How can you breathe underwater?

Question

SCUBA equipment is pretty amazing, allowing us to breathe underwater and explore.

But they have drawbacks: they're noisy, cumbersome, and time-limited.

It seems like a futuristic scenario would include something lightweight and elegant that would allow me to survive underwater.

The guys in the video here use something like that – https://tvtropes.org/pmwiki/pmwiki.php/VideoExamples/UnderwaterCity – but how would it work?

Edit: With thanks to the commentors below, I see four technological pathways to getting this done:

1. scrubbing CO2 from exhaled air (like a diving rebreather)
2. getting oxygen by electrolysis (like a submarine's life-support system)
3. getting dissolved oxygen out of the water (like a gill)
4. SCUBA that uses compressed air, but the diver can surface for one second every 30 minutes and it gulps in a full tank of fresh air (like a dolphin or whale)

Answer 1

Yucky or Bizarre?

There are a couple potential lines of technology that could possibly liberate future divers from cumbersome breathing apparatus on their faces.

IVUBA --- IntraVenous Underwater Breathing Apparatus. The technology exists to deliver oxygen directly to the bloodstream via injected O2 pouches. Blood oxygen levels are kept normal without resorting to breathing. The issue of scrubbing CO2 from the blood has also been addressed. A combination device could be constructed that would shunt a diver's blood into the machine, remove the CO2 and inject a bit of O2 before sending it back into the diver.

TRUBA --- TransRectal Underwater Breathing Apparatus. Yep. Swim like a turtle, my friend!

Um.

Research into mammalian anal respiration has demonstrated in the lab that a) some researchers have entirely too much time and money on their hands and b) that mice can butt breathe.

Maybe some day, your divers will too!

They'd still have to carry a device strapped to their back, and there would still be a breathing tube. It's just the breathing tube doesn't go into the mouth! Yay!

Answer 2

The Sabatier reaction is how CO2 is scrubbed to provide breathable air on the International Space Station: https://en.wikipedia.org/wiki/Sabatier_reaction#International_Space_Station_life_support

It happens when you combine CO2 with H2 at 300–400°C, 30 bar, in the presence of a nickel catalyst.

The three-way cycle

What I propose (and sanity-check me on this please), is a three-point cycle: the Sabatier reaction, electrolysis, and breathing.

1. the Sabatier reaction takes in CO2 from exhaled air, and H2 from electrolysis, and outputs methane and water. CO2 + 4H2 → CH4 + 2H2O
2. electrolysis takes in water, outputs H2 (to feed the Sabatier reactor) and O2 (to feed the person). 2H2O → 2H2 + O2
3. breathing takes in O2 and outputs CO2 (into the Sabatier reactor), on a one-mole-per-mole basis according to http://www.madsci.org/posts/archives/2004-09/1096283374.En.r.html

The problem

This is not a perfect closed loop system, unfortunately, because the Sabatier reaction requires four hydrogen molecules.

Solution 1

Add hydrogen at step 4:

1. Start with 2H20
2. Electrolyse that into 2H2 and O2
3. Breathe the O2. Now you have 2H2 and CO2
4. Add bottled hydrogen. Now you have 4H2 and CO2
5. That gives CH4 + 2H2O

According to Starfish Prime's answer here, our hero requires ~5mmol of oxygen per second, aka 0.3mol/min. So we need to add/consume 0.6 moles of hydrogen gas per minute at step 4, which (given H2's molecular weight of 2.01588g/mol) 1.209528 g/min or 72.57168 g/hour of hydrogen to keep the system running, scrubbing the aquanaut's CO2 and providing their O2. The system will produce 0.3mol/min of methane, 4.812738 g/min or 288.76428 g/hour, so if you take methane's energy density to be energy 50 MJ/kg, that's 14.438214 MJ/hour, or 4.010615 kilowatt hours per hour, better known as kilowatts, of methane fuel. According to Starfish Prime's answer already linked, the electrolysis required 2.38kW (and the temperature and pressure of the Sabatier reactor requires energy too). Note that you actually won't be able to use that methane until you get home, as you have no oxygen with which to combust it.

Solution 2

Use more water than is needed, offgas oxygen at step 3:

1. Start with the 2H20 from the end of this cycle. Add 2H20 from somewhere to
2. Electrolyse that into 4H2 and 2O2
3. Chuck out some O2. You now have 4H2 and O2.
4. Breathe the O2. Now you have 4H2 and CO2
5. The Sabatier reaction turns that into CH4 + 2H2O

Disadvantages: electrolysis is energy-expensive, as we've seen. By doubling the amount of water you're electrolysing, you waste 2.38kW. If our hero is swimming in the sea, she'll need to desalinate the water before electrolysing it, or die from chlorine gas inhalation. And she has enough contraptions on her back already without adding desalination! Probably better to carry a tiny bit of distilled water: the input (to get 0.3mol of O2 per min) is 0.6g of water per minute, 36g (aka millileters) of water per hour. Still waste the energy though. Another disadvantage is that you're blowing gas bubbles, which will spook some fish (a problem with normal SCUBA too); many sea-creatures are very sensitive to sounds.

