Reverse electrodialysis or pressure delayed osmosis unit with heat pump

Abstract

Methods and systems for generating electricity or hydrogen from thermal energy are disclosed. The method includes separating a first saline solution from a second saline solution through a selective permeable membrane, receiving thermal energy from a heat source through the first saline solution and / or the second saline solution, and mixing the first saline solution and the second saline solution in a controlled manner, the first saline solution being separated from the second saline solution as a salinity difference between the first saline solution and the second saline solution decreases. At least some salinity gradient energy is captured as electrical power. The method further includes transferring thermal energy from the first saline solution to the second saline solution by the heat pump, causing an increase in salinity difference between the first saline solution and the second saline solution. The methods and systems may 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 generally relates to salt gradient heat engine systems and methods for generating electricity and / or hydrogen from thermal energy. Background Art

[0002] Salinity gradient power (SGP) is energy generated by the difference in salt concentration between two naturally occurring fluids (typically freshwater and saltwater), for example, when a river flows into the ocean. Reverse electrodialysis (RED) can be used to recover energy from salinity gradients, for example, by passing a salt solution and freshwater through a stack of alternating cation and anion exchange membranes. The chemical potential difference between saltwater and freshwater generates a voltage across each membrane, and the total potential of the system is the sum of the potential differences across all membranes. Open-loop RED cells require a continuous source of saltwater and freshwater to maintain the salinity gradient. This constraint may limit the practical location of commercial-scale RED cells. In addition, open-loop RED cells are susceptible to contamination by minerals, microorganisms, or other foreign matter or substances in the water source. Closed-loop RED cells do not require a continuous source of concentrated and dilute salt solutions, but require continuous regeneration of the salinity difference between the concentrated and dilute solutions, which can be energy intensive and / or inefficient.

[0003] This document describes methods and systems directed to solving the above-referenced problems and / or other important problems. Summary of the Invention

[0004] A method for generating electricity from thermal energy is disclosed. The method comprises: separating a first salt solution from a second salt solution via a selectively permeable membrane; transferring thermal energy to the first salt solution and / or the second salt solution via a heat pump; and controlling mixing of the first salt solution and the second salt solution to capture at least some of the salinity gradient energy as electricity as the salinity difference between the first and second salt solutions decreases. The method may include regenerating the salinity difference between the first and second salt solutions 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.

[0005] The salt decomposition process may include providing at least a portion of a spent dilute solution formed from a first salt solution, wherein the spent dilute solution contains salt; heating the spent dilute solution to decompose the salt, thereby producing 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. When the method includes generating a third salt solution by membrane distillation, the method may further include mixing the third salt solution into the first salt solution and / or the second salt solution. The first and second salt solution processes may include circulating the solutions in a substantially or completely closed closed system.

[0006] The selectively permeable membrane may comprise graphene, graphene oxide, or reduced graphene oxide, optionally having nanopores therein. The selectively permeable membrane may be a single-layer sheet, a multi-layer sheet, or a cylinder.

[0007] The method may further comprise capturing the salinity gradient energy using reverse electrodialysis or pressure-retarded osmosis to drive a generator.

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

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

[0010] The method may include a regeneration process comprising applying a salt decomposition process, the salt decomposition process comprising: providing a spent dilute solution formed from a first salt solution, wherein the spent dilute solution contains salt; heating the spent dilute solution to decompose the salt, thereby producing a gaseous product; transferring the gaseous product to an absorber; and solidifying the gaseous product to reform into a salt precipitate in the spent concentrated solution within the absorber. By transferring the gaseous product to the absorber, the salt content of the spent dilute solution can be reduced to regenerate the first salt solution. The salt precipitate can be dissolved in the spent concentrated solution to regenerate a second salt solution, and optionally the spent dilute solution has a higher salt content than the first salt solution.

[0011] The method may include a regeneration process comprising electrodialysis, the electrodialysis comprising: providing a spent dilute solution formed from a first salt solution, wherein the spent dilute solution contains salt; providing a spent concentrated solution formed from a second salt solution; and supplying electricity to separate the salt into ions and to move the ions from the spent dilute solution to the spent concentrated solution. The salt content of the spent dilute solution is reduced to regenerate the first salt solution, while the salt content of the spent concentrated solution is increased to regenerate the second salt solution.

[0012] The method may include a regeneration process comprising applying an evaporation process, the evaporation process comprising: providing a spent concentrated solution formed from the second salt solution; heating the spent concentrated solution to generate water vapor; and transferring the water vapor to mix with the spent dilute solution. The salt content of the spent dilute solution is reduced to regenerate the first salt solution, while the salt content of the spent concentrated solution is increased to regenerate the second salt solution.

[0013] The method may include a regeneration process comprising applying a membrane distillation process, the membrane distillation process comprising: providing a membrane distillation vessel comprising a hydrophobic membrane having a waste concentrated solution on one side of the membrane and a waste dilute solution on an opposite side of the membrane; and warming the waste concentrated solution to generate water vapor. The water vapor permeates the hydrophobic membrane to mix with the waste dilute solution to regenerate a first salt solution, while the salt content of the waste concentrated solution is increased to regenerate a second salt solution.

[0014] The method may include a regeneration process comprising applying a forward osmosis process comprising: circulating the spent concentrate solution and the draw solution through a forward osmosis system to regenerate the second brine 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 capacitive mixing (CAP) system, or both PRO and CAP systems to generate additional electricity.

[0016] A system for generating electricity is disclosed. The system includes: a first salt solution; a second salt solution, wherein the second salt solution has a salinity different from the salinity of 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 selectively permeable membrane that separates the first salt solution from the second salt solution. The system may further include: at least one regeneration system. The selectively permeable membrane may be configured to control mixing of the first salt solution and the second salt solution, and further configured to capture at least some salinity gradient energy as electricity when the first salt solution and the second salt solution mix. The selectively permeable membrane may include graphene, graphene oxide, or reduced graphene oxide, and optionally contain nanopores therein. The selectively permeable membrane may be a single-layer sheet, a multi-layer sheet, or a cylinder containing graphene, graphene oxide, and / or reduced graphene oxide. The heat pump may be a vapor compression cycle, a thermoelectric refrigerator, a chemical absorption refrigerator, or other devices known in the art for simultaneous heating and cooling. The regeneration system includes one or more of the following: a salt precipitation system, a membrane distillation system, a salt decomposition system, an electrodialysis system, a forward osmosis system, evaporation, or any combination thereof.

[0017] When the regeneration system comprises a membrane distillation system, the membrane distillation system comprises: a container containing at least a portion of the first or second salt solution, the container being covered by a hydrophobic membrane; and a heat pump (or alternatively, a second heat pump) configured to warm the container and cool the opposite side of the hydrophobic membrane. The membrane distillation system is configured to generate a salt gradient at the membrane after warming and to produce a third salt solution in the container. The hydrophobic membrane may comprise polytetrafluoroethylene, polypropylene, or polyvinylidene fluoride, and may optionally be configured in a sandwich unit 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 a spent dilute solution formed from the first salt solution, wherein the spent dilute solution contains salt; a heat pump (or alternatively, a second heat pump) configured to warm the vessel; and a cold water stream configured to receive at least one gaseous product released from the vessel. The salt decomposition system can be configured to decompose the salt and then reform the salt precipitate in the cold water stream.

[0019] The salt decomposition system may include: a container configured to receive a spent dilute solution from a reverse electrodialysis cell, wherein the spent dilute solution contains salt; a heat pump configured to warm the container and generate a gaseous product containing the salt in the container; and an absorber configured to receive a spent concentrated solution from the reverse electrodialysis cell and receive the gaseous product. Salt in the gaseous product can be absorbed by the spent concentrated solution to regenerate a 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] When the regeneration system includes a membrane distillation system, the membrane distillation system may include: a membrane distillation container including a hydrophobic membrane; and a heat pump configured to warm the container and cool one side of the hydrophobic membrane. The membrane distillation system may be configured to generate a salt gradient at the hydrophobic membrane after warming, and to generate an ultra-dilute solution and regenerate a concentrated salt solution. The membrane distillation system may further include: a concentrate tank configured to receive a waste concentrated solution from the reverse electrodialysis cell and connected to the membrane distillation container; a diluent tank configured to receive a waste dilute solution from the reverse electrodialysis cell and connected to the membrane distillation container. The concentrate tank may be configured to receive a concentrated solution from the membrane distillation container, and the diluent tank may be configured to receive an ultra-dilute solution from the membrane distillation container. The hydrophobic membrane may include polytetrafluoroethylene, polypropylene, or polyvinylidene fluoride, and may be configured in a sandwich unit stack structure.

[0022] When the regeneration system includes an evaporation system, the evaporation system may include: an evaporator configured to receive the spent concentrated solution from the reverse electrodialysis cell and generate water vapor to regenerate the concentrated salt solution; a heat pump configured to provide heat energy to the evaporator; and a condenser configured to receive the spent dilute solution from the reverse electrodialysis cell and receive the water vapor generated by the evaporator. The water vapor is condensed and mixed with the spent dilute solution to regenerate the dilute salt solution.

[0023] When the regeneration system includes an electrodialysis system, the electrodialysis system can be configured to receive the spent dilute solution and the spent concentrated solution. Supplying power to the electrodialysis system causes ions to move from the spent dilute solution to the spent concentrated solution to regenerate the dilute salt solution and the concentrated salt solution.

[0024] The regeneration system may include a salt precipitation system and an electrodialysis system, wherein the salt precipitation system includes: a salt precipitator configured to receive a spent dilute solution from a reverse electrodialysis cell and precipitate salt from the spent dilute solution, and then supply the spent dilute solution to the electrodialysis system; a concentrate tank configured to receive a spent concentrated solution from the reverse electrodialysis cell and receive salt generated by the salt precipitator. The concentrate tank may be configured to receive the concentrated solution, and the dilute tank may be configured to receive the dilute salt solution from the electrodialysis system.

[0025] The regeneration system may include a forward osmosis system configured to receive a spent concentrated solution from the reverse electrodialysis cell and to regenerate the concentrated brine using a switchable solubility system. In the case of a forward osmosis system, the regeneration system may further include a diluent tank configured to receive the spent dilute solution from the reverse electrodialysis cell and to receive water from the switchable solubility system, wherein the water is mixed with the spent dilute solution to regenerate the dilute brine. The switchable solubility system may include: a draw solution that circulates through the forward osmosis system and produces a spent draw solution; a recovery device configured to receive the spent draw solution and add heat, wherein CO2 is released and water is produced; and a generator configured to receive a solution from the recovery device and add CO2 to regenerate the draw solution. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] The accompanying drawings, which are incorporated herein and constitute a part of this specification, illustrate specific embodiments of the present disclosure and do not limit the scope of the present disclosure.

[0027] Figure 1 An exemplary reverse electrodialysis (RED) system is shown.

[0028] Figure 2 An exemplary regeneration system is shown.

[0029] Figure 3 An exemplary thermal optimization system is shown.

[0030] Figure 4 An exemplary pressure-retarded permeation (PRO) system is shown.

[0031] Figure 5 An exemplary hydrogen generation system is shown.

[0032] Figure 6 A flow chart of a method for generating electricity from thermal energy.

[0033] Figure 7 A block diagram is shown of an example of internal hardware that may be used to contain or implement program instructions according to one embodiment.

[0034] Figure 8is an example of a system and method of the present disclosure comprising a RED or PRO battery in combination with a salt precipitation system, and connected to an external heat source.

[0035] Figure 9

[0014] Another example of a system and method of the present disclosure includes a RED or PRO battery in combination with a salt precipitation system. This system does not include an external heat source system, but heat is added to the diluate tank, for example, via a heat pump loop and / or waste heat.

[0036] Figure 10 is an example of a system and method of the present disclosure comprising a RED or PRO battery in combination with a salt decomposition system.

[0037] Figure 11 is an example of a system and method of the present disclosure that includes a RED or PRO battery in combination with a evaporation system.

[0038] Figure 12 is an example of a system and method of the present disclosure comprising a RED or PRO cell coupled with an electrodialysis system and salt precipitation.

[0039] Figure 13 is an example of a regeneration system incorporating a membrane distillation system and salt precipitation.

[0040] Figure 14 is an example of a regeneration system incorporated into a membrane distillation system.

[0041] Figure 15 is an example of a batch system for regeneration, where all operating tanks are working.

[0042] Figure 16 This is an example of water production using a steam condenser and a heat pump.

[0043] Figure 17 How to use Figure 16 Example of a system to generate cold and hot fluid tanks for use in a salinity gradient energy thermal system.

[0044] Figure 18 is an example of the systems and methods of the present disclosure comprising a RED or PRO cell combined with atmospheric water generation using a liquid desiccant dehumidification process.

