Electrodialysis reversal or pressure retarded osmosis cell with heat pump

Abstract

A method and system for generating electricity or hydrogen from thermal energy is disclosed. The method includes separating a first salt solution from a second salt solution by a selectively permeable membrane (104), receiving thermal energy from a heat source by the first salt solution and / or the second salt solution, mixing the first salt solution and the second salt solution in a controlled manner, and capturing at least a portion of the salinity gradient energy as electricity as the salinity difference between the first salt solution and the second salt solution decreases. The method further includes transferring thermal energy from the first salt solution to the second salt solution by a heat pump (320), increasing the salinity difference between the first salt solution and the second salt solution. The method and system can include regeneration processes such as membrane distillation, forward osmosis, electrodialysis, salt evaporation, and / or salt decomposition for further energy efficiency and power generation.

Description

[Technical Field]

[0001] The present technology relates generally to salt gradient heat engine systems and methods for producing electrical power and / or hydrogen from thermal energy. [Background technology]

[0002] Salinity gradient power is energy generated from the salinity difference between two fluids (usually freshwater and saltwater), which occurs naturally, such as when a river flows into the ocean. Reverse electrodialysis (RED) can be used to extract energy from the salinity gradient, for example, by passing saltwater and freshwater through a stack of alternating cation-exchange and anion-exchange membranes. The chemical potential difference between the saltwater and freshwater generates a voltage across each membrane, and the overall system potential is the sum of the potential differences across all membranes. An open-loop RED battery requires a continuous supply of saltwater and freshwater to maintain the salinity gradient. This constraint can limit the practical installation locations of commercial-scale RED batteries. Furthermore, open-loop RED batteries are susceptible to contamination from minerals, microorganisms, and other foreign particles and substances in the water source. A closed-loop RED cell does not require a continuous source of concentrated and dilute saline solutions, but it does require that the salinity difference between the concentrated and dilute solutions be continually regenerated, which can be energy intensive and / or inefficient.

[0003] SUMMARY Described herein are methods and systems for addressing the above and / or other problems. Summary of the Invention

[0004] A method for generating electrical power from thermal energy is disclosed. The method includes separating a first salt solution from a second salt solution with a selectively permeable membrane, transferring thermal energy to the first salt solution and / or the second salt solution with a heat pump, mixing the first salt solution and the second salt solution in a controlled manner, and capturing at least a portion of the salinity gradient energy as electrical power as the salinity difference between the first salt solution and the second salt solution decreases. This may include regenerating the salinity difference between the first salt solution and the second salt solution by applying a regeneration process selected from the group consisting of salt splitting, electrodialysis, membrane distillation, evaporation, forward osmosis, salt precipitation, or any combination thereof.

[0005] The salt decomposition process may include providing at least a portion of a spent dilute solution formed from a first salt solution, the spent dilute solution including salt; heating the spent dilute solution to decompose the salt and generate at least one gaseous product; transferring the at least one gaseous product to a cold solution; and solidifying the gaseous product to reform as a salt precipitate in the cold solution. If the method includes generating a third salt solution by membrane distillation, the method may further include mixing the third salt solution with the first salt solution and / or the second salt solution. The first salt solution and second salt solution methods may include circulating the solutions in a substantially or completely closed system.

[0006] The permselective membrane may be composed of graphene, graphene oxide, or reduced graphene oxide, and may optionally have nanopores. The permselective membrane may be a single thin sheet, a multi-layer sheet, or a cartridge.

[0007] The method may further include capturing salinity gradient energy using reverse electrodialysis or using pressure retarded osmosis to drive an electrical generator.

[0008] The method may include transferring thermal energy from a first salt solution to a second salt solution to precipitate salt in the first salt solution, and optionally further including introducing the precipitated salt into the second salt solution to increase a salinity difference between the first salt solution and the second salt solution.

[0009] The method may include using a portion of the generated electrical power to produce hydrogen gas by electrolysis.

[0010] The method may include a regeneration process applying salt decomposition, which may include providing a spent dilute solution formed from a first salt solution, the spent dilute solution including salt; heating the spent dilute solution to decompose the salt and generate gaseous products; transferring the gaseous products to an absorber; and solidifying and reforming the gaseous products as salt precipitates in the spent strong solution in the absorber. Transferring the gaseous products to the absorber reduces the salt content of the spent dilute solution to regenerate the first salt solution. The salt precipitate may be dissolved in the spent strong solution to regenerate a second salt solution, and optionally, the salt content of the spent dilute solution may be higher than that of the first salt solution.

[0011] The method may include a regeneration process including electrodialysis, which may include providing a spent dilute solution formed from a first salt solution, the spent dilute solution including salt, providing a spent concentrate solution formed from a second salt solution, and applying electricity to separate the salt into ions and transfer the ions from the spent dilute solution to the spent concentrate solution, reducing the salt content of the spent dilute solution to regenerate the first salt solution and increasing the salt content of the spent concentrate solution to regenerate the second salt solution.

[0012] The method may include a regeneration process that includes applying a process of evaporation, which may include providing a spent concentrate solution formed from the second salt solution, heating the spent concentrate solution to generate steam, and transporting the steam for mixing with the spent dilute solution, reducing the salt content of the spent dilute solution to regenerate the first salt solution and increasing the salt content of the spent concentrate solution to regenerate the second salt solution.

[0013] The method may include a regeneration process that includes applying a process of membrane distillation, which may include providing a membrane distillation vessel including a hydrophobic membrane having a spent strong solution on one side of the membrane and a spent dilute solution on the other side of the membrane, and warming the spent strong solution to produce water vapor that permeates the hydrophobic membrane and mixes with the spent dilute solution to regenerate the first salt solution and increase the salt content of the spent strong solution to regenerate the second salt solution.

[0014] The method may include a regeneration process including applying a process of forward osmosis, which may include circulating the spent concentrate solution and the draw solution through a forward osmosis system to regenerate a second saline solution and produce a spent draw solution, and circulating the spent draw solution through a switchable solubility system to regenerate the draw solution and produce water.

[0015] The method may include applying a pressure retarded osmosis (PRO) system, a volumetric mixing (CAP) system, or both a PRO system and a CAP system to generate additional power.

[0016] A system for generating electricity is disclosed. The system includes a first salt solution, a second salt solution, the second salt solution having a different salinity than the first salt solution, a heat pump configured to transfer thermal energy to the first salt solution and / or the second salt solution, and a permselective membrane separating the first salt solution from the second salt solution. The system may further include at least one regeneration system. The permselective membrane is configured to control mixing of the first salt solution and the second salt solution and may be further configured to capture at least a portion of the salinity gradient energy as electrical power when the first salt solution and the second salt solution are mixed. The permselective membrane may include graphene, graphene oxide, or reduced graphene oxide and may optionally include nanopores therein. The permselective membrane may be a single-layer thin sheet, a multi-layer sheet, or a cartridge comprising graphene, graphene oxide, and / or reduced graphene oxide. The heat pump may be a vapor compression cycle, a thermoelectric chiller, a chemical absorption chiller, or other device used in the art for simultaneous heating and cooling. The regenerative system may include one or more of a salt precipitation system, a membrane distillation system, a salt splitting system, an electrodialysis system, a forward osmosis system, evaporation, or any combination thereof.

[0017] When the regeneration system includes a membrane distillation system, the membrane distillation system includes a vessel containing at least a portion of the first or second salt solution, the vessel being covered with a hydrophobic membrane, and a heat pump (or alternatively, a second heat pump) configured to heat the vessel and cool the opposite side of the hydrophobic membrane. The membrane distillation system is configured to form a salt gradient across the membrane after heating to produce a third salt solution within the vessel. The hydrophobic membrane may include polytetrafluoroethylene, polypropylene, or polyvinylidene fluoride, and may optionally be configured in a sandwich cell stack configuration.

[0018] When the regeneration system includes a salt decomposition system, the salt decomposition system includes a vessel configured to receive at least a portion of the spent lean solution formed from the first salt solution, the spent lean solution including salt, a heat pump (or alternatively, a second heat pump) configured to heat the vessel, and a cold water stream configured to receive at least one gaseous product discharged from the vessel. The salt decomposition system may be configured to decompose the salt and thereafter reform a salt precipitate in the cold water stream.

[0019] The salt decomposition system may include a vessel configured to receive a spent lean solution from the reverse electrodialysis battery, the spent lean solution including salt, a heat pump configured to heat the vessel and generate a gaseous product including the salt in the vessel, and an absorber configured to receive the spent concentrate solution from the reverse electrodialysis battery and to receive the gaseous product, wherein the salt in the gaseous product can be absorbed by the spent concentrate solution to regenerate the concentrated salt solution.

[0020] The salt gradient heat engine system may include a salt precipitation system and a membrane distillation system, or an electrodialysis system and a salt precipitation system. The salt gradient heat engine system may further include a liquid desiccant dehumidification process.

[0021] If the regeneration system includes a membrane distillation system, the membrane distillation system may include a membrane distillation vessel including a hydrophobic membrane and a heat pump configured to heat the vessel and cool one side of the hydrophobic membrane. The membrane distillation system may be configured to form a salt gradient across the hydrophobic membrane after heating, generate an ultra-dilute solution, and regenerate a concentrated salt solution. The membrane distillation system may further include a concentrate tank configured to receive the spent concentrate solution from the reverse electrodialysis battery and connected to the membrane distillation vessel, and a dilute tank configured to receive the spent dilute solution from the reverse electrodialysis battery and connected to the membrane distillation vessel. The concentrate tank may be configured to receive the concentrate solution from the membrane distillation vessel, and the dilute tank may be configured to receive the ultra-dilute solution from the membrane distillation vessel. The hydrophobic membrane may be made of polytetrafluoroethylene, polypropylene, or polyvinylidene fluoride and configured in a sandwich cell stack configuration.

[0022] If the regeneration system includes an evaporation system, the evaporation system may include an evaporator configured to receive the spent strong solution from the reverse electrodialysis battery and generate water vapor for regenerating the strong salt solution, a heat pump configured to supply thermal energy to the evaporator, and a condenser configured to receive the spent weak solution from the reverse electrodialysis battery and receive the water vapor generated by the evaporator. The water vapor condenses and mixes with the spent weak solution to be regenerated as the weak salt solution.

[0023] If the regeneration system includes an electrodialysis system, the electrodialysis system may be configured to receive the dilute spent solution and the concentrated spent solution, and electricity is supplied to the electrodialysis system to transfer ions from the dilute spent solution to the concentrated spent solution, thereby regenerating the dilute salt solution and the concentrated salt solution.

[0024] The regeneration system may include a salt precipitation system and an electrodialysis system, the salt precipitation system including a salt precipitator configured to receive the spent dilute solution from the reverse electrodialysis battery and precipitate salts from the spent dilute solution before the spent dilute solution is supplied to the electrodialysis system, and a concentrate tank configured to receive the spent concentrate solution from the reverse electrodialysis battery and to receive salt produced by the salt precipitator, the concentrate tank may be configured to receive the concentrate solution and the dilute tank is configured to receive the dilute salt solution from the electrodialysis system.

[0025] The regeneration system may include a forward osmosis system configured to receive the spent concentrate solution from the reverse electrodialysis battery and regenerate the concentrated salinity solution using the switchable solubility system. In the forward osmosis system, the regeneration system may further include a lean solution tank configured to receive the spent lean solution from the reverse electrodialysis battery and to receive water from the switchable solubility system, where the water is mixed with the spent lean solution to regenerate the dilute salinity solution. The switchable solubility system may include a draw solution circulated through the forward osmosis system and producing the spent draw solution, a capture device configured to receive the spent draw solution and add heat, where CO2 is released and water is produced, and a generator configured to receive solution from the capture device and add CO2 to regenerate the draw solution.

