Is this possible ? 2

[nbucska]

Humid air contains more energy than the same amount of
dry air and water at the same temperature.

Would it be possible to take humid air ( e.g. at sea )
compress it, with heat exchenger cool it, precipitate
the water and recover the energy from heat and pressure
to drive the compressor.

Would this be useful for producing clean water?
Perhaps even end up with some energy left over ?


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[JStephen]

Sounds like a perpetual-motion device to me- things that are mixed don't easily unmix themselves- 2nd Law.

 

[djack77494]

nbucska,
The difficulty comes from your second step. You want to start with humid air (which does have more energy than the equivalent amounts of dry air and liquid water). You then compress this air. This is an energy intensive step, and inherently/practically has a fair amount of irreversibility (lost work) to it. Now comes step two - cool the compressed air. This removes much of the energy contained in the compressed humid gas, especially if accompanied by condensation. Furthermore, it is likely heat that will be rejected as waste heat to atmosphere or some heat sink. So what energy will you recover? The only energy to recover is what I've called "waste heat". That has low utility since its temperature will likely be quite low; it is not likely to be a source of useful energy. While I hate to sound so negative, I cannot be optimistic about this. Try analyzing the overall process using the second law of thermodynamics.
Doug

 

[nbucska]

djack:
The heat could be pumped from the compressed humid air to
heat up the dry air. I realize that efficiencies are
important but even if there is some energy loss,
it may be useful to make clean water if it require
less energy than distillation.

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[MintJulep]

Why not skip the compression and just go directly to the cool the air to condense the water?

 

[nbucska]

Mint:
To cool, you have to compress some gas so why not use
the air itself?


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[rb1957]

as other rsponders have already mentioned the question about efficiency, i'll leave that one and ask ...

why will the water be clean ? won't it have all the impurities (junk) that's floating around in the air now ?

and how much water are you looking to produce ? i don't know the %age volume of water in humid air, but i'm willing to speculate that it's not much.

we've speculated that this process isn't efficient, you've responded that maybe it is more efficient than distillation. maybe it is, maybe it isn't, maybe no-one has thought of this process before (i think that's unlikely), but certainly people have invested in distillation plants and i think they'd've considered the alternatives.

but what the heck, give it a whirl and see what happens.

 

[JStephen]

Why you have to compress the air: I think the intent is to cool it to ambient to condense water out, not chill it below ambient- requires only a heat exchanger, not a refrigeration unit.

I don't think it would be too hard to show that the whole process violated the 2nd law, though. Whip out that Thermodynamics textbook and go to work.

 

[MintJulep]

As this seems to be an "at sea" exercise, I'm thinking it may be possible to get a surface below the dew-point using only water, possibly enhanced by evaporative cooling. I haven't given it much thought, but certainly possible, at least a portion of the time. Obviously can't beat the second law, but might be able to get the required energy for free.

Thermal energy storage is another possibility.

Condensing water to drink out of air is a known technique. It is taught in survival schools, often called a "solar still".

I believe that sailing ships of yore collected the morning dew from the sails.

 

[zekeman]

Again, to get the water out you must ultimately drop the temperature below the dew point, so what do you do. You compress it and raise the temperature of the fluid which gets you nowhere, since you have to cool it or perform an expansion to cool it to where you would be in the first place and you have expended a high form of energy of compression to a more useless form of energy in the form of heat. So the best you can do is cool it without compression. Where are you going to get this magical cooling system-- maybe by evaporative cooling using spray or water tower cooling with no access to the heat energy you have extracted which went into the water used for cooling. Moreover, if by some magic you could get this energy, it would be in too low a form to do the useful work you imagine. There goes that perpetual motion machine.
And by the way there is no violation of the 2nd law of thermo, only the uselessness of the form of energy you have extracted.

 

[Yorkman]

What you could do is cool the sea air using an air to water heat exchanger,(a chilled water coil). Heres the interesting part with a question in it. How far below the ocean surface would you have to go to reach water that is say 40-45 degrees F? You could pump this to the surface to run through the chilled water coil to condense the moisture out of the air. The water may not have to be that cold (40_45) it would depend on the dewpoint of the air your working with. I think hurricane air would work best. But seriuosly, I have read about a similar application using deep lake water to get free cooling at a University back east (USA).
You still have the purity issues to deal with, and I know nothing is free, but as clean water becomes less and less available this might be an interesting process to explore.

I'm not a real engineer, but I play one on T.V.
A.J. Gest, York Int./JCI

 

[insult2injury]

Basically, a heat engine would be required to get usable energy out. In order to get any decent amount of work out, you need a substantial temperature difference. If you plan on using a compressor to establish that temperature difference, you will never be able to produce more work out than you are putting in. That can be proved by the second law.

 

[Yorkman]

Well, I looked up ocean temperatures. Thars cold water down thar. But you might have to go aways down to git it. Looking at a rough graph it could be as deep as 500 meters or more for the good stuff. On the other hand your ft/hd required wouldn't extend the full 500 meters would it. Just what ever hieght your miracle water maker is above the ocean surface. That might not be too bad!!

I'm not a real engineer, but I play one on T.V.
A.J. Gest, York Int./JCI

 

[Yorkman]

Nbucska,
When you get rich and famous don't forget the little guys out there bangin around these fool hardy notions.

I'm not a real engineer, but I play one on T.V.
A.J. Gest, York Int./JCI

 

[JStephen]

Okay, look at the original post. Package this process in a black box and here's what you get:
Humid air at ambient temperature comes in.
Dry air at ambient temperature (perhaps colder) goes out.
Water at ambient temperature goes out.
Heat goes out.
And is that process possible? I think not.

For that matter, if you have a black box that sits there sucking in humid air and putting out heat all on it's own, let's just hook it up to a steam engine and get some free energy off that heat while we're at it.

 

[IRstuff]

Sure, it's recoverable, it's just too low level to be usable.

A typical thermopile requires the temperature of a burning fuel to generate electricity.

TTFN

 

[KiwiMace]

I think djack77494 got it finished in the 2nd reply. This is an air-standard brayton thermodynamic cycle in which nbucska has asked can it be closed up to complete the cycle. djack said it right with what I can summarise as a No, as the quality of the heat source isn't good enough to drive what would be a turbine to run the air compressor.

So if you are going to condense water out of air, you should probably target cool humid air that requires slightly less energy removal to condense.

 

[nbucska]

york:
I was engineer for 30+ yrs, got 10+ patents which is not enough to be famous. For that you have to be a singer
or football player.

I used "clean" as shorthand for "reasonably saltfree
to be usable for drinking or irrigation"

Jstephen:
The mixture of oil and water has higher entropy but still
separates sponteneously.

The humid air has latent heat, too.

ALL:
Would the efficiency ( friction,heat losses ) be
better if it were scaled up?





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[IRstuff]

Nope, it's too low a level. Consider that it's about as warm as a human body. At that, it would more practical to use a human body, since its temperature is relative constant. And yet, cardiac pacemakers use batteries.

TTFN

 

[automatic2]

In a lossless closed loop of compression/decompression, you would need to add the energy required to transport the moisture through the loop. The transporting of moisture is the 'work', the loop would be the vehicle.

But alas, we must add in losses. As you are likely aware, mechanical methods are lossy, so calc your transport load, then add your losses.

If you had a waste heat source available, you'd have your transport energy.

attaching some form of your idea to an energy waste stream may actually decrease it's 'wastefullness'. You'll still be consuming but the net result may have benefit.