[0045] Figure 19 is an example of a system and method of the present disclosure comprising a RED or PRO cell in combination with a forward osmosis system. DETAILED DESCRIPTION

[0046] The present disclosure relates to salt gradient heat engine systems that utilize heat and generate electricity from the thermal energy. A salt gradient heat engine can be any system that utilizes thermal energy to generate or regenerate a salinity gradient and produce usable energy (e.g., electricity and / or hydrogen). Examples of salt gradient heat engine systems include RED and PRO cells. RED cells can incorporate more than one selectively permeable membrane and one or more electrodes, and are discussed in more detail below. PRO cells can incorporate one membrane and do not require one or more electrodes. PRO cells generate pressure rather than generating electricity directly from the salinity difference between a concentrated salt solution and a solution as RED cells do. The selectively permeable membrane of a PRO cell can be configured to preferentially allow a solvent rather than a solute to pass through the membrane, for example, from a dilute solution to a concentrated salt solution, so as to reduce the salinity difference between the solutions.

[0047] Both RED and PRO cells can include a concentrated salt solution separated from a dilute solution by a selectively permeable membrane. Furthermore, in both systems, the power ratio generated by the system is at least a function of the salinity difference between the concentrated salt solution and the dilute solution, and optionally also at least a function of the temperature of the concentrated salt solution.

[0048] A reverse electrodialysis system is disclosed, comprising 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 comprises a first membrane configured to be selectively permeable to cations and a second membrane configured to be selectively permeable to anions, the second membrane being spaced apart from the first membrane. The cell further comprises a concentrated salt solution disposed between the first membrane and the second membrane, the first membrane and the second membrane separating the concentrated salt solution from the dilute salt solution, such that the first membrane selectively allows cations to migrate toward the cathode and the second membrane selectively allows anions to migrate toward the anode, resulting in a voltage difference between the cathode and the anode. The first selectively permeable membrane and the second selectively permeable membrane may comprise ion exchange membranes.

[0049] Disclosed herein are multiple (e.g., 2-500, 2-200, 10-400, or 2-100) selectively permeable membranes for use in reverse electrodialysis systems. Some selectively permeable membranes restrict the ability of ionic components to diffuse freely. In contrast, cation exchange membranes (and anion exchange membranes) allow cationic components and anionic components to migrate or move in opposite directions, respectively. Each selectively permeable membrane can be made of an organic or inorganic polymer (e.g., an ion exchange resin) having charged (ionic) side groups. Each selectively permeable membrane can be made of graphene, reduced graphene oxide, or graphene oxide. The selectively permeable membrane can include graphene in a single-layer configuration or optionally stacked multilayer sheets, and optionally include nanopores. The selectively permeable 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 or other aspects of the graphene sheet. The single-layer or multilayer sheets can be stretched or otherwise constructed to change the permeability of the membrane.

[0050] The selectively permeable membrane can be a bipolar membrane (e.g., anionic on one side and cationic on the opposite side) which, when used, generates acids and bases from salts present in solution. The selectivity of the selectively permeable membrane can be based on size, charge, charge density, phase (e.g., hydrophobic / hydrophilic), or polarity.

[0051] The selectively permeable membrane may be a polymer composite membrane having oriented nanochannels, such as those disclosed in WO 2022 / 032236, which is incorporated herein by reference in its entirety. For example, the selectively permeable membrane may be a thin film composite membrane comprising: (i) a polymeric 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 arranged as hexagonal packed cylinders within the membrane, oriented parallel to the membrane surface, and existing as an H1 mesophase; wherein the cylinders are internally cross-linked within the cylinders; and wherein the cylinders are 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 polymeric membrane, film, or coating. In an embodiment, the porous support layer is polyacrylonitrile, polyvinylidene fluoride, polysulfone, polyamide, polyimide, polypropylene, anodized aluminum oxide, cellulose acetate, or a nonwoven fabric.

[0052] The salt gradient heat engine system may include a heat source configured to transfer thermal energy to a concentrated salt solution or a dilute salt solution; and a regeneration system comprising a heat pump. The heat pump may be any device known in the art for simultaneous heating and cooling, and optionally produces a coefficient of performance greater than about 1, or about 1 to about 10, about 1 to about 6, or about 3 to about 4. The heat pump may be a vapor compression cycle, a thermoelectric refrigerator, a chemical absorption refrigerator, or the like. The vapor compression cycle may be a screw type, an acoustic type, an air conditioner. The heat pump used herein may include a refrigerant that undergoes or does not undergo a phase change. The refrigerant may be CO2, helium, or any other refrigerant known for use in heat pumps. The heat pump may include a thermodynamic cycle. The thermodynamic cycle may include any combination of refrigerant and / or non-refrigerant cycles that provide the ability to heat and cool simultaneously. The heat pump may be a thermoacoustic heat pump, such as the thermoacoustic heat pump 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 a system disclosed in U.S. Patent No. 9,915,436, entitled "Heat Source Optimization System," or U.S. Patent No. 11,067,317, entitled "Heat Source Optimization System." Each of the aforementioned patents is incorporated herein by reference in its entirety. In some embodiments, a humidifier, a dehumidifier, a two-way ventilation fan, and / or a wet refrigerator may be used in conjunction with the heat pump to drive reverse electrodialysis. The heat pump may be fueled by any known heat exchange fluid, such as, but not limited to, water, a refrigerant, ethylene glycol, or oil.

[0053] Traditionally, humidity is detrimental to HVAC cooling because it adds a dead load to the system; energy is consumed by condensing water vapor, wasting energy and leading to energy inefficiency. Typically, when the desired temperature is set on the thermostat, any energy spent condensing water vapor is energy that could be used to cool the air. In this case, energy is consumed to condense the vapor into condensate, and the HVAC unit works against the latent heat of evaporation.

[0054] In contrast, according to the present disclosure, humidity in the environment can be highly advantageous because humidity provides additional energy to the system, which can be used to drive the reverse electrodialysis process and generate electricity, hydrogen, oxygen, and any combination thereof. For example, humidity can be introduced into the system and the latent heat of water vapor can be captured and used to drive the reverse electrodialysis and generate electricity. Similarly, as ice forms, the thermal crystallization of water can be used to drive the reverse electrodialysis process to generate electricity, hydrogen, oxygen, or any combination thereof.

[0055] The regeneration system may be configured to receive the dilute salt solution from at least one of the one or more units and remove heat energy from the dilute salt solution (via a heat pump), causing the dilute salt solution to precipitate salt. The regeneration system may be configured to, after causing the dilute salt solution to precipitate salt, circulate the dilute salt solution to at least one of the one or more units, introduce the precipitated salt into the concentrated salt solution, and cause the precipitated salt to dissolve in the concentrated salt solution.

[0056] The regeneration system may be configured to transfer at least some of the heat removed from the dilute salt solution back to the dilute salt solution after causing the dissolved salt in the dilute salt solution to precipitate. The regeneration system may be configured to transfer at least some of the heat removed from the dilute salt solution to the concentrated salt solution, causing the precipitated salt to dissolve in the concentrated salt solution. The heat source may be configured to transfer heat to the concentrated salt solution, causing the precipitated salt to dissolve in the concentrated salt solution. The concentrated salt solution may include an endothermic solution or an exothermic solution. The concentrated salt solution may include a substance having a nonlinear temperature-dependent solubility.

[0057] One or more other regeneration systems can be adopted. The regeneration system may include electrodialysis. In addition to the reverse electrodialysis system, electrodialysis can be further used for water purification. In the case of electrodialysis, the dilute salt solution can be further desalted using electrodialysis. For example, when precipitating salt, the concentration of the dilute salt solution is limited by the solubility curve. In order to further dilute the dilute salt solution, electrodialysis can be adopted. Any renewable electricity (for example, solar energy / wind energy, etc.) can be used to force electrodialysis to separate salt from the dilute salt solution and produce a further diluted stream. This provides energy storage. During the hours when the sun is out, the salt gradient using electrodialysis can be fully utilized, and then the energy from the salt gradient can be utilized after sunset. The battery / tank that can be used for electrodialysis is the same as that used for reverse electrodialysis. Figure 12 is an example of a system including electrodialysis and RED / PRO cells, which is discussed further below.

[0058] The reverse electrodialysis system may further include a control system configured to coordinate heat transfer between the one or more heat sources and the reverse electrodialysis system based on one or more measurements of the one or more heat sources or the state of the reverse electrodialysis system. The heat source includes one or more of geothermal heat, industrial waste heat, or solar heat.

[0059] A salt gradient heat engine system may include a salt decomposition system to generate a salt gradient. Salt decomposition can be employed in conjunction with reverse electrodialysis rather than the salt precipitation process disclosed herein. For example, a spent dilute solution (supplied by a RED cell) can be heated to a temperature above the temperature at which the salt decomposes (e.g., ammonium bicarbonate decomposes into CO2 and ammonia at approximately 60°C). As the solution is heated, the salt decomposes and leaves the spent dilute solution as gaseous products, which reduces the salt concentration in the spent dilute solution, thereby producing a regenerated dilute solution. A vacuum or, optionally, the application of a fan can help move the gaseous products toward a cold stream. The gaseous products (e.g., CO2 and ammonia) can be forced into a cold water stream to react and precipitate back into a solid salt form (e.g., CO2, ammonia, and cold water into ammonium bicarbonate salt). The solid salt (e.g., ammonium bicarbonate) precipitated from the solution can be delivered to a concentrated salt solution in the RED cell. In this process, a heat pump can be used to heat and cool the spent dilute solution and cold water vapor, respectively. The heat pump can be the same as or different from the heat pump used in other steps of the reverse electrodialysis system disclosed herein. Optionally, as the gaseous products precipitate in the cold stream, heat energy can be extracted and transferred to the spent dilute solution to decompose the salts. Additionally, heat energy can be used to increase the temperature of the precipitated solution to increase solubility and allow for superconcentrated solutions. Figure 10 is an example of a salt splitting process, which shows the solution flow between the RED cell and the vessel of the salt splitting process, and is discussed in further detail below.By applying the salt splitting process, it is possible to regenerate concentrated and dilute salt solutions by incorporating salt splitting.

[0060] The reverse electrodialysis system may include a second unit. The second unit may include a third membrane configured to be selectively permeable to cations and a fourth membrane configured to be selectively permeable to anions, the fourth membrane being spaced apart from the third membrane. The second unit may include a second concentrated salt solution disposed between the third membrane and the fourth membrane, the third membrane and the fourth membrane separating the second concentrated salt solution from a second dilute salt solution. The concentrated salt solution may include an endothermic solution, the second concentrated salt solution may include an exothermic solution, and the heat pump may be configured to transfer heat between the concentrated salt solution and the second concentrated salt solution.

[0061] Reverse electrodialysis systems can include membrane distillation systems. Membrane distillation (MD) is a heat-driven separation process in which liquids are rejected and only vapor molecules permeate through a porous hydrophobic membrane. The driving force in the MD process is the vapor pressure difference generated by the temperature difference across the hydrophobic membrane. The hydrophobic membrane must be inherently hydrophobic, or its surface can be modified to be hydrophobic. The hydrophobic membrane used for MD can be polytetrafluoroethylene (PTFE), polypropylene (PP), polyvinylidene fluoride (PVDF), or any combination thereof. Large surface area hydrophobic membranes can be used in a sandwich unit stack design similar to a RED stack. The stack of hydrophobic membranes can include only one or more types of hydrophobic membranes without electrodes. The use of a heat pump can improve the efficiency of the MD process by simultaneously heating and cooling, thereby generating a strong temperature gradient driving force. For example, a heat pump can be used to simultaneously heat the waste concentrated solution or the waste dilute solution to about 40°C to about 80°C on one side of the hydrophobic membrane while providing a cooling means (e.g., a cooling stream heat exchanger, an evaporator with a refrigerant, or cooling water) on the other side of the hydrophobic membrane to produce a dilute solution and to capture heat, e.g., from ambient conditions, geothermal heat, solar energy, or industrial waste heat. Membrane distillation allows for efficient generation of a salt gradient using the power of a heat pump, which can be used in the reverse electrodialysis system disclosed herein. The heat pump can be the same or different from the heat pump used in other steps of the reverse electrodialysis system disclosed herein. Membrane distillation can be used to generate a concentrated salt solution and, optionally, a dilute salt solution for introduction into the RED cell. Figure 13 is an example of a membrane distillation system, which shows the solution flow between the MD system and the RED system. It is possible to regenerate concentrated and dilute salt solutions by introducing membrane distillation.

[0062] The membrane distillation process utilized herein may be direct contact membrane distillation (DCMD), air gap membrane distillation (AGMD), vacuum membrane distillation (VMD), sweep gas membrane distillation (SWGMD), vacuum multi-effect membrane distillation (V-MEMD), permeate gap membrane distillation (PGMD), or a combination thereof.