[0026] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate certain embodiments of the present disclosure and are not intended to limit the scope of the disclosure. [Brief explanation of the drawings]

[0027] [Figure 1] FIG. 1 shows an example of an electrodialysis reversal (RED) system. [Figure 2] FIG. 2 shows an example of a playback system. [Figure 3] FIG. 3 shows an example of a thermal optimization system. [Figure 4] FIG. 4 shows an example of a pressure retarded osmosis (PRO) system. [Figure 5] FIG. 5 shows an example of a hydrogen generation system. [Figure 6] FIG. 6 is a flow chart of a method for generating electrical power from thermal energy. [Figure 7] FIG. 7 illustrates a block diagram of example internal hardware that may be used to store or execute program instructions according to an embodiment. [Figure 8] FIG. 8 is an example of a system and method of the present disclosure including a RED or PRO battery associated with a salt precipitation system and connected to an external heat source. [Figure 9] 9 is another example of the systems and methods of the present disclosure including a RED or PRO battery in conjunction with a salt precipitation system. This system does not include an external heat source system, but rather adds heat to the dilute solution tank, such as via a heat pump loop and / or waste heat. [Figure 10] FIG. 10 is an example of a system and method of the present disclosure including a RED or PRO battery in conjunction with a salt decomposition system. [Figure 11] FIG. 11 is an example of the systems and methods of the present disclosure including a RED or PRO battery in conjunction with an evaporation system. [Figure 12] FIG. 12 is an example of a system and method of the present disclosure including a RED or PRO battery in conjunction with an electrodialysis system and salt precipitation. [Figure 13] Figure 13 shows an example of a regeneration system incorporating a membrane distillation system and salt precipitation. [Figure 14] Figure 14 shows an example of a regeneration system incorporating a membrane distillation system. [Figure 15] Figure 15 is an example of a regenerative batch system with all operating tanks in operation. [Figure 16] Figure 16 shows an example of water production using a steam condenser and a heat pump. [Figure 17]FIG. 17 is an example showing how the system of FIG. 16 can be used to create cold and hot fluid tanks for use in a salinity gradient energy thermal system. [Figure 18] FIG. 18 is an example of a system and method of the present disclosure including a RED or PRO battery in conjunction with atmospheric water generation using a liquid desiccant dehumidification process. [Figure 19] FIG. 19 is an example of a system and method of the present disclosure including a RED or PRO battery in conjunction with a forward osmosis system. DETAILED DESCRIPTION OF THE INVENTION

[0028] The present invention relates to a salt gradient heat engine system that utilizes heat to generate electrical power from thermal energy. A salt gradient heat engine can be a system that utilizes thermal energy to generate or regenerate a salinity gradient to generate usable energy, such as electricity and / or hydrogen. Examples of salt gradient heat engine systems include the RED battery and the PRO battery. A RED battery may incorporate multiple permselective membranes and one or more electrodes, as described in more detail below. A PRO battery incorporates a single membrane and does not require one or more electrodes. A PRO battery generates pressure rather than generating electrical power directly from the salinity difference between a concentrated (or concentrated) salt solution, as in the RED battery. The permselective membrane in a PRO battery is configured to allow solvent, rather than solute, to pass through the membrane preferentially, reducing the salinity difference between the solutions, for example, from a dilute (or diluted) solution to a concentrated salt solution.

[0029] Both RED and PRO batteries can include a concentrated salt solution separated from a dilute solution by a selectively permeable membrane, and in both systems, the rate at which power is generated by the system is a function of at least the difference in salinity between the concentrated salt solution and the dilute solution, and optionally also at least the temperature of the concentrated salt solution.

[0030] The electrodialysis reverse system includes an anode, a cathode, and one or more cells disposed between the anode and the cathode. At least one of the one or more cells includes a first membrane configured to selectively permeate cations and a second membrane configured to selectively permeate anions, the second membrane being spaced apart from the first membrane. The cell further includes a concentrated salt solution disposed between the first and second membranes, the first and second membranes separating the concentrated salt solution from the dilute salt solution such that the first membrane selectively migrates cations toward the cathode and the second membrane selectively migrates anions toward the anode, creating a voltage difference between the cathode and the anode. The first and second permselective membranes may include ion exchange membranes.

[0031] The permselective membranes used in the reverse electrodialysis systems disclosed herein may be, for example, 2-500, 2-200, 10-400, or 2-100. Certain permselective membranes limit the ability of ionic components to diffuse freely. Instead, cation exchange membranes (and anion exchange membranes) allow cation and anion components, respectively, to migrate or move in opposite directions. Each permselective membrane can be made of organic or inorganic polymers with charged (ionic) side chains, such as ion exchange resins. Each permselective membrane can be made of graphene, reduced graphene oxide, or graphene oxide. The permselective membrane can include graphene configured into single or thin multilayer sheets, optionally stacked, and optionally containing nanopores. The permselective membrane can include graphene, reduced graphene oxide, or graphene oxide, and can be a cartridge, such as those commonly used in reverse osmosis water filtration systems. The permeability of the membrane can depend on the configuration and other characteristics of the graphene sheets. The single layer or thin multi-layer sheets can be stretched or otherwise configured to vary the permeability of the membrane.

[0032] The permselective membrane may be a bipolar membrane (e.g., anion on one side, cation on the other) that generates acids and bases from salts present in solution during use. The selectivity of a permselective membrane is determined by its size, charge, charge density, phase (e.g., hydrophobic / hydrophilic), or polarity.

[0033] The permselective membrane may be a polymer composite membrane with aligned nanochannels, such as that disclosed in WO 2022 / 032236, the entire contents of which are incorporated herein by reference. For example, the permselective membrane may be a thin film composite membrane comprising: (i) a polymer membrane, film, or coating comprising a layer having a first surface, a second surface, and a film thickness therebetween, and comprising cylindrical polymer fibers at least partially aligned as hexagonally packed cylinders within the film, aligned parallel to the film surface, and present as an H1 mesophase, the cylinders being internally crosslinked within the cylinders, and the cylinders being spatially arranged to provide channels between the cylinders for fluid flow through the membrane, film, or coating; and (ii) a porous support layer in contact with the polymer membrane, film, or coating. In embodiments, the porous support layer is polyacrylonitrile, polyvinylidene fluoride, polysulfone, polyamide, polyimide, polypropylene, anodized aluminum, cellulose acetate, or a nonwoven fabric.

[0034] A salt gradient heat engine system can include a heat source configured to transfer thermal energy to a concentrated or dilute salt solution and a regeneration system including a heat pump. The heat pump can be any device known in the art for simultaneous heating and cooling, optionally providing a coefficient of performance greater than about 1, or between about 1 and about 10, between about 1 and about 6, or between about 3 and about 4. The heat pump can be a vapor compression cycle, a thermoelectric chiller, a chemical absorption chiller, or the like. The vapor compression cycle can be a screw acoustic air conditioner. The heat pump used herein can include a phase-change or non-phase-change refrigerant. The refrigerant can be CO2, helium, or other refrigerant known for use in heat pumps. The heat pump can include a thermodynamic cycle. The thermodynamic cycle can include any combination of refrigerant and non-refrigerant cycles that provide simultaneous heating and cooling. The heat pump may be a thermoacoustic heat pump, such as that developed by Equium (https: / / www.pv-magazine.com / 2023 / 01 / 02 / residential-thermo-acoustic-heat-pump-produces-water-up-to-80-c / ). The heat pump may be the system disclosed in U.S. Pat. No. 9,915,436, "Heat Source Optimization System," or U.S. Pat. No. 11,067,317, "Heat Source Optimization System." Each of the above patents is incorporated herein by reference in its entirety. In certain embodiments, a humidifier, dehumidifier, bidirectional exhaust fan, and / or swamp cooler may be used in combination with the heat pump to drive electrodialysis reversal. The heat pump may be fueled by known heat exchange fluids, which may be, but are not limited to, water, refrigerant, glycol, oil, etc.

[0035] Traditionally, humidity is undesirable in HVAC cooling because it adds dead weight to the system. Condensing water vapor consumes energy, resulting in wasted energy and reduced energy efficiency. Typically, when a desired temperature is set on a thermostat for cooling, the energy used to condense water vapor is energy that could have been used to cool the air. In this scenario, energy is consumed to condense the vapor into condensed water, and the HVAC unit is working against the latent heat of vaporization.

[0036] In contrast, according to the present disclosure, humidity in the environment can be highly desirable because it provides additional energy to the system that can be used to drive the electrodialysis reverse process and generate electricity, hydrogen, oxygen, and any combination thereof. For example, humidity can be introduced into the system, and the latent heat of the water vapor can be captured and used to drive electrodialysis reverse and generate electricity. Similarly, the formation of ice can utilize the thermal crystallization of water to drive the electrodialysis reverse process and generate electricity, hydrogen, oxygen, or any combination thereof.

[0037] The regeneration system can be configured to receive the dilute salt solution from at least one of the one or more cells, remove heat energy from the dilute salt solution (by a heat pump), and precipitate salt in the dilute salt solution. The regeneration system may be configured to, after precipitating salt with the dilute salt solution, circulate the dilute salt solution through at least one of the one or more cells, introduce the precipitated salt into the concentrated salt solution, and dissolve the precipitated salt in the concentrated salt solution.

[0038] The regeneration system can be configured to return at least a portion of the thermal energy removed from the dilute salt solution to the dilute salt solution after precipitating the salt dissolved in the dilute salt solution. The regeneration system can be configured to transfer at least a portion of the thermal energy removed from the dilute salt solution to the concentrated salt solution to dissolve the precipitated salt in the concentrated salt solution. The heat source can be configured to transfer thermal energy to the concentrated salt solution to dissolve the precipitated salt in the concentrated salt solution. The concentrated salt solution can include an endothermic solution or an exothermic solution. The concentrated salt solution can include a substance having a solubility with a nonlinear temperature dependence.

[0039] One or more other regeneration systems may be employed. The regeneration system may include electrodialysis. In addition to electrodialysis reverse systems, electrodialysis can be used for water purification. Electrodialysis can further desalinate dilute salt solutions. For example, when precipitating salt, the concentration of the dilute salt solution is limited by its solubility curve. Electrodialysis can also be used to further dilute dilute salt solutions. Renewable electricity (such as solar or wind) can be used to perform electrodialysis to separate salt from the dilute salt solution and form a more dilute stream. This allows for energy storage. For example, electrodialysis can be used to charge a salt gradient during sunshine and then harness the energy from the salt gradient after sunset. The same battery / tank used for electrodialysis reverse can be used for electrodialysis. Figure 12 shows an example system including electrodialysis and a RED / PRO battery, which is described in more detail below.

[0040] The electrodialysis reversal system may further include a control system configured to regulate the transfer of heat between one or more heat sources and the electrodialysis reversal system based on one or more measurements of conditions of the one or more heat sources or the electrodialysis reversal system, where the heat sources include one or more of geothermal, industrial waste heat, and solar heat.

[0041] The salt gradient heat engine system may include a salt decomposition system for generating the salt gradient. Salt decomposition may be achieved by reverse electrodialysis instead of the salt precipitation process disclosed herein. For example, the spent dilute solution (supplied from the RED battery) may be heated to a temperature higher than the temperature at which the salt decomposes (e.g., ammonium bicarbonate decomposes into CO2 and ammonia at around 60°C). As the solution is heated, the salt decomposes, leaving the spent dilute solution as a gaseous product, reducing the concentration of salt in the spent dilute solution and generating a regenerated dilute solution. The application of a vacuum or an optional fan helps move the gaseous product into the cold stream. Pumping the gaseous products (e.g., CO2 and ammonia) into a cold water stream reacts and precipitates them back into solid salt form (e.g., CO2 and ammonia with cold water to form ammonium bicarbonate salt). The precipitated solid salt (e.g., ammonium bicarbonate) in the solution may be transferred to a concentrated salt solution within the RED battery. In this process, a heat pump may be used to heat and cool the spent dilute solution and cold water streams, respectively. The heat pump may be the same as or different from the heat pump used in other steps of the electrodialysis reversal system disclosed herein. Optionally, as the gaseous products precipitate in the cold stream, thermal energy may be extracted and transferred to the spent dilute solution to decompose the salt. Additionally, thermal energy may be used to increase the temperature of the precipitated solution, increasing solubility and resulting in an ultra-concentrated solution. Figure 10 is an example of a salt decomposition process showing the flow of solution between the RED battery and the salt decomposition process vessel, as described in more detail below. Application of the salt decomposition process allows for the regeneration of concentrated and dilute salt solutions by incorporating salt decomposition.

[0042] The electrodialysis reverse system can include a second cell. The second cell includes a third membrane configured to selectively permeate cations and a fourth membrane configured to selectively permeate anions, the fourth membrane being spaced apart from the third membrane. The second cell includes a second concentrated salt solution disposed between the third and fourth membranes, the third and fourth membranes separating the second concentrated salt solution from the second dilute salt solution. The concentrated salt solution can include an endothermic solution, and the second concentrated salt solution can include an exothermic solution, and the heat pump can be configured to transfer heat between the concentrated salt solution and the second concentrated salt solution.

