How many plants are needed to keep someone alive?

[factotum]

There's a fairly common trope in SF series where you have a space station of some kind, and the lives of the people on board are maintained by having green vegetation around to exchange carbon dioxide with oxygen. I was just wondering about this, and I ask the Playground if anyone knows: has anyone ever worked out just how large an area of green plants would be needed to maintain the oxygen levels for a single person? Just curiosity, mainly, because I was wondering how much of the interior of a station would have to be given over to vegetation and how much could be used for people.

[gomipile]

You might find this paper from 2003 interesting:

https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20040012725.pdf

Specifically, around the 11th page, it covers the use of plants to provide breathing oxygen.

[factotum]

Hmmmm...interesting article, but not quite what I'm after. It just says that "half the food provided must be growing plants to provide enough oxygen for the crew" without really specifying how much that is, unless I'm missing something from my quick skim read?

[gomipile]

Hmmmm...interesting article, but not quite what I'm after. It just says that "half the food provided must be growing plants to provide enough oxygen for the crew" without really specifying how much that is, unless I'm missing something from my quick skim read?

Earlier, it goes into mass fractions of oxygen, water, and food needed to support humans. If you combine those numbers with this, I think that'll get you closer.

[deuterio12]

There's a fairly common trope in SF series where you have a space station of some kind, and the lives of the people on board are maintained by having green vegetation around to exchange carbon dioxide with oxygen. I was just wondering about this, and I ask the Playground if anyone knows: has anyone ever worked out just how large an area of green plants would be needed to maintain the oxygen levels for a single person? Just curiosity, mainly, because I was wondering how much of the interior of a station would have to be given over to vegetation and how much could be used for people.

Too many open variables to provide a simple answer.

Different plants produce oxygen at different rates, and then the same plant's oxygen release can vary depending of the enviroment conditions (in particular sunlight).

Then different people may have different oxygen needs. Like people living in tall mountains can grow used to living with lower oxygen concentrations in the air.

And of course it also depends of whetever you want to keep that person at peak condition or just barely enough oxygen to keep them awake. If the human is expected to do a lot of heavy lifting they'll need more oxygen but if they just need to monitor a bunch of machines they'll need less and so on.

[factotum]

Well, surely you can just take the averages over a population (on both the plant and human side) and use those? @Grytorm: I'll have a closer look at the link you provided, but it's quite dense and I don't know when I'll have the time to do it. Thanks!

[Brother Oni]

Well, surely you can just take the averages over a population (on both the plant and human side) and use those? @Grytorm: I'll have a closer look at the link you provided, but it's quite dense and I don't know when I'll have the time to do it. Thanks!

Page 5, table 2 states that the standard crewman needs 0.84kg of oxygen daily
Page 11, Table 6 states that the standard crewman generates 1.00kg of carbon dioxide daily

Note that this CO2 is generated purely from ingested carbohydrates - all the respired O2 is excreted as water (see the Krebs Cycle and oxidative phosphorylation for further details).

Making the assumption that 80% of a fresh plant is water, that leaves 20% of the plant made of other elements and compounds. Assuming that 40% of this remainder is carbon, that makes 8g of carbon per 100g of plant material. Given photosynthesis generates 6 O2 per 6 CO2, that's 8g of O2 per 100g of plant material.

About 50% of the generated O2 is consumed by the plant itself (link (https://academic.oup.com/aob/article/94/5/647/151785)), so that's 4g O2 / 100g plant material grown available for people.

Dividing the daily needs by that generation rate:

(840g/4g)*100g = 21 kg of plant mass grown daily to sustain a single human.

The fastest growing plant I know is bamboo; from this paper (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5430563/), that type of bamboo accumulates ~80% of maximal biomass in the first 40 days of growth.
Reading the paper, a large fully grown tree of ~13m will have ~20 kg of above ground biomass in 40 days. Since biomass is typically measured dry, we don't have to factor in the whole plant mass:

20 kg * 40% carbon = 8kg of captured carbon in 40 days
8kg captured carbon = 8 kg of released oxygen per 40 days
8kg oxygen * 50% for plant use = 4kg per 40 days available for human use.
4kg / 40 days = 100g oxygen average per day available for human use per plant.

0.84 kg oxygen daily need / 0.1 kg average daily oxygen generation = 8.4 plants per person, rounding up to 9 plants.

