Evaluation of Pressure Retarded Osmosis for Energy Generation from Mine Water

Energy shortage and water scarcity are worldwide challenges, which drive the exploitation of sustainable alternative energy sources. Among various options, osmotic energy has attracted much attention in recent years as a renewable energy that could be harvested by a pressure retarded osmosis (PRO) process, where energy is produced when water permeates naturally from a low salinity stream to pressurized high salinity stream. However, challenges like concentration polarization, membrane fouling and lack of commercial PRO membranes have greatly impeded the practical application of PRO process for energy harvesting. Therefore, proper design of optimized PRO modules with high performance is vital for practical PRO operations. Firstly, a novel thin-film composite hollow fiber PRO membrane with high PRO performance was fabricated. As the hollow fiber membrane is made from polymeric materials, it may gradually deform over time under high pressure loading, or membrane “creeping”, where hollow fiber membranes are severely stretched due to applied pressure in the lumen and thus lose its selectivity, will occur. An attempt to analyze the membrane creeping phenomenon was conducted. The membrane creeping was evaluated via nanoindentation by using an atomic force microscopic (AFM) technique. A non-stop 200 hours PRO test and integrity evaluation were carried out to investigate the membrane performance under various operating pressures. The results showed that the membrane was able to produce a stable power density output of 19.2 W/m2 at 15.0 bar, using 1.0 M NaCl as the draw solution and DI water as the feed water. Membrane creeping was observed when the applied pressure exceeded the safe operation limit or the flux turning point, where the membrane flux started to increase with increasing applied pressure in the PRO mode. This caused an irreversible damage to the membranes. Essentially, this work identified a safe operating boundary for the in-house produced PRO hollow fiber membranes so as to achieve optimized PRO performances. It also provides guidance for practical applications of polymeric hollow fiber membranes in the PRO process. Secondly, the feasibility of the produced PRO membranes for practical PRO operation was examined. It was found that the fouling was severe when a real wastewater retentate from a local water reclamation plant was used as the feed solution of the PRO process. Therefore, low pressure nanofiltration (NF) pretreatment was adopted to treat the wastewater brine prior to feeding it into the PRO process. Three NF membranes were compared in terms of their membrane properties, quality of the NF permeates (i.e., PRO feed) and PRO membrane performances. Results showed that the PRO water flux could increase to 30.5 L/m2h at 16 bar applied pressure by using in-house made low-pressure NF hollow fiber membranes on the pretreated solution, in contrast to a lower water flux of 9 L/m2h in the case of untreated wastewater retentate. A systematic analysis of the water chemistry and various membrane characterization such as electron dispersed X-ray (EDX) and X-ray photoelectron spectroscopy (XPS) depth profiling has revealed that calcium salts, organic compounds, and silica were main contributors of membrane fouling in the PRO process. Low-pressure NF was found to be able to mitigate the fouling potential from multivalent ions and organic matters, but silica scaling in PRO remains a challenge which needs to be further addressed. Furthermore, the PRO membranes were also scaled-up to a two-inch module from a lab-scale half-inch module for a pilot-scale study. The produced PRO modules have a maximum effective area of 0.5 m2. By assessing the PRO performances of the modules with different sizes, external concentration polarization (ECP) was found to have significant impact on the flux reduction during module scale-up. Different module designs, including fiber bundles, distribution baffles and distribution tubes, were thus adopted as an attempt to boost the membrane performance. A power density of 8.9 W/m2 at 15 bar was obtained using tap water as feed and 1M NaCl solution as draw solution. PRO was also carried out using the developed two-inch module on a pilot-scale setup with actual wastewater retentate as feed solution. Low pressure nanofiltration was selected as the pretreatment of the wastewater retentate to mitigate fouling. A power density of larger than 8 W/m2 was obtained when pretreated wastewater retentate was used as the feed solution, implying high potential of PRO in the pilot scale. Nevertheless, full potential of PRO can only be realized by mitigating ECP, which could be achieved by proper module designs and will be conducted in the further endeavor.