Solution 3

Close the loop by making it a four-way cycle by adding methane pyrolysis

Sabatier reaction is CO2 + 4H2 → CH4 + 2H2O

Electrolysis is 2H2O → 2H2 + O2

Methane pyrolysis is CH4 → C + 2H2

This completes the loop; all inputs feed back in, apart from a little charcoal as a waste product. Specifically 0.3mol of charcoal per hour, or 3.6 grams.

The methane pyrolysis (according to Musamali, R., & Isa, Y. M. (2018). Decomposition of methane to carbon and hydrogen; a catalytic perspective. Energy Technology. doi:10.1002/ente.201800593) can be done with a catalyst at 850°C and "The energy required for the production of one mole of hydrogen (45.1kJ/mol (H2) at 1073K)". As we said above "we need to add/consume 0.6 moles of hydrogen gas per minute", that's an added requirement of 1623.6 kJ/hour, or 451 Watts. This is in addition to the 2380W required for electrolysis, and the energy to heat the Sabatier reactor. Overall you might need 3-4kW, which is quite a lot for a wearable apparatus, but not unheard of, e.g. here's a 3-3.5kW backpack apparatus. This is a lot more energy-efficient than solution 2.

A bonus of this is that the heat generated could feasibly be redirected to actively warm the user in cold water; hypothermia is a danger during long dives.

Answer 3

Simple and Practical

A long tube, called the umbilical, brings air from the surface down to the diver. Surface-supplied diving forms have a decent depth sometimes hundreds of feet. They are easy to understand and implement.

Compared to SCUBA, they solve to of your complaints: They have an unlimited diving time (at least in terms of air available) and they are not bulky or heavy.

SNUBA is a portmanteau of "snorkeling" and "scuba." It is a simple form of surface-supplied diving. You can be trained enough to use it effectively in only a few minutes, assuming you already know how to swim.

Answer 4

Intravenus respirocyte injector.

Respirocytes are a theoretical artificial red blood cell that act as a pressure tank for oxygen with 230+ times the capacity of a normal red bloodcell☆.

You carry a pack of it on your back which is fed intravenously. Couple this with a rebreathing apparatus and/or a mechanical gill to filter oxygen out of the water and you can last for a long time with one pack. Just dont expect to last indefinitely, the surface area needed to effectively filter enough air out of the water is too high.

☆https://en.m.wikipedia.org/wiki/Respirocyte

Answer 5

Put the gill membrane in the flippers:

Flippers can be 0.094m² each (source).

Even moving at 1m/s, the two of them will pass through 188 liters of water per second.

Say 5cc/l of oxygen per liter. The flipper meet 940cc per second.

Say a person needs 6 liters of oxygen a minute, 100cc/sec.

The flipper-membrane will need to be 10.6% efficient at extracting oxygen, which is within handwaving distance.

And obviously there's a tube carrying it from there to the mouth/nose.

The flippers have two advantages: move more than other body parts, and have a big surface-area.

Answer 6

I'd check liquid breathing out: https://www.youtube.com/watch?v=TIGCdA2YLyY. Despite still having to replenish your supply, you wouldn't have to for ages, and liquid breathing bypasses problems with some traditional solutions like the bends/decompression sickness and nitrogen narcosis. I can easily see some version of this allowing humans to be aquatic to the same degree that true marine mammals that dive deep and surface fast (e.g. sperm whales) are.

Answer 7

Breathe hydrogen and oxygen.

Consider water. H2O. 1 mole of water is 18ml. But let us consider 2 moles of water because the math is easier when we turn it to gas. 36 ml. Less than a shot glass.

Electrolyze the water to hydrogen and oxygen. 2 moles of water produce 2 moles of hydrogen gas and 1 mole of oxygen gas. A mole of gas at the surface is 22.4 liters so 67.2 l.

That is a lot of gas. My minute ventilation is 10 l/minute because I get excited and huff and puff. Scuba cylinders can hold about 2500 l air. Of course the liters I breathe at depth contain more air than those I breathe at the surface and so a cylinder does not last me 250 minutes.

If the cold fusion reactor in your fannypack can kick out the energy needed to do it, you can make all the gas you need out of water. 2500 l of air can be made from 1.3L water.

There remains only the issue of breathing a mix of 66% hydrogen and 33% oxygen. Hydrogen is not toxic. Your voice would be very squeaky. No smoking!