BY FORWARD OSMOSIS

CROSS-REFERENCE TO RELATED APPLICATIONS

[001] This application makes reference to and claims the benefit of priority of an application for "Electrokinetic Power Generation By Forward Osmosis" filed on May 25, 2011, with the United States Patent and Trademark Office, and there duly assigned serial number 61/489,836. The content of said application filed on May 25, 201 1, is incorporated herein by reference in its entirety for all purposes.

TECHNICAL FIELD

[002] The invention relates to a power generating device, and a method of generating power by forward osmosis. The invention also relates to a water desalination device comprising the power generating device, and a method of water desalination.

BACKGROUND

[003] With increasing challenge of global warming, energy related issue is recognized as one of the most coveted topics in recent years. Global warming may mainly be attributed to over-emission of greenhouse gases, such as carbon dioxide, which is a by-product generated from burning fossil fuels for power generation. Furthermore, geopolitical instabilities and depletion of fossil fuels have also exacerbated the situation.

[004] In view of the above, there is an ongoing need to explore alternative technologies to produce cleaner and greener energy sources. To meet this need, new technologies have been employed to harvest renewable energy from various sources such as solar, wind, tidal and wave. However, these alternative energy sources are not stable as they may experience intermittent energy delivery due to variation in weather conditions.

[005] One promising option to generate clean and renewable energy involves the use of salinity gradient, where tremendous amount of energy is released when river or fresh water meets with seawater. Current state of the art methods to harvest energy from salinity gradient include pressure retarded osmosis (PRO) and reversed electrodialysis (RED).

[006] In PRO, river/fresh water is separated from seawater by a semi-permeable membrane where only water molecules can flow from the fresh water side towards the seawater side by a naturally spontaneous process called forward osmosis (FO). Due to the water molecules movement, pressure may be built up on the seawater side in the form of hydrostatic pressure, which may be utilized to drive a water turbine to generate electricity in a similar manner as that for hydroelectrical generation.

[007] RED, on the other hand, uses a cationic exchange membrane (CEM) and an anionic exchange membrane (AEM) in combination to separate ions from a feed solution, such that an electrical potential between the two membranes is generated, and can be directly applied as electrical energy.

[008] However, these methods suffer from the following deficiencies. For example, for a PRO process, other than the need to build a PRO plant (similar to hydroelectric plant), PRO also requires the membrane to be strong enough to withstand the high pressure of the process, thereby incurring additional costs. Furthermore, since the pressure generated is then used to drive a turbine, additional pressure losses result due to process inefficiencies, which translate into higher capital and maintenance costs on the machines. The high pressures involved in the PRO process also put more stringent requirements on the parts and fittings for the system, thus making the system more costly. Furthermore, membrane fouling would deteriorate the membrane more severely in the pressurized environment, and chemical treatment on the fouling membrane would render it less environmental friendly and also incur extra cost for treatment.

[009] For a RED plant, for example, heavy ions would be trapped and accumulated in the ion exchange membrane. As a result, the membrane would need to be disposed off as chemical waste at the end of each production cycle, thereby incurring additional expenses. Furthermore, RED requires external pumping to pump the flow over tiny channels between the pair of ion exchange membrane. This requires high mechanical strength for the membrane and causes loss in pumping energy due to resistance within the tiny channels. Thus, to make RED self-sustainable, the energy produced must be sufficient to counter-balance the pumping work and other losses.

[0010] In view of the above, there remains a need for an improved power generating device, and a method of generating power that overcomes at least some of the above drawbacks. SUMMARY OF THE INVENTION

[0011] In a first aspect, the invention relates to a power generating device. The power generating device comprises

a) at least one first fluid reservoir containing a first fluid;

b) at least one second fluid reservoir containing a second fluid, the second fluid having a higher concentration of solute than the first fluid;

c) at least one forward osmosis cell comprising a semi-permeable membrane, wherein the semi-permeable membrane divides the cell into a first compartment and a second compartment, wherein the first compartment is in fluid communication with the first fluid reservoir and the second compartment is in fluid communication with the second fluid reservoir; and

d) at least one porous medium being arranged (i) between the first fluid reservoir and the first compartment, the porous medium being in fluid communication with the first fluid reservoir and the first compartment, (ii) between the second fluid reservoir and the second compartment, the porous medium being in fluid communication with the second fluid reservoir and the second compartment, or (iii) between the first fluid reservoir and the first compartment, the porous medium being in fluid communication with the first fluid reservoir and the first compartment, and between the second fluid reservoir and the second compartment, the porous medium being in fluid communication with the second fluid reservoir and the second compartment.

[0012] In a second aspect, the invention relates to a water desalination device. The water desalination device comprises

a) a power generating device according to the first aspect, wherein the second fluid reservoir further comprises an outlet, and

b) a pair of electrodes being arranged in the outlet of the second fluid reservoir of the power generating device,

wherein the pair of electrodes defines a channel for the second fluid to flow through and are arranged on opposing sides of the channel, the pair of electrodes being electrically connected to the porous medium of the power generating device, such that potential generated across the porous medium is applied to the pair of electrodes. [0013] In a third aspect, the invention relates to a method of generating power by forward osmosis. The method comprises

a) carrying out forward osmosis between a first fluid and a second fluid across a semi-permeable membrane, the second fluid having a higher concentration of solute than the first fluid, thereby producing a pressure difference across the semi-permeable membrane, and

b) driving the first fluid or the second fluid through a porous medium using the pressure difference produced by the forward osmosis to generate power via the porous medium.

[0014] In a fourth aspect, the invention relates to a method of water desalination. The method comprises

(a) carrying out forward osmosis between a first fluid and a second fluid across a semi-permeable membrane, the second fluid having a higher concentration of solute than the first fluid, thereby producing a pressure difference across the semi-permeable membrane,

(b) driving the first fluid or the second fluid through a porous medium using the pressure difference produced by the forward osmosis to generate power via the porous medium, and

(c) electrically connecting the porous medium to a pair of electrodes, wherein the pair of electrodes define a channel for the second fluid to flow through and are arranged on opposing sides of the channel, so as to apply the potential generated across the porous medium to the pair of electrodes to separate solute from the second fluid.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] The invention will be better understood with reference to the detailed description when considered in conjunction with the non-limiting examples and the accompanying drawings, in which:

[0016] Figure 1 is a schematic diagram of a forward osmosis process with the left-hand side representing the feed solution and the right-hand side denoting the draw solution.

