Which variables determine the exact output of this condensing device?

[anikad]

TL;DR

I'm looking to discuss the main variables that define the output/yield of a Atmospheric water generator device.

Good day all!
I've been researching on 'Atmospheric water harvester' (AWH) devices.
As I researched many functioning mechanisms on such devices, the most workable mechanism to me seems to be the one illustrated in the picture attached to this thread. (i.e running hot humid air over cool air using renewable energy source & collect the condensing water output.)

So my questions are twofold:
1) Am I right in thinking that this mechanism could be the most efficient to harvest water with no outside interference?
2) And what are the main variables that would affect the condensing water output we can get ..
In my mind, I suppose: the more mass volume of both hot & cold air we can get in contact per unit time, the better the condensing output?

Thanks a lot in advance for your input!

P.S: I'm new to the forum, but I intend to stick around & be an active participant here!

 

[Baluncore]

Welcome to PF.
You do not show where or how the humid air enters the system.
You do not show where the cool dry air is released.

Heating saturated air will not increase the water mass contained. You must cool moist air below the original cool entry temperature to get condensate out.

Heating air will reduce the Relative Humidity, but I see no liquid water supply available to saturate the heated air.

 

[anikad]

Thank you for your reply! Let me clarify the system.
The air is first made hot through solar collector on top part of the system.
The fan on the lower side of system sucks in ambient air & produces the cool air.
Both the hot humid air & cool air are made to get into contact, thereby generating the liquid condensate that is collected on the bottom.
Am I right to think that if I manage to create a system that generates more hot humid air from the top part per unit time, & simultaneously generate more cool air per unit time, I can keep increasing the condensate quantity?
If so, what would be the key constraints & other variables that affect the output.

Thanks again for your help.

 

[Baluncore]

The air is first made hot through solar collector on top part of the system.

Why heat the air coming in ?
You will only have to cool it down again below the input temperature, to reach 100% RH and so condense out some of the water as liquid.

 

[anikad]

Why heat the air coming in ?
You will only have to cool it down again below the input temperature, to reach 100% RH and so condense out some of the water as liquid.

The air is heated because as you mentioned earlier, the heated air has low RH. So the hot air (with the help of strong adsorbent material) sucks in moisture from ambient air outside in order to generate a flow of hot & humid air.
Am I still missing something?

 

[hutchphd]

Unless you are passing the heated air over a source of water (like heated sea water) it makes no sense to heat it. It will not "suck moisture" from the ambient air in a meaningful way.

 

[anikad]

The thing that's confusing me is what's exactly dictating the condensate output of this simple system.
My intuition tells me its the mass flow rate of both the hot, humid stream of air into the cool air stream in the heat exchanger.
I'd appreciate if you could confirm that or otherwise, & perhaps give a crude proportionality equation relating variables of this system & the condensate output.

 

[anikad]

Unless you are passing the heated air over a source of water (like heated sea water) it makes no sense to heat it. It will not "suck moisture" from the ambient air in a meaningful way.

Thanks for your input!
The idea was since this system would work under sunlight, we need to heat the air at one end & cool it at the other to create the temperature difference for condensate.
Since, the heated air will contain less water, the adsorbent material will suck some moisture from ambient air to compensate for us reducing moisture by heating the air.
In your suggestion- just increasing the ambient air mass flow rate (without heating it) in the heat exchanger would do the trick in increasing the condensate output?

 

[hutchphd]

You need to cool the humid air. If you cool it more (to a lower T) or cool more of it you will obtain more water. Heating it will not help.
The question is: what is your source of "cool"?

 

[anikad]

A simple fan at the bottom part of the system produces a cooler stream of air.
That's why I thought heating the air will create a greater temperature difference to increase water output. ( When you say cool it more, I suppose you're talking about a temperature difference too ?)

If you could share a simple physics equation that combines all those factors, that would solve the system design issue in a heartbeat.

 

[hutchphd]

A fan makes moving air but not cooler air. It feels cooler to use because it carries away our body heat via convection at skin and evaporation of sweat.
I fear this design is fundamentally flawed unless you have a ready source of cold air. Sorry.

 

[anikad]

The fan produces 'cooler air' because it's protected from the top part of the system that is directly exposed to sunlight.
I understand it will create a temperature difference just fine?

 

[hutchphd]

The cool air must have a temperature below the dew point temperature (look it up! ). The ambient temperature will necessarilly not get you that cold or you would have fog everywhere. This will not work as you envisage it.

 

[anikad]

Thank you for your input.
I disagree respectfully, because if I understood the laws of physics correctly, a hot stream of air combined with ambient fan should create condensation.
Can any other fellow member who's well versed in the topic please prove or disprove my point?

 

[Bystander]

a hot stream of air combined with ambient fan should create condensation.

PPMs are not allowed on this forum; you are perilously close to suggesting "something for nothing." See evaporative cooling, phase changes, latent heat,...

