Removing carbon dioxide from ambient air

Abstract

There is provided a system for removing carbon dioxide from ambient air. The system comprises a solid sorbent (2) configured to capture carbon dioxide from the atmosphere and a regeneration unit (1). The solid sorbent (2) moves along a path through the regeneration unit (1) from an inlet (10) to an outlet (17). The regeneration unit (1) comprises a heat source to generate a temperature gradient. The path through the regeneration unit (1) comprising an ascending channel (11) in which the temperature increases and a descending channel (16) in which the temperature decreases. Heat from the solid sorbent (2) moving along the descending channel (16) flows through a heat conductor (19) to solid sorbent (2) moving though the ascending channel (11).

Description

[0001] Removing Carbon Dioxide from Ambient Air

[0002] Technical Field of the Invention

[0003] The present invention relates to a System, Method and Apparatus for removing carbon dioxide from ambient air, so-called “Direct Air Capture” (DAC).

[0004] Background to the Invention

[0005] The difficulty of cutting global emissions of CO2 has led to serious consideration of direct air capture (DAC) of CO2 from the atmosphere as a means to reduce climate change. A large number of methods are currently under consideration and a review of the current state of play is set out in Wang E, et al. (2024) Reviewing direct air capture startups and emerging technologies. Cell Rep.: Physical Science 5:101791.

[0006] One method of direct capture of carbon dioxide from ambient air is adsorption by solid sorbents. The most widely-studied solid sorbents are supported amines, in which organic compounds containing primary amine groups ( — NH2) are bonded to porous substrates. The chemical bond formed to CO2 is strong enough to capture CO2 from the atmosphere, yet weak enough that it can be released (thus regenerating the sorbent) through partial evacuation and heating to = 100° C.

[0007] It is widely estimated that for large-scale DAC to become a realistic economic prospect, a cost of capture below USDIOO / ton is required. There appear to be three primary challenges to reaching this target with solid sorbents: high energy demand, slow adsorption kinetics, and low sorbent durability. Climeworks, a company that has commercialized DAC with amine-functionalized solid sorbents, is pursuing, as of 2019, an energy consumption of around 300 kJ / mol (3). At that energy consumption, capturing all of humanity’s ~ 35 Gt of annual CO2 emissions would require ~ 3xl020J of energy per year: about three times the annual primary energy consumption of the United States. Slow kinetics add to the cost by increasing the amount of sorbent required to achieve a specified total capture rate. It is widely accepted that physisorbent materials offer faster kinetics than chemisorbent materials, but have generally worse selectivity between CO2 and H2O. An object of the invention is to reduce the energy demand for DAC with solid sorbents, to provide an improvement over current DAC systems, and / or to provide alternative systems, methods or apparatus for removing carbon dioxide from ambient air.

[0008] Summary of the Invention

[0009] According to a broad aspect of the invention, there is provided a system for removing carbon dioxide from ambient air. The system may comprise a solid sorbent configured to capture carbon dioxide from the atmosphere and may comprise a regeneration unit. The solid sorbent may be arranged to move through the regeneration unit along a path from an inlet to an outlet. The regeneration unit may comprise at least one heat source configured to generate a temperature gradient. The path through the regeneration unit may include an ascending channel. The path through the regeneration unit may include a descending channel. The ascending channel may be a channel from the inlet along which temperature increases. As such, the solid sorbent may climb a temperature gradient as it moves along the ascending channel. The descending channel may be a channel to the outlet, along which temperature decreases. As such, the solid sorbent may descend a temperature gradient as it moves along the descending channel. The ascending channel and the descending channel may be connected by a heat conductor. As such, heat from the solid sorbent moving along the descending channel may flow through the heat conductor to the solid sorbent in the ascending channel. The heat conductor may physically separate the ascending channel from the descending channel. The ascending channel and the descending channel may be physically separated. The ascending channel and the descending channel may be thermally coupled. The ascending channel and the descending channel may be thermally coupled by the heat conductor. The heat conductor may block the exchange of gas between the ascending channel and the descending channels.

[0010] According to a first aspect of the invention, there is provided a system for removing carbon dioxide from ambient air, the system comprising: a solid sorbent configured to capture carbon dioxide from the atmosphere and a regeneration unit; the solid sorbent arranged to move through the regeneration unit along a path from an inlet to an outlet; the regeneration unit comprising at least one heat source configured to generate a temperature gradient; the path through the regeneration unit including an ascending channel and a descending channel; the ascending channel being a channel from the inlet along which temperature increases, such that the solid sorbent climbs a temperature gradient as it moves along the ascending channel; the descending channel, being a channel to the outlet, along which temperature decreases, such that the solid sorbent descends a temperature gradient as it moves along the descending channel; wherein the ascending channel and the descending channel are connected by a heat conductor; whereby heat from the solid sorbent moving along the descending channel flows through the heat conductor to the solid sorbent in the ascending channel wherein the heat conductor physically separates the ascending channel from the descending channel, whereby the ascending channel and the descending channel are physically separated and thermally coupled by the heat conductor.

[0011] In such an arrangement sensible heat (that is heat that is associated with changes of temperatures rather than changes of state) flows from cooling solid sorbent in the descending channel to warming solid sorbent in the ascending channel. Since the specific heat capacitiesof solids are generally much larger than those of gases, this approach (as opposed to recovery of heated gases for example) can be very efficient. Whilst not previously used in relation to DAC (to the inventor’s knowledge), counterflow heat exchange is a widely-applied method to recover heat, and the prospect that it offers could greatly reduce the energy demand for DAC with solid sorbents, and, additionally, open a largely unexplored parameter space for materials discovery and sorbent design: modest capacity, but fast kinetics and high longevity.

[0012] The ascending channel may be provided with a sufficient cross section to carry gas that desorbs from the sorbent, and that will comprise the product gas stream. The cross section of the ascending channel may gradually increase to provide space for gas desorbing from the sorbent as it moves along the ascending channel. The increasing cross section of the ascending channel may be configured to match gas flow velocity to sorbent velocity (i.e. to approximately match gas flow velocity to sorbent velocity) along the ascending channel. The cross section of the ascending channel may be configured such that the gas flow velocity is greater than the sorbent velocity. The system may comprise sheets of solid sorbent, the sheets of solid sorbent moving along the path through the regeneration unit. The sheets of solid sorbent may be substantially planar (that is their thickness is at least 5 time, at least 10 times, at least 15 times, either the width or length of the sheet of solid sorbent).

[0013] Each sheet may have a thickness of at least 1mm, 2mm, 3mm, or 10mm. Each sheet may have a thickness of no more than 100mm, 50mm, 30mm or 20mm. For example, each sheet may have a thickness of between 1mm and 100mm, such as between 3mm and 50mm, or between 2mm and 30mm, e.g. 10mm to 20mm.

