WO2025155741A1 - A self-contained, replicable module for direct air capture of carbon dioxide, powered by focused sunlight - Google Patents

Abstract

An apparatus for direct air capture of C0₂ includes a support frame; a plurality of trays defining a cargo region to contain CO₂ capture reactant therein; an oven comprising an optical window that allows concentrated sunlight to pass therethrough while confining heat delivered by said concentrated sunlight within said oven; a solar collector arranged to focus sunlight through said optical window into said oven; a storage vessel fluidly connected to said oven to receive CO₂ therefrom; and a control system configured to communicate with said mechanical assembly to control said sequential loading and removal of each of said plurality of trays based on at least one of a preprogrammed timing routine or CO₂ flow measurements.

Description

A SELF-CONTAINED, REPLICABLE MODULE FOR DIRECT AIR CAPTURE OF CARBON DIOXIDE, POWERED BY FOCUSED SUNLIGHT

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The present patent application claims priority benefit to U.S. Provisional Patent Application No. 63/621,425, filed on January 16, 2024, the entire content of which is incorporated herein by reference. All references cited anywhere in this specification, including the Background and Detailed Description sections, are incorporated by reference as if each had been individually incorporated.

BACKGROUND

1. Technical Field

[0002] The field of currently claimed embodiments of this invention relates to methods to remove carbon dioxide (CO2) directly from the atmosphere (Direct Air Capture, DAC), and concentrating it for sequestration.

2. Discussion of Related Art

[0003] The most advanced tests of Direct Air Capture today are based on a concept originated 25 years ago by Klaus Lackner (1999). Starting with a liquid solution of sodium hydroxide, CO2 is captured from the air as sodium carbonate. It is then precipitated out as calcium carbonate, CaCCh. The critical step in this process of calcination to separate CO2 in the form of pure gas is heating the CaCCh ~900°C. In the largest plants, fossil fuel is burned to provide the heat for this endothermic reaction.

[0004] Looking to the potential of using concentrated sunlight energy in this process, calcination using light energy was demonstrated in lab experiments by Craig (2010). As shown in FIG. 1, specimens of marble placed on a ceramic fiber board were heated first to 900°C and then further heated to drive off CO2 using focused light from a 5kW Xenon arc lamp. For a light flux of 430 kW/m² onto 6.5 mm marble stones set on fiber board, 8 minutes were needed to obtain complete calcination, equivalent to 23GJ per ton of CO2.

[0005] Craig’s approach using large size CaCO.3 pellets is deficient in that the light energy is not efficiently absorbed, and several times greater energy per unit mass was needed than should be the case for the known endothermic energy of the reaction. Abetter way to couple light energy to drive the reaction is needed if sunlight is to be used efficiently to power carbon capture in this way.

[0006] The potential for concentrating very large fluxes of solar energy to provide heat is illustrated by utility scale electrical generation plants (CSP) in which the light from a very large number of heliostats is directed to a single, black-coated cylindrical receiver atop a central tower. At the plant at Ivanpah (Tharp, 2018), each of three central receivers is surrounded by 58,000 heliostats of total mirror area 815,000 m², spread out over an area of 3.7 km², an area ratio of 22%. The thermal energy collected by a receiver over a typical year is 1,250,000 MWh, sufficient energy, if all could be utilized, to calcine 0.5 million tons/year/km² at the rate of 2.5 MWh/ton. But a major deficiency of present heliostat fields is that the sunlight concentration is not very high, yielding a receiver temperature of only 565°C, not hot enough for CaCCh calcination.

[0007] A demonstration of using concentrated sunlight for heating to > 900°C for calcination of CaCCh has been made by Guillot at the Odiello solar furnace. Highly concentrated sunlight obtained with a large secondary paraboloidal reflector was used to heat a metal box containing CaCO.3 particles agitated as a fluidized bed, to bring all the particles in thermal contact with the metal casing. However, this method was very inefficient, with only a few percent of the solar energy collected by the furnace converted to power the endothermic calcination reaction. In addition, the mechanical energy of the fluidization causes damage to calcium carbonate particles.

[0008] In related art, research toward achieving efficient solar-thermal conversion in thermochemical heat storage systems has resulted in development of black synthetic composites of calcium carbonate that include iron and manganese. These may efficiently be heated for calcination directly by concentrated sunlight. Teng et al have demonstrated in a laboratory furnace that such a composite with calcium, manganese and iron in the molar ratio 100-6-12, prepared with a porous, open structure, may be cycled through a loop of repeated calcination and carbonation with little degradation over 60 cycles (FIG. 3), given calcination at 700°C. Such material would be valuable for many repeated cycles of carbon capture and calcination, provided mechanical damage could be avoided.

[0009] A way to set out calcium hydroxide particles in trays for the capture phase of DAC has been demonstrated by the company Heirloom Carbon (McQueen) at a plant capable of a maximum sequestration rate of 1,000 tons of carbon dioxide per year. Here the CaCOs formed after capture of CO2 is removed from the trays and transported to a kiln for heating to > 900°C, when CO2 is driven off and captured. The remaining quicklime is slaked with water, and then returned to the trays to repeat the cycle of carbon capture. The calcination energy in this case is provided, not by burning fossil fuel, but by electricity sourced by renewable energy.

[0010] There are several deficiencies in this approach. The first is the high cost of electrical heating. The energy needed to turn one ton of CaCCh into quicklime, which releases 0.4 tons of concentrated CO2, is 3 GJ, i.e. the requirement is for 7.5 GJ (equal to 2,500 kWh) per ton of CO2 captured, similar to other solid capture methods. For electricity costing only $0.04/kWh, this still amounts to $100/ton of captured CO2. The amount of solar electricity needed at scale to power even 1 billion tons of CO2 removal per year would be 20 times the current total US solar production, requiring a huge, 30,000 km² of land area. A further deficiency is the mechanical damage to the CaCCh particles on removing them from the trays, collecting, transporting and loading them first into the kiln, and then back to the trays. A different approach is needed in order to both reduce energy consumption and to make practical use of many repeated cycles of fragile particles engineered for direct absorption of concentrated sunlight to power calcination.

