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> It's likely that it will either be impossible or prohibitively expensive to keep CO2 levels at normal Earth standards. I think we shouldn't use orbital space

by ajuc 5y ago

> It's likely that it will either be impossible or prohibitively expensive to keep CO2 levels at normal Earth standards. I think we shouldn't use orbital space station where space and energy is very constrained as model for lunar base (where space and energy will be quite easy to get by comparatively). Average person breaths out ~1kg of CO2 per day or ~400 per year [1]. Corn field absorbs ~4000 kg of CO2 yearly per 1000 m2 [2]. The same 1000 m2 field will produce ~4000 ears or 320 000 kcal yearly [3][4] Assuming 2000 kcal for average person (so 730 000 kcal yearly) - we need 2000 m2 of corn fields to produce food for that person and just 100m2 to absorb CO2 from that person. Food will be a more important constraint in a self-sustaining base. That means if we build lunar base in the lava tubes (that are often over 300 meters wide and go on for kilometers), we can keep them self-sustainable at density of 1 person per less than 10 meters of the lava tube. [1] https://www.nrdc.org/onearth/waiting-exhale#:~:text=So%20breathe%20easy.,CO2%20as%20his%20sedentary%20brethren https://www.nrdc.org/onearth/waiting-exhale#:~:text=So%20bre....) [2] https://www.canr.msu.edu/news/corn_fields_help_clean_up_and_protect_the_environment https://www.canr.msu.edu/news/corn_fields_help_clean_up_and_... [3] https://www.quora.com/How-many-ears-of-sweet-corn-are-in-an-acre https://www.quora.com/How-many-ears-of-sweet-corn-are-in-an-... [4] https://www.nutritionix.com/i/usda/corn-1-ear-medium-6-0.75-to-7-1-2-long-yields/513fceb575b8dbbc21001520 https://www.nutritionix.com/i/usda/corn-1-ear-medium-6-0.75-...

lazide 5y ago
That math seems a bit implausible if you look at it from a closed system perspective. For said 1000m^2 field, it has to get the carbon from somewhere to produce the food, that then powers 6 months worth of activity for said human. If a human via respiration can only provide a tiny fraction of it (per the inverse of the other statement regarding:amount of Co2 absorbed for the field), where does the rest come from? Decaying organic material? Soil? Both of those are problematic closed system wise. There is a lot of very dubious math in everything from ‘amount of water consumed’ and ‘amount of carbon absorbed’, etc. which if you dig into it really doesn’t add up. This feels like one of them?
- jjk166 5y ago
  You would have to bring in enough carbon for the initial setup. Once you have enough material to grow the first crop, you will decompose previous crops to provide carbon and other nutrients for subsequent crops. In a closed system, none of that is being destroyed. In a real system you will get some losses (absorbtion by structural materials, population growth, leaks, etc) but these would be small compared to the overall amount, and easily made up with resupply at first and mined material over the long term.
  - ajuc 5y ago
    It's so ironically steampunk that one of the main things we will supply to space stations at first will be coal :) And that each of them will need a big biofuel-fired powerplant just to produce food :)
    - runnerup 5y ago
      Could imagine synthetic diamond dust instead of coal.
- ajuc 5y ago
  That just shows that the actual problem in a base with self-sustaining food production will be the inverse - not enough CO2 to produce food instead of too much of it to breath. To solve this you need initial biomass (or CO2) and then after the harvest you burn or otherwise decompose everything you didn't eat to close the cycle and get your CO2 (and some of the energy) back. The easiest way to achieve that initial CO2 concentration would probably be to bring about ~3.5 tonnes of coal per person, burn it with oxygen mined from lunar soil generating energy for your initial base-building (24 MWh of heat about 40% of which you can convert to electric energy) and capturing the resulting 8 tonnes of CO2 for agriculture. Of course you'd bring reasonably pure carbon, not the cheap dirty stuff we dig from the ground with heavy metals and sulphur in it. Also catalyst would be needed to avoid producing carbon monoxide. Another concern is how much volume of air we need in these tubes to keep the atmosphere breathable and suitable for enough CO2 to supply plants. Assume the tube is a cylinder 150m in radius and 10 meter in height (height is horizontal here ;) ). That's 700 000 m3 of air which should weight about 857 500 kg. At 410.28 ppmv CO2 in air we can calculate that 0.0623240117 % of the air mass should be CO2, which is ~535 kg in that 10-meter section of the tube. This is a long way off the 8 tonnes of CO2 needed to grow the plants, but they don't want all of that CO2 at once anyway - so we store the waste plant matter from harvest and burn it slowly over the year to keep CO2 concentration in air breathable. We could also separate the atmospheres for plants and for people to optimize CO2 ppm for each, but that makes math more complicated :)

by ajuc 5y ago

lazide 5y ago

That math seems a bit implausible if you look at it from a closed system perspective. For said 1000m^2 field, it has to get the carbon from somewhere to produce the food, that then powers 6 months worth of activity for said human. If a human via respiration can only provide a tiny fraction of it (per the inverse of the other statement regarding:amount of Co2 absorbed for the field), where does the rest come from? Decaying organic material? Soil? Both of those are problematic closed system wise. There is a lot of very dubious math in everything from ‘amount of water consumed’ and ‘amount of carbon absorbed’, etc. which if you dig into it really doesn’t add up. This feels like one of them?

jjk166 5y ago
You would have to bring in enough carbon for the initial setup. Once you have enough material to grow the first crop, you will decompose previous crops to provide carbon and other nutrients for subsequent crops. In a closed system, none of that is being destroyed. In a real system you will get some losses (absorbtion by structural materials, population growth, leaks, etc) but these would be small compared to the overall amount, and easily made up with resupply at first and mined material over the long term.
- ajuc 5y ago
  It's so ironically steampunk that one of the main things we will supply to space stations at first will be coal :) And that each of them will need a big biofuel-fired powerplant just to produce food :)
  - runnerup 5y ago
    Could imagine synthetic diamond dust instead of coal.
ajuc 5y ago
That just shows that the actual problem in a base with self-sustaining food production will be the inverse - not enough CO2 to produce food instead of too much of it to breath. To solve this you need initial biomass (or CO2) and then after the harvest you burn or otherwise decompose everything you didn't eat to close the cycle and get your CO2 (and some of the energy) back. The easiest way to achieve that initial CO2 concentration would probably be to bring about ~3.5 tonnes of coal per person, burn it with oxygen mined from lunar soil generating energy for your initial base-building (24 MWh of heat about 40% of which you can convert to electric energy) and capturing the resulting 8 tonnes of CO2 for agriculture. Of course you'd bring reasonably pure carbon, not the cheap dirty stuff we dig from the ground with heavy metals and sulphur in it. Also catalyst would be needed to avoid producing carbon monoxide. Another concern is how much volume of air we need in these tubes to keep the atmosphere breathable and suitable for enough CO2 to supply plants. Assume the tube is a cylinder 150m in radius and 10 meter in height (height is horizontal here ;) ). That's 700 000 m3 of air which should weight about 857 500 kg. At 410.28 ppmv CO2 in air we can calculate that 0.0623240117 % of the air mass should be CO2, which is ~535 kg in that 10-meter section of the tube. This is a long way off the 8 tonnes of CO2 needed to grow the plants, but they don't want all of that CO2 at once anyway - so we store the waste plant matter from harvest and burn it slowly over the year to keep CO2 concentration in air breathable. We could also separate the atmospheres for plants and for people to optimize CO2 ppm for each, but that makes math more complicated :)