WO2023107006A2

WO2023107006A2 - Pressure retarded osmosis and systems integrating it for osmotic energy harvesting and storage

Info

Publication number: WO2023107006A2
Application number: PCT/SG2022/050886
Authority: WO
Inventors: Qianhong SHE; Dan Li
Original assignee: Nanyang Technological University
Priority date: 2021-12-06
Filing date: 2022-12-06
Publication date: 2023-06-15
Also published as: WO2023107006A3

Abstract

A pressure retarded osmosis (PRO) method and system, and systems integrating the PRO system for osmotic energy harvesting and storage, are disclosed herein. The PRO system is configurable as a semi-closed PRO system. The PRO system may be part of an integrated multi-functional desalination-osmotic energy storage (DOES) system. The system may be further configured to switch between any of a semi-closed PRO mode, a closed-loop PRO mode, a closed-circuit PRO mode, a semi-closed reverse osmosis (RO) mode, a closed-circuit RO mode, and a closed-loop RO mode.

Description

PRESSURE RETARDED OSMOSIS AND SYSTEMS INTEGRATING IT FOR OSMOTIC ENERGY HARVESTING AND STORAGE

The present application claims priority to the Singapore patent application no. 10202113519U filed on 6th December 2021, the contents of which are hereby incorporated by reference in entirety for all purposes.

TECHNICAL FIELD

[0001] The present disclosure relates to pressure-retarded osmosis, and more particularly to osmotic energy systems integrating pressure-retarded osmosis.

BACKGROUND

[0002] Osmotic energy, also called osmotic power, salinity-gradient energy, salinitygradient power or blue energy, is a renewable energy that may be generated from the mixing of two solutions with different salinities, such as the mixing of river water and seawater at estuaries, which can be extracted by pressure retarded osmosis (PRO) processes. However, the exploitation of osmotic energy is relatively under-explored owing to the low efficiency of conventional PRO processes.

SUMMARY

[0003] In one aspect, the present application discloses a system, the system comprising: a system, comprising: at least one membrane module, the at least one membrane module including: a high salinity (HS) channel configured to receive a HS influent stream for a HS stream in the at least one membrane module, the HS stream being configured to exit the at least one membrane module as a HS effluent stream; a low salinity (LS) channel configured to provide a LS flow path for a LS stream in the at least one membrane module, the HS stream having a level of salinity or an osmotic pressure that is higher than the LS stream; a membrane disposed between the HS channel and the LS channel, the membrane being a selectively permeable membrane, and in a cycle of an operation, a HS liquid flows through the HS channel and a LS liquid flows through the LS channel, the cycle being characterized by an osmotic pressure difference across the membrane between the HS stream and the LS stream; at least one HS tank, each of the at least one HS tank defining a maximum volume of the liquid processable by the system in the cycle, the at least one of the HS tank being in fluidic communication with at least one of the HS influent stream and the HS effluent stream; a first energy recovery device, wherein the first energy recovery device is configured to at least partially pressurize the HS influent stream based on energy recovered from at least a part of the HS effluent stream; and a HS fluid circuit coupling the at least one HS tank with the HS channel and the first energy recovery device, wherein the fluid circuit is configurable to provide a flow path through the HS channel for the cycle, wherein the system is operable with one HS tank in use or with two HS tanks in use, and wherein the system is configured to apply a hydraulic pressure to the HS influent stream, and wherein the hydraulic pressure is controllably variable within a range below the osmotic pressure difference across the membrane.

[0004] The system may further comprise at least one LS tank, wherein the at least one LS tank defines a maximum volume of the LS liquid processable by the system in the cycle. The system in which the at least one LS tank may be in fluidic communication with the LS influent stream and the LS effluent stream.

[0005] The system may further comprise a second energy recovery device coupled in parallel with the first energy recovery device, wherein each of the first energy recovery device and the second energy recovery device is operable to recover energy from different streams received from the HS effluent stream. The second energy recovery device may be configured to transfer the energy recovered from at least a part of the HS effluent stream to a third system, or wherein the second energy recovery device comprises a turbine, the turbine being operable based on energy recovered from at least a part of the HS effluent stream. In some examples, the second energy recovery device is a pressure exchanger. The system, wherein the operation may comprise at least two cycles, and wherein in a first of the at least two cycles, the HS influent stream may be supplied from an external source.

[0006] The system may be operable with the one HS tank in use, and wherein the HS effluent stream in a first of two cycles of a two-cycle operation is received by the one HS tank in use, and wherein in a second of the two cycles of the two-cycle operation, the HS influent stream is supplied from the one HS tank in use. In a last cycle of the two cycles, a final HS effluent stream is discharged out of the system. [0007] The system, wherein the system is operable with the two HS tanks in use, and wherein the HS effluent stream in a first cycle of at least three cycle is received by a first of the two HS tanks in use, and wherein in a second of the at least three cycles, the HS influent stream is supplied from the first of the two HS tanks in use, and wherein the HS effluent stream is received by a second of the two HS tanks in use, and wherein the first of the two HS tanks in use and the second of the two HS tanks in use alternately function as the HS influent tank and HS effluent tank, respectively in subsequent cycles prior to a last cycle of the at least three cycles. In the last cycle, the HS influent stream is supplied from the first of the two HS tanks in use if a total number of the at least three cycles is even and the HS influent stream is supplied from the second of the two HS tanks in use if the total number of the at least three cycles is odd, and wherein a final HS effluent stream is discharged out of the system.

[0008] The system in which the operation comprises at least two cycles, and wherein the osmotic pressure of the HS influent stream is constant in each of the at least two cycles, and wherein the osmotic pressure of the HS influent stream decreases between successive cycles of the at least two cycles.

[0009] The system, wherein the system is operable in a semi-closed (SC) pressure retarded osmosis (PRO) mode in which the hydraulic pressure is held constant during the cycle, and wherein the hydraulic pressure is changed between successive cycles.

[0010] The system described above, wherein the system is operable with the one HS tank in use, and wherein a volume of an initial HS liquid supplied from an external source is stored in the one HS tank in use, and wherein the HS influent stream is supplied to the HS channel and the HS effluent stream is received by the one HS tank in use, and wherein a final HS effluent in the one HS tank in use and in the HS channel is discharged out of the system.

[0011] The system being operable in a closed-loop PRO (CLPRO) mode in which a hydraulic pressure is applied to the HS influent stream, and wherein the hydraulic pressure is controllably variable within a range below the osmotic pressure difference across the membrane, and wherein the osmotic pressure of the HS influent stream and the osmotic pressure of the HS effluent stream both decrease continuously over a series of cycles. [0012] The system, wherein an initial HS influent is supplied from an external source to the HS channel without entering any of the at least one HS tank, and wherein at least one part of the HS effluent stream is circulated by a pump as the HS influent stream without going through a first energy recovery device. The system, wherein the at least one part of the HS effluent stream is in fluid communication with a second energy recovery device for energy production before being discharged out of the system, and wherein a final HS effluent in the HS channel is discharged out of the system without entering any of the at least one HS tank.

[0013] The system operable in a closed-circuit PRO (CCPRO) mode in which the HS influent stream is characterized by a hydraulic pressure that is lower than the osmotic pressure difference across the membrane, and wherein the hydraulic pressure is controllably variable within a range below the osmotic pressure difference across the membrane.

[0014] The system being configured to enable switching between any of the following: a semi-closed pressure retarded osmosis (SCPRO) mode, a closed-loop pressure retarded osmosis (CLPRO) mode, and a closed-circuit pressure retarded (CCPRO) mode, to controllably supply usable energy.

[0015] The system, further comprising a pump configured to contribute to the hydraulic pressure of at least one part of the HS influent stream independent of energy recovered from at least one part of the HS effluent stream, the system being operable in a reverse osmosis (RO) mode, and wherein the HS effluent stream is in exclusive fluidic communication with the first energy recover device, and wherein the at least one LS tank is not essential in the individual RO processes since the freshwater produced by an RO process can be supplied for use directly. In embodiments of the system having at least one LS tank for freshwater storage, the LS tank may be in exclusive fluidic communication with the LS effluent, and wherein the HS influent stream is characterized by a hydraulic pressure that is higher than the osmotic pressure.

[0016] The system may be operable in a semi-closed reverse osmosis (SCRO) mode in which the hydraulic pressure is controllably variable within a range above the osmotic pressure, and wherein the operation comprises at least two cycles, and wherein the hydraulic pressure is constant in each of the at least two cycles, and wherein hydraulic pressure is changed between successive cycles of the at least two cycles, and wherein the osmotic pressure of HS influent stream is constant in each of the at least two cycles, and wherein the osmotic pressure of the HS influent stream increases cycle by cycle.

[0017] The system may be operable in a closed-loop reverse osmosis (CLRO) mode in which the only one HS tank in use functions as the HS influent supplier and the HS effluent receiver, and wherein the hydraulic pressure is controllably variable within a range above the osmotic pressure.

[0018] The system, wherein the system is operable in a closed-circuit reverse osmosis (CCRO) mode in which at least one part of the HS effluent stream is circulated by a pump as the HS influent stream without going through a first energy recovery device, and wherein at least one part of the the fresh HS influent stream is supplied by the external source continuously to compensate for a volume of water permeating from the HS channel to the LS channel, and wherein the hydraulic pressure is controllably variable within a range above the osmotic pressure.

