The present disclosure relates generally to capturing CO2 from spacecraft crew cabin atmosphere for life support purposes.
The goal of the National Aeronautics and Space Administration (NASA) is to return humans to the surface of the moon, then journey to Mars and even beyond. In order to accomplish this ambitious goal, robust life support systems are required to operate without reliance on a resupply. The current air revitalization system on the International Space Station (ISS), the Carbon Dioxide Removal Assembly (CDRA), utilizes sorbent-based, temperature-swing adsorption (TSA) technology. However, CDRA has repeated replacement and maintenance costs due to adsorbent material degradation.
The instant disclosure relates to CO2 removal systems to succeed CDRA. Specifically, it involves forcing a phase change of CO2 from the cabin atmosphere by solidifying it onto a cold surface. Generating a cold surface can be accomplished via multiple methods, including cryogenic coolers and thermal radiators to deep space. CO2 deposition, or CDep, is highly reliable as it has no expendable materials, no vacuum is required, and needs minimal moving parts. CDep also potentially eliminates the need for a separate storage system to deliver pressurized, pure CO2 to an O2 generation system, such as the Sabatier processor currently on the ISS. A deposition system can also remove residual humidity in addition to CO2 via a multi-stage process, and can also significantly assist the trace contaminant control function. Whereas cryogenic cooling technologies are established and approaches for Mars atmosphere CO2 capture have been tested, there is a need for the application of cryogenic cooling to capturing CO2 from the crew cabin atmosphere for life support purposes and for systems that improve both the scale and complexity by incorporating multiple coolers that operate in parallel, alternating fashion to provide constant CO2 capture.
Described herein a system for spacecraft atmosphere CO2 capture includes a first heat exchanger configured to receive airflow from the spacecraft atmosphere and to cool said airflow via a first heat exchange with CO2-depleted air, a pre-cooler configured to receive and cool the airflow from the first heat exchanger, a second heat exchanger configured to receive the airflow from the pre-cooler and to cool said airflow via a second heat exchange with the CO2-depleted air, and first and second deposition coolers each configured to operate in a deposition mode in which CO2 from the airflow is deposited to generate said CO2-depleted air, and a sublimation mode in which deposited CO2 is sublimated into CO2 gas. A controller is configured to alternately cycle each of the first and second deposition coolers between the deposition mode and the sublimation mode, with the first cooler operating in deposition mode when the second cooler is operating in sublimation mode, and vice versa.
Also described herein is a method for spacecraft atmosphere CO2 capture, including cooling airflow from the spacecraft atmosphere in a first heat exchange with CO2-depleted air, cooling the airflow from the spacecraft atmosphere using a pre-cooler, cooling the airflow from the spacecraft atmosphere in a second heat exchange with the CO2-depleted air, and depositing CO2 from the airflow in first and second deposition coolers that are each alternately cycled between a deposition mode and a sublimation mode. In the deposition mode, CO2 from the airflow is deposited to generate the CO2-depleted air, and in the sublimation mode, deposited CO2 is sublimated into CO2 gas. When the first cooler is operating in deposition mode, the second cooler is operating in sublimation mode, and vice versa.
Also described herein is a machine-readable storage medium having stored thereon a computer program for controlling a system for spacecraft atmosphere CO2 capture, the computer program comprising a routine of set instructions for causing the system to perform the steps of cooling airflow from the spacecraft atmosphere in a first heat exchange with CO2-depleted air, cooling the airflow from the spacecraft atmosphere using a pre-cooler, cooling the airflow from the spacecraft atmosphere in a second heat exchange with the CO2-depleted air, and depositing CO2 from the airflow in first and second deposition coolers that are each alternately cycled between a deposition mode and a sublimation mode. In the deposition mode, CO2 from the airflow is deposited to generate the CO2-depleted air, and in the sublimation mode, deposited CO2 is sublimated into CO2 gas. When the first cooler is operating in deposition mode, the second cooler is operating in sublimation mode, and vice versa.
The accompanying drawings, which are incorporated into and constitute a part of this specification, illustrate one or more examples of embodiments and, together with the description of example embodiments, serve to explain the principles and implementations of the embodiments.
