Author Topic: Evaporative Cooling for Hot and Humid Climates?  (Read 19131 times)

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Norsman

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Evaporative Cooling for Hot and Humid Climates?
« on: February 10, 2011, 11:41:27 AM »
Hello Group,

It is well known that evaporative cooling (EC) is not very suitable for humid climates.  EC is however, the least costly of common (active) cooling techniques.  As a native of Florida, I have spent many hours researching how to make EC applicable in the humid southeast US.  There are two primary obstacles to employing EC in the southeast.  Additional (and undesirable) moisture in living spaces is the most notable disadvantage when IC is attempted in humid climates.  However, the greatest limitation is the lack of sufficient wet bulb depression at night.  Many people are of the opinion that the relative humidity (RH) in the southeast is too great.  This is true – but, mostly at night.  Most of the region has sufficiently low RH during the day – around 50% to 60% RH midday.  This is low enough to yield wet bulb temps 10 to 12 degrees lower than ambient dry bulb.  This is significant cooling, but pointless if it can only be accomplished during the day, as nighttime temps in the summer are higher than the human comfort range.

I constructed and tested a sub-scale EC prototype that I believed has a good chance of defeating the limitations associated with EC in the southeast US.  The prototype was a novel EC configuration that addressed the issue of insufficient wet bulb depression by decentralizing the evaporator and employing a high mass evaporative medium within the walls and floors, namely lava rock gravel.  I dubbed the system Integrated Evaporative (IE) cooling.  Lava rock gravel serves a triple role in the system: Firstly, it is the water containment vessel. Secondly, it is the evaporative medium.  Finally, lava rock serves as the heat transfer medium.  Wet lava rock has an extraordinarily high thermal capacity.  This feature in conjunction with the high volume of wet lava rock within the floor and walls provides very high thermal mass.  High thermal mass affords a pronounced “cave effect” – allowing the system to "coast" over idle (nighttime) periods.  Running the system only during the day is a doubling of cost savings.  Additionally, the prototype resolved the issue of unwelcomed additional moisture in living spaces by exhausting the moisture-laden process air outside the dwelling.  A bonus effect of “dynamic insulation“ is provided by the IE configuration.  It is generally agreed that solar radiation is the greatest source of heat within the interiors of dwellings.  The evaporator within the walls and floor of the IE system provides a 100% effective thermal barrier to infiltrating heat through the walls and floor.

The IE prototype has demonstrated a genuine potential to employ evaporative cooling in hot and humid climates.  More details as well as videos, photos, drawings and charts are available at my personal website (spam-free).  I am reluctant to reveal the URL as it may be a violation of policy for initial posts. I hope that I get replies to this post (affirmative or negative) and I believe I will be permitted to provide my website address in follow-ups to any replies.

joestue

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Re: Evaporative Cooling for Hot and Humid Climates?
« Reply #1 on: February 11, 2011, 12:41:32 AM »
yep.. so long as you're not trying to sell anything...

welcome to the forum btw.


bob g

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Re: Evaporative Cooling for Hot and Humid Climates?
« Reply #2 on: February 11, 2011, 02:36:16 AM »
you got my attention, lets hear about it

bob g
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thirteen

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Re: Evaporative Cooling for Hot and Humid Climates?
« Reply #3 on: February 11, 2011, 02:47:17 AM »
The size and the power needed will be a big question and the exhaust port to the outside may need an extension to get it away from the dwelling and direct it to a garden or flower bed area. Just an idea to toss or have a chuckle with.
MntMnROY 13

wpowokal

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Re: Evaporative Cooling for Hot and Humid Climates?
« Reply #4 on: February 11, 2011, 04:08:57 AM »
And the next post will be calling for investors, the latent heat of evaporation is well documented, then again I hope I am wrong.

allan
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Norsman

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Re: Evaporative Cooling for Hot and Humid Climates?
« Reply #5 on: February 11, 2011, 07:24:04 AM »
Thanks to the members that replied to my post.  I am not trying to sell anything, nor am I seeking investors.  I would like to get earnest and thoughtful reviews of the work that I've done.  My personal website is: http://www.inovaenergy.com/ There are no ads or requests for anything.  I have submitted a patent application , but I'm tired of waiting for it.  I've decided to publicly disclose the technology. (I'm not a wholehearted believer in patents anyway)  Any comments are welcome as long as they are rational reviews. There are many similar forums on the web - I chose Fieldlines to disclose my work because I have followed it for sometime and I believe I will get helpful suggestions.

zap

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Re: Evaporative Cooling for Hot and Humid Climates?
« Reply #6 on: February 11, 2011, 10:15:46 AM »
I skimmed a lot of your site and it's a very interesting system.