[0019] The system, configured to be sequentially or concurrently operable in a reverse osmosis (RO) mode and a pressure retarded osmosis (PRO) mode, wherein the RO mode is any one of a semi-closed pressure retarded osmosis (SCRO) mode, a closed-loop RO (CLRO) mode, a closed-circuit RO (CCRO) mode, and wherein the PRO mode is any one of a semi-closed PRO (SCPRO) mode, a closed-loop PRO (CLPRO) mode, and a closed- circuit PRO (CCPRO) mode.

[0020] The system having at least two HS tanks in use being configured to operate as an energy storage system in which the system is operable in the RO mode to convert an input energy into osmotic energy in form of a salinity difference between a final concentrated HS effluent and a final LS effluent, and wherein the system is operable in the PRO mode to convert the osmotic energy to electricity. The system in the RO mode may be configured to charge the energy storage system, and wherein the system in the PRO mode is configured to discharge the energy storage system. The system in which at least a part of the initial HS liquid is stored in the first of the at least two HS tanks, and wherein a part of the initial LS liquid is stored in the at least one LS tank prior to the energy storage system being discharged in the PRO mode. The system in the RO mode may be configured to produce the LS liquid in response to the at least one LS tank being empty of liquid. [0021] In a first charging-discharging cycle, the initial HS influent stream is supplied by the first of the two HS tanks for RO if charging is performed firstly or for PRO if discharging is performed firstly, wherein the initial HS influent stream of PRO is the final HS effluent of RO if charging is performed firstly, and wherein the initial HS influent stream of RO is the final effluent of PRO if the discharging is performed firstly, and wherein in any subsequent charging-discharging cycle, the final HS effluent of RO is the initial HS influent of PRO, and wherein the final HS effluent of PRO is the initial HS influent of RO.

[0022] The system, wherein the final HS effluent in both RO and PRO process is received by one of the at least two HS tanks.

[0023] The system in which the energy storage system has at least one HS tank, the energy storage system further comprising two HS reservoirs, wherein the initial HS influent stream supplied by a first of the two HS reservoirs and a final HS effluent stream is received by a second of the two HS reservoirs if the system is in the RO mode, and wherein the initial HS influent stream is supplied by the second of the two HS reservoirs and the final HS effluent stream is received by the first of the two HS reservoirs if the system is in the PRO mode. Each of the at least one HS tank has a smaller volume than that of either of the two HS reservoirs is used to provide intermediate storage and supply of the HS stream. The at least one membrane module may be configured to be operable in one of the PRO mode and RO mode. The system in which a first of the at least one membrane module is configured as a PRO membrane module, and wherein a second of the at least one membrane module is configured as a RO membrane module.

[0024] The system further comprising one or more treatment units, the one or more treatment units being configured to treat one or more streams upstream and/or downstream of the at least one membrane module.

BRIEF DESCRIPTION OF DRAWINGS

[0025] Fig. 1 is a schematic diagram of a closed-loop pressure retarded osmosis (CLPRO process;

[0026] Fig. 2 is the schematic diagram showing a semi-closed pressure-retarded osmosis (SC-PRO) system according to embodiments of the present disclosure;

[0027] Fig. 2A is the schematic diagram of the SCPRO process for energy production in the first cycle of a series of cycles;

[0028] Fig. 2B is the schematic diagram of the SCPRO process for energy production when the cycle number is even in a series of cycles, prior to a last cycle;

[0029] Fig. 2C is the schematic diagram of the SCPRO process for energy production when the cycle number is odd in a series of cycles, prior to a last cycle;

[0030] Fig. 2D is the schematic diagram of the SCPRO process for energy production in the last cycle when the total number of cycles is even in a series of cycles;

[0031] Fig. 2E is the schematic diagram of the SCPRO process for energy production in the last cycle when the total number of cycles is odd in a series of cycles;

[0032] Fig. 2F is the schematic diagram of the SCPRO process at the beginning of each operation to replenish the pipe and PRO membrane module with fresh HS solution;

[0033] Fig. 2G is a schematic diagram of another example of the system of Fig. 2 integrated with a second energy recovery device of another system;

[0034] Fig. 3 is a schematic diagram of the another embodiment of the system in a closed- circuit PRO (CCPRO) mode;

[0035] Fig. 4 is a schematic diagram of the present system that can operate in various reverse osmosis (RO) modes (including SCRO, CLRO, CCRO) and in various PRO modes (including SCPRO, CLPRO, CCPRO);

[0036] Fig. 4A is a schematic diagram of the system of Fig. 4 in a SCRO mode;

[0037] Fig. 4B is a schematic diagram of the system of Fig. 4 in a CLRO mode;

[0038] Fig. 4C is a schematic diagram of the system of Fig. 4 in a CCRO mode;

[0039] Fig. 5 is a schematic diagram of an osmotic battery system according to one embodiment, in which the system integrates RO and PRO to serve as an RO-PRO integrated osmotic battery system with two HS reservoirs and one LS reservoir;

[0040] Fig. 6 is a schematic diagram of another embodiment of the RO-PRO integrated osmotic battery system with two HS reservoirs, two intermediate HS tanks, and one LS reservoir;

[0041] Fig. 7 is a schematic diagram of another embodiment of the RO-PRO integrated osmotic battery system with separate membrane modules for RO and PRO;

[0042] Fig. 8 is a schematic diagram of a desalination-osmotic energy storage (DOES) system;

[0043] Fig. 9 is the osmotic membrane system with pretreatment and posttreatment in the LS side;

[0044] Fig. 10 is a diagram showing the specific energy production (SEP) and energy production efficiency (EPE) of SCPRO under different conditions;

[0045] Fig. 11 is a diagram showing a comparison of SEPs between different PRO processes; and

[0046] Fig. 12 is a diagram showing the roundtrip efficiency of the RO-PRO integrated system for energy storage..

DETAILED DESCRIPTION

[0047] Reference throughout this specification to “one embodiment”, “another embodiment” or “an embodiment” (or the like) means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment. Thus, the appearance of the phrases “in one embodiment” or “in an embodiment” or the like in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the described features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. In the following description, numerous specific details are provided to give a thorough understanding of embodiments. One skilled in the relevant art will recognize, that the various embodiments be practiced without one or more of the specific details, or with other methods, components, materials, etc. In other instances, some or all known structures, materials, or operations may not be shown or described in detail to avoid obfuscation.

[0048] The word “exemplary” is used herein to mean “serving as an example, instance, or illustration.” Any embodiment described herein as “exemplary” is not necessarily to be construed as preferred or advantageous over other embodiments. As used herein, the singular ‘a’ and ‘an’ may be construed as including the plural “one or more” unless apparent from the context to be otherwise.

[0049] The terms "about" and "approximately" as applied to a stated numeric value encompasses the exact value and a reasonable variance as will be understood by one of ordinary skill in the art, and the terms “generally” and “substantially” are to be understood in a similar manner, unless otherwise specified.

[0050] It will be understood by one skilled in the art that drawings of the various exemplary systems appended herewith are simplified diagrams to avoid obfuscation. In actual implementation, there may be additional or alternative components (such as but not limited to pipelines, pumps, valves, etc.) which are either not shown in the appended drawings or represented by dotted/dashed lines to better aid understanding. In the present disclosure, the term “continuous process” (not to be confused with the term “continuously”) refers to an operation in which all the liquid passing through a membrane module is freshly drawn from a source external of the system. In contrast, in a “non-continuous” operation, at least a part of the liquid passing through a membrane is drawn from within the system, e.g., from a tank or a reservoir forming part of the system. The terms “batch” and “closed-loop” may be used interchangeably in the present disclosure to describe a non-continuous operation in which liquid in the system is passed at least twice through a membrane module before the liquid is discharged out of the system or before the system takes in a fresh feed. In contrast to a batch operation, a multi-stage operation involves a continuous flow of liquid from one membrane module to another membrane module, i.e., the output from one membrane module is the input to a different membrane module. The term “semi-closed” as used herein refers to a non-continuous operation in which the liquid is drawn from a tank or reservoir and passed more than once through the membrane module (similar to a “batch” operation) before the liquid is returned to the same tank or reservoir. For the sake of brevity, as used herein, “PRO” refers to “pressure retarded osmosis”, “RO” refers to reverse osmosis, “SC” refers to “semi-closed”, “CL” refers to “closed loop, “CC” refers to closed circuit, and combinations thereof are to be understood in a similar manner.

[0051] Fig. 1, for example, is a schematic diagram of at least a part of a system 100 according to at least one embodiment of the present disclosure. The system 100 includes at least one membrane module 200. To avoid obfuscation, Fig. 1 shows an example with one membrane module 200. The membrane module 200 includes a high salinity (HS) channel 230 and a low salinity (LS) channel 250, with a membrane 240 disposed between the HS channel 230 and the LS channel 250. The membrane module 200 may be selected from various types of membrane modules, including but not limited to spiral-wound modules, hollow fiber membrane modules, etc. The membrane 240 is a semi-permeable membrane, i.e., the membrane 240 is selectively permeable such that it enables permeation of water (or solvent) while preventing solutes from passing therethrough. That is to say, the membrane 240 is a selectively permeable membrane that only allows solvent to permeate through but rejects solute permeation.