In the drawings:
and
Example embodiments are described herein in the context of a spacecraft atmosphere CO2 capture system and method. The following description is illustrative only and is not intended to be in any way limiting. Other embodiments will readily suggest themselves to those of ordinary skill in the art having the benefit of this disclosure. Reference will be made in detail to implementations of the example embodiments as illustrated in the accompanying drawings. The same reference indicators will be used to the extent possible throughout the drawings and the following description to refer to the same or like items.
In the description of example embodiments that follows, references to “one embodiment,” “an embodiment,” “an example embodiment,” “certain embodiments,” etc., indicate that the embodiment described may include a particular feature, structure, or characteristic, but every embodiment may not necessarily include the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with an embodiment, it is submitted that it is within the knowledge of one skilled in the art to effect such feature, structure, or characteristic in connection with other embodiments whether or not explicitly described. The term “exemplary” when used herein means “serving as an example, instance or illustration.” Any embodiment described herein as “exemplary” is not necessarily to be construed as preferred or advantageous over other embodiments.
In the interest of clarity, not all of the routine features of the implementations described herein are shown and described. It will be appreciated that in the development of any such actual implementation, numerous implementation-specific decisions must be made in order to achieve the developer's specific goals, such as compliance with application- and business-related constraints, and that these specific goals will vary from one implementation to another and from one developer to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking of engineering for those of ordinary skill in the art having the benefit of this disclosure.
In accordance with this disclosure, the components, process steps, and/or data structures described herein may be implemented using various types of operating systems, computing platforms, computer programs, and/or general purpose machines. Devices of a less general purpose nature, such as hardwired devices, field programmable gate arrays (FPGAs), application specific integrated circuits (ASICs), or the like, may also be used without departing from the scope and spirit of the inventive concepts disclosed herein. Where a method comprising a series of process steps is implemented by a computer or a machine and those process steps can be stored as a series of instructions readable by the machine, they may be stored on a tangible medium such as a computer memory device (e.g., ROM (Read Only Memory), PROM (Programmable Read Only Memory), EEPROM (Electrically Eraseable Programmable Read Only Memory), FLASH Memory, Jump Drive, and the like), magnetic storage medium (e.g., tape, magnetic disk drive, and the like), optical storage medium (e.g., CD-ROM, DVD-ROM, paper card, paper tape and the like) and other types of program memory.
Herein, reference to a computer-readable or machine-readable storage medium encompasses one or more non-transitory, tangible storage media possessing structure. As an example and not by way of limitation, a computer-readable storage medium may include a semiconductor-based circuit or device or other IC (such, as for example, a field-programmable gate array (FPGA) or an ASIC), a hard disk, an HDD, a hybrid hard drive (HHD), an optical disc, an optical disc drive (ODD), a magneto-optical disc, a magneto-optical drive, a floppy disk, a floppy disk drive (FDD), magnetic tape, a holographic storage medium, a solid-state drive (SSD), a RAM-drive, a SECURE DIGITAL card, a SECURE DIGITAL drive, or another suitable computer-readable storage medium or a combination of two or more of these, where appropriate. Herein, reference to a computer-readable storage medium excludes any medium that is not eligible for patent protection under 35 U.S.C. § 101. Herein, reference to a computer-readable storage medium excludes transitory forms of signal transmission (such as a propagating electrical or electromagnetic signal per se) to the extent that they are not eligible for patent protection under 35 U.S.C. § 101. A computer-readable non-transitory storage medium may be volatile, nonvolatile, or a combination of volatile and non-volatile, where appropriate.
Herein, “or” is inclusive and not exclusive, unless expressly indicated otherwise or indicated otherwise by context. Therefore, herein, “A or B” means “A, B, or both,” unless expressly indicated otherwise or indicated otherwise by context. Moreover, “and” is both joint and several, unless expressly indicated otherwise or indicated otherwise by context. Therefore, herein, “A and B” means “A and B, jointly or severally,” unless expressly indicated otherwise or indicated otherwise by context.
The basis of the phase change method of CO2 capture as described herein involves flowing cabin air through a chamber containing a cold surface that is below the deposition temperature of CO2, but above the condensation points of N2 and O2, allowing the CO2 to deposit. The CO2-free air then re-enters the cabin. Once the cold surface is considered saturated with solid CO2, the system switches to a parallel chamber. The solid CO2 in the first chamber then sublimes and is removed for storage or for use in other processes, such as Sabatier or other process for O2 regeneration for example.
