Cascading heat pump-driven temperature vacuum swing adsorption for direct air capture of carbon dioxide

Abstract

A sorbent reactor system for removing carbon dioxide from air is provided. The sorbent reactor system facilitates improved energy and thermal efficiency and includes a cascading heat pump. The cascading heat pump includes a plurality of heat exchangers fluidly connected to each other via a pipe network. The plurality of the heat exchangers are microchannels coated with sorbent material capable of adsorbing carbon dioxide from ambient air, with sorbent heat exchangers then disposed in vacuum chamber reactors fluidly connected to each other. A first heat transfer fluid flows and recirculates through the pipe network and heat exchangers of a first heat pump, transferring heat away from hot sorbent undergoing a cooldown process under vacuum and towards cool sorbent undergoing a desorption process under vacuum. A second heat transfer fluid flows and recirculates through a second heat pump to transfer energy from ambient air to the first heat transfer fluid.

Description

CASCADING HEAT PUMP-DRIVEN TEMPERATURE VACUUM SWINGADSORPTION FOR DIRECT AIR CAPTURE OF CARBON DIOXIDECROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to U.S. Provisional Application No. 63 / 665,743, filed on June 28, 2024; the disclosure of which is incorporated herein by reference.TECHNICAL FIELD

[0002] The invention relates to devices and systems for direct air capture of carbon dioxide. More specifically, the invention relates to using heat pumps as the source of thermal energy for driving temperature vacuum swing adsorption cycles in reactors that capture carbon dioxide from the atmosphere or that capture carbon dioxide from the exhaust of fossil fuel combustion devices.BACKGROUND

[0003] Global warming is posing devastating effects on our climate, health, and communities. Coastal flooding due to rising sea levels, extended wildfire seasons, as well as more destructive hurricanes are the direct impacts of climate change. Moreover, global food and water security are at stake. There is a consensus among scientists that global warming is directly linked to the increase in the level of greenhouse gases in the atmosphere. Carbon dioxide (CO2) is a major greenhouse gas, and its concentration in the atmosphere has sharply increased over the past century due to the burning of fossil fuels. While efforts are underway to decarbonize electrical grids by building renewable energy sources that do not emit greenhouse gases, phasing out fossil fuels for renewables will continue to take time, technological advancement, and significant investment. In the meantime, a 2022 IPCC report makes clear that the world is not on a trajectory to limit global warming above the 1.5°C threshold by 2050, and that in order to stave off the worst effects of climate disaster there exist the future means to remove CO2 from the atmosphere. Therefore, there is a growing need for technologies that can efficiently capture CO2 from the flue gas of power plants and other industrial processes and, increasingly, even from ambient air. The latter is known as direct air capture (DAC).

[0004] CO2capture processes commonly utilize some type of regenerable adsorbent bed to capture the CO2 from a gas or air stream (see, for example, Sanz-Perez, et al., Chemical Reviews, 2016, 116, 11840-11876, which is incorporated by reference in this disclosure in its entirety). A common approach can involve a first step of moving ambient air or flue gas through abed of a solid sorbent that is effective at capturing a significant portion of the CO2 contained therein. Once the sorbent reaches a level of significant saturation of CO2, it needs to be regenerated in a second step. During regeneration, the adsorbent bed is treated with, for example, heat, vacuum, steam, or some combination thereof to cause the CO2 to desorb from the sorbent. The released CO2 is subsequently captured, and the regenerated sorbent can then be returned to the first step and reused to capture more CO2. Due to the relatively low concentrations (currently 424 parts per million) of CO2 in ambient air, high volumes of ambient air need to be moved and processed in a DAC process, so the systems need to be highly efficient.

[0005] Common solid CO2 sorbents include various zeolites or molecular sieves; amine- functionalized silicious, inorganic, activated carbon, graphitic, metal organic framework (MOF) or polymeric supports; amine-functionalized carbon, glass, cellulosic, or polymeric fibers; and basic or weakly basic ion exchange resins (see, for example, Samanta, et al., Industrial & Engineering Chemistry Research, 2012, 51, 1438-1463, which is incorporated by reference in this disclosure in its entirety). In some cases, the solid CO2 sorbents are utilized in powder or pellet form in fluidized bed or packed bed configurations through which air is passed. In other cases, the solid CO2sorbents are utilized in fibrous webs, mats, or woven fabrics through which air is passed. In still other cases, the solid CO2sorbents are formed into structured monoliths or other structured forms such as sheets, films, membranes, or plates through or around which air may be passed. Sorbents structured in a sheet-type form may be conveniently mounted within a supporting cartridge to form parallel-plate contactor structures which may be mounted within an adsorbent reactor chamber. Gases and / or fluids may then be flowed through the contactors with a relatively low-pressure drop compared with, for example, packed bed configurations.

[0006] The captured CO2 is desorbed during the sorbent regeneration process, which usually involves some combination of applying heat and vacuum to the adsorbent bed to effect desorption. This process is commonly known as temperature vacuum swing adsorption (TVSA) and has been researched and reported in the literature (see, for example, Elfving, et al., ChemicalEngineering Journal, 2021, 404, 126337, which is incorporated by reference in this disclosure in its entirety).

[0007] A TVS A process requires large amounts of power and energy to repeatedly heat sorbent and desorb CO2, and prior art traditionally relied on electricity to generate steam as a major source of process heat and as a carrier to sweep away gaseous CO2 as it is being desorbed. See, for example, U.S. Patent No. 10,279,306 (Gebald, et al.). However, the capital cost for the power system equipment and the operating costs to generate electricity significantly increase the levelized costs of carbon capture technology, and some sorbent material degrades over time when exposed to moisture and steam. There is a need then for improved DAC systems and methods that use significantly less electricity and power while avoiding direct contact between sorbents and steam.

[0008] Heat pumps, such as air-source heat pumps, are known in the art to reduce the power requirements of heating. Air-source heat pumps comprise a heat transfer fluid in an enclosure that absorbs heat from ambient air. The heat transfer fluid passes through a vapor-compression refrigeration process, whereby the heat pump delivers or transfers the absorbed heat to a new location or space. Heat pumps reduce the amount of energy required to heat a system by taking advantage of the temperature differential between the outdoors and indoors. Heat pumps reduce reliance on fossil fuels by facilitating heat transfer through the work of expansion and compression of the heat transfer fluid using energy obtained from the air rather than only from a fuel source. Due to being capable of operating bidirectionally, heat pumps can act like air conditioners or heaters depending on operating mode, obviating the need for separate chillers and furnaces.

[0009] DAC systems can incorporate heat pumps to balance plant and heat transfer processes to reduce the fuel and electricity costs inherent to the heating and cooling of sorbent material required during CO2 desorption. However, these systems still use heat pumps to help generate steam for sorbent heating and CO2 removal, which as disclosed above, can degrade sorbent over time. There is a need then for DAC systems that make use of efficient heat sources, such as heat pump technology, to reduce energy usage while also efficiently transferring heat to sorbent materials without direct steam contact to avoid sorbent degradation.SUMMARY

[0010] The invention relates to systems and methods for capturing carbon dioxide (CO2) from the air using CCh-adsorbent material (also called “sorbent”). The systems and methods comprise a direct air capture (DAC) process system that functions as a heat pump. The systems and methods comprise interconnected heat pumps and heat exchangers for providing efficient temperature vacuum swing adsorption (TVSA) cycles without using steam in direct contact with the sorbent material as a heat transfer medium. The systems and methods of the invention comprising multiple interconnected heat pumps are sometimes referred to herein as “cascading heat pump systems”.

