I believe that the requirements for a single persons $\require{mhchem}\ce{O2}$/$\ce{CO2}$ balance run somewhere in the region of $8m^2$ of algae, which would seem to be possible in a single $\approx 1m^3$ bioreactor. Which is pretty handy, as it also produces algae which can be used towards food requirements and/or biofuel.

1) However I can't seem to find data for how much energy this would require?

2) Also can't find data on how long this algae would take to 'grow' to a stage where it could be harvested? As in what would the calories per month be from such a setup?

I know these are kinda... not 'space' questions, but I'm not sure people on a gardening forum will have information on this kind of setup either! Figured this question has probably occured to other people on here before now :)

  • $\begingroup$ I've read some of those already, but while they have similar questions they don't seem to have much in the way of answers (for the questions I had at least). One of them mentioned 1500W needed for 8m2 of algae, which at least sounds like it might be correct, though I'm not sure how they come to that value. I think it was something to do with sun power over that area of ground on earth. Actual growth times aren't mentioned at all, I suspect it's something people either don't think about or gloss over haha. $\endgroup$
    – nirurin
    Commented Nov 5, 2019 at 3:53
  • $\begingroup$ Okay good to hear it. I'm not proposing those as duplicates. By including them in a comment they become linked questions which can be helpful for future readers as well. It's an interesting question by the way! If you have no luck here, you could either move it to Biology SE or ask a different but related question there simultaneously. $\endgroup$
    – uhoh
    Commented Nov 5, 2019 at 4:09
  • 1
    $\begingroup$ Ahh sorry, I'm tired, my brain thought of it as "12 square feet" instead of 12x12. I wonder why LED grow lights are generally given much higher values. Even 'low light' crops like lettuce are said to be 30W per square foot, which would be 300w per square meter (i believe?) $\endgroup$
    – nirurin
    Commented Nov 6, 2019 at 1:36
  • 1
    $\begingroup$ I'll give Biology a try :) $\endgroup$
    – nirurin
    Commented Nov 6, 2019 at 1:59

1 Answer 1

  1. Energy requirement: BIG

According to Wikipedia https://en.wikipedia.org/wiki/Photosynthetic_efficiency , the maximum theoretical photosynthetic efficiency of plants using sunlight is 11%, but due to reflection and transmission is more typically 3-6%. Similar figures are presented for algae in https://www.sciencedirect.com/topics/chemistry/photosynthetic-efficiency#:~:text=Photosynthetic%20efficiency%20of%20microalgae%20is,microalgae%20biomass%20produced%20by%20photosynthesis

The free energy to convert a mole of CO2 to glucose and O2 is 114 kcal (0.13 kWh).

The ISS uses 0.84 kg of oxygen per person per day, or 26 moles. At, say, 4.5% efficiency this would require 77 kWh per day, or 3.3 kW continuously.

If this power is produced by solar cells (at 15% efficiency), it would require about 21 square meters (or 1/10th of a tennis court) while ISS is in direct sunlight. In LEO it would require twice that panel area (plus batteries) due to nocturnal eclipsing.

But LEDs are only 35% efficient at converting electricity to light, so adjust accordingly.

  1. Algae Growth:

For efficiency, the algae need to be constantly held at the optimum combination of temperature, nutrient concentration and algae density. This means a continuous process of adding nutrients and removing algae, not a batch process. If the bioreactor is producing 26 moles a day of oxygen, it would also be producing about 750 g per day of glucose, protein and oils. This would be about a person’s normal caloric intake… but only if you can choke down grass-clipping smoothies which you know contain your own excrement.

An algae bioreactor initially seems like a great idea for space travel. It is very elegant to turn waste CO2 and astronaut poop into oxygen and food. Unfortunately, the device’s mass and energy requirements make it impractical for voyages in the inner solar system.

Because of resupply difficulties, it will likely be mandatory for interstellar travel.


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