Optimal distance for mature bamboo is ~1 m so each plant needs about ~1.22 m2 of space (assuming you have to keep it away from the walls of the arboretum), making it 11m2. Adding in the 13m height of the plant, that's 143m3 per person.

Note that growing bamboo has a number of specific requirements (they like lots of water and warmth), plus factoring in soil depletion, slow initial growing, re-preparing the land again for new growth and infrastructure to supply everything, you're going to need a lot more space than that. There's also the issue of dealing with 180kg of dry bamboo every 40 days.

Edible plants would have a slower rate of growth, but the biomass could be recycled more efficiently. This is the harvest index mentioned in Page 11 of the NASA document and goes into some depth of the various systems and other variables you'd have to take into account.

Aquatic plants and things like phytoplankton could be more efficient but have their own issues like getting the CO2 and O2 in and out of solution and circulation.
Note that there's a number of critical assumption (primarily that 40% of captured CO2 from photosynthesis ends up as structural material), so tweak numbers accordingly if you don't believe my assumptions.

[factotum]

Wow, thanks! Sounds a reasonable assumption...it hadn't occurred to me that plants would only be absorbing CO2 when they were actively growing, though? Surely they still need to absorb some CO2 in order to make the sugars they live off even when they're not actively growing? Still, 143m^2 of actively growing bamboo is a good baseline, because anything else would require more plant matter.

[DavidSh]

Note that this CO2 is generated purely from ingested carbohydrates - all the respired O2 is excreted as water (see the Krebs Cycle and oxidative phosphorylation for further details).

How can this work? For example, glucose is C6H12O6. The carbon:oxygen ratio is 1:1. There isn't enough oxygen in it to fully oxidize all of the carbon to CO2.

[Brother Oni]

Wow, thanks! Sounds a reasonable assumption...it hadn't occurred to me that plants would only be absorbing CO2 when they were actively growing, though? Surely they still need to absorb some CO2 in order to make the sugars they live off even when they're not actively growing? Still, 143m^2 of actively growing bamboo is a good baseline, because anything else would require more plant matter.

When people consider plants for carbon capture for environmental management, it's only actively growing forests/plants that are counted, so that's what I based my calculations on.

From this page (https://projects.ncsu.edu/project/treesofstrength/treefact.htm) and this thread (https://www.reddit.com/r/askscience/comments/2fb2uz/have_we_ever_measured_the_caloric_intake_of_a_tree/), a tree generates 56g of glucose a day. 40% of glucose is carbon, so ostensibly that's (56g x 40%) = 22.4g of carbon captured daily.

Given 50% own use of oxygen for the plant, that leaves 11.2g available daily for human use.
0.84kg / 0.0112kg = 75 trees required to supply the daily oxygen for a single person. Since the link doesn't specify the size of the tree or its footprint, I'm stuck here on how much space is actually required.

A minor point of order though for the bamboo, it's 143 cubic metres per person not square metres. While on a planet you can generally ignore height for growing crops, this is not an option in a spacecraft or other fully enclosed environment.
While there's lots of 'empty' space underneath the foliage, it needs to be clear so that the plant can grow freely to absorb as much sunlight as possible (probably a giant park to help the astronaut's psychological wellbeing).


How can this work? For example, glucose is C6H12O6. The carbon:oxygen ratio is 1:1. There isn't enough oxygen in it to fully oxidize all of the carbon to CO2.

The main thing about biological reactions is that they always occur in aqueous solution, so if you're missing some oxygen or hydrogen from somewhere, it's coming from the water present in the environment.

Taking it from the very start with glycolysis (https://en.wikipedia.org/wiki/Glycolysis), glucose undergoes a number of reactions to make pyruvate:

C6H12O6 → 2 C3H3O3

As you can see we've already lost a number of hydrogen to the water (pyruvate is the conjugated base of pyruvic acid, which is going to happen in an aqueous solution) and to other helper molecules.

The pyruvate then gets chucked into the Krebs Cycle (https://en.wikipedia.org/wiki/Citric_acid_cycle), with the pyruvate getting processed entirely:

Pyruvate ion + 4 NAD+ + FAD + GDP + Pi + 2 H2O → 4 NADH + FADH2 + 4 H+ + GTP + 3 CO2

Again if you look there's no inhaled oxygen directly involved and the CO2 has gotten the other oxygen from the water.