[0014] The sheets of solid sorbent may incorporate channels through which air can flow. The sheets of solid sorbent may each comprise a plurality of sorbent vanes. The vanes may be arranged perpendicular to the planar surface of the solid sorbent sheets. The vanes may be arranged perpendicular to an axis defined by a path of travel by the sheet of solid sorbent though the ascending and descending channels. The vanes could for example be a series of parallel plates, or form a honeycomb configuration. Channels may be provided between parallel plates. Channels may be provided through the honeycomb structure. Each vane may have a thickness of at least 0.05mm, at least 0.1mm, at least 0.5mm or at least 1mm. Each vane may have a thickness of no more than 5mm, no more than 2mm or no more than 1mm. For example, each vane may have a thickness of between 0.05mm and 5mm, or between 0.1 and 1mm.

[0015] The sheets of solid sorbent may incorporate a thermally conductive material to increase the rate of heat flow across the sorbent sheet thickness. The vanes may comprise a thermally conductive core, to which layers of sorbent are bonded. The core may for example be aluminium, copper or graphite.

[0016] The vanes may comprise end caps. The end caps may comprise a thermally conductive material (preferably at least 10 W- m-1■ K-1). The end caps may be thermally coupled to the thermally conductive core. The end caps may be in contact with the thermally conductive core. The end cap may be provided on an edge of the vane proximal to the heat conductor when the sheet of solid sorbent is in the ascending channel. The end cap may be provided on an edge of the vane proximal to the heat conductor when the sheet of solid sorbent is in the descending channel. The end cap may have high wear resistance (for example by being made of harder materials than the heat conductor. The end cap may be formed of beryllium copper. The end cap may be formed of a titanium alloy. The end caps may extend over the one or more edges of the sorbent layers. The end cap may be provided over an edge of the sorbent proximal to the heat conductor when the sheet of solid sorbent is in the ascending channel. The end cap may be provided over an edge of the solid sorbent proximal to the heat conductor when the sheet of solid sorbent is in the descending channel.

[0017] The sheets or vanes may be rigid.

[0018] Providing the sorbent as sheets which are relatively thin, allows for a relatively fast and efficient transfer of heat from sheets which are descending the descending channel to sheets that are ascending the ascending channel. This is further enhanced by the provision of a thermally conductive core and further still by the provision of a wider cap which increases the surface area for thermal transfer between the vanes and the heat conductor.

[0019] The sheets of solid sorbent may be compressible from an open configuration for absorption of carbon dioxide from ambient air to a compressed configuration for desorption. For example, where formed of parallel vanes, the parallel vanes may be mounted for movement between positions closer and further from each other, e.g. slidably mounted. The system may comprise a sorbent compressor, configured to compress the sheets of solid sorbent on entry into the regeneration unit, so as to reduce the amount of entrained air entering the regeneration unit. Reducing the amount of entrained air improves the purity of the desorbed carbon dioxide

[0020] Preferably, in the open configuration the channels are at least 0.1mm, at least 0.3mm or at least 0.45mm across. Preferably in the compressed configuration the vanes are no more than 0.1mm, no more than 0.05mm, or no more than 0.01mm across.

[0021] The heat conductor may be formed from a metal. The metal may be, or may comprise, aluminium or copper. The heat conductor may have a thermal conductivity of at least 100 W m-1K-1, preferably at least 200 W m-1K-1and more preferably at least 400 W m-1K-1-. Alternatively, where the sheets of solid sorbent comprise magnets, the heat conductor may have a thermal conductivity of at least 30 W m-1K-1. The use of magnets will increase the rate of heat transfer between the sheets of solid sorbent and the heat conductor, thereby reducing the efficiency requirement of the heat transfer through the heat conductor.

[0022] The solid sorbent ascending the ascending channel and the solid sorbent descending the descending channel may be configured to be tightly thermally coupled to the heat conductor. The solid sorbent ascending the ascending channel and the solid sorbent descending the descending channel may be separated from the heat conductor by no more than 0.1mm along the majority of the length of the ascending and descending channels. More preferably, the solid sorbent ascending the ascending channel and the solid sorbent descending the descending channel may be arranged in mechanical contact with the heat conductor along the majority of the length of the ascending and descending channels.

[0023] The sheets of solid sorbent may comprise magnets. The magnets may impart a contact force, urging the sheets of solid sorbent against the heat conductor. Additionally, magnets are a convenient way to have a distributed contact force, allowing the sheets of solid sorbent to conform to any smooth undulations of the heat conductor surface that may be present. Where the sheets of solid sorbent comprise magnets, the heat conductor may be formed of, or comprise a magnetic element, for example steel.

[0024] The heat conductor may be a heat conducting panel. The heat conducting panel may extend along the majority of the length of the ascending channel and the descending channel.

[0025] Such an arrangement is a very straightforward way for heat to pass from the solid sorbent in the descending channel to a corresponding point in the ascending channel.

[0026] The regeneration unit may comprise an outflow channel, the outflow channel in fluid communication with the ascending channel at or near the top of the temperature gradient. Desorbed product gas may flow out from the ascending channel via the outflow channel. The “product gas,” in the outflow channel, may be mostly CO2, but may also include N2, O2, and H2O.

[0027] The regeneration unit may comprise a constriction in the ascending channel of the path after the outflow channel (preferably immediately after the outflow channel). The constriction may restrict the amount of desorbed gas that passes through the constriction with the sorbent into the descending channel. As such, gas which desorbs as the sorbent moves along the ascending channel (preferably including most of the CO2 which desorbs from the sorbent flows along the ascending channel (e.g. at least 50%, 60%, 70%, 80%, 90% or 95% of the CO2 which desorbs from the sorbent)), flows from the ascending channel to the outflow channel.

[0028] The cross section of the constriction may be no more than 1mm, no more than 500 micrometres, no more than 200 micrometres, no more than 100 micrometres or no more than 50 micrometres wider and / or taller than the sorbent. The cross section of the constriction may be at least 5 micrometres, 10 micrometres or 20 micrometres wider and / or taller than the sorbent. For example, the cross section of the constriction may be 20-100 micrometres wider and 20-100 micrometres taller than the sorbent.

[0029] The cross section of the channel may abruptly increase downstream of the constriction. Alternatively it may gradually increase downstream of the constriction. The latter arrangement may offer better gas fluid dynamics, whilst the former may be simpler to construct.