[0052] A plurality of valves (valve 405, valve 406, valve 408, valve 423, valve 424, valve 425, valve 426, valve 427, valve 429) may be used to control the flow path of the HS influents streams and the HS effluent streams. The paired valves may optionally be replaced by three-way switches that can also be used to govern flow directions. Additionally, upstream of the energy recovery device 106 and a high-pressure pump 411, a low-pressure pump (not shown) can be installed for the intake of the HS solution. It is not indispensable in general, but preferred in large-scale applications.

[0053] For ease of reference, the system 100 may be described as being composed of a HS side 103 and a LS side 105, with the HS channel 230 forming part of the fluid circuit on the HS side 103, and with the LS channel 250 being part of the LS side 105. The LS side 105 may be used in a variety of configurations, as will be made clear from the following description. In operation, the LS stream 415 in the LS channel 250 is characterized by a level of salinity lower than that of the HS stream 413 in the HS channel 230. In this example, the system 100 is shown operating in a pressure-retarded osmosis (PRO) mode in which a volume of water from the LS channel 250 is drawn across the membrane 240 to the HS channel 230. The LS effluent stream 433 out of the LS channel 250 is characterized by a higher degree of salinity than the LS influent stream 431. The LS influent stream 431 may be delivered by a pump 432 to the LS channel 250. The LS effluent 433 may be discharged directly.

[0054] As with the LS side 105 of the system 100, the HS side 103 of the system 100 may be used in a variety of configurations. For example, the HS side 103 may include one or more HS tanks 300. In one configuration, only one HS tank 310 (300) is in use. Any other HS tank 320 may be disregarded by closing off the pipelines leading to/from the any other tank 320 throughout the PRO process such that the system 100 essentially operates with only one tank on the HS side 103. In this example, by closing the valve 406, valve 423, and valve 425, a second HS tank 320 will be closed off, leaving a first HS tank 410 in use throughout the operation. For the sake of brevity, in the present disclosure, reference to a system having only one tank (such as the system 100 of Fig. 1) may refer to a system that may be physically coupled to one or more than one tank and in which all but one tank are closed off from use throughout the entire operation of the system. In other examples, more than one HS tank 300 is used. It will also be understood by one of ordinary skill in the art that there may be one or more feed inlets 407 and one or more discharge outlets 427/429, such that the point at which fresh feed is added to the system 100 and/or fluid is discharged from the system 100 can be controlled. In other words, the same system 100 can be configured to implement different embodiments of a PRO process.

[0055] The PRO process enabled by the system 100 is characterized by a volume of a solution that can be cycled through the HS channel 230 of the membrane module 200 for an N number of cycles, with the osmotic pressure difference (across the membrane 240). For PRO to occur, the hydraulic pressure applied to the HS influent stream 412 (at the inlet of the HS channel 230) is kept below the osmotic pressure difference across the membrane 240. In other words, according to embodiments of the present disclosure, the maximum hydraulic pressure that is applied at the Nth cycle is lower than the maximum hydraulic pressure that is applied at the (N-l)th cycle.

[0056] One configuration of the system 100 that produces a decreasing osmotic pressure difference across the membrane 240 is shown in Fig. 1. Throughout the whole PRO operation, also referred to as a closed-loop PRO (CLPRO) operation, the HS effluent stream 416 is discharged to the same HS tank 310 and mixes with the residual HS solution in the tank. That is, the HS influent stream 403/410/412 is continuously diluted throughout the PRO operation. The hydraulic pressure applied to the HS influent stream 412 may be provided by a pressure exchanger 409 and a pump 411. Energy recovery devices, such as a pressure exchanger 409 and a hydro-turbine 420, are arranged in the HS side 103 for pressure (or mechanical) energy recovery and electrical power generation, respectively. The pressure exchanger 409 is configured to at least partially pressurize the HS influent stream 412 based on energy recovered from at least a part of the HS effluent stream 418. At least a part of the energy extracted from the HS effluent stream 419 out of the membrane module 200 may be used to operate a hydroturbine 420 to generate electricity and to transfer the energy to a third system. Optionally, the HS effluent stream 416 may be split into multiple streams 418/419 by a flow controller 417. The flow rate of the HS effluent stream 418 directed towards the pressure exchanger is preferrably to equal to that of the influent stream 403 flowing to the pressure exchanger 409, while the flow rate of the effluent stream 419 flowing to the hydro-turbine 420 is preferably about the same as the permeate flow rate in the membrane module 200. In addition, due to the inevitable energy loss in the pressure exchanger 409, the circulation pump 411 (which also serves as a booster pump) is positioned downstream of the pressure exchanger 409 it to increase the hydraulic pressure of the HS influent stream 412 to the target hydraulic pressure value.

[0057] Another preferred embodiment of the system 100 that can be used to perform a PRO operation of the present disclosure is schematically illustrated in Fig. 2. The system 100 includes a first HS tank 310 and a second HS tank 320, which are in liquid communication with the membrane module 200 and defines a maximum volume of a liquid processable by the system 100 in a first cycle of the liquid through the HS channel 230. The first HS tank 310 and the second HS tank 320 are configured for the storage and collection of HS influents and effluents alternately in each cycle of the PRO process.

[0058] For the sake of brevity, similar aspects of different embodiments will not be repeated. For example, it can be appreciated that the system 100 of Fig.2 similarly includes a plurality of valves (valve 405, valve 406, valve 408, valve 423, valve 424, valve 425, valve 426, valve 427, valve 429) for controlling the flow path of HS influents (stream 403, stream 404 and stream 407) and HS effluents (stream 421 and stream 422).

[0059] As illustrated in Fig. 2A, in the first cycle of the PRO process proposed herein, also referred to as a semi-closed PRO (SC-PRO) process, the valve 408, valve 424 and valve 426 are open, and the other valves are closed. The fresh HS solution (stream 407) is directed into the pressure exchanger 409, in which the pressures of the stream 407 and the HS effluent (stream 418) are exchanged. After the further elevation of pressure in stream 410 by the booster pump 411, the stream 412 with a hydraulic pressure lower than the osmotic pressure difference across the membrane 240 is transported into the HS channel 230. Water molecules (or solvent) pass through the membrane 240 from the LS channel 250 to the HS channel 230, resulting in an increased volume of the pressurized HS solution. The diluted HS effluent (stream 416) may be separated into two parts by the flow controller 417. One of the stream (stream 418) at the same flow rate as the stream 412 is transferred to the pressure exchanger 409 for energy exchange. The other one (stream 419) at the same flow rate of permeate flows through the hydro-turbine 420 to produce electricity. The HS effluents after flowing through the two energy recovery devices are discharged to the HS tank 310. As for the LS side, the LS solution 431 is continuously supplied into the LS channel 250 by the circulation pump 432.

[0060] In the second cycle, asillustrated by Fig. 2B, the HS tank 310 becomes the HS influent container (feed tank), and the HS tank 2 is used to store the HS effluent (receiver tank). Therefore, the valve 405, valve 423 and valve 425 are open, the others are closed. The PRO process is similar to the previous cycle. The stream 403 is pumped into the HS channel 230 of the membrane module 200 after being pressurized by the pressure exchanger 409 and the booster pump 411. The pressure of stream 403 is adjusted to be lower than that in the previous cycle. The water permeation from the LS channel 250 to the HS channel 230 leads to the further dilution of the HS solution (stream 416). The stream 418 and stream 419 are depressurized by the two energy recovery devices and then discharged into the HS tank 320. The LS stream 251 is constantly provided into the LS channel 250 during the whole process. Electric power is generated when the extra pressurized HS effluent (stream 419) passes through the hydro-turbine 420. [0061] In the third cycle as shown in Fig. 2C, the respective functions of the two HS tanks are interchanged, such that the HS tank 310 is used to store the HS effluent while the HS tank 320 turns into the HS influent container. In this case, the valve 406, valve 424 and valve 426 are open, the others are closed. The HS influent becomes stream 404 and flows from the HS tank 320 to the HS channel 230 of the membrane module 200 with a constant hydraulic pressure applied by the pressure exchanger 409 and the booster pump 411. The applied hydraulic pressure is further decreased compared to the previous cycle and does not exceed the osmotic pressure difference across the membrane 240 between the diluted HS and LS solutions out of the membrane module 200. The pressurized HS solution out of the membrane module 200 is divided into two streams (stream 418 and stream 419) for energy recovery and power generation respectively. The depressurized HS effluents (stream 421 and stream 422) are stored in the HS tank 310 as the HS influent of next cycle. At the same time, the LS solution 251 is continuously fed into the LS channel 250 during the whole PRO process by the circulation pump 432.

