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Thread: technicil issues, providing oxygen

  1. #1
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    technicil issues, providing oxygen

    I am going to cut and paste some discussion from other threads and more it to a new topic so it is easier for others to find. Please feel free to add your own comments.


    The power consumption of various oxygen production schemes.

    Oxygen concentrator direct from Mars atmosphere. The current medical oxygen concentrators use roughly 300W to produce 5 liters per minute. On Mars the over atmosphere must be compressed 15 times to get to 1 barr, then would also need to be an absorbent CO2 to minimize that first, exposing the used absorbent to the outside near vacuum and concentrated should refresh it for the next cycle. That should add little extra power but should not more than double to power consumption. Worst case 3KWh for 25 liters of O2.

    Water extraction from soil. Using the microwave extractor, 10KW for maybe half an hour, for 5KWh plus rover power, to produce 60 liters. From that, 0.008KWh per mole which is small addition. So 5KWh plus for 22.4 liters of O2.

    So it looks like they are much closer than I thought, but the concentrator still looks a little better.


    StarFarer42:
    Mike, you've mentioned this oxygen concentrator idea before but it won't work. I'm sure medical oxygen generators work great but they also have the advantage of operating in an environment that already has 20 kPa partial pressure of O2. On Mars, O2 makes up just 0.14% of the atmosphere (not the 0.6% figure you gave in a different thread). Also, the average atmospheric pressure is just 0.6 kPa which means the partial pressure of O2 is 0.6 kPa * 0.0014 = 0.00084 kPa. To get the 20 kPa partial pressure of O2 that humans need to live you'd have to concentrate the available oxygen almost 24,000 times! It's just not practical.

    Mauldred:
    It's much more practical to get O2 from electrolysis of water. And yes, for the skepticals, there are large quantities of water on Mars, in the form of ice just beneath a thin regolith layer in many places. Phoenix only had to remove a few millimeters of regolith to find water ice underneath.
    http://news.bbc.co.uk/2/hi/science/nature/7465419.stm
    Each time there's a meteorite crashing on Mars the splashes expose large quantities of ice: http://news.nationalgeographic.com/n...e-picture.html
    The method to extract water from the ground without drilling has already been explained.

    me:
    Starfarer42, you are quite correct. I was talking on the phone while trying to write this up and left out the compression step. You are correct that neither step will work well without first reaching near Earth normal pressures. Also I have been seeing different partial pressures on Mars for nitrogen and oxygen. I did find to reputable (better than wikipedia, which is where I probably got the .6% number), I tend now to agree. The oxygen will be 0.0063 time lowered overall pressure of 0.007, it will need concentrating and compressing of over 20,000 times its initial pressure. But it does take MUCH less energy to compress a near vacuum than air that is already dense or under high pressure. I still think it will be less power, and mechanically easier and more reliable to compress Mars are, absorb out the CO2 (to be added to the garden to enhance growth rates) then of the remaining Argon, Nitrogen, Oxygen atmosphere, run the concentrator a little higher in the cycle to get better purity oxygen from 20 to 1 instead of 3 to 1 Nitrogen. I am not sure if the zeolite would capture the Argon as well. And the argon might be useful for TIG welding, industrial processes, and maybe even Mars Suit mixtures to avoid nitrogen narcosis problems in the outside suit run at substantially lower pressure than inside the habitat.

    I realise that the rovers must still be sent out to get water, and currently it looks like electrolyses to make oxygen once the water has been collected in a small price to pay in the power budget, and it also produces hydrogen, which can be used to converting CO2 back to O2 and methane, but I still like the greater mechanical simplicity of a stationary oxygen generator instead of driving a rover all over the place to get to fresh regions where the water has not be all 'exploited; already.

    But if both systems or so close in power budget, then maybe they should each be the redundant backup for the other.

    Mike
    MythBuster, Rocket Scientist, roboticist, Mars One Applicant (did not pass round 1)
    http://applicants.mars-one.com/profile/d6d61b0b-5736-43ff-8fa9-8378ba8b48a1
    Mike's descriptive term of the week: "Buried Bigelows" <=> inflatable living structures, vacuum rated, Mars gravity rated, deployed in deep trenches and covered with 5 meters of regolith
    Note: Not affiliated with Bigelow Aerospace, accept to honor their technological success in LEO

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    Having both systems so that one of them could be a backup solution is a good idea, plus the astronauts will need water anyway for their own consumption, and compressing CO2 can also be useful to make O2 and CH4 (with the Sabatier reaction as proposed by Dr Zubrin) that's easier to stock than H2. O2 and CH4 can be used as a backup energy source in case of dust storm for instance.

    Zubrin relies on a nuclear reactor though. So maybe the solar panels' energy output will remain a problem.
    Last edited by Mauldred; 09-09-2013 at 10:32 AM.
    "A still more glorious dawn awaits; not a sunrise, but a galaxyrise. A morning filled with four hundred billion suns, the rising of the Milky Way." - Carl Sagan
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    Interesting how the oxygen concentrator works. New to me, although I've heard of swing processors. Perhaps the concentrator could be most useful the other way around, to concentrate Nitrogen. The MO ECLSS will need to produce large quantities of buffer gas (nitrogen/ argon mix) apart from oxygen, and this may be a better way to do it than using cryogenics.