[0011] There thus remains a need for improved methods and systems to remove carbon dioxide (CO2) directly from the atmosphere. SUMMARY

[0012] An apparatus for direct air capture of CO2 according to an embodiment of the current invention includes a support frame; a plurality of trays defining a cargo region to contain CO2 capture reactant therein and to be disposed in said support frame so as to be exposed to air for CO2 capture; an oven disposed proximate to said support frame and arranged to receive each of said plurality of trays in succession, said oven comprising an optical window that allows concentrated sunlight to pass therethrough while confining heat delivered by said concentrated sunlight within said oven; a solar collector arranged to focus sunlight through said optical window into said oven; a mechanical assembly to operatively engage said frame and each of said plurality of trays to sequentially load and remove each tray from said oven; a storage vessel fluidly connected to said oven to receive CO2 therefrom; and a control system configured to communicate with said mechanical assembly to control said sequential loading and removal of each of said plurality of trays based on at least one of a preprogrammed timing routine or CO2 flow measurements. The apparatus constitutes a self-contained module for direct air capture of CO2, in which throughout the reactant remains in the same, flat, level tray, while the energy required to power the endothermic release process is delivered directly to the tray in the form of sunlight focused by adjacent mirrors.

[0013] A method of direct air capture of CO2 according to an embodiment of the current invention includes providing a solar optical system configured to collect and direct sunlight to a focus; providing a sealable oven arranged such that a portion thereof intercepts said focus so as to receive focused sunlight from said solar optical system; loading a tray containing a reactant therein into said sealable oven, said reactant being exposed to air; and sliding said tray within said sealable oven through said focus. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] The present disclosure, as well as the methods of operation and functions of the related elements of structure and the combination of parts and economies of manufacture, will become more apparent upon consideration of the following description and the appended claims with reference to the accompanying drawings, all of which form a part of this specification, wherein like reference numerals designate corresponding parts in the various figures. It is to be expressly understood, however, that the drawings are for the purpose of illustration and description only and are not intended as a definition of the limits of the invention.

[0015] FIG. 1 is a figure from Craig, 2010, showing calcination of CaCCh using light as the energy source;

[0016] FIG. 2 is a figure showing the Odiello solar collector used to power calcination of CaCCh;

[0017] FIG. 3 is a figure from Teng et al 2022 showing repeated cycles of calcium looping;

[0018] FIG. 4 is a figure from Heirloom (2022) showing direct air capture made by solid reactant particle in trays;

[0019] FIG. 5 is a schematic illustration of a complete, self-contained module for Direct Air Capture according to an embodiment of this invention;

[0020] FIG. 6A is a schematic plan view of the module of FIG. 5;

[0021] FIG. 6B and FIG 6C are schematic diagrams of an oven and heat-exchanger coupling according to an embodiment of this invention;

[0022] FIG. 7 is a schematic illustration showing trays of reactant being hydrated according to an embodiment of this invention; [0023] FIG. 8 is a schematic diagram of a control system according to an embodiment of this invention;

[0024] FIG. 9 is a plan view showing a detail of modules arrayed in a field according to an embodiment of this invention;

[0025] FIG. 10 is a schematic illustration providing a 3D view of a complete module of the apparatus according to an embodiment of this invention;

[0026] FIG. 11 is a schematic detail illustration of a section of a module according to an embodiment of this invention; and

[0027] FIG. 12 is a schematic detail of a plan view of a field of modules according to an embodiment of this invention.

DETAILED DESCRIPTION

[0028] Some embodiments of the current invention are discussed in detail below. In describing embodiments, specific terminology is employed for the sake of clarity. However, the invention is not intended to be limited to the specific terminology so selected. A person skilled in the relevant art will recognize that other equivalent components can be employed, and other methods developed without departing from the broad concepts of the current invention. All references cited anywhere in this specification, including the Background and Detailed Description sections, are incorporated by reference as if each had been individually incorporated.

[0029] An embodiment of the current invention provides a complete, self-contained module for direct air capture (DAC) of CO2. Porous solid reactant is spread out in trays which are co-located with an oven for calcination to recover pure CO2 for burial, and with two mirrors to collect the required solar energy. One mirror is turned by a heliostat to direct sunlight into a similarly-sized, adjacent, fixed concave paraboloidal mirror that further directs the sunlight downward to an intense focus. At the focus is located a horizontal, long, sealed oven which has at its center a window to allow entry of the focused light. The long trays of reactant are transferred one at a time into the oven. After each tray is inserted, the oven is sealed and the tray is moved slowly along under the focus, to heat the full length of the tray. CO2 driven from the reactant is collected at high concentration and removed from the oven. The oven is then opened and the tray is returned to the frame which is moved to bring the next tray into position for transfer into the oven. The trays at other times remain in place in the frame for capture from the atmosphere. The cycle is repeated for trays one after another through the day, as long as the sun is shining. Embodiments of this invention can avoid the collection and transfer of DAC reactant to large-scale kilns, as practiced in prior art.

[0030] The trays remain horizontal on being transferred smoothly into and out of the oven, minimizing mechanical damage to the reactant. In an embodiment, the capturing reactant is calcium hydroxide. Iron and manganese may be added to both strengthen it and darken it so that in the following calcination step to recover CO2, the heating is efficiently done directly with the concentrated sunlight. It is projected that the capture/cal cination cycles may be repeated 100 times over a year before the reactant needs to be replaced. In existing (2025) tests of DAC technology at scales of 1,000 tons per year or greater, the large energy requirement is met by either burning fossil fuel or using renewable electricity. An embodiment of this invention is three times more efficient in its use of sunlight to obtain the required energy directly, rather than from solar generated electricity.