[0043] A reverse electrodialysis system can include a membrane distillation system. Membrane distillation (MD) is a thermally driven separation process in which liquid is rejected and only vapor molecules permeate a porous hydrophobic membrane. The driving force behind the MD process is the vapor pressure difference generated by a temperature difference across the hydrophobic membrane. The hydrophobic membrane must be inherently hydrophobic or its surface must be modified to be hydrophobic. Hydrophobic membranes for MD can be polytetrafluoroethylene (PTFE), polypropylene (PP), polyvinylidene fluoride (PVDF), or any combination thereof. The large surface area of ​​hydrophobic membranes can be used in a sandwich-type cell stack design similar to the RED stack. A stack of hydrophobic membranes can contain only one or more types of hydrophobic membranes without electrodes. A heat pump can be used to simultaneously heat and cool the membrane, creating a strong temperature gradient driving force, thereby increasing the efficiency of the MD process. For example, a heat pump can be used to heat the spent concentrated solution or the spent diluted solution to about 40°C to about 80°C on one side of the hydrophobic membrane while simultaneously providing cooling means, such as a cold stream heat exchanger, refrigerant-loaded evaporator, or chilled water, on the other side of the hydrophobic membrane to generate the diluted solution. Heat can be sourced from, for example, ambient conditions, geothermal, solar, or industrial waste heat. Membrane distillation can leverage the power of a heat pump to efficiently generate a salt gradient for use in the electrodialysis reversal system disclosed herein. The heat pump can be the same or different from the heat pumps used in other steps of the electrodialysis reversal system disclosed herein. Membrane distillation can be used to generate a concentrated salt solution for introduction into the RED battery, and optionally, a diluted salt solution. Figure 13 is an example of a membrane distillation system showing the flow of solutions between the MD system and the RED system. The incorporation of membrane distillation allows for the regeneration of concentrated and diluted salt solutions.

[0044] The membrane distillation process utilized here can be direct contact membrane distillation (DCMD), air gap membrane distillation (AGMD), vacuum membrane distillation (VMD), sweeping gas membrane distillation (SWGMD), vacuum multi-effect membrane distillation (V-MEMD), permeate gap membrane distillation (PGMD), or a combination thereof.

[0045] A membrane distillation system may include a vessel containing at least a portion of a first or second salt solution, the vessel being covered with a hydrophobic membrane that allows vapor permeation, and a heat pump configured to heat the vessel and cool the opposite side of the membrane. The membrane distillation system may be configured to create a salt gradient across the membrane after warming to produce a third salt solution (i.e., a concentrated or concentrated solution) within the vessel. The membrane distillation system may also include a second heat pump for heating the vessel and, optionally, a cooling device on the opposite side of the vessel and the membrane. The cooling device may be a cold stream, a heat exchanger, a refrigerant loop, or may operate, for example, by condensing vapor back to a liquid. Figure 13 is an example of a membrane distillation process, showing how a hydrophobic membrane forms a salt gradient between a warm concentrated solution and a cold dilute solution, as described in more detail below. The membrane distillation system may include at least a portion of a first or second salt solution separated by a hydrophobic membrane that allows vapor permeation. In this embodiment, the solution is supplied to both sides of a hydrophobic membrane, and a heat pump is configured to heat one side of the membrane, while a cooling device is provided on the opposite side of the membrane to cool the solution.

[0046] The reverse electrodialysis system includes a microbial reverse electrodialysis cell (MREC) as disclosed in U.S. Patent No. 9,112,217, the entire contents of which are incorporated herein by reference. In this embodiment, microorganisms generate electricity to power the RED system. The MREC includes multiple electrogenic microorganisms arranged within a RED battery, which can assist in the production of hydrogen and electricity through the oxidation of organic matter at the anode and the reduction of oxygen at the cathode. Because microorganisms thrive in warm environments, the use of a heat pump can improve the ability of the microorganisms to remove electrode overpotentials. Electrode overpotentials lead to significant energy losses due to thermodynamically unfavorable electrode reactions. Furthermore, utilizing the thermal energy provided by the heat pump improves reaction rates and reduces the amount of membranes required to generate the same amount of energy in the RED. For example, in residential environments, organic waste, such as sewage from a septic tank, can be converted into usable energy using microbial reverse electrodialysis. The use of ultraviolet light in any of the salt gradient heat engine systems disclosed herein can limit microbial growth and contamination within the system.

[0047] In a salt gradient heat engine system, the RED and PRO can operate together continuously or in a batch system. When operating in a batch system, the electrodialysis reversal system can include multiple precipitators and / or multiple tanks in a parallel or stacked arrangement. When operating in a batch system, the precipitators, tanks, and stacks as a whole can be of different sizes and operated in series, parallel, or a combination of both, and can be operated in countercurrent, crosscurrent, or parallel current. For example, the system can include two or more precipitators, each producing a regenerated dilute solution, which are combined before flowing into the RED battery. And / or the system can include two or more dissolving tanks, each producing a regenerated concentrated salt solution, which are combined before flowing into the RED battery. When operating as a batch system, the salt solution can be passed through the RED battery stack multiple times (multiple passes) to increase efficiency. This allows parts of the system to be shut down, for example for maintenance, while the rest of the system continues to operate.

[0048] In one embodiment, a single vessel may be both a precipitator (e.g., precipitating salts from a spent solution and removing a regenerated dilute solution) and a dissolving tank (e.g., adding spent solution to a vessel containing precipitated salts, which can then dissolve the salts in the solution to create a regenerated concentrated solution), where instead of removing the precipitated salts, the salts are left in the vessel and used to create a regenerated concentrated salt solution.

[0049] The concentrated and dilute solutions are in separate loops. However, the two solutions can be mixed in a controlled manner (controlled mixing within the RED / PRO stack). Additionally, uncontrolled mixing osmosis (water flow) can occur between the concentrated and dilute solutions within the RED / PRO stack. Osmosis can transfer or move some of the water in the dilute solution into the concentrated solution within the RED / PRO stack. This water movement through osmosis can create a controlled flow to balance the total volume in the dilute and concentrated tanks. Without this, the volume in the concentrated tank would continue to increase. Controlled mixing or flow between the loops can be achieved by incorporating valves that operate to ensure both loops contain the same volume of solution.

[0050] A method for generating electrical power from thermal energy is disclosed. The method includes separating a first salt solution from a second salt solution using a selectively permeable membrane. The method includes receiving thermal energy from a heat source using the first salt solution and / or the second salt solution. The method includes mixing the first salt solution and the second salt solution in a controlled manner and capturing at least a portion of the salinity gradient energy as electrical power as the salinity difference between the first salt solution and the second salt solution decreases. The method includes transferring thermal energy from the first salt solution to the second salt solution using a heat pump, thereby increasing the salinity difference between the first salt solution and the second salt solution.

[0051] The method may include capturing salinity gradient energy using reverse electrodialysis. The method may further include capturing the salinity gradient energy by pressure-retarded osmosis to drive a generator. In some embodiments, the first salt solution and the second salt solution each circulate in a closed system. Transferring thermal energy from the first salt solution to the second salt solution may precipitate salt in the first salt solution. The method may include introducing the precipitated salt into the second salt solution to increase the salinity difference between the first salt solution and the second salt solution. The method may include using a portion of the generated electricity to produce hydrogen gas, and optionally oxygen gas, by electrolysis. In some examples, transferring thermal energy from the first salt solution to the second salt solution includes transferring thermal energy from the first salt solution, which is at a lower temperature than the second salt solution.

[0052] The method may further include adjusting the transfer of heat from one or more heat sources to the first salt solution and / or the second salt solution based on one or more measurements of the one or more heat sources or conditions of the first salt solution and / or the second salt solution, wherein the heat sources may include one or more of geothermal heat, industrial waste heat (e.g., from a power plant), exhaust from transportation vehicles (e.g., cars, ships, trucks), solar heat, etc.

[0053] The method can include reversing the circulation of the first and second salt solutions in a closed system by applying solenoid valves to both ends of the closed loop system. Reversing the circulation does not stop energy production, but rather causes the membrane to wear more evenly on opposite sides, potentially extending the service life of the permselective membrane.

[0054] In the following description, conventional features of the disclosed technology that would be apparent to one skilled in the art are omitted or only briefly described. Reference to various embodiments does not limit the scope of the claims appended hereto. Moreover, any examples described herein are non-limiting and merely set forth some of the many possible embodiments of the appended claims. Furthermore, specific features described herein can be used in combination with other features described, in each of the various combinations and permutations possible. Those skilled in the art will know, using the present invention in combination with routine experimentation, how to achieve other results not specifically disclosed in the examples or embodiments.

[0055] It is also understood that the terminology used in the description is for the purpose of describing particular versions or embodiments only and is not intended to limit the scope of the present disclosure, which is limited solely by the appended claims. Unless otherwise defined herein, all terms shall be interpreted as broadly as possible, including the meaning implied by the specification and the meaning understood by one of ordinary skill in the art and / or as defined in dictionaries, treatises, etc. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. Although any methods, devices, and materials similar or equivalent to those described herein can be used in the practice or testing of embodiments of the present disclosure, preferred methods, devices, and materials are described herein. All references cited herein are incorporated by reference in their entirety.

[0056] As used in this specification and the appended claims, the singular forms "a," "an," and "the" include the plural, and reference to a particular numerical value includes at least that particular value unless the context clearly dictates otherwise. Ranges may be expressed herein as from "about" or "approximately" one particular value and / or to "about" or "approximately" another particular value. When such ranges are expressed, another embodiment includes from the one particular value and / or to the other particular value. Similarly, when values ​​are expressed as approximations, by use of the antecedent "about," it will be understood that the particular value forms another embodiment. It is also understood that all spatial references, e.g., horizontal, vertical, top, upper, lower, below, left, right, etc., are for illustrative purposes only and can be varied within the scope of the disclosure. For example, the terms "upper" and "lower" are relative and are used only in the context of each other and do not necessarily mean "superior" and "inferior." Generally, similar spatial references of different features or components indicate similar spatial orientations and / or locations. That is, each "first end" is located or oriented at the same end of the device.

[0057] The systems and methods described in this disclosure are generally directed to efficiently extracting usable energy from the salt concentration difference between two solutions by precisely regulating and controlling their mixing. The systems can be used to generate electricity directly, hydrogen gas or hydrogen and oxygen gas, which can be used as fuel to generate mechanical (and / or electrical) power, or to generate pressure and / or gravitational potential energy, both of which can be used to drive turbines or perform other useful work. Systems come in a variety of sizes and power outputs. Some embodiments may be configured to generate power at the scale of a single residence or commercial building. In some examples, systems include industrial power generation systems that supply power to a regional or national grid. In some examples, systems not only (or instead of) generating electricity, but also supply hydrogen fuel, for example, to power a fleet of vehicles.

[0058] FIG. 1 illustrates an example of a system 100 for generating electricity from a salinity gradient. The example system 100 includes a reverse electrodialysis (RED) battery 110. The RED battery 110 includes a cathode 112 and an anode 114 separated by one or more cells 150, 150a, 150b. Each cell 150 contains a salt solution 130, i.e., a liquid mixture of a solvent and salt, where the salt is dissolved into anionic and cationic components, and the ionic components are free to move with respect to one another. Each ion may have a single charge or multiple charges. In some examples, the solvent and solute are water and sodium chloride (NaCl), respectively. The dissociated ions of NaCl are Na + and Cl -, each carrying a single charge. Other solvents and solutes that form liquid mixtures containing anions and cations that are free to move relative to one another can also be used. Solvents can be organic or inorganic liquids, including, but not limited to, water, alcohol, benzene, and glycerin. Salt solutions can be exothermic or endothermic. That is, as the solution forms, it absorbs heat, as when potassium chlorate (KClO3) or potassium nitrate (KNO3) dissolves in water, or it releases heat, as when calcium chloride (CaCl2) dissolves in water.

[0059] As shown in FIG. 1, a salt solution 130 is separated from a dilute solution 140 (i.e., a solution with a lower solute concentration than the salt solution) by permselective membranes 104, 104a-d. The salt solution 130 is separated from the dilute solution 140 by cation exchange membranes (104a, 104c) on one side of a cell 150, and from the dilute solution 140 by anion exchange membranes (104b, 104d) spaced apart from the cation exchange membranes (104a, 104c) on the other side of the cell 150. The salt solution 130 is disposed in the space between the cation exchange membranes (104a, 104c) and the anion exchange membranes (104b, 104d). Without these membranes 104, the salt solution 130 would freely diffuse into the dilute solution 140, equalizing the salt concentrations of the two solutions. As negatively charged ions migrate toward the anode 114 and positively charged ions migrate toward the cathode 112, a potential difference (voltage) is created across the cells 150. The total voltage of the battery 110 includes the voltage of each cell 150.