[0063] The membrane distillation system may include: a container containing at least a portion of a first or second salt solution, the container being covered by a hydrophobic membrane that allows vapor permeation; a heat pump configured to warm the container; and cooling the opposite side of the membrane. The membrane distillation system may be configured to generate a salt gradient at the membrane after warming and produce a third salt solution (i.e., concentrated) in the container. The membrane distillation system may include a second heat pump to warm the container, and optionally a cooling device on the side of the membrane opposite the container. The cooling device may be a cold stream, a heat exchanger, a refrigerant circuit, or otherwise operate, for example, to condense the vapor back into a liquid. Figure 13This is an example of a membrane distillation process, which shows a hydrophobic membrane that creates a salt gradient between a warm, concentrated solution and a cold, dilute solution, as discussed in further detail below. The membrane distillation system can 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 solutions are provided on either side of the hydrophobic membrane, and a heat pump is configured to warm one side of the membrane; a cooling device is provided on the opposite side of the membrane to cool the solution.

[0064] The reverse electrodialysis system may include a microbial reverse electrodialysis electrolysis cell (MREC), as disclosed in U.S. Patent No. 9,112,217, which is incorporated herein by reference in its entirety. In this embodiment, the microorganisms generate electricity to drive the RED system. The MREC may include a variety of electrogenic microorganisms disposed in a RED cell that assist in the production of hydrogen and electricity through oxidation of organic matter at the anode and reduction of oxygen at the cathode. Because microorganisms thrive in warmer conditions, the use of a heat pump may increase the ability of the microorganisms to remove electrode overpotentials. Electrode overpotentials promote significant energy losses due to thermodynamically unfavorable electrode reactions. Additionally, the leveraged thermal energy provided by the heat pump promotes greater reaction kinetics, which reduces the amount of membrane required to produce the same amount of energy in the RED. For example, in a residential environment, organic waste (e.g., sewage from a septic tank) can be converted into usable energy using microbial reverse electrodialysis. Ultraviolet light can be used with any of the salt gradient heat engine systems disclosed herein to limit any microbial growth / fouling within the system.

[0065] In a salt gradient heat engine system, the RED and PRO can be operated together continuously or in a batch system. When operating in a batch system, the reverse electrodialysis system can include multiple precipitators and / or multiple tanks arranged in parallel or in a stack. When operating in a batch system, the precipitators, tanks, and stack as a whole can be of different sizes and operated in series, parallel, or a combination of both, and can operate in countercurrent, crosscurrent, or co-flow. For example, the system can include two or more precipitators, each producing a regenerated dilute solution that is combined before flowing into the RED cell; and / or the system can include two or more dissolution tanks, each producing a regenerated concentrated salt solution that is combined before flowing into the RED cell. When operating as a batch system, the salt solution can be passed through the RED cell stack several times (multiple passes) to improve efficiency. This provides the ability to stop operation in one part of the system, for example, for maintenance, while maintaining operation in the rest of the system.

[0066] In one embodiment, a single vessel can be both a precipitator (e.g., precipitates salt from spent solution and removes a regenerated, dilute solution) and a dissolver (e.g., adds spent solution to a vessel containing precipitated salt and then dissolves the salt into the solution so that a concentrated solution can be regenerated). Here, the salt remains in the vessel and is used to prepare a regenerated, concentrated salt solution, rather than removing the precipitated salt.

[0067] The concentrated solution and the dilute solution will be in separate circuits. However, there may be some controlled mixing of the two solutions (controlled mixing within the RED / PRO stack). In addition, some uncontrolled mixing (water flow) may occur between the concentrate and dilute solutions within the RED / PRO stack. Due to osmosis, some water in the dilute solution may be transferred or migrated into the concentrated solution in the RED / PRO stack. Because of this movement of water due to osmosis, there may be a controlled flow that balances the total volume in the dilute and concentrate tanks. Otherwise, the volume in the concentrate tank continues to increase. The controlled mixing or flow between the circuits can be controlled by incorporating a valve that operates to ensure that both circuits have the same solution volume.

[0068] A method for generating electricity from thermal energy is disclosed. The method includes separating a first salt solution from a second salt solution via a selectively permeable membrane. The method includes receiving thermal energy from a heat source via the first salt solution and / or the second salt solution. The method includes controlling mixing of the first salt solution and the second salt solution, capturing at least some salinity gradient energy as electricity 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 via a heat pump, causing the salinity difference between the first salt solution and the second salt solution to increase.

[0069] The method may include using reverse electrodialysis to capture salinity gradient energy. The method may further include using pressure-retarded osmosis to drive a generator to capture salinity gradient energy. In some embodiments, the first saline solution and the second saline solution are each circulated in a closed system. Transferring heat energy from the first saline solution to the second saline solution may cause salt to precipitate from the first saline solution. The method may include introducing the precipitated salt into the second saline solution, causing the salinity difference between the first saline solution and the second saline solution to increase. The method may include using a portion of the generated electricity to produce hydrogen and, optionally, oxygen through electrolysis. In some examples, transferring heat energy from the first saline solution to the second saline solution includes transferring heat energy from the first saline solution, which is cooler than the second saline solution.

[0070] The method may further include coordinating heat transfer from one or more heat sources to the first saline solution and / or the second saline solution based on the one or more heat sources or one or more measurements of the state of the first saline solution and / or the second saline solution. The heat source may include one or more of geothermal heat, industrial waste heat (e.g., power plant heat), exhaust from transportation vehicles (e.g., cars, ships, trucks), or solar heat.

[0071] The method may include reversing the circulation flow of the first saline solution and the second saline solution in the closed system. This may be achieved by applying solenoid valves at both ends of the closed loop system. Reversing the circulation flow does not stop energy generation and extends the use of the selectively permeable membrane, causing the membrane to wear substantially equally on its two opposite lateral sides.

[0072] The following discussion omits or briefly describes only conventional features of the disclosed technology that would be apparent to one skilled in the art. Reference to various embodiments does not limit the scope of the appended claims. In addition, any examples set forth in this specification are intended to be non-limiting and merely illustrate some of the many possible embodiments of the appended claims. Furthermore, specific features described herein may be used in combination with other described features in each of a variety of possible combinations and permutations. One of ordinary skill in the art will know how to use the present invention in combination with routine experimentation to achieve other results not specifically disclosed in the examples or embodiments.

[0073] It is also understood that the terms used in this specification are only for the purpose of describing a particular form or embodiment, and are not intended to limit the scope of the present disclosure, which will be limited only by the appended claims. Unless specifically defined otherwise herein, all terms should be given their broadest possible interpretation, including the meaning implied by this specification and the meaning understood by those skilled in the art and / or the meaning defined in dictionaries, papers, etc. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as those of ordinary skill in the art are generally understood. 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 now described. All references mentioned herein are incorporated by reference in their entirety.

[0074] As used in the specification and including the appended claims, the singular forms "a (kind) (the indefinite article a, an)" and "the" include plural forms, and unless the context clearly dictates otherwise, references to a particular numerical value include at least that particular value. Ranges may be expressed herein as from "about" or "approximately" one particular value and / or to "about" or "approximately" another particular value. When such a range is expressed, another embodiment includes from one particular value and / or to other particular values. Similarly, when a value is expressed as an approximation by using the antecedent "about", it will be understood that the particular value forms another embodiment. It is also understood that all spatial references, for example, horizontal, vertical, top, upper, lower, bottom, left and right, are for illustrative purposes only and may be changed within the scope of the present disclosure. For example, references to "upper" and "lower" are relative and are used only relative to one another in context, and not necessarily "upper" and "lower". Generally, similar spatial references to different aspects or components indicate similar spatial orientation and / or positioning, ie, each "first end" is at or directed toward the same end of the device.

[0075] The systems and methods described in this disclosure generally relate to efficiently extracting usable energy from the salt concentration difference between two solutions via the precise arrangement and controlled mixing of two solutions. The system can be used to directly generate electricity, or generate hydrogen, or hydrogen and oxygen, which can be used as a fuel for generating mechanical (and / or electrical) power, or generate pressure and / or gravitational potential energy, any of which can be used to drive a turbine or perform other useful work. The system covers a range of sizes and power outputs. Some embodiments can be configured to generate power on the scale of a single residential or commercial building. In some instances, the system includes an industrial power generation system that provides electricity to a regional or national power grid. In some instances, the system provides hydrogen fuel, for example, to power a fleet of vehicles, as well as (or instead of) generating electricity.

[0076] Figure 1 An exemplary system 100 for generating power from a salinity gradient is shown. The exemplary system 100 includes a reverse electrodialysis (RED) cell 110. The RED cell 110 includes a cathode 112 and an anode 114 separated by one or more cells 150 (150a, 150b). Each cell 150 contains a saline solution 130, i.e., a liquid mixture of a solvent and a salt that has been dissolved into its anionic and cationic constituents such that the ionic components are free to move relative to each other. Each ion can have a single charge, or can have multiple variations. In some examples, the solvent and solute are water and sodium chloride (NaCl), respectively. The dissociated ions of NaCl are NaCl, each having a single charge. + and Cl -Other solvents and solutes can be used that also form a liquid mixture comprising anions and cations that are free to move relative to each other. The solvent can be an organic or inorganic liquid, including but not limited to water, alcohol, benzene, and glycerol. The salt solution can be exothermic or endothermic. That is, as the solution forms, the solution can absorb heat, such as when potassium chlorate (KClO3) or potassium nitrate (KNO3) is dissolved in water, or release heat, such as when calcium chloride (CaCl2) is dissolved in water.

[0077] like Figure 1 As shown, a salt solution 130 is separated from a dilute solution 140 (i.e., a solution having a lower solute concentration than the salt solution) by a selectively permeable membrane 104 (104a-d). The salt solution 130 is separated from the dilute solution 140 on one side of the cell 150 by a cation exchange membrane (104a, 104c) and on the other side of the cell 150 by an anion exchange membrane (104b, 104d), which is spaced apart from the cation exchange membrane (104a, 104c). The salt solution 130 is disposed in the space between the cation exchange membrane (104a, 104c) and the anion exchange membrane (104b, 104d). In the absence of these membranes 104, the salt solution 130 would freely diffuse into the dilute solution 140, thereby equalizing the salinity of the two solutions. The movement of negatively charged ions toward the anode 114 and the movement of positively charged ions toward the cathode 112 causes a potential difference (voltage) across the cell 150. The total voltage of the battery 110 includes the voltage of each cell 150 .

[0078] Ions may tend to accumulate near the membrane 104. This accumulation may hinder the process of power generation. To counteract this accumulation, the system 100 may apply agitation or mixing forces to the salt solution 130 and / or the dilute solution 140, causing the ions to be more evenly (uniformly) distributed throughout the solutions 130, 140. In some examples, the system 100 applies acoustic vibrations to one or more solutions 130, 140 to enhance the uniformity of the solutions 130, 140. The system 100 may apply acoustic vibrations to ion accumulation areas of the cell 150 (e.g., near one or more membranes 104) to effectively enhance the uniformity of the one or more solutions 130, 140.

[0079] like Figure 1As shown, the electrodes (e.g., cathode 112 and anode 114) are surrounded by a dilute solution 140. Alternatively, the electrodes 112 and 114 may be surrounded by a rinse solution (e.g., an aqueous solution) that circulates in a closed loop between the electrodes 112 and 114, separating the rinse solution from the salt solution 130 and the dilute solution 140. In such an arrangement, the outer selectively permeable membranes 104 (i.e., the membranes 104 closest to the respective electrodes 112 and 114 of the RED cell 110) are of the same type (e.g., two anion exchange membranes or two cation exchange membranes). For example, the cation exchange membrane 104 may separate the rinse solution surrounding the cathode 112 from the salt solution 130, and the cation exchange membrane 104 may also separate the rinse solution surrounding the anode 114 from the dilute solution 140. In this arrangement, cations that migrate from the salt solution 130 to 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 the respective electrodes 112 and 114 can both be anion exchange membranes. In this case, anions circulate from the anode 114 (in the rinse solution) to the cathode 112, where they 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 balanced 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 and 114. Because both electrodes 112 and 114 are surrounded by the same electrolyte (the rinse solution) in this arrangement, the rinse solution may form a resistive load between the electrodes 112 and 114 through which current can flow, thereby 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.

[0080] The current generated by the cell 110 is a function of the rate of ion movement, which 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) and (at least) the temperature of the salt solution 130, as well as aspects of the membrane 104. The temperature of the salt solution 130 affects the rate at which ions in the salt solution 130 move toward (and across) the membrane 104 due to the increased kinetic energy of the ions at higher temperatures. According to the Nernst equation, the power generated is a function of the logarithm of the ratio of the salinities of the salt solution 130 and the dilute solution 140. However, as ions move from the salt solution 130 to the dilute solution 140, the salinity of the dilute solution 140 increases, while the salinity of the salt solution 130 decreases. Therefore, 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 cell 110, the salinity difference can be continuously regenerated by refreshing the spent salt solution 130 and / or the spent dilute solution 140. To this end, spent brine 130 and / or spent dilute solution 140 may be circulated between RED cell 110 and the regeneration system (e.g., in a closed loop). Alternatively, spent brine 130 and / or spent dilute solution 140 may be continuously replenished, for example, from a natural source such as a river or ocean or bay.