Pre-treatment of wastewater retentate to mitigate fouling on the pressure retarded osmosis (PRO) process

The global issues of water scarcity and climate change are driving the utilities sector, primarily water and electricity, to transit toward more sustainable sources. Water supply increasingly relies on energy-intensive desalination in recent years, hence renewable energy is suggested to be used as its power origin under the decarbonization framework. However, the intermittent availability of most renewables (e.g., solar energy and wind energy) escalates the gap between supply and demand in not only electricity but also water sectors. To address this issue, osmotic energy becomes a promising choice as it can be stored easily by depositing two solutions with salinity gradient in separate containers and converted into electricity when needed by pressure-retarded osmosis (PRO). However, the low capacity and efficiency of conventional single-stage PRO (SSPRO) processes deteriorate its practical feasibility. First of all, two novel PRO configurations and operating modes, atmospheric batch PRO (ABPRO) and semi-closed PRO (SCPRO), are designed to improve the energy production performance of PRO. Both ABPRO and SCPRO can realize 100% energy production efficiency (EPE) ideally facilitated by the multi-cycle operation and variable-pressure mode. However, a significant decay of performance is observed in all PRO processes under practical conditions. ABPRO shows a comparable performance with SSPRO at high PRO water recoveries (PR) due to the entropy generation caused by mixing. Due to the avoidance of mixing by storing the influent and effluent of draw solution (DS) in two separate tanks, SCPRO features the highest SEP and EPE at PR > 0.3. Utilizing a 1.2 M NaCl solution as the DS and a 0.05 M NaCl solution as the feed solution (FS), the EPE of SCPRO approaches 67.6% at PR = 0.5, which is approximately 15% higher than SSPRO and 8% higher than ABPRO. Furthermore, effective solutions to achieve the coordinated management of energy and water are crucial to establish a sustainable society. Therefore, a desalination-osmotic energy storage (DOES) system integrating reverse osmosis (RO) and PRO is proposed in this study to further enhance the practical feasibility of PRO and strengthen the water-energy resilience. The DOES system features multiple functions, which uses the renewables or waste energy as the power origin to produce water via RO and utilizes partial RO permeate and brine to generate electricity via PRO. Employing the superior semi-closed (SC) mode in DOES substantially reduces energy losses arising from over-pressurization in RO and under-pressurization in PRO. Benefiting from the variable-pressure mode, the DOES system is characterized by 100% energy efficiency in ideal scenarios. The practical maximum efficiency can be maintained at ~75% in the SC mode for both RO and PRO stages at high RO recovery (R) and permeate utilization ratios (∅), attributed to the flexible adjustment of the number of cycles in the SC mode for performance optimization. The specific energy production (SEP), termed as the total energy normalized by the RO permeate volume, approaches 0.48 kWh·m-3 at R = 0.5 and ∅ = 0.5, which is doubled when R and ∅ increased from 0.5 to 0.75. Finally, the module-scale performance of the DOES system is simulated with the comprehensive consideration of inefficiencies. The results reveal that an increase in R can lead to higher SEC, SEP, and PD simultaneously. However, given the SEP defined as the total energy production normalized by the volume of RO permeate used in PRO in this chapter, it may exhibit an initial rise followed by a decline when ∅ increases. In addition, an increase in PD due to the rise of water flux may result in a deterioration of energy efficiency. Nevertheless, this trade-off between PD and energy efficiency can be mitigated by operating DOES at a higher R. As R rises from 0.5 to 0.75, the EPE, SEP and PD escalate from 62.6%, 0.83 kWh·m-3 and 11.6 W·m-2 to 65.3%, 1.29 kWh·m-3 and 18.1 W·m-2, respectively, At a constant ∅ of 0.50 and a fixed water flux of 14 LMH. The energy production capacity at R = 0.75 is comparable to that of a pumped hydro system with a vertical height of approximately 530 meters while DOES has less topographical constraints and construction loads. Given its various performance advantages and multi-functionality, the DOES system with the novel SC mode can potentially enable it to effectively enhance energy and water resilience and contribute to a low carbon future.