[0062] The operation of the following cycles (except the last cycle) in a series of SC-PRO cycles is essentially the sequential repetition of the second cycle in Fig. 2B and the third cycle in Fig. 2C. Except for the first cycle and the last cycle, the operation of all the cycles with an even sequence number follows the process shown in Fig. 2B, while the operation of all the cycles with an odd sequence number follows the method described in Fig. 2C. In the last cycle of a batch (the term “batch” as used herein refers as a series of cycles in the operation, not to be confused with a conventional batch process), the diluted HS solution will be discharged by opening the valves 427 and 429 and closing the valves 423, 424, 425, and 426. If the last cycle number is even, the process of the last cycle is indicated in Fig. 2D, in which the valve 405, valve 427, and valve 429 are open and other valves are closed. The flow paths or the operational process is similar to that in Fig. 2B, except that the depressurized HS effluents including the stream 421 and stream 422 are discharged outside directly rather than collected in the HS tank 320. If the total cycle number is odd, the last cycle is as shown in Fig. 2E. The valve 406, valve 427, and valve 429 are open but the others are closed in this case. The flowpaths or the operational process is similar to that of the third cycle depicted in Fig. 2C, except that the two depressurized HS effluents (stream 421 and stream 422) are discharged out of the system 100. [0063] As shown in Fig. 2F, when the HS solution in the HS tanks 310/320 is drained in the last cycle, a new SC-PRO process (a new series of SC-PRO cycles) can start with the introduction of fresh HS solution (stream 407) into the system 100. Since some residual diluted HS solution likely still exists in the membrane module 200 and in the pipelines, the valves 427 and 429 may be kept opened while the valves 423, 424, 425, and 426 remain closed so that the residual diluted HS solution can be fully discharged out of the system 100 with the introduction of fresh HS solution. After that, a new series of SC-PRO cycles begins with the first cycle as shown in Fig. 2A, followed by repeating the cycles as described above. During the SC-PRO operation of all cycles, the LS stream 431/415 remains a continuous circulation in the LS channel 250.

[0064] The PRO process of Figs. 2A - 2F is referred to herein as a semi-closed PRO (SC- PRO) process as the flow of liquid is broken or “paused” over the course of the entire PRO process. Preferably, the SC-PRO process is effected by the use of two or more HS tanks 310/320. The two HS tanks 310/320 are used as the influent tank and the effluent tank alternately from cycle to cycle. In a series of cycles in one SC-PRO process, the PRO process will be operated over multiple cycles with the HS effluent in each cycle (except the last cycle) serving as the HS influent of the next cycle. A cycle ends when all the liquid in the influent tank has been drawn out, passed through the membrane module 200, and the resulting diluted HS effluent has been collected in the effluent tank. This HS effluent serves as the HS influent of the subsequent cycle. This enables the applied hydraulic pressure (or the target hydraulic pressure value) to be selected at different times over the whole SC-PRO process so that the applied hydraulic pressure more closely track the changes in the osmotic pressure difference across the membrane 240 over the whole PRO process. In some embodiments, the applied hydraulic pressire is incrementally reduced from the initial cycle to the last cycle in a stepwise manner (e.g., between successive cycles). The value of the applied hydraulic pressure (or the target hydraulic pressure value) in each cycle may be selected from a range of values lower than the osmotic pressure of the HS effluent out of the PRO module in that cycle. Owing to the higher efficiencies and energy recovery of the proposed system 100, it may further be feasible to replace the hydro-turbine 420 with another energy recover device 106 such that energy recovered thereby can be used to operate an external system 500 (Fig. 2G). [0065] In contrast, the conventional single-stage PRO process is a continuous process and a process in which a volume of liquid passes through each membrane module only once. While it is known that the HS solution undergoes continuous dilution along the HS stream in the membrane module 200, the actual conditions within the membrane module are complex and difficult to predict. In the conventional PRO system, it is not possible to track the changes in the osmotic pressure such that the applied hydraulic pressure is set at a constant value below the lowest possible osmotic pressure throughout the entire conventional PRO process. As a result, in the convention PRO process, there is considerable loss in efficiency from the under-pressurization of the membrane module. The multi-stage PRO process can reduce the energy loss caused by the under-pressurization by applying different hydraulic pressures in different stages according to the corresponding osmotic pressure difference. However, the multi-stage system may require multiple energy recovery devices between different stages, which leads to higher capital cost and bigger footprint.

[0066] Fig. 3 shows another embodiment of the system 100 in which the system is operating in a closed-circuit pressure retarded osmosis (CCPRO) mode. High salinity (HS) solution 407 is introduced into the HS channel 413 until it gradually fills the HS channel 413. Then the LS solution 431 is pumped into the system via the circulation pump (CP) 432. During the CCPRO process, the hydraulic pressure is lower than the osmotic pressure, water in the LS liquid permeates to the HS side 103, diluting the HS liquid 413. The diluted HS effluent 416 of greater volume is divided into two streams, one effluent stream 418 is recirculated in the membrane module 200 at the HS influent flow rate, the other effluent stream 419 goes through the hydro-turbine 420 at the permeate flow rate to generate electricity. The process continues until the pre-set water permeation volume is achieved. After the pre-set water permeation volume is achieved, the final disposal will be performed by opening valves 427 and 429. All the final HS effluents 416 will be discharged out of the system 100, but some of the diluted liquid remains in the piping system and membrane module 200. Therefore, the fresh HS solution 407 is introduced, so that the remaining HS solution continues to flow out till it is fully replaced by the fresh HS draw solution. Then the next operation can start immediately.

[0067] In the CCPRO process, the applied hydraulic pressure is variable, which changes with the decreasing osmotic pressure difference across the membrane 240, which can improve the energy production efficiency. The first of energy recovery devices 409 (generally, pressure exchanger) is not required in CCPRO, thus avoiding the energy loss due to the device inefficiency. The continuous discharge of the HS effluent stream 419 in the CCRO process results in the salinity gradient loss, leading to the reduction of osmotic energy that can be extracted.

[0068] Fig. 4 schematically illustrates an embodiment of the system 100 configured to switch between various PRO modes (including SCPRO, CLPRO, CCPRO) and various RO modes (including SCRO, CLRO, CCRO) after further comprising a high-pressure pump 434. The detailed operations of the CLPRO, SCPRO, and CCPRO are described above with reference to Figs. 1 to 3. The detailed operations of SCRO, CLRO, and CCRO will be described with reference to Figs. 4A - 4C.

[0069] Fig. 4A schematically illustrates the system 100 in SCRO mode. During this process, valve 446 is always open. In the first cycle of SCRO, valve 408 is open and then the initial HS influent 407 taken from an external source is divided into two streams (stream 436 and stream 437) by a flow controller 435. Stream 436 with a same flow rate as the permeate 433 is introduced into the HS channel 413 after being pressurized by the pump 434, while stream 437 is pressurized by an energy recovery device 409 (generally, a pressure exchanger) and a pump 411 before running into the HS channel 230 as HS stream 413. Under an applied hydraulic pressure higher than the osmotic pressure, the water in the HS liquid 412 passes through the membrane 240 into the LS channel 250 to form the LS stream (RO permeate) 433. The HS effluent stream 416 passes through the energy recovery device 409 to exchange the pressure energy with the HS stream 437 and then discharged into the HS tank 310 by opening the valve 424. If the total number of cycles of SCRO is two (2), in the second cycle, the process is almost the same as that of the first cycle, except that the HS influent is taken from the HS tank 310 and the HS effluent 416 is discharged out of the system 100 during which the valves 405 and 429 are open. If the total number of cycles of SCPRO is higher than two (2), in the second cycle of SCRO, the HS influent is taken from the HS tank 1, the depressurized HS effluent 421 is discharged into the HS tank 320, during which the valves 405 and 423 are open. The operation of the following cycles is essentially the repetition of the RO process during which the HS tank 310 and HS tank 320 function as the HS liquid supplier and HS liquid receiver alternately, except that the final HS effluent 421 is discharged out of the system 100 in the last cycle. The HS influent stream of last cycle is taken from the HS tank 320 when the total number of cycles is odd and is taken from the HS tank 310 when the total number of cycles is even. During the SCRO process, the hydraulic pressure is unchanged in each cycle but is changed (increased) between successive cycles. In the cycle or cycles between the initial cycle and the last cycle of the SCRO operation, if the HS influent stream 412 is drawn exclusively from the first HS tank 310 in a cycle, the HS effluent stream 416 of the same cycle is directed into the second HS tank 320 exclusively. In the next cycle, the HS influent stream 412 is drawn exclusively from the second HS tank 320 and the HS effluent stream 416 is directed into the first HS tank 310 exclusively. In the subseqeuent cycle, the first HS tank 310 again serves as the sole HS influent supply tank and the second HS tank 320 again serves as the sole HS effluent receiving tank. In yet the next cycle, the second HS tank 320 again serves as the sole HS influent supply tank and the first HS tank 310 again serves as the sole HS effluent receiving tank. The HS tanks 300 take turns to alternately serve as the HS influent supply tank and the HS effluent receiving tank from cycle to cycle, with the HS influent stream 412 being drawn solely from the HS influent supply tank without mixing with the HS effluent.

[0070] Fig. 4B schematically illustrates the system 100 operating in the CLRO mode. At the beginning of a batch (i.e., a series of cycles in one operation) of CLRO, a volume of the initial HS liquid is taken from an external source by only opening valves 408 and 405 and is collected in the HS tank 310 (not introduced into the membrane module 200 directly), and then the supply of the initial HS liquid from the external source ends in this batch of CLRO. During the desalination process, valves 405, 446, 424 keep open. The HS influent stream 403 from the HS tank 310 is split into two streams (stream 436 and stream 437) by the flow controller 435. Stream 436 with a same flow rate as the permeate 433 is induced into the HS channel 230 after being pressurized by a pump 434, while stream 437 with the same flow rate as the HS effluent stream 416 is pressurized by an energy recovery device 409 (generally pressure exchanger) and a pump 411 before running into the HS channel 230. Under an applied hydraulic pressure higher than the osmotic pressure, the water in the HS liquid 412 passes through the membrane 240 into the LS channel 250 to form the LS stream (RO permeate) 433. The HS effluent stream 416 goes through the energy recovery device 409 to exchange the pressure energy with the HS influent stream 437 and then is discharged into the HS tank 310 to mix with the residual HS liquid. When the pre-set water permeation volume is achieved, the process ends and the final concentrated HS liquid in the HS tank 310 is discharged out of the system by only opening valves 426 and 427. In the CLRO process, the hydraulic pressure is variable and increases with the increasing osmotic pressure of the HS liquid.

