Could airplanes use water vapor to replace liquids?
November 11, 2016 10:00 AM   Subscribe

Would it be possible for an airplane to produce potable water from the air vapor outside of the plane?

Considering how much priority there seems to be on reducing takeoff weight by airlines, it seems that it would be a considerable savings if you could jettison the water carried onboard by way of sodas, juice, coffee, water bottles, that blue toilet water, etc. Is it possible to build a system that would take water vapor from outside of the plane and filter it so it was drinkable?

I am very much not an aerospace engineer and imagine this must be completely impossible, but I would love to hear the science behind why not.

posted by amicamentis to Travel & Transportation (11 answers total) 1 user marked this as a favorite
I'm no engineer either, but one problem I foresee is that the capturing and filtering systems would require energy to run, which would presumably come from either fuel or batteries, both of which are heavy.
posted by If only I had a penguin... at 10:09 AM on November 11, 2016 [1 favorite]

There'd be some added drag, but you'd take the air onboard through an inlet, slow the air down (important, or else the water that comes out could be blown away), chill the air so water condenses, and then extract and filter the water that you've taken in while letting the dried air get pushed out by a new batch of air. One option would be to have a supercold surface that gets scraped constantly for ice-- you could freeze the precipitation out.

But this would be an increased expense of fuel because of the increase in drag... trivial, maybe, but so might be the amount of water you'd get.

Once the plane is above the clouds, which is where it spends most of its time, the air is drier and drier as you go up. Plus, it's already cold (partly because, being dry air, it can't hold heat worth a damn), and to distill the water, you'd have to get it colder.

Someone would have to run numbers, but maybe there's some sweet spot in the altitude where planes can safely fly (visibility okay) without hitting each other (altitude zone is at least X-thousand feet thick), where the planes are flying low enough to recover water, while not so low that they're encountering thicker air and burning more fuel. My suspicion is that the zone where water is recoverable cheaply and efficiently does not overlap with the zone where the air's not so thick that fuel efficiency, such as it is now, goes completely out the window.
posted by Sunburnt at 10:11 AM on November 11, 2016 [2 favorites]

Any weight savings gained by not having to carry the water on takeoff would have to be offset by the extra weight of the new equipment to capture, condense and filter the water, also.
posted by radwolf76 at 10:15 AM on November 11, 2016 [3 favorites]

A couple of thoughts:
Air, in general, gets colder as you gain altitude. See Lapse Rate
Pressure decrease with altitude. See Pressure Altitude. This reduces the density, there is simply less air to hold water at higher altitudes.
Cold Air holds less moisture. See CIPM-2007 (pdf) for formulas. If you'd like this formula as an Excel Add-in MeMail me.
Atmospheric Water Generation typically relies on devices with large surface areas facing into the wind.

I think you'd have to run the numbers for how much water you need and how quickly you need to generate it. My gut feeling is that the amount of surface area needed would create so much drag that the plane wouldn't be able to fly, never mind the energy required to purify the water and wherever else needs to be done (pumps, etc.).
posted by Confess, Fletch at 10:28 AM on November 11, 2016 [3 favorites]

I don't think there is as much water in the air as you might believe. Here are some numbers I was playing with:

Let's say cruising altitude is 36,000ft, and the air temperature is -56.5°C (-69.7°F). The vapor pressure of water (assuming 100% humidity, which might be flawed based on Sunburnt's comment above) would be 1.722Pa.

So, one liter of air would hold 9.56E07 moles of water, which is 17 micrograms of water, or 1.7nanoliters of water per liter of air. Which means you'd need to process 13,917,647 liters of air to get an 8oz glass of water.

