Half a croissant, on a plate, with a sign in front of it saying '50c'
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radiative cooled water condenser

water from air
  [vote for,

XPrize have announced a new challenge: turn air into water (see link). The device must be able to produce at least 2,000 liters of water a day from the atmosphere, using completely renewable energy, for at most 2 cents a liter.

This got me thinking about air well condensers (see link). Basically, something that gets cold over night such that water in the air condenses onto it.

The problem with air well condensers is they cannot get cold enough to condense large quantities of water.

So my idea is to repurpose and slightly modify BunsenHoneydew's idea "Cosmic Background Refrigeration".

So I propose a large trough (maybe 10 metres wide and hundreds of metres long) with a reflective coating on the inner surface. The cross-section of the trough could be parabolic or a V-shape. The outer surface of the trough has a thick layer of insulation.

The trough could be covered with panes of glass to decrease the convective heating when we are trying to radiatively cool the mass.

A large mass (e.g. 3 metre diameter) roughly cylindrical porous ceramic material is supported within, and extends the length of, the trough. The pores are large enough to allow the flow of air through it (e.g. about 5cm). This could be as simple as rocks stuck together with concrete. Alternatively the mass could be a phase change material with a melting point below the dew point (e.g. water in a container).

The trough would be covered during the day with insulated lids to prevent the mass from getting hot. The lids are taken off at night to allow the radiative cooling of the mass to occur.

Inlets would be opened when the mass was below dew point to allow air to flow through the condenser. Dew would form on the mass and trickle down the supports into the trough which is then collected into a reservoir.

xaviergisz, Oct 24 2016

Xprize water http://water.xprize.org/
[xaviergisz, Oct 24 2016]

Air well (condenser) https://en.wikipedi...ir_well_(condenser)
[xaviergisz, Oct 24 2016]

Cosmic Background Refrigeration
[xaviergisz, Oct 24 2016]

A review: dew water collection from radiative passive collectors http://download.spr...f2246bc903e260ef3f7
Radiative dew condensers have been shown to collect up to 0.6 mm/day/m² [xaviergisz, Oct 25 2016]

waterseer http://waterseer.org/
Similar to bungston's idea. Be warned, there is some scepticism on the internet about the claim it can produce 37 litres per day. [xaviergisz, Oct 30 2016]

Cheap plastic film cools whatever it touches up to 10°C http://www.sciencem...ver-it-touches-10-c
This plastic coating would make this idea more efficient [xaviergisz, Feb 26 2017]

A Device That Can Pull Drinking Water From the Air Just Won the Latest XPrize https://science.sla...n-the-latest-xprize
[xaviergisz, Oct 22 2018]

Exploiting radiative cooling for uninterrupted 24-hour water harvesting from the atmosphere https://advances.sc...ntent/7/26/eabf3978
[xaviergisz, Aug 25 2021]


       I'll just set out a tarp in the Amazon and wait for my $1.5mil.   

       I think I'll be able to beat a litre every 42 seconds.(2kl/d)
FlyingToaster, Oct 24 2016

       Hmm, I like this. I've got a remote site with usually humid air where we have to run an R/O plant with expensive diesel.
AusCan531, Oct 25 2016

       One of the competition guidelines is: "The entire unit should be on a platform which is at least 20 inches from the ground and the area below the platform should be completely visible through the cameras during testing".   

       I guess that eliminates the possibility of building a high- mass air well or one that uses a ground source heat pump.
scad mientist, Oct 25 2016


       If you aim your radiator into space (deep space or otherwise as long as it is off planet), you will also help fight global warming.
popbottle, Oct 26 2016

       I thought the idea would be to use radiaoactive waste to heat air, then pump it through an air-cooled pipe to condense out the water.
Voice, Oct 27 2016

       I don't like that 20 inches off the ground thing. I suppose it is to prevent cheating. Cooling air is a fine way to get the water out. The coolth needs to come from somewhere and so a large conductive structure extending deep underground makes sense. I was envisioning an enormous curving copper-coated aluminum marvel.
bungston, Oct 27 2016

       How about this scheme   

       1. The wingbeats of wild bees (or some comparable renewable energy source) drives a trickle compressor which fills aluminum scuba tanks with air. Tanks get hot because that is what happens when you compress air.   