[0071] Fig. 4C schematically illustrates the system 100 operating in a CCRO mode. In the CCRO mode, valves 408 and 444 remain open. The HS influent 407 is supplied continuously with a same flow rate as the permeate 433 by an external source and is introduced into the HS channel 413 after being pressurized by a pump 434. The HS effluent is circulated and further pressurized by a pump 411 as a part of the HS influent without going through any energy recovery devices. The HS stream 439with the target hydraulic pressure is mixed with the pressurized fresh HS influent 438, becoming the HS influent stream 412 and entering the HS channel 413. When the water permeation volume reaches the pre-set value, the HS effluent is discharged out of the system 100 by opening valve 429. During the CCRO process, the hydraulic pressure builds up with the increasing osmotic pressure of HS stream 412.

[0072] Fig. 5 schematically illustrates an embodiment of the system 100 configured as a RO-PRO integrated osmotic battery system 600, with two HS reservoirs 310’, 320’ and one LS reservoir 440, operable in various RO and PRO modes. The osmotic battery system 600 integrating RO and PRO can be used for energy storage and production on a grid scale. RO processes driven by the input energy serve as an energy storage process (also called charging process), during which an initial HS liquid is separated into a concentrated HS liquid and a LS liquid (RO permeate), indicating that the input energy is converted into the osmotic energy which is stored in the form of salinity difference between two liquids. PRO processes serve as an energy production process (also called discharging process), during which the osmotic energy is converted into electricity by mixing the HS and LS liquids produced by RO processes. It should be noted that in the osmotic battery, there are no water and salt losses theoretically. Unlike the individual RO process for desalination in which the RO permeate is supplied for use directly, in the osmotic battery, the RO permeate produced in the charging process will be stored for the further energy production by PRO. During the PRO process, the freshwater produced by RO passes through the membrane from the LS channel 250 to the HS channel 230 under a hydraulic pressure lower than the osmotic pressure difference across the membrane 240. The increased volume of HS liquid goes through the energy recovery device 420 to produce electricity when the energy recovery device is a hydroturbine or to transfer the energy to a third system when the energy recovery device is a pressure exchanger. When the volume of water permeation during RO and PRO processes is the same, the salinity of the initial HS influent of RO would be equal to that of the final HS effluent of PRO, and the salinity of the initial HS influent of PRO would be equal to that of the final HS effluent of RO.

[0073] At the initial time, the osmotic battery system 600 requires a volume of HS liquid stored in the HS reservoir 310’. An initial LS liquid is not essential since the LS liquid can be produced by RO processes first. If there is still the initial LS liquid in the LS reservoir 440 at the initial time, the PRO (discharging) process can be performed first to generate electricity by converting the osmotic energy stored between the salinity of HS and LS liquids, otherwise the RO (charging) process should be performed first to charge the osmotic battery (to produce the concentrated HS liquid and RO permeate (LS liquid) used for electricity production by PRO) before it can generate electricity by PRO.

[0074] Various RO and PRO modes can be performed based on the configuration shown in Fig. 5, including the SC mode, the CL mode and the CC mode. The detailed operation processes of SCRO, CLRO, CCRO, SCPRO, CLPRO and CCPRO are almost the same as described in Fig. 4A, Fig. 4B, Fig. 4C, Fig.2 (including Fig.2A to Fig. 2G), Fig. 1 and Fig. 3, respectively, except that the LS effluent is collected or circulated in the LS reservoir 440, the initial HS influent is provided by one of the HS reservoirs 3107320’, not the external source, and the final HS effluent is received by one of the HS reservoirs according to the operation of the specific mode (for example, as for CLRO-CLPRO mode, both the initial HS influent and the final HS effluent are stored in the HS reservoir 310’; as for the CCRO- CCPRO mode, the HS influent of CCRO, which is the HS effluent of CCPRO, is stored in the HS reservoir 310’, while the HS effluent of CCRO, which is the HS influent of CCPRO, is stored in the HS reservoir 320’; as for the SCRO-SCPRO mode, it depends on the number of cycles of SCRO and SCPRO; etc.).

[0075] Fig. 6 schematically illustrates an RO-PRO integrated osmotic battery system 600/100 with two HS reservoirs 310’, 320’, two intermediate HS tanks 310,320, and one LS reservoir 440, in which the system 100 can switch and operate between various RO modes and PRO modes. Compared with the osmotic battery shown in Fig.5, two HS tanks 310,320 can be added to the system 100 to enhance its feasibility and practicality. In this system, the two HS reservoirs 310’, 320' are only responsible for the separate storage of initial HS liquid and the final HS liquid, the two HS tanks 310,320 are responsible for any of the intermediate HS stream. Similarly, all of the SC mode, CL mode and CC mode can be applied in this system 100, and the detailed operation is the same as that described in Fig. 4A, Fig. 4B, Fig. 4C, Fig.2 (including Fig.2A to Fig. 2G), Fig. 1 and Fig. 3, except that the initial HS influent of RO processes is taken from the HS reservoir 310’, the final HS effluent of RO processes is discharged to the HS reservoir 320’, the initial HS influent of PRO processes is taken from the HS reservoir 320’ and the final HS effluent of PRO processes is discharged to the HS reservoir 310’. For example, if the 3-cycle SCRO and SCPRO are applied in the system 100 and the charging process is performed first, a volume (not larger than the volume of the HS tanks) of HS influent is taken from the HS reservoir 310’ and introduced into the membrane module 200 after the pressurization by the pump 434 and energy recovery device 409. The concentrated HS effluent is collected in the HS tank 310 after the depressurization in energy recovery device 409, which is the HS influent of the second cycle of SCRO. Then the HS effluent of the second cycle of SCRO is collected in the HS tank 320 and would be the HS influent of the third cycle, while the HS effluent of the third cycle (the final HS effluent of SCRO) is discharged into the HS reservoirs 320’. Next, in the discharging process, the initial HS influent is taken from the HS reservoir 320’ and introduced into the membrane module 200 after the pressurization by the energy recovery device 409. The diluted HS effluent is collected in the HS tank 310 after the depressurization in energy recovery devices 409 and 420, which would be the HS influent of the second cycle of SCPRO. Then the HS effluent of the second cycle of SCPRO is collected in the HS tank 320 and would be the HS influent of the third cycle, while the HS effluent of the third cycle (the final HS effluent of SCPRO) is discharged into the HS reservoir 310’.

[0076] The configuration (as outlined in the dotted-dashed box) 700 is the same as that shown in Fig. 4. This embodiment of the system 100 only requires additionally two HS reservoirs 310’, 320’ and one LS reservoir 440 to form a closed system for energy storage and production. In Fig. 6, the HS reservoirs 310’ and 320’ serve as the initial HS influent supplier (replacing the “external source” in Fig. 4) and the final HS effluent container (instead of discharging all the final HS effluent out of the system in Fig, 4), respectively. The HS tanks 310, 320 has a smaller volume than that of each of the HS reservoirs 310’, 320’, which can reduce the duration time of each operation and improve the flexibility of switching between different modes.

[0077] In actual implementation, if it is not required for the system 100 be operable in all of the various possible modes described, the devices and pipelines that are not required in the desired modes of operation can be removed (instead of being switched off/closed off).

[0078] Fig. 7 schematically illustrates an RO-PRO integrated osmotic battery system 600 with separate membrane modules for the RO process and the PRO process. In this embodiment, different osmotic membranes can be used in the RO mode and in the PRO mode. The exemplary embodiment as shown in Fig. 7 provides two separate membrane modules 200,200’ - one for RO and another for PRO. The specific operation can be similar to that described above, except that the HS influent 412 is pumped into the membrane module 200 (configured with a RO membrane 240) by opening the valve 442 and valve 443 in RO modes; and the stream 412 is transported into the other membrane module 200’ (configured with a PRO membrane 240’) by opening the valve 442’ and valve 443’ in PRO modes.

[0079] Similarly, if it is found that some of the operational modes are not required in actual implementation, according to the actual situation, the unnecessary devices and pipelines may be omitted so that the system only runs in the desired mode(s).

[0080] According to an embodiment of the present disclosure, the system 100 may be configured as a desalination-osmotic energy storage (DOES) system (Fig. 8). The DOES system as proposed herein is a multi-purpose system that can be used for desalination (water production), osmotic energy storage and electricity generation. The configuration is similar to that of osmotic battery system as shown in Fig.7, but the DOES system is not a completely closed-system. The DOES system is configured so that it can introduce a fresh HS liquid 407’ from an external source by opening valve 408’, discharge a part of the HS effluent stream (e.g., stream 421 and 422) out of the system by opening valves 427 and/or 429, and supply a part of the LS liquid 448 collected in the LS reservoir 440 for use (e.g., as domestic water) by opening valve 447.