Where oxygen is involved, is in oxidative phosphorylation (https://en.wikipedia.org/wiki/Oxidative_phosphorylation), where oxygen is used to help perform some of the transformative steps in the Krebs Cycle, primarily the final step in the electron transport chain where it acts as an electron sink:

https://upload.wikimedia.org/wikipedia/commons/0/06/Complex_IV.svg

There's a fair few floating hydrogen ions about, but whether that's coming from the Krebs Cycle or just from solution is anybody's guess. Basic chemistry teaches this:

 H2O ⇌ OH− + H+

In actuality, it's a little more complicated (it involves hydronium (https://en.wikipedia.org/wiki/Hydronium)), so to confirm my initial statement, if you have a biological reaction in solution and there's magically appearing/disappearing oxygen or hydrogen, it's coming from the water. :smallbiggrin:

If you're unhappy with the Wikipedia links, I can dig out my biochem textbooks and cite the book, page and reference.

[factotum]

A minor point of order though for the bamboo, it's 143 cubic metres per person not square metres. While on a planet you can generally ignore height for growing crops, this is not an option in a spacecraft or other fully enclosed environment.
While there's lots of 'empty' space underneath the foliage, it needs to be clear so that the plant can grow freely to absorb as much sunlight as possible (probably a giant park to help the astronaut's psychological wellbeing).


We're talking space station here, so there's going to be plenty enough headroom for the bamboo to reach its maximum height.

[Xyril]

We're talking space station here, so there's going to be plenty enough headroom for the bamboo to reach its maximum height.

Wait, what?

The level of what a reasonably plausible space station would be varies based on the level of technology and economic development of the setting, but keep in mind that in most realistic settings, space will always be subject to some limits. A space station might give you more room to work with than a ship, but I don't think it's reasonable to assume "plenty of room."

Just some figures to keep in mind, some bamboo species reach nearly 100 feet, and depending on how they're lit/grown, I'm not 100% certain that having the ceiling or lighting located at 100 feet and one inch would necessarily make for optimal growing conditions. Even in more utopian, idealistic settings such as Star Trek (where even the utilitarian Klingon warships featured officers' quarters bigger than my old dorm room), you had space stations such as Deep Space Nine where even the largest space (the maybe three or four story high Promenade) probably wouldn't give you 100 feet of headroom.

In either hard sci-fi or a plausible near future, space carries a real trade off. Would we be able to create a 100 foot high space for plants? Absolutely, especially if we have no alternatives for providing a lasting, economically efficient source of oxygen for the station. Would that be "plenty of headroom" that happens to be built into the station? Absolutely not--if we didn't need that space for something as vital as oxygen generation, we would be using it for something slightly less vital, but still useful.

[farothel]

I think if they are ever going to do this on a large scale, it will not be with actual plants like bamboo but with algea, which are much easier to grow and take a lot less place. Or even bacteria (either cyanobacteria who have the same systems) or with genetically engineered bacteria of other genuses. The same goes with food for crew. Vat-grown stuff will be the way to go for this.

[factotum]

Wait, what?

The level of what a reasonably plausible space station would be varies based on the level of technology and economic development of the setting, but keep in mind that in most realistic settings, space will always be subject to some limits. A space station might give you more room to work with than a ship, but I don't think it's reasonable to assume "plenty of room."


I'm thinking of the O'Neill cylinder type of space station (e.g. Babylon 5) where the station is basically a hollow tube with habitation and planted areas on the inner surface. Such a station would likely be a kilometre across or more, so there would, indeed, be *plenty* of headroom for a plant species that only reaches 30m. If you had a station with much more restrictive headroom then you obviously wouldn't use such a large plant--Brother Oni only specified bamboo because it's one of the fastest growing plant species around, AFAIK.

[Xyril]

I'm thinking of the O'Neill cylinder type of space station (e.g. Babylon 5) where the station is basically a hollow tube with habitation and planted areas on the inner surface. Such a station would likely be a kilometre across or more, so there would, indeed, be *plenty* of headroom for a plant species that only reaches 30m. If you had a station with much more restrictive headroom then you obviously wouldn't use such a large plant--Brother Oni only specified bamboo because it's one of the fastest growing plant species around, AFAIK.

