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Comment by wcoenen | Hacker News Reader

From the article:

> You might be familiar with this if you know about electrolysis. On Earth this process is commonly used in manufacturing, such as to produce aluminium. An electrical current is passed through a liquid form of aluminium oxide (commonly called alumina) via electrodes, to separate the aluminium from the oxygen. In this case, the oxygen is produced as a byproduct.

This is currently not the industrial reality.

Aluminum is typically produced via the Hall–Héroult process which involves a carbon source and emits CO2. (Alternative processes which only emit oxygen are possible, but presumably those are more expensive.)

https://en.wikipedia.org/wiki/Hall%E2%80%93H%C3%A9roult_proc...

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One of the first and most immediate challenges for any Moon or Mars base is an excess of CO2.

It won't cause lunar warming. But it will still be a problem and require significant energy and other resources.

It's likely that it will either be impossible or prohibitively expensive to keep CO2 levels at normal Earth standards.

See also: the ISS which operates with much higher atmospheric CO2 levels (up to 5000ppm) than would be considered healthy on earth (up to 1000ppm).

> It's likely that it will either be impossible or prohibitively expensive to keep CO2 levels at normal Earth standards.

I think we shouldn't use orbital space station where space and energy is very constrained as model for lunar base (where space and energy will be quite easy to get by comparatively).

Average person breaths out ~1kg of CO2 per day or ~400 per year [1]. Corn field absorbs ~4000 kg of CO2 yearly per 1000 m2 [2]. The same 1000 m2 field will produce ~4000 ears or 320 000 kcal yearly [3][4]

Assuming 2000 kcal for average person (so 730 000 kcal yearly) - we need 2000 m2 of corn fields to produce food for that person and just 100m2 to absorb CO2 from that person. Food will be a more important constraint in a self-sustaining base.

That means if we build lunar base in the lava tubes (that are often over 300 meters wide and go on for kilometers), we can keep them self-sustainable at density of 1 person per less than 10 meters of the lava tube.

[1] https://www.nrdc.org/onearth/waiting-exhale#:~:text=So%20bre....)

[2] https://www.canr.msu.edu/news/corn_fields_help_clean_up_and_...

[3] https://www.quora.com/How-many-ears-of-sweet-corn-are-in-an-...

[4] https://www.nutritionix.com/i/usda/corn-1-ear-medium-6-0.75-...

That math seems a bit implausible if you look at it from a closed system perspective. For said 1000m^2 field, it has to get the carbon from somewhere to produce the food, that then powers 6 months worth of activity for said human. If a human via respiration can only provide a tiny fraction of it (per the inverse of the other statement regarding:amount of Co2 absorbed for the field), where does the rest come from? Decaying organic material? Soil? Both of those are problematic closed system wise.

There is a lot of very dubious math in everything from ‘amount of water consumed’ and ‘amount of carbon absorbed’, etc. which if you dig into it really doesn’t add up. This feels like one of them?

That just shows that the actual problem in a base with self-sustaining food production will be the inverse - not enough CO2 to produce food instead of too much of it to breath.

To solve this you need initial biomass (or CO2) and then after the harvest you burn or otherwise decompose everything you didn't eat to close the cycle and get your CO2 (and some of the energy) back.

The easiest way to achieve that initial CO2 concentration would probably be to bring about ~3.5 tonnes of coal per person, burn it with oxygen mined from lunar soil generating energy for your initial base-building (24 MWh of heat about 40% of which you can convert to electric energy) and capturing the resulting 8 tonnes of CO2 for agriculture.

Of course you'd bring reasonably pure carbon, not the cheap dirty stuff we dig from the ground with heavy metals and sulphur in it. Also catalyst would be needed to avoid producing carbon monoxide.

Another concern is how much volume of air we need in these tubes to keep the atmosphere breathable and suitable for enough CO2 to supply plants.

Assume the tube is a cylinder 150m in radius and 10 meter in height (height is horizontal here ;) ). That's 700 000 m3 of air which should weight about 857 500 kg. At 410.28 ppmv CO2 in air we can calculate that 0.0623240117 % of the air mass should be CO2, which is ~535 kg in that 10-meter section of the tube. This is a long way off the 8 tonnes of CO2 needed to grow the plants, but they don't want all of that CO2 at once anyway - so we store the waste plant matter from harvest and burn it slowly over the year to keep CO2 concentration in air breathable.