[0081] Figure 2An exemplary regeneration system 200 is shown. The exemplary regeneration system 200 includes a salt removal subsystem 210. The regeneration system 200 circulates spent dilute solution 140 from the RED cell 110 in a closed loop through the salt removal subsystem 210 and back to the RED cell 110 as renewed dilute solution 140. In some examples, the salt removal subsystem 210 evaporates the solvent of the dilute solution 140 and then condenses it, and then circulates the condensed solvent back to the RED cell 110 as renewed dilute solution 140. The system can evaporate the solvent until the remaining dilute solution 140 is close to or even below the solubility limit. The remaining dilute solution 140 can then be reintroduced into the spent salt solution 130 to renew the salt solution 130. In some examples, the salt removal subsystem 210 evaporates the solvent of the salt solution 130 and then condenses it, and then circulates the condensed solvent back to the RED cell 110 as renewed dilute solution 140. In a closed-loop RED cell 110, the salt removal subsystem 210 can evaporate and then condense both the salt solution 130 and the dilute solution 140, circulate the condensed solvent from both solutions back to the RED cell 110 as a renewed dilute solution 140, and circulate the remaining solution back to the RED cell 110 as a renewed (concentrated) salt solution 130. In some examples, the salt removal subsystem 210 reduces the salinity of the dilute solution 140 through a salt precipitation process rather than (or in addition to) evaporation to regenerate the salinity difference between the salt solution 130 and the dilute solution 140 in the RED cell 110. Other methods of regenerating the salinity gradient include freezing the spent dilute solution 140, for example, by eutectic chilled crystallization (ECC), or using microfiltration and / or membrane separation. In some examples, multiple methods are advantageously combined. For example, a precipitation or freezing stage can be augmented by a subsequent membrane filtration stage to optimize the total energy required to separate salt from the spent dilute solution 140.

[0082] like Figure 2 As shown, the exemplary salt removal subsystem 210 removes salt from the spent dilute solution 140 by causing salt precipitation. Generally, the ability of a solvent to dissolve a solute increases with increasing temperature. Conversely, lowering the temperature of a solution below a temperature known as the saturation point (the temperature at which the solution is at its maximum salinity) generally causes solute precipitation. The exemplary salt removal subsystem 210 includes a heat transfer device 216 that is configured to cool the spent dilute solution 140 to a temperature below the saturation point. If the spent dilute solution 140 is an exothermic solution, the dilute solution 140 will cool further as the salt precipitates. In some examples, the heat transfer device 216 also heats the renewed dilute solution 140, for example, back to the temperature of the spent dilute solution 140 before cooling. That is, the heat transfer device 216 can transfer some or all of the heat energy removed from the spent dilute solution 140 back to the renewed dilute solution 140, as with Figure 2As shown by the arrows associated with the heat transfer device 216. In this way, the temperature of the refreshed dilute solution 140 entering the RED cell 110 is substantially the same as the temperature of the spent dilute solution 140 leaving the RED cell 150.

[0083] In exemplary salt removal subsystem 210, precipitated salt 212 is settled to the bottom of salt removal subsystem 210, for example, in dense solid form. In some instances, salt removal subsystem 210 includes a conveying device 230, which is configured to transport precipitated salt 212 away from salt removal subsystem 210. Conveying device 230 can be a conveyor belt, a pump, an Archimedean screw or other devices or systems configured to physically transport precipitated salt 212 away from salt removal subsystem 210. For example, if salt is in solid form, conveying device 230 can be a mechanical system capable of transporting solid materials. In some instances, removed salt 212 is transported to salt replenishment subsystem 220, which reintroduces (for example, redissolves) salt into waste brine 130 at this salt replenishment subsystem, thereby renewing waste brine 130. In a manner similar to how the regeneration system 200 circulates the dilute solution 140, the regeneration system 200 can also circulate the spent brine 130 from the RED cell 110 in a closed loop through the salt replenishment subsystem 220 and back to the RED cell 110 as renewed brine 130. The salt replenishment subsystem 220 can increase the salinity of the brine 130 by re-dissolving the salt removed by the salt removal subsystem 210, thereby regenerating the salinity difference between the brine 130 and the dilute solution 140 in the RED cell 110.

[0084] As described above, a solvent's ability to dissolve a solute generally increases with increasing temperature. Thus, a higher temperature of the salt solution 130 allows for higher salinity levels and an accompanying greater difference in salinity between the salt solution 130 and the dilute solution 140. A solubility curve is a graph 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 can vary linearly with temperature over a wide temperature range (e.g., the entire range over which the solvent is liquid). In some instances, the amount of solute that a solvent can dissolve can vary nonlinearly with temperature. In these cases, even within a narrow temperature range, the amount of solute that a solvent can dissolve can increase, for example, by a factor of five or more. System 100 can be configured to operate RED cells 110 within a temperature range where the salinity of the salt solution 130 is high. Dissolving additional salt may require transferring additional heat to the salt solution 130. Additionally, the system 100 can maintain the temperature of the RED cell 110 at a point above the solubility point to provide a "safety margin" to avoid undesirable precipitation if the salt solution 130 cools below the solubility point.

[0085] The salt replenishment subsystem 220 may receive thermal energy from one or more heat sources configured to increase the temperature of the salt solution 130, for example, to allow additional salt to dissolve. For example, the salt replenishment subsystem 220 may receive waste heat from the heat transfer device 216 of the salt removal subsystem 210. The salt replenishment subsystem 220 may also be configured to receive thermal energy from other heat sources, such as Figure 2 220. Examples of heat sources include, but are not limited to, geothermal heat, industrial waste heat, solar heat, combustion heat, vapor compression cycle waste heat (e.g., from a heat pump), chemical reaction heat, or other forms of heat that cannot be easily or efficiently converted to a usable form by conventional means (e.g., by driving a turbine).

[0086] A thermal optimization system can be used to optimize the use of thermal energy associated with the power generation system 100 described herein. The thermal optimization system is further described in U.S. Patent No. 11,067,317, which is incorporated by reference in its entirety. The thermal optimization system can transfer thermal energy from one or more heat sources to one or more radiators. Examples of radiators include the interior of a residential or office space during the cooler seasons of the year, heated swimming pools, saunas, and steam rooms. In these examples, the system can be configured to regulate the temperature by adjusting the transfer of thermal energy to the radiators. For example, the thermal optimization system can monitor the temperature of the heated space and / or heated water and use processor-based logic to regulate the heat transfer, such as (but not limited to) running one or more PID feedback loops and / or expert systems. During the hotter seasons, the interior space can be a heat source. In this case, the processor-based control system can regulate the transfer of thermal energy leaving these spaces, thereby regulating the temperature.

[0087] Figure 3An exemplary thermal optimization system 300 is shown. The exemplary optimization system 300 includes one or more heat sources 305, one or more heat sinks 310, and one or more RED cells 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 systems that generate electricity from salinity gradients (e.g., capacitive hybridization (CAP)) instead of (or in addition to) RED cells 110. The optimization system 300 may include one or more pumps 320 configured to transfer heat from one location to another. In some examples, the optimization system 300 uses vapor compression refrigeration to transfer heat from the heat source to the heat sink. That is, the optimization system 300 may compress the refrigerant to transfer thermal energy from the refrigerant to the heat sink (e.g., via a heat exchanger configured to absorb and distribute the thermal energy). The optimization system 300 can then transfer the compressed refrigerant (e.g., by pumping the compressed refrigerant through piping configured for this purpose) to a heat source and allow the refrigerant to expand, thereby absorbing 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 that redirects the transfer of thermal energy. The optimization system 300 can control the reversing valve to change the direction of heat transfer, for example, cooling the interior space during the day when the ambient temperature is relatively high, and then reversing the heat flow to heat the interior space at night as the ambient temperature drops.

[0088] The operation of the thermal optimization system 300 may be coordinated by a control system 350 having a processor, for example, as described below with respect to Figure 7 The processor executes instructions that cause the control system 350 to coordinate heat transfer between the heat source 305, the heat sink 310, and the RED battery 110 (or PRO or CAP system), for example, based on prevailing conditions. The control system 350 may also coordinate heat transfer within the power generation system 100, for example, between subsystems such as the salt removal subsystem 210 and the salt replenishment subsystem 220. The control system 350 may transmit or receive signals 352 (352a-c) to or from the heat source 305, the heat sink 310, and the RED battery 110 (or PRO or CAP system). The control system 350 may receive signals 352 indicating, for example, one or more conditions or states of the heat source 305, the heat sink 310, and the RED battery 110 (or PRO system). For example, the signals 352 may indicate measured quantities, such as temperature and / or pressure, or they may indicate user input, such as a target temperature for the heated interior space. Temperatures associated with power generation system 100 may include the temperatures of salt solution 130 and dilute solution 140 in the RED cells, in salt removal subsystem 210 , and / or in salt replenishment subsystem 220 , respectively.

[0089] Control system 350 can be used to transmit control signals, for example, to control the speed of compressors and / or pumps, the direction of reversing valves, the operating speed of salt delivery / transfer device 230, and the like, to achieve a specified target temperature and / or power output. For example, control system 350 can adjust the temperature of a salt solution to be at or near its solubility limit. In this way, control system 350 can effectively and simultaneously control and adjust heat transfer between multiple heat sources 305 and heat sinks 310 based on prevailing conditions and user settings, while optimizing the output of power generation system 100. Furthermore, control system 350 can influence the level of vapor compression cycle waste heat generated by one or more heat pumps 320 and transfer that waste heat to one or more heat sinks and / or RED batteries 110, thereby effectively recapture its own waste heat for power generation or other purposes. Thus, optimization system 300 can transfer heat from any or all of the multiple heat sources under various dynamic conditions (e.g., as conditions change throughout the year or throughout the day) and / or based on the needs of RED batteries 110. Furthermore, control system 350 can configure power generation system 100 to store excess energy. For example, when demand for electricity is low, the control system 350 can configure the RED cell 110 to generate a portion of its energy output as hydrogen to be used as fuel at a later time, rather than as electricity for use at the time of generation. Furthermore, in the case of a PRO system, the control system 350 can configure the rate at which pressure is converted into electricity, for example by controlling the rate of flow through a turbine. In this way, the control system 350 can retain some energy, for example, in the form of gravitational potential energy, when demand for electricity is low, and convert a larger amount of gravitational potential energy into electricity when demand for electricity is high.

[0090] In some embodiments, system 100 includes a PRO system instead of (or in addition to) the RED battery 110 described above. Figure 4An exemplary PRO system 120 is shown. The exemplary PRO system 120 also includes a saline solution 130 separated from a dilute solution 140 by a selectively permeable membrane 104c, similar to the RED battery 110 described above. However, rather than generating electricity directly from the salinity difference between the saline solution 130 and the dilute solution 140, the PRO system generates pressure, which can take the form of gravitational potential energy. Therefore, the PRO system may not include electrodes. The selectively permeable membrane of the PRO system can be configured to preferentially allow solvent to pass through the membrane rather than solute, for example, from the dilute solution 140 to the saline solution 130, in order to reduce the salinity difference between the solutions. In the PRO system, the selectively permeable membrane can be a hollow fiber membrane that allows water to permeate rather than salt. In the PRO system, the selectively permeable membrane can be a hollow fiber membrane, a spiral-wound membrane, a flat sheet membrane, or a combination thereof. As a result, as the solvent migrates across the membrane 104c into the saline solution 130, the pressure and / or average height of the solvent in the saline solution 130 can increase over time. The system 100 can convert the increased pressure and / or gravitational potential energy of the brine 130 into a more usable form of power, such as electrical energy, for example, by directing the (elevated) brine 130 through a turbine, paddle wheel, or other mechanical device suitable for generating electricity. The system 100 can then circulate the spent brine 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 in a partial vacuum to reduce the boiling point) and subsequent condensation. The solvent recovery system can then circulate the condensed solvent back to the PRO system 120 as a renewed or "top-up" weak solution 140, and circulate the renewed brine (after removing excess solvent) back to the PRO system 120 as a renewed brine 130. Alternatively (or additionally), the solvent recovery system may cause solutes to precipitate from the spent brine 130, then recycle the spent brine back to the PRO system 120 as "make-up" weak solution 140, and introduce the precipitated salts back into the brine 130, e.g., via Figure 2 As with the RED battery 110 described above, the rate at which the PRO system 120 generates electricity is a function of at least the salinity difference between the salt solution 130 and the dilute solution 140 and at least the temperature of the salt solution 130.