[0031] To have a significant impact on climate change, the volume of DAC must be at a rate not insignificant compared to the current 40 billion tons annual increment in atmospheric CO2 and the 1 trillion ton excess already in the atmosphere, and the cost must approach $100 per ton captured. In some embodiments of the current invention, the cost of all the materials used is consistent with this cost target. The small size of its individual modules has the advantage that the design can be evolved over many iterations to develop the highest efficiency and lowest manufactured and installed cost when advanced to high volume replication. At the estimated capture rate of 10 tons of CO2 per year for an embodiment for a module of the invention, it would have to be replicated 4 billion times to capture 40 billion tons per year, enough to remove in 25 years the existing trillion ton atmospheric excess that causes global warming.

[0032] FIG. 5 and FIG. 6A are schematic 3-dimensional and plan views of a complete module of an embodiment of the current invention. CO2 is captured by a reactant set out in trays such as 1-4 located in a cylindrical frame 20. Only a few of the trays shown in cylindrical frame 20 are label. Each tray has a number and is assigned to a location in the frame with the same number. A central fan 30 draws air into the frame’s central region to flow out radially out along the length of said trays. Adjacent to the cylindrical frame 20 is a horizontal elongated sealable oven 100 which receives trays one at a time 5 after their reactant has become saturated with CO2. Heating in the sealed oven 100 releases CO2 from their reactant, which is transported via the pipe 120 for storage and subsequent burial.

[0033] In an embodiment, between the cylindrical frame 20 and the sealable oven 100 is a heat exchanger 110 configured to hold two trays in thermal proximity. In heat exchanger 110 heat is transferred from a hot tray in cooling location 112 to a cold tray in a heating location 111. The heat exchanger 110 is equipped with a mechanism to translate it so either location may be placed adjacent to the oven. FIG 6B and FIG 6C show a sealed bellows 115 included between the oven 100 and the heat exchanger 110 to allow this motion without breaking the CO2 containment seal. CO2 extracted then passes through the heat exchanger before being collected for storage and burial. FIG 6B and FIG 6C show also a central window 105 into the oven 100 which is located at the focus 230 of the solar collector to allow entry of concentrated sunlight to heat the reactant under the window.

[0034] In operation, as an example, while tray 6 is being heated in the oven to extract CO2, tray 7 is extracted from location 7 in the cylindrical frame 20 and placed into the heating side location, 111, of the heat exchanger. Here it is heated by a the tray 5, newly extracted from the sealable oven 100 and located in the cooling side 112 to heat tray 7. At the end of CO2 extraction from tray 6, the cylindrical frame 20 is rotated to bring location 5 in line with the cooling side 112 to receive back its tray 5. The heat exchanger 110 is then translated in position to receive the processed tray 6 from the oven into the cooling side 112, as shown in FIG 6B, then translated back to position the heating side 111 for insertion of the heated tray 7 into the oven 100, as shown in FIG 6C . The cylindrical frame 20 is now rotated to bring tray 8 into position for insertion into the heating side 111 of the heat exchanger 110. This cycle is repeated in sequence to process all the trays through the heat exchanger 110 and the oven 100.

[0035] In this embodiment, heat transfer in heat exchanger 110 is by radiation, as shown in Figure 7. Rotating blackened copper wheels are located directly above the two trays, with diameter wide enough to span both. Each wheel as it rotates comes into thermal equilibrium between radiative heat gained from the hot tray and that lost to the cold tray. The wheels reach an equilibrium temperature T_e given by

Te = ((Th⁴-Tc⁴)/2) ^1/4

For example, at a time when the hotter tray is at absolute temperature Th=l,000°K and the colder one is at To = 500°K, the wheel will reach an equilibrium temperature of 984K, radiating heat mostly down to the colder tray at a the 984K black-body radiation flux of 53 kW/m². If the copper wheels are 4 mm thick, they have heat capacity of 13.8 kJ/C, and parts over the cooler tray will cool at about 4°C/sec. A rotation speed of 15 rpm will be sufficient to keep them sufficiently isothermal.

[0036] The cylindrical frame 20 is located between a heliostat 210 and a paraboloidal reflector 220, as shown in FIGS. 5 and 6A. Incoming rays of sunlight 201 are reflected as parallel rays 202 by the heliostat 210 having a flat reflector 211. The flat reflector 211 is oriented by the heliostat’s dual-axis drive 212 so that the reflected rays 202 are directed parallel to the fixed axis of a paraboloid reflector 220, no matter the position of the sun in the sky and the direction of incoming sunlight rays 201. The reflected rays 202 strike the off- axis segment of the paraboloid reflector 220 and are directed downward as rays 203 to converge at the focus 230 of the reflector, which lies vertically below the center of the paraboloid reflector 220. Here they form an intense image of the solar disc, coincident with the central entrance window 105 of the oven 100. [0037] In an embodiment of the invention, the reactant is initially calcium hydroxide. FIG. 7 is a detail of trays 10, 11, 12 and 13 used to hold the reactant in cylindrical frame 20 during this process. After absorbing CO2 over a period of days from air, the reactant becomes calcium carbonate. This is calcined in the oven to become lime, calcium oxide. After return to the cylindrical frame 20, radial water sprays, such as 14, 15, 16 and 17 in FIG. 7 are installed over each tray, 10, 11, 12 and 13 which are used to slake the lime and return it to calcium hydroxide to repeat the capture process.

[0038] An embodiment of a module control system is shown in the diagram of FIG. 8.

Example module of specific size operating under representative solar flux in an embodiment

[0039] Here we analyze the performance of a module embodiment according to the design of FIG. 5, whose concentrator optics have the specific dimensions given in Table 1. The general concepts of the current invention are not limited to only this embodiment.