I was actually thinking about Babylon 5 in particular, but I wasn't 100% sure about the dimensions. IIRC, they had at least something like 60 "decks" worth of station along the outer wall of the station, possibly some other crap in between, then a central park area with transit going along the axis and sufficient radius for a fall from out of one of the transit cars into the "ground" of the park to be fatal.

Again though, the same issue stands--that isn't "free" or "spare" room. It's enclosed, pressurized space in one of the most shielded parts of the station where you can pick from an effective gravity value anywhere on the spectrum from free fall to station normal. In terms of today's scientific research, micro-gravity is a very interesting research space--many of the scientists who send projects up to the ISS or on cubesats do so specifically because it's the only way to get away from Earth normal and see what happens.

Granted, in any setting where you could lose four Babylon stations and still have the resources and willpower to build the fifth one, launching ships out of gravity wells has probably become economical (that, or it was a singular goal of an authoritarian state directing the collective economic output of an effectively endless supply of cheap, expendable labor) and putting labs in space won't be as hard as it is now. Still, building a science ship, outfitting it with superior radiation shielding and whatever else you take for granted in the middle of a giant station, and making provisions for an isolated, full time research team seems like a waste when you could just build some extra decks above the B5 park, take advantage of existing infrastructure, and have the added bonus of letting your scientists interact with people during their off hours.

From a military standpoint, space that is essentially "uphill" from external weapons systems and entry points, and essentially as far away as possible from outside attack and incursion, probably has high value as well. Personally, I'd stick high ranking and essential personnel there too, but apparently sci-fi disagrees with me.

I'm not saying the park wouldn't make sense, just that having all that airspace is a meaningful trade-off, and not a freebie.

[Brother Oni]

I'm not saying the park wouldn't make sense, just that having all that airspace is a meaningful trade-off, and not a freebie.

I agree, as you have to consider the psychological needs of both the civilians and military personnel. As I understand it, even the illusion of open airspace is very conducive for mental health - even prisoners get some time outside.

Being permanently enclosed in cramped quarters takes a special kind of person - the patrol time of nuclear class submarines are only limited by the onboard supplies they can cram onboard before embarking and the psychological endurance of the crew - prospective crew are heavily screened for mental health before serving and even then bubbleheads are regarded as a bit of a weird and unique bunch by the rest of the Navy.

[paddyfool]

It's worth noting that attempts to live in such a sealed environment as would be required here have not proved terribly easy. See, for instance, the history of Biosphere 2: https://en.m.wikipedia.org/wiki/Biosphere_2

[factotum]

I'm not saying the park wouldn't make sense, just that having all that airspace is a meaningful trade-off, and not a freebie.

If it's supplying oxygen to the entire station then it's hardly a "freebie" anyway? It's a critical part of the station that just happens to be organic rather than mechanical.

[gomipile]

If it's supplying oxygen to the entire station then it's hardly a "freebie" anyway? It's a critical part of the station that just happens to be organic rather than mechanical.

You are referring to a benefit or use of the volume, Xyril is talking about engineering costs of that volume. The two are not equivalent, though they would be considered together and hopefully balanced by the engineers.

[farothel]

That's why I mentioned algea. They can be grown in a much smaller volume, although they wouldn't give the same morale effect as other plants.

[Brother Oni]

That's why I mentioned algea. They can be grown in a much smaller volume, although they wouldn't give the same morale effect as other plants.

They also have the issue of getting the CO2/O2 in and out of solution, along with the significant infrastructure involved - a 1 cubic metre bioreactor will weigh 1 tonne for the water alone, before factoring in the vessel, algae and other infrastructure. A separate issue is engineering - you have to put specialised daylight wavelength bulbs in at regular intervals due to the limited penetration of light through water and the physical blocking effects of the other algae.

This page (https://academic.oup.com/bioscience/article/60/9/722/238034) indicates that an algae pond of 3600 acres can capture 80% of the daily CO2 emissions of a 200MW natural gas power station during daylight hours (assuming a population density of 20g dry weight algae per square metre).

This report (https://ieaghg.org/docs/General_Docs/Reports/2012-08.pdf) indicates that gas powered power plants emits 348kg CO2 per MWh, so:

200MWh x 348kg CO2 = 69600 kg CO2
69600 kg CO2 x 80% = 55680 kg CO2 per 3600 acre
55680 kg CO2 / 3600 acre = 15.4667 kg CO2 per acre
15.4667 kg CO2 per acre = 0.0038 kg CO2 per square metre = 3.82 g CO2 per square metre

Assuming 50% availability of O2 for human use and no losses in getting it out of solution:

3.82 g CO2 / square metre * 50% = 1.91 g O2 per square metre

From above, an astronaut need 840g O2 a day so:

840 g / 1.91 g per square metre = 439.9 square metres of algae at a concentration of 20g dry weight per square metre.