[0081] There are two main operation methods of the DOES system: (i) where the HS reservoir 310’ is not in use, and (ii) where the HS reservoir 310’ is used.

[0082] (i) In the case where the HS reservoir 310’ is not required or not in use: All the initial HS influent needed for the RO process is taken from the external source (e.g., sea), which is introduced into the membrane module or the HS tank 310 directly. All of the final concentrated HS liquid or a part of the final concentrated HS liquid and a part of the freshwater produced by RO are stored in the HS reservoir 320’ and the LS reservoir 440, respectively, the other part of the HS liquid is discharged out of the system 100, and the other part of the freshwater is supplied for use. Then during the PRO process, the HS liquid stored in the HS reservoir 320’ is introduced as the initial HS influent of PRO, the freshwater stored in the LS reservoir 440 is circulated continuously. The final diluted HS effluent of PRO is discharged out of the system 100.

[0083] (ii) In the case where the HS reservoir 310’ is configured or in use: A volume of the HS solution is stored in the HS reservoir 310’ at the initial time. During the RO process, the initial HS influent is taken from the HS reservoir 310’ first. A part of the freshwater produced by RO is stored in the LS reservoir 440 for electricity generation and the other is supplied for use directly. Only a part of the concentrated HS effluent is stored in the HS reservoir and the other is discharged out of the system 100, which is in order to make sure that the salinity of the diluted HS effluent after PRO is the same as that of the initial HS liquid stored in the HS reservoir 310’. Then during the PRO process, a volume of the HS liquid 404’ stored in the HS reservoir 320’ is used as the initial HS influent, and the final diluted HS effluent is received by the HS reservoir 310’ (instead of being discharged out of the system 100). In this case, the initially stored HS liquid in the HS resevior 310’ would be consumed. When the HS liquid in the HS reservoir 310’ runs out, the fresh HS liquid 407’ from an external source can be introduced into the HS reservoir 310’.

[0084] Another advantage of the system 100 is its adaptability for use in any one of multiple modes (the SC mode, CL mode and CC mode). [0085] As described, advantageously, the system 100 can be switched between different modes of operation, e.g., between a PRO (energy production) mode and a RO (water production and energy storage) mode. In the present disclosure, when the DOES system 600 may be said to be in an RO mode , involving the production and storage of a brine (or a solution of relatively high salinity) and the production, storage, and supply of a fresh water solution (or a solution of relatively low salinity) via an RO operation and preferably a SCRO operation (although not limited to such). The DOES system 600 may be described as being in a PRO mode when the stored osmotic potential energy is converted into electrical energy or to perform other work. In the present disclosure, the discharging mode is preferably a SCPRO operation (although not limited to such). This enables the DOES system 600 to control the supply of energy in a controllable manner. One example is illustrated by Table 1 below.

[0086] In actual implementation, if it is not required for the system 100 be operable in all of the various possible modes described, the devices and pipelines that are not required can be removed (instead of being switched off/closed off). For example, if the system 100 would be required to operate in the CC mode, the valve 444 and the pipeline connected thereto may be removed.

[0087] Owing to the membrane rejection, salt leaking from HS solution to LS solution will occur unavoidably, leading to energy losses in the energy production processes. In one example, pretreatment and/or posttreatment can be applied in the LS side to remove the solutes in the LS solution to an acceptably low level such that the energy loss in PRO for energy production is reduced. As shown in Fig. 9, the pre-treatment device 451 is arranged upstream of the inlet of the LS channel 250 to deionize or purify the LS solution during the PRO process, and it works when the valve 449 is closed and the valve 450 is opened. Although the solute leakage will not stop due to the concentration gradient, the pre-treatment device 451 can remove the solutes in the LS incoming stream 431 before the LS stream 431 enters into the membrane module 200. Alternatively, if the LS side is a closed loop with a reservoir or tank for storing and collecting the LS solution in the LS side, the post treatment device 447 may be connected to the outlet of the LS channel 250 (and be operatational in both RO mode and PRO mode). In the RO mode, the salinity of the permeate will be reduced through the post treatment device 447 before the permeate is discharged to the reservoir or tank 300/310/320/3107320’. In the PRO mode, the reverse salt diffusion from HS solution to the LS solution may still exist, which can be addressed by the post-treatment device 447. In other words, the post treatment helps to ensure that the LS solution collected in the reservoir or tank has low salinity and is clean. The post treatment may be implemented by closing valve 445 and opening valve 446. In another scenario, both pre-treatment and posttreatment can be concurrently operational by closing valve 445 and valve 449, and opening valve 446 and valve 450 in the LS loop. This helps to ensure the LS stream entering the membrane module has relatively low salinity to reduce energy loss in the PRO mode. Treatment units which can be selected to implement the pre-treatment or post treatment include but are not limited to brackish water reverse osmosis (BWRO), electrodialysis (ED), electrode capacitive deionization (CDI), electro-deionization (EDI) and so on.

[0088] The system 100 as disclosed herein has a great potential for practical applications. For example, it can be utilized as a high-efficiency energy harvesting system to supply electricity for human use. [0089] There are many options of HS and LS solutions in practice, which can be classified into three types roughly, natural resources (e.g., seawater and river water), wastewater (e.g., RO brine, brackish water, recycled water) and any synthesized solutions with salinity differences for energy production by PRO. In addition to the natural salinity-gradient pair (e.g., seawater vs. river water), there are many salinity-gradient pairs available in industry, such as industrial brine vs. wastewater, seawater desalination brine vs. wastewater. In addition, it can be combined with the desalination process, such as the RO mode, to compensate the energy consumption through producing extra power. In other words, the present system 100 provides a new and viable way of exploiting such natural salinitygradient pairs as a source of renewable energy. Furthermore, the PRO process integrated with any one or more of the following to enables the system 100 to control energy production and/or energy storage according to the real-time and/or predicted needs: CLPRO, CCPRO, CCPRO, SCRO, CLRO, and CCRO. Other type of energy such as solar, wind, tidal and excess electricity can be input in RO process for desalination and to separate the initial salt water into a fresh water and a more concentrated salt water. In this manner, renewable energy can be converted into osmotic power and stored in the form of the salinity gradient between the concentrated salt water and the freshwater. Part of the freshwater stored can also be supplied for use. In response to energy demand, the osmotic power stored between the concentrated salt water and the residual freshwater can be extracted by SCPRO processes to generate electricity.

[0090] The energy production performance of SCPRO of the present disclosure is evaluated by calculating the specific energy production (SEP=Ep/VHS,f), that is, the produced energy (E_p) normalized by the final HS solution volume (Vus,f). Another useful parameter is the energy efficiency (r|pRo=SEP/SEP_max), that is, the ratio of the realistic SEP with the considerations of inefficiencies to the theoretical maximum SEP (SEPmax). A 1.2 mol/L NaCl solution which has a similar osmotic pressure of SWRO brine is used as the initial HS liquid and the freshwater ( 0 mol/L) is used as the LS liquid in the simulation. The salt rejection of the membrane is assumed as 100%. The water permeation ratio or the water recovery ratio (R=V_p/VHS,f) is the proportion of the permeate volume (Vp) to the final HS solution volume (Vus,f). Besides the ideal case, three levels of inefficiencies are involved in the calculation. In a basic case, the efficiencies of booster pump (qp), pressure exchanger (T|PX) and hydraulic turbine (T|HT) are 0.85, 0.95 and 0.90, respectively, and the pressure loss is 0.5 bar. In a good case, r|p, qpx and T|HT are set as 0.90, 0.98 and 0.95 respectively, and the pressure loss is 0.1 bar. Furthermore, the r|p, r|px and T|HT are also set as 0.95, 0.99 and 0.97, respectively, with no pressure loss, for an aspiring case, which are expected to be realized in the future with the development of related technologies.

[0091] The SEP and energy efficiency of SCPRO processes under the optimal cycles are shown in Fig. 10. The dotted line represents the ideal SEP obtained in the SCPRO system with perfect equipment and infinite cycles, which is equal to the thermodynamic maximum specific energy that can be extracted by PRO. The SEP increases with increasing water recovery firstly and then decreases with further increasing water recovery. The highest SEP of 0.61 kWh m'³ reaches at the water permeation ratio of 0.65, which shows excellent energy production capacities. Although inefficiencies have significantly negative influences on the power generation, the SEP of 0.37 kWh m'³ still can be achieved in the SCPRO system at the water recovery of 0.60 under the basic case, and the energy efficiency remains stable at around 61%. The SEP can reach up to be 0.46 kWh m'³ with an efficiency of 76% in the SCPRO process under the good conditions at the water recovery of 0.65, which is approximately equivalent to that of a 160 meter high pumped hydro energy storage system. Furthermore, a higher SEP of 0.51 kWh/m³ and energy efficiency of 84% can be achieved in the SCPRO in the aspiring case. The energy efficiency at different recoveries is relatively stable due to the availability to optimize the performance by flexibly adjusting the number of cycles in SCPRO. Depending on the initial salt concentration and water recovery, the energy production capacity can range above 0.57 kWh/m³ to above 3.03 kWh/m³ (equivalent to over 1000 metres water height of pumped hydro storage at the same energy density). The results demonstrate that SCPRO has great potential to achieve high energy efficiency in large-scale osmotic energy harvesting.