Table 1. Dimensions of an embodiment according to the current invention

[0040] The heliostat reflector 210 with the above dimensions is conveniently assembled from 3 facets, each a standard size rectangular float glass mirror, 2.4 x 3.3 m, set side by side. The paraboloid is conveniently assembled from three facets of the same size, together with three more each 2.4 m x 1.1 m, all three cut from 1 standard rectangle. These facets will be curved by slumping, as are the curved facets of parabolic trough solar reflectors.

[0041] There are 50 reactant trays in each of the 4 layers, for a total of 200. They are made preferably from silica-surfaced ceramic fiber board 1.5 mm thick. FIG. 7, shows four such trays, 10, 11, 12 and 13, in two levels of the cylinder frame 20. Their flat base sheets, 1.5 m long x 150 mm wide, have vertical black side walls about 25 mm high, to contain the reactant particles and provide stiffness and absorb light. The mass of each tray with these dimensions is 0.5 kg.

[0042] The reactant to be calcined, CaCCh, will be placed in the trays to a depth of around 3 mm. The total area in all 200 trays is 40.5 square meters and the total reactant volume equal to 0.081 m³. For porous reactant having 50% of the solid density of 2,710 kg/m³, the total module mass of CaCCh is 220 kg, for a heat capacity of 200,000 JAC. The mass of all 200 fiber board trays, with the dimensions given above, is 44 kg, and the heat capacity is 19,000 JAC, 10% of that of the reactant.

[0043] The power of the concentrated sunlight delivered to the reactant at any one time depends on the flux of incoming direct sunlight, the heliostat mirror area, 24 m², and a number of additional factors which reduce the transmitted power, estimated as follows: cosine factor from projected angle, averaging 80%; shadowing by neighbors, averaging 85%; reflectivity losses at the two mirrors, averaging 85%; and transmission losses at the oven window, 95%. Based on these estimates, the average effective module sunlight collection area is 24 x 0.80 x 0.85 x 0.85 x 0.95 = 13.2 m². As an example of the collected energy, if the solar flux at direct normal incidence is 760 W/m², representative of an average annual value, the power delivered to the focus will be 10.0 kW.

[0044] The oven is designed to trap the maximum amount of the sunlight energy entering the window, and convert it to heat, raising the reactant temperature to an optimum value, which may be >900 °C if needed. It can be chosen to best power the endothermic calcination reaction while minimizing structural degradation. The window is made with no more than the area needed to transmit the solar disc focus, so as to minimize outgoing loss of heat and light. This minimum area is set by the angular size of the image of the sun’s disc formed by the paraboloidal reflector. For the dimensions of the module mirrors given in Table 1, a ray tracing model shows that the solar focus is elliptical, requiring a horizontal window surface with dimension of 75 mm x 65 mm, for an area 0.00387 m². The window is made of fused silica with an elliptical opening of this size. The heat lost by black body radiation from the window of this area is, for an oven temperature of 900°C, equal to 0.42 kW, amounting to 4% of the input power. The light lost out of the window by reflection depends on the details of the reactant, which will preferably be modified to be optically absorbent, and the oven geometry near the window. This loss is estimated to be also no more than 4%.

[0045] There is additional heat lost by thermal conduction through the insulated walls of the oven. To calculate this, we assume that heat is transferred back about 3 m to the CO2 collection pipe near the sealed entry to the heat exchanger 110 by the flow of hot CO2 and that this will maintain the first 1.5 m length of the oven at 900°C. The oven wall, made of ceramic fiber insulation with thickness 0.2 m, extends around the inner oven perimeter of 0.5 m. The ceramic fiber insulation has a conductivity of 0.15 W/m² per °C/m, thus the heat loss due to conduction for 900°C oven temperature is 0.15 W/m°C x (1.5 x 0.6) m² x 900/0.2 °C/m = 600W. Thus, if the oven is calcining at 900°C, the combined thermal loss from window radiation and oven conduction is 1.2 kW, 12% of the solar input energy.

[0046] Given the optical and thermal efficiencies calculated above for a module, its annual yield of CO2 capture is estimated as follows for a location, such as Phoenix, Az, where the annual total of direct sunlight at normal incidence is 2,500 kWh/m². Then for the effective module collection area given above of 13.3 m², the total annual solar energy delivered to the module focus is 33,250 kWh. Given also conductive and radiative losses totaling 12%, the energy absorbed by reactant in the oven will be 25,000 kWh/year. Adopting the rate for the calcium chemistry of 2,500 kWh per ton of CO2 captured, the projected yield per module is thus 10.6 tons/year of captured CO2 .

Example of a field of modules according to an embodiment of this invention

[0047] FIG. 9 shows a detail of a field of modules oriented with the axis from the heliostat to the paraboloid aimed due south, and with the dimensions given in Table 1. For a rectangular grid as shown, we require a vertical spacing of 14 m in the N-S direction, and 8 m in the E-W direction to accommodate the sweep circles 214 to ensure no heliostat collisions. This leaves a width of 2.2 m between each heliostat 200 and the adjacent next paraboloidal mirror 220 to the north provided that the heliostats are turned to the north at 15 degree elevation. This is wide enough for robotic vehicle access in E-W lanes for mirror cleaning and service.

[0048] The ground area occupied by each module is 114 m². Thus, the density of modules laid out in this way is 8,770 per km². Given that each heliostat has a reflector area of 24 m², the ratio of reflector to ground area is 21%, similar to that achieved in CSP plants in which heliostats direct light to a central receiver.

[0049] The potential for direct air capture at large scale, is estimated as follows for sites with solar annual resource of 2,500 kWh/m² at direct normal incidence. Given the module yield above, and assuming a thermal requirement rate of 2,500 kWh per ton of CO2 captured, the yield of 10.6 tons per year per module translates to 93,000 tons per year of captured CO2 per km² of field area. Set out on sunny lands around the world with area totalling 700 x 700 km, a total of 4 billion module replicas would capture 40 billion tons per year, enough to remove in 25 years the measured trillion ton excess that causes global warming.