[0011] In some embodiments of the invention, the DAC process system functions as a first heat pump. The first heat pump comprises a plurality of reactors, each of the plurality of reactors enclosing: at least one sorbent heat exchanger and a sorbent material capable of adsorbing CO2 from the atmosphere. The sorbent material can be applied or coated on at least one surface of each sorbent heat exchanger. The first heat pump further comprises a pipe network having a plurality of pipes and control valves. The pipe network fluidly connects the at least one sorbent heat exchanger of each of the plurality of reactors to at least one other sorbent heat exchanger of another of the plurality of reactors. The first heat pump further comprises a first compressor fluidly connected to the plurality of reactors via the pipe network and a first expansion valve fluidly connected to the plurality of reactors via the pipe network. In some embodiments of the invention, the system further comprises at least one second heat pump, creating a cascading heat pump system. Each of the at least one second heat pump comprises a pump heat exchanger in thermal contact with the pipe network of the first heat pump, a source heat exchanger fluidly connected to the pump heat exchanger, a second compressor fluidly connected between the pump heat exchanger and the source heat exchanger, and a second expansion valve fluidly connected between the pump heat exchanger and the source heat exchanger. The system further comprises a first fluid that flows and recirculates through the first heat pump, and a second fluid that flows and recirculates through the at least one second heat pump in a closed loop and is in periodic thermal contact with the first fluid via the pump heat exchanger.

[0012] In some embodiments of the invention, the first fluid and the second fluid can be refrigerants. In some embodiments of the invention, the first fluid can include a different chemical substance than the second fluid. In some embodiments of the invention, the first fluidcan be a synthetic fluid, such as a hydrofluoroolefin, or a natural fluid, such as butane or isobutane. In some embodiments of the invention, the second fluid can be a synthetic fluid, such as a hydrofluoroolefin or a natural fluid, such as propane.

[0013] In some embodiments of the invention, the sorbent heat exchangers can be microchannel heat exchangers that comprise a manifold, a plurality of parallel tubes, and a plurality of fins. The plurality of parallel tubes can be connected at either end to the manifold which holds the plurality of parallel tubes in place. The plurality of fins can be disposed between, and connected to, adjacent parallel tubes. The sorbent material can be applied directly on the surfaces of the plurality of fins and the plurality of parallel tubes, and the first fluid can flow through the manifold to the plurality of parallel tubes and is in thermal contact with the plurality of fins. The sorbent material can be a coating on the surface of the sorbent heat exchangers. The sorbent material can be a solid amine-based sorbent. The plurality of parallel tubes and the plurality of fins of the microchannel heat exchangers can be formed of a lightweight thermally conductive metal, such as aluminum, and the plurality of parallel tubes can be directly joined to the plurality of fins. In some embodiments of the invention, the plurality of fins of the microchannel heat exchangers can have a serpentine shape or a wave shape.

[0014] In some embodiments of the invention, the control valves can include a plurality of first control valves and second control valves, each of the plurality of the first and second control valves can include three ports, and each of the three ports can be selected from one of: a reactorside port, a first pipe-side port, or a second pipe-side port. Each port of the plurality of the first and second control valves can alternate between an open position and a closed position. The opening and closing of the ports of the plurality of the first and second control valves can selectively divert the first fluid within the pipe network either: to a sorbent heat exchanger, from a sorbent heat exchanger, or bypassing a sorbent heat exchanger. The plurality of first control valves can control flow of the first fluid into the sorbent heat exchangers from the outlet of the first compressor and out of the sorbent heat exchangers to the inlet of the pump heat exchanger. The plurality of second control valves can control throughput flow of the first fluid to and from the first expansion valve.

[0015] In some embodiments of the invention, each of the reactors can be vacuum chambers including a plurality of walls forming an enclosure, at least one frame defining an opening of the vacuum chamber, at least one door disposed in each frame, where the doors are actuatable foropening and closing the vacuum chambers, and a pump system for evacuating air from the vacuum chamber when the door is closed.

[0016] In some embodiments of the invention, the pump heat exchanger can raise the temperature of the first fluid prior to the first fluid entering the first compressor by transferring heat from the second fluid to the first fluid via the pump heat exchanger. The first compressor can compress the first fluid such that the first fluid output downstream from the first compressor exhibits a temperature between 80 °C and 100 C. The boiling temperature of the second fluid can be lower than the ambient temperature of outside air, and the condensing temperature of the second fluid can be warmer than the boiling temperature of the first fluid. The first expansion valve can lower the pressure of the first fluid such that the first fluid output downstream from the first expansion valve exhibits a temperature between 40°C and 50°C.

[0017] Since heat pumps provide a highly efficient means for transferring heat, the cascading heat pump systems in accordance with the invention are highly energy efficient. Using a cascading heat pump system in accordance with the invention, the effective coefficient of performance (COPcffcctivc) of the cascading heat pump system is greater than 1. In some embodiments of the invention, the COPeffective of the cascading heat pump system is greater than 1.5. In some embodiments of the invention, the COPeffective of the cascading heat pump system is greater than 2. In some embodiments of the invention, the COPeffective of the cascading heat pump system is greater than 2.5.

[0018] In some embodiments of the invention, each reactor undergoes one of three modes selected from: CO2 adsorption, CO2 desorption, or active cooling. The first fluid that flows via the pipe network from a sorbent heat exchanger can undergo active cooling to another sorbent heat exchanger undergoing CO2 desorption, and the first fluid can transfer heat from warmer sorbent heat exchangers to colder sorbent heat exchangers.BRIEF DESCRIPTION OF THE DRAWINGS

[0019] These and other objects, features, and advantages of the invention will be more fully appreciated or become better understood when considered in conjunction with the accompanying drawings, where:

[0020] FIG. 1 shows a schematic view of a direct air capture (DAC) process system that acts as a first heat pump in accordance with the invention.

[0021] FIG. 2 shows a schematic view of a direct air capture process system comprising a plurality of heat pumps creating a cascading heat pump system in accordance with the invention.

[0022] FIG. 3 shows a thermodynamic vapor compression refrigeration cycle of a first heat pump comprised in the cascading heat pump system of FIG. 2.

[0023] FIG. 4 shows a thermodynamic vapor compression refrigeration cycle of a second heat pump comprised in the cascading heat pump system of FIG. 2.

[0024] FIG. 5 shows an energy flow diagram of a cascading heat pump system in accordance with the invention.

[0025] FIG. 6 shows the relationship between pressure and enthalpy for a first fluid utilized in a first heat pump in accordance with the invention.

[0026] FIG. 7 shows the relationship between pressure and enthalpy for a second fluid utilized in a second heat pump in accordance with the invention.

[0027] FIG. 8 shows a schematic view of a cascading heat pump system including a separation tank in accordance with the invention.

