The sun is great, and vital to nearly all of Earth's processes.  What bothers me is that the sun also makes the air really hot!  As a result, millions of people turn to air conditioning to cool their homes, cars, offices, etc.  Traditional air conditioning units use massive amounts of energy, making electricity bills skyrocket in the summertime. 

What if there was a way to cool your home, car, or workplace without the need for a big, loud, and energy-hungry machine?  What if this device also was entirely solar powered with no moving parts?  Sound to good to be true? 

This Instructable will cover the method I used to build a prototype Solar Air Conditioning Unit.  This unit is a prototype, and was only ever meant as a proof of concept that my idea could work.  There is a lot of testing that needs to be done to see how this idea can be used in more practical applications.
  • Introduction
  • "Why?"
  • Theory of Operation
  • Gather Materials
  • The Build Pt. 1 - Cutting Wood
  • The Build Pt. 2 - Gluing
  • The Build Pt. 3 - Metalwork
  • The Build Pt. 4 - PVC
  • The Build Pt. 5 - Miscellaneous
  • Using the Unit
  • Going Further and Conclusion
  • Videos!
.:| This Instructable was intended to be entered into the Green Technology Contest.  However, because this project uses no electronics, it was not accepted.  I entered this into the Hurricane Laser Contest instead, and if you like this Instructable, it would be awesome if you would vote for it in the contest!  Having a laser cutter would be awesome because it would allow me to build things better and more quickly to be able to share them with the community! |:.
7/17/2012:  Thanks so much for the feature!!

Step 1: "Why?"

I wanted to try to keep the Intro fairly short and sweet, and more formal.  But why build this, at a personal level?

-I'll take this point to say that if you don't care about specifically why I built this, then move on to the next step, no hard feelings : )

My car sits in the sun all day, and has a black interior.  I'm sure you already know that dark colored things tend to get hotter when they sit in the sun than lightly colored things.  Additionally, windows do a funny thing when light hit them, and serve as a sort of insulation, so that the air in my car actually gets hotter than the outside air temperature.  Both of these factors cause me to start sweating almost immediately upon entering my car, which isn't very attractive when going to work, nor very comfortable.

Therefore, I was thinking of different ways I could cool down my car while it sits in the driveway.  Leaving all my windows down isn't very practical for safety reasons and letting bugs in.  A foil sun shade that would sit on my windshield would be annoying to put up and take down.  I also had a vague idea of how a Solar Air Conditioning Unit could work, and decided that a car would be the perfect test medium for my concept!  The car would allow me to attempt to cool a small amount of space, in a fairly controlled setting, and potentially save me from a very uncomfortable commute.

Step 2: Theory of Operation

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.  Cohesionmeans 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!
Jump over here (step 11) to see my video on how it works: https://www.instructables.com/id/DIY-Solar-Air-Conditioning-Unit/step11/Videos/
More on Convection:
More on Evaporation:
More on Cohesion/Adhesion:

Step 3: Gather Materials

You'll need a few different tools and materials to build your Solar Air Conditioning Unit.  I'm sure all of these could be substituted and worked around in some way, but here is what I used.  Keep in mind that this unit will be made to fit my car.  I tried to keep track of prices as best  I could, because cost is always a consideration with my projects.  I had a lot of the materials already.

  1. MDF Sheet.  |  1/4" thick.  2'x4'.  $7 or so.
  2. Glass sheet.  |  11.5"x20.5".  $5 At local hardware store
  3. Sheet Metal.  |  Recovered from old microwaves.
  4. 1" PVC Pipe.  |  A small section.  I had this already.
  5. 1/2" PVC Pipe  |  A small section.  I had this already.
  6. 1" PVC-1/2" Elbow.  |  The 1" side was a slip coupling, and the 1/2" side was threaded.  $0.80 or so.
  7. 1/2" PVC Male-thread Coupler.  $0.80 or so.
  8. 1" PVC End Cap.  $0.50 or so.
  9. PVC Primer and Cement.  |  I had these already.
  10. Wood Glue.  |  $3-4.
  11. Caulk.  |  This was left over from another project.  Outdoor Acrylic-Silicone I believe.
  12. Matte Black Paint  | $6 at craft store.
  13. Mesh.  |  A splatter protector from the Dollar Store. $1
  14. Small wood screws.  |  $2 at local hardware store.
  15. Fabric.  |  From an old t-shirt.
Total cost: $27.10
  1. Dremel Rotary Tool.
  2. Oscillating Cutting Tool.
  3. Jigsaw.
  4. Drill.
  5. Various hand tools: knives, scissors, screwdrivers, etc.

Step 4: Build Pt. 1 - Cutting Wood

..No, it's not technically wood that I am using.  It's MDF, as stated in the previous step.  MDF stands for "Medium Density Fiberboard".  I had been wanting to see what it was like to work with MDF for a while, because in a lot of cases it can be more durable than wood.  This was also very easy to cut and work with.  I used mainly the jigsaw and oscillating cutter for this step.

