Potential Game-Changer for Generating Propellant on Mars.
Copyright 2023 Robert Clark
As an educator it’s quite exciting that an important unsolved problem may be one university or high school students, or even amateur scientists can address.
A possibly limiting technology for a manned Mars mission is the high energy requirements to produce the return propellant on Mars. Robert Zubrin estimated for a Starship-sized vehicle as it requiring possibly 10 football fields worth of solar panels, or possibly even a nuclear reactor placed on Mars:
Marcus House in his video series also discusses the problem:
SpaceX Starship can return from Mars without surface refilling.
Slide from the video showing proposed size of Mars solar panels for ISRU propellant production:
Beginning at about the 12 minute point, House runs the numbers and considers it so daunting, at least for initial missions, that he suggests it might be better instead to transport the propellant from Earth with Starship tanker flights all the way to Mars. This of course discounts the entire advantage of using Mars generated propellant to achieve a smaller mission size.
The problem is the method envisioned would be by either splitting CO2 into carbon and oxygen and/or H2O into hydrogen and oxygen by electrolysis to be used for the propellant. However, these are energy intensive operations, which explains why in the reverse direction you get so much energy out when you combine them for combustion.
What the researchers for this new idea are proposing instead is getting the oxygen for Mars atmosphere by filtering out the free O2 already there:
Feb 19, 2022 Breathing Mars Air: Stationary and Portable O2 Generation. Ivan Ermanoski Arizona State University The basis for TSSD is a two-step thermally-driven cycle operating below ~260 C. thermal swing sorption/desorption (TSSD)--to generate oxygen from the Mars atmosphere with 10x less energy than the state of the art. Our approach is motivated by thermodynamics: the minimum theoretical work to separate oxygen from the Mars atmosphere is ~30-50 times lower than to obtain it by splitting carbon dioxide. Efficiency: TSSD is expected to be ~10x more efficient than MOXIE. For MOXIE, the target power requirement for oxygen propellant production is 30 kW. The TSSD estimate is only 4 kW; i.e., 90% less than MOXIE. Applying TSSD in rovers, the estimated power for oxygen pro-duction is only ~50 W/person. https://www.nasa.gov/directorates/spacetech/niac/2022/Breathing_Mars_Air/
Here’s the breakdown of the Martian atmosphere:
Atmosphere of Mars. General information[2] Average surface pressure 610 Pa (0.088 psi; 4.6 mmHg; 0.0060 atm) Mass 2.5x1016 kg[1] Composition[3][4] Carbon dioxide 95% Nitrogen 2.8% Argon 2% Oxygen 0.174% Carbon monoxide 0.0747% Water vapor 0.03% (variable) https://en.wikipedia.org/wiki/Atmosphere_of_Mars
The authors so far are proposing just getting O2 from the Martian air. But note there is also free carbon monoxide(CO) in its atmosphere. Then it may also be possible to filter out CO from Mars air. This is quite important because CO can be made to combust with O2:
1.)2CO+O2→2CO2;ΔH=−569kJ/mol
However, when CO can be obtained from low energy filtration from the Martian atmosphere then free hydrogen for propulsion can be obtained by the reaction:
2.)CO + H2O → CO2+ H2, ΔH = -41 kJ/mol
This uses water that both orbiters and landers have shown to be wide spread on Mars even in low latitude locations in ice form.
You can then get methane, if that is the preferred fuel over hydrogen, by reacting the free hydrogen with CO2 by the famous Sabatier reaction:
3.)∆H = −165.0 kJ/mol
The nice thing about this proposal is that any decent high school or university lab or even well-appointed amateur lab can test it out themselves. Key question: what is the method requiring least energy to concentrate and filter out the O2 and CO when in the concentrations known in the Martian atmosphere?
About how to filter out the O2 and CO, three possibilities come to mind:
Of game-changing importance is that rather than football fields of solar cells or even a nuclear power plant being needed to generate the propellant, it may be the process of producing the propellant actually generates power.
But if this is so easy, why has this not been done before? Mars atmosphere is almost all CO2, so the obvious thing to investigate is getting O2 by splitting CO2 into carbon and O2 by electrolysis.
On the other hand, O2 is only a little more than 1/1000th of Mars atmosphere, which itself is only 1/100ths Earth’s air density. So the oxygen there would be in the range of only 1/100,000th the density we normally see on Earth. So this would require an extreme level of densification to be usable.
However, in this regard, it may be sufficient to just cool the oxygen to liquid form which is needed for the propellant application anyway. This might not require too high energy input since at night the temperatures drop to well below freezing on Mars.
The most optimal generation and separation/filtration methods have to take into account the conditions on Mars. For instance in eq. (2.) the energy released of -41 kJ/mol is when the water is in gaseous form. It's much less if it is liquid or frozen water form. But in this regard it is notable temperatures on Mars surface do reach above the melting point of water on large swaths of Mars during the the mid afternoon hours on Mars.
It had been frequently said that liquid water can not exist on Mars because Mars landers had shown the air temperatures were always below freezing. But what was missed was the actual surface temperatures were often well above the air temperatures. This is a rather obvious point that's easily understood when you compare for example on Earth the temperature of the air on a hot Summer day with what the temperature on the ground might be, especially if the ground is a dark surface absorbing sunlight rather than reflecting it. See the graphs from the Spirit and Opportunity MER rovers:
But because of the low atmospheric pressure the time at which the water will remain liquid is relatively short, even if the ground temperature will be above the melting point for hours. This is good for our propellant generation purposes though since we will get the water in gaseous form without additionally adding energy.
Hydrogen fuel from Mars to Earth?