[0017] Figure 2 is a schematic diagram for streaming potential and streaming current resulted from water flow across a micro-channel with electric double layer (EDL, κ '). [0018] Figure 3 is a schematic diagram of a self-sustainable electrokinetic power generation unit utilizing the forward osmosis (FO) effect in the pumping mode. Electrokinetic streaming potential is generated through a porous media column.

[0019] Figure 4 is a schematic diagram of a self-sustainable electrokinetic power generation unit utilizing the forward osmosis (FO) effect in the suction mode. Electrokinetic streaming potential is generated through a porous media column.

[0020] Figure 5 is a schematic diagram of a multiple stack configuration of electrokinetic (EK) power generation units in series and in parallel and multiple stack configuration of forward osmosis (FO) in parallel configuration.

[0021] Figure 6 is a photograph of an experimental setup demonstrating the electrokinetic power generation capability of the invention. In the embodiment shown, deionized water (DI) water was used as feed solution, and various molarity concentrations ranging from 0.5 M to 4 M of sodium chloride (NaCl) solutions were used as draw solutions. Forward osmosis (FO) semi-permeable membrane was provided by HTI® (Albany, USA).

[0022] Figure 7 is a graph showing generated streaming potential at various molarity concentrations of NaCl draw solution.

[0023] Figure 8 is a graph showing current versus potential curve for various molarity concentrations of NaCl draw solution. The figure also shows the streaming current when potential = 0, and the streaming potential when streaming current - 0.

[0024] Figure 9 is a graph showing power densities versus potential curve for various molarity concentrations of NaCl draw solution.

[0025] Figure 10 are photographs showing (A) top view; and (B) front view of an alternate experimental setup for generating direct current (DC) power source via the forward osmosis induced electrokinetic potential technology according to the invention. In the embodiment shown, DI water was used as the feed solution and NaCl solutions were used as the draw solution. A pressure sensor is added between the electrokinetic power generator module and the forward osmosis module to quantify the pressure developed across the porous media column.

[0026] Figure 11 is a schematic diagram of an alternate electrokinetic-forward osmosis power generating system depicted in Figure 10.

[0027] Figure 12 is a graph showing FO fluxes versus draw solution concentration for three conditions: (i) FO open flux without porous media attached (diamond symbol); (ii) FO flux with glass porous media (square symbol); and (iii) FO flux with polyethylene porous media (circular symbol).

[0028] Figure 13 is a graph showing pressure versus flow rate relationship across different types of porous media column of glass and polyethylene.

[0029] Figure 14 are graphs showing (A) streaming potential, and (B) streaming current, as a function of flow rate across glass and polyethylene porous media column.

[0030] Figure 15 is a graph showing measured total channel resistance across various draw solution concentrations.

[0031] Figure 16 is a graph showing projected power density versus FO induced flux across each type of porous media of glass and polyethylene.

[0032] Figure 17 are graphs showing (A) streaming potential and (B) streaming current of glass porous media connected in series; and (C) streaming potential and (D) streaming current of glass porous media connected in parallel, under different applied flow rate.

[0033] Figure 18 is a graph showing the streaming potential with respect to the forward osmosis induced water flow rate across the porous medium where electricity is generated. Both the experimental data and the calculated results are included for comparison, and it shows that the model predictions in general correlate well to the trend demonstrated from the experimental observations.

[0034] Figure 19 is a graph showing the streaming current with respect to the forward osmosis induced water flow rate across the porous medium where electricity is generated. Both the experimental data and the calculated results are included for comparison, and it shows the model predictions in general correlate well to the trend demonstrated from the experimental observations.

[0035] Figure 20 is a comparison table of the performances of various types of renewable energy. Ref. [1] to [4] are references from which the performance values of PRO and RED processes are obtained for comparison purposes. Ref. [1 ] denotes Achilli, A. et al., Journal of Membrane Science, 2009. 343(1-2): p. 42-52. Ref. [2] denotes Lee, K.L. et al., Journal of Membrane Science, 1981. 8(2): p. 141-171. Ref. [3] denotes Loeb, S. et al, Desalination, 2002. 150(2): p. 205-205. Ref [4] denotes Post, J.W. et al, Journal of Membrane Science, 2007. 288(1-2): p. 218-230.

[0036] Figure 21 is a schematic diagram of a capacitive deionization (CDI) process showing A) deionization/desalting process; and B) regeneration process. [0037] Figure 22 is a schematic diagram of a CDI self-sustainable integrated desalination system with direct current (DC) power according to an embodiment of the invention.

[0038] Figure 23 is a schematic diagram of a power generating device using forward osmosis under continuous mode.

DETAILED DESCRIPTION OF THE INVENTION

[0039] In a first aspect the present invention refers to a power generating device. As used herein, the term 'power generating device" refers to a device or apparatus that is able to convert kinetic energy or potential energy (also referred to as Gibbs free energy of mixing) to electrical energy. Accordingly, the terms "power" and "electrical energy" are used interchangeably. Generally, power that is generated may be quantified in terms of the potential (or voltage) and current that is produced by the device using the relationship set out in Equation (1)

[0040] Power = voltage x current Equation (1)

[0041] The power generating device of the present invention utilizes the principles of forward osmosis and electrokinetic effects to generate power. Generally, forward osmosis refers to a natural phenomenon where water is drawn from a feed solution by a draw solution having a higher concentration of a solute across a semi-permeable membrane. The difference in concentration of the solute between the feed solution and the draw solution generates an osmotic pressure gradient, which drives water transport across the membrane.