[0092] The comparison of SEPs between different PRO processes under the case of using the good devices (r|p=0.90, T|PEX=0.98, and T|HT=0.95, and the pressure loss = 0.1 bar) is demonstrated in Fig. 11. It is worth noting that the number of cycles is configured at each water permeation ratio in the SCPRO mode since the number of cycles can be optimized without any reconstruction of the system. As shown in Fig. 11, when the water recovery is lower than 0.55, the CCPRO mode performs better than any other modes. However, when the water recovery is higher than 0.55, SCPRO is superior to others. Moreover, although the performance of multi-stage PRO (5-stage) at high water recoveries (>0.7) is comparable to that of SCPRO and superior to that of CLPRO, CCPRO and single-stage PRO, it requires significantly larger footprint and more devices than all the other modes (SCPRO, CLPRO and CCPRO), which is not cost-efficient. Nevertheless, it is worth noting that when the conditions (e.g., device quality, pressure loss, salinity of the initial HS solution) change, the optimal operating mode at different water recoveries may also change. The system as disclosed herein can be flexibly switched to the desired mode to optimize efficiency.

[0093] As for the system operating as an osmotic battery that can operate at various RO (i.e., SCRO, CLRO and CCRO) and PRO (i.e., SCPRO, CLPRO and CCPRO) modes for energy storage and production, a 0.6 mol/L NaCl solution which has the similar osmotic pressure of seawater is used as the initial HS influent in the simulation. The salt rejection of the membrane is also assumed as 100%. The water permeation ratio (ROB=V_P/VHS,O) is defined as the proportion of the permeate volume (VP=VP,RO=VP,PRO) in the RO/PRO process to the initial HS solution volume (VHS,O). Generally, in the energy storage and production process, the input energy is converted into the osmotic power by the RO process, and the permeate obtained from the RO process returns to the HS solution during the PRO process for energy production, which means that the permeate volume and the water permeation ratio in the RO and PRO modes are the same. The definition of the round-trip efficiency (r|_rOundtrip=EP/EC) is the ratio of the energy production (EP) in the PRO process to energy consumption (EC) in the RO process. The inefficiencies including the device efficiencies (r|p=0.90, T|PEX=0.98, and T|HT=0.95) and pressure loss (0.1 bar) are considered.

[0094] The results are presented in Fig. 12. As shown in Fig. 12, the round-trip efficiency of the osmotic battery system integrating CCRO and CCPRO is the highest when the water permeation ratio is lower than 0.55, but which decreases from around 67% to 56% when the water permeation ratio increases from 0.1 to 0.55. Then the SCRO-SCPRO integrated osmotic battery system outperforms with a stable roundtrip efficiency of around 56%. Nevertheless, it is worth noting that when the conditions (e.g., device quality, pressure loss, salinity of the initial HS solution) change, the optimal operating mode at different water permeation ratios may also change. The osmotic system proposed herein can be flexibly switched to the desired mode to optimize efficiency. [0095] The osmotic battery system proposed herein uses hydro-turbine to produce electricity and uses salinity gradients for energy storage. Therefore, it has all the advantages of pumped hydro storage, i.e., it is safe, sustainable, and green (free from carbon emissions). Moreover, the osmotic battery sysrem is not constrained by topographical conditions. It can be installed on the ground, subterranean (underground), or floating in offshore (e.g., using neutrally buoyant chambers), or even on offshore islands to save mainland footprint. Floating solar photovoltaic can be deployed on the reservoirs of the system when it is implemented in coastal and offshore areas.

[0096] The present application provides an osmotic battery system which provides sustainable energy production with high energy storage efficiency. Besides, the osmotic battery system is easily switchable between RO and PRO operations and various RO and PRO modes (e.g. SC, CL, and CC modes), as desired. Separation of HS influent and HS effluent saves energy consumption and increases round-trip efficiency. The proposed system 100 can be used not only for grid storage of intermittent renewable energy and excess electricity, but also for energy-efficient production of fresh water from saline waters (e.g., seawater) and large scale storage of fresh water.

[0097] Advantageously, the present system 100 can use pre-treated seawater or synthetic salt water. This reduces membrane fouling, increases the useful life of the apparatus and reduces the cost of maintenance.

[0098] Alternatively described, in another aspect, the system 100 includes: at least one membrane module 200, the at least one membrane module 200 including: a high salinity (HS) channel 230 configured to receive a HS influent stream 412 for a HS stream 413 in the at least one membrane module 200, the HS stream 413 being configured to exit the at least one membrane module 200 as a HS effluent stream 416; a low salinity (LS) channel 250 configured to provide a LS flow path for a LS stream 415 in the at least one membrane module 200, the HS stream 413 having an osmotic pressure higher than the LS stream 415; and a membrane 240 disposed between the HS channel 230 and the LS channel 250, the membrane 240 being a selectively permeable membrane, where in an operation having at least one cycle of a liquid through the HS channel 230, each of the at least one cycle is characterized by an osmotic pressure difference across the membrane 240; two tanks 300, each of the two tanks 300 defining a maximum volume of the liquid processable by the system in any one of the at least one cycle, at least one of the two tanks 300 being in fluidic communication with at least one of the HS influent stream 412 and the HS effluent stream 416; a first energy recovery device 106, wherein in the operation the first energy recovery device 106 is configured to at least partially pressurize the HS influent stream 412 based on energy recovered from at least a part of the HS effluent stream 416; and a HS fluid circuit coupling each of the two tanks 300 with the HS channel 230 and the first energy recovery device 106, wherein the fluid circuit is configurable to provide a flow path through the HS channel 230 for the at least one cycle, and wherein the system 100 is operable in a pressure retarded osmosis (PRO) mode in which a hydraulic pressure is applied to the HS influent stream 412, and wherein the hydraulic pressure is controllably variable within a range below the osmotic pressure difference across the membrane 240. The hydraulic pressure may be held constant in each of the at least one cycle with the hydraulic pressure being changed or reduced between cycles or on completion of any one of the at least one cycle.

[0099] Alternatively described, the system 100 is configurable to operate in a semi-closed PRO (SCPRO) mode in which a flow of liquid is broken or “paused” over the course of a series of cycles of liquid through the membrane module 200. Preferably, the system 100 is operable with two HS tanks 310/320 in use as the influent tank and the effluent tank respectively, with each of the two HS tanks 300 alternately serving as the influent tank and the effluent tank in successive cycles. A cycle ends when all the liquid in the influent tank has been drawn out, passed through the membrane module 200, and the resulting diluted HS effluent 416 has been collected in the effluent tank. This HS effluent 416 serves as the HS influent 412 of the second cycle. The system 100 in operation includes applying a varying hydraulic pressure (or the target hydraulic pressure value) over the series of cycles making up one PRO operation, such that the the applied hydraulic pressure tracks the changes in the osmotic pressure difference across the membrane 240 over the whole PRO process. In some embodiments, the applied hydraulic pressire is incrementally reduced from the initial cycle to the last cycle in a stepwise manner (e.g., between successive cycles). The value of the applied hydraulic pressure (or the target hydraulic pressure value) in each cycle may be selected from a range of values lower than the osmotic pressure of the HS effluent out of the PRO module 200 in that cycle. [0100] The system 100 may be configured to switch between operating in the SCPRO mode and operating in a RO mode. The system 100 may be configured to switch between operating in the SCPRO mode and operating in any one of the following modes: a CLPRO mode, a CCPRO mode, a SCRO mode, a CLRO mode, and a CCRO mode. The switching may be effected by controlling the state of one or more valves, flow controllers, and/or devices in the system.

[0101] All examples described herein, whether of apparatus, methods, materials, or products, are presented for the purpose of illustration and to aid understanding, and are not intended to be limiting or exhaustive. Various changes and modifications may be made by one of ordinary skill in the art without departing from the scope of the invention as claimed.

Claims

1. A system, comprising: at least one membrane module, the at least one membrane module including: a high salinity (HS) channel configured to receive a HS influent stream for a HS stream in the at least one membrane module, the HS stream being configured to exit the at least one membrane module as a HS effluent stream; a low salinity (LS) channel configured to provide a LS flow path for a LS stream in the at least one membrane module, the HS stream having a level of salinity or an osmotic pressure that is higher than the LS stream; a membrane disposed between the HS channel and the LS channel, the membrane being a selectively permeable membrane, and in a cycle of an operation, a HS liquid flows through the HS channel and a LS liquid flows through the LS channel, the cycle being characterized by an osmotic pressure difference across the membrane between the HS stream and the LS stream; at least one HS tank, each of the at least one HS tank defining a maximum volume of the liquid processable by the system in the cycle, the at least one of the HS tank being in fluidic communication with at least one of the HS influent stream and the HS effluent stream; a first energy recovery device, wherein the first energy recovery device is configured to at least partially pressurize the HS influent stream based on energy recovered from at least a part of the HS effluent stream; and a HS fluid circuit coupling the at least one HS tank with the HS channel and the first energy recovery device, wherein the fluid circuit is configurable to provide a flow path through the HS channel for the cycle, wherein the system is operable with one HS tank in use or with two HS tanks in use, and wherein the system is configured to apply a hydraulic pressure to the HS influent stream, and wherein the hydraulic pressure is controllably variable within a range below the osmotic pressure difference across the membrane.