[0050] This is a lot of land, but it is 3 times less that that needed if the energy to do the same job were to be sourced as solar electricity from PV modules. These convert sunlight into electricity with only 20% efficiency, for a yield of 400MWh year per acre for single axis tracking, equal to 100,000 MWh/year per km² (Bollinger reference, 2022). Adopting again the same energy requirement of 2,500 kWh per ton, and allowing for 80% thermal efficiency for calcining, the electrical energy needed for even one billion tons of CO2 removed would be 3,125 billion kWh. US current solar electricity generation at utility scale (2022) totals 150 billion kWh/year from a land area of 1,500 km². Thus, to power direct air capture of just 1 billion tons per year of CO2 by solar electricity, using the same calcium chemistry, would require 30,000 km² land area, three times the area required when using modules according to this embodiment of this invention.

[0051] Smaller volumes of removal won’t do much given that the present atmospheric content of CO2, at 410 ppm, is 3,300 Gtons. To reduce global warming, this needs to be reduced by 1000 Gtons, back to 300 ppm, even after fossil fuel generation has been abandoned.

[0052] Some embodiments of the current are directed to:

1. An apparatus for direct air capture of CO2 comprising: solid reactant spread out in long, narrow trays stacked one above the other; solar focusing optics; and a sealable oven, wherein CO2 is captured from ambient air in in repeated cycles of three steps, while said trays are maintained horizontal, as follows:

1) said trays of reactant are exposed to the air, when they absorb CO2;

2) said trays are placed sequentially into and then out of said oven, each tray being heated with concentrated sunlight to drive off the CO2, which is collected at high concentration, and then

3) said trays are processed with gas or vapor as necessary to become again absorptive.

2. The solar focusing optics according to 1 comprising:

1) a dual axis heliostat with planar reflector and

2) a fixed, off-axis section of a paraboloidal mirror, and wherein these elements together direct sunlight downward to a local high intensity focus collocated with a window into said oven.

3. The sealable oven according to 1 including: a long, narrow horizontal floor along which said similarly long and narrow trays can be slid, so their full length gradually passes under a central window, and wherein the transmitted focused sunlight is concentrated at high intensity onto only a small portion of said tray at any one time, thereby strongly heating and efficiently driving off CO2 from that portion. 4. The sealable oven according to 1 further including a pipe to collect CO₂ from the entry end of the oven, and wherein hot CO2 released from the reactant on heating flows back across and pre-heats the unprocessed reactant as it approaches the region of intense heating under the window.

5. The reactant in trays according to 1 comprising: initially calcium hydroxide Ca(OH)2 (slaked lime), which after exposure to the air becomes calcium carbonate, CaCC , and which after heating to drive off CO2 becomes quicklime, CaO, and which is slaked with water vapor to become once again calcium hydroxide.

6. The reactant according to 1 having depth in said trays of 1 - 5 mm, and wherein after many repeated cycles breaks down to take the form of small particles, and may be replenished as needed with fresh reactant with larger particle size.

7. The trays according to 1 having length in the range 1-3 m and width 100 - 300 mm, and being made of refractory ceramic board with thickness in the range 1 - 2 mm.

8. The trays according to claims 1 and 5 supported in a rack equipped with water vapor jets, and wherein water vapor is introduced above each tray of CaO for slaking, at a rate such that the exothermic slaking reaction maintains the reactant at a temperature in the range 50 - 150°C, hot enough that convective and evaporative cooling will allow the slaking process to be completed in less than 2 hours.

FURTHER EMBODIMENTS

[0053] The reference numerals used in this section refer to FIGS. 10 to 12 only.

[0054] FIGS. 10 and 11 are schematic 3D views of a complete module of an embodiment of the current invention in which CO₂ is captured by a reactant set out in long trays 41 - 48, located between a heliostat 10 and a paraboloidal reflector 20. Sunlight 1 is reflected as parallel light 3 by the heliostat 10 having a flat reflector 11. The reflector 11 is oriented by the heliostat’s dual-axis drive 2 so that the reflected light 3 is directed parallel to the fixed axis of a paraboloid 20, no matter the position of the sun in the sky and tire direction of incoming sunlight. Tire light 3 strikes the off-axis segment 21 of the paraboloid 20. The segment 21 is oriented so that the central ray 3 strikes the center 4 of the segment and is directed vertically downward as ray 5 to the focus of the paraboloid 6. All the other parallel rays of reflected sunlight are reflected also to the same focus 6.

[0055] The long reactant trays 41 - 47 are supported by a frame 40 shown in a schematic detail in FIG. 11.

[0056] As shown in FIG. 10, located beneath the paraboloid 21 is a long horizontal oven 30, with a central entrance window 6 coincident with the focus 6. All the sunlight at the intense focus 6 passes through the window. The oven 30 has within it a long, narrow horizontal floor along which trays 41-47 may be slid. The trays are introduced one at a time into the oven. As illustrated in FIG. 11, the tray 47 is about to be slid into the oven through the narrow opening 31, which will then be sealed. The tray is then moved into the oven 30 along the floor until the intense focused sunlight strikes the entering end on the tray and drives the release of CO2 from that end. The tray is then slowly advanced, with only a small portion being processed at any one time, but over time the full length of the tray is processed while all the CO2 is collected at the end of the oven near 31. On this completion, the access 31 is opened, and the tray retracted and returned to the holding frame 40. The holding frame 40 is then translated horizontally, bringing the adjacent tray 48 into position opposite the entrance 31, when it is slid in for processing. After the trays in the top layer are all processed in this way, the holding frame 40 is raised to bring the second tray layer in line with the oven entrance 31, and then all the trays in that layer are processed. Proceeding in this way, all the trays are processed in turn, a process that may take several days, depending on the solar flux through each day.

[0057] The tray support rack 40 is equipped with water vapor jets above each tray, which are used to slake the CaO lime in trays that have been calcined. The rate of slaking is set such that this exothermic reaction maintains the reactant hot enough that convective and evaporative cooling will allow the slaking process to be completed in less than 2 hours.