[0042] An illustration of a forward osmosis process is shown in Figure 1, which is a schematic diagram of a forward osmosis cell comprising a semi-permeable membrane. The semi-permeable membrane divides the cell into a first compartment (left hand side of the semi-permeable membrane) containing a feed solution, and a second compartment (right hand side of the semi-permeable membrane) containing a draw solution. The difference in concentration of the solute between the feed solution and the draw solution generates an osmotic pressure gradient across the semi-permeable membrane, which drives water transport across the membrane in the direction from the first compartment to the second compartment in the direction shown. When water is being drawn from the feed side, solutes such as contaminant and ions may be retained by the membrane. The draw effect by forward osmosis may create suction at the feed side, and a hydrostatic head of water column may be built up on the draw side. [0043] With the above in mind, the power generating device of the present invention comprises at least one first fluid reservoir containing a first fluid, and at least one second fluid reservoir containing a second fluid. The first fluid may form the feed solution and the second fluid may form the draw solution. The first reservoir and the second reservoir may each comprise a feed line and an outlet for fluid flow. Generally, the first fluid and second fluid are aqueous liquids. For example, the first fluid and the second fluid may separately be an aqueous solution that is being treated for the purpose of waste water processing or purified water recovery, such as seawater, brine and other saline solutions, brackish water, mineralized water, tap water, ground water, and industrial waste water. The fluid that is present in the reservoir may be supplied directly from one or more of the above-mentioned sources. For example, the feed line of the first reservoir may be connected to a brackish water supply, while the feed line of the second reservoir may be connected to a seawater supply. In other embodiments, the first fluid and the second fluid may be contained in the respective reservoirs. Accordingly, the power generating device of the present invention may be a stand- alone, self-sufficient device, where connections to external supply of the first fluid and the second fluid are not required.

[0044] The first fluid and the second fluid may comprise a solute. Generally, either one of or both the first fluid and the second fluid may comprise a solute, provided the concentration of the solute in the second fluid is higher than that in the first fluid. The term "solute" as used herein refers one or more substances that are dissolved in a solvent. In embodiments where the solvent is water, the solute comprises an ionic salt species. Examples of ionic salt species include, but are not limited to sodium chloride, magnesium chloride, calcium chloride, potassium chloride, potassium nitrate, ammonium nitrate, sodium di-hydrogen phosphate, and combinations thereof. In one embodiment, the solute is sodium chloride.

[0045] The ionic salt species may dissociate in water into their constituent ions. For example, sodium chloride may dissociate in water to form sodium ions and chloride ions. In so doing, an electrolyte solution may be formed. In various embodiments, both the first fluid and the second fluid contain a solute, with the second fluid having a higher concentration of solute than the first fluid. For example, the first fluid may be fresh water and the second fluid may be seawater. As another example, the first fluid may be brackish water and the second fluid may be seawater. In other embodiments, only the second fluid contains a solute. For example, the first fluid may be deionized water and the second fluid may be ground water or tap water. In embodiments where the solute is sodium chloride, the term "salinity" may also be used to refer to the concentration of sodium chloride, where a higher salinity refers to a higher concentration of sodium chloride in the fluid.

[0046] A difference in solute concentration is required between the first fluid and the second fluid for forward osmosis to take place. The concentration of solute in the first fluid and the second fluid may be any suitable amount that allows forward osmosis to take place. In various embodiments where the solute comprises sodium chloride and the first fluid is deionized water, for example, concentration of sodium chloride in the second fluid may be in the range of about 0.1 M to about 10 M, such as 0.1 M to about 5 M or about 0.5 M to about 4 M. In one embodiment, when the first fluid is formed substantially of deionized water, the concentration of sodium chloride in the second fluid is about 4 M.

[0047] The power generating device of the present invention includes at least one forward osmosis cell having a semi-permeable membrane. The semi-permeable membrane divides the forward osmosis cell into a first compartment and a second compartment. As used herein, the term "semi -permeable membrane" refers to a semi-permeable material that selectively allows certain species to pass through it while retaining others within or on the material. A membrane therefore functions like a filter medium to permit a component separation by selectively controlling passage of the components from one side of the membrane to the other side. Examples of membrane types include tubular membranes, hollow fiber membranes and flat-sheet membranes. Spiral wound membranes, which have been previously used in reverse osmosis (RO) seawater desalination, may also be used.

[0048] Tubular membranes and hollow fiber membranes typically assume the form of hollow tubes of circular cross-section, whereby the wall of the tube may function as the membrane. Generally, a tubular membrane module comprises membrane tubes placed into support pipes, for example, porous stainless steel of fiber glass reinforced plastic pipes. In various embodiments, the feed solution or the first fluid flows down the tube bore, while the draw solution or the second fluid flows on the outer side of the porous support pipe. Due to the difference in concentration of solute in the first fluid and the second fluid, forward osmosis takes place across the semi-permeable membrane, and the permeate is collected on the outer side of the porous support pipe across the membrane, where it is joined with the second fluid flow. Accordingly, the first compartment corresponds to the region defined by the tube bore at which the feed solution or first fluid flows. The second compartment corresponds to the region defined by the other side of the porous support pipe at which the permeate and the second fluid flow. In alternate embodiments in which the first fluid flows on the outer side of the porous support pipe and the second fluid flows in the tube bore, the configuration of the first compartment and the second compartment are reversed. The diameters of tubular membranes are typically in the range of about 1 cm to about 2.5 cm. In some modules, the membranes are cast directly on the porous pipes, while in others they are prepared separately as tubes and then installed into the support pipes.

[0049] For hollow fiber membranes, the selective layer is present as, or as part of, the fiber wall. In various embodiments, the feed solution or the first fluid is passed into the housing and along the outer cylindrical surfaces of the tubes. The draw solution or the second fluid flows through the center of the membrane tubes. Due to the difference in concentration of solute in the first fluid and the second fluid, forward osmosis takes place across the semipermeable membrane, and the feed water in the feed solution may permeate the membrane tubes, and join the draw solution or the second fluid to travel through the center of all membrane tubes, until it reaches one end of the housing. Retentate that does not permeate the membrane tubes may exit the housing module through a retentate outlet that is located at this other end of the housing. Accordingly, the first compartment may be defined by the region along the outer cylindrical surfaces of the tubes where the first fluid flows, and the second compartment may be defined by the center of the membrane tubes where the second fluid and the permeate flows. In alternative embodiments where the first fluid flows in the center of the membrane tubes, and the second fluid flows along the outer cylindrical surfaces of the tubes, the first compartment may be defined as the center of the membrane tubes where the first fluid flows, and the second compartment may be defined as the region along the outer cylindrical surfaces of the tubes where the second fluid flows.