The ISS is currently maintained at an average CO2 partial pressure of 3.0 mmHg, but an even lower partial pressure of 2.0 mmHg, or approximately 2600 ppm assuming atmospheric pressure, will be required to maintain crew health on future missions. Also, for a 4-crew mission, the CO2 removal system must remove 4.16 kg of CO2 per day.
Multiple operating parameters dictate the CDep design. The first consideration is temperature. Utilizing the Clausius-Clapeyron equation and known conditions at the triple point, the deposition temperature can be estimated for any CO2 partial pressure. At a partial pressure of 2.0 mmHg, the deposition temperature of CO2 is about 142K. If a lower partial pressure is desired, then the deposition temperature decreases. Therefore, the operating temperature of the cold surface must be below this deposition temperature in order to both reduce the CO2 partial pressure as well as overcome heat transfer effects to allow CO2 to deposit.
When using a Stirling cooler, the surface temperature is dictated by the cooling power. A Stirling cooler operates by using a piston and displacer to repeatedly compress and expand a working fluid, typically helium, across two heat exchangers and a regenerator. This cycle generates a temperature gradient, therefore producing a cold surface. As the desired operating temperature increases, so does the amount of cooling power generated (at the same input electrical power). However, to generate the amount of cooling power required to directly collect 4.16 kg CO2 per day, about ten times the current electrical power used to run CDRA would be needed. To mitigate this discrepancy, methods to increase thermal efficiency must be employed. CDep utilizes the cooled, CO2-free air exiting the system to precool incoming air via air-to-air heat exchangers (HXs). HXs with sufficient effectiveness would reduce the electrical power needed to be equivalent to CDRA.
The second parameter that affects the system design is pressure drop. As the mass of solid CO2 accumulates onto the cold surface, the pressure drop across the chamber is expected to increase slowly. If CO2 ice buildup chokes the flow, then pressure increases exponentially. This change in rate of pressure drop increase may have a strong influence on cycle time—that is, time that one cooler is operating in deposition mode while the other is operating in sublimation mode, as detailed below.
A third operating parameter is the low thermal conductivity of the deposited CO2, which insulates the cold surface from the inlet air stream, thus reducing the ability to collect more CO2 over time. The decrease in CO2 capture efficiency due to CO2 ice buildup will also greatly influence the operating cycle time.
Another operating parameter to consider is cabin air inlet flow rate. The higher the inlet flow rate, the more CO2 is exposed to the cold surface, but the more cooling power is required to cool the air and deposit a sufficient amount of CO2. Therefore the amount of CO2 captured increases, but capture efficiency decreases.
In system 10, input cabin air at atmospheric pressure is provided to a first air-to-air heat exchanger 12 via flow path segment 14. Airflow through the system 10 can be driven by blower or similar airflow device (not shown). The flow path segment 14, as well as other segments and components described herein, are kept as short as possible to minimize thermal loss, and may be insulated, for example using Cryogel Z, which is aerogel suspended in a fiberglass blanket, and has a thermal conductivity of 0.014 W/mK. In certain embodiments, half-inch stainless steel tubing is used for air flow, and quarter inch tubing for CO2 flow.
First heat exchanger 12 begins the cooling of the cabin air, through a heat exchange with return, CO2-depleted air as described below. It will be appreciated that CO2-depleted air is air from which CO2 has been partially, completely, or substantially completely removed by the system 10. In certain embodiments, the heat exchanger 12 may be a shell-and-tube type device. Additional heat exchangers, forming multiple heat exchange stages at this or other junctures in the flow path, may be provided.
The cooled input cabin air from heat exchanger 12 is then provided to a pre-cooler 16 via flow path segment 18 for further cooling. Generally, pre-cooler 16 operates by running incoming air across a cooled surface, for example in the form of a finhead, as further detailed below. In certain embodiments, pre-cooler 16 is also configured to capture water, along with certain volatile organic compounds (VOCs), or any other compounds, trace contaminants, or the like, that may need to be removed. Such water and VOC capture can be conducted elsewhere in the flow circuit, additionally or in the alternative. In certain embodiments, pre-cooler 16 can be one of multiple pre-coolers or pre-cooler stages that can be used at this or other junctures in the flow path.