First and foremost... how do address mold and mildew growth?

You show a "graph" from coolerado.  Theirs is a hybrid system.
A simple dual stage evaporation system will work under higher humidity than a single stage system.  How does you system compare to a dual stage evaporation system?

Norsman

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Re: Evaporative Cooling for Hot and Humid Climates?
« Reply #7 on: February 11, 2011, 01:34:22 PM »
I skimmed a lot of your site and it's a very interesting system.

First and foremost... how do address mold and mildew growth?

You show a "graph" from coolerado.  Theirs is a hybrid system.
A simple dual stage evaporation system will work under higher humidity than a single stage system.  How does you system compare to a dual stage evaporation system?

Thanks zap for viewing my website and for your comments.

In the prototype I made no effort to contain the water.  I expect that water containment can be easily accomplished in full-scale.  An array of water-tight plenums (something like stovepipe) in the wall is one way - no more complicated than plumbing.  Modern dwellings have high pressure water in the walls and it's fully contained.  My plan is to have water fully contained within the walls.  There is a potential for mold within the evaporative circuit, but this can be readily mitigated with the periodic introduction of biocides in the open circuit. 

Dual-stage, indirect coolers provide human comfort without introducing more humidity to living spaces.  This is a feature shared by the Integrated Evaporative (IE) system and the Coolerado system.  The Coolerado and other "dew point" evaporative cooling systems (like the Dutch Statiq system) use multiple stages to reintroduce sensible cooling from the secondary stage back to the primary stage.  This step lowers the wet buld temperature in the primary stage and improves cooling potential.  Improved cooling in the primary stage means lower temps in the second stage and a cyclic effect is initiated.  However, as effective at defeating wet bulb limitation they may be, commercial dew point coolers can not be employed in hot and humid climates.  Unlike commercial dew point coolers, the IE system relies on high thermal mass to provide continuous comfort in humid climates.  In the daytime, when there is sufficient dew point depression, commercial dew point coolers are fully capable of providing human comfort in humid climates (like the southeastern United States).  At night, the ambient temperature may be uncomfortably warm, however there is typically too little dew point depression for adequate cooling.  High thermal mass is the feature that permits the IE system to be employed in climates where no other evaporative system can function satisfactorily.





 

   

zap

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Re: Evaporative Cooling for Hot and Humid Climates?
« Reply #8 on: February 12, 2011, 09:15:42 AM »
Norsman,

I would be interested to see your current system's performance if it were reversed 'head to toe'.
Instead of using a bed of gravel in the floor, move the gravel up to the ceiling.
Although this method wouldn't be the ideal setup for heat if dual mode operation was desired, I would think the efficiency of the system would be impoved.

Although you discuss condensation and water recovery inside the system, I saw nowhere on your site where interior condensation was mentioned.  How much interior condensation was observed in the prototype?

Norsman

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Re: Evaporative Cooling for Hot and Humid Climates?
« Reply #9 on: February 12, 2011, 10:30:08 AM »

I would be interested to see your current system's performance if it were reversed 'head to toe'.
Instead of using a bed of gravel in the floor, move the gravel up to the ceiling.
Although this method wouldn't be the ideal setup for heat if dual mode operation was desired, I would think the efficiency of the system would be improved.

Although you discuss condensation and water recovery inside the system, I saw nowhere on your site where interior condensation was mentioned.  How much interior condensation was observed in the prototype?

zap,

I'm glad you find this idea interesting.  You're right. Since heat rises it would make sense to have the evaporator in the ceiling.  But, I think that would create more problems than it solves.  As far as interior condensation goes, and I bet you're thinking sweating walls and floor.  That could possibly be a problem in full-scale, but not a daily occurrence, because no evaporative cooler can cool below the dew point.  Here's what could happen though; on a hot summer day when the dew point is low the system could be working at optimum.  Then, the weather changes and brings a spike in dew point, raising the temp at where water vapor condenses to a point higher than the concurrent temp of the floor and walls.  In this scenario, you would have sweaty floors and walls.  This scenario could be prevented by having low relative humidity within the dwelling to begin with.  I think a successful IE system will have to marry a dehumidifier.  There are a few solar powered dehumidifiers that would keep the operating costs down. 