[0050] Flat-sheet membranes, on the other hand, are formed from one or more sheets of membrane material placed adjacent to or bonded to one another. The sheets of membrane material may be packed in a housing. In use, the feed solution or the first fluid may be introduced on one side of the membrane material, while the draw solution or the second fluid may be introduced on the other side of the membrane material. In various embodiments, the first fluid and the second fluid may flow in a tangential direction to the surface of the membrane to allow sufficient time for mass transfer of the solute to take place. The first fluid and the second fluid may flow in the same direction i.e. co-current flow, or in opposite directions i.e. counter-current flow. As a result of differences in concentration of solute in the first fluid and the second fluid, forward osmosis takes place across the membrane material. In various embodiments where the first fluid is brackish water and the second fluid is seawater, for example, the feed water in the first fluid may permeate the membrane sheet, where the permeate is joined with second fluid in the other side of the membrane sheet. Accordingly, in these embodiments, the first compartment corresponds to the side of the membrane at which the first solution flows, and the second compartment corresponds to the side of the membrane at which the second solution and the permeate flows.

[0051] Spiral wound membrane refers to a structure in which a membrane sandwich, which may be formed from two flat sheet membranes separated by a flat sheet porous channel spacer member, is wound about a permeate collection tube to assume a cylindrical configuration. The spiral wound membrane may be packed in a cylindrical housing to form a cylindrical module. In various embodiments, a feed solution or first fluid is passed into one end of the cylindrical module, where the first fluid flows along one side of the wound membrane sandwich. A draw solution or second fluid is introduced into the same or different end of the cylindrical module, where the second fluid flows in the channel spacer member. Due to forward osmosis that takes place as a result of difference in concentration of solute between the first fluid and the second fluid, the feed water from the first fluid permeates the membrane, and passes into the channel spacer member as permeate. The permeate and the second solution travels in a spiral, until it reaches the center of the module, where the permeate and the second solution flow through small holes present in the collection tube, and exit the module through an outlet. As the same time, retentate that does not permeate the membrane may exit the module through an outlet at the opposite end of the module. Accordingly, the first compartment of the forward osmosis cell comprising a spiral wound membrane as the semi-permeable membrane corresponds to the region at one side of the wound membrane sandwich in which the first fluid flows. The second compartment of the forward osmosis cell on the other hand corresponds to the region encompasses by the channel spacer member at which the second fluid and the permeate flow.

[0052] In one embodiment, the semi-permeable membrane is a hollow fiber membrane. The inventors of the present invention have found that the use of a hollow fiber membrane results in a higher power output, which may be due to the higher surface to volume ratio of the membrane. In another embodiment, the semi-permeable membrane is a spiral bound membrane, which may be preferred in some applications as these membranes are packed in module form for ease of replacement and maintenance.

[0053] The first compartment of the forward osmosis cell is in fluid communication with the first fluid reservoir, and the second compartment of the forward osmosis cell is in fluid communication with the second fluid reservoir. In such a configuration, the first fluid that is contained in the first fluid reservoir flows from the first fluid reservoir into the first compartment of the forward osmosis cell, and is in contact with the semi-permeable membrane. In various embodiments, the first compartment of the forward osmosis cell comprises an outlet, to allow the first fluid to flow out from the first compartment. In some embodiments, the outlet of the first compartment is connected to the first reservoir, such that part of the first fluid is allowed to flow back to the reservoir. By injecting a fresh supply of the first fluid into the forward osmosis cell, a concentration gradient between the first fluid and the second fluid may be maintained to provide for a continuous mode of operating the power generating device.

[0054] Similarly, on the other side of the semi-permeable membrane, the second fluid that is contained in the second fluid reservoir flows from the second fluid reservoir into the second compartment of the forward osmosis cell, and is in contact with the semi-permeable membrane. The second compartment of the forward osmosis cell may comprise an outlet, to allow the second fluid to flow out from the second compartment. In some embodiments, the outlet of the second compartment is connected to the second reservoir, such that part of the second fluid is allowed to flow back as a recycle stream to the reservoir. As in the case for the first fluid, provision of a fresh supply of the second fluid into the forward osmosis cell maintains a concentration gradient between the first fluid and the second fluid so as to provide for a continuous mode of operating the power generating device.

[0055] In various embodiments, the difference in concentration of the solute between the first fluid and the second fluid generates an osmotic pressure gradient across the semipermeable membrane, which drives water transport across the membrane from the first compartment to the second compartment, while retaining the solute in the first compartment. The first fluid and the second fluid may flow in a co-current configuration, i.e. flowing tangential to the membrane and in the same direction. The first fluid and the second fluid may alternatively flow in a counter-current configuration, i.e. flowing tangential to the membrane but in opposite directions, to allow time for the solute concentration to equilibrate between the first fluid and the second fluid, and to provide a constant osmotic pressure gradient along the semi-permeable membrane. In so doing, the forward osmosis process may be more efficient.

[0056] The power generating device of the present invention is further based on the principle of electrokinetic effects to generate power.

[0057] Figure 2 is a schematic diagram illustrating the principle of an electrokinetic effect as a result of fluid flow through a channel. The channel, having length L and radius r, may be made from a dielectric material such as glass. When a feed solution such as river or fresh water is allowed to flow through the channel, surface of the channel wall may be charged due to a combination of factors, such as surface ionization of the channel wall, as well as ion adsorption and ion dissolution of the feed solution. An electric double layer (EDL with its thickness denoted by κ ^"1 ) may be formed. The counter ions in the diffuse layer (mobile part) of the electric double layer (EDL) are carried towards the downstream end, resulting in an electrical current in the pressure-driven flow direction. This current is called the streaming current Is- Due to the migration of net amount of cations and anions, a potential difference between the inlet and outlet of the channel, called the streaming potential Vs, may be built up along the channel. Power is thereby generated as a result of the streaming current and streaming potential that is produced in the channel.

[0058] The power generating device of the present invention comprises at least one porous medium being arranged (i) between the first fluid reservoir and the first compartment, the porous medium being in fluid communication with the first fluid reservoir and the first compartment, (ii) between the second fluid reservoir and the second compartment, the porous medium being in fluid communication with the second fluid reservoir and the second compartment, or (iii) between the first fluid reservoir and the first compartment, the porous medium being in fluid communication with the first fluid reservoir and the first compartment, and between the second fluid reservoir and the second compartment, the porous medium being in fluid communication with the second fluid reservoir and the second compartment.

[0059] By arranging the at least one porous medium between and in fluid communication either of, or both, the first and second fluid reservoirs and the corresponding first and second compartments of the forward osmosis cell, the driving force that is generated as a result of forward osmosis through the semi-permeable membrane drives the first fluid and/or the second fluid through the pores in the porous medium. A streaming current and streaming potential is produced across the porous medium due to electrokinetic effects of the fluid flow through the porous medium. As a result, power is generated.

