[wolfekeeper (OP)]

I noticed that water is concentrated by the weather, but it's constantly trying to evaporate again (except on very humid days).

I wondered if it's possible to use evaporation to generate energy; when I spray water (if it's not too humid) it cools the air down, and cool air contracts, so I think you could use the cooling to pull on a piston and make energy to power your house (or theoretically make a water powered car!)

Would this really work?

[lightarrow]

Yes, it was discovered in the XVIII century:
http://en.wikipedia.org/wiki/Steam_engine

[wolfekeeper (OP)]

That's not the same thing at all.

[peppercorn]

More specifically:
atmospheric engine

Also, ask yourself how much work it takes to continually spray water.

[CliffordK]

You might take a look at the Stirling engine.

In most cases they aren't particularly powerful.  But, the engine is basically an enclosed steam engine with an evaporation component and a condensation component.

http://en.wikipedia.org/wiki/Stirling_engine

Keep in mind that the atmosphere is not homogenous.  It is relatively warm down at ground level where water evaporates.  The water vapor is lighter than air, and rises to the cooler upper troposphere where it is too cold for the water to exist in vapor form, and clouds condense.  Then, perhaps rain will fall from the clouds back down to the ground level.  This cycle becomes part of the heat flux of the earth, cooling the surface, and transporting the heat to the upper troposphere.

Potentially a Stirling type engine can be built (with the proper solvent) with any temperature gradient.  There are proposals to make similar engines utilizing the deep sea temperatures which are close to 0°C at depth, and much warmer at the surface.

Actually, it looks like the Stirling Engine often just uses the gas phase, and does not require a gas/liquid phase. The "regenerator" seems to function in place of the valves.

http://www.animatedengines.com/vstirling.shtml

But, a similar engine could be built with gas/liquid phase changes, and a solvent that tended to condense at the temperatures in the engine.  But, then you have something similar to the Newcomen steam engine mentioned above.

[wolfekeeper (OP)]

The difference is that those engines have a specific fuel source, they are what is somewhat facetiously referred to as an 'external combustion engine' whereas this simply relies on the cooling effect of water.

And I agree that pumping the water would take some power but I'm pretty sure it's a lot less than the energy generated from the latent heat of vapourisation of water- the cooling effect creates liters of difference of volume whereas you're pumping just a few cc of water volume, and the extra pressure for injection isn't high enough to make it not work.

[CZARCAR]

just compare the btu content of water thermal changes vs burning gasoline which is about 140,000 btu/gal.

[wolfekeeper (OP)]

The latent heat of vapourisation of water isn't actually that shabby about 2.2MJ/litre or 9700 BTU per gallon. And while I would suppose you would need a lot more of it, water is very, very cheap.

[Geezer]

As Lightarrow already pointed out, it's quite an old idea. More specifically, the "atmospheric engine" was driven by atmospheric pressure acting on a whopping great piston!

 http://en.wikipedia.org/wiki/Newcomen_steam_engine

[wolfekeeper (OP)]

That worked the other way, the atmospheric engine worked by condensing water vapour. This would work by evaporating water to make water vapour.

[Geezer]

That worked the other way, the atmospheric engine worked by condensing water vapour. This would work by evaporating water to make water vapour.

Ah! Right - but you'll end up with a heat engine of sorts whatever you do. As Clifford suggests, another way to do that is with a Stirling engine which operates as long as it has a source of heat and a means to dump the heat, and that could be, or rely on, some sort of evaporative cooler.

Another approach might be to emulate what weather does and use evaporation to lift a mass of water, condense it, maybe with an evaporative cooler, then run it downhill to drive a turbine. Surely somebody tried that? I'm not sure whether a scheme like that constitutes a heat engine or not. It probably does, but it's a bit less obvious.

The snag with that is the large amount of energy required to turn water into water vapour. On the other hand, if the energy is free (solar?), who cares.

[peppercorn]

I'm not sure whether a scheme like that constitutes a heat engine or not. It probably does, but it's a bit less obvious.