       2. Let tanks cool off as they rest in cool and renewable zephyrs.   

       3. Let air out thru condenser baffles which capture moisture released by air, now colder than ambient because decompressing.
bungston, Oct 27 2016

       Go ask some desert dwellers and ask how their ancestors used to do it..
not_morrison_rm, Oct 27 2016

       //Go ask some desert dwellers and ask how their ancestors used to do it..//   

       I asked. They don't remember.
AusCan531, Oct 28 2016

       [AusCan] In Australia, Don't you have to ask where to walk?
wjt, Oct 28 2016

       //I've got a remote site with usually humid air where we have to run an R/O plant with expensive diesel.//   

       [Auscan] - I'm curious where that is, because we have the same problem with our project - at least for the moment and maybe forever. A fairly common story in the northern parts of Australia, I would imagine.   

       I always pondered how good you could get using distillation - using insulation and reheating to maximise efficiency. Presumably R/O beats it but I wonder by how much, because in many places, heat energy is cheaper than electrical energy.
Custardguts, Oct 30 2016

       //I asked. They don't remember.   


       Anyway, looking on the net I found "diesel is very hygroscopic" but unless diesel engines can run on a water/diesel mix like a very bad steam engine...it's all work in progress...<hits thumb with a wallaby>.
not_morrison_rm, Oct 31 2016

       //Go ask some desert dwellers and ask how their ancestors used to do it...//   

       //I asked. They don't remember//   

       You're asking the wrong ones. I have personally asked many times, and they did it quite effectively - but in limited quantities and with a moderate success rate. You need to remember that in a nomadic hunter-gatherer society, almost all of your available time is spent gathering the resources for survival - in plenty you thrive, and in drought/scarcity - you suffer. The population in a given area, given the enormous time periods we're dealing with, reached equilibrium with what the available resources and technologies permitted - in the case of Australian first peoples, this was generally a very small number, given the complete lack of domesticable plants and animals, limiting them to hunting and gathering, and some small, limited examples of emergent farming in specific locations. A group I know, of perhaps 100 individuals would range across an area maybe 75km X 25km throughout a year or multi year cycle. The landscape would not support any more than that, through scarcity of resources.   

       So I've been in some rather forlorn places (in pristine condition, mostly) where you or I would die rather quickly of thirst, sun exposure, or starvation - but that used to support a modest number of Aboriginals in the past. Having had the privilege of spending quite some time with traditional owners, one of my standard/favourite questions to ask is, where would you find water (or food) around here? The answers are always fascinating. Sometimes it's "no good here, we just walk through to over that way", and sometimes it's "just behind that sand dune, see that high ground back there with the green trees? Dig under those shady trees at the bottom there, there's water in the ground I reckon". Or sometimes the frustrating "There's a hole behind those rocks back there with plenty of water - bit smelly but". Or many variants on those lines. I could only wish to ever develop the bush sense some of these folks have.   

       Probably the point to all that is that traditional owners are a great font of knowledge for how to find enough water to survive on. Not enough water to live a 21st century western lifestyle on.
Custardguts, Oct 31 2016

       When I try to put together something requiring lots of quanities and then work math on it, it is not much fun (for me) and there often turn out to be errors of orders of magnitude. Also aside from incompetence there is sloth.   

       I would be interested in reading math from any capable baker laying out the energy budget for a compressed air scheme as above: the energy required to compress into scuba tanks in one day the quantity of air containing 2000 L water.
bungston, Nov 01 2016

       ^ well, theoretically none(ish), since you'd be recycling the coolth and the pumping through the next batch.   

       // Not enough water to live a 21st century western lifestyle on.// I am now going to google "How often did the Romans bathe ?".
FlyingToaster, Nov 01 2016

       //the energy required to compress into scuba tanks in one day the quantity of air containing 2000 L water.//   

       I'll try my hand at this calculation.   

       • a cubic meter of air has 10ml of water.
• 2000 litres of water requires 200,000m³ of air.
• 200,000m³ of air is compressed to 980m³ at 3000psi
• the work to compress gas is given by the equation:

       W is work in Joules
n is number of moles
R is the gas constant = 8.3 JK^-1mol^-1
V1 is initial volume and V2 is final volume
there are approximately 40 moles per cubic metre in gas.
T is temperature

       W = 40 * 200,000 * 8.3 * 290 * ln (980/200,000)
W = 1e11 J
W = 27e6 Wh

       Assuming you can buy electricity for $0.20/kWh, this would cost $5,400. Of course you can recover a lot of the energy when you decompress it, so not quite that expensive.
xaviergisz, Nov 01 2016

       Thanks for the math, [xavier]
Voice, Aug 25 2021


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