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Recently, I saw a video on Cody'sLab, where an “algae panel” was constructed. This is set up like a solar panel and uses algae to convert light and air into food and oxygen.

This got me wondering how useful such a device would be in long term space life. How much algae would you need in order to support one person’s calorie and oxygen needs?

Your answer should include:

  • the mass of the algae;
  • the mass of water needed to support it;
  • the mass and size of any containment or other support systems needed.

You can assume that the craft is located at a distance from the Sun equal to the mean distance of Earth from the Sun. You can use any algae species you want, however it must of course be edible.

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    $\begingroup$ There are a number of questions on this site that ask a similar question, but none have answers. $\endgroup$
    – Fred
    Apr 27, 2022 at 18:51
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    $\begingroup$ You don't ask for much! Don't forget a cost and schedule estimate. $\endgroup$ Apr 27, 2022 at 19:03
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    $\begingroup$ Just because it's a hard question doesn't mean it isn't a good question. $\endgroup$
    – GdD
    Apr 28, 2022 at 8:12
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    $\begingroup$ @GdD from my question: “You can assume that the craft is located at a distance from the Sun equal to the mean distance of Earth from the Sun.” $\endgroup$
    – Topcode
    Apr 28, 2022 at 14:36
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    $\begingroup$ It would help if I learned to read at some point @Topcode. I'll work on that. $\endgroup$
    – GdD
    Apr 28, 2022 at 14:37

1 Answer 1

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I was recently involved in a feasibility study for using Chlorella vulgaris algae for generating medical oxygen using solar energy in off-grid medical facilities.

Bottom line: it requires one cubic meter (one metric ton) of nutrient medium per person. The mass of the algae itself is trivial compared with the growth medium.

Light is toxic to the photosynthetic enzymes in algae. Sunlight needs to either be attenuated or intermittent. Light intensity decreases as it passes through the medium. If you are attempting to minimize the mass of the medium, you will need to use LED light source (and the attendant solar panels) rather than direct sunlight.

The algae themselves attenuate light so either algae density needs to be low (lots of medium) or LEDs must be close together (lots of LEDs).

Algae are not very efficient at converting light to O2. That’s not what evolution selected them for. Overall, they are 12% energy efficient at separating CO2 into O2 and carbohydrates. The bioreactor process is power intensive, so it becomes a major power load on a spacecraft.

The bioreactor needs to be mixed, but very gently since the algae are fragile. On Earth this is usually done with bubbles rising through the medium, but the bubble size needs to be controlled to prevent shear forces on the algae. In microgravity, bubble mixing is not available. Mixing the cubic meter of medium in small passages is a non-trivial problem.

Separating O2 bubbles from the medium is easy if gravity is present. Not so in microgravity.

High O2 productivity is dependent on keeping the algae on a particular point of their growth curve by regulating density, light intensity and nutrient levels. The bioreactor will need attention, maintenance and problem solving. The astronaut becomes a farmer.

The bioreactor produces a large amount of green goo and consumes significant amounts of electrolyte nutrients. In theory a closed loop could be engineered where astronauts could poop in the bioreactor and eat the goo. You might want to run that past them first.

To answer your questions:

Mass of algae and medium: 1 metric ton per person

Size of container: 1 cubic meter (within spaceship’s temperature controlled pressure hull).

Mass of containment, solar panels, lights, pressure hull volume and control systems: 1 metric ton per person

So (in very round numbers) you are looking at 2 metric tons per person for the bioreactor.

According to NASA each person aboard the ISS consumes 0.84 kg O2 per day. 2 tons of O2 per person would last 6.5 years at that rate. From a mass budget perspective it makes more sense to use O2 boil-off from propellant tanks than to carry a bioreactor for interplanetary voyages. Boil-off would also be much more reliable.

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  • $\begingroup$ Hey, sorry to bother you. I was wondering if it is possible for you to provide a link to the study mentioned? Your answer is great but I would like to be able to get a more detailed look at this topic $\endgroup$
    – Topcode
    Aug 26, 2022 at 1:56
  • $\begingroup$ @Topcode ... Our work was not pursued to publication because it rapidly became obvious it would not produce a workable solution for providing medical oxygen in off-grid locations. Too bad. Results of NASA's work are available through NASA nasa.gov/mission_pages/station/research/news/… Reports are couched in marketing terms like "possibly", "potential", "future". Any hard numbers like kilowatthours per mole of oxygen or mass per astronaut or hours of maintenance per day of life support are pretty thin. $\endgroup$
    – Woody
    Aug 26, 2022 at 2:19
  • $\begingroup$ Ah, well that sucks. Thanks for getting back to me so soon though. $\endgroup$
    – Topcode
    Aug 26, 2022 at 2:24
  • $\begingroup$ @Topcode ... I'd be happy to "CHAT" about it except I can't find "CHAT" in the menu structure of the SE site. Did you have any particular questions? $\endgroup$
    – Woody
    Aug 26, 2022 at 2:27
  • $\begingroup$ Click the SE logo in the top right, should be a chat button next to the current stack site. Click that and you can start a room. $\endgroup$
    – Topcode
    Aug 26, 2022 at 2:29

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