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DIY Solar Air Conditioning Unit

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Step 2: Theory of Operation

Picture of Theory of Operation
parts diagram.png
There's some awesome scientific stuff that makes our Solar Air Conditioning Unit possible!!  The three major principles at work are convection, evaporation, and adhesion-cohesion.

1.  Convection:
     But what does that mean!?  Hot air rises.  As the hotter air rises, it can draw cooler air up with it.  Convection cycles can occur when hot air rises, and draws cooler air from lower elevation.  When the hot air rises and loses energy it begins to fall.  This is the bare bones explanation.  There's a ton more information out there from people who know more than I do.  I did a lot of reading before starting this project, and I'd encourage you to do likewise.  I've listed some sources for more information at the end of this step for your convenience : )

2.  Evaporation:
     We are using water in this project as our cooling agent.  Evaporation is when a substance changes from liquid to gas at it's surface.  Links to fun evaporation information down below.

3.  Cohesion and Adhesion:
     Cohesion and Adhesion are two properties of water, and closely related.  Cohesion means that water tends to stick to itself, or try to stick together.  You can see this by placing small drops of water onto wax paper.  You'll notice that the drops tend to ball up.  This is because of the strong polar bonds in the water that attract individual molecules to one another.  Adhesion means that water sticks to other stuff.  This effect is best seen by dipping part of a paper towel into a small amount of water, and watching as the water makes its way up the paper towel.
(*7/23/2012: shannonlove was nice enough to point out that a more accurate term for this water property is "capillary action".  I was going from what I remembered in my chemistry classes, however doing a search for "capillary action" may lead to more helpful information.)

These three principles make the Unit what it is.  The sun will shine on the unit and heat the air at the bottom of the shaft.  The air will rise, and also draw in more air from the bottom.  As the air rises, it will cause water in a small chamber to evaporate.  When the water evaporates, it will take energy away from the heated air in order to change from a liquid to a gas.  This will cause the surrounding air to get cooler.  The water is brought into contact with the moving air by a piece of fabric.  The fabric will pull up water from the chamber to increase the surface area of the water with respect to the moving air.  As the water evaporates and the fabric gets dry, more water will be drawn up the fabric due to the awesomeness of cohesion and adhesion.

Got it?  Awesome.  Let's get building!
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Jump over here (step 11) to see my video on how it works: http://www.instructables.com/id/DIY-Solar-Air-Conditioning-Unit/step11/Videos/
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More on Convection:
     http://en.wikipedia.org/wiki/Convection
     http://en.wikipedia.org/wiki/Convective_heat_transfer
     http://www.uoguelph.ca/geology/geol2250/glossary/HTML%20files/convection.html
     http://www.youtube.com/watch?v=7xWWowXtuvA
More on Evaporation:
     http://en.wikipedia.org/wiki/Evaporation
     http://www.chem4kids.com/files/matter_evap.html
     http://science.howstuffworks.com/evaporation-info.htm
More on Cohesion/Adhesion:
     http://www.appstate.edu/~goodmanjm/rcoe/asuscienceed/background/waterdrops/waterdrops.html
     http://en.wikipedia.org/wiki/Cohesion_%28chemistry%29
     http://ga.water.usgs.gov/edu/adhesion.html
     http://science.howstuffworks.com/environmental/earth/geophysics/h2o7.htm
 
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SAWWAS721 year ago
Could the effectivness of your design be increased by using a small solar powered fan? I just curious.
:-)

fozzy13 (author)  SAWWAS721 year ago
Yes! It would just need to be placed at the bottom near where the air intake would be.

Thanks for reading and commenting! : )
I used to work for Thermacore Inc. and we used to build air to air heat exchanger for large outside electronics cabinets for getting the internal air temp as close to the outside temperature as we could. The only moving part was a fan inside and out to move air over the fins. It was made up of 24 thermosiphons with methanol as a working fluid.
fozzy13 (author)  foreverdisturbed1 year ago
That sounds like an awesome project! Even if it was for a job, it sounds like fun to me. Thanks for sharing!

Thanks for the read and the comment!! : )
I worked there for 18 years and built all types of electronics cooling products and they were all different which did make it interesting and challenging.
A minor quibble:

Firstly, the movement of water up the fabric is caused by capillary action.  While technically the sum of cohesive and adhesive forces in most liquids, that is not the way it is measured, calculated and termed. People looking to learn more should search on "capillary action" instead of adhesion and cohesion because the latter terms are seldom used in reference to water. 

Secondly, a surprising amount of the cooling done by this system will actually be performed by compression and then decompression of the air as it is drawn through a narrow opening into the bottom chamber and then allowed to expand. 

This latter effect can be employed alone to produce surprising levels of cooling. Buckminster Fuller discovered that geodesic domes would auto cool from this effect if he put small opening in the very top and allow the heated air on the surface of the dome to pull air inside the dome out through open door ways. Later it was discovered that the traditional domed architecture of many desert dwelling peoples also employed this effect. 

If you intentionally draw the air down a small tube configured like a radiator, you may find that you don't need any water at all to produce useful levels of cooling. The compressed air will be warmer than the ambient so it will cool as it passes down the tube. When it reaches the end of the tube, it will decompress and cool even further. 





This design will work as an evaporative AC. They are common to cool entire homes in the western desert areas as they work well in arid environments and poorly in humid environments. It is simply evaporation that cause the cooling and not compression of the air. The reason that more common AC units use freon is that cooling and expansion by air is very limited. It would take a lot of energy to compress air and release it through a narrow opening to get limited results. If you look at a capillary tube in a normal AC it can not function with air at all as moisture would freeze and plug the tube. Freon contains no moisture and when AC units are built they are evacuated by deep vacuum before freon is added.
fozzy13 (author)  glorybe1 year ago
Thanks for the comment and insight!! : )
fozzy13 (author)  shannonlove1 year ago
You make a good point about capillary action! I was merely going from the terms I remembered from my chemistry classes.

I'm aware of Charles's Law, however it would only play a miniscule part in this particular setup. This is because the outer window-box is not completely air-tight. Therefore, when air is drawn into the system it is not at a significantly smaller volume than it will be as it moves through the convection shaft.
-Unless you are referring to the air as it expands moving from the unit into the car, in which case you'd be correct!

Geodesic domes are awesome!

Thanks for the read and the comment! : )
ramatern1 year ago
This would work a lot better dual-channel. Make the wet cloth the boundary between an outer channel (heated by the sun, rising as a result, and evaporating the water in the cloth) and an inner channel (cooled by the cold cloth, falling as a result). So the inner channel circulates inside air, cooling it - and the outer channel circulates outside air, evaporating water increasing humidity - which is no problem, since that air is being exhausted anyway. A small solar fan or fans to increase the flow rate in the respective channels could increase the efficiency, at the cost of greater complexity.

My family had a swamp cooler in California that worked this way. The evaporation cooled a heat exchanger - the moistened air did not flow into the building, as it does on many swamp coolers.
fozzy13 (author)  ramatern1 year ago
A dual-channel would be awesome! I didn't think about it as I was building it, however it would be interesting, and fairly easy to add a second channel on the back of the base board.

I had considered using a heat exchanger, but decided not to due to the scale of the unit. : )

Thanks for the read and the comment!! : )
dkersting1 year ago
I'm havind a hard time to visualize this paragraph: "These three principles make the Unit what it is. The sun will shine on (...) adhesion."

Maybe someone could do a small scheme showing the flow of the wather?
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