But, clouds have lots more water than that, you say! Let's run that assumption: cumulus clouds hold 0.5g water per cubic meter. So, to get an 8oz (237ml) glass of water, you'd need to process 474 cubic meters of water, which is 474,000 liters of air. Much more reasonable, but still kind of bonkers, and to get this rate of extraction you'd need to be flying through clouds constantly. I don't think pilots like to do that.
posted by sparklemotion at 10:36 AM on November 11, 2016 [10 favorites]

running with sparklemotion's numbers, if you are flying at mach 0.85 (which seems to be what airliners do) that's about 1000km/h so a 1m^2 collector would sweep up a million m^3 every hour, or a billion (1e9) litres. so you'd get about 70 glasses of water per hour.

that's assuming that there's some 100% efficient way to do the extraction, which as others have pointed out is extremely unlikely. still, it's interesting that a reasonable size intake (a square metre) gives just 70 glasses of water per hour when flying as fast as reasonably possible through clouds. i'm a little surprised it's not higher.
posted by andrewcooke at 12:37 PM on November 11, 2016 [3 favorites]

Per this page the Boeing 787 has a total potable water capacity of 270 gallons (1022 liters) which is 4320 8 oz cups. The longest flight of a 787 is about 14 hours which means that it would have to produce about 308.6 cups per hour. Per andrewcooke's numbers that's a 4.4 square meter intake which doesn't seem so bad.

This doesn't take into account the additional water that is 'stored' in sodas and bottles of water. On the good side they probably don't use all the water in the tanks and you could make some sort of waterless toilets that saved some more. Also I hope nobody needs any water while the plane is on the ground.
posted by Confess, Fletch at 1:16 PM on November 11, 2016 [1 favorite]

The main problem here seems to be that there are a lot of liquids on a plane that aren't drinking water. I think it's within the realm of possibility to develop a technology that would condense and filter water from water vapor, for airline use. But... what happens when someone orders a gin and tonic, or an orange juice, or a diet coke? Getting from drinking water to Chardonnay would basically require Star Trek replicator technology.

I suppose you could posit a situation where such a technology were developed, and airlines decided to forgo all other beverages onboard aside from in-flight produced drinking water. But that seems so unreasonably distant from the world we live in today as to make your question meaningless.
posted by Sara C. at 1:30 PM on November 11, 2016

tang was good enough to get us to the moon, dammit!
posted by andrewcooke at 2:03 PM on November 11, 2016 [3 favorites]

I am an aerospace engineer, and I have some thoughts. First off, there are always two questions to answer with something like this - is it technically possible, and is it realistically (or, say, economically) possible?

Technically, yes, such a system could work. There's even precedent for it - the pressurization system on board the airplane. The cabin stays at a breathable atmosphere by drawing air off the "compression" side of the engines (pre-combustion, that is), mixing it with partly recirculated cabin air, and then redistributing it. There are large temperature changes involved, and maybe not the way you think - the engines do take in very cold air, but the act of compressing it heats it up dramatically, so you actually have to chill the air again to get it to a comfortable temperature before it goes in the cabin. The system already has drain lines in place to collect condensed moisture and dispose of it (or rehumidify the air partly), so tapping in to that source of water would be fairly easy.

Realistically - no, this will likely never happen. What you're trying to do - save a few hundred pounds in water and soft drinks - would be outweighed (sorry!) by the extra weight of plumbing, purifiers, filters, and whatever other equipment would be needed to harvest this water. It would probably add weight to the plane overall. Furthermore, it's not just takeoff that weight factors in - fuel efficiency is weight-dependent through all phases of flight. So if you were to take off without this water and then harvest it in the air, now you have the weight of the condenser system plus the weight of liquid that you were trying to save in the first place.

There's also the added cost to manufacture and maintain such a system, which over the lifetime of the aircraft would be orders of magnitude greater than the fuel costs of carrying around a drinks cart. The one thing that these kinds of questions never acknowledge is that operating and maintaining equipment is the single most expensive part of a product lifecycle, usually more than design and acquisition combined. Adding complexity adds cost.

The other big point - you can choose to not fly with the drinks in the galley. If you install one of these systems, it stays there forever. If you really must save a few hundred pounds, it's a lot easier to leave a galley cart behind than it is to rip out your water condenser system.

There's also something to be said from a customer service angle. You certainly could force Air Force flight crews to drink this stuff, but your average paying customer is probably going to be turned off by only being offered condensed cloud water unless your marketing department is really savvy.
posted by backseatpilot at 2:50 PM on November 11, 2016 [9 favorites]

I was wondering if it was more feasible to reclaim water exhaled into the cabin air, but based on backseatpilots comments, maybe not.
posted by SemiSalt at 4:23 PM on November 11, 2016 [1 favorite]

« Older What is going on with health insurance customer...   |   Help me find a routine to do during the day Newer »
This thread is closed to new comments.