Cooled air flow from pre-cooler 16 is then directed via flow path segment 20 to a second heat exchanger 22 for additional cooling, for example to a temperature just above the CO2 deposition temperature, by way of a heat exchange with the return CO2-depleted air. Additional heat exchangers, forming multiple heat exchange stages at this juncture in the flow path, may be provided.
The air from second heat exchanger 22 is then alternately directed to first or second coolers 24 and 26, via respective flow path segments 28 and 30. It will be understood that coolers 24 and 26 may be referred to as deposition coolers, even though both deposition and sublimation operations may be performed by them. Like pre-cooler 16, the deposition coolers 24 and 26 operate by running incoming air across a cooled surface, for example in the form of a finhead, as further detailed below, in conditions conducive to CO2 deposition.
The system 10 is configured to provide continuous CO2 capture. While one of deposition coolers 24 or 26 is operating in deposition mode, the other is operating in sublimation mode, generating CO2 gas, which is directed out of the cooler through flow path segment 32. This alternating operation of coolers 24 and 26 is managed by controller 34, which may also control flow through valves 36a1, 36a2, 36b1, 36b2, 36c1, 36c2 (collectively 36), opening or closing them as necessary to establish the appropriate flow streams. Thus during the deposition cycle of cooler 24 (sublimation of cycle of cooler 26), controller 34 opens valves 36a1 and 36a2, and closes valves 36b1 and 36b2, to effect flow through cooler 24 and direct CO2-depleted return air to heat exchangers 22 and 12, by way of flow paths 28, 38 and 42. Valves 36a1 and 36a2, and valves 36b1 and 36b2, may be for example any suitable pneumatic valves. During this cycle, while cooler 24 is effecting CO2 deposition, cooler 26 is sufficiently warmed to effect sublimation of previously-deposited CO2, with controller 34 opening valve 36c2 and closing valve 36c1 to direct the sublimated CO2 gas out through flow path 32. Valves 36c1 and 36c2 may be for example any suitable solenoid valves. Conversely, during the deposition cycle of cooler 26 (sublimation of cycle of cooler 24), controller 34 opens valves 36b1 and 36b2, and closes valves 36a1 and 36a2, to effect flow through cooler 26 and direct CO2-depleted return air to heat exchangers 22 and 12, by way of flow paths 30, 40 and 42. During this cycle, while cooler 26 is effecting CO2 deposition, cooler 24 is sufficiently warmed to effect sublimation of the previously-deposited CO2, with controller 34 opening valve 36c1 and closing valve 36c2 to direct the sublimated CO2 gas out through flow path 32. In certain embodiments, a dedicated valve controller (not shown) can be provided, and its operation can synchronized with that of controller 34 and with the cyclic operation of the coolers 24 and 26.
The durations of the deposition cycles of coolers 24 and 26 can be timer-controlled, or controlled by controller 34 as a function of feedback from various points in the system, relating to parameters such as CO2 ice growth in the coolers 24 and 26, or CO2 concentration in the cabin air, or any other parameters directly or indirectly affecting operation and throughput. In addition, while alternate operation of two coolers 24 and 26 is described, additional numbers of coolers can be added to the circuit and suitably synchronized to increase throughput, and some cycles may alternate completely (180 degrees) or partially.
As described herein, in certain embodiments, controller 34 includes a microprocessor (μP) executing a computer program stored in a machine-readable storage medium (memory) for controlling system 10 for spacecraft atmosphere CO2 capture based on received feedback, the computer program comprising a routine of set instructions for causing the system to perform the steps of cooling airflow from the spacecraft atmosphere in a first heat exchange with CO2-depleted air, cooling the airflow from the spacecraft atmosphere using a pre-cooler, cooling the airflow from the spacecraft atmosphere in a second heat exchange with the CO2-depleted air, and depositing CO2 from the airflow in first and second deposition coolers that are each alternately cycled between a deposition mode and a sublimation mode. In the deposition mode, CO2 from the airflow is deposited to generate said CO2-depleted air, and in the sublimation mode, deposited CO2 is sublimated into CO2 gas. When the first cooler is operating in deposition mode, the second cooler is operating in sublimation mode, and vice versa.