[0060] Ions may tend to accumulate near the membrane 104. This accumulation may interfere with the power generation process. To combat this accumulation, the system 100 may apply a stirring or mixing force to the salt solution 130 and / or dilute solution 140 to cause the ions to become more uniformly (homogenously) distributed throughout the solutions 130, 140. In some examples, the system 100 may apply sonic vibrations to one or more of the solutions 130, 140 to increase the homogeneity of the solutions 130, 140. The system 100 may apply sonic vibrations to areas of the cell 150 where ions accumulate, such as near one or more membranes 104, to effectively increase the homogeneity of the one or more solutions 130, 140.

[0061] As shown in FIG. 1 , the electrodes (e.g., cathode 112, anode 114) are surrounded by a dilute solution 140. Alternatively, the electrodes 112, 114 may be surrounded by a rinse solution, which is circulated in a closed loop between the electrodes 112, 114, separating the rinse solution from the salt solution 130 and the dilute solution 140. In such an arrangement, the outer permselective membranes 104 (i.e., the membranes 104 closest to each electrode 112, 114 of the RED battery 110) are the same type (e.g., both anion exchange membranes or both cation exchange membranes). For example, a cation exchange membrane 104 can separate the rinse solution surrounding the cathode 112 from the salt solution 130, and a cation exchange membrane 104 can separate the rinse solution surrounding the anode 114 from the dilute solution 140. In this configuration, cations migrating from the salt solution 130 into the electrolyte surrounding the cathode 112 are recycled to the anode 114, where they may pass through the cation exchange membrane 104 and enter the dilute solution 140. Similarly, the membranes 104 closest to each electrode 112, 114 may both be anion exchange membranes, in which case anions (in the rinse solution) are circulated from the anode 114 to the cathode 112, where anions pass through the anion exchange membrane 104 and enter the dilute solution 130. In either configuration, a reduction reaction occurs at the cathode 112, and a balancing oxidation reaction occurs at the anode 114. In some examples, the rinse solution includes a supporting electrolyte to enhance the reactions at the electrodes 112, 114. In this configuration, because both electrodes 112, 114 are surrounded by the same electrolyte (the rinse solution), the rinse solution forms a resistive load between the electrodes 112, 114, and current flows through that load, potentially reducing the power output of the RED battery 110. In some examples, the rinse solution and / or associated circulation system may be configured to have a high resistance relative to the output load of the RED battery 110 or the internal resistance of the cells 150 of the RED battery 110. For example, the rinse solution circulation path may be configured to be relatively long.

[0062] The current generated by the battery 110 is a function of the rate of ion migration, which in turn is a function of several factors, including the salinity gradient (i.e., the difference in salinity between the salt solution 130 and the dilute solution 140), (at least) the temperature of the salt solution 130, and the characteristics of the membrane 104. The temperature of the salt solution 130 affects the rate at which ions in the salt solution 130 migrate toward (and across) the membrane 104 due to the increased kinetic energy of ions at higher temperatures. According to the Nernst equation, the power generated is a function of the logarithm of the salinity partition ratio between the salt solution 130 and the dilute solution 140. However, as ions migrate from the salt solution 130 to the dilute solution 140, the salinity of the dilute solution 140 increases and the salinity of the salt solution 130 decreases. Thus, the gradient between the "spent" salt solution 130 and the "spent" dilute solution 140 decreases. To maintain the current (and therefore power output) of the battery 110, the salinity difference can be continuously regenerated by refreshing the spent salt solution 130 and / or the spent lean solution 140. To this end, the spent salt solution 130 and / or the spent lean solution 140 may be circulated (e.g., in a closed loop) between the RED battery 110 and a regeneration system. Alternatively, the spent salt solution 130 and / or the spent lean solution 140 may be continuously replenished from a natural source, such as, for example, a river, ocean, or bay.

[0063] FIG. 2 illustrates an example regeneration system 200. The example regeneration system 200 includes a salt removal subsystem 210. The regeneration system 200 circulates the spent dilute solution 140 in a closed loop from the RED battery 110 through the salt removal subsystem 210 and back to the RED battery 110 as a refreshed dilute solution 140. In some examples, the salt removal subsystem 210 evaporates and condenses the solvent from the dilute solution 140 and circulates the condensed solvent back to the RED battery 110 as a refreshed dilute solution 140. The system may evaporate the solvent until the remaining dilute solution 140 approaches or falls below its solubility limit. The remaining dilute solution 140 can be reintroduced into the spent salt solution 130 to refresh the salt solution 130. In some examples, the salt removal subsystem 210 evaporates and condenses the solvent from the salt solution 130 and circulates the condensed solvent back to the RED battery 110 as a refreshed dilute solution 140. In a closed-loop RED battery 110, the salt removal subsystem 210 can evaporate and condense both the salt solution 130 and the lean solution 140, circulate the condensed solvent from both solutions back to the RED battery 110 as the refreshed lean solution 140, and circulate the remaining solution back to the RED battery 110 as the refreshed (concentrated) salt solution 130. In some examples, the salt removal subsystem 210 reduces the salt concentration of the lean solution 140 through a process of salt precipitation rather than (or in addition to) evaporation, regenerating the salinity difference between the salt solution 130 and the lean solution 140 in the RED battery 110. Other methods for regenerating the salinity gradient include, for example, freezing the spent lean solution 140 via eutectic chilled crystallization (ECC) or using microfiltration and / or membrane separation. In some examples, multiple methods are advantageously combined. For example, the precipitation or freezing step may be augmented by a subsequent membrane filtration step to optimize the total energy required to separate the salt from the spent dilute solution 140 .

[0064] As shown in FIG. 2 , the example salt removal subsystem 210 removes salts from the used dilute solution 140 by precipitating the salts. Generally, the ability of a solvent to dissolve a solute increases with increasing temperature. Conversely, lowering the temperature of a solution below what is called the saturation point (the temperature at which the solution's salt concentration is at its maximum) typically results in the solute precipitating. The example salt removal subsystem 210 includes a heat transfer device 216 configured to cool the used dilute solution 140 to a temperature below the saturation point. If the used dilute solution 140 is an exothermic solution, the dilute solution 140 further cools as the salts precipitate. In some examples, the heat transfer device 216 heats the refreshed dilute solution 140, for example, to the temperature of the used dilute solution 140 before cooling. That is, the heat transfer device 216 can transfer some or all of the thermal energy removed from the used dilute solution 140 back to the refreshed dilute solution 140, as indicated by the arrow associated with the heat transfer device 216 in FIG. 2 . In this way, the temperature of the refreshed dilute solution 140 entering the RED battery 110 will be substantially the same as the temperature of the spent dilute solution 140 exiting the RED cell 150 .

[0065] In the illustrated salt removal subsystem 210, the precipitated salt 212 settles to the bottom of the salt removal subsystem 210, for example, in the form of a dense solid. In some examples, the salt removal subsystem 210 includes a conveying device 230 configured to convey the precipitated salt 212 out of the salt removal subsystem 210. The conveying device 230 may be a belt, a pump, an Archimedes' screw, or other device or system configured to physically transport the precipitated salt 212 out of the salt removal subsystem 210. For example, if the salt is a solid, the conveying device 230 may be a mechanical system capable of transporting solid materials. In some examples, the removed salt 212 is conveyed to the salt replenishment subsystem 220, where the salt is reintroduced (e.g., redissolved) into the spent salt solution 130 to refresh the spent salt solution 130. Similar to how the regeneration system 200 circulates the dilute solution 140, the regeneration system 200 can also circulate the spent salt solution 130 in a closed loop from the RED battery 110 through the salt replenishment subsystem 220 to the RED battery 110 and back as refreshed salt solution 130. The salt replenishment subsystem 220 can increase the salinity of the salt solution 130 through the process of redissolving salt removed by the salt removal subsystem 210, thereby regenerating the salinity difference between the salt solution 130 and the dilute solution 140 within the RED battery 110.

[0066] As previously mentioned, the ability of a solvent to dissolve a solute generally increases with increasing temperature. Thus, the higher the temperature of the salt solution 130, the higher the salt concentration and the greater the difference in salt concentration between the salt solution 130 and the dilute solution 140. A solubility curve is a plot of the amount of solute that a particular amount of solvent can dissolve as a function of temperature. In some instances, the solubility curve associated with a solution is linear. That is, the amount of solute that a solvent can dissolve may vary linearly with temperature across a wide range of temperatures (e.g., the entire range in which the solvent is liquid). In some instances, the amount of solute that a solvent can dissolve may vary nonlinearly with temperature. In such cases, the amount of solute that a solvent can dissolve may increase, for example, five-fold or more, even within a narrow temperature range. The system 100 can be configured to operate the RED battery 110 within a temperature range in which the salt concentration of the salt solution 130 is high. Dissolving additional salt may require additional heat to be transferred to the salt solution 130. Additionally, the system 100 may maintain the temperature of the RED battery 110 above the solubility point to provide a "safety margin" to avoid undesirable precipitation if the salt solution 130 is cooled below the solubility point.

[0067] The salt replenishment subsystem 220 can receive thermal energy from one or more heat sources configured to raise the temperature of the salt solution 130, for example, to dissolve additional salt. For example, the salt replenishment subsystem 220 can receive waste heat from the heat transfer device 216 of the salt removal subsystem 210. The salt replenishment subsystem 220 can also be configured to receive thermal energy from other heat sources, as indicated by the arrow associated with the salt replenishment subsystem 220 in FIG. 2. Example heat sources include, but are not limited to, geothermal heat, industrial waste heat, solar heat, heat from combustion, vapor compression cycle waste heat (e.g., heat pumps), heat from chemical reactions, or other forms of heat that are not easily or efficiently converted into a usable form by conventional means, such as driving a turbine.

[0068] A thermal optimization system can be used to optimize the use of thermal energy by the power generation system 100 described herein. Thermal optimization systems are further described in U.S. Pat. No. 11,067,317, the entire contents of which are incorporated herein by reference. A thermal optimization system transfers thermal energy from one or more heat sources to one or more heat sinks. Examples of heat sinks include the interior of residential or office spaces during cooler months of the year, heated swimming pools, saunas, steam rooms, etc. In these examples, the system can be configured to regulate temperature by adjusting the transfer of thermal energy to the heat sinks. For example, the thermal optimization system can monitor the temperature of the heated spaces and / or heated water and adjust the transfer of heat using processor-based logic, such as, but not limited to, implementing one or more PID feedback loops and / or expert systems. During hotter months, interior spaces may be heat sources. In this case, a processor-based adjustment system can adjust the transfer of thermal energy from these spaces to regulate their temperature.

[0069] FIG. 3 illustrates an example of a thermal optimization system 300. The example optimization system 300 includes one or more heat sources 305, one or more heat sinks 310, and one or more RED batteries 110, as described above. In some examples, the power generation system 100 includes a pressure-retarded osmosis (PRO) system (discussed in more detail below) or other system for generating electricity from a salinity gradient (e.g., capacitive mixing (CAP)) instead of (or in addition to) the RED batteries 110. The optimization system 300 may include one or more pumps 320 configured to move heat from one location to another. In some examples, the optimization system 300 transfers heat from the heat source to the heat sink using vapor compression refrigeration. That is, the optimization system 300 can compress a refrigerant and transfer thermal energy from the refrigerant to the heat sink (e.g., via a heat exchanger configured to absorb and distribute thermal energy). The optimization system 300 transfers a compressed refrigerant to a heat source (e.g., by pumping the compressed refrigerant through purpose-configured piping) and expands the refrigerant to absorb thermal energy from the heat source (e.g., via a heat exchanger configured to provide thermal energy from the heat source). The vapor compression refrigeration system may include a reversing valve or other controllable device to change the direction of the thermal energy transfer. The optimization system 300 may control the reversing valve to change the direction of heat transfer, for example, to cool an interior space during the day when the ambient temperature is relatively high, and then reverse the heat flow to heat the interior space at night when the ambient temperature drops.