In my prototype, I used fibercement panels as the inner wall sheathing.  This material has the thermal conductance attribute that I wanted, but had the undesirable characteristic of seeping water through to the inner surface.  For this reason, I had 100% humidity within the "living space" of the prototype the entire time - running or idle.  Should I build another test model, I will place the wet gravel in containment devices (I like the idea of stovepipe cast into concrete walls) so that it cannot leak, to the interior or exterior. 

Tritium

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Re: Evaporative Cooling for Hot and Humid Climates?
« Reply #10 on: February 12, 2011, 01:28:43 PM »
Ok,  How can I use this in an existing structure without major remodeling?

Thurmond

dnix71

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Re: Evaporative Cooling for Hot and Humid Climates?
« Reply #11 on: February 12, 2011, 02:08:37 PM »
Tritium's questions was also mine. In new construction this makes sense assuming you can import enough lava rock. Maybe a proper substitute for lava rock would make this easier to do.

This is still not something that would retrofit easily.

ruddycrazy

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Re: Evaporative Cooling for Hot and Humid Climates?
« Reply #12 on: February 12, 2011, 02:24:42 PM »
Yonks ago in the Aussie outback guy wet a hessian sack and found with the wind blowing thru it had a cooling effect so the koolgardie fridge was born. Todays evaporative coolers also use the same principle. A guy over here years ago published a design on the net where the evap cooler was palced at ground level and a void was installed near the roof space to get rid of the rising heat. I modified an old evap cooler for my shed and did the same thing and man it does keep the shed cool and bearable when it's 43C in the shade.

zap

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Re: Evaporative Cooling for Hot and Humid Climates?
« Reply #13 on: February 12, 2011, 03:32:08 PM »
About the only way you could easily implement this system into an existing structure would be if major renovation is already planned or underway... I'm thinking a meth lab where the house is usually almost completely gutted.
I've only done some very rough figures in my head but I come up with somewhere in neighborhood of $4/sqft for new construction and $6-8/sqft for retrofit and I'm almost positive I'm way low.

There's basically no way to add the system as shown to old construction if it's slab-on-grade (which is what got me to thinking about the attic installation) and even a basement or crawl space would present quite a few challenges.

Filling every stud cavity would be doable but... wow.  You could install duct work that's been prefilled with the lava rock but you'd still need a manifold for the air top and bottom either way and that would stick out like a sore thumb unless the whole wall was furred out.

The attic system might work all by itself without any need to touch the exterior walls but I would need to do some more figuring as to how much someone would have to beef up the ceiling structure.

Norsman

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Re: Evaporative Cooling for Hot and Humid Climates?
« Reply #14 on: February 12, 2011, 08:14:36 PM »
I see no way to retrofit with this concept.  Actually, I don't believe it's commercially viable with today's relatively low energy costs.  The exception may be a DIY one-off in Cob or Adobe.  (I would love to see an Integrated Evaporative enthusiast give it a try.)  As it stands right now, I think the only commercial application may be agriculture - poultry or dairy farming or an active "zeer pot" device for perishable farm products.  The day will come when AC will be a luxury in hot climates.  We may see an exodus from the southeast US, unless Integrated Evaporative or some other economical cooling technique comes along.

bob g

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Re: Evaporative Cooling for Hot and Humid Climates?
« Reply #15 on: February 12, 2011, 08:51:04 PM »
there are already numerous evap cooling systems that work well in the SW US climate, many were abandoned with the advent of dirt cheap electric refrigeration systems.

however in my opinion they will make a comeback, and should have done so in a big way long before now,, its just that most folks have forgotten how effective they were and because they are poorly understood today by contractors and are a fraction of the cost, there is no incentive to use them,, at least yet.

in my opinion, it is not necessary to use the volcanic rock and circulate cooled air into the thermal mass when it is much easier to simply cool concrete with cooled water flowing through embedded pex tubing.

in many climates you can sufficiently cool water in a cooling tower overnight to remove heat from the interior thermal mass (walls, floors and in some cases ceilings) to flywheel through the next days heat easily without fear of biology projects growing within the thermal mass.

given enough thermal mass and enough temp differentials AC is really unnecessary in all but the most humid climates.