[0091] The RED system may be a capacitive (CAP) system instead of (or in addition to) the RED battery and / or PRO system. The CAP system is an electrode-based technology for generating electrical energy from a salinity gradient. Electricity generation using a CAP system is based on cycles of charging and discharging electrodes. The electrodes in a CAP system are sequentially exposed to two solutions with a large difference in salinity. The CAP system charges energy in the form of a salinity gradient and extracts energy from the salinity difference, and benefits from the voltage rise that occurs between two electrodes immersed in a saline solution when the salt concentration of their saline solution changes. Power generation depends primarily on the following properties of each electrode: the magnitude of the potential rise when the salinity changes, and the potential in the high-salinity solution. The electrodes can be identical so that the flow of the system can be reversed and the polarity can be changed without stopping electricity generation. The CAP system may include one or more selectively permeable membranes as disclosed herein.

[0092] like Figure 1 As shown, 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 can be separated by a single selectively permeable membrane 140 (e.g., a cation exchange membrane or an anion exchange membrane). The selective movement of ions across the single membrane 140 causes a potential difference (voltage) across the membrane 140. Figure 1 Similar to the embodiments of the present invention, the current generated by a single membrane embodiment of the RED cell 150 (e.g., a flow pump) is also a function of the rate of ion movement, and the rate of ion movement is a function of several factors, including the salinity gradient between the salt solution 130 and the dilute solution 140, (at least) the temperature of the salt solution 130 (due to the increase in kinetic energy of ions at higher temperatures), and aspects of the single membrane 104.

[0093] In some embodiments, the system includes a first RED cell 110a configured to use an exothermic salt solution 130a and a second RED cell 110b configured to use an endothermic salt solution 130b. The system can transfer heat generated by dissolving solutes in the exothermic salt solution 130a to the endothermic solution 130b to replace the heat absorbed while dissolving the solutes.

[0094] In some embodiments, a portion of the generated electricity is used to produce hydrogen, for example, by decomposing water through electrolysis. For example, when the dilute solution is water, a potential difference of 1.23 volts can be applied to the water to decompose the water into hydrogen and oxygen. The oxygen can be supplied and pumped to 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. The salt solution or the dilute solution (or both) can be decomposed by electrolysis. Figure 5An exemplary RED cell 110 configured to generate hydrogen is shown. The released hydrogen and oxygen can "bubble" to surfaces near the cathode 112 and anode 114 of the RED cell 110, respectively. The system 100 can separate the oxygen and hydrogen (e.g., by physically separating the cathode 112 and anode 114), capture and store the hydrogen (e.g., by capturing the hydrogen as it bubbles to surfaces near the cathode), and then transport the hydrogen to a suitable location by appropriate means for use as a fuel. In these cases, the system 100 must replenish or "top up" the solvent lost due to electrolysis, for example, from a stream or other freshwater source. The RED cell 110 can be configured to allow electrolysis to occur naturally. That is, the RED cell 110 can be configured to generate a sufficient potential difference to induce electrolysis of its solvent. In some examples, the regeneration system uses electrolysis to renew the salinity of the spent brine 130, while simultaneously replenishing fresh water as a renewed dilute solution 140 circulated to the RED cell 110 (or PRO or CAP system). In some examples, the system includes a separate reservoir of water (e.g., not a solvent for the electrolysis RED cell, PRO, or CAP system) for generating hydrogen via electrolysis. The separate water reservoir can have its own "make-up" source while the RED cell or PRO system maintains a closed loop. In these embodiments, the water reservoir can also serve as a heat reservoir or play another role in the thermal optimization process.

[0095] Figure 6 A flowchart 600 is shown of an exemplary method for generating electricity from thermal energy. At step 602, the exemplary method includes separating a first salt solution 130 from a second salt solution 140 via a selectively permeable membrane 104. The selectively permeable membrane 104 can be configured to provide controlled mixing of the first salt solution 130 and the second salt solution 140 so as to capture salinity gradient energy in a more useful form when mixing occurs. In some examples, the selectively permeable membrane 104 is configured to preferentially allow the solvent of the first salt solution 130 to pass through the membrane and into the separated second salt solution 140, for example, as in a PRO system. In some examples, the selectively permeable membrane 104 is configured to preferentially allow anions or cations of the first salt solution 130 to pass through the membrane and into the separated second salt solution 140, for example, as in a RED battery 110.

[0096] At step 604, the exemplary 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 may allow 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 RED battery 110 may have an efficiency of approximately 30% (i.e., 30% of the thermal energy transferred to the RED battery 110 is converted into electricity or other usable energy forms). The heat pump 320 may have a coefficient of performance (COP) between 3 and 4 (i.e., the heat pump 320 may require 1 KW of power to absorb 2-3 KW of power from the heat source and transfer 3-4 KW to the heat sink (the sum of the input power and the heat power absorbed 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 that are not easily converted to a useful form of energy, such as heat sources less than 300°C). In some predicted examples, the heat source may be the result of an industrial process that would otherwise simply export waste heat to the environment. At 30% efficiency, the RED battery 110 may generate 1.2 KW of electricity from the 4 KW of transferred heat. In this predicted example, 1 KW of electricity may be used to power the heat pump 320, leaving 200 kW of electricity is used for other purposes. Thus, in this predictive example, the combined system 100 of RED battery 110 and heat pump 320 produces a net output of 200 W of electricity, with no net power input other than 3 kW of "waste" heat. In cases where the waste heat is the result of an industrial process, the combined system 100 of RED battery 110 and heat pump 320 is expected to produce a net output of 200 W while providing the benefit of cooling the waste heat by 3 kW before exporting it to the environment. The expected net efficiency of the combined system 110 can be further amplified with improvements to the efficiency of the RED battery 110.

[0097] Furthermore, the power generation system 100 can improve the effective coefficient of performance (COP) of a heat pump 320 (e.g., a heat pump used to heat or cool a living space) by capturing some of the waste energy generated by one or more heat pumps and converting the waste energy into electricity to power the heat pump 320. 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 source to a heat sink. If the heat sink does not require the full 4.5 kW of power (3 kW of pumped heat plus 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 for conversion to power for the heat pump 320, thereby increasing the effective COP of the heat pump 320. Furthermore, the control system 350 can configure the power generation system 100 to convert some amounts of waste energy into a form that can be stored for later use, for example, if the instantaneous demand for electricity is greater than the amount of electricity that can be generated. For example, the PRO system can retain waste energy in the form of unreleased pressure and / or gravitational potential energy for release at a future time, e.g., when demand for electrical energy is greater. Similarly, instead of generating a certain amount of electrical energy, the RED cell 110 can generate hydrogen for use as fuel at a future time. Thus, waste heat from the heat pump 320 can be flexibly captured and released, further increasing the effective COP of the heat pump.

[0098] At step 606, the exemplary method includes mixing the first saline solution 130 with the second saline solution 140 in a controlled manner. At step 608, the exemplary method includes capturing at least some of the salinity gradient energy as electricity. As described above, the RED battery 110, the PRO system, or the CAP system can be configured to convert the salinity gradient energy into a more useful form as the solutions (130, 140) mix. At step 610, the exemplary method includes transferring heat energy from the first saline solution to the second saline solution via the heat pump 320. At step 612, the exemplary method includes causing the salinity difference between the first saline solution and the second saline solution to increase. As described above, the heat pump 320 can cool the spent dilute solution 140, causing salt to precipitate from the dilute solution 140, thereby renewing the dilute solution. The heat pump can transfer heat from the spent dilute solution 140 to the spent saline solution 130, enhancing the process of dissolving the salt introduced into the saline solution 130. Alternatively (or additionally), heat pump 320 may heat spent brine 140, causing brine 140 to evaporate, thereby refreshing the brine. The evaporated solvent may be condensed (e.g., by cooling the heat pump) as the solvent vapor circulates back to RED cell 110 as a refreshed dilute solution.

[0099] Figure 8An exemplary regeneration system including a salt precipitation system is shown. In this embodiment, the salt precipitation system includes a salt precipitator 880, three heat exchangers 882, 884, 838 (although alternative embodiments may include one heat exchanger, two heat exchangers, or more than three heat exchangers), and an external heat source system 890. The regeneration system circulates spent dilute solution 840 from the RED or PRO cell 810 through the salt precipitation system in a closed loop and returns it to the RED or PRO cell 810 as renewed dilute solution 845. Optionally, the renewed dilute solution 845 may be stored for a period of time in a tank 846 for holding dilute solution before being directed to the RED or PRO cell 810. The spent dilute solution 840 is directed to the salt precipitator 880, where a heat exchanger 884 is applied to remove heat energy from the spent dilute solution 840, causing salt 885 to precipitate, which further dilutes the solution within the salt precipitator 880, thereby providing a regenerated dilute solution 845 (which may also be referred to as renewed dilute solution). Salt 885 is directed to a concentrate tank 836 (or a tank configured to regenerate spent concentrate solution) that holds spent concentrate solution 830. Within concentrate tank 836, salt 885 is dissolved in spent concentrate solution 830, optionally with the application of thermal energy via a heat exchanger 838 or the like, to increase the salt concentration and produce regenerated concentrate solution 835 (which may also be referred to as renewed concentrate solution).

[0100] The salt precipitation system reduces the salinity of the spent dilute solution 840 through a salt precipitation process rather than (or in addition to) evaporation, and uses the precipitated salt 885 to increase the salinity of the spent concentrated solution 830 to regenerate the salinity difference between the concentrated solution 835 and the dilute solution 845 so that the feed stream can be directed to and used in the RED or PRO cell 810.

[0101] like Figure 8 As shown, the exemplary salt precipitation system removes salt from the spent dilute solution 840 by causing salt precipitation. Lowering the temperature of the solution below a temperature known as the saturation point (the temperature at which the solution is at its maximum salinity) generally causes solute precipitation. The system includes a heat transfer device (shown as a heat exchanger 884) Figure 8In the embodiment of the present invention, the heat transfer device is configured to cool the spent dilute solution 840 to a temperature below the saturation point. The heat exchanger 882 passively exchanges heat between the renewed dilute solution 845 and the spent dilute solution 840. The heat exchanger 882 heats the renewed dilute solution 845, for example, back to the temperature of the spent dilute solution 840 before it was cooled, since the temperature of the solution in the diluent tank 846 can optimally be between approximately 30°C and 50°C. In other words, the heat exchanger 882 can transfer some or all of the heat energy removed from the spent dilute solution 840 back to the renewed dilute solution 845. The spent dilute solution 840 can be between approximately 30°C and 40°C, and it will be reduced to approximately 5°C in the precipitator. In this way, the temperature of the renewed dilute solution 845 entering the cell 810 is substantially the same as the temperature of the spent dilute solution 840 leaving the cell 810. The heat exchanger 882 can perform both cooling and heating functions as described above, or it can be positioned in the system to heat the regenerated dilute solution 845 only after it leaves the precipitator 880. For example, it can heat the stream of regenerated weak solution 845 by blowing hot air over it. Control valves 811 and 812 are included in the system to control and balance the volume of the solution flow. Pumps 813 and 814 are included in the system to move the solution and allow it to flow to the RED or PRO cell 810. Pumps 813 and 814 can be any such pump or device known in the art for performing work on a fluid to move the fluid, 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 between systems of the present disclosure. Those skilled in the art will appreciate that one or more control valves may be added to any embodiment of the present disclosure. Heat exchangers 884 and 838 transfer heat from the precipitator 880 or tank 836, respectively, to the heat source system 890. Heat exchangers 884 and 838 can be coils immersed in the precipitator 880 or tank 836, respectively, with liquid circulated within the coils, or each heat exchanger can be any other heat exchanger known in the art, such as a shell-and-tube heat exchanger.

[0102] Figure 8 An external heat source system 890 is included, comprising a heat exchanger 891 and a heat pump 892 for adding heat to the system. The external heat source system 890 can be used with the salt precipitation system to supply thermal energy to the concentrate tank 836. The external heat source system 890 is depicted as having a closed circuit, optionally a refrigerant circuit, to generate warming and cooling for the precipitator 880 and the concentrate tank 836. The heat exchanger 891 optionally draws low-grade heat (e.g., below about 100°C, or below about 200°C) or ultra-low-grade heat (e.g., ambient air, or below about 80°C) directly from the air or an external heat source. The external heat source supplies external heat to the heat source system 890, which can ultimately be converted into electricity by the RED / PRO battery 810.

[0103] In other embodiments, heat source system 890 is not present, and instead high grade heat (eg, steam, or from any source that supplies heat above about 100° C.) may be added directly to the concentrate tank to heat it. Figure 9 Shown includes a salt precipitation system without Figure 8 An example of a regenerative system for an external heat source system is included in . Figure 9 As the salt precipitator 880 is cooled, thermal energy is released, optionally as heat 991. Heat 991 may be removed by any means known in the art and directed to any source, released, or directed back into the system via heat 992. Heat 992, 993 may be introduced into the system from ambient energy, a heat pump, or elsewhere in the system (e.g., heat 991).