[0070] The operation of the thermal optimization system 300 may be coordinated by a control system 350 including a processor, for example, as described below with respect to FIG. 7. The processor executes instructions that cause the control system 350 to adjust heat transfer between the heat source 305, the heat sink 310, and the RED battery 110 (or the PRO or CAP system), for example, based on current conditions. The control system 350 may adjust heat transfer between subsystems within the power generation system 100, such as the salt removal subsystem 210 and the salt replenishment subsystem 220. The control system 350 can send and receive signals 352, 352a-c, between the heat source 305, the heat sink 310, and the RED battery 110 (or the PRO or CAP system). The control system 350 can receive signals 352, 352a-c, that indicate one or more conditions or states of, for example, the heat source 305, the heat sink 310, and the RED battery 110 (or the PRO or CAP system). For example, signal 352 may represent a measured quantity such as temperature and / or pressure, or may represent a user input such as a target temperature for the heated interior space. Temperatures associated with power generation system 100 include the temperatures of salt solution 130 and lean solution 140 in the RED battery, salt removal subsystem 210, and / or salt replenishment subsystem 220, respectively.

[0071] The control system 350 is used to send control signals, such as compressor and / or pump speeds, reversing valve direction, and salt delivery / transport device 230 operating speed, to achieve a commanded target temperature and / or power output. For example, the control system 350 can adjust the temperature of a salt solution to be at or near its solubility limit. In this way, the control system 350 can optimize the power generation system 100's output while simultaneously efficiently controlling and regulating heat transfer between multiple heat sources 305 and heat sinks 310 based on current conditions and user settings. Furthermore, the control system 350 can influence the level of vapor compression cycle waste heat generated by one or more heat pumps 320 and transfer the waste heat to one or more heat sinks and / or the RED battery 110, thereby efficiently recovering the waste heat for power generation or other purposes. In this way, the optimization system 300 can transfer heat from any or all of the various heat sources under various dynamic conditions (e.g., changing conditions throughout the year or day) and / or based on the demand of the RED battery 110. Additionally, control system 350 can configure power generation system 100 to store excess energy. For example, when demand for electrical energy is low, control system 350 can set RED battery 110 to generate a portion of its energy output as hydrogen gas to be used later as fuel, rather than as electrical energy used at the time of generation. Additionally, in the case of a PRO system, control system 350 can configure the rate at which pressure is converted to electricity, for example, by controlling the flow rate through the turbine. In this way, control system 350 can retain some energy, for example, in the form of gravitational potential energy, when power demand is low, and convert more gravitational potential energy to electrical energy when power demand is high.

[0072] In some embodiments, the system 100 includes a PRO system instead of (or in addition to) the RED battery 110 described above. FIG. 4 illustrates an example of a PRO system 120. The example PRO system 120, like the RED battery 110 described above, also includes a salt solution 130 separated from a dilute solution 140 by a permselective membrane 104c. However, rather than generating power directly from the salinity difference between the salt solution 130 and the dilute solution 140, the PRO system generates pressure, which may be in the form of gravitational potential energy. Thus, the PRO system may not include electrodes. The permselective membrane of the PRO system may be configured to allow solvent, rather than solute, to preferentially pass through the membrane, e.g., from the dilute solution 140 to the salt solution 130, to reduce the salinity difference between the solutions. In the PRO system, the permselective membrane may be a hollow fiber membrane that is permeable to water but not to salt. In the PRO system, the permselective membrane may be a hollow fiber membrane, a spiral-wound membrane, a flat-plate membrane, or a combination thereof. As a result, the pressure and / or average height of the solvent within the salt solution 130 may increase over time as the solvent migrates through the membrane 104c and into the salt solution 130. The system 100 can convert the increased pressure and / or gravitational potential energy of the salt solution 130 into a more usable form of power, such as electrical energy, by, for example, directing the (rising) salt solution 130 to a turbine, paddlewheel, or other suitable mechanism for generating electricity. The system 100 can then circulate the spent salt solution 130 through a solvent removal system similar to the salt removal subsystem 210 described above. For example, the solvent recovery system can remove excess solvent by evaporation (optionally under partial vacuum to lower the boiling point) followed by condensation. The solvent recovery system then circulates the condensed solvent back to the PRO system 120 as a refreshed or "make-up" lean solution 140 and circulates the refreshed salt solution (after the excess solvent has been removed) back to the PRO system 120 as the refreshed salt solution 130.Alternatively (or additionally), the solvent recovery system may precipitate solutes from the spent salt solution 130 and then circulate the spent salt solution back to the PRO system 120 as a "make-up" lean solution 140, introducing the precipitated salts back into the salt solution 130, for example, via the transfer device 230 of FIG. 2. Similar to the RED battery 110 described above, the rate (or speed) of power produced by the PRO system 120 is a function of at least the salinity difference between the salt solution 130 and the lean solution 140 and at least the temperature of the salt solution 130.

[0073] The RED system may be a capacitive electrochemical (CAP) system instead of (or in addition to) the RED battery and / or the PRO system. The CAP system is an electrode-based technology used to generate electrical energy from a salinity gradient. Power generation using a CAP system is based on cycles of charging and discharging electrodes. The electrodes of a CAP system are sequentially exposed to two solutions with significantly different salinity concentrations. The CAP system charges and extracts energy from the salinity difference in the form of a salinity gradient, utilizing the voltage increase that occurs between two electrodes immersed in a salt solution when its salt concentration changes. The amount of power generated depends primarily on the following characteristics of each electrode: the amplitude of the potential increase due to the change in salinity and the potential within the high-salinity solution. The electrodes may be identical so that the flow in the system can be reversed and the polarity changed without stopping electricity generation. The CAP system may include one or more selectively permeable membranes, as disclosed herein.

[0074] As shown in FIG. 1 , the salt solution 130 is separated from the dilute solution 140 by cation exchange membranes (104a, 104c) on one side of the cell 150 and by anion exchange membranes (104b, 104d) on the other side of the cell 150. Alternatively, the salt solution 130 and the dilute solution 140 may be separated by a single permselective membrane 140 (e.g., a cation exchange membrane or an anion exchange membrane). The selective movement of ions across the single membrane 140 generates a potential difference (voltage) across the membrane 140. As with the embodiment of FIG. 1 , the current generated by a single membrane embodiment (e.g., a flow pump) of the RED cell 150 is also a function of the rate of ion movement, which is a function of several factors, including (at least) the salinity gradient between the salt solution 130 and the dilute solution 140, the temperature of the salt solution 130 (because higher temperatures increase the kinetic energy of ions), and the characteristics of the single membrane 104.

[0075] In some embodiments, the system includes a first RED battery 110a configured to use an exothermic salt solution 130a and a second RED battery 110b configured to use an endothermic salt solution 130b. The system can transfer heat generated by dissolving a solute in the exothermic salt solution 130a to the endothermic solution 130b to replace the heat absorbed in dissolving the solute.

[0076] In some embodiments, a portion of the generated power is used to produce hydrogen gas, for example, by splitting water via electrolysis. For example, if the dilute solution is water, a potential difference of 1.23 volts can be applied to the water to split the water into hydrogen and oxygen. The oxygen can be supplied and pumped into a space, such as a building or other indoor area. The hydrogen and / or oxygen can be stored for later use, for example, as a battery. Either the salt solution or the dilute solution (or both) can be split by electrolysis. FIG. 5 shows an example of a RED battery 110 configured to produce hydrogen. The released hydrogen and oxygen gases can “bubble” to the surface near the cathode 112 and anode 114, respectively, of the RED battery 110. The system 100 separates the oxygen and hydrogen gases (e.g., by physically separating the cathode 112 and anode 114), captures and stores the hydrogen (e.g., by capturing it as it bubbles to the surface near the cathode), and then transports the hydrogen by suitable means to a suitable location for use as a fuel. In such cases, the system 100 must replenish or “top up” the solvent lost in the electrolysis, for example, from a stream or other freshwater source. The RED battery 110 may be configured to allow electrolysis to occur naturally; that is, the RED battery 110 may be configured to generate a potential difference sufficient to cause electrolysis of the solvent. In some examples, the regeneration system uses electrolysis to refresh the salinity of the spent salt solution 130 when makeup fresh water is circulated to the RED battery 110 (or PRO or CAP system) as the refreshed dilute solution 140. In some examples, the system includes a separate water reservoir to produce hydrogen gas by electrolysis (e.g., rather than electrolyzing the solvent in the RED battery or PRO or CAP system). The separate water reservoir may have its own “top up” source, while the RED battery or PRO system remains a closed loop. In these embodiments, the water reservoir may also function as a heat reservoir or play other roles in the thermal optimization process.

[0077] FIG. 6 illustrates a flowchart 600 of an example method for generating electrical power from thermal energy. In step 602, the example method includes separating a first salt solution 130 from a second salt solution 140 using a permselective membrane 104. The permselective membrane 104 is configured to provide controlled mixing of the first salt solution 130 and the second salt solution 140, such that mixing can more usefully capture salinity gradient energy. In some examples, the permselective membrane 104 is configured to preferentially allow the solvent of the first salt solution 130 to pass through the membrane into the second salt solution 140, as in a PRO system. In some examples, the permselective membrane 104 is configured to preferentially allow either the anions or cations of the first salt solution 130 to pass through the membrane into the second salt solution 140, as in a RED battery 110.

[0078] In step 604, the example method includes receiving thermal energy from a heat source. The power generated by the RED battery 110 (or PRO system) is a function of temperature. The received thermal energy allows the RED battery 110 to continue operating (e.g., generating electricity). In some examples, the control system 350 is configured to regulate the amount of thermal energy received and configure which heat source 305 provides the thermal energy. In some examples, the control system 350 is configured to transfer waste heat from one or more heat pumps 320 to the RED battery 110. In some embodiments, the power generation system 100 provides some or all of the power to operate the one or more heat pumps 320. As a prophetic example, the efficiency of the RED battery 110 may be approximately 30% (i.e., 30% of the thermal energy transferred to the RED battery 110 is converted to electricity or other usable forms of energy). The heat pump 320 may have a coefficient of performance (COP) of 3-4 (i.e., the heat pump 320 may take 2-3 KW of power from the heat source and require 1 KW of power to transfer 3-4 KW to the heat sink (the sum of the input power and the thermal power taken from the heat source). For example, a heat pump 320 with a COP of 4 may require 1 KW of power to transfer a total of 4 KW of heat to the RED battery 110. The heat pump 320 may transfer heat from low-grade or "waste" heat sources, e.g., sources below 300°C, that are not easily converted into a useful form of energy. In some contemplated examples, the heat source may come from an industrial process that would otherwise simply emit waste heat into the environment. At 30% efficiency, the RED battery 110 can generate 1.2 KW of power from 4 KW of transferred heat. In this contemplated example, 1 KW of power can be used to power the heat pump 320, leaving 200 W of power for other purposes. Thus, in this contemplated example, the combined RED battery 110 and heat pump 320 system 100 produces a net output of 200 W of power with no net input of power other than the 3 KW of "waste" heat.If the waste heat comes from an industrial process, the combined RED battery 110 and heat pump 320 system 110 is projected to produce a net power output of 200 W while also providing the benefit of cooling the waste heat by 3 KW before releasing it to the environment. The projected net efficiency of the combined system 110 can be further amplified by improving the efficiency of the RED battery 110.

[0079] Additionally, the power generation system 100 can capture a portion of the waste energy generated by one or more heat pumps and convert the waste energy into electrical energy to power the heat pump 320, thereby improving the effective coefficient of performance (COP) of the heat pump 320, such as a heat pump used to heat or cool a residential space. For example, a heat pump with a heating COP of 3 may require 1.5 KW of power to pump 3 KW of heat from a heat source to a heat sink. If the heat sink does not require the full 4.5 KW of power (3 KW of pumped heat and up to 1.5 KW of waste heat), the control system 350 can configure the optimization system 300 to transfer some or all of the waste heat to the RED battery 110 and convert it into electricity for the heat pump 320, improving the effective COP of the heat pump 320. Additionally, the control system 350 can configure the power generation system 100 to convert some of the waste energy into a form that can be stored for later use, such as when the instantaneous demand for power is greater than the amount of power that can be generated. For example, a PRO system may retain waste energy in the form of unreleased pressure and / or gravitational potential energy and release it in the future, for example, when demand for electrical energy increases. Similarly, instead of producing an amount of electrical energy, the RED battery 110 may produce hydrogen gas to be used as a fuel in the future. Thus, waste heat from the heat pump 320 can be flexibly captured and released to further increase the effective COP of the heat pump.