Example module of specific size operating under representative solar flux [0058] Here we analyze the performance of a module embodiment according to the design of FIG. 10, whose concentrator optics have the specific dimensions given in Table 2.

Table 2. Dimensions of an embodiment according to the current invention

[0059] The heliostat reflector 11 with these dimensions is conveniently assembled from 3 facets, each a standard size rectangular float glass mirror, 2.4 x 3.3 m, set side by side. The paraboloid is conveniently assembled from the three facets of the same size, together with three more each 2.4 m x 1.1 m, all three cut from 1 standard rectangle.

[0060] The 90 reactant trays are made from a high temperature alloy such as Inconel in the form of a thin flat sheet, 100 microns thick. The flat base sheet, 3 m long x 150 mm wide, has folded up vertical side walls about 25 mm high, to contain the reactant particles and provide stiffness and absorb light. The mass of each tray with these dimensions is 0.49 kg. As shown in FIG. PPA-3, The trays are arrayed in six layers, one above the other, with 100 mm vertical separation. Each layer has 15 trays separated by 50 mm laterally, for 200 mm center-to-center spacing. [0061] The reactant to be calcined, CaCO.3, will be place in the trays to a depth of around 2 mm. The total area in all 90 trays is 40.5 square meters and the total reactant volume equal to 0.081 m³. For 80% of the solid density of 2,710 kg/m³, the total module mass of CaCCh is 220 kg, for a heat capacity of 200,000 J/C. The mass of all 90 Inconel trays, with the dimensions given above, is 44 kg, and the heat capacity 19,000 J/C, 10% of that of the reactant.

[0062] The power of the concentrated sunlight delivered to the reactant at any one time depends on the flux of incoming direct sunlight, the heliostat mirror area, 24 m², and a number of additional factors which reduce the transmitted power estimated as follows: projected angle, (cosine factor), averaging 80%; shadowing by neighbors, averaging 85%; reflectivity losses at the two mirrors, averaging 85%; and transmission losses at the oven window, 95%. Based on these estimates, the average effective module sunlight collection area is 24 x .8 x .85 x .85 x .95 = 13.2 m². As an example of the collected energy, if the solar flux at direct normal incidence is at 760 W/m², representative of an average annual value, the power delivered to the focus will be 10.0 kW.

[0063] The oven is designed to trap the maximum amount of the sunlight energy entering the window, and convert it to heat, raising the reactant temperature to > 900 °C, and powering the endothermic calcination reaction. Some of light is absorbed directly by the reactant, some is reflected, to be absorbed by black coating on the inside surfaces of the tray and oven. The window is made with no more than the area needed to transmit the solar focus to minimize outgoing loss of heat and light. This minimum area is set by the angular size of the image of the sun’s disc formed by the paraboloidal reflector. For the dimensions of the module mirrors given in Table 2, a ray tracing model shows that the solar focus is elliptical, with dimension on the horizontal window surface of 75 mm x 65 mm, for an area 0.00387 m². The window is made of fused silica with an opening of this dimension. The heat lost by black body radiation from the window of this area is, for an oven temperature of 900°C, equal to 0.42 kW, and at 1000°C equal to 0.58 kW, amounting to 4% - 6% of the input power. The light lost out of the window by reflection depends on the details of the reactant, which will preferably be modified to be optically absorbent, and the oven geometry near the window. This loss is estimated to be no more than 4%.

[0064] There is additional heat lost by thermal conduction through the insulated walls of the oven. To calculate this, we assume that heat is transferred back 3 m to the CO2 collection pipe near the sealed entry by radiation and the flow of hot CO2, and that this will maintain the first 3 m length of the oven at 900°C. The oven wall, made of ceramic fiber insulation with thickness 0.2 m, extends around the inner oven perimeter of 0.6 m. The ceramic fiber insulation has a conductivity of 0.15 W/m² per °C/m, thus the heat loss due to conduction is 0.15 W/m°C x (3 x 0.6) m² x 900/0.2 °C/m = l,200W. Thus, the total thermal loss from window radiation and oven conduction when the oven is calcining at 900°C is 2 kW, 20% of the solar input energy.

[0065] Given the optical and thermal efficiencies calculated above for a module, its annual yield of CO2 capture is estimated as follows for a location, such as Phoenix, Az, where the annual total of direct sunlight at normal incidence is 2,500 kWh/m². Then for the effective module collection area given above of 13.3 m², the total annual solar energy delivered to the module focus is 33,250 kWh. Given also conductive and radiative losses totalling 20%, the energy absorbed by reactant in the oven will be 26,600 kWh/year. Adopting the rate for the quicklime chemistry of 2,500 kWh per ton of CO2 captured, the yield per module is thus 10.6 tons/year of captured CO2 .

Example of a field of modules according to this invention

[0066] FIG. 12 shows a detail of a schematic plan view of a field of adjacent modules according to an embodiment of this invention, each one being of the specific size given above. The circles shown of 7.7 m diameter mark the clearance needed around each heliostat to make collisions impossible, no matter the different orientations. The spacing between modules shown is 13.9 m in the N-S direction, and 8.2 m in the E-W direction. This leaves a width of 3.7 m between the heliostat and paraboloidal mirrors, provided that the heliostats are turned to 15-degree elevation. This is wide enough for robotic vehicle access for mirror cleaning and service. [0067] The ground area occupied by each module is 114 m². Thus, the density of modules laid out in this way is 8,770 per km². Given that each heliostat has a reflector area of 24 m², the ratio of reflector to ground area is 21%, similar to that achieved in CSP plants in which heliostats direct light to a central receiver.