[0030] One or more openings may be provided between the ascending channel and the descending channel part- way along the temperature gradient to divert initially desorbed gas away from the ascending channel. The initially desorbed gas may include nitrogen and oxygen, which are typically more weakly bound to the sorbent and thus likely to desorb at a lower average temperature than carbon dioxide and water. This can increase the concentration of carbon dioxide in the desorbed gas in the ascending channel because the initially desorbed gas is likely to comprise mostly nitrogen and oxygen. Where the heat conductor separates the ascending channel from the descending channel, the opening may be provided in the heat conductor. The regeneration unit may comprise a constriction in the ascending channel of the path after the or each opening. The constriction may be arranged immediately after the or each opening. The constriction may restrict the initially desorbed gas, including desorbed nitrogen and oxygen from passing through the constriction with the sorbent, such that the initially desorbed gas, including desorbed nitrogen and oxygen is diverted through the or each respective opening.

[0031] The cross section of the constriction may be no more than 1 millimetre, no more than 500 micrometres, no more than 200 micrometres, no more than 100 micrometres or no more than 50 micrometres wider and / or taller than the sorbent. The cross section of the constriction may be at least 5 micrometres, 10 micrometres or 20 micrometres wider and / or taller than the sorbent. For example, the cross section of the constriction may be 20-100 micrometres wider and 20-100 micrometres taller than the sorbent.

[0032] The cross section of the channel may abruptly increase downstream of the constriction. Alternatively it may gradually increase downstream of the constriction. The latter arrangement may offer better gas fluid dynamics, whilst the former may be simpler to construct.

[0033] A gas-permeable screen may be provided between the ascending channel and the descending channel part-way along the temperature gradient to divert initially desorbed gas away from the ascending channel. The initially desorbed gas may include desorbed nitrogen and oxygen, which desorb at a lower temperature than carbon dioxide and water. This diversion can increase the concentration of carbon dioxide in the desorbed gas in the ascending channel, because the initially desorbed gas is likely to comprise mostly nitrogen and oxygen. Where the ascending channel and the descending channel are separated by the heat conductor, the gas permeable screen may be provided in the heat conductor.

[0034] The gas permeable screen may be a mesh or foam. For example, the gas permeable screen may be wire sponge, e.g. steel wool. The gas permeable screen may be a (smooth) sheet having two surfaces. There may be an array of holes between the two surfaces. The regeneration unit may comprise a constriction in the ascending channel of the path in the region of the gas permeable screen. As such, initially desorbed gas, including nitrogen and oxygen may be forced to pass through the gas permeable screen.

[0035] The cross section of the constriction may be no more than 1 millimetre, no more than 500 micrometres, no more than 200 micrometres, no more than 100 micrometres or no more than 50 micrometres wider and / or taller than the sorbent. The cross section of the constriction may be at least 5 micrometres, 10 micrometres or 20 micrometres wider and / or taller than the sorbent. For example, the cross section of the constriction may be 20-100 micrometres wider and 20-100 micrometres taller than the sorbent.

[0036] The cross section of the channel may abruptly increase downstream of the constriction. Alternatively it may gradually increase downstream of the constriction. The latter arrangement may offer better gas fluid dynamics, whilst the former may be simpler to construct.

[0037] The regeneration unit may comprise a constriction at the inlet to the ascending channel of the path. The constriction may restrict the amount of ambient air entering the regeneration unit with the sorbent.

[0038] The cross section of the constriction may be no more than 200 micrometers, no more than 100 micrometers or no more than 50 micrometers wider and / or taller than the sorbent. The cross section of the constriction may be at least 5 micrometers, 10 micrometers or 20 micrometers wider and / or taller than the sorbent. For example, the cross section of the constriction may be 20-100 micrometres wider and 20-100 micrometres taller than the sorbent.

[0039] The cross section of the channel may abruptly increase downstream of the constriction. Alternatively it may gradually increase downstream of the constriction. The latter arrangement may offer better gas fluid dynamics, whilst the former may be simpler to construct.

[0040] The regeneration unit may comprise a replenishment channel in fluid communication with the descending channel. The replenishment channel may be in fluid connection with the descending channel at or near the top of the temperature gradient. Ambient air may be provided to the sorbent as it moves along the descending channel from the replenishment channel. Accordingly, at least nitrogen and oxygen from the ambient air that passes through the replenishment channel may be adsorbed into the sorbent. This may generate heat which may flow through the heat conductor to the sorbent in the ascending channel.

[0041] The descending channel may be configured to allow ambient air to be provided to the sorbent. The descending channel may be configured to allow ambient air to flow from a descending channel outlet to, or near to, the top of the temperature gradient.

[0042] The regeneration unit may comprise a constriction at the outlet from the descending channel of the path. The constriction may restrict the amount of air that is drawn through the replenishment channel. This restriction in air flow (without which the motion of the sorbent could draw more air into the replenishment channel and then out through the outlet) can reduce demands on the counterflow heat exchange (i.e. ensure that more sensible heat is transferred from the sorbent in the descending channel to the sorbent in the ascending channel and less is lost to the environment)

[0043] The cross section of the constriction may be no more than 1 millimetre, no more than 500 micrometres, no more than 200 micrometres, no more than 100 micrometres or no more than 50 micrometres wider and / or taller than the sorbent. The cross section of the constriction may be at least 5 micrometres, 10 micrometres or 20 micrometres wider and / or taller than the sorbent. For example, the cross section of the constriction may be 20-100 micrometres wider and 20-100 micrometres taller than the sorbent.

[0044] The ascending channel may be maintained at a reduced pressure (that is, a pressure below atmospheric pressure). The ascending channel may be maintained at a pressure of less than 1 atm, preferably less than 0.75 atm, more preferably less than 0.5 atm, for example, less than 0.25 atm, such as less than 0.1 atm. The descending channel may be maintained at a reduced pressure (that is, a pressure below atmospheric pressure). The descending channel may be maintained at a pressure of less than 1 atm, preferably less than 0.75 atm, more preferably less than 0.5 atm, for example, less than 0.25 atm, such as less than 0.1 atm.

[0045] An ascending channel inlet may comprise an airlock. A descending channel outlet may comprise an airlock. The regeneration unit may comprise a parallel heat flow channel. The ascending channel may comprise a parallel heat flow channel. The parallel heat flow channel may be configured to regulate the regulate the temperature of the ascending and / or descending channels. The parallel heat flow channel may extend from the top of the temperature gradient towards the bottom of the temperature gradient. The parallel heat flow channel may be comprised of a solid thermally conductive material (for example copper). The parallel heat flow channel may comprise a bar. The cross-section of the bar may decrease as the bar extends down the temperature gradient. The cross-section of the bar may be varied along the temperature gradient in order to regulate the amount of heat that flows along this parallel channel. The parallel heat flow channel may comprise a tubular element with a circulating fluid.