[0104] Figure 10An exemplary regeneration system including a salt decomposition system is shown. In this embodiment, the salt decomposition system includes an absorber 1080, a salt decomposition vessel 1081, two heat exchangers 1082, 1084 (although alternative embodiments may include one heat exchanger, or more than two heat exchangers), and a heat pump 1013. Heat is added to the salt decomposition. The regeneration system circulates spent dilute solution 1040 from the RED or PRO cell 1010 in a closed loop through the salt decomposition vessel 1081, where heat is applied to warm the tank and form vapor and gaseous salt 1085, and returns to the RED or PRO cell 1010 as renewed dilute solution 1045. Heat exchanger 1084 is positioned between the spent dilute solution stream 1040 and the renewed dilute solution stream 1045 to transfer thermal energy. Spent concentrated solution 1030 circulates from the RED or PRO cell 1010 in a closed loop through the absorber 1080 and then returns to the RED or PRO cell 1010 as renewed concentrated solution 1035. Heat exchanger 1082 is positioned to transfer thermal energy between the spent concentrated solution stream 1030 and the renewed concentrated solution stream 1035. When heat is applied to vessel 1081, gaseous products (e.g., decomposed gaseous salts) 1085 are released from the vessel and received by adsorber 1080, where the salts decomposed within the vapor are absorbed into the spent concentrated solution 1035 to increase the salt concentration of the solution. In the salt decomposition vessel 1081, salts are removed from the spent dilute solution 1040, leaving a regenerated dilute stream 1045. Heat 992, 993 can be introduced into the system from ambient energy, a heat pump, or elsewhere in the system. Heat 991 can be removed and used in the heat pump or released. The heat pump 1013 can heat and cool simultaneously (see heat input 991 and heat output 993) and use heat / thermal energy 994 from another source. A control valve 1011 is included in the system to control and balance the volume of the solution flow. Salt decomposition vessel 1081 separates gas from liquid; it may include a flash tank or stripping tower to decompose the salt in solution. Salt decomposition vessel 1081 and absorber 1080 may be any vessel or group of vessels, tanks, and / or towers that perform the functions described herein and are readily understood by those skilled in the art.

[0105] Figure 11 is another example of a regeneration system that includes an evaporation system. Figure 11 and Figure 10The embodiment of differs in that it incorporates an evaporator 1091 and a condenser 1090. In this example, spent concentrated solution 1030 is circulated from the RED or PRO cell 1010 through the evaporator 1091 in a closed loop, where heat 992, 993 is applied, causing the solution to boil and water to evaporate as vapor 1093, leaving a more concentrated solution—a newer concentrated solution—that is, directed back to the RED or PRO cell 1010. Vapor 1093 is directed from evaporator 1091 to condenser 1090, where the container is cooled and the vapor is condensed into water and mixed with spent dilute solution 1040 to produce a newer dilute solution 1045, which is directed back to the RED or PRO cell 1010 in a closed loop. A heat pump 1013 provides a refrigeration effect on the condenser 1090. Traditionally, a chilled water circuit would be used in place of this heat pump, and any heat input into the system would be lost. By incorporating a heat pump as shown, the heat is recovered, making the process more energy-efficient. The evaporator 1091 and the condenser 1090 can be any tank, vessel, tower, or combination thereof that performs the functions described herein and is readily understood by those skilled in the art.

[0106] Figure 12 is an example of a regeneration system that includes electrodialysis and salt precipitation. The dilute solution 1141 and the concentrated solution 1131 are directed to the electrodialysis system 1190, where electricity is supplied to force the salt to separate from the dilute solution 1141 by moving ions from the dilute to the concentrate, which produces an ultra-dilute solution 1147. By using electrodialysis, the electricity is used to generate a more concentrated solution 1132 and an ultra-dilute solution 1147. The spent dilute solution 1140 is desalted in the precipitator 1180, leaving the dilute solution 1141 (having a lower salt concentration than the spent dilute solution 1140). By using electrodialysis, the dilute solution 1141 is further diluted outside the solubility curve to produce an ultra-dilute solution 1147 and create a larger salt gradient. As shown in FIG. Figure 12 As depicted in FIG, the ultra-dilute solution flows from the electrodialysis tank to the diluent storage tank 1146 where it is mixed with the dilute solution 1141 (if any) from the precipitator 1180. In an alternative embodiment, the system can be configured to flow the ultra-dilute solution 1147 directly into the cell 1110, with valves employed and positioned to alter the flow of the solution, as will be readily apparent to one skilled in the art.

[0107] The spent dilute solution 1140 is directed to a cooled (heat removed 1191) salt precipitator 1180. The removal of heat from the spent dilute solution 1140 causes salt 1185 to precipitate, which further dilutes the solution within the salt precipitator 1180, thereby providing a dilute solution 1147, which can be sent to a storage tank 1146 and / or further diluted by electrodialysis. The regenerated dilute solution 1145 (which may also be referred to as a renewed dilute solution) has the same salt concentration as the super-dilute solution 1147, or has a lower salt concentration than the dilute solution 1141 after mixing with the super-dilute solution 1147 in tank 1146. The regenerated dilute solution 1145 flows back to the cell 1110 to utilize the salinity gradient. The salt 1185 is combined with the spent concentrated solution 1130 to increase its salinity and then directed to the concentrate storage tank 1136. In the concentrate storage tank 1136, salt 1185 is dissolved in the spent concentrate solution 1130, optionally by applying any heat 1192 or other thermal energy source, to increase the salt concentration and produce a concentrated solution 1131, which is then directed to the electrodialysis system 1190 to increase its salt concentration and produce a more concentrated solution 1132. The more concentrated solution 1132 may have a higher or equal salt concentration than the regenerated concentrate solution 1135 (which may also be referred to as a renewed concentrate solution). The concentrate tank 1136 may have excess salt (not shown) at its bottom, which, if present, can be used as a battery to allow the process to continue operating when excess heat is temporarily unavailable.

[0108] The salt precipitation system reduces the salinity of the spent dilute solution 1140 by a salt precipitation process, rather than (or in addition to) evaporation, and uses the precipitated salt 1185 to increase the salinity of the spent concentrated solution 1130 to increase the salinity difference between the concentrated solution 1131 and the dilute solution 1141. Figure 2 Concentrated solution 1131 and diluted solution 1141 are further processed by electrodialysis to increase the salinity difference between the solutions before being reintroduced into cell 1110. The electricity 1171 generated by RED or PRO cell 1110 can be used, stored, and / or used as electricity 1170 for operating electrodialysis system 1190 by any means known in the art.

[0109] And in Figure 12 In some embodiments, the electrodialysis system 1190 uses a different tank than the RED or PRO cell 1110, while in other embodiments, electrodialysis can use the same tank as that used for reverse electrodialysis.

[0110] Figure 13 is incorporated into the membrane distillation system 1390 (the electrodialysis system 1190 is Figure 12The membrane distillation system 1390 includes a heat pump (not shown) that evaporates water that is able to permeate through a porous hydrophobic membrane (not shown) into the dilute solution to reduce its salt concentration, thereby producing an ultra-dilute solution 1347. The ultra-dilute solution 1347 can be directly fed into the cell 1110 or into a dilute tank as shown, where valves are employed and positioned to change the flow of the stream. Figure 13 As depicted in FIG, the ultra-dilute solution 1137 flows from the membrane distillation system 1390 to the diluate storage tank 1146 where it is mixed with the dilute solution 1141 (if any) from the precipitator 1180. In an alternative embodiment, the system can be configured to flow the ultra-dilute solution 1347 directly into the cell 1110, with valves being employed and positioned to alter the flow of the solution, as will be readily apparent to one skilled in the art.

[0111] Within the concentrate storage tank 1136, salt 1185 is dissolved in the spent concentrate solution 1130, optionally by applying any heat 1192 or other thermal energy source, to increase the salt concentration and produce a concentrated solution 1131, which is then directed to the membrane distillation system 1390 to increase its salt concentration and produce a more concentrated solution 1332. The more concentrated solution 1332 can have a higher or equal salt concentration than the regenerated concentrated solution 1135 (which can also be referred to as a renewed concentrated solution).

[0112] Figure 14 is an example of a regeneration system that incorporates a membrane distillation system 1390 without salt precipitation. The spent dilute solution 1140 is directed to a dilute storage tank 1146 where it is cooled (i.e., heat energy 1191 is removed) and directed to the membrane distillation system 1390. The spent concentrated solution 1130 is directed to a concentrate storage tank 1136 where it is warmed (i.e., heat energy 1192 is put into the system) and directed to the membrane distillation system 1390. Heating and cooling of the tanks may optionally be supplied by one or more heat pumps (not shown). The spent dilute stream 1140 and the spent concentrate stream 1130 are directed to the membrane distillation system 1390, which operates using a vapor pressure differential generated by the temperature difference between the streams across a hydrophobic membrane (not shown). Regenerated dilute solution 1445 and regenerated concentrated solution 1435 are produced by operation of the membrane distillation system 1390, which are then circulated back to the RED or PRO cell 1110, optionally returning through the storage tanks 1146, 1136, as shown. Figure 14 shown.

[0113] Figure 15This is an example of a RED / PRO tank combined with a regeneration system, where multiple tanks are operated 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 dissolution tank), or can be inoperative at any given time. For example, tank 1501 can initially operate as a cooling tank to precipitate salts from the spent dilute solution, leaving a regenerated dilute solution 1545, which can then be directed to a diluent tank 1546. Tank 1501 can then be operated as a heating tank, where the spent concentrated solution 1530 is directed to a tank containing the precipitated salts, which is then warmed to dissolve the salts into solution, leaving a regenerated concentrated solution 1535, which can then be directed to a concentrate tank 1536. At other times, tank 1501 can be inoperative when it requires maintenance or for any other reason. This change in operation can be applied to any operating tank in the batch system as needed.

[0114] A valve system 1520 separates the tanks from the RED / PRO cell 1510 and operates by directing the flow of all or a portion of each of the spent dilute solution 1540 and the spent concentrate solution 1530 to one or more operating tanks 1501, 1502, 1503, 1504. A second valve system 1522 separates the operating tanks 1501, 1502, 1503, 1504 from the storage tanks and operates by directing the flow of regenerated dilute solution 1545 and regenerated concentrate solution 1535 to respective storage tanks 1546, 1536: the diluent tank 1546 receives and holds the regenerated dilute solution 1545, and the concentrate tank 1536 receives and holds the regenerated concentrate solution 1535. Although the Figure 15 Four operating tanks are shown, but fewer or more than four tanks can be connected to the system and operated in a batch system.

[0115] Figure 16 A method for producing water is shown using 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 the 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, the system can be used in a home or other building to dehumidify the ambient air (or extract water from the ambient air) and optionally generate heat that can be converted into electricity by the RED / PRO battery.

[0116] Figure 17 Merge Figure 16as a method of producing a hot water tank 1620 and a cold water tank 1625, which can be used to warm and cool the solution used in the salinity gradient heat engine system 1610 disclosed herein; optionally including a regeneration system (e.g. Figure 8-15 ). The evaporator 1603 cools the water to create a cold tank 1625, while the condenser 1601 heats the water to create a hot tank 1620. A heat transfer medium 1613 (e.g., water, glycol, oil, refrigerant) flows from the cold tank 1625 through the salinity gradient heat engine system 1610 in a closed loop, and a heat transfer medium 1614 flows from the hot tank 1620 through the salinity gradient heat engine system 1610 in a closed loop. Although optional, a radiator 1640 is shown and used to absorb or dissipate excess heat from the system. If the system generates more heat than required, it can be transferred to the radiator, which can, for example, release heat 1631 to the atmosphere. Although optional, other heat sources 1630 are shown that provide thermal energy or heat 1631 to the system. Each other heat source can be any means known in the art, such as, but not limited to, a heat exchanger, industrial steam, radiator, ambient, coil, convection.

[0117] The salinity gradient system disclosed herein can generate water (also referred to herein as atmospheric water generation). Atmospheric water generation is the process of extracting water from the air using a variety of techniques, such as condensation, adsorption, and cooling. The concept of atmospheric water generation is based on the fact that even in arid and desert regions, the atmospheric environment contains significant amounts of water vapor. When combined with RED / PRO batteries, heat pumps, and closed-loop processes, both energy and water can be generated.

[0118] One means of atmospheric water generation is through condensation, which involves cooling the air to a temperature below the dew point / condensation temperature, causing water vapor in the air to condense into liquid water. This process is commonly used in thermodynamic cycles, such as dehumidifiers and heat pumps. The process depends on the temperature and humidity of the air, which is readily understood by those skilled in the art, for example, by consulting a psychometric chart.

