 and is attracting attention for its ongoing PRO pilot plant with a scale of 460 m3/d, using concentrated seawater and treated sewage water. In this study, we evaluated the feasibility of introducing PRO in existing desalination plants. The feasibility was examined based on technology, operation, and economy. Based on the number of seawater desalination plants in each country and the electricity charges, it was determined whether the introduction of PRO would be viable.

Optimization of pressure retarded osmosis process and estimation of Indian blue energy capacity

The combination of long coastline (about 7500 km) and abundant river water (9.76 × 1011 m3/year) flowing into the sea across the southern peninsula of the Indian sub-continent provides the best conditions for blue energy extraction. The objective of this study is to estimate the Indian blue energy potential by mixing river and seawater before discharging into the sea through the optimized pressure-retarded osmosis (PRO) process. The model-based optimization approach is solved to establish an optimal process condition for integrated PRO + ERD (energy recovery device) process with respect to actual Indian river and seawater salinity. The PRO experiments are performed using syntactic seawater and river water as a draw solution (DS) and feed solution (FS), respectively. The PRO process optimization strategy is proposed to maximize the net power produced using river and seawater by manipulating operating conditions and module length. Three different objective functions are evaluated to maximize the extraction of the blue energy generation. The optimized PRO process was able to generate a net power of 0.07 kWh per 1m3 of river water supplied to the PRO system. The blue energy generation from the Indian major rivers discharging along the Indian coastal area was estimated to be 12.34 TWh/year.