[0028] FIG. 9 shows an example of a sorbent reactor comprised in a cascading heat pump system in accordance with the invention.

[0029] FIG. 10 shows a microchannel heat exchanger for use inside a reactor of a cascading heat pump system in accordance with the invention.DETAILED DESCRIPTION

[0030] The invention relates to systems and methods for capturing carbon dioxide (CO2) from the air using CCh-adsorbent material (also called “sorbent”). In some embodiments of the invention, the systems and methods comprise a direct air capture (DAC) process system that functions as a heat pump. In some embodiments of the invention, the systems and methods comprise interconnected heat pumps and heat exchangers for providing efficient temperature vacuum swing adsorption (TVSA) cycles without using steam in direct contact with the sorbent material as a heat transfer medium. The systems and methods of the invention comprising multiple interconnected heat pumps are sometimes referred to herein as “cascading heat pump systems”.

[0031] The systems and methods of the invention comprise a DAC process system that functions as a heat pump. A diagram of an exemplary embodiment of the DAC process system thatfunctions as a heat pump in accordance with the invention is shown in FIG. 1 . The DAC process system that functions as a heat pump is referred to herein as “a first heat pump”. The first heat pump 10 comprises a first compressor 11; a plurality of first control valves 12a, 12b, and 12c; a plurality of sorbent heat exchangers 13a, 13b, and 13c; a plurality of second control valves 14a, 14b, and 14c; a first expansion valve 15; and a plurality of reactors 16a, 16b, and 16c that house the plurality of sorbent heat exchangers 13a, 13b, and 13c. While three first control valves, three reactors housing three sorbent heat exchangers, and three second control valves are illustrated in FIG. 1, any number of first control valves, reactors, sorbent heat exchangers, or second control valves can be included in the first heat pump.

[0032] In some embodiments of the invention, the first heat pump 10 may further comprise one or more second heat pump(s), creating a cascading heat pump system. An example embodiment of the cascading heat pump system of the invention wherein the first heat pump 10 further comprises a second heat pump 30 is shown in FIG. 2. The second heat pump 30 comprises a second compressor 20, a pump heat exchanger 21, a second expansion valve 22, and a source heat exchanger 23 formed in a closed loop. In some embodiments of the invention, the second heat pump 30 provides heat to the first heat pump 10 via the pump heat exchanger 21.

[0033] In an exemplary embodiment of the invention, each of the components of the cascading heat pump system can be fluidly connected together. In this context “fluidly connected” means that the cascading heat pump system components share a pipe network that allows fluid to flow freely between components. The fluid that flows through the components of the cascading heat pump system may be a heat transfer fluid such as, but not limited to, a synthetic or organic fluid in various phases of matter, i.e. liquid, gas, or a mixture thereof. In the context of the invention captured in this application, any heat transfer fluid shall be referred to as a fluid. When a heat transfer fluid flows from a cold source to a hot source it is a cold fluid, and when a heat transfer fluid flows from a hot source to a cold source it is a hot fluid. The cascading heat pump system may comprise a plurality of heat pumps, each with its own heat transfer fluid. Heat transfer fluid makes thermal contact with an element of the cascading heat pump system when it transfers heat to that element via an interstitial element, such as via a heat exchanger.

[0034] The flow of fluid through the fluidly connected components of the first heat pump 10 transfers heat from components comprised in the cascading heat pump system to other system components in such a manner that, in aggregate, the first heat pump 10 functions as a distributedcascading heat pump, siphoning waste energy from hot spots to where it can be reclaimed and repurposed for thermodynamic efficiency. This is in distinction with the definition of a heat pump as a discrete device, one that uses work to transfer heat from a cool space to a warm space by transferring thermal energy using a refrigeration cycle, instantiated within the confines of a single enclosure. In the context of the invention, components comprised in the cascading heat pump system may be literal heat pump devices or may be a plurality of distributed elements that function, behave, or act like heat pumps, and this distinction will be made clear throughout.

[0035] In some embodiments of the invention, the first heat pump 10 comprises a first compressor 11 fluidly connected via a pipe network with a plurality of sorbent heat exchangers 13a, 13b, and 13c (collectively 13a-c) as shown in FIG. 1 and FIG. 2. The first heat pump 10 may comprise any number of sorbent heat exchangers, though FIG. 1 and FIG. 2 disclose an example embodiment in which three sorbent heat exchangers 13a-c are fluidly connected in parallel. The pipe network fluidly connecting the first compressor 11 and the plurality of sorbent heat exchangers 13a-c may further comprise a plurality of first control valves 12a, 12b, and 12c (collectively 12a-c). In some embodiments of the invention, the plurality of first control valves 12a-c are three-way valves having an outlet, an inlet, and a bypass. Each sorbent heat exchanger is respectively associated with a first control valve.

[0036] As shown in FIG. 1 and FIG. 2, the first heat pump 10 comprises three first control valves 12a, 12b, and 12c and each of the first control valves 12a, 12b, and 12c is associated with each of the sorbent heat exchangers 13a, 13b, and 13c, respectively. The first heat pump 10 may further comprise a first expansion valve 15 fluidly connected via the pipe network with the plurality of sorbent heat exchangers 13a-c. The pipe network fluidly connecting the first expansion valve 15 and the plurality of sorbent heat exchangers 13a-c may further comprise a plurality of second control valves 14a, 14b, and 14c (collectively 14a-c). Each sorbent heat exchanger is associated with a second control valve; FIG. 1 and FIG. 2 show an embodiment of the invention in which three second control valves 14a, 14b, and 14c are present and associated with sorbent heat exchangers 13a, 13b, and 13c, respectively. The plurality of first control valves 12a-c and the plurality of second control valves 14a-c can each be independently open or closed, and they can be configured to direct fluid flow in a particular direction. The actuation of the plurality of first control valves 12a-c and the plurality of second control valves 14a-c directs the flow of heattransfer fluid into-, out of-, and between- the plurality of sorbent heat exchangers 13a-c inside the plurality of reactors 16a, 16b, and 16c (collectively 16a-c).

[0037] In some embodiments of the invention, the first heat pump 10 may further comprise a second heat pump 30 as shown in FIG. 2, creating a cascading heat pump system. The second heat pump 30 is fluidly connected via a pipe network to the first compressor 11 and the plurality of sorbent heat exchangers 13a-c. The second heat pump 30 comprises a second compressor 20, a pump heat exchanger 21, a second expansion valve 22, and a source heat exchanger 23 formed in a closed loop. A second fluid flows through the closed loop of the second heat pump 30 such that the second fluid flows through the second compressor 20, the pump heat exchanger 21, the second expansion valve 22, and the source heat exchanger 23. The first fluid is distinct from the second fluid, and the two do not mix in any of the fluid connections comprised in the cascading heat pump system. In some embodiments of the invention, the first fluid and the second fluid are different chemical substances. Second heat pump 30 transfers heat to the first fluid while it flows from the outlet of at least one sorbent heat exchanger 13a-c and through the pump heat exchanger 21 before it enters the first compressor 11. In some embodiments of the invention, the cascading heat pump system further comprises at least one additional second heat pump with each additional second heat pump comprising a second compressor, a pump heat exchanger, a second expansion valve, and a source heat exchanger.