 

[Lnewqban]

Please, look for "Desiccant dehumidifiers" here:
https://www.esmagazine.com/articles/100811-choosing-the-right-dehumidification-technology

Also, please see:
https://www.engineeringtoolbox.com/air-heating-cooling-mixing-d_26.html

 

[hutchphd]

I disagree respectfully, because if I understood the laws of physics correctly, a hot stream of air combined with ambient fan should create condensation.

I don't mind a disagreement, and you are free to waste as much of your own time as you wish.
Please let me know how this works out.

 

[hmmm27]

Thank you for your input.
I disagree respectfully, because if I understood the laws of physics correctly, a hot stream of air combined with ambient fan should create condensation.
Can any other fellow member who's well versed in the topic please prove or disprove my point?

What's the source of the diagram ?

[edit: it doesn't show the flow of the ambient air, only "recirculating air" ; can't find it on the web, either]

 

[Baluncore]

That's why I thought heating the air will create a greater temperature difference to increase water output.

I cannot see how you could get any water condensate from the device as it is designed.
Has it yet produced a single drop? That would be a zillion times more efficient than zero.

If you could share a simple physics equation that combines all those factors, that would solve the system design issue in a heartbeat.

I would look at using a pressure reduction to condense water, then pump the pool of liquid water from the low pressure chamber, before allowing the pressure to rise again.

 

[anikad]

What's the source of the diagram ?

[edit: it doesn't show the flow of the ambient air, only "recirculating air" ; can't find it on the web, either]

Hi, thanks for your input.
The device design is from Google (Alphabet) which they open-sourced recently.
For the hot air input, the ambient air is just sucked in from outside the device.
For the cool air input, the ambient air is again sucked in via a fan at the bottom.

 

[anikad]

Please, look for "Desiccant dehumidifiers" here:
https://www.esmagazine.com/articles/100811-choosing-the-right-dehumidification-technology

Also, please see:
https://www.engineeringtoolbox.com/air-heating-cooling-mixing-d_26.html

Thank you! Can you summarize your conclusion as related to this device here?
I'm arguably more of a systems design specialist than an engineer

 

[anikad]

I cannot see how you could get any water condensate from the device as it is designed.
Has it yet produced a single drop? That would be a zillion times more efficient than zero.

I would look at using a pressure reduction to condense water, then pump the pool of liquid water from the low pressure chamber, before allowing the pressure to rise again.

Quite interesting! Can you explain a quick physics mechanism as to how pressure reduction condense water or share a suitable link on the web?
& is it a more (or less) efficient mechanism to condense water than the temperature drop / temperature difference method we're discussing from the device above?

 

[Tom.G]

A variation comes to mind than MIGHT be an enhancement.

Having the Hot air flowing over the Absorbent bed may improve the moisture extraction. But then again, you are heating the incoming cool moist air, which makes it less likely to give up its moisture.

It looks like you will just have to build the thing and try it, rather than have it play mind games with you!

And please let us know what you find, it could turn out to be useful.

Cheers,
Tom

p.s. Maybe it needs to be a batch process rather than continuous. Run the incoming ambient over the absorbent to saturate it, then run heated air over the absorbent to extract the moisture for the cold plate.

 

[hmmm27]

ffffs. The lengths people will go not to worry the pretty little heads with such inconveniences as "science" is unbelievable.

Hi, thanks for your input.
The device design is from Google (Alphabet) which they open-sourced recently.
For the hot air input, the ambient air is just sucked in from outside the device.
For the cool air input, the ambient air is again sucked in via a fan at the bottom.

Yeah, and thanks loads for making me have to google for the original link.

Garbage article from google/x-company ; it does contain a larger resolution diagram of the device where - given an existing (but shallow) knowledge of the existence of desiccation dehumidifiers - it's easier to figure out what's actually happening

A directly related article/paper in Nature which looks possibly interesting and I might read tomorrow if the headache from looking at the crap article subsides.

It's a desiccant dehumidifier : they exist ; I think the "new" thing is the man-portable paradigm.

The inside air and the outside air do not mix.

There's a turntable (close to the B mark on the diagram) which contains a desiccant.

As it turns, outside air is blown across the surface, which sucks out some moisture because that's what desiccants do. (The outside air then leaves the device, never to be seen from, again).

It continues to turn, the slightly soggy surface passing through the curtain to the inside, where hot(ter) internal air is blown across it, evaporating some of the water. The particular patch of turntable we're looking at then returns to the outside as fresh, shiny and dry.

Meanwhile the inside air goes and does the heat exchange thing, which cools it and causes some of the water vapour to condense. It then returns (through the other side of the heat-exchange) to the turntable.

Yes, it works. The article says so, and even says the prototype does "150ml/h/m^2", though it mentions neither what the m^2 refers to (presumably one of either heating or desiccant surfaces), nor the environmental conditions under which the measurement/calculation took place.

Efficient ? compared to what ?

It doesn't look like a prototype so much as a proof of concept. In my pretty much totally uneducated-on-the-subject-matter opinion it could be done without the fans, though : probably less efficient, but niftier IMHO. On the other hand, you could use the (presumed) solar panels which run the thing to charge a cellphone to call for a pizza. That bit's efficient in respect that the waste heat from electricity generation is actually being used (to heat the internal air).

