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Comment by ChuckMcM | Hacker News Reader

I am surprised no one has mentioned replacing the adsorption CO2 scrubbers on space craft[1] with something that is powered by electricity. The article claims 130 grams of CO2 removed from the air per kilowatt-hour. Astronauts might expel 40 - 50 grams of CO2 per hour into the air, so 500wHrs of power keeps the air breathable forever? That is a good deal. For reference the ISS has a crew of 6 and has 84 - 120kW of power capacity [2].

[1] Study problem on CO2 removal from NASA -- https://www.nasa.gov/pdf/519338main_AP_ED_Chem_CO2Removal_St...

[2] https://www.nasa.gov/mission_pages/station/structure/element...

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The reaction consumes water, which isn't exactly abundant on a space station or space vessel. You can regain the water by burning the alcohol, which can be done in a combustion chamber without reintroducing the CO2 to the air of the habitat. That's a rather exothermic reaction – and I'm guessing that the bionic leaf also generates excess heat – which might be a problem. Space vessels can only cool down by radiating heat, which I gather to be a quite complex problem. Nevertheless, it might be workable for all I know...

You're overcomplicating this hugely. It's much simpler to burn the alcohol within the astronauts, converting it to a small amount of heat, CO2 and happiness.

I don't think they will enjoy drinking isopropanol or isobutanol.

Can confirm, have drunken ethanol before.. the after effects are devastating

You missed the key distinction here, ethanol is not the greatest substance, but is relatively safe for animals. Other side chain alcohols metabolize into byproducts like formaldehyde and well, are highly toxic. This whole distinction is the basis for the sale of "denatured alcohol" / "wood alcohol" and its much cheaper price.

"In some countries, sales of alcoholic beverages are heavily taxed. In order to avoid paying beverage taxes on alcohol that is not meant to be consumed, the alcohol must be "denatured", or treated with added chemicals to make it unpalatable."

[1] https://en.wikipedia.org/wiki/Denatured_alcohol

I assure you, the after effects of drinking most alcohols are much more devastating than your average spirit.

For reference:

ethanol (Grain alcohol) rat, oral 7,060 mg/kg

isopropanol rat, oral 3,600 mg/kg

isobutanol rat, oral 2,460 mg/kg

Hardy harr harr harr...

Please tell us more...!

Can it not lead to blindness?

Ethanol is normal drinking alcohol. You are thinking of methyl alcohol, methanol.

As little as 10 mL of pure methanol, ingested, is metabolized into formic acid, which can cause permanent blindness by destruction of the optic nerve. 30 mL is potentially fatal, although the median lethal dose is typically 100 mL (3.4 fl oz) (i.e. 1–2 mL/kg body weight of pure methanol)..

Why can't the heat be converted to electricity?

Because heat is pure entropy; it's what you ultimately end up with after electricity has been used to do work, and the process is irreversible due to the Second Law of Thermodynamics: https://en.wikipedia.org/wiki/Second_law_of_thermodynamics#G...

Only a difference in heat levels can still be converted to electricity, for instance by reverse Peltier effect: https://en.wikipedia.org/wiki/Thermoelectric_cooling

And on a space station, most of your heat differential is created by active cooling, so recovering that via Peltier elements is a bit… counter productive.

The Second Law of Thermodynamics just means the conversion can't be a hundred percent efficient. Indeed, I didn't think of that as a problem.

I think you need a temperature gradient to generate electricity from heat

The space station is in, uh, space. I'm fairly certain a temperature gradient could be arranged.

It's actually the complete opposite - space is not "infinitely cold" it's more like "infinitely isolating" - like the perfect giant thermos. Space suits for example have a whole layer of water tubes just to cool down the astronaught, otherwise they will pass out from their own body's temperature.

Indeed, black body radiation isn't very effective at dissipating heat!

The ISS has an 28 tons active cooling system that barely radiates away as much heat as a one ton HVAC unit on Earth (70kW).

It's hard to generate a temperature gradient when you're blasted from the unfiltered Sun on one side, and reflected IR from Earth.

And surrounded by a near vacuum.

And yet thermos has somehow made billions by assuming the opposite.

Some of it probably could be, but the effiency of those processes are low enough that you'd still have a heat problem. Also, recall that the energy originated with the solar panels to start with. It's a very wasteful way of generating electricity compared to the solar panels, so although it might be worth considering to recover some of the energy investment and eliminate a fraction of the heat, the net output of such a process will probably be an order of magnitude lower than the initial energy investment.

Nit: I believe you meant 500W. 500wHrs would only keep the air breathable for an hour.

Doesn't change your conclusion though!

A rule of thumb for remembering whether you want to say "power" or "energy".

If you could replace "power" with "horsepower", you're on the right track!

If you could say "gallons of gas" instead of "power", you mean energy!

wHrs is energy (and kind of a dumb unit. Watts is already joules per second, so you're saying (joules/second)*hours. It would make more sense to just say joules.

You are correct, 500W for 1 hour (.5kWh) would completely counter the breathing of one crew member for that 1 hour.

This leaf isn't generating oxygen though. It's generating alcohols. I wonder what it would take to generate oxygen out of the alcohol.

It does actually. The alcohols are mainly made from the carbon in CO2 and the hydrogen in H2O. Isobutanol is (CH3)2CHCH2OH, and isopropanol is C3H8O. So that's a lot of O left over.

Roughly speaking, energy + CO2 = carbon + oxygen. Left to right is photosynthesis. Right to left is fire.

[edit: do be careful when applying this equation in practice! Often, hitting CO2 with a big hammer just yields CO, or carbon monoxide. It's poisonous!]

CO2 scrubbers in spacecraft don't generate oxygen either. They only absorb CO2.

https://www.quora.com/How-do-the-CDRA-and-Vozdukh-systems-sc...

I bet this could be a great unintended outcome for those astronauts :)

This seems like it would work much better on Submarines. You could concoct a Nuclear Submarine that never needs to surface!

Actually nuclear submarines already make their own oxygen with electrolysis so they don't ever need to (nor do they) surface, except to exchange personnel or to load additional food.

Of course, they also need separate CO2 scrubbers to keep the air breathable, which consume a lot of power, use caustic and hazardous chemicals, and make the whole ship smell pretty bad.

An artificial leaf that removed CO2 without these downsides might justify a refit.

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