[0046] According to another aspect of the invention, there is provided a method of removing carbon dioxide from ambient air. The method may comprise absorbing carbon dioxide from ambient air onto a solid sorbent and passing the solid sorbent though a regeneration unit. The method may comprise the solid sorbent moving through the regeneration unit along a path from an inlet to an outlet along a temperature gradient. The method may comprise the solid sorbent passing (preferably first) along an ascending channel being a channel from the inlet along which temperature increases. As such, the sorbent may climb a temperature gradient as it moves along the ascending channel. The method may comprise the solid sorbent (preferably subsequently) passing along a descending channel, being a channel to the outlet, along which temperature decreases. As such, the sorbent may descend a temperature gradient as it moves along the descending channel. The ascending channel and the descending channel may be connected by a heat conductor (preferably the ascending channel and the descending channel are physically separated and thermally coupled by the heat conductor). As such, heat from the solid sorbent moving along the descending channel may flow through the heat conductor to the sorbent in the ascending channel.

[0047] According to a second aspect of the invention, there is provided a method of removing carbon dioxide from ambient air, the method comprising: absorbing carbon dioxide from ambient air onto a solid sorbent and passing the solid sorbent though a regeneration unit; wherein the solid sorbent moves through the regeneration unit along a path from an inlet to an outlet along a temperature gradient; the solid sorbent passing first along an ascending channel being a channel from the inlet along which temperature increases, such that the sorbent climbs a temperature gradient as it moves along the ascending channel; the solid sorbent then passing along a descending channel, being a channel to the outlet, along which temperature decreases, such that the sorbent descends a temperature gradient as it moves along the descending channel; wherein the ascending channel and the descending channel are connected by a heat conductor; whereby heat from the solid sorbent moving along the descending channel flows through the heat conductor to the sorbent in the ascending channel.

[0048] The method may be a method of operating the system of the first aspect of the invention. Any optional features described above in relation to the first aspect of the invention may optionally be included in the second aspect of the invention. Likewise, the optional aspect of the second aspect set out below may be included in the system of the first aspect of the invention.

[0049] Preferably gas including carbon dioxide desorbs from the sorbent as it moves along the ascending channel and desorbed carbon dioxide flows to the top of the temperature gradient, to reduce the partial pressure of water as it desorbs. This encourages desorption of water improving regeneration of the sorbent.

[0050] According to another embodiment of the invention, there is provided a regeneration unit for removing adsorbed carbon dioxide from a sorbent. The regeneration unit may comprising a path from an inlet to an outlet. The regeneration unit may comprise at least one heat source which generates a temperature gradient. The path through the regeneration unit may include an ascending channel. The path through the regeneration unit may include a descending channel. The ascending channel may be a channel from the inlet along which temperature increases. As such, the sorbent may climb a temperature gradient as it moves along the ascending channel. The descending channel may be a channel to the outlet, along which temperature decreases. As such, the sorbent may descend a temperature gradient as it moves along the descending channel. The ascending channel and the descending channel may be connected by a heat conductor. As such, heat from the solid sorbent moving along the descending channel may flow through the heat conductor to the sorbent in the ascending channel.

[0051] According to a third aspect of the invention there is provided a regeneration unit for removing adsorbed carbon dioxide from a sorbent, the regeneration unit comprising: a path from an inlet to an outlet; and at least one heat source which generates a temperature gradient; the path through the regeneration unit including an ascending channel and a descending channel; the ascending channel being a channel from the inlet along which temperature increases, such that the sorbent climbs a temperature gradient as it moves along the ascending channel; the descending channel, being a channel to the outlet, along which temperature decreases, such that the sorbent descends a temperature gradient as it moves along the descending channel; wherein the ascending channel and the descending channel are connected by a heat conductor; whereby heat from the solid sorbent moving along the descending channel flows through the heat conductor to the sorbent in the ascending channel.

[0052] The apparatus may be a regeneration unit for use in the system of the first aspect of the invention. Any optional features described above in relation to the first aspect of the invention (or the second aspect of the invention) may optionally be included in the third aspect of the invention. Likewise, the optional aspect of the third aspect set out below may be included in the system of the first aspect of the invention (or the method of the second aspect).

[0053] The apparatus may further comprise a track configured to transport the sorbent along the path from the inlet to the outlet. The track may for example be a conveyer belt.

[0054] In another aspect of the invention there is provided a sheet of solid sorbent for removing carbon dioxide from ambient air. The sheet of solid sorbent may be compressible from an open configuration for absorption of carbon dioxide from ambient air to a compressed configuration for desorption.

[0055] According to a fourth aspect of the invention, there is provided a sheet of solid sorbent for removing carbon dioxide from ambient air, the sheet of solid sorbent being compressible from an open configuration for absorption of carbon dioxide from ambient air to a compressed configuration for desorption.

[0056] The sheet of solid sorbent according to the fourth aspect of the invention may be for use in the system of the first aspect of the invention. Any optional features of the sheet described in relation to the first aspect of the invention may optionally be included in the fourth aspect of the invention.

[0057] Detailed Description of the Invention

[0058] In order that the invention may be more clearly understood one or more embodiments thereof will now be described, by way of example only, with reference to the accompanying drawings, of which:

[0059] Figure 1 is a diagram of a system according to the invention, including a regenerator unit and solid sorbent;

[0060] Figure 2 shows an isometric view of a sorbent sheet of the system of figure 1 in a first configuration for adsorption;

[0061] Figure 3 shows an isometric view of sorbent sheets of the system of figure 1 in a second configuration for desorption; and

[0062] Figure 4 shows a cross section through a system according to a second embodiment of the invention;

[0063] Figure 5 shows a cross section through an alternative to the system according to the first embodiment; and

[0064] Figure 6 shows a cross section through a system according to a third embodiment of the invention.

[0065] With reference to Figure 1, a system for removing carbon dioxide from ambient air is shown. The system includes a regeneration unit 1 and a series of solid sorbent sheets 2, which adsorb carbon dioxide (and other gases) from ambient air, and desorb the carbon dioxide (and other gases) in the regeneration unit 1. The system may comprise a further adsorber unit (not shown), arranged to filter ambient air and pass it across the sorbent sheets 2. An exemplary adsorber unit is disclosed in WO2023 / 230353A1.

[0066] In the embodiment of the invention, the solid sorbent is provided in thin sheets 2 to allow rapid thermalisation, although those skilled in the art may envisage alternative formats. In the present example, as best seen in figures 2 and 3, each sheet 2 is formed of a series of vanes 3. In this embodiment the sheets 2 are formed of a series of parallel vanes 3 which are mounted for movement between a first configuration shown in figure 2 and a second configuration shown in figure 3. Each sheet has a thickness (d) of between 10-20mm, so each vane has a width (d) of 10- 20mm. Each vane has a thickness of 0.1 -1mm and is made up of a solid thermally conductive core 4 of e.g. aluminium, copper or graphite of thickness ai with a layer 5 of solid sorbent of thickness a2 bonded on either side.