Not to denigrate the people who designed and put it together; I'm not going to pretend I could do it.

 

[anikad]

A variation comes to mind than MIGHT be an enhancement.

Having the Hot air flowing over the Absorbent bed may improve the moisture extraction. But then again, you are heating the incoming cool moist air, which makes it less likely to give up its moisture.

It looks like you will just have to build the thing and try it, rather than have it play mind games with you!

And please let us know what you find, it could turn out to be useful.

Cheers,
Tom

p.s. Maybe it needs to be a batch process rather than continuous. Run the incoming ambient over the absorbent to saturate it, then run heated air over the absorbent to extract the moisture for the cold plate.

Hi Tom,
Yes, that's my exact goal to build this system.
I am just hoping to get key physical relations behind the heat exchanger mechanism of the device that's in accordance with the laws of physics. (so that I know what to focus on to increase the yield of condensate output)
But no luck so far- some members above even claim I'm trying to build a PPM, which obviously it isn't by any stretch.

 

[anikad]

ffffs. The lengths people will go not to worry the pretty little heads with such inconveniences as "science" is unbelievable.Yeah, and thanks loads for making me have to google for the original link.

Garbage article from google/x-company ; it does contain a larger resolution diagram of the device where - given an existing (but shallow) knowledge of the existence of desiccation dehumidifiers - it's easier to figure out what's actually happening

A directly related article/paper in Nature which looks possibly interesting and I might read tomorrow if the headache from looking at the crap article subsides.

It's a desiccant dehumidifier : they exist ; I think the "new" thing is the man-portable paradigm.

The inside air and the outside air do not mix.

There's a turntable (close to the B mark on the diagram) which contains a desiccant.

As it turns, outside air is blown across the surface, which sucks out some moisture because that's what desiccants do. (The outside air then leaves the device, never to be seen from, again).

It continues to turn, the slightly soggy surface passing through the curtain to the inside, where hot(ter) internal air is blown across it, evaporating some of the water. The particular patch of turntable we're looking at then returns to the outside as fresh, shiny and dry.

Meanwhile the inside air goes and does the heat exchange thing, which cools it and causes some of the water vapour to condense. It then returns (through the other side of the heat-exchange) to the turntable.

Yes, it works. The article says so, and even says the prototype does "150ml/h/m^2", though it mentions neither what the m^2 refers to (presumably one of either heating or desiccant surfaces), nor the environmental conditions under which the measurement/calculation took place.

Efficient ? compared to what ?

It doesn't look like a prototype so much as a proof of concept. In my pretty much totally uneducated-on-the-subject-matter opinion it could be done without the fans, though : probably less efficient, but niftier IMHO. On the other hand, you could use the (presumed) solar panels which run the thing to charge a cellphone to call for a pizza. That bit's efficient in respect that the waste heat from electricity generation is actually being used (to heat the internal air).

Not to denigrate the people who designed and put it together; I'm not going to pretend I could do it.

Thanks for the details.
If I'm correct, the following dependencies hold true as relating to condensate output:
1) The hotter the inside air blowing- the more it can extract moisture & the more the yield.
2) The more rounds of heat exchange process takes place per unit time, the more the yield.
3) The lower I can get the temperature of the ambient air on the bottom, the more the yield.

Is that true? If there's some physics wizard in here, an exact equation relating all those variables in the system would help massively!

 

[hutchphd]

Having the Hot air flowing over the Absorbent bed may improve the moisture extraction. But then again, you are heating the incoming cool moist air, which makes it less likely to give up its moisture.

Here is a more reasonable design using a spongiform dessicant:
https://www.kqed.org/science/1924858/new-device-produces-water-from-thin-air-no-electricity-required

 

[Tom.G]

I recall there was a much older thread here that referenced the turntable dehumidifier, but I can't find it now.

Here are a couple slightly related threads that you might find interesting:
https://www.physicsforums.com/posts/6264085/
https://www.physicsforums.com/posts/6194175/

Cheers,
Tom

 

[anikad]

Here is a more reasonable design using a spongiform dessicant:
https://www.kqed.org/science/1924858/new-device-produces-water-from-thin-air-no-electricity-required

That certainly looks a better design than Google's.
Can you confirm this: Will the MOF material adsorb more moisture per unit time when it's dry & less when it's getting wet(er)?
Or the rate of adsorption & desorption doesn't depend on the amount of moisture in the MOF dessicant?

 

[anikad]

I recall there was a much older thread here that referenced the turntable dehumidifier, but I can't find it now.

Here are a couple slightly related threads that you might find interesting:
https://www.physicsforums.com/posts/6264085/
https://www.physicsforums.com/posts/6194175/

Cheers,
Tom

Thanks for sharing that!
If I understand correctly, these devices will not produce condensate at night , since there's no solar power to heat the turntable dessicant (and thus no temperature difference) ?