[0060] The streaming current and potential, hence the amount of power, that may be generated, may be controlled by varying, for example, the porosity of the porous medium and the length of the channels in the porous medium.

[0061] The pores in the porous medium may be regular or irregularly shaped. The size of the pores may be characterized by their mean diameter. The term "diameter" as used herein refers to the maximal length of a straight line segment passing through the center of a figure and terminating at the periphery. Accordingly, the term "mean diameter" refers to an average diameter of the pores, and may be calculated by dividing the sum of the diameter of each pore by the total number of pores. The mean diameter of the pores in the porous medium can range between about 10 nm to about 10 μτη, such as about 10 run to about 500 nm, about 100 nm to about 1 μιη, about 500 nm to about 1 μηι, about 1 μιη to about 10 μηι, or about 5 μπι to about 10 μηι. In various embodiments, the pores in the porous medium may be at least substantially monodisperse, in order that there is uniform distribution of fluid flow through the pores.

[0062] The pores in the porous medium may form channels. Depending on the size of the pores, the channels in the porous medium may also be termed as nano-channels or micro- channels. Generally, the length of the micro-channels may range from about a few milimeters to about a few centimeters, such as about 1 mm to about 5 cm, for example about 1 mm to about 2 cm, about 1 mm to about 1 cm, or about 1 cm to about 5 cm.

[0063] The porous medium may comprise a dielectric material. In some embodiments, the porous medium consists substantially or consists of a dielectric material. Examples of dielectric material include, but are not limited to, glass, a polymer such as polyethylene, ceramic, silicon oxide, silicon nitride, silicon oxynitride, and combinations thereof. In various embodiments, the dielectric material is glass. In other embodiments, the dielectric material is polyethylene.

[0064] Using a power generating device according to various embodiments of the present invention, streaming potential in the range of few tens to hundreds of millivolts have been recorded, with corresponding streaming current in the range of hundreds of nanoampere. The values of streaming potential or streaming current obtained may depend on the membrane material type, configuration of the membrane (flat or hollow fiber), and concentration gradient applied across the forward osmosis membrane, the type of material used and the size of pores (which determine the number of channels in parallel) in the porous medium, as well as the type of solution used for the fluids. In some embodiments, the projected power density is in the order of 10 ¹ W/m ² , which is comparable to the power density produced by pressure retarded osmosis (PRO) and reverse electrodialysis (RED).

[0065] In various embodiments, the porous medium is arranged between the first fluid reservoir and the first compartment. It has been found by the inventors of the present invention that a higher power performance is obtained when the porous medium is arranged in such a configuration i.e. in the suction mode.

[0066] Figure 23 is a schematic diagram of a power generating device according to an embodiment of the invention. In the embodiment shown, a forward osmosis cell comprising a spiral wound membrane is used, and the power generating device is being operated under continuous mode. The first fluid reservoir comprises a feed solution as the first fluid, while the second fluid reservoir comprises a draw solution as the second fluid. As depicted in the figure, the first fluid and the second fluid are each pumped by a separate rotary pump across the forward osmosis cell.

[0067] As described above, the first fluid may flow along one side of the wound membrane, and the second fluid may flow in the channel spacer member. A porous medium, indicated as the E Power Generator Module in the diagram, is being arranged between the first fluid reservoir and the first compartment, wherein the porous medium is in fluid communication with the first fluid reservoir and the first compartment. The porous medium may alternatively, or additionally, be arranged between the second fluid reservoir and the second compartment, wherein the porous medium is in fluid communication with the second fluid reservoir and the second compartment. Due to differences in concentration of solute between the first fluid and the second fluid, forward osmosis takes place across the membrane, and feed water is drawn across the membrane from the first fluid to the second fluid. The pressure different produced by the forward osmosis acts in combination with the pressure exerted on the first fluid by the rotary pump, to drive the first fluid through the porous medium to generate power via the porous medium.

[0068] As depicted in the figure, a part of the first fluid may be channeled into the first fluid reservoir, while the rest is being returned as a recycle stream to combine with a fresh stream of first fluid, for flow into the porous medium. The second fluid containing permeate, on the other hand, may be channeled back into the second fluid reservoir after flow through the forward osmosis cell. By carrying out the process flow as described above, power generation by the power generating device may take place continuously. In some embodiments, the first fluid reservoir and the second fluid reservoir may be connected to a continuous source of fluid supply, such as a river stream and the sea, such that concentration of the first fluid and the second fluid may be maintained at a substantially constant level. Accordingly, a constant osmotic pressure gradient may be maintained across the semipermeable membrane.

[0069] Depending on the type of application, the power generating device may comprise two or more forward osmosis cells and/or two or more porous media. For example, two or more forward osmosis cells may be stacked and connected in parallel to produce a higher water flow rate. In some embodiments, two or more porous media may be stacked and arranged between the fluid reservoir and the forward osmosis cell in parallel to generate a higher current flow. In other embodiments, two or more porous media may be stacked and arranged between the fluid reservoir and the forward osmosis cell in series to generate a higher potential across the porous media. The forward osmosis cells and the porous media may be stacked in any suitable combination and configuration, depending on the amount of power required for generation, and which in turn depends on the specific application.

[0070] A power generating device according to various embodiments of the present invention is advantageous in that it is self sustainable. Unlike convention power generation methods such as power generation by burning fossil fuel, the process is environmental friendly as there is zero emission. Furthermore, given that the power generating device may be operated with no mechanical moving parts, harzards and risks to personnel associated with high pressure operations are eliminated. The non-use of mechanical moving parts also means that maintenance is relatively simple and fuss free. As power generation is independent on the forces of nature, power generation using the power generating device of the invention may be continuous and stable, in contrast to convention green technology methods that use solar, wind and wave energy which suffer from intermittent power generation. Furthermore, the power generation device is scalable to meet user requirements depending on the type of operation. For example, in microscale operations, the power generation device may be made as a standalone, portable unit having a small footprint, while for larger scale operations, the power generation device may be stacked to provide a larger power source. More importantly, as the power generating device may use fresh water and sea water as the input fluids, these sources are plentiful in nature and may be obtained relatively easily, thereby providing a mass producible, cheap and economical source of power.