[0067] In the first configuration, the sorbent vanes 3 are arranged for adsorption, in an open configuration with channels having a relatively large gap aj between each vane 3. This allows a relatively large amount of air to pass through the space between the vanes 3, and therefore to a relatively rapid adsorption of carbon dioxide from ambient air. In the second configuration, the sorbent vanes 3 are arranged for desorption in a compressed configuration, with the distance across the channels reduced, so there is a relatively small gap between each vane 3. This compressed configuration is adopted on, or prior to, entry into the regeneration unit 1, in order to reduce the amount of entrained air, between the vanes, that enters the regeneration unit 1. In order to move between the two configurations, the vanes 3 may be slidably mounted, for example. Alternatively, pivoting arrangements could be envisaged or even provision of sorbent on a compressible, sponge like material which can be compressed by squeezing.

[0068] As a further alternative, the sorbent sheets 2 may not be compressible, and may instead maintain the same configuration, with the same distance across the channels both in adsorption and desorption. The latter approach may be less efficient, but could result in cheaper and / or more robust sorbent sheets, and the possibility of using geometry that may be less well suited for compression, e.g. honeycomb. As illustrated in figure 3, the vanes 3 may include one or more thermally conductive caps 6 extending from the core 4 of each vane 3, so as to enhance heat transfer to / from the sheets 2. The thermally conductive caps may for example be formed from the same material as the core 4. Alternatively, the thermally conductive caps may be formed from may wear resistant materials, that is materials which resists wear when rubbing against other components in the regeneration unit 1, for example beryllium copper or a titanium alloy.

[0069] In some embodiments, the sorbent sheets 2 may be provided with one or more magnets 7 arranged in, or proximate to the thermally conductive caps 6.

[0070] The solid sorbent may be any suitable sorbent. The inventors have modelled a system according to the invention employing zeolite 13X as the sorbent. Zeolite 13X was selected because of the large quantity of available data on this compound. However, unless there is a preceding dessicant stage, the high affinity of zeolites for H2O would limit their application to DAC at ambient temperatures of = -60°C or below — Antarctic conditions. A counterflow setup could also be applied with supported amines, and the design implications of the slow adsorption rate are discussed below. In the inventor’s opinion, based on present knowledge the most promising route for economical DAC is using the counterflow system of the invention operating in cold but not Antarctic conditions of -20 to 0 °C using new physisorbent materials.

[0071] Referring once again figure 1, which illustrates the system of the invention. Once the loading of carbon dioxide is at or near its maximum following adsorption, e.g. by blowing of ambient air 28 through the channels in the thin sheets 2 of solid sorbent, the sheets 2 of solid sorbent are delivered (e.g. by tracks or a conveyer belt, not shown) to an inlet 10 of the regeneration unit 1 or “counterflow machinery”. The inlet is opens to an ascending channel 11, which is a channel along which temperature ascends, from the inlet 10. Note that the terms ascending and descending refer to temperature, not height, unless the context provides otherwise. The regeneration unit 1 is provided with one or more heat source (not shown) in order to form a temperature gradient represented in figure 1 by the expression temperature gradient which is lowest at the inlet and highest at or near a fluid connection between ascending channel 11 and an outflow channel 13, at the opposite end of the ascending channel 11 from the inlet 10. As the sheets 2 move along the ascending channel 11 of the counterflow machinery, they climb the temperature gradient and gas desorbs. Constrictions — openings just larger than the sorbent sheets — guide the flow of the desorbed gas. At the top of the temperature gradient, desorbed gas is diverted into the outflow channel 13. It is advantageous not to divert desorbed CO2 before it reaches the top of the gradient, because it is likely that the last gas species to desorb from the sorbent will be H2O, and the presence of a large amount of CO2 gas will reduce the partial pressure of H2O and encourage its desorption.

[0072] As H2O in the outflow channel 13 cools, it can be expected to condense. To prevent condensation of H2O on the walls of the outflow channel 13, a portion of the product stream (after separation of the H2O) might be re-directed back into the outflow channel to provide a dry boundary layer 14 as a barrier to condensing H2O.

[0073] At the top of the temperature gradient 12, the sorbent sheets 2 move from the ascending channel 11 to a descending channel 16. The descending channel 16, is a channel from the top of the temperature gradient to an outlet 17. As the name suggests, temperature descends along the descending channel 16 to the outlet 17.

[0074] Notably, the ascending and descending channels are physically separated by a heat conductor 19. The heat conductor 19 may for example be a panel of heat conducting material, such as copper (which has a thermal conductivity in excess of 400 W m-1K-1) or aluminium. Where the solid sorbent sheets 2 comprise magnets 7, the heat conductor 19 may instead be formed of or comprise a magnetic material, for example steel. As the sorbent sheets 2 pass along the ascending channel (up the temperature gradient) and then along the descending channel (down the temperature gradient), they maintain close thermal contact with the heat conductor 19 (i.e. preferably physical contact, or less preferably separation by only a very small air gap (as) e.g. less than 0.1mm). As such, sensible heat can transfer from the sorbent sheets 2 descending the temperature gradient in the descending channel 16 to sorbent sheets 2 ascending the temperature gradient in the ascending channel 11. This thermal contact between descending and ascending sheets results in a counterflow of heat, in the opposite direction to the flow of sorbent sheets 2 (and gas), which can lead to much greater efficiency of the system, owing to reduced thermal demand from the heat source.

[0075] Where present, the magnets 7 may be used to enhance the thermal contact between the sorbent sheets 2 and the heat conductor 19. The magnets 7 are attracted to the heat conductor 19 (or where this is made of a non-magnetic material, a co-located magnetic element, this urged the sorbent sheets 2 towards the heat conductor 19 such that the air gap (as) is further reduced, or the thermally conductive cap 6 bears against the heat conductor 19 (at least within the bounds of manufacturing tolerances).

[0076] In this embodiment of the invention, a replenishment channel 18 supplies ambient air to the sheets as they enter the descending channel 16. The purpose of this flow is to maintain the pressure within the counterflow system at atmospheric pressure: the descending sheets 2, having a very low CO2 loading, and will have a higher affinity for N2 and O2 than the ascending sheets 2, and if no replenishment gas is supplied the pressure in the descending channel 16 would fall. Usefully, the heat generated by adsorption of N2 and O2 flows into the ascending channel 11 via the heat conductor 19, reducing energy demand.

[0077] In other embodiments, such as that shown in figure 5, the descending channel 16 may be configured to provide ambient air to the sorbent sheets 2 as they enter the descending channel 16. This may be achieved by providing a gap 18a between the sorbent sheets and an opposing wall 19a on the opposite side of the descending channel 16 to the heat conductor 19.