[0068] The potential for direct air capture at km scale, similar to that of the CSP plants is estimated as follows, for a site with a solar annual resource of 2,500 kWh/m² at direct normal incidence. Given the module yield above, and assuming a thermal requirement rate of 2,500 kWh per ton of CO2 captured, the yield of 10.6 tons per year per module translates to 93,000 tons per year of captured CO2 per km² of field area. For capture of 1 billion tons per year, a ground area of around 10,000 square km will be needed. This is 0.1% of total land area of the USA, about 0.25% of US agricultural land area, or about 1% of the US land with low agricultural value but high solar flux, in California, Nevada, Arizona, New Mexico and Texas.

[0069] This is a lot of land, but it is 3 times less that that needed if the energy to do the same job were to be sourced as solar electricity from PV modules. These convert sunlight into electricity with only 20% efficiency, for a yield of 400MWh year per acre for single axis tracking, equal to 100,000 MWh/year per km². (Bollinger reference, 2022). Adopting again the same energy requirement of 2,500 kWh per ton, and allowing for 80% thermal efficiency for calcining, the electrical energy needed per Gton of CO2 removed would be 3,125 billion kWh. US current solar electricity generation at utility scale (2022) totals 150 billion kWh/year from a land area of 1,500 km². Thus, to power direct air capture 1 Gton of CO2 by solar electricity using the same lime chemistry would require 30,000 km² land area, three times the area required when using modules according to this invention.

[0070] Smaller volumes of removal won’t do much, given that the present atmospheric content of CO2, at 410 ppm, is 3,300 Gtons. To reduce global warming, this needs to be reduced by 1000 Gtons, back to 300 ppm, even after fossil fuel generation has been abandoned.

[0071] Some embodiments of the current are directed to: 1. An apparatus for direct air capture of CO₂ comprising: solid reactant spread out in long, narrow trays; solar focusing optics; and a sealable oven, wherein CO2 is captured from ambient air in in repeated cycles of three steps, while said trays are maintained horizontal, as follows:

1) said trays of reactant are exposed to the air, when they absorb CO2;

2) said trays are placed sequentially into and moved further into and then out of said oven, each tray being heated with concentrated sunlight to drive off the CO2, which is collected at high concentration, and then

3) said trays are processed with gas or vapor as necessary to become again absorptive.

2. The solar focusing optics according to 1 comprising:

1) a dual axis heliostat with planar reflector and

2) a fixed, off-axis section of a paraboloidal mirror, and wherein these elements together direct sunlight downward to a local high intensity focus.

3. The sealable oven according to 1 including: a long, narrow horizontal floor along which said similarly long and narrow trays can be slid, so their full length gradually passes under a central window, and wherein the transmitted focused sunlight is concentrated at high intensity onto only a small portion of said tray at any one time, thereby strongly heating and efficiently driving off CO2 from that portion.

4. The sealable oven according to 1 further including a pipe to collect CO2 from the entry end of the oven, and wherein hot CO2 released from the reactant on heating flows back across and pre-heats the unprocessed reactant as it approaches the region of intense heating under the window.

5. The reactant in trays according to 1 comprising: initially calcium hydroxide Ca(0H)2 (slaked lime), which after exposure to the air becomes calcium carbonate, CaCCh, and which after heating to drive off CO2 becomes quicklime, CaO, and which in said step 3 is slaked with water vapor to become once again calcium hydroxide.

6. The reactant according to 1 having depth in said trays of 1 - 3 mm, and wherein after many repeated cycles breaks down to take the form of small particles, and may be replenished as needed with fresh reactant with larger particle size.

7. The trays according to 1 having length in the range 2 - 4 m and width 100 - 300 mm, and being made of refractory metal alloy such as Inconel with thickness in the range 100 - 200 microns.

8. The trays according to 1 and 5 supported in a rack equipped with water vapor jets, and wherein water vapor is introduced above each tray of CaO for slaking, at a rate such that the exothermic slaking reaction maintains the reactant at a temperature in the range 50 - 150°C, hot enough that convective and evaporative cooling will allow the slaking process to be completed in less than 2 hours.

[0072] Some further embodiments of the current are directed to:

9. An apparatus for providing concentrated sunlight onto a horizontal surface comprising: a heliostat with a planar reflector and a fixed, off-axis section of a paraboloidal mirror, and wherein said planar reflector reflects sunlight into said fixed, off-axis section of said paraboloidal mirror which directs the sunlight downward to said focus.

10. An apparatus for direct air capture of CO2 comprising: a solar optical system configured to collect and direct sunlight to a focus; a sealable oven arranged such that a portion thereof intercepts said focus so as to receive focused sunlight from said solar optical system, wherein said sealable oven being configured to receive a tray of reactant that is exposed to air and is movable through said focus such that said reactant heats up sufficiently along a length thereof to absorb CO2 from said air.

11. The apparatus according to 10, wherein said solar optical system is a dual axis heliostat comprising a planar reflector and a fixed, off-axis section of a paraboloidal mirror, and wherein said planar reflector and said fixed, off-axis section of said paraboloidal mirror together collect and direct sunlight to said focus.

12. The apparatus according to 10 or 11, further comprising a plurality of trays disposed on a support frame and disposed proximate said sealable oven, said sealable oven defining an oven entrance to allow each of said plurality of trays to be sequentially loaded into and removed from the sealable oven, wherein each tray of said plurality of trays are structured to be able to contain a reactant therein to be exposed to air and to pass through said focus such that said reactant heats up sufficiently along a length thereof to absorb CO2 from said air.

[0073] The embodiments illustrated and discussed in this specification are intended only to teach those skilled in the art how to make and use the invention. In describing embodiments of the invention, specific terminology is employed for the sake of clarity. However, the invention is not intended to be limited to the specific terminology so selected. The above-described embodiments of the invention may be modified or varied, without departing from the invention, as appreciated by those skilled in the art in light of the above teachings. It is therefore to be understood that, within the scope of the claims and their equivalents, the invention may be practiced otherwise than as specifically described. REFERENCES

KS Lackner, P Grimes and HJ Ziock , 1999, Carbon Dioxide Extraction from Air?, LAUR-99-5113, Los Alamos National Laboratory.