[0071] In a second aspect, the invention relates to a water desalination device. The water desalination device includes a power generating device according to the first aspect, wherein the second fluid reservoir further comprises an outlet.

[0072] Besides the use of forward osmosis and electrokinetic effects which is utilized in power generation to power the water desalination device, the water desalination device further includes the use of a capacitive deionization (CDI) process for water desalination.

[0073] As used herein, the term "capacitive deionization" refers to an electrochemical process, in which an electric potential is applied to a pair of electrodes to adsorb the ions that present in a fluid flowing between the electrodes onto the surface of the electrodes. The adsorption may arise from electric double layer (EDL) that is formed when the surface of the porous electrodes is in contact with an electrolyte solution.

[0074] Figure 21 is a schematic diagram of a capacitive deionization (CDI) process showing A) deionization/desalting process; and B) regeneration process. Structurally, a capacitive deionization device may comprise the following components: 1) a pair of ion adsorption porous electrodes, 2) a channel/spacer for allowing a fluid to flow through, and 3) a pair of current collectors for providing electric potential or direct current (DC) voltage.

[0075] Referring to Figure 21A, when potential is applied across the electrodes, ions in the flowing fluid may migrate to the oppositely charged electrodes and are held within the electric double layer on the surface of the electrodes. This is known as the deionization or desalting process. In various embodiments, seawater comprising sodium chloride as solute is used as the fluid. Upon application of a potential across the electrodes, sodium ions and chloride ions that are present in the fluid migrate to the oppositely charged electrodes, thereby desalting the seawater in the process.

[0076] As depicted in Figure 2 IB, for example, upon removal of the applied potential, the adsorbed ions will be released back to the flowing fluid. This process is known as the regeneration process, and may be used to regenerate the electrodes to produce a more concentrated effluent stream.

[0077] With the above in mind, the water desalination device of the present invention includes a pair of electrodes, wherein the pair of electrodes is being arranged in the outlet of the second fluid reservoir of the power generating device. The pair of electrodes defines a channel for the second fluid to flow through and is arranged on opposing sides of the channel. The pair of electrodes is electrically connected to the porous medium of the power generating device, such that potential generated across the porous medium is applied to the pair of electrodes. Materials that may be used to form the electrodes are generally porous. Examples of material that may be used to form the electrodes include, but are not limited to carbon aerogel, porous carbon particles, carbon nanotubes and carbon nanofibers, carbon cloth and/or sheet.

[0078] As mentioned herein, fluid flow is initiated by the at least one forward osmosis cell of the power generating device as a result of osmotic pressure gradient generated across the semi-permeable membrane. The osmotic pressure gradient generated may drive water across the membrane from the first compartment to the second compartment of the forward osmosis cell due to the forward osmosis phenomenon. Depending on the number of porous medium used and the arrangement of the porous medium, either of or both the first fluid and the second fluid pass through the porous medium to generate a streaming potential across the porous medium. As described previously, the streaming potential is generated due to electrokinetic effects, which happens as a result of hydrodynamic flow carried net charge density in the electric double layer of the surface of the pores in the porous medium. Such generated electrokinetic potential from the porous medium may be used as an energy source in the form of potential or direct current (DC) voltage directly applied to the pair of electrodes.

[0079] In various embodiments, two or more pairs of electrodes may be used. The two or more pairs of electrodes may be stacked in parallel to form a capacitive deionization stack, defining multiple channels for an increased volume of fluid flow.

[0080] In some embodiments, a voltage switch may be included in the water desalination device to control applied voltage to the capacitive deionization stack cyclically. For example, the "on" mode of the switch may be employed for deionization (for water desalination), and the "off mode of the switch may be used for regeneration (to produce a more concentrated effluent stream). The voltage switch may be used in conjunction with flow valves to separate the desalted and concentrated stream. For example, part of the desalted stream and concentrated stream may be channeled back to the first fluid reservoir and the second fluid reservoir respectively to replenish the depleted volume. [0081] The water desalination device according to an aspect of the invention is advantageous in that fluid flow may be driven purely by the forward osmosis process without the need for any external pumping. In embodiments whereby pumping is used, for example, to obtain a continuous mode of water desalination, the fluid flow that is driven by forward osmosis may act in combination with external pumping for energy efficiency. This in turn translates into lower energy consumption, and lesser maintenance, thereby resulting in lower costs. Furthermore, the device is self-sustaining as the potential that is generated across the porous medium may be applied across the pair of electrodes for capacitive deionization of the fluid to take place. In addition, the low potential operation (of a range about 1 V to about 2 V) means that no electrolysis may take place. Besides use of the device as a standalone unit, the device may also be used as an additional recovery system on existing desalination plants.

[0082] According to a third aspect, the present invention relates to a method of generating power by forward osmosis. The method comprises carrying out forward osmosis between a first fluid and a second fluid across a semi-permeable membrane, the second fluid having a higher concentration of solute than the first fluid, thereby producing a pressure difference across the semi-permeable membrane. Suitable materials that may be used for the solute have already been described above.

[0083] The method further comprises driving the first fluid or the second fluid through a porous medium using the pressure difference produced by the forward osmosis to generate power via the porous medium. In various embodiments, the porous medium comprises a dielectric material. For example, dielectric material may be selected from the group consisting of glass, polymer, ceramic, silicon oxide, silicon nitride, silicon oxynitride, and combinations thereof. In one embodiment, the dielectric material is glass.

[0084] The method of generating power by forward osmosis may be a continuous method. For example, the first fluid may be fed into the system at the same rate at which the second fluid is removed from the system. In so doing, power may be generated on a continuous basis. An example of a power generating unit operating under continuous mode such as that shown in Figure 23 has already been described above.

[0085] According to a fourth aspect, the invention relates to a method of water desalination. The method comprises carrying out forward osmosis between a first fluid and a second fluid across a semi-permeable membrane, the second fluid having a higher concentration of solute than the first fluid, thereby producing a pressure difference across the semi-permeable membrane. The method also comprises driving the first fluid or the second fluid through a porous medium using the pressure difference produced by the forward osmosis to generate power via the porous medium, and electrically connecting the porous medium to a pair of electrodes, wherein the pair of electrodes define a channel for the second fluid to flow through and are arranged on opposing sides of the channel, so as to apply the potential generated across the porous medium to the pair of electrodes to separate solute from the second fluid.