The propellant generation from taking atmospheric oxygen and carbon monoxide might actually generate energy. But this means we could actually produce hydrogen and methane at low cost, unlike the electrolysis methods that require high energy input. This shows the importance of this in general not just for Mars. It could be a means of producing clean energy in the form of hydrogen being transported from Mars to Earth. Again, it is remarkable that this is something high school and university teams can contribute to establishing its feasibility.
But for it to be feasible, the energy requirements for producing the O2 and CO will have to be improved. The NASA release on the new TSSD method touts its reduced energy requirements over the usual electrolysis method, MOXIE, giving its power requirements as 4 kW. But the news release doesn't say how much oxygen is produced by the process for that power. This article allows us to estimate it:
MARCH 3, 2022 BY ANDY TOMASWICK
Mars Explorers are Going to Need air, and Lots of it. Here’s a Technology That Might Help Them Breath Easy.
...
All that power consumption makes for an expensive system. A MOXIE machine that can create 2 kg O2 per hour (enough to support two explorers) would require around 25 kW of power, or slightly less than the average American house uses per day. While that may not seem like a lot, utilizing solar energy is a much more difficult prospect on Mars. Any early solar farm built to run the MOXIE system would dwarf the habitat it could supply with oxygen for just two astronauts.
Enter the TSSD, which nicely eliminates all three major problems with MOXIE. The system itself relies on a thermochemical pumping system, which relies on heat differentials to move the atmosphere to the appropriate place, eliminating the need for a mechanical pump. It also doesn’t suffer from carbon fouling as it doesn’t break apart CO2. Lastly, it doesn’t require too much energy, with Dr. Ivan Ermanoski, the PI on the NIAC funded project and a research professor at Arizona State University, expecting that it will be 90% more efficient than MOXIE.
TSSD needing 1/10th the power of MOXIE would suggest 2 kg per hour of O2 for 2,500 watts, or 1 kg per hour for 1,250 watts. The process operating at an hour would require 1,250w*3,600s = 4,500,000 joules, 4.5 MJ, of energy per kg of O2 generated.
Then we also need to separate out the CO. The CO content in the Mars atmosphere is about half that of the O2 so as a first order estimate we can estimate it as taking 9,000,000 joules per kg generated, though to get the actual value would take closer examination of the whole process used to generate the O2 and how it would have to be adapted to generate the CO.
To get an idea of how much total energy this is in practical terms for both O2 and CO production, equation 1.) gives the energy generated by combusting CO in O2 as 569,000 joules per mol of O2. Since O2 is 32 grams per mol, this is 569,000J/32gm = 17,800 joules/gm or 17.8 MJ per kg of O2. But the CO needed for the reaction is 2 moles for each mole of O2, and since they are close to the same molecular weight you need about 2 kg of CO for each kg of oxygen.
So you need 4.5 MJ of energy to generate a kg of O2 and then 2*9 MJ = 18 MJ to generate the needed 2 kg of CO. This is a total of 22.5 MJ required to put in to get out the 17.8 MJ you would get by combusting the CO with the O2.
For only producing the H2 by equation 2.), you really need only the CO but the molecular weight of the CO is 15 times that of the H2 so you would need 15 times as much CO and its high energy cost by the TSSD process for each kg of H2 produced.
Then the major impediment for making this feasible is finding low energy methods of separating out O2 and CO at approx. 1 part per thousand concentrations from a CO2 atmosphere only 1/100th as dense as Earths.
TSSD is about 1/10th as power intensive as the usual electrolysis method. To put this in perspective, instead of 10 footballs fields of solar cells to run the propellant generating plant for the Starship on Mars, it would only take 1 football field worth of solar cells. The energy requirements probably need to be reduced again by at least another factor of 10.
Some recent advances in gas separation by absorption of surfaces might improve on the energy requirements of TSSD:
Weird Crystal Can Absorb All The Oxygen In A Room — And Then Release It Later
This could potentially make fuel cells, space travel, and scuba diving a lot more efficient.
BY SARAH FECHT | PUBLISHED OCT 2, 2014 1:15 AM EDT
...
Researchers from the University of Southern Denmark say they’ve invented a crystal that pulls oxygen out of the air and even water. Apparently, just a spoonful of the stuff can suck up all the oxygen in a room.
The crystal is a salt made from cobalt*, and it appears to be capable of holding oxygen at a concentration that is 160 times higher than the air we breathe. The paper notes that “an excess” of the substance would bind up to 99 percent of the oxygen in a room.
But what’s more remarkable is that the crystal can later release the oxygen when exposed to heat or low-oxygen conditions. In a press release, study author Christine McKenzie likens it to the hemoglobin in our blood, which uses iron to bind and release oxygen in the human body.
Unlike the windows of your house, nanoscale holes in graphene (named as “nanowindows”) can selectively choose which type of air molecules can pass through.
Scientists from Shinshu University and PSL University, France, theoretically proved concerted motion of the nanowindow-rim to selectively allow molecules to pass, in an energy-efficiently and fast way (Nature Communications, “Air separation with graphene mediated by nanowindow-rim concerted motion”). This brings up new possibilities to create an advanced molecular separation membrane technology.
The mechanism of separation by nanowindows is that the atomic vibration of the nanowindow-rim changes the effective nanowidow size. When the rim of one side is deviated and the other is deviated to the opposite direction, the effective nanowindow size becomes larger than when the rim does not move. This effect is very predominant for molecules of oxygen, nitrogen, and argon, inducing an efficient separation of oxygen from air.
Alternatively, this is low enough that a space elevator can do it using currently available materials. Still another possibility is by using a railgun, i.e., electromagnetic accelerator.
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