  4. #4
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    Quote Originally Posted by Mauldred View Post
    O2 and CH4 can be used as a backup energy source in case of dust storm for instance.
    this is a good idea and Zubrin makes a very good case for using this as the fuel for rovers in his "The Case for Mars".
    MO can also use this in fuel cells to produce electricity, but as a compact fuel for external powered operation it would be very handy.

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    I had always wanted fuel cells to make it up to at least the complexity of methane, because hydrogen is so hard to store. I did a quick search and it looks like methane fuel cells are currently lower efficiency, need to run at very high temps (600 to 800C), and may require adding or refreshing the membranes or electrodes with exotic metals. But if they are that good now, by the time the design is frozen and hardware built, they may be much more efficient and durable.

    Also note that when running electrolytically derived hydrogen (or methane derived from the hydrogen and CO2) it will all the oxygen (for H2, 1.5 times for methane!), so a second source of oxygen may be beneficial. On the other hand, having an embarrassment of riches of hydrogen doesn't sound like a problem!

    Mike
    MythBuster, Rocket Scientist, roboticist, Mars One Applicant (did not pass round 1)
    http://applicants.mars-one.com/profile/d6d61b0b-5736-43ff-8fa9-8378ba8b48a1
    Mike's descriptive term of the week: "Buried Bigelows" <=> inflatable living structures, vacuum rated, Mars gravity rated, deployed in deep trenches and covered with 5 meters of regolith
    Note: Not affiliated with Bigelow Aerospace, accept to honor their technological success in LEO

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    I have a number of concerns about relying on concentrating O2 from the ambient Martian atmosphere:

    1 - You're starting with such a low ambient pressure that it can only charitably be called an "atmosphere" in comparison to the vacuum of space. If you could produce a pressure that low in a laboratory on Earth you'd call it high grade vacuum. There's just not much material to start with which leads to...
    2 - You need such a high compression factor -- 20,000 times or more -- that I have trouble believing you could manage it. You say that's not a problem and I'd like to see a reference on that because...
    3 - So far I can find only *one* reference to concentrating oxygen on Mars. It seems legit but it must be a very under-appreciated idea if nobody else is talking about it.
    4 - I've been reading everything I can find about how you would colonize Mars and live off the land. So far none of the web-sites and books I've read even mention this as a possibility. Are the authors unaware of this idea or do they feel it's so unrealistic that it doesn't even merit a mention?
    5 - Where does the oxygen come from? Is there some natural process that replenishes it? How much can you pull from the atmosphere before you find there's none left?
    6 - The atmosphere is mostly CO2. What do you do with all of this waste? Can you vent it back to the outside air? Wouldn't that interfere with the process of concentrating the oxygen?
    7 - Is this proven technology? Remember that Mars One's modus operandi is to use existing technology as much as possible.

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    Here's my thinking about "mining" oxygen on Mars. This is based on material I've been reading -- mostly Zubrin's "The Case for Mars" -- but also several other books and web-sites.

    1 - It starts with water. The pioneers need water for drinking and growing plants so they have to collect it anyways. Fortunately it's readily available because the colony has been situated at roughly 40 degrees N latitude where the soil is about 4% to 5% frozen water.
    2 - You electrolyze the water into hydrogen and oxygen. Compress the oxygen and store it in tanks.
    3 - The hydrogen is valuable stuff but it's hard to store. Fortunately we can use a Sabatier reactor to combine it with CO2 from the atmosphere to produce methane and water.
    4 - Compress and store the methane. Send the water back to the electrolysis units to be recycled.

    If you think about it, you're getting oxygen from *both* the water and the Martian atmosphere which is a win-win scenario. You can take the waste heat generated by the Sabatier reactor and use it to heat the water prior to electrolyzing it, which actually reduces the overall energy required compared to electrolysis alone. The methane you get as a by-product is useful stuff too. It can be used as rocket fuel, or in a fuel cell, or it can be fed through further chemical processes to create plastics.

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    So, oxygen would be needed for the first habitat, but after the first colonists arrive would there not be oxygen production from the first set of plants? How long would it take for plant produced oxygen to take over from technologically produced oxygen? Seems to me oxygen would only need to be estracted from soil water or air for the first set of buildings, till the first colonists arrive with the biological method. Or am I missing something here...?

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    Making enough oxygen from plants will be hard to arrange because the plants will be using artificial lighting to produce the oxygen, and only after they have enough for their own needs. The overall energy requirements for oxygen are lower by producing it directly. You also need an artificial source of oxygen to keep it at the right concentration in the base atmosphere.
    Making oxygen will be a part of living on Mars for many decades to come, I think.

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    Interestingly, perchlorates, which can kill the astronauts anytime if they enter into contact with untreated Martian soil, can also be used to produce the oxygen they require to live.

    But the process seems not so simple because the thermal decomposition of Mg(ClO4)2 produces MgCl2 and O2 but also MgO and Cl2, which we don't want, so we must find a way to catalyse the transformation in O2 and MgCl2 to avoid the formation of MgO and Cl2.

    http://www.lpi.usra.edu/meetings/lpsc2011/pdf/1909.pdf
    "A still more glorious dawn awaits; not a sunrise, but a galaxyrise. A morning filled with four hundred billion suns, the rising of the Milky Way." - Carl Sagan
    "The good thing about science is that it's true whether you believe in it or not." - Neil DeGrasse Tyson

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