Plasma-based Oxygen Generation in Space

Roadmap Creators: | Lanie McKinney | Roy Weinstock

Technology Roadmap Sections and Deliverables

This technology roadmap has the unique identifier:

2PBOGS - Plasma-based Oxygen Generation in Space

The number 2 denotes that this is a Level 2 technology roadmap at the product level. In reference to our technology, Level 1 encompasses all oxygen generation technologies that can be used in space, Level 2 is product level. Level 3, the system level, could reference the plasma source needed for conversion and Level 4, the subsystem level could represent the the kind of separation subsystem used.

Roadmap Overview

Oxygen generation in space is a critical enabling technology to support the development of human exploration of the Moon and Mars. NASA's Artemis program plans to head back to the Moon this decade in a phased approach to build up lunar activity, including a commercial presence, to serve as a proving ground for human missions to Mars. The space industry is set to be valued at ~1.8 trillion USD by 2035, with increasing access to space enabled through the recent reduction of launch costs. Increasingly longer stays off-world at destinations of increasing distance from Earth require technologies that recycle and utilize available resources, called in-situ resource utilization (ISRU), to make large mission architectures feasible and enable dramatic cost-savings. Notably, the most near-term large-scale oxygen demand will be driven by the Starship HLS vehicle's propellant needs on the Moon. There are other uses for oxygen as well as different resources available to extract oxygen, namely H2O, CO2, and Regolith, which differ between the Moon and Mars.

Non-thermal plasma reactors are emerging power-to-gas technologies with the potential to facilitate various chemical synthesis processes with hardware commonality and redundancy. In plasma-based systems, electrical power is used to ionize a feedstock gas, creating a highly reactive environment that leverages electron excitation chemistry to break stable molecular bonds and produce value-added products. Plasma reactors operate at non-equilibrium conditions, allowing for lower-temperature operation and instantaneous start-up, making them adaptable to intermittent power availability. Unlike other technologies, plasma reactors can process any feedstock gas, and have electrical settings which can be tuned to optimize or favor particular products. It has been demonstrated that plasma reactors can reduce regolith and convert water and CO2 into oxygen {CITE}, which means that plasma-based conversion technologies are extensible to both near and far-term missions at various locations on the Moon and Mars.

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Design Structure Matrix (DSM) Allocation

The market level DSM for Oxygen Generation includes candidate technologies which can extract oxygen from a variety of resources available in space, namely CO2, water, and regolith, and shows that plasma-reactors can be combined with other conversion technologies for enhanced performance. At the system level, key enabling technologies are the resource acquisition system (3RA), the plasma reactor (3PR), separation system (3SS), and environmental control system (3ECS). At level 4, the enabling technologies include 5 different kinds of plasma sources (3MWPS, 3NP, 3DCHVPS, 3ACHVPS, 3RFPS) which generate plasmas in different ways that have different characteristics, and 3 relevant kinds of separation technologies (3SM, 3VSA, 3CD). The DSM at this level clearly demonstrates the cluster of interdependencies around the selection of the the plasma source and elements of the overall reactor configuration. Different combinations of level 4 elements, like the plasma source, reactor geometry, and separation subsystem may lead to novel performance.

Roadmap Model using OPM

We provide an Object-Process-Diagram (OPD) of the 1SRG roadmap in the figure below. This diagram captures the main product of the roadmap (Space Resource Generation Systems), and decomposes the possible common subsystems of level 2 technologies into their level 3 systems (acquisition, conversion, separation stages, control system), its characterization by Figures of Merit (FOMs) as well as the main processes (of Acquiring, Converting, Separating) through a myriad of possible routes.

An Object-Process-Language (OPL) description of the roadmap scope is auto-generated and given below. It reflects the same content as the previous figure, but in a formal natural language.

Figures of Merit

The table below show a list of FOMs that can be used to assess Space Resource Generation Technologies.

Figure of Merit	Unit	Description
Production Rate	kg/hr	the rate of generating a target product
Lifetime Embodied Energy	MJ/kg	the thermodynamic sum of past, present and future work required to create, operate, maintain and decommission a system per kg of product produced
Specific Energy Consumption	kWh/kg	total energy required to produce a kg of product
Purity	%	percentage of the target product in separated product stream
Launch-adjusted Atom Economy	%	ratio of mass of useful product generated to the total mass of reactants and launched mass needed

Production Rate

At it's simplest, a space resource generation technology must meet a demand for a product, described as a production rate. Production rates can be constrained by the performance of the technology itself or also by the available inputs, as space is a resource constrained environment.

Lifetime Embodied Energy

This FOM is defined as “...the thermodynamic sum of past, present and future work required to create, operate, maintain and decommission a system, including appropriate shares of indirect contributions from upstream systems as well as from other systems in a system-of-systems [1].” This metric has been shown to reproduce results from Equivalent System Mass analyses, the industry standard for trading advanced life support systems, while decoupling from launch mass (which is becoming an outdated cost proxy). While similar in units to SEC, this FOM captures value (as a proxy for cost) across a whole architecture or lifetime. This is an extremely relevant metric to the space industry for determining what the real cost of developing infrastructure and operating a space resource generation technology over longer time periods to identify promising business use cases.

Specific Energy Consumption

This FOM describes the energy required of a chemical conversion process per kg of value-added output produced. It is an energy efficiency metric which can allow comparison between different technologies which may use different chemical pathways. With production rate, these FOM’s can be multiplied to imply a power requirement.

Purity

This FOM describes how pure the useful product stream is after separation of the output stream, which is an evaluation of conversion and separation performance.