[0078] An optional feature of the regeneration unit 1 illustrated in figure 1 is a diversion at comparatively low temperature from the ascending channel 11 to the descending channel 16. In the ascending channel 11, it is likely that the gas that desorbs at the lower-temperature end of the temperature gradient will have a high concentration of N2 and O2, so to raise the concentration / purity of CO2 in the product stream it is helpful to divert this initially-desorbed gas.

[0079] With sensible heat recovery, regardless of the sorbent it will almost certainly be more energy-efficient to drive desorption through a temperature swing alone, rather than a combined temperature and vacuum swing, because the total energy consumption depends only weakly on the temperature at the top of the gradient.

[0080] Accordingly, in the setup illustrated in Figure 1, the entire system operates at essentially atmospheric pressure, so that gas flows can be directed with simple constrictions; the cross section of each constriction may be 20-500 micrometres wider and 20-500 micrometres taller than the sorbent sheets 2. Obviously the closer the constrictions are, the better, subject to tolerances, which should be sufficiently large to avoid restricting movement of the sheets through the constrictions.

[0081] The first constriction 20 that the sorbent sheets 2 meet is arranged at the inlet 10. This constriction serves to restrict the amount of ambient air that enters the system with the sorbent sheets. The second constriction 21 is provided part-way along the ascending channel 11 immediately after an opening 22 in the heat conductor, between the ascending channel 11 and the descending channel 16, through which initially desorbed gas 23, which desorbs at a comparatively low temperature (with a correspondingly high concentration of Nitrogen and Oxygen) is diverted from the ascending channel 11 to the descending channel 16.

[0082] Precisely where to arrange this second constriction 21 and the corresponding opening 22 along the temperature gradient will be a matter of selection for those skilled in the art, balancing purity (which would tend towards a later diversion) against yield (which would tend towards an earlier diversion). If the sorbent has low affinity for N2 and O2, constriction 21 and opening 22 may be omitted.

[0083] In the embodiment shown in figure 1, there is an abrupt enlargement of the channel after the first constriction 19 and second constriction 21. In general it is important to ensure that the ascending channel 11 has a sufficient cross section to carry gas that desorbs from the sorbent sheets 2, and that will comprise the product gas stream. In one embodiment (not shown) the cross section of the ascending channel may gradually increase to provide space for gas desorbing from the sorbent as it moves along the ascending channel. The increasing cross section of the ascending channel may be configured to match gas flow velocity to sorbent sheet 2 velocity along the ascending channel. The third constriction 24 is arranged immediately after the opening to the outflow channel 13 at the top of the temperature gradient, such that the sorbent sheets 2, from which product gas 25 has desorbed continue through the constriction, whilst product gas 25, which is rich in carbon dioxide is diverted to the outflow channel 13.

[0084] In the embodiment of figure 1, there are no constrictions in the descending channel 16 (where species from ambient air 26 from the replenishment channel 18 will be adsorbed rather than desorbed), until the outlet 17. A fourth constriction 27 is provided at the outlet 17; this constriction 27 will restrict the amount of air that is drawn through the replenishment channel 18. Without the restriction 27, the motion of the sorbent sheets 2 could draw more air into the replenishment channel and then out through the outlet, and this added flow could increase demands on the counterflow heat exchange. In an alternative, less preferred embodiment, shown in figure 5 and described below, the regeneration unit 1 could be provided without a constriction 27 on the outlet, and without a replenishment line 18. The descending channel 16 could instead have a large enough cross-section that ambient air / gas can flow upward while the sorbent 2 moves downward. That is, the replenishment flow could occur through the descending channel 16.

[0085] In the illustrated embodiment of figure 1, product gas descends the temperature gradient in the outflow channel to a separation region 29 where water is separated from the carbon dioxide product stream, by suitable techniques known to those skilled in the art, and water 30 flows along one conduit 31, whilst the carbon dioxide product stream 32 flows along another conduit 33, after which it may be liquified, or otherwise sequestered.

[0086] For a counterflow setup to be effective, the residence time of the sorbent sheets 2 in the counterflow setup of the regeneration unit 1, where heat passes from sheets 2 in the descending channel 16 to the ascending channel 11 should be much longer than the thermalization time across the sorbent sheet 2. The inventors have modelled scenarios taking into account heat capacity of the sheets 2, thickness and thermal conductivity and determined that with thin sheets of sorbent 2, the residence time in the regeneration unit 1 would be commercially tolerable. Figure 4 shows a second embodiment of the invention, which is a modification of the apparatus of figure 1. The system of the second embodiment of the invention is identical except that the aperture 22 and second constriction 21 are not provided. Instead, in order to divert initially desorbing gas, the ascending channel has a gas- permeable screen 34 between the ascending channel and the descending channel partway along the temperature gradient. The gas permeable screen 34 may be a mesh or foam. For example, the gas permeable screen may be wire sponge, e.g. steel wool, or a planar sheet with a plurality of holes extending through. It is designed to allow desorbing gas to pass through it from the ascending channel to the descending channel 16, whilst allowing sensible heat to pass through it from the descending channel 16 to the ascending channel.

[0087] The screen 34 provides enough of a barrier to gas flow that mixing between the gas in the ascending and descending channels is prevented. In order to ensure that desorbing gas passes through the screen 34 and does not simply remain in the ascending channel 11, the regeneration unit 1 includes a long constriction 35 in the ascending channel 11 of the path through which the sorbent sheets 2 flow in the region of the gas permeable screen 34, such that initially desorbed gas, including nitrogen and oxygen is forced to pass through the gas permeable screen 34 into the descending channel. After the screen 34, the ascending channel 11 opens up to create an air channel, so that gas that desorbs at that point flows up the temperature gradient. After the screen 34, the barrier 19 between the ascending and descending channels becomes an airtight membrane once again, exactly as described with reference to figure 1 after the second constriction 21.

[0088] It will be appreciated by those skilled in the art that the figures show large gaps around the sorbent sheets 2, e.g. between the sheets 2 and the heat conductor 19 as well as between leading and following sheets 2 in the regeneration unit. This, of course, is merely for ease of illustration and ease of understanding on the part of the reader; in practice, sheets will enter the apparatus in close succession optionally in physical contact and remain close to (or touching) one another, and the heat conductor as they pass through the counterflow machinery. In some cases, the sorbent sheets may be separated between the descending channel outlet 17 and a distal end of conveyor carrying the sorbent sheets 2. In this portion of the sorbent sheet’ s cycle they may move at increased speed to prevent ambient airflow from passing through both sheets.