[0038] Each of the plurality of sorbent heat exchangers 13a-c may comprise a heat exchanger coated with a sorbent material. Each sorbent heat exchanger is structured to exhibit a form factor that maximizes surface area while minimizing thermal mass. Examples of suitable sorbent heat exchanger structure shapes include microchannels or micro-fins formed into parallel shells, tubes, or plates. Preferably, the sorbent heat exchangers are formed of finned microchannels. Said microchannel heat exchangers (MCHEs) may be made of a lightweight thermally conductive metal that has been extruded into flat tubes through which flows the first fluid. In some embodiments of the invention, the MCHEs are made of aluminum. The space between the flat tubes that form a MCHE comprises a metal serpentine fin that is attached at either end to an adjacent flat tube. In some embodiments of the invention, the metal serpentine fin is attached to the end of the adjacent flat tube via brazing. In some embodiments of the invention, the metal serpentine fin is attached to the end of the adjacent flat tube via welding. A structure of adjacent flat tubes with intervening serpentine fins may then be bordered by manifolds. In someembodiments of the invention, the serpentine fins are coated with a sorbent material. In some embodiments of the invention, the serpentine fins are coated with a sorbent material via a dipping process. In some embodiments of the invention, up to 1000 square meters of sorbent material is disposed and coated onto the microchannels per cubic meter of the MCHE.

[0039] The sorbent materials may comprise, consist of, or consist essentially of a zeolite or molecular sieve; amine-functionalized siliceous, inorganic, activated carbon, graphitic, metal organic framework (MOF) or polymeric supports; amine-functionalized carbon, glass, cellulosic, or polymeric fibers; basic or weakly basic ion exchange resins; polyamine compounds or polyamine polymers; or mixtures thereof. In some embodiments of the invention, the sorbent material comprises a weakly basic ion exchange resin. In some embodiments of the invention, the sorbent material comprises a polyamine compound or polyamine polymer, optionally supported on a solid support. In some embodiments of the invention, the sorbent material comprises a MOF compound. In some embodiments of the invention, the sorbent material comprises a solid amine-based sorbent.

[0040] Each of the plurality of sorbent heat exchangers may be located inside a reactor that can be in an open position (open to the air) or a closed position (sealed to prevent the ingress of outside air). When the reactor is in the open position, the sorbent may adsorb CO2 from the air. When the reactor is in the closed position, the sorbent may be heated in order to desorb (release) CO2 into the enclosure. The plurality of sorbent heat exchangers 13a-c may be actively or passively heated or cooled when the reactors 16a-c are in either the open or closed position.

[0041] First compressor 11 and second compressor 20 may be driven by separate electric motors. In some embodiments of the invention, the separate electric motors are powered by electricity generated by renewable energy sources. The same or additional power sources can further supply power to the plurality of reactors 16a-c for actuating valves and opening and closing doors, as well as to fans (not shown) that may be disposed to route ambient air to the sorbent enclosed in the reactor, as well as to power a vacuum pump system (not shown) connected to the plurality of reactors 16a-c for evacuating air and introducing vacuum conditions to the plurality of sorbent heat exchangers 13a-c.

[0042] The first fluid is constantly recirculating within the closed loop of the first heat pump 10 while the cascading heat pump system is operating and can pass into or out of any system component fluidly connected via the pipe network. However, the first fluid is circulated androuted according to the open and closed states of the plurality of first control valves 12a-c and the plurality of second control valves 14a-c. The plurality of first control valves 12a-c and the plurality of second control valves 14a-c control the delivery of the first fluid to each of the plurality of sorbent heat exchanger 13a-c, such that the source of the first fluid entering a given sorbent heat exchanger is selected from being either: (1) the outlet of the first compressor 11 or (2) the outlet of the first expansion valve 15, as determined by the selected operating mode of each sorbent heat exchanger control valve, the temperature of the sorbent heat exchanger, and present ambient air conditions. The ports of the plurality of first control valves 12a-c and the plurality of second control valves 14a-c operate in a binary fashion and are always either “open” or “closed”.

[0043] In some embodiments of the invention, the plurality of first control valves 12a-c and the plurality of second control valves 14a-c comprise T-valves having three ports: a reactor-side port that connects to a sorbent heat exchanger and two pipe-side ports that connect to the pipe network. Flow of the first fluid can be bidirectional through any control valve. The ports of the control valves are configured so as to divert or direct fluid flow in a desired direction. If, for example, the reactor-side port of a first control valve 12a, 12b, or 12c is closed and the pipe-side ports are open, the first fluid passing through the first control valve will divert back into the pipe network, where it can be delivered to a sorbent heat exchanger 13a-c in a different reactor 16a-c or to the pump heat exchanger 21 connected with the second heat pump 30.

[0044] In some embodiments of the invention, the first fluid flowing out of the first compressor 11 can be returned to the inlet of the first compressor 11 through a pipe-side port of a first control valve 12a, 12b, or 12c by way of the pump heat exchanger 21 connected with the second heat pump 30. If a first control valve is configured such that the first fluid bypasses a reactor, this can ensure that the first fluid stays sufficiently hot so as not to condense inside the first compressor 11. In addition, a hot first fluid may flow into one or more of the plurality of sorbent heat exchangers that is / are in the process of being heated to desorb CO2 under vacuum. In addition, a cold first fluid may flow into one or more of the plurality of sorbent heat exchangers that is / are in the process of being cooled down for a subsequent adsorption step. The first fluid that exits sorbent heat exchanger 13a, 13b, or 13c may flow into the inlet of another sorbent heat exchanger, the inlet of the pump heat exchanger 21 connected with the second heat pump 30, or the inlet of the first expansion valve 1 . The first fluid exiting the first expansion valve 15subsequently passes through one or more of the plurality of sorbent heat exchangers that is / are in the process of being cooled under vacuum.

[0045] FIG. 2 shows an example operating state of a cascading heat pump system. The cascading heat pump system comprises a first heat pump 10 wherein the ports of first control valve 12a and second control valve 14a are configured to allow a hot first fluid, in the form of a superheated vapor, to flow from the outlet of first compressor 11 into sorbent heat exchanger 13a. In some embodiments of the invention, the hot first fluid has a temperature between 80°C to 120°C, 90°C to 110°C, or 95°C to 105°C. Inside sorbent heat exchanger 13a, the first fluid makes thermal contact with the sorbent material via the thermally conductive elements of the sorbent heat exchanger. This thermal contact causes the first fluid to heat the sorbent to a temperature whereby adsorbed CO2 is desorbed and released. The first fluid in sorbent heat exchanger 13a is subsequently extracted from the sorbent heat exchanger and flows to the first expansion valve 15, which expands the first fluid (now a superheated vapor) and cools the first fluid into a mixture of saturated vapor and liquid before recirculating the first fluid to a different sorbent heat exchanger.