[0086] The invention illustratively described herein may suitably be practiced in the absence of any element or elements, limitation or limitations, not specifically disclosed herein. Thus, for example, the terms "comprising", "including", "containing", etc. shall be read expansively and without limitation. Additionally, the terms and expressions employed herein have been used as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding any equivalents of the features shown and described or portions thereof, but it is recognized that various modifications are possible within the scope of the invention claimed. Thus, it should be understood that although the present invention has been specifically disclosed by preferred embodiments and optional features, modification and variation of the inventions embodied therein herein disclosed may be resorted to by those skilled in the art, and that such modifications and variations are considered to be within the scope of this invention.

[0087] The invention has been described broadly and genetically herein. Each of the narrower species and subgeneric groupings falling within the generic disclosure also form part of the invention. This includes the generic description of the invention with a proviso or negative limitation removing any subject matter from the genus, regardless of whether or not the excised material is specifically recited herein.

[0088] Other embodiments are within the following claims and non- limiting examples. In addition, where features or aspects of the invention are described in terms of Markush groups, those skilled in the art will recognize that the invention is also thereby described in terms of any individual member or subgroup of members of the Markush group. EXPERIMENTAL SECTION

[0089] Example 1: General process scheme [0090] Principles of forward osmosis (FO) and electrokinetic processes are utilized in the present invention. A forward osmosis process is integrated with the streaming potential/streaming current generated from an electrokinetic process to generate power which may be used directly as a form of electricity.

[0091] Figures 3 and 4 illustrate two embodiments of a self-sustainable power generation unit that is designed on the basis of the present invention. In Figure 3, the power generation unit is in a pumping mode. In Figure 4, the power generation unit is in a suction mode. Electrokinetic streaming potential is generated through a porous media column, where water in the feed solution is pumped or drawn respectively through the column.

[0092] The forward osmosis induced pressure (or pump) in the forward osmosis unit causes water to flow due to osmotic pressure gradient. By connecting a porous media column to the forward osmosis unit, water is driven through the pores of the porous media column to generate electrokinetic streaming potential across the column. The porous media may comprise various dielectric materials, such as glass, polymer, and ceramic. The size of the pores in the porous media may generally be at the micro and/or nano-scale level. Due to the wide range of relatively inexpensive materials that may be used as well as the ease of fabrication of the porous media, costs of manufacturing may be greatly reduced compared to the state of the art methods in which the channels are custom manufactured through techniques such as micro fabrication. Since the pores are present in the porous media as an array of channels arranged in parallel, streaming current may be improved greatly.

[0093] Figure 5 shows individual power generation units connected or stacked in series or in parallel manner. When the power generation units are connected in series, such multiple- stack of electrokinetic power generation is expected to produce higher streaming potential. On the other hand, when the power generation units are stacked in parallel, larger streaming currents are expected. Therefore, by stacking the power generation units such as that shown in Figure 5, both the streaming potential and the streaming current may be scaled up to a desired value for practical applications.

[0094] Figure 22 is a schematic diagram of a CDI self-sustainable integrated desalination system with DC power according to an embodiment of the invention.

[0095] Example 2: Experimental setup of power generation device

[0096] Figure 6 shows an actual experimental setup of a power generating device according to an embodiment of the present invention to demonstrate the functionality of such self-sustainable electrokinetic power generation unit. The experimental setup comprises four parts, including (i) and (ii) two chambers containing respectively the feed and draw solutions, (iii) a porous media column for electrokinetic power generation, and (iv) a forward osmosis module for generating water flow. A forward osmosis membrane is sandwiched in the forward osmosis module to separate the feed solution from the draw solution. The forward osmosis module and porous media holder were custom made in house. The forward osmosis membrane was provided with compliments from Hydration Technology Innovations (HTI, Albany, OR). This membrane is widely utilized in various forward osmosis experiments for sea water desalination, and it is the only commercial FO membrane available in the market. It is actually a flat sheet/plate and frame membrane with a thickness of about 200 μηι. The

2 2

membrane is able to generate water fluxes ranging from 5 liter/m .hr to 20 liter/m .hr (LMH) depending on the concentration or molarity of the draw solution used.

[0097] In the demonstration experiment, sodium chloride (NaCl) solutions of 0.5 M to 4 M were used as draw solution. The feed solution was deionized (DI) water. Polyethylene (PE) porous media column of diameter 20 mm, thickness of 8 mm and 20 μηι pore size was used. A pair of silver/silver chloride (Ag/AgCl) electrode meshes was placed at the front and back of the porous media column. The two electrodes were connected to a source meter (Keithley 2612 A) for recording the streaming potential and streaming current generated through the porous media column when water flow was induced by the forward osmosis module.

[0098] Example 3: Results

[0099] Preliminary results obtained from the power generating device disclosed herein are depicted in the graphs shown in Figures 7 to 9.

[00100] Figure 7 is a graph showing generated streaming potential at various molarity concentrations of NaCl draw solution. The forward osmosis induced pressure has been fully built up to drive water flow across the porous media. A higher molarity of the draw solution may produce higher osmotic pressure gradient, which translates into a higher water transport flux. From the results obtained and summarized in Figure 7, it may be seen that a 4 M NaCl draw solution generates the highest streaming potential. Therefore, it may be concluded that a higher concentration of draw solution can produce a larger streaming potential. It is noted that approximately 120 mV was recorded from the experiment using 4 M NaCl draw solution. [00101] Once the streaming potential reached steady value, current test was carried out to measure the current loading of the experimental power generating device disclosed herein. Sweeping of a known voltage to the test unit would produce current output. Figure 8 is a graph showing the current-potential curve of the experimental test unit shown in Figure 6. The highest current generated is approximately about 0.1 μΑ at 3 M NaCl draw solution. In addition, Figure 9 depicts the produced power density versus potential curve at various molarity concentrations of NaCl draw solution. It is noted that the power density produced by this experimental power generating device is of order about mW/m ³ . The concept of this invention has been demonstrated that it is capable of generating power.

[00102] Example 4: Alternative experimental setup of power generation device

[00103] An alternative experimental setup is shown in Figures 10 and 11. In this configuration, forward osmosis fluxes are computed by measuring the change in weight per unit time per membrane area in the feed reservoir to provide a volumetric flow induced across the porous media column. Meanwhile, a pressure sensor is also added between the electrokinetic power generator module and the forward osmosis module in order to quantify the pressure developed across the porous media column. This allows characterization of pressure versus flow rate relationship of different porous media columns.