[0119] Another means of atmospheric water generation is through adsorption, which uses a desiccant material such as silica gel or zeolite to absorb water from the air. Once the desiccant material has absorbed water and optionally becomes saturated, it can be heated to release water, which can be collected and used. In a similar manner, a liquid desiccant is a substance with a high affinity for water molecules and can be used to remove water from the air, which can be referred to as a liquid desiccant dehumidification process. The method may comprise passing air (optionally having a medium to high humidity content, for example, greater than about 30%; not dry or arid) over a surface coated with a liquid desiccant, which absorbs water from the air. When the absorbed water is removed from the liquid desiccant, it can be regenerated to its original state. Any solid desiccant material or liquid desiccant known in the art can be used herein.

[0120] Liquid desiccant dehumidification processes can be closed-loop systems that include two separate air handling units: one for dehumidification (also known as a conditioner) and one for regeneration (also known as a generator). The dehumidification unit typically consists of an absorber, where the liquid desiccant is sprayed or coated onto a surface, and a fan or blower circulates the moist air over the surface. As the air passes over the surface, the liquid desiccant absorbs moisture from the air, leaving dry air that is discharged into the conditioned space.

[0121] The regeneration system can include a separate container containing the used liquid desiccant, and a heat source (in this case, a heat pump). In this embodiment, the absorbed moisture is removed from the liquid desiccant through a process called regeneration, in which heat is applied to the liquid desiccant to drive off the absorbed water. The removed water vapor can be condensed via a heat pump to produce water (which can be potable water in the case of added filtration (e.g., reverse osmosis or any other system known in the art)), or it can be added to a dilute solution to increase the salinity gradient of the RED / PRO cell. This produced water can additionally or alternatively be used to generate hydrogen, as it can be used as feed for electrolysis and / or RED.

[0122] Typically, atmospheric water generation is highly energy intensive because it fights the latent heat of water vaporization to generate water. In combination with a closed loop salinity gradient engine system such as RED, this latent heat can be harnessed and converted into hydrogen and / or electricity rather than being released to the atmosphere. Some advantages of the liquid desiccant dehumidification process include its ability to extract water from low humidity levels in hot and humid climates, and its ability to be regenerated using waste heat, solar energy and / or heat pumps. Using a heat pump alone to condense water vapor from dry, hot climates would be far more difficult than using a desiccant in combination with a heat pump. The combination of a liquid desiccant desiccant dehumidification process with a heat pump and a salinity gradient engine system allows for the generation of large amounts of water and energy even in very dry and hot climates.

[0123] Figure 18 is an example of atmospheric water generation using a liquid desiccant dehumidification process. In this example, water is drawn from the air and absorbed by the desiccant within the conditioner 1812. The used (weakened) desiccant 1820 is transferred to the generator 1811, where the desiccant is regenerated by evaporation of the water, thereby regenerating the strong desiccant 1821, which is redirected back to the conditioner 1812. A heat pump 1882 is employed to transfer heat between the weakened desiccant stream 1820 and the regenerated strong desiccant stream 1821. The vapor 1884 removed from the generator 1811 can be directed to a vapor condenser 1815 where it is cooled and liquefied to produce water 1885 for any purpose, including potable water (using a filtration system).

[0124] Figure 18 Utilizing a method of generating a hot water tank 1813 and a cold water tank 1814, the hot water tank 1813 and the cold water tank 1814 can be used to warm and cool a solution (e.g., water) used in a salinity gradient heat engine system 1810 disclosed herein; optionally including a regeneration system (e.g., Figure 8-15 ). Heat exchanger 1884 cools the water to create cold tank 1814, while heat exchanger 1883 heats the water to create hot tank 1813. A heat transfer medium 1816 (e.g., water, glycol, oil, refrigerant) flows from heat exchangers 1883, 1884 into heat pump 1802 in a closed loop. One or more radiators and other heat sources may be incorporated to provide heating or cooling to one or both of hot tank 1813 or cold tank 1814. Hot tank 1813 provides hot water as heat supply stream 1830 to generator 1811 and salt gradient heat engine system 1810 in a closed loop, which returns to hot tank 1813 as stream 1831 at a lower temperature. Cold tank 1814 provides cold water as cold supply stream 1832 to steam condenser 1815, regulator 1812, and salt gradient heat engine system 1810 in a closed loop, which returns to cold tank 1814 as stream 1833 at a higher temperature.

[0125] The salinity gradient system disclosed herein may include forward osmosis (FO) as a means of regenerating spent dilute and concentrated solutions from RED / PRO cells, i.e., a regeneration system. In a forward osmosis system, a feed solution (e.g., spent concentrated solution) may be placed on one side of a semipermeable membrane, and a draw solution may be placed on the other side of the semipermeable membrane. The draw solution may be any solution with a higher osmotic pressure than the feed solution; it may include different salts, synthetic salts, or it may be substantially the same as the feed solution, but at a higher concentration. A salt gradient is generated to draw water from the spent concentrated solution, which is then regenerated by a switchable solubility system incorporating the draw solution. The osmotic pressure gradient created by the draw solution draws water molecules from the feed solution through the membrane, while leaving salts and other contaminants on the side with the feed solution. This allows for the regeneration of the concentrated solution, which can be directed back into the RED / PRO cell. The membrane used in the FO system may be any membrane that can be used for PRO cells. However, for FO, the membrane does not have to be designed to withstand as high pressures as the PRO membrane.

[0126] Forward osmosis systems can be incorporated into switchable solubility systems, which utilize a draw solution and a reversible reaction between carbon dioxide (CO2) and water to produce solutions with switchable solubility properties. These switchable solubility solutions can switch between hydrophobic and hydrophilic forms. Methods for preparing switchable solubility solutions may include dissolving an amine in water and producing a solution with a specific pH level. For example, the amine may be 1-cyclohexylpiperidine, N-methyldipropylamine, 4-(diethylamino)ethyl butyrate, N,N-dimethylphenethylamine, N,N-diethylbutylamine, etc. As mentioned herein, a draw solution is a solution with a high osmotic pressure 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, which causes the pH of the solution to drop. This change in pH causes the solubility of the amine to change, making it either readily soluble or poorly soluble in water. Therefore, the solubility of the amine can be changed by the presence of CO2 and reversed by applying heat. Specifically, at low pH (e.g., below about 7, or between 7 and 1), amines become readily soluble in solution. Conversely, when the pH is higher (e.g., above 7), amines become poorly soluble in water and are difficult to dissolve in solution. The solubility of CO2 is temperature-dependent. As temperature increases, the solubility of CO2 decreases, allowing water and CO2 to separate from the amine solution. Once the water is removed, the solution is cooled to increase the solubility of CO2, allowing the amine to completely dissolve and increase solubility.

[0127] Using a switchable solubility system as described above allows for the regeneration of the draw solution while also utilizing the power of a heat pump, which can provide both heating and cooling. As the solubility of the draw solution decreases, the water can be separated and sent out to be mixed with the spent dilute solution from the RED / PRO cell. By removing the water, this concentrates and regenerates the draw solution used in the forward osmosis system, thereby regenerating the spent concentrated solution from the RED / PRO cell.

[0128] When a salinity gradient engine system is operated near an industrial facility, such as a power plant, it can provide multiple benefits. Power plants generate excess waste heat and release CO2 as an undesirable byproduct in the process. This industrial CO2 can be used in the forward osmosis system disclosed herein to control the solubility of the draw solution.

[0129] exist Figure 19 In FIG. 1 , the regeneration system circulates spent dilute solution 1940 from RED or PRO cell 1910 in a closed loop through diluent tank 1946, where water 1970 is added to produce regenerated dilute solution 1945, which is directed back to RED or PRO cell 1910. Spent concentrate solution 1930 is also circulated from RED or PRO cell 1910 in a closed loop through forward osmosis system 1980 to produce regenerated concentrate solution 1935, which is directed back to RED or PRO cell 1910. A switchable solubility system 1920 is also depicted. Draw solution 1955 is circulated through forward osmosis system 1980 and used to operate the system, producing spent draw solution 1950. In recovery unit 1960, heat 1992 is added to spent draw solution 1950 and CO2 is released, causing the solubility of the spent draw solution to decrease, and water 1970 can be separated and decanted. When CO2 is added back into the system in the generator 1965, the solubility of the solutes in the draw solution increases (e.g., salts can be redissolved in the solution; it should be concentrated to make the osmotic pressure high). To increase the solubility of CO2, the solution is cooled (heat energy is removed from the system 1991).

[0130] It should be understood that the various aspects disclosed herein may be combined in combinations different from those specifically presented in the description and drawings. It should also be understood that, depending on the example, some actions or events of any process or method described herein may be implemented in a different order, may be added, combined, or omitted altogether (e.g., all described actions or events may not be necessary to implement the technology). In addition, although for clarity, some aspects of the present disclosure are described as being implemented by a single module or unit, it should be understood that the technology of the present disclosure may be implemented by a combination of units or modules associated with, for example, a RED cell, a PRO system, a CAP system, a hydrogen generation subsystem, a salt precipitation subsystem, an evaporation subsystem, etc.

[0131] In one or more instances, the described techniques can be implemented in hardware, software, firmware, or any combination thereof. If implemented in software, the functions can be stored as one or more instructions or codes on a computer-readable medium and executed by a hardware-based processing unit. The computer-readable medium may include a non-transitory computer-readable medium, which corresponds to a tangible medium such as a data storage medium (e.g., RAM, ROM, EEPROM, flash memory, or any other medium that can be used to store desired program code in the form of instructions or data structures and can be accessed by a computer).

[0132] Instructions may be executed 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 circuits. Thus, as used herein, the term "processor" may refer to any of the aforementioned structures or any other physical structure suitable for implementing the described techniques. Furthermore, these techniques may be fully implemented in one or more circuits or logic elements.

[0133] Figure 7 Exemplary hardware that can be used to contain or implement program instructions is shown. Bus 710 serves as the main information path (highway) interconnecting the other illustrated components of the hardware. Central processing unit (CPU) 705 is the central processing unit of the system and performs the calculations and logical operations required to execute the program. Figure 7 The CPU 705, in combination with one or more of the other elements disclosed herein, is an example of a processor (as such terms are used in this disclosure). Read-only memory (ROM) and random access memory (RAM) constitute examples of non-transitory computer-readable storage media 720, storage devices, or data storage (as such terms are used in this disclosure).

[0134] Program instructions, software, or interactive modules for providing an interface and implementing any queries or analyses associated with one or more data sets may be stored in the storage device 720. Optionally, the program instructions may be stored on a tangible, non-transitory computer-readable medium, such as an optical disc, a digital disc, a flash memory, a memory card, a universal serial bus (USB) drive, an optical storage medium, and / or other recording medium.

[0135] An optional display interface 730 may allow information from bus 710 to be displayed in audio, video, graphic, or alphanumeric format on a display 735. Communication with external devices may occur using various communication ports 740. Communication port 740 may connect to a communication network, such as the Internet or an intranet.

[0136] The hardware may further include an interface 745 that allows data to be received from input devices such as a keyboard 750 or other input devices 755 such as a touch screen, remote control, pointing device, video input device and / or audio input device.

[0137] It will be appreciated that various of the above-disclosed and other features and functions, or alternatives thereof, may be desirably combined into many other different systems or applications or combinations of systems and applications. Moreover, numerous presently unforeseen or unanticipated substitutions, modifications, variations, or improvements therein may be subsequently made by those skilled in the art, which are also intended to be encompassed by the following claims.

Claims

1. A method for generating electricity from thermal energy, comprising: separating the first salt solution from the second salt solution by a selectively permeable membrane; transferring heat energy to the first salt solution and / or the second salt solution via a heat pump; as well as The first saline solution and the second saline solution are mixed in a controlled manner to capture at least some salinity gradient energy as electricity as the salinity difference between the first saline solution and the second saline solution decreases.

2. The method of claim 1 , further comprising 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 decomposition, electrodialysis, membrane distillation, evaporation, forward osmosis, salt precipitation, or any combination thereof.

3. The method of any of the preceding claims, further comprising applying a pressure-retarded osmosis (PRO) system, a capacitive mixing (CAP) system, or both a PRO and CAP system to generate additional electricity.

4. The method of any one of the preceding claims, wherein the method comprises generating a third saline solution by membrane distillation, and further comprising mixing the third saline solution into the first saline solution and / or the second saline solution.

5. The method of any preceding claim, wherein the selectively permeable membrane comprises graphene, graphene oxide, or reduced graphene oxide.

6. The method of claim 5, further comprising applying a stretching force to the selectively permeable membrane to adjust the permeability of the membrane.

7. The method of claim 5, wherein the selectively permeable membrane is a single-layer sheet, a multi-layer sheet, or a cylinder containing graphene, graphene oxide, and / or reduced graphene oxide.