[0089] In use, sorbent sheets 2 first absorb carbon dioxide (and most likely some other species too) from ambient air. Many sorbent sheets 2 may be provided, such that at any one time, most of them are outside the regeneration unit absorbing carbon dioxide, rather than inside it - for example, there could be ten times as many sheets 2 outside as inside, and they could spend more than 30 minutes, or more than an hour outside the regeneration unit 1, absorbing carbon dioxide. Having entered the regeneration unit 1, the sheets 2 then move along a track into the regeneration unit, passing through the first constriction 20 as they enter the inlet and start to heat up, as they move along the ascending channel 11. If the sorbent sheets are compressible, they are compressed on or before entry. The increase in heat as they ascend the ascending channel 11 causes gases (mostly nitrogen and oxygen) to desorb and these gases 23 pass through to the descending channel via the opening 22 in the first embodiment, or the screen 34 in the second embodiment. As the sorbent sheets 2 continue up the temperature gradient in the ascending channel 11, gas continues to desorb, now a greater proportion of carbon dioxide, and towards the top of the temperature gradient, water (in a gaseous state). The sorbent sheets then pass through a third constriction 24, whilst desorbed product gas 25 passes into the outflow channel 13. In the described embodiments, all the way along the ascending channel, the sorbent sheets are in close thermal contact with the thermally conducting panel 19, and therefore absorb heat not only from the heat source which is heating the channel 11, but from solid sorbent sheets 2 that are counter-flowing down the descending channel 16. Having reached the top of the temperature gradient, sheets begin to flow down the descending channel, absorbing some species from ambient air 26 from the replenishment channel 18 and transferring heat to colder sorbent sheets 2 on the opposite side of the heat conductor 19 that are ascending the ascending channel 11. The sheets move relatively slowly through the regeneration unit 1, they may for example reside in the regeneration unit 1 for about 5 minutes or more and thus (a) sheets in the ascending channel have sufficient time to desorb most or all of the adsorbed carbon dioxide whilst moving along the channel and (b) sheets in the descending channel have sufficient time to transfer most or all of their elevated heat to sheets in the ascending channel. As such, there is a counterflow of heat, in the opposite direction to that of the sorbent sheets 2 (that is the sorbent sheets move from the ascending channel to the descending channel whereas heat moves from the descending channel to the ascending channel, which results in a more efficient system.

[0090] In the above described embodiments desorption relies upon a temperature swing to remove carbon dioxide. Figure 6 shows a third embodiment which is a modification of the first embodiment. In this embodiment a pressure differential is used to enhance the desorption process. The system of the third embodiment of the invention is identical to the first embodiment except that the ascending channel inlet 10 and descending channel outlet 17 are provided with airlocks 110,117. Both air locks have substantially the same structure and as such, only one will be described in detail. In further embodiments it is also envisioned that a single airlock may be provided with entering and exiting sorbent sheets alternating (thereby avoiding pressure cycles with no sorbent sheet in the airlock.

[0091] The airlocks may also be configured to act as constrictions, removing the need for separate constrictions 20.

[0092] The inlet airlock 110 comprises a chamber 111 with a first airlock door 112 and an opposite second airlock door 113. The airlock chamber 111 has substantially the same profile as the ascending channel 11. The first airlock door 112 provided communication between the airlock chamber 111 and the exterior of the regeneration unit 1 and the second airlock door 113 provides communication between the airlock chamber 111 and the ascending channel 11. The airlock chamber 111 is also in fluid communication with a pump (not shown) operable to reduce the pressure in the airlock chamber 111.

[0093] In order to maintain a lower pressure in the ascending 11 and descending 16 channels, the replenishment channel 18 is provided with a pressure regulator 50 with delivers replenishment air at the operating pressure of the regenerating unit 1.

[0094] In use, the third embodiment is again identical to the first embodiment, expect that the ascending 11 and descending channels 16 are held at a pressure of approximately 0.1 atmospheres, this allows desorption of the CO2 to occur at lower temperatures, thereby reducing the amount of energy required to heat the sorbent sheets 2. To maintain this reduced pressure while introducing the sorbent sheets 2 into the regeneration unit 1 the sorbent sheets enter via the inlet airlock 110. First, if not already closed the second airlock door 113 is closed isolating the airlock chamber 111 from the ascending channel 11, ambient air is then introduced into the airlock chamber 111 until it is at ambient pressure. Then the first airlock door 112 is opened and the sorbent sheet 2 is moved into the airlock chamber 111 and the first airlock door 112 is closed. The airlock chamber 111 is then pumped out to a pressure of 0.1 atmospheres so that it is equalised with the ascending channel 11. Next the sorbent sheet 2 is moved through the ascending chamber as described above and the process is repeated for the next sorbent sheet.

[0095] A further advantage of operation in a reduced atmosphere is an improvement in the durability of the solid sorbent, expected sorbents are less like to degrade due to the reduced operating temperature.

[0096] When a sorbent sheet 2 reaches the end of the descending channel 16 the process is done in reverse with the outlet airlock 117.

[0097] The one or more embodiments are described above by way of example only. Many variations are possible without departing from the scope of protection afforded by the appended claims.

[0098] For example, the system described above does not include a desiccant stage. However, it is envisaged that it be augmented with a desiccant stage, which could render otherwise unsuitable sorbents, such as Zeolite 13X as suitable.

Claims

CLAIMS1. A system for removing carbon dioxide from ambient air, the system comprising: a solid sorbent configured to capture carbon dioxide from the atmosphere and a regeneration unit; the solid sorbent arranged to move through the regeneration unit along a path from an inlet to an outlet; the regeneration unit comprising at least one heat source configured to generate a temperature gradient; the path through the regeneration unit including an ascending channel and a descending channel; the ascending channel being a channel from the inlet along which temperature increases, such that the solid sorbent climbs a temperature gradient as it moves along the ascending channel; the descending channel, being a channel to the outlet, along which temperature decreases, such that the solid sorbent descends a temperature gradient as it moves along the descending channel; wherein the ascending channel and the descending channel are connected by a heat conductor; whereby heat from the solid sorbent moving along the descending channel flows through the heat conductor to the solid sorbent in the ascending channel wherein the heat conductor physically separates the ascending channel from the descending channel, whereby the ascending channel and the descending channel are physically separated and thermally coupled by the heat conductor.

2. A system according to claim 1 wherein the ascending channel is provided with a sufficient cross section to carry gas that desorbs from the sorbent, and that will comprise the product gas stream.

3. A system according to claim 2 wherein the cross section of the ascending channel gradually increases to provide space for gas desorbing from the sorbent as it moves along the ascending channel; and wherein the increasing crosssection of the ascending channel is configured to approximately match gas flow velocity to sorbent velocity along the ascending channel.