[00104] Example 5: Comparison between different types of porous media column

[00105] Further to the preliminary results obtained earlier by using polyethylene porous columns, tests were also carried out using glass based porous columns. Comparisons between different types of materials, channel geometries such as pore size, porosity and tortuosity, and surface properties have been made and presented below.

[00106] Figure 12 is a graph showing FO fluxes versus draw solution concentration for three conditions: (i) FO open flux without porous media attached (diamond symbol); (ii) FO flux with glass porous media (square symbol); and (iii) FO flux with polyethylene media (circular symbol), and serves to illustrate the fluxes induced across two types of porous media columns at various NaCl draw solution concentrations. As a baseline, the open fluxes without porous media column show much higher value. This is due to hydrodynamic drag induced by flow through the porous medium column. The magnitude of fluxes in glass porous media is relatively higher than those in polyethylene porous media, which may be attributed to the differences in channel geometries, structural and surface properties. In terms of channel geometries, although the porous glass media have similar pore size and surface area as the polyethylene porous media, the thickness of glass is only half that of polyethylene. Glass is also much a rigid material compared to polymer polyethylene; the latter may swell in liquid and thus probably disturbs the flow profile across it. Moreover, the surface of glass porous media is hydrophilic and thus should carry higher electrical charges than polymer porous media with hydrophobic surface.

[00107] With the fluxes obtained, volumetric flow rate can be easily computed using the size of membrane. Together with the pressure measurement, the pressure versus flow rate relationship across each porous media can be characterized and are illustrated in Figure 13. Figure 13 is a graph showing pressure versus flow rate relationship across different types of porous media column of glass and polyethylene. As can be seen from the figure, pressure developed across glass porous media column is always higher, which may be attributed to its higher tortuosity and lower porosity as it is more closely packed in glass than polyethylene porous media.

[00108] Figure 14 A and 14B show the streaming potential and streaming current across each type of porous media column (glass and polyethylene) at various flow rates. Specifically, the flow rate is the self induced solely by the forward osmosis mechanism. Results show that both streaming potential and streaming current are linearly proportional to the induced flow rate. It is expected that as higher flow rate results in higher flow velocity where more ions can be transported downstream and consequently higher electrical power can be generated.

[00109] Although similar values of streaming potential were generated for the porous media tested, the streaming current measured for glass is much higher than that for polyethylene. This may largely be attributed to the higher number of channels per unit surface area of the glass porous media as compared to that for polyethylene, as the glass porous media is more densely packed. Hence, more channels are connected in parallel and higher current may be generated in the glass porous media column. It may also be due to higher surface charge density of glass.

[00110] The total resistance across each type of porous media were also measured, and it was found that glass based porous media column has much lower electrical resistance than polymer based polyethylene porous media column such as that shown in Figure 15. Significantly, polyethylene porous media has recorded resistance of at least one order magnitude higher than glass at mega Ohm level. Resistance may be attributed to various factors such as channel geometries, surface properties, and the chemical properties between the electrolyte solution and channel surface. Using circuit theory, it is possible to keep the total electrical resistance within merely a few tens of Ohms per unit area (a higher number of channels per unit area will give rise to lower resistance, as discussed in Example 6) of the porous medium employed. Based on experimental results obtained, it was computed that the power performance of a power generating device disclosed herein may reach an order of magnitude of 10 ¹ W/m ² under different forward osmosis fluxes induced. Consequently, development and adaptation of better performance membrane and more effective porous media are the keys for enhancing the power generation performance.

[00111] Example 6: Power density calculation

[001 12] Figure 16 is a graph showing projected power density versus FO induced flux across each type of porous media of glass and polyethylene.

[00113] Maximum power generated can be calculated as [00114] P _ma Equation (2) [001 15] where the corresponding maximum output voltage is half of the streaming potential voltage ^v s , and ^R Totai is the total electrical resistance of the porous media column. Since a porous medium can be considered as an array of microchannels connected in parallel, the total resistance is equivalent to

1 _ 1 1 1

[001 16] — ^{+ +} ' " ^{+ ~} R ^~ Equation (3)

Total channel, \ channel,! channel, N

[001 17] Assumin that all channels are identical, namely

[001 18] ~ & Equation (4)

[00119] and substituting Equation (4) into Equation (3), we get

[00120] R _Total =—

M Equation (5)

[00121] where N is the number of channels consisted in the porous medium which can be calculated by

[00122] # =— y

m Equation (6) [00123] Where ^a is the average pore size, A _e is the effective cross-sectional area of the porous medium described as a factor of the original cross sectional area ^A as:

[00124] A _e = A Equation (7)

Vr

[00125] where Ψ and ^τ are the porosity and tortuosity of the porous medium, respectively. Hence based on the method described above, the total resistance per unit area of the porous medium can be reduced to a value of merely few tens of Ohms. Furthermore, as the total current generation is the summation of individual channels, power density is only dependent on the magnitude of liquid flux across the porous medium generated by forward osmosis alone.

[00126] Example 7: Multiple configurations connected in parallel and in series

[00127] As mentioned in Example 1, one way of elevating the power performance of a power generating device described herein may be carried out by connecting the electrokinetic power generator module in parallel and in series analogous to common batteries connected in the same manner.

[00128] Experiments were carried out to verify these concepts with total of up to four electrokinetic power generator modules employed. Since glass porous media have been demonstrated to be superior to the polyethylene porous media, the experiments were conducted by utilizing glass porous media only.

[00129] From the results obtained, augmentation of streaming potential is achieved by connecting the power generator modules in series. On the other hand, augmentation of current is realized by connecting the modules in parallel. The potential and current produced is almost proportional to the number of electrokinetic power generator employed as well as the applied flow rate across the porous media. Hence, it is possible to scale up the system according to the specific power requirements of various applications. Figure 20 is a comparison table of the performances of various types of renewable energy.

[00130] A power generating device of the invention may be used in applications ranging from a micro to a macro scale level. In a micro scale level, it may be used to power up micro devices such as a micro pump, and micro sensor. In a medium scale level, it can be regarded as a portable power sources for electronic devices, home electrical appliance, and lighting, for example. In a much bigger scale, this invention can lead to building of a supplementary power plant to supply additional power that complements the main power plants such as a hydropower plant, and nuclear plant.