ymmv, imho, etc

bob g
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Norsman

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Re: Evaporative Cooling for Hot and Humid Climates?
« Reply #16 on: February 13, 2011, 09:57:59 AM »
Thanks bob g,

We are in agreement.  Cooling towers are a good way to yield wet bulb temps, and pex tubing would be a very simple way to deliver the cooling.


in my opinion, it is not necessary to use the volcanic rock and circulate cooled air into the thermal mass when it is much easier to simply cool concrete with cooled water flowing through embedded pex tubing.

in many climates you can sufficiently cool water in a cooling tower overnight to remove heat from the interior thermal mass (walls, floors and in some cases ceilings) to flywheel through the next days heat easily without fear of biology projects growing within the thermal mass.

given enough thermal mass and enough temp differentials AC is really unnecessary in all but the most humid climates.

However, absorbent rock is essential to what my prototype does.  The primary difference between what I've done and what you suggest is the aspect of "dew point cooling" (there are commercial dew point cooling systems) or what I prefer to call "defeating wet bulb limitation".  I invite you to view the video of the IE prototype on the first page of http://www.inovaenergy.com/present.html.  Stop the video on the last few frames.  I have some psychrometric calculations there.  I took the video early in July.  The day I took the video, I did not record weather data.  But, as the psychrometric calculations indicate, it would take a RH of ~38% to provide a temp drop of 20°F as is indicated in the video.  Searching weather stations (weatherunderground), I can't find a record of RH that low, in July, in my location (32401).  It appears that I'm defeating wet bulb limitation.  I explain how I think my prototype does this on page two of inovaenergy.com.  The key to the process is, in fact, the lava rock or some other absorbent rock (like brick rubble). 

BTW is it permitted to provide links to sites like weatherunderground?  There is also a handy psychrometric calculator that I would like to link.  Each of these sites have Google ads.

dnix71

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Re: Evaporative Cooling for Hot and Humid Climates?
« Reply #17 on: February 13, 2011, 01:12:25 PM »
The real practical limitation of evaporative cooling is the inability to lower humidity. The reason you use less energy to run is the large amount of energy required to dehumidify the air.

That's also exactly why freon compressor a/c systems are so popular. If you live in south Florida even if the air is cool at night if the humidity is high enough it's still miserable weather to try and sleep in. Same goes for England but cold and wet instead.

Just dehumidifying the air without cooling is enough in most climates to make it livable.

joestue

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Re: Evaporative Cooling for Hot and Humid Climates?
« Reply #18 on: February 13, 2011, 01:35:58 PM »
someone will suggest dehumidifying the air then sending it through the evap cooling system in 3...2...   ;D

"defeating wet bulb limitation"
great claims...

Quote
Lava rock and the liquid water it contains have greater thermal conductance and far greater specific heat capacity than air or water vapor. These physical characteristics permit the loss of latent heat to impart sensible cooling more to the lava rock and less to the air stream.

interesting.
i've got a way to test this.
slit a heat pipe down the side and bond it to a heatsink, with a water supply to keep the sinthered copper powder saturated.
then blow air across it. (with a duct covering the top of course)
what you're saying is the heat pipe should get colder than the air correct?

bob g

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Re: Evaporative Cooling for Hot and Humid Climates?
« Reply #19 on: February 13, 2011, 02:01:42 PM »
i am familiar with the double stage, and multistage evap systems
wherein the first stage cools via evap, that cooled air is sent through a
second stage with an air to air exchanger which take the humidity out of the room air
then that air is cooled again with another evap stage which adds back humidity

that sort of system can cool down to or below the dew point from what i gather.

there is also a rotary exchanger wherein water is misted through one half of the disc
metal fibers and the room return air is forced through cooling the fibers as the water is evaporated out,, then the cooled fibers turn into the airflow back into the room where the air is cooled via dehumidification of the air.

i have been studying up on various methods of evap cooling and there are some very interesting things that have been done in the past that worked remarkably well in a variety of climates.

bob g
research and development of a S195 changfa based trigenerator, modified
large frame automotive alternators for high output/high efficiency project X alternator for 24, 48 and higher voltages, and related cogen components.
www.microcogen.info and a SOMRAD member

Norsman

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Re: Evaporative Cooling for Hot and Humid Climates?
« Reply #20 on: February 13, 2011, 08:41:13 PM »
dnix71,
Your observation is very accurate.