4. A system according to any preceding claim comprising sheets of solid sorbent, the sheets of solid sorbent moving along the path through the regeneration unit; wherein each sheet has a thickness of 2mm to 30mm and incorporates channels through which air can flow.

5. A system according to claim 4 wherein the sheets of solid sorbent each comprise a plurality of sorbent vanes; wherein each vane has a thickness of between 0.1mm and 1mm, and each vane comprises a thermally conductive core, to which layers of sorbent are bonded.

6. A system according to claim 5 wherein the sheets of solid sorbent are compressible from an open configuration for absorption of carbon dioxide from ambient air to a compressed configuration for desorption; wherein in the open configuration the channels are at least 0.1mm across and in the compressed configuration the channels are no more than 0.05mm across.

7. A system according to claim 5 or 6 wherein each vane comprises thermally conductive end caps, wherein the end caps are thermally coupled to the thermally conductive core and are provided over an edge of the sorbent layer.

8. A system according to any preceding claim wherein the heat conductor is formed from a metal having a thermal conductivity of at least 100 W m-1K-1.

9. A system according to any preceding claim wherein the solid sorbent ascending the ascending channel and the solid sorbent descending the descending channel are configured to be tightly thermally coupled to the heat conductor; the solid sorbent ascending the ascending channel and the solid sorbent descending the descending channel being separated from the heat conductor by no more than 0.1mm along the majority of the length of the ascending and descending channels.

10. A system according to any preceding claims wherein the heat conductor is a heat conducting panel which extends along the majority of the length of the ascending channel and the descending channel.

11. A system according to any preceding claim wherein the regeneration unit comprises an outflow channel, the outflow channel in fluid communication with the ascending channel at or near the top of the temperature gradient, whereby desorbed product gas flows out from the ascending channel via the outflow channel; the system further comprising a constriction in the ascending channel of the path immediately after the outflow channel, to restrict the amount of desorbed gas that passes through the constriction with the sorbent into the descending channel, such that gas which desorbs as the sorbent moves along the ascending channel flows from the ascending channel to the outflow channel.

12. A system according to any preceding claim wherein one or more openings are provided between the ascending channel and the descending channel part-way along the temperature gradient to divert initially desorbed gas, including nitrogen and oxygen, which desorb at a lower temperature than carbon dioxide and water, away from the ascending channel; and wherein a constriction is provided in the ascending channel of the path immediately after the or each opening to restrict the initially desorbed gas, including desorbed nitrogen and oxygen from passing through the constriction with the sorbent, such that the initially desorbed gas, including desorbed nitrogen and oxygen is diverted through the or each respective opening.

13. A system according to claim 11 or 12 wherein the cross section of the or each constriction is 20-500 micrometres wider and 20-500 micrometres taller than the sorbent.

14. A system according to any of claims 1 to 11 wherein a gas-permeable screen is provided between the ascending channel and the descending channel part-way along the temperature gradient to divert initially desorbed gas, including desorbed nitrogen and oxygen, which desorb at a lower temperature than carbon dioxide and water away from the ascending channel; wherein the gas permeable screen is a mesh or foam.

15. A system according to claim 14 wherein the regeneration unit comprises a constriction in the ascending channel of the path in the region of the gas permeable screen, such that initially desorbed gas, including nitrogen andoxygen is forced to pass through the gas permeable screen; wherein the cross section of the constriction is 20-500 micrometres wider and 20-500 micrometres taller than the sorbent.

16. A system according to any preceding claim wherein the regeneration unit comprises a constriction at the inlet to the ascending channel of the path, to restrict the amount of ambient air entering the regeneration unit with the sorbent.

17. A system according to any preceding claim comprising a replenishment channel in fluid communication with the descending channel at or near the top of the temperature gradient, whereby ambient air is provided to the sorbent as it moves along the descending channel from the replenishment channel and at least nitrogen and oxygen from the ambient air that passes through the replenishment channel are adsorbed into the sorbent so as to generate heat which flows through the heat conductor to the sorbent in the ascending channel.

18. A system according to claim 17 wherein the regeneration unit comprises a constriction at the outlet from the descending channel of the path to restrict the amount of air that is drawn through the replenishment channel.

19. A system according to claim 15, 16 or 18 wherein the cross section of the or each constriction is 20-100 micrometres wider and 20-100 micrometres taller than the sorbent.

20. A system according to any preceding claim wherein the ascending channel and descending channel are maintained at a pressure less than atmospheric pressure.

21. A system according to claim 20 wherein the pressure in the ascending channel and descending channel is less than 0.5 atmospheres.

22. A method of removing carbon dioxide from ambient air, the method comprising: absorbing carbon dioxide from ambient air onto a solid sorbent and passing the solid sorbent though a regeneration unit; wherein the solid sorbent moves through the regeneration unit along a path from an inlet to an outlet along a temperature gradient;the solid sorbent passing first along an ascending channel being a channel from the inlet along which temperature increases, such that the sorbent climbs a temperature gradient as it moves along the ascending channel; the solid sorbent then passing along a descending channel, being a channel to the outlet, along which temperature decreases, such that the sorbent descends a temperature gradient as it moves along the descending channel; wherein the ascending channel and the descending channel are connected by a heat conductor; whereby heat from the solid sorbent moving along the descending channel flows through the heat conductor to the sorbent in the ascending channel wherein the heat conductor physically separates the ascending channel from the descending channel, whereby the ascending channel and the descending channel are physically separated and thermally coupled by the heat conductor.

23. A method according to claim 22 wherein gas including carbon dioxide desorbs from the sorbent as it moves along the ascending channel and desorbed carbon dioxide flows to the top of the temperature gradient, to reduce the partial pressure of water as it desorbs.

24. A regeneration unit for removing adsorbed carbon dioxide from a sorbent, the regeneration unit comprising: a path from an inlet to an outlet and at least one heat source which generates a temperature gradient; the path through the regeneration unit including an ascending channel and a descending channel; the ascending channel being a channel from the inlet along which temperature increases, such that the sorbent climbs a temperature gradient as it moves along the ascending channel; the descending channel, being a channel to the outlet, along which temperature decreases, such that the sorbent descends a temperature gradient as it moves along the descending channel;wherein the ascending channel and the descending channel are connected by a heat conductor; whereby heat from the solid sorbent moving along the descending channel flows through the heat conductor to the sorbent in the ascending channel wherein the heat conductor physically separates the ascending channel from the descending channel, whereby the ascending channel and the descending channel are physically separated and thermally coupled by the heat conductor..

25. A regeneration unit according to claim 24 further comprising a track configured to transport the sorbent along the path from the inlet to the outlet.

26. A sheet of solid sorbent for removing carbon dioxide from ambient air, the sheet of solid sorbent being compressible from an open configuration for absorption of carbon dioxide from ambient air to a compressed configuration for desorption.

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