2. The system according to claim 1, further comprising at least one LS tank, wherein the at least one LS tank defines a maximum volume of the LS liquid processable by

32 the system in the cycle. The system according to claim 2, wherein the at least one LS tank is in fluidic communication with the LS influent stream and the LS effluent stream. The system according to claim 1, the system further comprising a second energy recovery device coupled in parallel with the first energy recovery device, wherein each of the first energy recovery device and the second energy recovery device is operable to recover energy from different streams received from the HS effluent stream. The system according to claim 4, wherein the second energy recovery device is configured to transfer the energy recovered from at least a part of the HS effluent stream to a third system, or wherein the second energy recovery device comprises a turbine, the turbine being operable based on energy recovered from at least a part of the HS effluent stream. The system according to any one of claims 1 to 5, wherein the operation comprises at least two cycles, and wherein in a first of the at least two cycles, the HS influent stream is supplied from an external source. The system according to any one of claims 1 to 6, wherein the system is operable with the one HS tank in use, and wherein the HS effluent stream in a first of two cycles of a two-cycle operation is received by the one HS tank in use, and wherein in a second of the two cycles of the two-cycle operation, the HS influent stream is supplied from the one HS tank in use. The system according to claim 7, wherein in a last cycle of the two cycles, a final HS effluent stream is discharged out of the system. The system according to any one of claims 1 to 6, wherein the system is operable with the two HS tanks in use, and wherein the HS effluent stream in a first cycle of

33 at least three cycle is received by a first of the two HS tanks in use, and wherein in a second of the at least three cycles, the HS influent stream is supplied from the first of the two HS tanks in use, and wherein the HS effluent stream is received by a second of the two HS tanks in use, and wherein the first of the two HS tanks in use and the second of the two HS tanks in use alternately function as the HS influent tank and HS effluent tank, respectively in subsequent cycles prior to a last cycle of the at least three cycles. The system according to claim 9, wherein in the last cycle, the HS influent stream is supplied from the first of the two HS tanks in use if a total number of the at least three cycles is even and the HS influent stream is supplied from the second of the two HS tanks in use if the total number of the at least three cycles is odd, and wherein a final HS effluent stream is discharged out of the system. The system according to any one of claims 1 to 10, wherein the operation comprises at least two cycles, and wherein the osmotic pressure of the HS influent stream is constant in each of the at least two cycles, and wherein the osmotic pressure of the HS influent stream decreases between successive cycles of the at least two cycles. The system according to any one of claims 1 to 11, wherein the system is operable in a semi-closed (SC) pressure retarded osmosis (PRO) mode in which the hydraulic pressure is held constant during the cycle, and wherein the hydraulic pressure is changed between successive cycles. The system according to any one of claims 1 to 5, wherein the system is operable with the one HS tank in use, and wherein a volume of an initial HS liquid supplied from an external source is stored in the one HS tank in use, and wherein the HS influent stream is supplied to the HS channel and the HS effluent stream is received by the one HS tank in use, and wherein a final HS effluent in the one HS tank in use and in the HS channel is discharged out of the system. The system according to claim 13, the system being operable in a closed-loop PRO (CLPRO) mode in which a hydraulic pressure is applied to the HS influent stream, and wherein the hydraulic pressure is controllably variable within a range below the osmotic pressure difference across the membrane, and wherein the osmotic pressure of the HS influent stream and the osmotic pressure of the HS effluent stream both decrease continuously over a series of cycles. The system according to any one of claims 1 to 5, wherein an initial HS influent is supplied from an external source to the HS channel without entering any of the at least one HS tank, and wherein at least one part of the HS effluent stream is circulated by a pump as the HS influent stream without going through a first energy recovery device. The system according to claim 15, wherein the at least one part of the HS effluent stream is in fluid communication with a second energy recovery device for energy production before being discharged out of the system, and wherein a final HS effluent in the HS channel is discharged out of the system without entering any of the at least one HS tank. The system according to any one of claims 15 to 16, the system being operable in a closed-circuit PRO (CCPRO) mode in which the HS influent stream is characterized by a hydraulic pressure that is lower than the osmotic pressure difference across the membrane, and wherein the hydraulic pressure is controllably variable within a range below the osmotic pressure difference across the membrane. The system according to any one of claims 1 to 17, the system being configured to enable switching between any of the following: a semi-closed pressure retarded osmosis (SCPRO) mode, a closed-loop pressure retarded osmosis (CLPRO) mode, and a closed-circuit pressure retarded (CCPRO) mode, to controllably supply usable energy. The system according to claim 1, further comprising a pump configured to contribute to the hydraulic pressure of at least one part of the HS influent stream independent of energy recovered from at least one part of the HS effluent stream, the system being operable in a reverse osmosis (RO) mode, and wherein the HS effluent stream is in exclusive fluidic communication with the first energy recovery device, and wherein the HS influent stream is characterized by a hydraulic pressure that is higher than the osmotic pressure. The system according to claim 19, wherein the system is operable in a semi-closed reverse osmosis (SCRO) mode in which the hydraulic pressure is controllably variable within a range above the osmotic pressure, and wherein the operation comprises at least two cycles, and wherein the hydraulic pressure is constant in each of the at least two cycles, and wherein hydraulic pressure is changed between successive cycles of the at least two cycles, and wherein the osmotic pressure of HS influent stream is constant in each of the at least two cycles, and wherein the osmotic pressure of the HS influent stream increases cycle by cycle. The system according to claim 19, the system being operable in a closed-loop reverse osmosis (CLRO) mode in which only one HS tank in use functions as the HS influent supplier and the HS effluent receiver, and wherein the hydraulic pressure is controllably variable within a range above the osmotic pressure. The system according to claim 19, wherein at least one part of a fresh HS influent stream is supplied by the external source continuously to compensate for a volume of water permeating from the HS channel to the LS channel, and wherein a final HS effluent of a series of cycles is discharged out of the system. The system according to claim 22, wherein the system is operable in a closed-circuit reverse osmosis (CCRO) mode in which at least one part of the HS effluent stream is circulated by a pump as the HS influent stream without going through a first energy recovery device, and wherein at least one part of the fresh HS influent stream is supplied by the external source continuously to compensate for a volume of water permeating from the HS channel to the LS channel, and wherein the hydraulic pressure is controllably variable within a range above the osmotic pressure.

36 The system according to any one of claims 1 to 23, the system being configured to be sequentially or concurrently operable in a reverse osmosis (RO) mode and a pressure retarded osmosis (PRO) mode, wherein the RO mode is any one of a semiclosed pressure retarded osmosis (SCRO) mode, a closed-loop RO (CLRO) mode, a closed-circuit RO (CCRO) mode, and wherein the PRO mode is any one of a semiclosed PRO (SCPRO) mode, a closed-loop PRO (CLPRO) mode, and a closed- circuit PRO (CCPRO) mode. The system according to claim 24, the system with at least two HS tanks in use being configured to operate as an energy storage system in which the system is operable in the RO mode to convert an input energy into osmotic energy in form of a salinity difference between a final concentrated HS effluent and a final LS effluent, and wherein the system is operable in the PRO mode to convert the osmotic energy to electricity. The system according to claim 25, wherein the system in the RO mode is configured to charge the energy storage system, and wherein the system in the PRO mode is configured to discharge the energy storage system. The system according to claim 25 or 26, wherein at least a part of the initial HS liquid is stored in the first of the at least two HS tanks, and wherein a part of the initial LS liquid is stored in the at least one LS tank prior to the energy storage system being discharged in the PRO mode. The system according to claims 27, wherein the system in the RO mode is configured to produce the LS liquid in response to the at least one LS tank being empty of liquid. The system according to claim 28, in a first charging-discharging cycle, the initial HS influent stream is supplied by the first of the two HS tanks for RO if charging is performed firstly or for PRO if discharging is performed firstly, wherein the initial HS influent stream of PRO is the final HS effluent of RO if charging is performed

37 firstly, and wherein the initial HS influent stream of RO is the final effluent of PRO if discharging is performed firstly, and wherein in any subsequent chargingdischarging cycle, the final HS effluent of RO is the initial HS influent of PRO, and wherein the final HS effluent of PRO is the initial HS influent of RO. The system according to claim 29, wherein the final HS effluent in both RO and PRO process is received by one of the at least two HS tanks. The system according to claim 30, the energy storage system with at least one HS tank, the energy storage system further comprising two HS reservoirs, wherein the initial HS influent stream supplied by a first of the two HS reservoirs and a final HS effluent stream is received by a second of the two HS reservoirs if the system is in the RO mode, and wherein the initial HS influent stream is supplied by the second of the two HS reservoirs and the final HS effluent stream is received by the first of the two HS reservoirs if the system is in the PRO mode. The system according to claim 31, wherein each of the at least one HS tank having a smaller volume than that of either of the two HS reservoirs is used to provide intermediate storage and supply of the HS stream. The system according to claim 24, wherein the at least one membrane module is configured to be operable in one of the PRO mode and the RO mode. The system according to claim 24, wherein a first of the at least one membrane module is configured as a PRO membrane module, and wherein a second of the at least one membrane module is configured as a RO membrane module. The system according to any one of claims 1 to 34, the system further comprising one or more treatment units, the one or more treatment units being configured to treat one or more streams upstream and/or downstream of the at least one membrane module.

38