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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 :)

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nirurin
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  • Check the answers to 1: How many plants would be needed to produce oxygen enough for 20 humans? 2: Is space suit using algae for oxygen production possible? 3: Oxygen self-supply by plants 4: How feasible is it to grow algae on Mars to use as food, plastic and oxygen production? 5: What would be the ideal hydroponic crops for long duration travels? – uhoh Nov 05 '19 at 00:42
  • 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.

     – nirurin Nov 05 '19 at 03:53
  • 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. – uhoh Nov 05 '19 at 04:09
  • I've been doing separate research too, but when I look up the standard led wattage for a square meter it's somewhere around 300W...

    I realise now that the 'space suit using algae' answer says 300W as well, but it then gives the wattage for 8m2 as "1000-1500w", when it should be more like 2000-2500w. Perhaps a typo. Including pumps etc, I suspect 2500W is closer to the right answer.

    I may well try and ask a variation of this on the Biology SE, specifically for growth rates perhaps. I'll see if anyone else weighs in on here. I'm glad you find it interesting too!

     – nirurin Nov 05 '19 at 04:25
  • I have some generic illustrations of the spectral absorption of some chlorophylls in Are two colors (red + blue) necessary for LED grow lights, or would either color be sufficient?. Sunlight at 1 AU is about 1360 W/m^2, about 900 W/m^2 sea level (at noon if your surface normal points at the Sun) and about 40% of that (350 W/m^2) is in the visible. But chlorophyl can only use a fraction of even the visible spectrum. So if the LEDs were spectrally optimized, then I would guess 100 W/m^2 of that red/purple LED light is way more than enough! – uhoh Nov 05 '19 at 06:47
  • A quick check here and especially here suggests that for red+blue LED lighting the power might be 20-30 W/m^2. monochromatic LEDs are very roughly speaking 50% efficient, so the electrical power required might be double that. Also, solar spectral data available in links found in this answer – uhoh Nov 05 '19 at 07:15
  • Thanks for the data on those, although the algae that seems 'the best' (or at least the one that I've found to be the ideal for this use), is actually a more rare blue-green algae. Which means it makes use of a wider range of the light spectrum than normal plants. Not sure if that's an advantage or not, it may be part of the reason why they grow so fast and are so efficient (relatively speaking). – nirurin Nov 05 '19 at 22:31
  • The link that led to this paper (https://arxiv.org/ftp/arxiv/papers/1406/1406.3016.pdf) ends up with an example that is 307W for a 12' by 12' area, which is a little bigger than a square meter. So 300W still seems like a conservative overestimate for a square meter. I haven't seen anywhere that seems to corroborate the other link saying 30W per square meter, that seems extremely unlikely. – nirurin Nov 05 '19 at 22:40
  • That's 13 times bigger than a square meter! 12' means 12 feet or about 3.66 meters, so that area is about 13.2 m^2, and 300 W / 13.2 m^2 is about 23 W/m^2. – uhoh Nov 06 '19 at 01:30
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    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?) – nirurin Nov 06 '19 at 01:36
  • excellent question! I'm not sure where it would be best asked as a separate question in SE. Possibly Biology SE or Sustainability SE? – uhoh Nov 06 '19 at 01:38
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    I'll give Biology a try :) – nirurin Nov 06 '19 at 01:59

1 Answers1

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  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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