Chapter 11 - Fundamentals of Pressure Retarded Osmosis

Organic fouling in pressure retarded osmosis: Experiments, mechanisms and implications

Recently, reverse osmosis (RO) is the most common process for seawater desalination. A common problem in both RO and thermal processes is the high energy requirements for seawater desalination. The one energy saving method when utilizing the osmotic power is utilizing pressure retarded osmosis (PRO) process. The PRO process can be used to operate hydro turbines for electrical power production or can be used directly to supplement the energy required for RO desalination system. This study was carried out to evaluate the performance of both single-stage PRO process and two-stage PRO process using RO concentrate for a draw solution and RO permeate for a feed solution. The major results, were found that increase of the draw and feed solution flowrate lead to increase of the production of power density and water permeate. Also, comparison between CDCF and CDDF configuration showed that the CDDF was better than CDCF for stable operation of PRO process. In addition, power density of two-stage PRO was lower than the one of single-stage. However, net power of two-stage PRO was higher than the one of single-stage PRO.

Energy can be generated from two streams of different salt concentration using the osmotic pressure difference. Different methods have been proposed to harvest this energy. Pressure retarded osmosis (PRO) is investigated as a viable method and most promising technology. In PRO process, pure water permeates through a semi permeable membrane from the low hydrostatic pressure stream (feed solution) to the higher hydrostatic pressure stream (draw solution) due to the osmotic pressure difference. This increases the volume flow rate of the pressurized draw stream and energy is obtained by depressurizing the draw stream through a hydro turbine. In this study a one-dimensional computational model is developed to precisely estimate the power production under different operating conditions. Different feed and draw solution concentrations are used to estimate the power production from PRO. The maximum power density (power per unit membrane area), using available membrane characteristics, obtained from seawater–freshwater streams is 2.6 W/m2 and for the disposed brine–seawater streams is 9.1 W/m2. The performance of PRO process is very sensitive to the membrane characteristics in particular to the water permeability and PRO module configuration.

Pressure Retarded Osmosis (PRO): Past experiences, current developments, and future prospects

Osmotic power generation by pressure retarded osmosis using seawater brine as the draw solution and wastewater retentate as the feed

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

【Pressure retarded osmosis (PRO) process is one of membrane processes for harvesting renewable energy by using salinity difference between feed and draw solutions. Power is generated by permeation flux multiplied by hydraulic pressure in draw side. Membrane fouling phenomena in PRO process is presumed to be less sever, but it is inevitable. Membrane fouling in PRO process decreases water permeation through membrane, resulting in significant power production decline. This study intended to investigate the effect of hydraulic pressure in PRO process on alginate induced organic fouling as high and low hydraulic pressures (6.5 bar and 12 bar) were applied for 24 h under the same initial water flux. In addition, organic fouling in draw side from the presence of foulant (sodium alginate) in draw solution was examined. As major results, hydraulic pressure was found to be not a significant factor affecting in PRO organic fouling as long as the same initial water flux is maintained, inidicating that operating PRO process with high hydraulic pressure for efficient energy harvesting will not cause severe organic fouling. In addition, flux decline was negligible from the presence of organic foulant in draw side.】

Evaluation of apparent membrane performance parameters in pressure retarded osmosis processes under varying draw pressures and with draw solutions containing organics

Potential of osmotic power generation by pressure retarded osmosis using seawater as feed solution: Analysis and experiments

Impact of hydraulic pressure and pH on organic fouling in pressure retarded osmosis (PRO) process

Osmotic power production from salinity gradient resource by pressure retarded osmosis: Effects of operating conditions and reverse solute diffusion