[0046] In addition, FIG. 2 shows the ports of first control valve 12b and second control valve 14b configured to allow a cool first fluid (a mixture of superheated vapor and liquid) to flow from the outlet of the first expansion valve 15 through sorbent heat exchanger 13b. In some embodiments of the invention, the cool first fluid has a temperature between 30°C to 60°C, 35°C to 55°C, or 40°C to 50°C. Inside the sorbent heat exchanger, the first fluid makes thermal contact with the sorbent material via the thermally conductive elements of the sorbent heat exchanger. Because sorbent heat exchanger 13b is presently undergoing a cooldown period and is warmer than the first fluid, the first fluid boils when heat is transferred to the first fluid from the sorbent heat exchanger, thereby cooling sorbent heat exchanger 13b. In some embodiments the sorbent heat exchanger 13b is cooled to a temperature between 30°C to 60°C, 35°C to 55°C, or 40°C to 50°C. When the first fluid subsequently exits sorbent heat exchanger 13b, the first fluid (now a warmer vapor) flows to the pump heat exchanger 21 connected with the second heat pump 30 and makes thermal contact with the second fluid in the second heat pump 30 via the thermally conductive elements of the pump heat exchanger. In this way heat is transferred to the first fluid from the second fluid before the first fluid recirculates to the first compressor 11.

[0047] In addition, FIG. 2 shows the ports of first control valve 12c and second control valve 14c configured to prevent the first fluid from flowing into or out of sorbent heat exchanger 13c while it is undergoing adsorption under exposure to ambient air. The flow of ambient air through the sorbent heat exchanger 13c is enough to cool it to ambient temperature. Ambient atmospheric conditions can vary by season, location, and sea level but are commonly understood to exhibit temperatures between -40°C and 40°C and to exhibit pressures between 0.8 and 1.1 bar. In some embodiments of the invention, ambient conditions during operation of the heat pumps comprise temperatures between -10°C and 35°C and pressures of approximately 0.9 and 1.1 bar. In some embodiments of the invention, ambient conditions during operation of the heat pumps comprise temperatures between 10°C and 35°C and pressures of approximately 1.0 bar.

[0048] The possible configurations of operating states of the DAC process system of the invention or of the cascading heat pump system of the invention are not limited to the permutations depicted in FIG. 1 or FIG. 2, respectively, as all isolation valves and bypass valves comprised in the first heat pump 10 may be configured to allow any number of sorbent heat exchangers to be heated, cooled, or isolated as individuals or as groups in any suitable combination.

[0049] The first fluid flows through the pipe network in different states, depending on its temperature and pressure. The condensing temperature of the first fluid at the first compressor outlet pressure T1 is selected to be in the range of 80°C to 120°C, between 90°C to 110°C, between 95°C to 105°C, between 90°C to 100°C, or alternatively between 85°C to 95°C. Being a pure fluid (not a mixture of chemically different fluids), the first fluid changes from vapor to liquid at a constant temperature. The boiling temperature of the first fluid after being expanded to low pressure at constant temperature is selected to be in the range of 30°C to 60°C, between 35°C to 55°C, or alternatively between 40°C to 50°C. Preliminary analysis suggests that the higher end of this range is likely to be preferred, with the maximum of the range determined by the highest pressure ratio producible by the compressor.

[0050] The second fluid flows and recirculates through the second heat pump 30 in different states, depending on its temperature and pressure. Being a pure fluid (rather than a mixture of chemically different fluids), the condensing temperature of the second fluid remains constant during the transition from vapor to liquid. So that heat can be transferred to the heat exchanger circuit acting as a heat pump, the condensing temperature of the second fluid is selected to be afew degrees warmer than the boiling temperature of the first fluid. To facilitate a heat transfer from the ambient outdoor air to the second fluid, the boiling temperature of the second fluid is selected to be several degrees colder than the outdoor air temperature. In some embodiments of the invention, the second heat pump 30 may draw heat from sources other than ambient outdoor air including from bodies of water or from the ground. In some embodiments of the invention, the second heat pump 30 may draw heat from low-grade heat sources such as waste heat from industrial processes or from data centers.

[0051] The first and second fluids may be refrigerants and organic or synthetic fluids. The first fluid may comprise a hydrofluoroolefin such as, but not limited to, R1234yf or a natural fluid like n-butane (“butane”) or isobutane. The second fluid may comprise a hydrofluoroolefin such as, but not limited to, R1233zd or a natural fluid like propane. Hydrofluoroolefins are preferable as heat transfer fluids in heat pumps due to their favorable temperatures of condensation and boiling point under the pressure conditions required during cyclic phase changes. Synthetic hydrofluoroolefins have high volumetric heat capacity, are non-flammable, and reduce deleterious environmental impacts compared to ozone-depleting hydrofluorocarbons. Hydrocarbons such as isomers of butane or propane are also acceptable and efficacious alternatives.

[0052] The plurality of sorbent heat exchangers 13a-c are enclosed in individual of the plurality of reactors 16a-c, respectively, and operate in one of three modes according to the stages of a TVSA cycle: (1) CO2 adsorption while exposed to ambient air, (2) CO2 desorption while heated under vacuum, or (3) cooling under vacuum (a transition mode after desorption and prior to adsorption). The plurality of sorbent heat exchangers 13a-c may operate in parallel or in sequence, or in combination thereof, in one of the three prescribed operating modes, such that simultaneously at least one sorbent heat exchanger may be operating in any given mode. The number of sorbent heat exchangers operating in one of the three modes at any one time is not limited in practice. The operating mode of each sorbent heat exchanger may be determined by a repeating sequence of steps of predetermined time intervals. Alternatively, the start and stop times of operating modes as well as the quantity of the plurality of reactors 16a-c operating in a given mode at any one time may be sequenced by a logic controller which takes as inputs the data from temperature, pressure, and air-flow sensors disposed on the surface of, or in proximity to, the sorbent heat exchangers.

[0053] FIGS. 3 and 4 show the counterclockwise thermodynamic cycles of the first and second fluids, respectively, as the first and second fluids undergo phase changes at various stages within the first and second heat pumps of the cascading heat pump system. The cycles are represented, in a manner typical for thermodynamic analysis, by plots of the relationship between enthalpy of the fluid and the log of its pressure. A saturation curve, the line separating the difference between the fluid existing as a two-phase mixture (combination saturated vapor and liquid, represented by the area under the curve) or a superheated vapor phase (represented by the area outside the curve), is also plotted. The bold lines in the figures are isotherms: lines representing fluid conditions as a function of enthalpy and pressure while at constant temperature. The directional arrows indicate the order of condition and phase changes as the first and second fluids move through their respective closed loops in the first and second heat pumps, respectively. Both the first and second fluids may be selected such that the first and second fluids undergo a phase change from vapor to liquid and vice versa at a constant temperature.

[0054] FIG. 3 shows that the first fluid reaches a temperature Tl and a pressure Pl after being compressed by compressor 11 (this is node A in the graph). When the first fluid transfers heat to a colder sorbent heat exchanger undergoing desorption, the sorbent heat exchanger is heated to a temperature Th (Th is colder than Tl; this occurs at node B in the graph). When the first fluid receives heat from a warmer sorbent heat exchanger, the sorbent heat exchanger cools to a temperature T3 and the first fluid cools to a temperature Tc (Tc is colder than Th while still being above ambient temperature; this is node C in the graph). When the pressure of the first fluid is decreased by first expansion valve 15, the first fluid exhibits a lower pressure P2 and a lower temperature T3 (T3 < T2 < Tl; this is node D in the graph). The temperature and pressure of the first fluid throughout the cascading heat pump system is dependent on the type of fluid used as the first fluid.