[0080] In step 606, the exemplary method includes mixing the first salt solution 130 and the second salt solution 140 in a controlled manner. In step 608, the exemplary method includes capturing at least a portion of the salinity gradient energy as electrical power. As described above, the RED battery 110 or the PRO or CAP system can be configured to convert the salinity gradient energy into a more useful form as the solutions (130, 140) are mixed. In step 610, the exemplary method includes transferring thermal energy from the first salt solution to the second salt solution via a heat pump 320. In step 612, the exemplary method includes increasing the salinity difference between the first salt solution and the second salt solution. As described above, the heat pump 320 can cool the used dilute solution 140, precipitating salt from the dilute solution 140 and refreshing the dilute solution. The heat pump transfers heat from the spent lean solution 140 to the spent salt solution 130 to enhance the process of dissolving salt introduced into the salt solution 130. Alternatively (or in addition), the heat pump 320 may heat the spent salt solution 140 to evaporate the salt solution 140 and refresh the salt solution. The evaporated solvent may be condensed (e.g., cooled by the heat pump) as the solvent vapor is circulated back to the RED battery 110 as a refreshed lean solution.

[0081] 8 shows an example of a regeneration system including a salt precipitation system. In this embodiment, the salt precipitation system includes a salt precipitator 880, three heat exchangers 882, 884, and 838 (alternative embodiments may include one, two, or more than three heat exchangers), and an external heat source system 890. The regeneration system circulates spent dilute solution 840 in a closed loop from a RED or PRO battery 810 through the salt precipitation system and back to the RED or PRO battery 810 as refreshed dilute solution 845. Optionally, the refreshed dilute solution 845 may be stored for a period of time in a dilute solution holding tank 846 before being sent to the RED or PRO battery 810. The spent dilute solution 840 is sent to a salt precipitator 880, where a heat exchanger 884 removes thermal energy from the spent dilute solution 840 to precipitate salts 885, further diluting the solution in the salt precipitator 880 and providing a regenerated dilute solution 845 (which may also be referred to as a refreshed dilute solution). The salts 885 are sent to a concentrate tank 836 (or a tank configured to regenerate the spent concentrate solution) that contains the spent concentrate solution 830. Within the concentrate tank 836, the salts 885 are dissolved in the spent concentrate solution 830, optionally by application of thermal energy, such as via a heat exchanger 838, to produce a regenerated concentrate solution 835 (which may also be referred to as a refreshed concentrate solution) with an increased salinity.

[0082] The salt precipitation system reduces the salinity of the spent dilute solution 840 through the process of salt precipitation rather than (or in addition to) evaporation, and uses the precipitated salt 885 to increase the salinity of the spent concentrate solution 830, regenerating the salinity difference between the concentrate solution 835 and the dilute solution 845, and directing the stream to the RED or PRO battery 810 for use therein.

[0083] As shown in FIG. 8 , an exemplary salt precipitation system removes salts from a used dilute solution 840 by precipitating the salts. Reducing the temperature of the solution below what is known as the saturation point (the temperature at which the solution's salt concentration is greatest) typically results in precipitation of the solute. The system includes a heat transfer device, shown in FIG. 8 as heat exchanger 884, configured to cool the used dilute solution 840 to a temperature below the saturation point. Heat exchanger 882 passively exchanges heat between the refreshed dilute solution 845 and the used dilute solution 840. Because the temperature of the solution in the dilute solution tank 846 can optimally be about 30° C. to about 50° C., the heat exchanger 882 heats the refreshed dilute solution 845 to, for example, the temperature of the used dilute solution 840 before cooling. That is, the heat exchanger 882 can return some or all of the thermal energy removed from the used dilute solution 840 to the refreshed dilute solution 845. The spent dilute solution 840 may be at about 30°C to about 40°C and is cooled to about 5°C within the precipitator. In this way, the temperature of the refreshed dilute solution 845 entering the battery 810 is substantially the same as the temperature of the spent dilute solution 840 leaving the battery 810. The heat exchanger 882 may perform both cooling and heating functions as described above, or may be positioned within the system to only heat the regenerated dilute solution 845 after it leaves the precipitator 880. For example, the regenerated dilute solution 845 stream may be heated by blowing hot air through it. Control valves 811, 812 are incorporated into the system to control and balance the flow rate of the solution. Pumps 813, 814 are included in the system to move the solution through the RED or PRO battery 810. The pumps 813, 814 may be any pump or device known in the art that acts on a fluid to move it, such as, but not limited to, a diaphragm pump, a centrifugal pump, or a peristaltic pump. The number of control valves and heat pumps may vary among the disclosed systems, and one skilled in the art will understand that one or more control valves may be added to any embodiment of the present disclosure.Heat exchangers 884, 838 transfer heat from precipitator 880 or tank 836, respectively, to heat source system 890. Heat exchangers 884, 838 may be submerged within precipitator 880 or tank 836, respectively, and may be coils with a liquid circulating within the coil, or may be other heat exchangers known in the art, such as shell and tube heat exchangers.

[0084] FIG. 8 includes an external heat source system 890 that includes a heat exchanger 891 and a heat pump 892 that add heat to the system. The external heat source system 890 can be used in conjunction with a salt precipitation system to provide thermal energy to the concentrate tank 836. The external heat source system 890 is shown with a closed loop, optionally a refrigerant loop, to heat and cool the precipitator 880 and the concentrate tank 836. The heat exchanger 891 draws low-grade heat (e.g., below about 100° C., or below about 200° C.) or very low-grade heat (e.g., ambient air, or below about 80° C.), optionally directly from the air or from an external heat source. The external heat source supplies the external heat to the heat source system 890, which is ultimately converted to electricity by the RED / PRO battery 810.

[0085] In other embodiments, heat source system 890 is not present, and instead high-grade heat (e.g., steam, or any heat source that provides heat above about 100°C) can be added directly to heat the concentrate tank. FIG. 9 shows an example of a regeneration system that includes a salt precipitation system, but without the external heat source system included in FIG. 8. In FIG. 9, as salt precipitation device 880 cools, thermal energy is released, optionally as heat 991. Heat 991 can be removed, sent to any source, released, or returned to the system via heat 992 by any means known in the art. Heat 992, 993 can be introduced into the system from ambient energy, a heat pump, or elsewhere in the system (e.g., heat 991).

[0086] FIG. 10 shows an example of a regeneration system including a salt decomposition system. In this embodiment, the salt decomposition system includes an absorber 1080, a salt decomposition vessel 1081, two heat exchangers 1082, 1084 (alternative embodiments may include one heat exchanger or more than two heat exchangers), and a heat pump 1013. Heat is applied to decompose the salt. The regeneration system circulates the spent lean solution 1040 from the RED or PRO battery 1010 in a closed loop through the salt decomposition vessel 1081, where heat is applied to warm the tank, forming vapor and gaseous salt 1085, which is returned to the RED or PRO battery 1010 as refreshed lean solution 1045. The heat exchanger 1084 is positioned to transfer thermal energy between the spent lean solution stream 1040 and the refreshed lean solution stream 1045. The spent strong solution 1030 circulates in a closed loop from the RED or PRO battery 1010 through the absorber 1080 and then returns to the RED or PRO battery 1010 as a refreshed strong solution 1035. A heat exchanger 1082 is positioned to transfer thermal energy between the spent strong solution stream 1030 and the refreshed strong solution stream 1035. As heat is added to the vessel 1081, gaseous products (e.g., decomposed gaseous salts) 1085 are released from the vessel and received by the absorber 1080, where the decomposed salts in the vapor are absorbed by the spent strong solution 1035, increasing the salt concentration of the solution. Salts are removed from the spent lean solution 1040 in the salt decomposition vessel 1081, leaving a regenerated lean stream 1045. Heat 992, 993 can be introduced into the system from ambient energy, a heat pump, or elsewhere in the system. Heat 991 is removed and can be used in a heat pump or released. The heat pump 1013 can heat and cool simultaneously (see heat input 991 and heat output 993) or can use heat / thermal energy 994 from another source. A control valve 1011 is incorporated into the system to control and balance the flow rate of the solution. The salt decomposition vessel 1081 separates the gas from the liquid and may include a flash tank or stripper column to decompose the salts in the solution.Salt decomposition vessel 1081 and absorber 1080 may be any vessel or group of vessels, tanks, and / or columns that perform the functions described herein and are readily understood by one of ordinary skill in the art.

[0087] Figure 11 is another example of a regeneration system including an evaporation system. The embodiment of Figure 11 differs from the embodiment of Figure 10 by incorporating an evaporator 1091 and a condenser 1090. In this example, spent concentrate 1030 circulates in a closed loop from a RED or PRO battery 1010 through an evaporator 1091, where heat 992, 993 is added to boil the solution and evaporate water as steam 1093, leaving a more concentrated solution, or refreshed concentrate, which is returned to the RED or PRO battery 1010. The steam 1093 is sent from the evaporator 1091 to a condenser 1090, where the vessel is cooled and the steam is condensed to water, which mixes with the spent lean solution 1040 to form refreshed lean solution 1045, which is returned to the RED or PRO battery 1010 in a closed loop. A heat pump 1013 provides a cooling effect to the condenser 1090. Traditionally, this heat pump would be replaced by a chilled water loop, and all heat input into the system would be lost. Incorporating a heat pump as shown recovers heat, improving the energy efficiency of the process. The evaporator 1091 and condenser 1090 may be any tank, vessel, column, or group thereof that performs the functions described herein and is readily understood by one of ordinary skill in the art.

[0088] FIG. 12 shows an example of a regeneration system involving electrodialysis and salt precipitation. Dilute solution 1141 and concentrated solution 1131 are sent to electrodialysis system 1190, where electricity is applied to separate salts from dilute solution 1141 by transferring ions from the dilute solution to the concentrated solution, producing ultra-dilute solution 1147. Using electrodialysis, electricity is used to produce a more concentrated solution 1132 and ultra-dilute solution 1147. The spent dilute solution 1140 is then desalted in precipitator 1180, leaving dilute solution 1141 (which has a lower salt concentration than the spent dilute solution 1140). Dilute solution 1141 is further diluted beyond its solubility curve using electrodialysis to produce ultra-dilute solution 1147, creating a larger salt gradient. 12, the ultra-dilute solution flows from the electrodialysis tank to the dilute solution storage tank 1146 where it is mixed with the dilute solution 1141 (if present) from the precipitator 1180. In another embodiment, as can be readily envisioned by one skilled in the art, the system may be configured to allow the ultra-dilute solution 1147 to flow directly to the battery 1110, employing valves and positioning to redirect the solution flow.

[0089] The spent dilute solution 1140 is sent to the salt precipitator 1180 where it is cooled (heat 1191 is removed). Removing thermal energy from the spent dilute solution 1140 precipitates salt 1185, further diluting the solution in the salt precipitator 1180 to produce dilute solution 1147. This dilute solution may be sent to a storage tank 1146 or further diluted by an electrodialysis process. The regenerated dilute solution 1145 (also called a refreshed dilute solution) has the same salinity as the ultra-dilute solution 1147 or a lower salinity than the dilute solution 1141 after being mixed with the ultra-dilute solution 1147 in tank 1146. The regenerated dilute solution 1145 flows back to the battery 1110 to take advantage of the salinity gradient. The salt 1185 is mixed with the spent concentrate solution 1130 to increase its salinity and is then sent to the concentrate storage tank 1136. In the concentrate storage tank 1136, salt 1185 is dissolved into the spent concentrate 1130, optionally with the application of heat 1192 or other thermal energy source, to increase the salinity and produce concentrate 1131, which is then sent to an electrodialysis system 1190 to increase the salinity and produce a more concentrated solution 1132. The more concentrated solution 1132 may have a higher or equal salinity than the regenerated concentrate 1135 (which may also be referred to as a refreshed concentrate). The bottom of the concentrate tank 1136 may contain excess salt (not shown). If excess salt is present, the concentrate tank 1136 may function as a battery to keep the process running when extra heat is temporarily unavailable.

[0090] The salt precipitation system reduces the salinity of the spent dilute solution 1140 through the process of salt precipitation rather than (or in addition to) evaporation, and uses the precipitated salt 1185 to increase the salinity of the spent concentrate solution 1130, increasing the salinity difference between the concentrate solution 1131 and the dilute solution 1141. In this embodiment, unlike FIG. 2, the concentrate solution 1131 and the dilute solution 1141 are further processed by electrodialysis to increase the salinity difference between the solutions before being reintroduced into the battery 1110. The electricity 1171 generated by the RED or PRO battery 1110 can be used, stored, and / or used as electricity 1170 for operation of the electrodialysis system 1190 by any means known in the art.

[0091] Although in FIG. 12 a different tank is used for the electrodialysis system 1190 than the RED or PRO battery 1110, in other embodiments the same tank may be used for electrodialysis as is used for electrodialysis reversal.