Chamber 48 of cooler 24 contains a finhead 50 mounted on a cold tip 52. The cold tip cools the finhead 50 to the appropriate temperate to induce deposition of CO2 on the finhead. Cooling of the cold tip 52 can be accomplished by a cooling source 54, which can be for example a cryogenic cooler as explained below, and/or thermal radiator to deep space. Use of piston coolers and Stirling coolers is also contemplated. The cooling source 54 is precisely controllable, for example by controller 34 (
The required cooling power of deposition cooler 24, as well as pre-cooler 16 and deposition cooler 26 at one or more crew scale, may be determined based on an energy balance calculator. Input parameters include air flow rate required to remove 1 kg CO2/day (initial estimate of 5 Standard Cubic Feet Per Minute, “SCFM”), inlet CO2 concentration, cold surface temperature required, estimated HX effectiveness, and CO2 capture efficiency. In certain embodiments, for the pre-cooler 16, a Janis SC-10 provides sufficient power and can be used. For the deposition coolers 24 and 26, two Sunpower Cryotel GTs can be used. The Janis cooler is air-cooled, but the Cryotels utilize a water jacket to reject heat, so a circulating coolant loop may be supplied via a Koolance EXC-800.
In certain embodiments, the chamber 48 enclosing the cold tips and attached finheads may be manufactured by modifying ConFlat (CF) unions for the Sunpower Cryotel deposition coolers 24 and 26, and a Klein Flange union for the Janis pre-cooler 16.
In the operation of cooler 24 in deposition mode, air flow is directed into chamber 48 in the axial direction (A) from inlet 44, and passes across the cooling of finhead 50 for cooling thereby to the CO2 deposition temperature, depositing its CO2 load on the finhead. The CO2-free air is then ejected from the chamber 48, out through outlet 46. In the operation of cooler 24 in sublimation mode, finhead 50 is warmed sufficiently to cause CO2 ice that has accumulated on the finhead 50 during the previous deposition cycle to sublimate into the chamber 48 for expulsion, motivated by pressure due to the phase change of the CO2 from solid to gas. The density of solid CO2 is approximately 1500 kg/cubic meter while the density of gaseous CO2 is approximately 1.98 kg/cubic meter, yielding a volumetric expansion ratio of approximately 750:1. By selectively controlling the seal of the chamber 48 in which this expansion is occurring, pressure is allowed to build up to desired levels.
While embodiments and applications have been shown and described, it would be apparent to those skilled in the art having the benefit of this disclosure that many more modifications than mentioned above are possible without departing from the inventive concepts disclosed herein. The invention, therefore, is not to be restricted based on the foregoing description. This disclosure encompasses all changes, substitutions, variations, alterations, and modifications to the example embodiments herein that a person having ordinary skill in the art would comprehend. Similarly, where appropriate, the appended claims encompass all changes, substitutions, variations, alterations, and modifications to the example embodiments herein that a person having ordinary skill in the art would comprehend. Moreover, reference in the appended claims to an apparatus or system or a component of an apparatus or system being adapted to, arranged to, capable of, configured to, enabled to, operable to, or operative to perform a particular function encompasses that apparatus, system, or component, whether or not it or that particular function is activated, turned on, or unlocked, as long as that apparatus, system, or component is so adapted, arranged, capable, configured, enabled, operable, or operative.
This application claims priority under 35 U.S.C. §§ 119 and 120 and 37 CFR 1.78(a) from U.S. Provisional Pat. App. No. 62/871,684 filed on Jul. 8, 2019, the contents of which are incorporated herein by reference in their entirety.
The invention described herein was made in the performance of work under a NASA contract and by (an) employee(s) of the United States Government and is subject to the provisions of Public Law 96-517 (35 U.S.C. § 202) and may be manufactured and used by or for the Government for governmental purposes without the payment of any royalties thereon or therefore. In accordance with 35 U.S.C. § 202, the contractor has elected not to retain title.
Number | Date | Country | |
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62871684 | Jul 2019 | US |