Just dehumidifying the air without cooling is enough in most climates to make it livable.

My original concept for the prototype that I built was to capitalize on your suggestion completely.  I designed a three pronged approach. First, a desiccant wheel to dry the process air and living space air.  Second, a solar thermal collector to regenerate the desiccant wheel. Third, a sub-scale dwelling that would incorporate the absorbant gravel to provide the thermal inertia required to "coast" overnight.  I built the "house" first.  While I was constructing a solar trough and regenerating oven, I decided to turn on the pump and fan of the house and see what would happen by itself.  I was astounded.  The interior temps in the daytime were below wet bulb.  At first, I was sure that my inexpensive thermometer was at fault.  So I bought a lab-grade thermometer.  It confirmed the cheap one.  Next, I bought a good quality data logger and it too confirmed the others.  I finished the solar trough and the regenerating oven, but I was never able to have them working together.  I used lithium chloride as the desiccant in the regenerator and it proved very problematic.  It is far more corrosive than I expected and caused the (steel framed) regenerator to fail constantly.  I can provide pictures and drawings of everything if anyone's interested.

Norsman

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Re: Evaporative Cooling for Hot and Humid Climates?
« Reply #21 on: February 14, 2011, 10:50:02 AM »
Good idea joestue,

interesting.
i've got a way to test this.
slit a heat pipe down the side and bond it to a heatsink, with a water supply to keep the sinthered copper powder saturated.
then blow air across it. (with a duct covering the top of course)
what you're saying is the heat pipe should get colder than the air correct?

But not exactly what I'm claiming.:)  I'm claiming that the same effect that is provided by the Maisotsenko cycle http://www.idalex.com/technology/index.htm via a complex multistage device is duplicated by the IE system in a simple single-stage system.  Here's how the “M-cycle” works:  Sensible cooling is provided by the loss of latent heat from the airstream in the first (wet) stage.  Heat exchangers are employed to transfer sensible cooling to a secondary (dry) stage.  A portion of this dry, sensibly cooled air is routed back to the primary stage where it cools the incoming air and lowers the wet bulb temperature of the primary stage.  Lower dry bulb in the first stage means lower cooling potential as well.  A cyclic effect is initiated that allows the system to approach dew point temps.  A third stage is used to deliver cool, dry air to living spaces.

In the IE system, evaporation occurs inside lava rock, not in the airstream directly.  Since lava rock, and the remaining liquid water inside, have much greater thermal conductance and far greater specific heat capacity than air or water vapor (in the airstream), the lava rock (and remaining liquid water) will soak up the sensible cooling more readily than the airstream.   The sensibly cooled gravel gives up a portion of the sensible cooling to the subsequent airstream.  This lowers the wet bulb temperature of the airstream and initiates a cyclic effect exactly like the M cycle; it's just far more simple. 

An evaporatime medium with high thermal mass and vaporizing water indirectly are key requirements for a single stage.  Both are satisfied beautifully and simply with lava rock gravel.



Tritium

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Re: Evaporative Cooling for Hot and Humid Climates?
« Reply #22 on: February 14, 2011, 12:57:18 PM »
3A zeolite is an excellent desiccant.

Thurmond

joestue

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Re: Evaporative Cooling for Hot and Humid Climates?
« Reply #23 on: February 14, 2011, 03:40:59 PM »
3A zeolite is an excellent desiccant.

Thurmond

Quote
But not exactly what I'm claiming.  I'm claiming that the same effect that is provided by the Maisotsenko cycle http://www.idalex.com/technology/index.htm via a complex multistage device is duplicated by the IE system in a simple single-stage system.  Here's how the “M-cycle” works:  Sensible cooling is provided by the loss of latent heat from the airstream in the first (wet) stage.  Heat exchangers are employed to transfer sensible cooling to a secondary (dry) stage.  A portion of this dry, sensibly cooled air is routed back to the primary stage where it cools the incoming air and lowers the wet bulb temperature of the primary stage.  Lower dry bulb in the first stage means lower cooling potential as well.  A cyclic effect is initiated that allows the system to approach dew point temps.  A third stage is used to deliver cool, dry air to living spaces.