[0055] FIG. 4 shows that the second fluid reaches a temperature T4 at Pressure P3: this occurs when the second fluid is changed to a superheated vapor by second compressor 20 (this is node E in the graph). When the second fluid transfers heat to the first fluid inside heat pump heat exchanger 21, it condenses at temperature T5 and saturation pressure P3 (this is node F in the graph). The condensation temperature T5 of the second fluid remains higher than ambient temperature Te. When the second fluid passes through second expansion valve 22 it becomes a mixture of saturated vapor and liquid at saturated pressure P4 and temperature T6 (this is node Gin the graph). When the second fluid passes through heat source heat exchanger 23, the second fluid boils at pressure P4 and temperature T6, (this is node H in the graph). Temperature T6 is colder than ambient temperature Te. The temperature and pressure of the second fluid throughout the cascading heat pump system is dependent on the type of fluid used as the second fluid.

[0056] FIG. 5 shows a schematic that displays the flow of energy in and out of the cascading heat pump system of the invention. Energy flows are diagrammed as either Work (W) or Heat (Q), and the energy flows depicted are demonstrative of the system states shown in FIG. 2 (i.e., with a first sorbent heat exchanger undergoing CO2 desorption, a second sorbent heat exchanger undergoing a cooldown transition post-desorption, and a third sorbent heat exchanger undergoing CO2 adsorption).

[0057] The benefits of the DAC process system and cascading heat pump system in accordance with the invention are evident when the ratio of energy) output from the cascading heat pump (in the form of heat) to energy input (in the form of work) is calculated. This ratio is also known in the art of heat pumps and refrigeration systems as the Coefficient of Performance for heating (“COP”). FIGS. 6 and 7 disclose the plotted relationship between enthalpy and pressure of the first fluid and the second fluid, respectively; while qualitatively similar to the diagrams shown in FIGS. 3 and 4, the difference is that FIG 6 and FIG. 7 highlight and callout the energy quantities in Work (W) and Heat (Q) that are added to the cascading heat pump system at each stage.Nodes A-D in FIG. 6 and nodes E-H in FIG. 7 are identical to those described in FIG. 3 and FIG. 4, respectively, indicating the phase composition and conditions of the first and second fluid for different operating modes of the carbon capture reactors.

[0058] In FIGS. 5, 6, and 7, heat flow QHI is the thermal energy transferred from the first heat pump 10 to a sorbent heat exchanger undergoing desorption (13a) and being raised to temperature Th. Heat QHI represents the sensible heat that is added to the sorbent and the heat exchanger as well as the latent heat required to desorb carbon dioxide from the sorbent. Heat flow Qci is the thermal energy transferred to the first heat pump 10 from a sorbent heat exchanger undergoing a cooldown transition (13b) and being lowered from temperature Th to temperature Tc. Heat flow QAI is the thermal energy lost to the air from a sorbent heat exchanger undergoing adsorption (13c). Qc2 is thermal energy transferred from ambient air to the second heat pump 30 via source heat exchanger 23. QH2 is thermal energy transferred to the first heat pump 10 from the second heat pump 30 (i.e., from the second fluid to the first fluid) via pumpheat exchanger 21 . Wi is the work supplied by an electric motor to a shaft that runs the first compressor 11. W2 is the work supplied by an electric motor to a shaft that runs the second compressor 20. Note that Wi and W2 do not include energy losses in torque transmission from the motor.

[0059] Conservation of energy for the control volume containing the first fluid (Rl) dictates that:

[0060] Conservation of energy for the control volume containing the second fluid (R2) further dictates:

[0061] Conservation of energy for the control volume containing the sorbent heat exchangers further dictates:

[0062] Conservation of energy for the cascading heat pump system requires that the net heat lost to ambient air must equal the sum of the energy supplied to the compressors, which is found by adding equations (1) through (3):

[0063] The ratio of useful input work to output work, or the effective coefficient of performance for heating the cascading heat pump system, can be expressed as equation (5):

[0064] The effective coefficient of performance (COPeffective) of the cascading heat pump system of the invention is greater than 1 because QH1is greater than 1 + W2, which is demonstrated by combining equations (1) and (2):(6)

[0065] A heating system that supplies the same amount of heat QHI using ideal resistance heating without heat pumps has an effective coefficient of performance (COPeffective) of one. Using a cascading heat pump system in accordance with the invention, COPeffective > 1. In some embodiments of the invention, the COPeffective of the cascading heat pump system is greater than 1.5. In some embodiments of the invention, the COPeffective of the cascading heat pump system is greater than 2. In some embodiments of the invention, the COPeffective of the cascading heat pumpsystem is greater than 2.5. This means that the same amount of heat can be supplied by the system with less energy input by taking advantage of the heat in ambient air. The COPeffective of the cascading heat pump system of the invention ultimately depends on many factors, including the thermodynamic and transport properties of the first and second fluids, the isentropic efficiencies of the first and second compressors, the size of the sorbent heat exchangers, the temperature required for CO2 desorption, the temperature required after cooling, and the average ambient air temperature where the system is sited.

[0066] An alternative embodiment of a cascading heat pump system in accordance with the invention is shown in FIG. 8. The cascading heat pump system may comprise a plurality of components fluidly connected in a closed loop forming a first heat pump 10 as in the embodiment shown in FIG. 2. However, the first heat pump 10 of this embodiment may further comprise a separation tank 40 that separates a first fluid flowing downstream of a first expansion valve 15 into a liquid phase and a saturated vapor phase. The liquid phase of the first fluid may then flow into a plurality of sorbent heat exchangers 13a-c, while the vapor phase of the first fluid may then be returned to the first compressor 11.

[0067] An example of the plurality of reactors 16a-c housing the plurality of sorbent-coated heat exchangers 13a-c for capturing CO2 are depicted in FIG. 9. The reactors enclosing the sorbent heat exchangers may be vacuum chambers 104 comprising a plurality of walls 103 that form an enclosure, at least one frame 102 defining an opening of the vacuum chamber 104, at least one door 101 disposed in each frame 102, and at least one seal 105 for sealing the door 101 in the frame 102 when closed, wherein the doors 101 are actuatable for opening and closing the vacuum chambers 104. The reactors may also comprise a pump system (not shown) for evacuating air from the vacuum chamber 104 when the door 101 is closed. The pump system may be activated when the sorbent heat exchanger is to undergo desorption or cooling under vacuum.