[0092] FIG. 13 is an example of a regeneration system incorporating a membrane distillation system 1390 and salt precipitation in place of the electrodialysis system 1190 of FIG. 12. The membrane distillation system 1390 includes a heat pump (not shown) that evaporates water that may permeate the dilute solution through a porous hydrophobic membrane (not shown) to reduce the salinity and produce an ultra-dilute solution 1347. The ultra-dilute solution 1347 can be sent directly to the battery 1110 or to a dilute solution tank, as shown, with valves employed and configured to redirect the stream flow. As shown in FIG. 13, the ultra-dilute solution 1137 flows from the membrane distillation system 1390 to a dilute solution storage tank 1146, where it is mixed with the dilute solution 1141 (if present) from the precipitator 1180. In another embodiment, as can be readily envisioned by one skilled in the art, the system can be configured to direct the ultra-dilute solution 1347 to the battery 1110, with valves employed and configured to redirect the solution flow.

[0093] Within concentrate storage tank 1136, salt 1185 is dissolved into spent concentrate 1130, optionally with the application of heat 1192 or other thermal energy source, to increase the salinity and produce concentrate 1131, which is then sent to membrane distillation system 1390 to increase the salinity and produce more concentrated solution 1332. More concentrated solution 1332 may have a higher or equal salinity than regenerated concentrate 1135 (which may also be referred to as refreshed concentrate).

[0094] Figure 14 is an example of a regeneration system incorporating a membrane distillation system 1390 without salt precipitation. The depleted lean solution 1140 is sent to a lean storage tank 1146 where it is cooled (i.e., thermal energy 1191 is removed) and sent to the membrane distillation system 1390. The depleted concentrated solution 1130 is sent to a concentrate storage tank 1136 where it is warmed (i.e., thermal energy 1192 is input into the system) and sent to the membrane distillation system 1390. Heating and cooling of the storage tanks can optionally be provided by one or more heat pumps (not shown). The depleted lean stream 1140 and the depleted concentrated stream 1130 are sent to the membrane distillation system 1390, which operates using the vapor pressure difference created by the temperature difference between the streams across a hydrophobic membrane (not shown). A regenerated lean solution 1445 and a regenerated concentrated solution 1435 are produced by operation of the membrane distillation system 1390 and are then circulated to the RED or PRO battery 1110 via storage tanks 1146, 1136 as needed, as shown in FIG. 14 .

[0095] FIG. 15 shows an example of a RED / PRO tank combined with a regeneration system in which multiple tanks operate in a batch system. Each of the operating tanks 1501, 1502, 1503, and 1504 can be a cooling tank (i.e., a precipitator), a heating tank (i.e., a dissolving tank), or can be inactive at any time. For example, tank 1501 can initially operate as a cooling tank, precipitating salts from the spent dilute solution to leave a regenerated dilute solution 1545, which can then be sent to dilute tank 1546. Tank 1501's operation can then be changed to a heating tank, and the spent concentrate solution 1530 can be sent to a tank that holds the precipitated salts, which can then be warmed to dissolve the salts into solution, leaving a regenerated concentrate solution 1535, which can then be sent to concentrate tank 1536. At other times, tank 1501 may require maintenance or be inactive for other reasons. This operational change can be applied to any operational tank in the batch system as needed.

[0096] The valve system 1520 operates by isolating the tanks from the RED / PRO battery 1510 and directing all or a portion of the flow of spent dilute solution 1540 and spent concentrate solution 1530 to one or more operating tanks 1501, 1502, 1503, 1504, respectively. The second valve system 1522 operates by isolating the operating tanks 1501, 1502, 1503, 1504 from the storage tanks and directing the flow of regenerated dilute solution 1545 and regenerated concentrate solution 1535 to respective storage tanks 1546, 1536. The dilute tank 1546 receives and holds the regenerated dilute solution 1545, and the concentrate tank 1536 receives and holds the regenerated concentrate solution 1535. While four tanks are shown operating in this Figure 15, fewer or more than four tanks can be connected to the system to operate as a batch system.

[0097] FIG. 16 illustrates a method for producing water utilizing a vapor condenser 1601, a compressor or heat pump 1602, and an evaporator 1603. A refrigerant 1612 circulates through the vapor condenser 1601, the heat pump 1602, and the evaporator 1603. As shown, the evaporator 1603 removes water vapor from ambient air, releasing water and air that is cooler and drier than the incoming ambient air. The condenser releases latent heat, which can be used in the salinity gradient energy system 1610 disclosed herein. For example, this system can be used to dehumidify (or remove moisture from) ambient air in homes and other buildings, and optionally generate heat that can be converted to electricity using a RED / PRO battery.

[0098] FIG. 17 incorporates the system of FIG. 16 as a method for producing a hot water tank 1620 and a cold water tank 1625 that can be used to heat and cool solutions used in a salinity gradient engine thermal system 1610 disclosed herein, optionally including a regeneration system such as those shown in FIGS. 8-15. An evaporator 1603 cools water to produce the cold water tank 1625, and a condenser 1601 heats water to produce the hot water tank 1620. A heat transfer medium 1613 (e.g., water, glycol, oil, refrigerant) flows from the cold tank 1625 through the salinity gradient engine system 1610 in a closed loop, and a heat transfer medium 1614 flows from the hot tank 1620 through the salinity gradient engine thermal system 1610 in a closed loop. An optional heat sink 1640 is shown to absorb or dissipate excess heat from the system. If the system is generating more heat than necessary, that heat can be transferred to the heat sink, for example, to the atmosphere, where it can be dissipated as heat 1631. Optionally, other heat sources 1630 are also shown supplying thermal energy or heat 1631 to the system, including, but not limited to, any means known in the art, such as, for example, heat exchangers, industrial steam, radiators, ambient temperature, coils, convection, etc.

[0099] The salinity gradient system disclosed herein is capable of producing water (also referred to herein as atmospheric water generation). Atmospheric water generation is the process of extracting water from the air using various techniques such as condensation, adsorption, and cooling. The concept of atmospheric water generation is based on the fact that the atmospheric environment contains significant amounts of water vapor, even in arid desert regions. When combined with a RED / PRO battery, a heat pump, and a closed-loop process, this can produce energy and water.

[0100] One means of producing atmospheric water is condensation, which involves cooling air below the dew point / condensation temperature, causing the water vapor in the air to condense into liquid water. This method is often used in thermodynamic cycles such as dehumidifiers and heat pumps. This process is dependent on the temperature and humidity of the air, which can be easily understood by those skilled in the art, for example, by reviewing a psychometric chart.

[0101] Another means of generating atmospheric water is adsorption. Adsorption uses a desiccant, such as silica gel or zeolite, to absorb moisture from the air. Once the desiccant absorbs moisture and, if necessary, becomes saturated, it can be heated to release the water, which can then be collected and used. Similarly, liquid desiccants, which are substances with a high affinity for water molecules, can be used to remove moisture from air in what may be referred to as a liquid desiccant dehumidification process. This process may involve passing air (optionally, air with a moderate to high humidity content, greater than about 30%, and not dried) over a surface coated with a liquid desiccant, which absorbs moisture from the air. Removing the absorbed water from the liquid desiccant allows it to be regenerated to its original state. Any solid or liquid desiccant known in the art may be used herein.

[0102] A liquid desiccant dehumidification process can be a closed-loop system that includes two independent air-handling units: a dehumidifying unit (also called a conditioner) and a regenerating unit (also called a generator). The dehumidifying unit typically consists of an absorber, where liquid desiccant is sprayed or applied to a surface and a fan or blower circulates moist air over the surface. As air passes over the surface, the liquid desiccant absorbs moisture from the air, and dry air is expelled into the conditioned space.

[0103] The regeneration system may include a separate vessel containing spent liquid desiccant and a heat source (in this case, a heat pump). In this embodiment, absorbed moisture is removed from the liquid desiccant through a process called regeneration, in which heat is applied to the liquid desiccant to remove the absorbed moisture. The removed water vapor can be condensed via a heat pump to produce water (which, with additional filtration (e.g., reverse osmosis or other known systems used in the art), may be potable) or added to a dilute solution to increase the salinity gradient of the RED / PRO battery. This product water can be used as a raw material (or feed) for electrolysis and / or RED, and thus can additionally or alternatively be used to produce hydrogen.

[0104] Typically, atmospheric water production consumes a significant amount of energy because it counteracts the latent heat of vaporization of water to produce it. When combined with a closed-loop salinity gradient engine system such as RED, the latent heat can be harnessed and converted into hydrogen and / or electricity rather than released into the atmosphere. The advantages of the liquid desiccant dehumidification process include the ability to extract water from low humidity levels, even in hot, humid climates, and the ability to use waste heat, solar energy, and / or heat pumps for regeneration. Condensing water vapor from dry, hot climates using only a heat pump would be much more difficult than using a heat pump and desiccant in combination. Combining the liquid desiccant dehumidification process with a heat pump and salinity gradient engine system can produce significant amounts of water and energy, even in very dry, hot climates.

[0105] Figure 18 is an example of atmospheric water generation using a liquid desiccant dehumidification process. In this example, water is extracted from the air and absorbed by a desiccant in a conditioner 1812. Spent (weakened) desiccant 1820 is transported to a generator 1811, where the desiccant is regenerated by evaporation of water, thereby regenerating a strong desiccant 1821, which is returned to the conditioner 1812. A heat pump 1882 is used to transfer heat between the weakened desiccant 1820 and the regenerated strong desiccant 1821 streams. Steam 1884 removed from the generator 1811 is sent to a steam condenser 1815, where it is cooled and liquefied to produce water 1885 for any purpose, including drinking water (subject to a filtration system).

[0106] FIG. 18 utilizes a method for fabricating a hot water tank 1813 and a cold water tank 1814, which can be used to heat and cool a solution (e.g., water) used in a salinity gradient engine thermal system 1810 disclosed herein, optionally including a regeneration system as shown in FIGS. 8-15. Heat exchanger 1884 cools water to create the cold tank 1814, and heat exchanger 1883 heats water to create the hot tank 1813. A heat transfer medium 1816 (e.g., water, glycol, oil, refrigerant) flows from heat exchangers 1883, 1884 to heat pump 1802 in a closed loop. One or more heat sinks and other heat sources can be incorporated to provide heating or cooling to either or both of the hot tank 1813 or cold tank 1814. The hot tank 1813 supplies hot water to the generator 1811 and the salt gradient heat engine system 1810 in a closed loop as hot supply stream 1830, which returns to the hot tank 1813 as a lower temperature stream 1831. The cold tank 1814 supplies cold water to the steam condenser 1815, to the conditioner 1812, and to the salt gradient heat engine system 1810 in a closed loop as cold supply stream 1832, which returns to the cold tank 1814 as a higher temperature stream 1833.

[0107] The salinity gradient system disclosed herein can include forward osmosis (FO) as a means of regenerating spent dilute and concentrated solutions from a RED / PRO battery, i.e., as a regeneration system. In a forward osmosis system, a feed solution, such as a spent concentrate solution, can be placed on one side of a semipermeable membrane, and a draw solution can be placed on the other side of the semipermeable membrane. The draw solution can be any solution with a higher osmotic pressure than the feed solution. The draw solution can contain a different salt, a synthetic salt, or be essentially the same as the feed solution but at a higher concentration. A salt gradient is created to draw water from the spent concentrate solution, which is then regenerated by a switchable solubility system incorporating the draw solution. The osmotic gradient created by the draw solution draws water molecules from the feed solution through the membrane, while salts and other contaminants remain in the feed solution. This allows the concentrate solution to be regenerated and returned to the RED / PRO battery. The membrane used in the FO system can be any membrane that may be used in a PRO battery. However, in the case of FO, the membrane does not need to be designed to withstand as high a pressure as the PRO membrane.

[0108] A forward osmosis system can incorporate a switchable solubility system, which utilizes a reversible reaction between the draw solution and carbon dioxide (CO2) and water to form a solution with switchable solubility properties. These switchable solubility solutions can switch between hydrophobic and hydrophilic forms. A method for forming a switchable solubility solution involves dissolving an amine in water to form a solution with a specific pH level. Examples of amines include 1-cyclohexylpiperidine, N-methyldipropylamine, ethyl 4-(diethylamino)butanoate, N,N-dimethylphenethylamine, and N,N-diethylbutylamine. A draw solution, as referred to herein, is a solution with a high osmolality or concentration to draw water across a semipermeable membrane. Therefore, a switchable solubility solution may be the draw solution in a forward osmosis system. When CO2 is introduced into the switchable solubility solution, it reacts with the amine to form a salt, lowering the pH of the solution. This change in pH alters the solubility of the amine, increasing or decreasing its solubility in water. Therefore, the solubility of amines can be altered by the presence of CO2, a change that is reversible with the application of heat. Specifically, when the pH is low (e.g., below about 7 or between 7 and 1), amines become more soluble in solution. Conversely, when the pH is high (e.g., above 7), amines become less soluble in water and less readily dissolve in solution. CO2 solubility is temperature dependent. As the temperature increases, CO2 solubility decreases, and water and CO2 separate from the amine solution. Once the water is removed, the solution is cooled to increase the solubility of CO2, allowing the amine to fully dissolve and increase its solubility.