Considering that daylight doesn't penetrate much more than 1 metre in fresh water (link (http://jeb.biologists.org/content/jexbio/10/4/293.full.pdf)), that's basically 440 cubic metres per person. I don't think algae can be used without some very clever engineering.

[Xyril]

I agree, as you have to consider the psychological needs of both the civilians and military personnel. As I understand it, even the illusion of open airspace is very conducive for mental health - even prisoners get some time outside.


True, but the illusion of open airspace might not actually require that much space. It's pretty amazing what we can do with VR already, and I imagine the technology has the potential for a few more decades of substantial refinement before hitting any sort of plateau. Of course, tricking the part of your brain that hates living in a space coffin isn't necessarily as easy as tricking the part of your brain that wants to pretend to be a space marine for twenty minutes.

[Cormac Mac Art]

This is a really interesting topic. Thank's for bringing it up!

[Andor13]

3.82 g CO2 / square metre * 50% = 1.91 g O2 per square metre

From above, an astronaut need 840g O2 a day so:

840 g / 1.91 g per square metre = 439.9 square metres of algae at a concentration of 20g dry weight per square metre.

Considering that daylight doesn't penetrate much more than 1 metre in fresh water (link (http://jeb.biologists.org/content/jexbio/10/4/293.full.pdf)), that's basically 440 cubic metres per person. I don't think algae can be used without some very clever engineering.

I think it's pretty reasonable to assume that it is not a large engineering burden to create greater efficiencies than a completely passive pond. Placing that same cubic meter of water into 1cm thick clear panels, for example, is going to massively increase effective surface area, and since you're going to need to be circulating the liquid anyway for gas exchange, presumably it's also easy to optimize nutrient flow for our little green buddies. I seem to recall that algae can reproduce at absurd rates under the proper conditions.

The Nasa article indicates that because respiration and photosynthesis are inverse reactions then if the total plant biomass grown = the food intake of the crew then the O2 consumed by the crew and the CO2 consumed by the plants will balance exactly. So the plants supplying half the crew's food comes from the observation that plants examined in the articles cited are about 50% edible by mass, what they refer to as the Harvest Index. Even if you use a completely inedible crop like bamboo, then the mass grown should still equal the food consumed.

[factotum]

I think it's pretty reasonable to assume that it is not a large engineering burden to create greater efficiencies than a completely passive pond. Placing that same cubic meter of water into 1cm thick clear panels, for example, is going to massively increase effective surface area

Or you could have the stuff flowing through 1m diameter transparent pipes with sunlight tubes in between, I guess?

[Brother Oni]

I think it's pretty reasonable to assume that it is not a large engineering burden to create greater efficiencies than a completely passive pond. Placing that same cubic meter of water into 1cm thick clear panels, for example, is going to massively increase effective surface area, and since you're going to need to be circulating the liquid anyway for gas exchange, presumably it's also easy to optimize nutrient flow for our little green buddies. I seem to recall that algae can reproduce at absurd rates under the proper conditions.


The problem is that the link which mentions the use of algae ponds for CO2 capture, doesn't mention their depth or the species, only the dry cell weight concentration.

Say we pick an algae earmarked for biofuel potential, Chlorella vulgaris. From this link (https://moodle.polymtl.ca/pluginfile.php/269951/mod_resource/content/0/weiqi%20fu%EF%BC%8C%20passol-Maximizing%20biomass%20productivity%20and%20cell%2 0density%20of%20Chlorella%20vulgaris%20by%20using% 20light-emitting%20diode-base%20photobioreactor.pdf), the optimal concentration of chlorella vulgaris is 20g dry weight per litre without light intensity shenanigans.

1 L over 1 square metre means you only need a panel 0.1 cm thick to hit that target of 439.9 m2 per person per day.

From the paper, their largest reaction vessel was no more than 4cm diameter, so presumably your panels can't get more than 2cm thick without the yield efficiency dropping off, making (439.9 m2 x 0.1cm / 2 cm) 22.0 m2 per person per day the smallest you could go. That's still about double the 11m2 bamboo footprint, but with a significantly reduced vertical footprint.