In the IE system, evaporation occurs inside lava rock, not in the airstream directly.  Since lava rock, and the remaining liquid water inside, have much greater thermal conductance and far greater specific heat capacity than air or water vapor (in the airstream), the lava rock (and remaining liquid water) will soak up the sensible cooling more readily than the airstream.   The sensibly cooled gravel gives up a portion of the sensible cooling to the subsequent airstream.  This lowers the wet bulb temperature of the airstream and initiates a cyclic effect exactly like the M cycle; it's just far more simple. 

An evaporatime medium with high thermal mass and vaporizing water indirectly are key requirements for a single stage.  Both are satisfied beautifully and simply with lava rock gravel.

@Tritium There's no desiccant involved afaik..

i'll have to sit and look at the math on
http://www.idalex.com/technology/how_it_works_-_technological_perspective.htm
it makes sense to me, their process.

the problem is that your system can't cool below the wet bulb temperature.
in effect what you're saying is that if you stick a thermometer inside a piece of lava rock and blow air over it the temperature will drop below the wet bulb temperature.
which, if true, is awesome.
how is a piece of lava rock any different than polyester / sinthered copper powder/fiberglass?

Quote
Since lava rock, and the remaining liquid water inside, have much greater thermal conductance and far greater specific heat capacity than air or water vapor (in the airstream), the lava rock (and remaining liquid water) will soak up the sensible cooling more readily than the airstream.   The sensibly cooled gravel gives up a portion of the sensible cooling to the subsequent airstream.  This lowers the wet bulb temperature of the airstream and initiates a cyclic effect exactly like the M cycle; it's just far more simple. 

i can see that working if you have a system where the lava rock is cycled through such a system
But you cant have the lava rock give up sensible cooling to the air and at the same time absorb it from the air without changing the conditions by which it does so.

Tritium

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Re: Evaporative Cooling for Hot and Humid Climates?
« Reply #24 on: February 14, 2011, 04:02:56 PM »
joestue,  The desiccant comment was in reference to post #20 by norseman where he said he used lithium chloride as a desiccant (another system).

Thurmond

dnix71

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Re: Evaporative Cooling for Hot and Humid Climates?
« Reply #25 on: February 14, 2011, 10:18:36 PM »

Norsman

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Re: Evaporative Cooling for Hot and Humid Climates?
« Reply #26 on: February 15, 2011, 02:18:00 PM »
I see that I'm getting just what I asked for - thoughtful comments. Thanks joestue.

But you cant have the lava rock give up sensible cooling to the air and at the same time absorb it from the air without changing the conditions by which it does so.

 :) Here's how I read the above quote: It's impossible to have countercurrent heat transfer in the same place at the same time.  :-\  I can't argue with that.  What is happening...if not as I described?  If you viewed the video on the first page of inovaenergy.com, you saw the prototype yield a 92 °F to 72°F drop.  To go from 92°F dry bulb to 72°F wet bulb requires a RH of no greater than 38%.  To go from 92°F dry bulb to 72°F dew point requires a RH of no greater than 52%.  I can find no history of RH as low as 38% in July for my area.  I've attached a Mollier plot of the video weather state – plotted to wet bulb and plotted to dew point.

How is a piece of lava rock any different than polyester / sinthered copper powder/fiberglass?

Absorbency primarily, plus highly irregular surface and high thermal mass.  Lava rock, much like activated carbon, has a very high surface area relative to mass.  Maybe it the “Nusselt” effect ?  I came to the single-stage dew point cooling hypothesis when I was searching patents in preparation for disclosure.  One of the European (WO/2003/091633) dew point cooling patents describes a similar set-up where Portland cement is used as the absorbent medium and it is specified that no water is allowed to become atomized in the air directly as this would permit cooling to the wet bulb only.  The European system made no effort to incorporate high mass.

United States Patent 7861549 states:

Quote
“the dewpoint cooler according to the invention is superior in the sense that its performance is substantially improved by the various stated characterizing aspects. Important is the use of said surface-enlarging break-up means, which make a substantial contribution toward the heat transfer between the heat-conducting wall and the respective flow-by media. The characteristic quantity here is the so-called Nusselt number, which is a measure for this heat transfer and can reach very high values according to the invention.  It is pointed out with emphasis that the wetting unit must be embodied such that no or at least negligible atomization occurs, but that there must be for instance an intermittent liquid flow which directly keeps the hydrophilic coating wet. Only in this way is the operation as dewpoint cooler with high efficiency ensured.”