We could also separate the atmospheres for plants and for people to optimize CO2 ppm for each, but that makes math more complicated :)

You would have to bring in enough carbon for the initial setup. Once you have enough material to grow the first crop, you will decompose previous crops to provide carbon and other nutrients for subsequent crops. In a closed system, none of that is being destroyed. In a real system you will get some losses (absorbtion by structural materials, population growth, leaks, etc) but these would be small compared to the overall amount, and easily made up with resupply at first and mined material over the long term.

It's so ironically steampunk that one of the main things we will supply to space stations at first will be coal :) And that each of them will need a big biofuel-fired powerplant just to produce food :)

Could imagine synthetic diamond dust instead of coal.

Interesting, I didn't know this about ISS. I've fought a tiring battle to get my CO2 levels below 500ppm consistently, and it's night and day compared to the 800-1200ppm I used to measure. Can't even imagine what 5000ppm is like!

Have you felt benefits from getting your levels lower? What steps did you take? Do you have a sensor you recommend?

Yes. I feel much more alert, but oddly I have a much harder time falling asleep. I've since started regular low-dose melatonin which helps a lot.

In particular I find I am much less groggy in the mornings, and stepping outside doesn't feel like a breath of fresh air.

The changes I made are not options for most. I have an industrial rooftop HVAC unit for a space about 5x larger than my home. The fans run continually and there is no fresh air exchange per se but part of the return duct runs through the attic which is not air-tight, more of an open cavity. I change the air filter every 3 months and keep the returns clean by vacuuming regularly. I try not to close doors to encourage circulation, and my bedroom door had an vent to the hall above it -- not sound or light proof, but doesn't restrict airflow.

The key that I've found is airflow. A closed room and a bunch of fans won't help you, but circulating the air of your entire home will make a big difference. It's important that your home breathes. An airtight home is actually very unhealthy, though heating/cooling is more efficient.

Lastly, cooking is the worst offender for poor air quality. Having a stove-top exhaust hood that actually filters and vents outside is so important. Cooking without exhaust or fresh air can easily bring the kitchen and surrounding areas near 1000ppm in under an hour. I keep my exhaust vent on more often than not and it makes a big difference. Negative air pressure is good: air is pulled in from outside.

Not the OP, but I've been using a model by Awair for a while now (looks like they've released a new product that's a bit more expensive since I got mine).

I work in a very small office shed, and if I leave the door and window closed tight it will quickly get over 2000 ppm (45-60 min). At that point I'm typically feeling more tired, and a bit foggy.

By keeping the window cracked I can keep the readings down in the 800-1100 range, and I feel much better.

You may want to look into wall mounted recuperators. They provide decent ventilation while not loosing too much heat.

Get some plants?

Office plants can't absorb 1kg of CO2 per day

You can just open a window and circulate air in the home to lower CO2 levels. I use a uHoo sensor https://getuhoo.com/

How has the uHoo sensor worked for you?

So, I bought one and it was great, but then randomly the CO2 sensor seemed to stop working after a few weeks. It stayed at the exact same ppm level without changing for a while. I sent an email to customer support and they shipped me a new one for free, and didn't even require me to send back the old one. Haven't had any issues since. I'm pretty happy with it.

Also interested in the answer to this!

My CO2 sensor self calibrates to show 400ppm as outdoor / minimal concentration. I was very upset to find out that it's not correct. In the last 5 years outdoor concentration rose from ~400 to ~420. Guess my kids won't be able to maintain < 500ppm.

I remember discussing this a while back on HN, and one issue was that consumer-grade CO2 meters aren't reliably calibated.

So just curious for my own designs, how certain are you about those ppm numbers you listed?

I have tested quite a few against calibrated commercial hardware at this point and they’re not as poorly calibrated for CO2 as you would think. The particulate counts and VOCs are usually only good for comparative measurements from the same device in my experience, and even then the drift is quite large.

I am desperately looking for one for the car, do you have anything to suggest?

Also, I am finding products that seem to require an Android or iOS device to work: no. They must be independent. If they had a webserver, nice idea, but all connections should be unrequired and switchable to off.

You might need to specify which kinds of gasses or particles you need to measure.

I would have assumed CO2 from the context, but since you mentioned cars I'm wondering if you mean CO, VOCs, and/or PM32.

Correctly guessed, mainly CO2 (breathing). I did not know that CO can be relevant in the car. But of course, the more is evalued, the better. In fact, for example, I had similar concerns for CO for the house (free flames).

My understanding is that in a car, CO is normally a concern only if exhaust gas is somehow leaking into the passenger compartment.