[0068] A profile view of an exemplary MCHE 200 in accordance with the invention is shown in FIG. 10. At least one sorbent heat exchanger is housed inside each reactor comprised within the first heat pump 10 of the cascading heat pump system. In an exemplary embodiment, one sorbent heat exchanger (e.g., 13a, 13b, or 13c) can be housed within one reactor (e.g., 16a, 16b, or 16c). In another exemplary embodiment, two or more sorbent heat exchangers can be housed within one reactor. In some embodiments of the invention, the MCHE 200 comprises a manifold 230, aplurality of parallel tubes 220, and a plurality of fins 210. The plurality of parallel tubes 220 are connected at either longitudinal end to the manifold 230; the manifold 230 serving to hold the plurality of parallel tubes 220 in place, provide structural support, and contain inlet and outlet connections to the pipe network. In some embodiments of the invention, the plurality of fins 210 have a serpentine or sine wave shape and are disposed between, and connected to, adjacent parallel tubes 220. The plurality of fins 210 are directly joined to the plurality of parallel tubes 220 and each set of fins 210 is disposed between two adjacent parallel tubes 220. The plurality of fins 210 may be attached to the plurality of parallel tubes 220 via a brazing process or other suitable welding process. Sorbent material is applied directly on the surfaces of the plurality of fins 210 and plurality of parallel tubes 220 as a coating. The MCHE 200 is configured such that the first fluid flows through the manifold 230 into the plurality of parallel tubes 220 such that the first fluid is in thermal contact with the plurality of parallel tubes 220 and the plurality of fins 210. Because the first fluid flows within the plurality of parallel tubes 220 during operation of the first heat pump 10 (not shown in FIG. 10), and because the sorbent material is coated onto the surfaces of the plurality of parallel tubes 220 and the plurality of fins 210, each MCHE 200 thereby facilitates thermal contact between the first fluid and the sorbent material. The MCHE 200 conducts heat in a direction (either cooling or heating the sorbent material) according to the temperature differential between the first fluid and the MCHE 200.

[0069] The foregoing detailed description of the certain exemplary embodiments has been provided for the purpose of explaining the principles of the invention and its practical application, thereby enabling others skilled in the art to understand the invention for various embodiments and with various modifications as are suited to the particular use contemplated. This description is not necessarily intended to be exhaustive or to limit the invention to the precise embodiments disclosed. The specification describes specific examples of accomplishing a more general goal that also may be accomplished in another way. Those skilled in the art will appreciate that the features described above can be combined in various ways to form multiple variations of the invention. All of the references cited herein are incorporated by reference in their entirety.

[0070] Exemplary Embodiments of the Invention

[0071] El. A cascading heat pump system for removing carbon dioxide (CO2) from the atmosphere, the system comprising:a first heat pump, the first heat pump comprising: a plurality of reactors, each of the plurality of reactors enclosing: at least one sorbent heat exchanger, and a sorbent material capable of adsorbing CO2 from the atmosphere, wherein the sorbent material is applied or coated on at least one surface of each of the at least one sorbent heat exchanger; a pipe network having a plurality of pipes and control valves, wherein the pipe network fluidly connects the at least one sorbent heat exchanger of each of the plurality of reactors to at least one other sorbent heat exchanger of another of the plurality of reactors; a first compressor fluidly connected to the plurality of reactors via the pipe network; and a first expansion valve fluidly connected to the plurality of reactors via the pipe network; at least one second heat pump, each of the at least one second heat pump comprising: a pump heat exchanger in thermal contact with the pipe network of the first heat pump; a source heat exchanger fluidly connected to the pump heat exchanger; a second compressor fluidly connected between the pump heat exchanger and the source heat exchanger; and a second expansion valve fluidly connected between the pump heat exchanger and the source heat exchanger; a first fluid that flows and recirculates through the first heat pump; and a second fluid that flows and recirculates through the at least one second heat pump in a closed loop and is in periodic thermal contact with the first fluid via the pump heat exchanger.

[0072] E2. The cascading heat pump system of El, wherein the first fluid and the second fluid are refrigerants, and wherein the first fluid comprises a different chemical substance than the second fluid.

[0073] E3. The cascading heat pump system of E l, wherein the first fluid is a synthetic fluid or a natural fluid.

[0074] E4. The cascading heat pump system of El or E2, wherein the second fluid is a synthetic fluid or a natural fluid.

[0075] E5. The cascading heat pump system of any one of E1-E4, wherein each of the at least one sorbent heat exchangers is a microchannel heat exchanger that comprises a manifold, a plurality of parallel tubes, and a plurality of fins, wherein the plurality of parallel tubes areconnected at either end to the manifold which holds the plurality of parallel tubes in place, wherein the plurality of fins are disposed between, and connected to, adjacent parallel tubes, wherein the sorbent material is applied directly on the surfaces of the plurality of fins and the plurality of parallel tubes, and wherein first fluid flows through the manifold to the plurality of parallel tubes and is in thermal contact with the plurality of fins.

[0076] E6. The cascading heat pump system of any one of E1-E5, wherein the sorbent material is a coating on the surface of the at least one sorbent heat exchanger.

[0077] E7. The cascading heat pump system of any one of E1-E6, wherein the sorbent material is a solid amine-based sorbent.

[0078] E8. The cascading heat pump system of any one of E5-E7, wherein the plurality of parallel tubes and the plurality of fins of each of the microchannel heat exchangers are formed of aluminum, and wherein the plurality of parallel tubes is directly joined to the plurality of fins.

[0079] E9. The cascading heat pump system of any one of Ef-E8, wherein the plurality of control valves comprise a plurality of first control valves and a plurality of second control valves, wherein each of the plurality of first control valves and the plurality of second control valves comprise three ports, and wherein each of the three ports is selected from one of the group consisting of: a reactor-side port, a first pipe-side port, or a second pipe-side port, and wherein each port of the plurality of first control valves and the plurality of second control valves alternate between an open position and a closed position.

[0080] E10. The cascading heat pump system of E9, wherein the opening and closing of the ports of the plurality of first control valves and the plurality of second control valves selectively diverts the first fluid within the pipe network either: to a sorbent heat exchanger, from a sorbent heat exchanger, or bypassing a sorbent heat exchanger.

[0081] El l. The cascading heat pump system of any one of E1-E10, wherein an effective coefficient of performance of the cascading heat pump system is greater than 1.

[0082] E12. The cascading heat pump system of E9, wherein the plurality of first control valves controls flow of the first fluid into the at least one sorbent heat exchanger from an outlet of the first compressor and out of the at least one sorbent heat exchanger to an inlet of the pump heat exchanger.

[0083] El 3. The cascading heat pump system of E9, wherein the plurality of second control valves controls throughput flow of the first fluid to and from the first expansion valve.

[0084] El 4. The cascading heat pump system of any one of El -El 3, wherein each of the plurality of reactors are vacuum chambers comprising: a plurality of walls forming an enclosure; at least one frame defining an opening of the vacuum chamber; at least one door disposed in each frame, wherein the at least one door is actuatable for opening and closing the vacuum chamber; and a pump system for evacuating air from the vacuum chamber when the at least one door is closed.

[0085] E15. The cascading heat pump system of any one of E1-E14, wherein the pump heat exchanger raises the temperature of the first fluid prior to the first fluid entering the first compressor by transferring heat from the second fluid to the first fluid via the pump heat exchanger.

[0086] E16. The cascading heat pump system of any one of E1-E15, wherein the first compressor compresses the first fluid such that a first fluid output downstream from the first compressor exhibits a temperature between 80°C and 100°C.

[0087] E17. The cascading heat pump system of any one of E1-E16, wherein a boiling temperature of the second fluid is lower than an ambient temperature of outside air, and wherein a condensing temperature of the second fluid is warmer than a boiling temperature of the first fluid.