8. The method of claim 5 or 7, wherein the selectively permeable membrane contains nanopores in the graphene, graphene oxide, and / or reduced graphene oxide.

9. The method of claim 1, further comprising applying acoustic vibrations to the first saline solution or the second saline solution to cause the first saline solution or the second saline solution to become more homogenous.

10. The method of claim 9, wherein the acoustic vibration is applied adjacent to the selectively permeable membrane.

11. The method of claim 1 , further comprising coordinating, by a processor, heat transfer from the one or more heat sources to the first saline solution and / or the second saline solution based on one or more measurements of the one or more heat sources or the state of the first saline solution and / or the second saline solution.

12. The method of claim 11, wherein the heat source comprises one or more of geothermal heat, industrial waste heat, or solar heat.

13. The method of claim 1 further comprising capturing the salinity gradient energy using reverse electrodialysis.

14. The method of claim 1 further comprising capturing the salinity gradient energy using pressure-retarded osmosis to drive a generator.

15. The method of any preceding claim, wherein the first saline solution and the second saline solution are each circulated in a closed system.

16. The method of any one of the preceding claims, wherein transferring thermal energy from the first saline solution to the second saline solution causes the first saline solution to precipitate salt.

17. The method of any preceding claim, further comprising introducing the precipitated salt into the second saline solution, causing the salinity difference between the first saline solution and the second saline solution to increase.

18. The method of any preceding claim, further comprising using a portion of the generated electricity to produce hydrogen by electrolysis.

19. The method of any of the preceding claims, wherein transferring thermal energy from the first saline solution to the second saline solution comprises transferring thermal energy from the first saline solution that is cooler than the second saline solution.

20. The method of any of the preceding claims, wherein a thermal optimization system is configured to coordinate heat transfer from the one or more heat sources to the first and / or second saline solutions based on one or more measurements of the one or more heat sources or a state of the first and / or second saline solutions.

21. The method of any preceding claim, wherein the heat source comprises one or more of geothermal heat, industrial waste heat, or solar heat.

22. The method of claim 2, wherein the regeneration process comprises applying a salt decomposition process, the salt decomposition process comprising: i) providing a spent dilute solution formed from the first salt solution, wherein the spent dilute solution contains salt; ii) heating the spent dilute solution to decompose the salts to produce gaseous products; and iii) transferring the gaseous products to an absorber; and iv) solidifying the gaseous product to reform into a salt precipitate in the spent concentrated solution within the absorber; The salt precipitate is dissolved in the spent concentrated solution, thereby regenerating the second salt solution.

23. The method of claim 22, wherein the spent dilute solution has a higher salt content than the first salt solution; and The salt content of the spent dilute solution is reduced by transferring the gaseous product to the absorber to regenerate the first salt solution.

24. The method of claim 2, wherein the regeneration process comprises applying an electrodialysis process, the electrodialysis process comprising: i) providing a spent dilute solution formed from the first salt solution, wherein the spent dilute solution contains salt; ii) providing a spent concentrated solution formed from the second salt solution; iii) supplying electricity to separate the salt into ions and to move the ions from the spent dilute solution to the spent concentrated solution; The salt content of the spent dilute solution is reduced to regenerate the first salt solution, while the salt content of the spent concentrated solution is increased to regenerate the second salt solution.

25. The method of claim 2, wherein the regeneration process comprises applying an evaporation process, the evaporation process comprising: i) providing a spent concentrated solution formed from the second salt solution; ii) heating the waste concentrated solution to generate water vapor; as well as iii) diverting the water vapor to mix with the spent dilute solution; and The salt content of the spent dilute solution is reduced to regenerate the first salt solution, while the salt content of the spent concentrated solution is increased to regenerate the second salt solution.

26. The method of claim 2, wherein the regeneration process comprises applying a membrane distillation process, the membrane distillation process comprising: i) providing a membrane distillation vessel comprising a hydrophobic membrane having a spent concentrated solution on one side of the membrane and a spent dilute solution on an opposite side of the membrane; as well as ii) warming the waste concentrated solution to generate water vapor; The water vapor permeates the hydrophobic membrane to mix with the waste dilute solution to regenerate the first salt solution, while the salt content of the waste concentrated solution is increased to regenerate the second salt solution.

27. The method of claim 2, wherein the regeneration process comprises applying a forward osmosis process comprising: circulating the spent concentrate solution and the draw solution through a forward osmosis system to regenerate the second brine and produce a spent draw solution; The spent draw solution is circulated through a switchable solubility system to regenerate the draw solution and produce water.

28. The method of claim 27, wherein the forward osmosis process further comprises: The water is mixed with the spent dilute solution to regenerate the first brine solution.

29. A salt gradient heat engine system for generating electrical power, comprising: a first saline solution; a second saline solution, wherein the salinity of the second saline solution is different from the salinity of the first saline solution; a heat pump configured to transfer thermal energy to the first salt solution and / or the second salt solution; as well as a selectively permeable membrane that separates the first saline solution from the second saline solution, the selectively permeable membrane being configured to control mixing of the first saline solution and the second saline solution, the selectively permeable membrane being further configured to capture at least some salinity gradient energy as electricity when the first saline solution and the second saline solution mix.

30. The system of claim 29, further comprising a regeneration system selected from the group consisting of a salt precipitation system, a membrane distillation system, an evaporation system, a forward osmosis system, a salt decomposition system, an electrodialysis system, and any combination thereof.

31. The system of claim 29 or 30, wherein the heat pump is a vapor compression cycle, a thermoelectric refrigerator, a chemical absorption refrigerator, or other device for simultaneous heating and cooling.

32. The system of any one of claims 29-31, wherein the selectively permeable membrane comprises graphene, graphene oxide, or reduced graphene oxide.

33. The system of claim 32, wherein the selectively permeable membrane is stretchable.

34. The system of claim 30, wherein the regeneration system comprises the salt precipitation system, the salt precipitation system being configured to: Receive waste dilute solution; removing heat energy from the spent dilute solution by the heat pump, causing salt to precipitate from the spent dilute solution and regenerating a dilute salt solution; After regenerating the dilute salt solution, circulating the dilute salt solution; introducing the precipitated salt into the spent concentrated solution; as well as The salt causing the precipitation is dissolved in the spent concentrated solution and a concentrated salt solution is regenerated.

35. The system of claim 34, wherein the regeneration system is further configured to transfer at least some of the heat energy removed from the dilute saline solution back to the dilute saline solution after causing precipitation of dissolved salt in the dilute saline solution.

36. The system of claim 34, wherein the regeneration system is further configured to transfer at least some of the heat energy removed from the spent dilute solution to the spent concentrate solution, causing the precipitated salt to dissolve in the spent concentrate solution.

37. The system of claim 34, wherein the heat source is configured to transfer thermal energy to the spent concentrate solution, causing the precipitated salt to dissolve in the spent concentrate solution.

38. The system of claim 30, wherein the concentrated salt solution includes a substance having a non-linear temperature-dependent solubility.

39. The system of claim 30, wherein the regeneration system comprises the salt precipitation system, and The salt precipitation system comprises: a salt precipitator configured to receive the spent dilute solution and precipitate salt from the spent dilute solution to regenerate the dilute salt solution; a concentrate tank configured to receive the spent concentrated solution and to receive salt generated by the salt precipitator to regenerate a concentrated salt solution; as well as One or more heat exchangers.

40. The system of claim 32, further comprising a dilution tank configured to receive the dilute salt solution from the salt precipitator.

41. The system of claim 32, further comprising a heat source system configured to supply thermal energy to the concentrate tank.

42. The system of claim 30, wherein the regeneration system comprises the membrane distillation system, and Wherein the membrane distillation system comprises: a membrane distillation vessel comprising a hydrophobic membrane; as well as a heat pump configured to warm the container and cool one side of the hydrophobic membrane, The membrane distillation system is configured to generate a salt gradient at the hydrophobic membrane after warming and produce an ultra-dilute solution and a regenerated concentrated salt solution.

43. The system of claim 42, further comprising: a concentrate tank configured to receive the spent concentrate solution and connected to the membrane distillation vessel; a diluate tank configured to receive the spent dilute solution and connected to the membrane distillation vessel; wherein the concentrate tank is configured to receive the concentrated solution after regeneration in the membrane distillation vessel, and the diluent tank is configured to receive the ultra-dilute solution from the membrane distillation vessel.

44. The system of claim 42, wherein the hydrophobic membrane comprises polytetrafluoroethylene, polypropylene, or polyvinylidene fluoride and is configured in a sandwich unit stack configuration.

45. The system of claim 30, wherein the regeneration system comprises the salt decomposition system, The salt decomposition system comprises: a container configured to receive a spent dilute solution, wherein the spent dilute solution contains salt, a heat pump configured to warm the container and generate a gaseous product containing a salt in the container, and An absorber is configured to receive the spent concentrated solution and the gaseous product, wherein the salt in the gaseous product is absorbed by the spent concentrated solution to regenerate a concentrated salt solution.

46. The system of claim 30, wherein the regeneration system comprises the evaporation system, The evaporation system comprises: an evaporator configured to receive the spent concentrated solution and generate water vapor to regenerate the concentrated brine; a heat pump configured to provide thermal energy to the evaporator; A condenser is configured to receive the spent dilute solution and the water vapor generated by the evaporator, wherein the water vapor is condensed and mixed with the spent dilute solution to be regenerated into a dilute salt solution.

47. The system of claim 30, wherein the regeneration system comprises the electrodialysis system, wherein the electrodialysis system is configured to receive a spent dilute solution and a spent concentrate solution; and Power is supplied to the electrodialysis system, causing ions to move from the spent dilute solution to the spent concentrated solution to regenerate the dilute salt solution and the concentrated salt solution.

48. The system of claim 47, wherein the regeneration system further comprises a salt precipitation system, wherein the salt precipitation system comprises: a salt precipitator configured to receive a spent dilute solution and precipitate salt from the spent dilute solution before supplying the spent dilute solution to the electrodialysis system; a concentrate tank configured to receive the spent concentrate solution and to receive salt generated by the salt precipitator; wherein the concentrate tank is configured to receive a concentrated solution after regeneration by electrodialysis, and the diluent tank is configured to receive the dilute salt solution from the electrodialysis system.

49. The system of claim 30, wherein the regeneration system comprises the salt precipitation system and the membrane distillation system.

50. The system of claim 30, wherein the regeneration system comprises electrodialysis and the salt precipitation system.

51. The system of claim 30, wherein the regeneration system comprises the forward osmosis system configured to receive the spent concentrate solution and regenerate a concentrate brine using a switchable solubility system.

52. The system of claim 51, wherein the regeneration system further comprises a diluate tank configured to receive spent dilute solution and to receive water from the switchable solubility system, wherein the water is mixed with the spent dilute solution to regenerate the dilute brine solution.

53. The system of claim 51 or 52, wherein the switchable solubility system comprises: a draw solution, the draw solution circulating through the forward osmosis system and producing a spent draw solution; a recovery unit configured to receive the spent draw solution and add heat, wherein CO2 is released and water is produced; as well as A generator is configured to receive the solution from the reclaimer and add CO2 to regenerate the draw solution.

54. The system of any one of claims 30-53, wherein the first membrane and the second membrane comprise ion exchange membranes.

55. The system of any one of claims 30-54, further comprising: The second unit includes: a third membrane configured to be selectively permeable to cations; a fourth membrane configured to be selectively permeable to anions, the fourth membrane being spaced apart from the third membrane; and A second concentrated salt solution is provided between the third membrane and the fourth membrane, and the third membrane and the fourth membrane separate the second concentrated salt solution from the second dilute salt solution, wherein: The concentrated salt solution includes an endothermic solution; The second concentrated salt solution comprises an exothermic solution; and The heat pump is configured to transfer heat between the concentrated saline solution and the second concentrated saline solution.

56. A system as described in any of claims 30-55, wherein the thermal optimization system includes a control system, which is configured to coordinate heat transfer between the one or more heat sources and the reverse electrodialysis system through a processor based on one or more measurement results of the state of the one or more heat sources or the reverse electrodialysis system.

57. The system of claim 37 or 41, wherein the heat source comprises one or more of geothermal heat, industrial waste heat, or solar heat.

58. The system of any one of claims 30-57, wherein the heat pump is a vapor compression cycle, a thermoelectric refrigerator, a chemical absorption refrigerator, or other devices known in the art for simultaneous heating and cooling.

59. The system of any one of claims 30-55, further comprising a thermal optimization subsystem configured to coordinate heat transfer from the one or more heat sources to the first and / or second saline solutions based on one or more measurements of the one or more heat sources or a state of the first and / or second saline solutions.

60. The system of claim 59, wherein the heat source comprises one or more of geothermal heat, industrial waste heat, or solar heat.

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