Unless your car has the fans off and the windows rolled up for a really long time, I'd be surprised if CO2 was something you need to worry about. Maybe if you're sleeping in it with the windows rolled up?

CO in the house is definitely a concern. If you have a fireplace, woodstove, or furnace with a properly functioning chimney, you're unlikely to run into problems AFAIK. But people die every winter because they used a propane grill, or kerosene heater, or gas stove/oven in their house without proper ventilation. Or used a wood stove with ineffective / blocked chimney. So in colder climates having a functioning CO meter seems like a no-brainer.

Exactly. In the car, it may happen in winter that you could be working for some time with little ventilation. You may at some point feel dizzy, and realize you would like a meter to get the feedback that can teach you be more aware of the air quality, to regulate the balance between letting the hot air inside escape and letting the fresh air (both senses) in.

In the house, similar awareness could be beneficial.

But I have not yet found a decent product. It is unacceptable that one is supposed to have a Google account or similar in order to read a ruler or configure a blender.

I'd like to share a thought on this, which you may find helpful. I apologize if it comes across as presumptuous; I'm not good at finding the right wording for this kind of thing. It's a cautionary tale from my own life that may or may not resonate with you.

Sometimes I've had similar lines of questioning. Where it occurs to me that something might be a risk worth addressing, and so I'd better gather data until I'm confident everything is okay or that I'll have a way to notice when the risk becomes real.

Every(?) time I've gone down that route, I later concluded that the concern really wasn't that big a deal, and that I'd just be hyper-focused on it because of anxiety or A.D.D. I.e., in the overall scheme of things, the true risks that deserve priority are pretty evident: good sleep and body weight, exercise, not smoking, keeping my finances in order, etc. I have a tendency to forget those primary issues when I start focusing on something that could, potentially, under some circumstances become a problem.

When I go down those rabbit trails, I end up reading a lot of articles, buying some unnecessary stuff from Amazon, planning for a project that realistically I'd never complete, and just worrying a lot.

If there's any takeaway from all this (for me, at least), it's "don't sweat the (probably) small stuff".

All my CO2 meters read about the same as the one in my grow room that controls the CO2. I don’t think that is a big problem.

Any chance you could share some of your findings? They'd be super helpful to some of us.

Not GP (nor a metrologist) but 50% reduction is surely the important thing anyway, in a home setting, not the absolute figures? I don't think the curve could be much off, since they work by counting particles, so if it's reading high at 800ppm then it should be reading just as high at 500ppm right?

I agree that presumably less CO2 is better than more.

I'm thinking that the specific CO2 concentrations matter if one wants to relate the readings to various research on human mental performance.

The Soviets solved this problem in their closed ecosystem experiments of the 1970s using 8 m^2 of algae per person:

https://en.wikipedia.org/wiki/BIOS-3

It’s take much less energy to maintain Earthlike CO2 levels in a habitat than it already does to make the oxygen that astronauts consume.

Wow, I pity the poor astronauts that have to do science inna state of dizziness !

Forgive my ignorance, but why is excess CO2 such a challenge? Isn't it extremely easy to simply vent it into space/the nearly-empty moon atmosphere?

Carbon isn't plentiful on the moon's surface so you want to conserve it for long duration missions/permanent habitats. Unfortunately storing it as CO2 in the air isn't great for human health.

You need food production anyway for a sustainable moon base. After the harvest store waste plant matter and burn it over the next season to keep the CO2 concentration in air good for plants and for you. It's a closed cycle if you eat/burn everything you grew each growing season, and you can keep the air CO2 concentration stable by adjusting how much waste matter you burn vs how much CO2 is captured by the plants.

You need about 2000 m2 of corn to feed one person over 1 year and just 100 m2 of corn will capture the CO2 exhaled by that 1 person in 1 year. Assume we live in horizontal lava tubes 300m wide, let's say 10 meters of that tube per person to have some margin of error. That's <535 kg of CO2 in air in that section of the tube and ~8000 kg of CO2 captured in corn planted there the moment before harvest.

It's doable IMHO. At least napkin math checks out.

Corn is a great plant optimized for production per acre. On the moon it may be more beneficial to optimize for carbon stability.

Get plants with a more favorable Edible Carbon::Total Carbon ratio.

A plant that can be harvested continuously would also be beneficial.

The ideal plant may be something simple like grass. We can't eat it, so would need another intermediate step. Possibly Cows or Sheep. These would be grown for milk and not meat.

So one day there really could be such a thing as moon cheese.

if we figure out efficient methods for carbon capture planetside, couldn't we reuse those methods for any non-terrestial habitat?

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