Algae panels aren't as nice to look at though. :smalltongue:

Edit: With regard to the algae concentration disparity in the lab and the algae ponds, I suspect that 20g dry weight per litre is probably not feasible outside of small scale lab conditions with tender loving care and you'd probably ideally want a lower concentration so that your algae has space to grow into as it captures the CO2.

[Knaight]

Edit: With regard to the algae concentration disparity in the lab and the algae ponds, I suspect that 20g dry weight per litre is probably not feasible outside of small scale lab conditions with tender loving care and you'd probably ideally want a lower concentration so that your algae has space to grow into as it captures the CO2.

While industrial scaling does tend to lose efficiency (which I'm sure you know given your profession) due to concepts like "well mixed" having a tendency to get hilarious as simplifications this particular case is probably less of an issue. A glorified CSTR at low flow rate operating at the high end of viable concentration is probably going to work fine, though keeping weight low is potentially fun.

Also fun is dealing with the accumulated dead algae in the outlet stream, trying not to waste live algae by having them in the outlet stream where you suddenly care much more about that due to mass restrictions, and what the inevitable recycle process there means for the inlet stream. Granted, unseen complications and complications worse than they seem can always crop up, but there's not a lot of reason to think that this particular design would be too bad. It's fairly similar to a lot of the e. coli bioreactors used for production of organics, except that you don't have to separate out the oxygen from the water the way you do with most products.

[Brother Oni]

While industrial scaling does tend to lose efficiency (which I'm sure you know given your profession) due to concepts like "well mixed" having a tendency to get hilarious as simplifications this particular case is probably less of an issue. A glorified CSTR at low flow rate operating at the high end of viable concentration is probably going to work fine, though keeping weight low is potentially fun.

Also fun is dealing with the accumulated dead algae in the outlet stream, trying not to waste live algae by having them in the outlet stream where you suddenly care much more about that due to mass restrictions, and what the inevitable recycle process there means for the inlet stream. Granted, unseen complications and complications worse than they seem can always crop up, but there's not a lot of reason to think that this particular design would be too bad. It's fairly similar to a lot of the e. coli bioreactors used for production of organics, except that you don't have to separate out the oxygen from the water the way you do with most products.

I'm more than happy to concede expertise in the industrial scale engineering side of things to those who know more (lab scale and scale up are my experience) and working with live critters is completely out of my field of expertise.

I'd agree with your proposal that a CSTR with the relevant filters (I can hear the hysterical laughter now) pumping the live algae out into the panels with a CO2 rich water stream (and have regular CO2 feeds in/O2 filters out to maximise speed of yield return), then filtration of O2 before collection back into the CSTR could be viable as a handwaving CO2 recycling / O2 generation system. Just don't approach NASA with the idea unless you want to be laughed at. :smalltongue:

[Andor13]

The problem is that the link which mentions the use of algae ponds for CO2 capture, doesn't mention their depth or the species, only the dry cell weight concentration.

Say we pick an algae earmarked for biofuel potential, Chlorella vulgaris. From this link (https://moodle.polymtl.ca/pluginfile.php/269951/mod_resource/content/0/weiqi%20fu%EF%BC%8C%20passol-Maximizing%20biomass%20productivity%20and%20cell%2 0density%20of%20Chlorella%20vulgaris%20by%20using% 20light-emitting%20diode-base%20photobioreactor.pdf), the optimal concentration of chlorella vulgaris is 20g dry weight per litre without light intensity shenanigans.

1 L over 1 square metre means you only need a panel 0.1 cm thick to hit that target of 439.9 m2 per person per day.

From the paper, their largest reaction vessel was no more than 4cm diameter, so presumably your panels can't get more than 2cm thick without the yield efficiency dropping off, making (439.9 m2 x 0.1cm / 2 cm) 22.0 m2 per person per day the smallest you could go. That's still about double the 11m2 bamboo footprint, but with a significantly reduced vertical footprint.

Algae panels aren't as nice to look at though. :smalltongue:

Edit: With regard to the algae concentration disparity in the lab and the algae ponds, I suspect that 20g dry weight per litre is probably not feasible outside of small scale lab conditions with tender loving care and you'd probably ideally want a lower concentration so that your algae has space to grow into as it captures the CO2.