Launch-adjusted Atom Economy

This is a measure of a conversion technology's sustainability, or its ability to minimize waste streams through recycling available resources. It is launch-adjusted so as to reward better recycling and penalize when resources must be launched to refuel. This metric is relevant for comparing conversion architectures that may involve different chemical pathways which make better use of launched materials and/or not require launched materials at all, and may produce less waste or unusable products. This also aligns with NASA’s space sustainability goals and may support long duration missions where local resources are unavailable or a particular resource is scarce.

LaAE = (product mass [kg/mol] ) / (launch + reactant mass [kg/mol])

Note

These Figures of Merit are more general than other FOMs used to describe particular technologies, because other metrics will have different fundamental limits which depend heavily on the technology. For example, faradaic efficiency describes how efficiently an electrolytic cell performs compared to theoretical limits, but this would not be a fair comparison to a reported first law efficiency (energy out/energy in) of a thermal conversion process. These FOMs were chosen to compare different space resource generation technologies at the market level.

Governing Equations

All conversion processes must conserve matter, which is why the available resources constrain the products that can be made. Broadly speaking, all conversion processes require some energy. The first law of thermodynamics ensures that energy is conserved within a system ΔU=Q−W, where ΔU is the change in internal energy of the system, Q is the heat added to the system, and W is the work done on the system. The second Law introduces the concept of entropy (S) and sets limits on the efficiency of conversion processes. The entropy change in a system is: ΔS= Q/T, where T is the temperature at which heat is transferred. All conversion processes can be described by these laws, and fundamental limits on each depend on the mechanism by which the conversion is occurring. It depends on if it is a chemical reaction, or purely a thermal process involving a change of state, or something else. ​

Time Evolution of Figures of Merit

Because many space resource generation proposals require interplanetary travel to the Moon and Mars, at the market-level these technologies are in the initial proof-of-concept phase. The only demonstration of such a technology off-world is MOXIE (Mars OXygen ISRU Experiment) which produced oxygen aboard the perseverance rover [2]. But we have been using oxygen generation technologies in space on the ISS which can be modeled over time, though it is important to note that this technology is more advanced compared to other space resource generation systems. Additionally, there are currently very few adopters of this technology (because there are very few space stations), so there isn’t a lot of data.

Increasing the Launch-adjusted atom economy FOM first became desirable due to the continuous operation of space stations like Mir and the ISS and the expensive cost of launch to supply life-support consumables. The first example is burning alkali chlorate “candles,” a form of solid oxide fuel generation [ https://en.wikipedia.org/wiki/Vika_oxygen_generator]: 60% of what is launched and used creates usable oxygen, so this leads to an atom economy of around 30%. Introducing water electrolysis to the ISS improved the atom economy, because 89% of what is launched is used as oxygen [3]. But because water is also a demand on the crew and this consumes water, it still has to be launched and results in an atom economy of 42%. An atom economy of 47% is achieved with a Sabatier reactor recovering some of the water and reducing the required launch mass, but still does not reach full conversion of CO2, and produces Methane which is currently vented [4]. A series bosch reactor, projected to reach maturity by around 2035, has a theoretical SAE limit of 73%, and paired with a carbon utilization technology could increase more [5]. Plotting the hypothetical maturation date, we see this may be the beginnings of stagnation, because where beyond full oxygen recovery there may be less desire to improve this FOM. The theoretical limit is 100%. In terms of oxygen generation and recovery technologies, it appears to be somewhere between rapid progress and slowing down (shown in the linear trend line) in improving how well we recycle launched oxygen-containing materials into space, approximately equal to about 1.11% improvement in LaAE per year using a linear regression.

The specific energy input required per kg of product has also increased over time, with the updated technologies and additions of reactors in series, but in the case of space station operations where energy can be renewably generated by solar panels, this has been favorable to minimize the launch mass. Interestingly, the decrease in specific energy input for MOXIE offers new value to stakeholders who are not concerned with full oxygen recovery, but instead want an efficient conversion process to scale up production.

Bibliography

[1] Lordos, G. C., Hoffman, J. A., and Summers, S. E., “Towards a Sustainable Industrial Development of Mars: Comparing Novel ISRU / ISM Architectures Using Lifetime Embodied Energy,” 2018 AIAA SPACE and Astronautics Forum and Exposition, 2018. https://doi.org/10.2514/6.2018-5125

[2] Hoffman, J. A., Hecht, M. H., Rapp, D., Hartvigsen, J. J., SooHoo, J. G., Aboobaker, A. M., McClean, J. B., Liu, A. M., Hinterman, E. D., Nasr, M., Hariharan, S., Horn, K. J., Meyen, F. E., Okkels, H., Steen, P., Elangovan, S., Graves, C. R., Khopkar, P., Madsen, M. B., Voecks, G. E., Smith, P. H., Skafte, T. L., Araghi, K. R., and Eisenman, D. J., “Mars Oxygen ISRU Experiment (MOXIE)—Preparing for Human Mars Exploration,” American Association for the Advancement of Science, Vol. 8, No. 35, 2022, p. eabp8636. https://doi.org/10.1126/sciadv.abp8636

[3] “Vika Oxygen Generator,” Wikipedia, Oct 06 2023. https://en.wikipedia.org/wiki/Vika_oxygen_generator

[4] Takada, K., Hornyak, D., Garr, J., Keuren, S. V., Faulkner, C., and ElSherbini, A., “Status of the Advanced Oxygen Generation Assembly,” presented at the 52nd International Conference on Environmental Systems, Calgary. https://ntrs.nasa.gov/citations/20230008013

[5] “SpaceCraft Oxygen Recovery (SCOR) - NASA.” Retrieved 8 October 2024. https://www.nasa.gov/spacecraft-oxygen-recovery-scor/