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Author Topic: Could a solar powered cooling system be used to collect water from the air?  (Read 4121 times)

Offline thedoc

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ian pritchard asked the Naked Scientists:
   My question, in hot places where water is scarce would this work ?

we have all enjoyed taking a cool beer out of the fridge on a hot day and no sooner we have done so, condensation can be seen dripping off the outside of the can

now taking for granted that we have this fridge set up and running in the desert full of beer, would we still see any condensation gathering on the can ?

and if so, surely a solar powered cooling system could be adapted to gather water

i guess i'm wrong but would like to know why if so

ps

at least the beer would be cold ;-)
What do you think?
« Last Edit: 15/10/2015 12:50:02 by _system »


 

Offline alancalverd

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Entirely feasible.
 

Offline chiralSPO

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I am imagining a scenario in which there is a refrigeration unit directly under some solar panels. These panels will serve two purposes in the desert: collecting solar energy, and providing shade to the unit. On average you should be able to capture about 150 usable watts per square meter (averaged over 24 hours). This would actually be able to provide for almost one standard household refrigerator per square meter.

I'm not sure how large an area you would need to generate the water required (this would likely be very high purity and sterile water, so it would be best suited for drinking, and too pure/too expensive for agriculture). Water condensation releases about 2.3 MJ per liter, and if we estimate that the heat pumps are able to move 2 J of thermal energy per 1 J of energy from the solar panels (this could be a terrible estimate, someone else can correct this if it is!), then given 150 watts (150 J/second) of power, and an endless supply of already saturated air (bad assumption in the desert, I know, I'll get there) you could expect a maximum of 12 L of water per square meter per 24 hour day. This number will decrease dramatically one you include the energy required to cool the air to it's dewpoint (and this will depend on how much moisture is actually in the air, and how hot it is) and energy required to move the air through the system (which will also depend on how much water is in the air)

Maybe you could get 4 L per day per square meter, so a 5x5 meter shed could potentially provide 5L of potable water per day to each of 20 people.

Check out this website which talks about the physics of heat pumps, and goes on to talk specifically about solar-powered refrigeration, and even has a discussion of condensation further down: http://physics.ucsd.edu/do-the-math/2012/06/heat-pumps-work-miracles/
 
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Offline alancalverd

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Assume a coefficient of performance of about 2. Your 150W refrigerator can therefore shift about 300W of heat. Condensing 1 gram of water requires about 2300 joules of heat to be extracted, so you can condense 300/2300 =  0.13 gram per second. If the water content of air is about 5% you need to pass 20 x 0.13 = 2.6 gram of air per second over your condenser - about 2 liters/second. You can probably achieve this with a 2W fan, which won't impose a significant load on your power supply.
 
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Offline chris

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Bravo - excellent analyses!

But how much water is there in desert air?
 

Offline alancalverd

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Not less than 1% by mass over a hot desert. Rather less over a cold desert.
 

Offline wolfekeeper

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Assume a coefficient of performance of about 2. Your 150W refrigerator can therefore shift about 300W of heat. Condensing 1 gram of water requires about 2300 joules of heat to be extracted, so you can condense 300/2300 =  0.13 gram per second. If the water content of air is about 5% you need to pass 20 x 0.13 = 2.6 gram of air per second over your condenser - about 2 liters/second. You can probably achieve this with a 2W fan, which won't impose a significant load on your power supply.

You'd also have to cool the air down to the dew point.

That ideally involves some kind of counter-flow heater exchanger, or lots of extra energy. Building the condenser underground would also help.

edit: heat capacity of air is 1 J/g/K, so 2l/second of air (~1.2kg/m^3) would need another ~96W to cool it down by 40 C, so as to liberate ~2.6g of water plus the condensation energy you already calculated.

So you're looking at ~250W to get 2.6 g/second. That's a metre square or so of solar panel.
« Last Edit: 18/10/2015 15:27:33 by wolfekeeper »
 

Offline wolfekeeper

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No, no. It's not an expensive facility at all. Individual Peltier devices can give you 60W of cooling:

e.g. http://tetech.com/peltier-thermoelectric-cooler-modules/high-performance/

It's like a litre bottle sized device, that could produce (say) a litre of water per hour, when connected to a solar panel, even in the desert.
« Last Edit: 18/10/2015 16:09:14 by wolfekeeper »
 

Offline wolfekeeper

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We're talking about drinking water, not water for crops; it's not a replacement for rain.

Some people have to walk miles to get to drinking water, and then they have to carry it back again. If you had a device that produced perhaps several gallons of water a day, that might well be worth it.

edit: It seems to be an off-the-shelf technology:

https://en.wikipedia.org/wiki/Atmospheric_water_generator
« Last Edit: 18/10/2015 17:52:32 by wolfekeeper »
 

Offline wolfekeeper

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> The 54 watt "LC-061" unit on that website you gave has a unit cost of $289.00 -without shipping costs to Rajastan.

> It would be more cost effective to ship them San Pellegrino mineral water.

That device isn't optimised to do this, but even assuming it was, how do you reckon that? Taking your ridiculous comment literally, if the mineral water costs $1 per litre, the system would pay for itself in a few months, and may well last decades.

I actually think if you have to walk miles to get drinking water, that costs extra calories, and it may well be more cost effective to sell the extra food you consume and burn to do that, and use it to buy an atmospheric water generator and solar panel instead. I'm not even exaggerating.
 

Offline wolfekeeper

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I'm not claiming that it's some great breakthrough, or a great business opportunity, but it may well be valuable for people in Africa and India.

The reason it might make more sense now is not the condensing side; it's the solar panel. Solar panels are getting ridiculously cheap, and for people cut off from water and electricity grids who are in sunny climates, they can make a big the difference.

Solar panels are actually more powerful than crops for generating energy. So if you're using muscle power to collect water, is it more cost effective to do that, or to use a solar panel to power the water collection. If the solar panels are cheap enough, there comes a point where you're better off with the solar panel. This wouldn't be feasible if powered from a diesel generator for example.
 

Offline alancalverd

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Why don't you get a life, Pecos?
 

Offline alancalverd

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The current relative humidity in Ramadi, Iraq is 17%. Alice Springs, Australia has 23%.
The saturation water content of air at 40 C is 8%, so a RH of 17% gives us .016 gram of water per liter of air, requiring a flow of 10 l/s to extract the maximum water that can be condensed by a 150W system.

However Wolfekeeper has a point - you won't actually get condensation above the dewpoint, which, from 17% RH and 40C, is 15C - we need another 200W  of cooling, so another solar panel, please.

You might do better to have the solar panel charge a battery, and use the electricity to drive your condensers at night: desert frosts are not unusual in the Middle East.
 

Offline wolfekeeper

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If the air is at 40C, then you don't really want to cool the air down to ~5C, extract the water, and then dump that back out at 5C; that's a waste, you really want to warm it back up to nearly 40C again and then dump it. So based on that idea you should be able to deal with the dew point issue using a recuperator:

https://en.wikipedia.org/wiki/Recuperator

Possibly a row of thermal wheels might work well:

https://en.wikipedia.org/wiki/Thermal_wheel

So if that works, you shouldn't need much more cooling in a desert than in a temperate climate.

That's important because both the solar panel and the refrigerator are relatively expensive.
 

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