I think you'ld want to select your algae for maximum edibility/nutritional value. Which, since this is a SF scenario, would presumably be easy to engineer into whatever algae you want, but still, let's assume half that density.

The algae paper would then give us a yield of 10g dry weight of algae per liter per day.
The NASA paper indicates we want 206 g of food per person per day (dehydrated, since we are using dry weight to figure algae production.)

So 21 liters of production volume per person per day would roughly balance out oxygen production, CO2 absorption, and gives you all your bulk food requirements. That's a single 1m by 1m by 2cm panel, per person per day. Considerably more compact that bamboo, and all the nori you can eat.

[Grey_Wolf_c]

all the nori you can eat.

This is starting to sound more and more like Discworld Dwarven Bread.

GW

[Brother Oni]

The NASA paper indicates we want 206 g of food per person per day (dehydrated, since we are using dry weight to figure algae production.)

Can I ask where you get that value of 206g from?

Page 5 Table 2 says an average male crewman needs 0.62kg food solids a day and 3400 Calories a day. Assuming they're eating pure carbohydrate at 4 Calories per gram that's still 850g worth of pure starch/sucrose, let alone the much less energy dense nori which has 35 Calories per 100grams.



So 21 liters of production volume per person per day would roughly balance out oxygen production, CO2 absorption, and gives you all your bulk food requirements. That's a single 1m by 1m by 2cm panel, per person per day. Considerably more compact that bamboo, and all the nori you can eat.

Again, I'm not following your math:

22.0 m2 by 2 cm panel is (220000 cm2 x 2 cm) 440,000 cm3 = 440,000 mL = 440 Litres at a density of 20g dry weight per litre.
Halve the density, you double the volume, so 880 litres at 10g dry weight per litre. Sticking with 2cm deep panels means you'll also need to double the surface area, so 44.0m2 per person daily.

The C. vulgaris paper, the maximal rate of cell growth was ~1.5 g dry cell weight per litre per day with a starting biomass concentration of 5 g dry cell weight per litre. Figure 5A seems to indicate rate of growth is broadly independent of the starting concentration, so assume that 1.5 g DCW per litre per day is still applicable at 10 g DCW per litre.

880 litres would therefore generate an extra 1.32kg dry cell weight of algae a day, which if processed into nori with 100% efficiency, gives you 462 Calories which is about 14% of a male astronaut's needs a day.

This is assuming you set the flow rate on the panels so that it takes 24 hours for algae to transit across; any faster than that and you'll need to look up more detailed rate of growth and carbon capturing variables. Similarly if you start making the panels thicker, you'll need to look up light penetration into water and the light blocking effects of algae.

[halfeye]

I can't believe it'll be anything other than lettuce that they try first, it's fast growing and moderately edible. Mind you, lack of variety will make any food appalling in a matter of weeks.

[Knaight]

I'd agree with your proposal that a CSTR with the relevant filters (I can hear the hysterical laughter now) pumping the live algae out into the panels with a CO2 rich water stream (and have regular CO2 feeds in/O2 filters out to maximise speed of yield return), then filtration of O2 before collection back into the CSTR could be viable as a handwaving CO2 recycling / O2 generation system. Just don't approach NASA with the idea unless you want to be laughed at. :smalltongue:

In terms of hiding major engineering problems in plain sight the term "relevant filters" is a thing of beauty.

Also my proposal is that the panels are the CSTRs, and that instead of the central rotor you use something else to mix them better suited for slow laminar flow. You've got a large, fairly flat panel with a liquid-algae outlet and inlet stream, using nutrient enriched water on inlet, at a slow trickle. Then the panels themselves are mixed as evenly as possible. You also wouldn't need the regular feeds in or out (though an oxygen filter per CSTR would be needed as an outlet stream, given that we're basically building the separator in).

There's a level of handwaving here, of course, but no more than for the rest of this spaceship.

*Which does run into a laminar mixing issue, another source of fun. There's a lot of these.

[abdulbasit]

If you have a long-term plan of growing trees to produce oxygen, 1 tree for every 10 people is enough. One mature tree with enough branches and tree can produce enough oxygen in a season for 10 people to breathe clean oxygen for a complete year.

[Chronos]

One mature tree produces zero oxygen, on net. Plants only produce net oxygen while they're still growing.

[truemane]

Metamagic Mod: how many Necroposters are required to keep an expired thread alive?