Bolinger, M. and G. Bolinger. 2022. Land Requirements for Utility-Scale PV: An Empirical Update on Power and Energy Density. IEEE Journal of Photovoltaics carboncredits.com website

R.A. Craig. 2010 Investigating the use of concentrated solar energy to thermally decompose limestone. - digital.library.adelaide.edu.au

J. Tharp, K.R. Anderson, 2018, Simulation and Lessons Learned from the Ivanpah Solar Plant Project, Solar Energy Society

L. Tenga, Y. Xuana, Y. Daa, X. Liua amd Y. Ding, 2019, Modified Ca-Looping materials for directly capturing solar energy and high-temperature storage, Energy Storage Materials www. el sevi er. com/1 ocate/ensm

N McQueen, M.Ghoussoub, J Mills, and M Scholten, 2022, A scalable direct air capture process based on accelerated weathering of calcium hydroxide, www.heirloomcarbon.com.

E Guillot, M Tessonneaud, JL Sans, G Flamant, 2020, Solar calcination at pilot scale in a continuous flow multistage horizontal fluidized bed, Solar Energy, Elsevier

Claims

We claim:

1. An apparatus for direct air capture of CO; comprising: a support frame; a plurality of trays defining a cargo region to contain CO2 capture reactant therein and to be disposed in said support frame so as to be exposed to air for CO2 capture; an oven disposed proximate to said support frame and arranged to receive each of said plurality of trays in succession, said oven comprising an optical window that allows concentrated sunlight to pass therethrough while confining heat delivered by said concentrated sunlight within said oven; a solar collector arranged to focus sunlight through said optical window into said oven; a mechanical assembly to operatively engage said frame and each of said plurality of trays to sequentially load and remove each tray from said oven; a storage vessel fluidly connected to said oven to receive CO2 therefrom; and a control system configured to communicate with said mechanical assembly to control said sequential loading and removal of each of said plurality of trays based on at least one of a preprogrammed timing routine or CChflow measurements; wherein the apparatus constitutes a self-contained module for direct air capture of CO2, in which throughout the reactant remains in the same, flat, level tray, while the energy required to power the endothermic release process is delivered directly to the tray in the form of sunlight focused by adjacent mirrors.

2. The apparatus according to claim 1, further comprising a fluid delivery system arranged at least partially within a portion of said frame to deliver fluid to react with said CO2 capture reactant when contained within each tray after having been returned from said oven, wherein said control system is further configured to communicate with said fluid delivery system to deliver fluid to expose said CO2 capture reactant to restore CO2 capture ability thereto.

3. The apparatus according to claim 2, further comprising a CO2 capture reactant loaded within each tray of said plurality of trays.

4. The apparatus according to claim 3, wherein said CO2 capture reactant comprises calcium hydroxide, wherein on capture of CO2 from air said calcium hydroxide becomes calcium carbonate, wherein CO2 driven off within said oven turns said calcium carbonate into calcium oxide, and wherein said fluid is water that turns said calcium oxide into calcium hydroxide.

5. The apparatus according to claim 4, wherein said CO2 capture reactant further comprises an added metal to blacken said CO2 capture reactant so as to increase absorption of sunlight and to increase mechanical stability to extend a number of cycles before replacement as needed.

6. The apparatus according to claim 5, wherein said added metal comprises at least one of iron or manganese.

7. The apparatus according to claim 1, wherein said frame is a cylindrical frame having at least a portion of said plurality of trays arranged in a circular pattern and said mechanical assembly is configured to rotate said frame stepwise for said sequential loading and removal of each of said plurality of trays into and out of said oven.

8. The apparatus according to claim 7, wherein said frame said frame is a cylindrical frame having at least a portion of said plurality of trays arranged in a plurality of stacked circular patterns, and wherein said mechanical assembly is further configured to step said cylindrical frame between said plurality of stacked circular patterns.

9. The apparatus according to claim 8, further comprising a fan arranged centrally to said cylindrical frame to flow air radially out along horizontal lengths of each of said plurality of trays.

10. The apparatus according to claim 1, wherein said plurality of trays are arranged substantially horizontally relative to a local ground upon which said apparatus is supported.

11. The apparatus according to claim 10, wherein said solar collector comprises: a heliostat comprising a planar reflector; and a fixed, off-axis section of a paraboloidal mirror; wherein said planar reflector is arranged to reflect sunlight into said fixed, off-axis section of said paraboloidal mirror, and wherein said fixed, off-axis section of said paraboloidal mirror is arranged to further reflect the sunlight downward to enter said oven at a focus located at said optical window, said optical window being arranged substantially horizontally relative to said local ground.

12. The apparatus according to any of claims 1 - 9, further comprising: a heat exchanger positioned between said frame and said oven, wherein said heat exchanger is configured to hold two of said trays in thermal proximity, one of said two trays being received newly extracted from said frame to be heated by the second tray of said two trays, and wherein said second of said two trays is to be cooled by said first tray before being returned to said frame.

13. The apparatus according to claim 12, further comprising a sealed bellows between said oven and said heat exchanger, wherein said sealed bellows provides a flow of hot CO2 from said oven through said heat exchanger before extraction of said CO2 for storage.

14. A method of direct air capture of CO2 comprising: providing a solar optical system configured to collect and direct sunlight to a focus; providing a sealable oven arranged such that a portion thereof intercepts said focus so as to receive focused sunlight from said solar optical system; loading a tray containing a reactant therein into said sealable oven, said reactant being exposed to air; and sliding said tray within said sealable oven through said focus.

15. The method according to claim 14, further comprising removing said tray from said sealable oven.

16. The method according to claim 15, further comprising sequentially loading, sliding and removing each of a plurality of trays to thereby absorb CO2 from said air in the reactant contained within each of said plurality of trays.

17. The method according to claim 16, further comprising treating said reactant in each tray of said plurality of trays to restore CO2 absorbing ability of said reactant after being removed from said sealable oven.