[0109] Using a switchable solubility system as described above allows the draw solution to be regenerated while also utilizing the power of a heat pump that can provide heating and cooling simultaneously. As the solubility of the draw solution decreases, the water can be separated and sent to mix with the spent dilute solution from the RED / PRO battery. Removing the water concentrates and regenerates the draw solution used in the forward osmosis system, which regenerates the spent concentrated solution from the RED / PRO battery.

[0110] Operating a salinity gradient engine system near industrial facilities, such as power plants, offers several advantages. Power plants generate both excess waste heat and release CO2 as an unwanted by-product in their processes. This industrial CO2 can be used in the forward osmosis systems disclosed herein to control the solubility of the draw solution.

[0111] In FIG. 19, the regeneration system circulates spent lean solution 1940 in a closed loop from the RED or PRO battery 1910 through a lean tank 1946, where water 1970 is added to form a regenerated lean solution 1945, which is returned to the RED or PRO battery 1910. Spent concentrate solution 1930 is also circulated in a closed loop from the RED or PRO battery 1910 through a forward osmosis system 1980, producing a regenerated concentrate solution 1935, which is returned to the RED or PRO battery 1910. A switchable solubility system 1920 is also shown. Draw solution 1955 is circulated through the forward osmosis system 1980 and used to operate the system, producing spent draw solution 1950. In a recovery device 1960, heat 1992 is added to the spent draw solution 1950, releasing CO2, thereby reducing the solubility of the spent draw solution and allowing water 1970 to be separated and decanted. When CO2 is returned to the system by generator 1965, the solubility of the solutes in the draw solution increases (e.g., salts must be redissolved in the solution, and the solution must be concentrated to increase its osmotic pressure). To increase the solubility of CO2, the solution is cooled (thermal energy 1991 is removed from the system).

[0112] It should be understood that the various features (or aspects) disclosed herein may be combined in combinations other than those specifically presented in the description and accompanying drawings. It should also be understood that, depending on the example, certain acts or events of any of the processes or methods described herein may be performed in a different order, added, combined, or omitted entirely (e.g., not all described acts or events may be required to implement the techniques). Furthermore, while certain features of the present disclosure are described for clarity as being performed by a single module or unit, it should be understood that the techniques of the present disclosure may be implemented by a combination of units or modules associated with, for example, a RED battery, a PRO system, a CAP system, a hydrogen generation subsystem, a salt precipitation subsystem, an evaporation subsystem, etc.

[0113] In one or more examples, the described techniques may be implemented in hardware, software, firmware, or any combination thereof. If implemented in software, the functions may be stored as one or more instructions or code on a computer-readable medium and executed by a hardware-based processing unit. Computer-readable media may include non-transitory computer-readable media, corresponding to tangible media such as data storage media (such as RAM, ROM, EEPROM, flash memory, or other media that can be used to store desired program code in the form of instructions or data structures and that can be accessed by a computer).

[0114] The instructions may be carried out by one or more processors, such as one or more digital signal processors (DSPs), general-purpose microprocessors, application specific integrated circuits (ASICs), field programmable logic arrays (FPGAs), or other equivalent integrated or discrete logic circuitry. Accordingly, the term "processor," as used herein, may refer to any of the foregoing structures or other physical structures suitable for carrying out the described techniques. The techniques may also be implemented entirely in one or more circuits or logic elements.

[0115] Figure 7 illustrates examples of hardware that can be used to store or execute program instructions. Bus 710 serves as the primary information highway interconnecting the other illustrated components of hardware. Central processing unit (CPU) 705 is the central processing unit of the system and performs the calculations and logical operations necessary to execute programs. CPU 705, alone or in combination with one or more of the other elements disclosed in Figure 7, is an example of a processor, as that term is used within this disclosure. Read-only memory (ROM) and random access memory (RAM) constitute examples of non-transitory computer-readable storage medium 720, memory devices, or data storage bodies, as that term is used within this disclosure.

[0116] Program instructions, software, or interactive modules for providing an interface and for performing queries or analyses related to one or more datasets may be stored in memory device 720. Optionally, the program instructions may be stored on a tangible, non-transitory computer-readable medium such as a compact disc, digital disc, flash memory, memory card, Universal Serial Bus (USB) drive, optical disc storage medium, and / or other storage medium.

[0117] An optional display interface 730 allows information from the bus 710 to be displayed in audio, visual, graphic, or alphanumeric format on a display 735. Communication with external devices can occur using various communication ports 740. The communication ports 740 can be connected to communication networks such as the Internet or an intranet.

[0118] The hardware may also include an interface 745 that allows for receiving data from input devices such as a keypad 750 or other input devices 755 such as a touch screen, remote control, pointing device, video input device, and / or audio input device.

[0119] It will be appreciated that various of the above-disclosed or other features and functions, or alternatives thereof, may be desirably combined into many other different systems or applications or combinations of systems and applications, and that various presently unforeseen or unanticipated substitutions, modifications, changes, or improvements may subsequently occur to those skilled in the art, which are intended to be encompassed by the following claims.

Claims

1. A method of generating electricity from thermal energy, Separating the first saline solution from the second saline solution using a selectively permeable membrane. Transferring thermal energy to the first saline solution and / or the second saline solution by a heat pump, Mixing the first saline solution and the second saline solution in a controlled manner, and capturing at least some of the saline concentration gradient energy as power as the saline concentration difference between the first saline solution and the second saline solution decreases, and The salt concentration difference between the first saline solution and the second saline solution is restored by applying a regeneration process selected from the group consisting of salt decomposition, electrodialysis, membrane distillation, evaporation, forward osmosis, salt precipitation, or any combination thereof. Methods that include...

2. The method according to claim 1, further comprising generating a third saline solution by membrane distillation, and mixing the third saline solution with the first saline solution and / or the second saline solution.

3. The method according to claim 1, wherein thermal energy is transferred from the first saline solution to the second saline solution, thereby precipitating the salt in the first saline solution.

4. The method according to claim 1, further comprising increasing the difference in salt concentration between the first salt solution and the second salt solution by introducing the precipitated salt into the second salt solution.

5. The method according to claim 1, further comprising using a portion of the generated electricity to generate hydrogen gas by electrolysis.

6. The method according to claim 1, wherein the heat source includes one or more of geothermal energy, industrial waste heat, or solar heat.

7. The aforementioned regeneration process, i) To provide a used dilute solution formed from the first saline solution, which contains salt, ii) Heat the used dilute solution to decompose the salt and generate a gaseous product. iii) Transferring the gaseous product to the absorber, and iv) Solidifying the gaseous product and reforming it as a salt precipitate in the used concentrated solution in the absorber. A salt decomposition process comprising the application of a salt decomposition process in which the salt precipitate dissolves in the used concentrated solution to regenerate the second salt solution. The method according to claim 1, including the method described in claim 1.

8. The used dilute solution has a higher salt content than the first saline solution. The method according to claim 7, wherein the salt content of the used dilute solution is reduced by transferring the gaseous product to the absorber, thereby regenerating the first saline solution.

9. The aforementioned regeneration process, i) To provide a used dilute solution formed from the first saline solution, which contains salt, ii) To provide a used concentrated solution formed from the second saline solution, iii) Supplying electricity to separate the salt into ions, and moving the ions from the used dilute solution to the used concentrated solution. An electrodialysis process comprising applying an electroosmotic process, wherein the salt content of the used dilute solution decreases, thereby regenerating the first saline solution, and the salt content of the used concentrated solution increases, thereby regenerating the second saline solution. The method according to claim 1, including the method described in claim 1.

10. The aforementioned regeneration process, i) To provide a used concentrated solution formed from the second saline solution, ii) Heating the used concentrated solution to generate steam, and iii) Transferring the water vapor in order to mix it with the used dilute solution. An evaporation process is applied in which the salt content of the used dilute solution decreases, thereby regenerating the first salt solution, and the salt content of the used concentrated solution increases, thereby regenerating the second salt solution. The method according to claim 1, including the method described in claim 1.

11. The aforementioned regeneration process, i) To provide a membrane distillation vessel including a hydrophobic membrane, wherein the membrane has a used concentrated solution on one side and a used dilute solution on the opposite side, and ii) Heating the used concentrated solution to generate steam. A membrane distillation process comprising the following: the water vapor permeates the hydrophobic membrane and mixes with the used dilute solution to regenerate the first saline solution, and the saline content of the used concentrated solution increases to regenerate the second saline solution. The method according to claim 1, including the method described in claim 1.

12. The aforementioned regeneration process, The used concentrated solution and draw solution are circulated through a forward osmosis system to regenerate the second saline solution and generate a used draw solution. The used draw solution is circulated through a switchable solubility system to regenerate the draw solution and generate water. Applying a forward penetration process that includes The method according to claim 1, including the method described in claim 1.

13. The aforementioned forward penetration process, The water is mixed with the used dilute solution to regenerate the first saline solution. The method according to claim 12, further comprising:

14. A salt gradient heat engine system for generating electricity, The first saline solution and A second saline solution, wherein the saline concentration of the second saline solution is different from that of the first saline solution. A heat pump configured to transfer thermal energy to the first saline solution and / or the second saline solution, A selective permeable membrane for separating the first saline solution from the second saline solution, configured to control the mixing of the first saline solution and the second saline solution, and further configured to capture at least a portion of the saline concentration gradient energy as electrical power when the first saline solution and the second saline solution are mixed, A regeneration system selected from the group consisting of membrane distillation systems, evaporation systems, forward osmosis systems, salt decomposition systems, electrodialysis systems, and any combination thereof. A system that includes this.

15. The regeneration system includes a salt precipitation system, and the salt precipitation system is Receive the used dilute solution, The heat pump removes thermal energy from the used dilute solution, precipitates salt from the used dilute solution, and regenerates the dilute salt solution. After regenerating the dilute salt solution, the dilute salt solution is circulated. The precipitated salt is introduced into the used concentrated solution, and The precipitated salt is dissolved in the used concentrated solution, and the concentrated salt solution is regenerated. The system according to claim 14, configured as described above.

16. The regeneration system includes a salt precipitation system, and the salt precipitation system is A salt precipitation apparatus configured to receive a used dilute solution, precipitate salt from the used dilute solution, and regenerate the dilute salt solution, A concentrated solution tank configured to receive used concentrated solution and salt produced by the salt precipitation device, thereby regenerating the concentrated saline solution, One or more heat exchangers and The system according to claim 14, including the system described in claim 14.

17. The regeneration system includes a membrane distillation system, and the membrane distillation system is A membrane distillation vessel containing a hydrophobic membrane, A heat pump configured to heat the container and cool one side of the hydrophobic film, The system according to claim 14, comprising, wherein the membrane distillation system is configured to form a salt gradient on the hydrophobic membrane after heating, produce an ultradilute solution, and regenerate the concentrated saline solution.

18. The regeneration system includes the salt decomposition system, and the salt decomposition system is A container configured to receive a used dilute solution, wherein the used dilute solution contains salt, A heat pump configured to heat the container and generate a gaseous product containing salt inside the container, An absorber configured to receive a used concentrated solution and the gaseous product, wherein the salt in the gaseous product is absorbed by the used concentrated solution to regenerate the concentrated salt solution. The system according to claim 14, including the system described in claim 14.

19. The regeneration system includes the evaporation system, and the evaporation system is An evaporator configured to receive a used concentrated solution and generate steam to regenerate the concentrated saline solution, A heat pump configured to supply thermal energy to the evaporator, A condenser configured to receive a used dilute solution and the water vapor produced by the evaporator, wherein the water vapor condenses and mixes with the used dilute solution to regenerate the dilute saline solution. The system according to claim 14, including the system described in claim 14.

20. The regeneration system includes the electrodialysis system, The electrodialysis system is configured to receive used dilute solution and used concentrated solution. The system according to claim 14, wherein electricity is supplied to the electrodialysis system to transfer ions from the used dilute solution to the used concentrated solution, thereby regenerating the dilute saline solution and the concentrated saline solution.

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