I'm pretty certain it still is a heat engine.  And as such it must be governed by Carnot's law - which equates to the efficiency of said device being low, compared with, say a superheated steam turbine.

Of course, the other key factor - cheapness (of a truly massive scale solar 'lifter') might just make it a winner.

.... ah ha!
Water Theatre

[wolfekeeper (OP)]

On the contrary, it's not formally a heat engine, a heat engine has a hot source and a cold sink, and these are missing. The source of energy here is essentially purely the low entropy, not any combustion, nor cooling.

However, you could formally construct a heat engine from it by formally adding those in a mathematical model, in particular adding a condenser/cold sink and a hot source/heat exchanger to heat the air back up again and cycle it back to form a closed system.

In that sense it's constrained by Carnot, but because you're not supplying the heat in the intended use, the Carnot number you get matters relatively little.

[Joe L. Ogan]

If you will devise a simple, inexpensive system for extracting the two component parts two parts hydrogen and one part oxygen, you will have more power than has ever been made available to mankind before.  Thanks for comments.  Joe L. Ogan

[peppercorn]

Maybe I'm missing something ... ?

It has a hot reservoir - the sun's heat (no need for combustion)
It has a cold reservoir - the cooler sprayed water (BTW is this open or closed loop?)

Heat moves from the Hot to the Cold doing work as it goes (hopefully).
Sounds like a Carnot heat engine to me.

[Geezer]

On the contrary, it's not formally a heat engine, a heat engine has a hot source and a cold sink, and these are missing.

Perhaps I don't understand your system, but I don't think it would work at all without a difference in temperature. If so, it's probably a heat engine.

[wolfekeeper (OP)]

Maybe I'm missing something ... ?

It has a hot reservoir - the sun's heat (no need for combustion)
It has a cold reservoir - the cooler sprayed water (BTW is this open or closed loop?)

Heat moves from the Hot to the Cold doing work as it goes (hopefully).
Sounds like a Carnot heat engine to me.

Depends where you draw the system box exactly, but you've failed to draw it correctly to make a heat engine; since the water is being used up. A heat engine is a closed cycle that you can go around as many times as you like that generates work from the difference between the hot and cold reservoirs. But if there's something being used up in the cycle then it's not a heat engine.

And note that a Carnot cycle doesn't even exist in the real world, it generates work infinitely slowly.

[Geezer]

I noticed that water is concentrated by the weather, but it's constantly trying to evaporate again (except on very humid days).

I wondered if it's possible to use evaporation to generate energy; when I spray water (if it's not too humid) it cools the air down, and cool air contracts, so I think you could use the cooling to pull on a piston and make energy to power your house (or theoretically make a water powered car!)

Would this really work?

I think we better start again.

When you say "this", what did you actually have in mind? A diagram would help a lot.

[lightarrow]

Depends where you draw the system box exactly, but you've failed to draw it correctly to make a heat engine; since the water is being used up. A heat engine is a closed cycle that you can go around as many times as you like that generates work from the difference between the hot and cold reservoirs. But if there's something being used up in the cycle then it's not a heat engine.

Water absorbs heat from air and so vaporizes and cools the air. Ok. How can you say this generates a pressure difference? Did you add vapour partial pressure to the cooled air to make the computation? And even if this makes a pressure difference, ok, this is called "meteorology" and you can exploit pressure differences = wind to extract eolic energy. It's however a heat engine: the hot source is the part of athmosphere heated by the sun, the cold source is the cold part of the athmosphere which is not...

[wolfekeeper (OP)]

It generates reduced pressure if you constrain it, and then let normal atmospheric pressure push down on a piston or a turbine until it's at ambient pressure. You can then clear out the air and put fresh air in and do it again.

As described it's not strictly a heat engine, since it's not closed cycle and it's just driven by the low entropy of the water and the air, but you can certainly formally consider it as part of a heat engine, if you supply the heat and cold from elsewhere and use them to close the cycle.