[0088] E18. The cascading heat pump system of any one of E1-E17, wherein the first expansion valve lowers the pressure of the first fluid such that a first fluid output downstream from the first expansion valve exhibits a temperature between 40°C and 50°C.

[0089] E19. The cascading heat pump system of E5, wherein the plurality of fins of each of the microchannel heat exchangers has a serpentine shape or a wave shape.

[0090] E20. The cascading heat pump system of any one of E1-E19, wherein each of the plurality of reactors undergoes one of three modes selected from: carbon dioxide adsorption, carbon dioxide desorption, or active cooling, wherein the first fluid flows via the pipe network from a sorbent heat exchanger undergoing active cooling to another sorbent heat exchanger undergoing carbon dioxide desorption, and wherein the first fluid transfers heat from warmer sorbent heat exchangers to colder sorbent heat exchangers.

[0091] E21. The cascading heat pump system of any one of E1-E20, wherein the first fluid is a hydrofl uorool efi n .

[0092] E22. The cascading heat pump system of any one of E1-E20, wherein the first fluid is butane or isobutane.

[0093] E23. The cascading heat pump system of any one of E1-E20 or E22, wherein the second fluid is a hydrofluoroolefm.

[0094] E24. The cascading heat pump system of any one of E1-E21, wherein the second fluid is propane.

Claims

What is claimed is:

1. A cascading heat pump system for removing carbon dioxide (CO2) from the atmosphere, the system comprising: a first heat pump, the first heat pump comprising: a plurality of reactors, each of the plurality of reactors enclosing: at least one sorbent heat exchanger, and a sorbent material capable of adsorbing CO2 from the atmosphere, wherein the sorbent material is applied or coated on at least one surface of each of the at least one sorbent heat exchanger; a pipe network having a plurality of pipes and control valves, wherein the pipe network fluidly connects the at least one sorbent heat exchanger of each of the plurality of reactors to at least one other sorbent heat exchanger of another of the plurality of reactors; a first compressor fluidly connected to the plurality of reactors via the pipe network; and a first expansion valve fluidly connected to the plurality of reactors via the pipe network; at least one second heat pump, each of the at least one second heat pump comprising: a pump heat exchanger in thermal contact with the pipe network of the first heat pump; a source heat exchanger fluidly connected to the pump heat exchanger; a second compressor fluidly connected between the pump heat exchanger and the source heat exchanger; and a second expansion valve fluidly connected between the pump heat exchanger and the source heat exchanger; a first fluid that flows and recirculates through the first heat pump; and a second fluid that flows and recirculates through the at least one second heat pump in a closed loop and is in periodic thermal contact with the first fluid via the pump heat exchanger.

2. The cascading heat pump system of claim 1, wherein the first fluid and the second fluid are refrigerants, andwherein the first fluid comprises a different chemical substance than the second fluid.

3. The cascading heat pump system of claim 2, wherein the first fluid is a synthetic fluid or a natural fluid.

4. The cascading heat pump system of claim 2, wherein the second fluid is a synthetic fluid or a natural fluid.

5. The cascading heat pump system of claim 1, wherein the each of the at least one sorbent heat exchangers is a microchannel heat exchanger that comprises a manifold, a plurality of parallel tubes, and a plurality of fins, wherein the plurality of parallel tubes are connected at either end to the manifold which holds the plurality of parallel tubes in place, wherein the plurality of fins are disposed between, and connected to, adjacent parallel tubes, wherein the sorbent material is applied directly on the surfaces of the plurality of fins and the plurality of parallel tubes, and, wherein first fluid flows through the manifold to the plurality of parallel tubes and is in thermal contact with the plurality of fins.

6. The cascading heat pump system of claim 5, wherein the sorbent material is a coating on the surface of the at least one sorbent heat exchanger.

7. The cascading heat pump system of claim 1, wherein the sorbent material is a solid amine-based sorbent.

8. The cascading heat pump system of claim 5, wherein the plurality of parallel tubes and the plurality of fins of each of the microchannel heat exchangers are formed of aluminum, and wherein the plurality of parallel tubes are directly joined to the plurality of fins.

9. The cascading heat pump system of claim 1, wherein the plurality of control valves comprise a plurality of first control valves and a plurality of second control valves, wherein each of the plurality of first control valves and the plurality of second control valves comprise three ports, and wherein each of the three ports is selected from one of the group consisting of: a reactorside port, a first pipe-side port, or a second pipe-side port, and wherein each port of the plurality of first control valves and the plurality of second control valves alternate between an open position and a closed position.

10. The cascading heat pump system of claim 9, wherein the opening and closing of the ports of the plurality of first control valves and the plurality of second control valves selectively diverts the first fluid within the pipe network either: to a sorbent heat exchanger, from a sorbent heat exchanger, or bypassing a sorbent heat exchanger.

11. The cascading heat pump system of any one of claims 1-10, wherein an effective coefficient of performance of the cascading heat pump system is greater than 1.

12. The cascading heat pump system of claim 9, wherein the plurality of first control valves controls flow of the first fluid into the at least one sorbent heat exchanger from an outlet of the first compressor and out of the at least one sorbent heat exchanger to an inlet of the pump heat exchanger.

13. The cascading heat pump system of claim 9, wherein the plurality of second control valves controls throughput flow of the first fluid to and from the first expansion valve.

14. The cascading heat pump system of any one of claims 1-10, wherein each of the plurality of reactors are vacuum chambers comprising: a plurality of walls forming an enclosure; at least one frame defining an opening of the vacuum chamber; at least one door disposed in each frame, wherein the at least one door is actuatable foropening and closing the vacuum chamber; and, a pump system for evacuating air from the vacuum chamber when the at least one door is closed.

15. The cascading heat pump system of any one of claims 1-10, wherein the pump heat exchanger raises the temperature of the first fluid prior to the first fluid entering the first compressor by transferring heat from the second fluid to the first fluid via the pump heat exchanger.

16. The cascading heat pump system of claim 15, wherein the first compressor compresses the first fluid such that a first fluid output downstream from the first compressor exhibits a temperature between 80°C and 100°C.

17. The cascading heat pump system of claim 15, wherein a boiling temperature of the second fluid is lower than an ambient temperature of outside air, and wherein a condensing temperature of the second fluid is warmer than a boiling temperature of the first fluid.

18. The cascading heat pump system of any one of claims 1-10, wherein the first expansion valve lowers the pressure of the first fluid such that a first fluid output downstream from the first expansion valve exhibits a temperature between 40°C and 50°C.

19. The cascading heat pump system of claim 5, wherein the plurality of fins of each of the microchannel heat exchangers has a serpentine shape or a wave shape.

20. The cascading heat pump system of any one of claims 1-10, wherein each of the plurality of reactors undergoes one of three modes selected from: carbon dioxide adsorption, carbon dioxide desorption, or active cooling,wherein the first fluid flows via the pipe network from a sorbent heat exchanger undergoing active cooling to another sorbent heat exchanger undergoing carbon dioxide desorption, and wherein the first fluid transfers heat from warmer sorbent heat exchangers to colder sorbent heat exchangers.

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