(*7/23/2012:  tafelice and isaacwilk were nice enough to point out that MDF has the tendency to absorb moisture easily, which I didn't know when I began this build.  It's a good idea to make sure you coat the Unit with a clear coat, or paint of some sort once it's built to try to minimize this.)

I began by cutting my largest shape.  In this case I had to try to match the shape of my car window.  I did this by measuring the general dimensions of the window, and gradually cutting down the edges and adjusting the curve to match as best I could.  it's not perfect, but it fit somewhat nicely.  I'll refer to this as the "base board".

After knowing how big my window was, I was able to identify how large the outer box of the Unit could be.  I marked this on the base board, and measured to cut for sides of my box, and thin strips for the top of the box, where the glass would be mounted later.  Notches were cut out of all the pieces so that they would fit together nicely to be glued together.

There's not too much I can do to describe this step.  It was a lot of tedious measuring and cutting, as I made it up as I went along.

Step 5: Build Pt. 2 - Gluing

At this point I glued the box together with wood glue and used some caulk to try to make it as airtight as possible.  I did not glue it to the base board, but instead cut notches into the bottom of each side, and cut holes into the base board so that the box can slide slightly into the base board.  This will allow it to be removed.

The window was installed at this point.  I wanted to use a piece of glass I found in the trash from an old window, but shattered it in the process of trying to cut it to size.  Getting a piece from the hardware store was much easier, however it ended up being slightly too small for the box I had just built.  It get around this I used the trimming I had taken from the window to add some bulk to the sides so that it could be securely caulked in place.  The metal trimming was really dirty, so I cleaned it off with an angle grinder while I was cutting it.  Caulk dries very quickly when you place it in the sun in 90+ degree weather.

Step 6: Build Pt. 3 - Metalwork

I wanted to use sheet metal for the main convection shaft.  This is because metal conducts heat well, and so it would better heat the air than say, MDF or plastic.  I used sheet metal from the housing of a microwave that I took apart.  I cut it to size with an angle grinder, and cut a few pieces that were riveted together to form a sort of box.

The ends of the box were bent out with pliers, and holes were drilled along the sides so that it could be attached to the base board.  A small amount of caulk was placed along the edges to try to make a good seal with the base board, but the two were not caulked together, so that it could be easily taken apart and repaired if necessary.

It's important to note that I did not attach the metal piece to the base board at this time.  We will do that later.

Step 7: Build Pt. 4 - PVC

I decided to build the water holder/trough out of PVC pipe, since it's easy to work with, easily available, and inexpensive.

A small section of 1/2" diameter PVC will go through a hole cut in the top of our metal container to a male adapter coupling and into a 90-degree elbow, which has a 1" slip-coupling on the other side.  The section of 1" PVC will have a slit cut in the top (via the oscillating cutter) for the fabric to go into, and be capped off with an end cap.  I used PVC primer and cement for the smooth coupling pieces and end cap, but did not glue the threaded piece in case I need to take it apart for repairs.  I used a 1/2" end cap, not glued in place, to top off the 1/2" pipe.

I cut a small piece of sheet metal, again from the microwave housings, and bent it into a bracket to hold the pipe in place with rivets on one end.  On the other end the pipe simply sticks through the hole in the metal box, however glue or caulk could be used to permanently affix it.

Step 8: Build Pt. 5 - Miscellaneous

There are a few unrelated things that still need to be done before this is operational.

1.  A small piece of fabric must be cut and fit into the cut made in the 1" PVC pipe.  Mine was from an old t-shirt.

2.  A hole must be cut in the base board for the air to be able to move into the car.  This was done with the oscillating tool, and was about the same width as the fabric piece.

3.  A hole was cut into the bottom of the outer box, and a piece of mesh glued in place to allow air to be drawn in from the bottom.

4.  Everything must be assembled.  The piece of fabric was put into position, from the pipe to the edge of the metal box.  Holes were drilled, and small wood screws were used to hold the metal box to the base board.  The bottom of the metal box, as well as the wooden outer box were box painted black.

Step 9: Using the Unit

Once everything is put together, the fun part starts!

To use the unit. I simply place it in my car window, add water to the spout above the metal housing, and put the window-box in place over it. 

This project could definitely use some in-depth testing, which I haven't had too much time for.  However, the first time I used this unit, my car's internal temperature dropped 10°F within only 40 minutes!  This was without any noticeable change in outside temperature, shading, or other factors. 
Update 7/31/12:  On most occasions, the Unit drops the car's internal temperature by 2-4°F.  Not nearly as awesome as the first test, however considering Ohio is not the ideal location where evaporative cooling would be most efficient, and the Unit is very small, the drop in temperature is still a success.

However, the thermometers I bought at the Dollar Store can hardly be called high-precision instruments, and one actually broke when trying to measure the initial temperature of the car one day because it was so hot.  When I can find affordable precise temperature monitoring equipment I'll do more testing.

Step 10: Going Further and Conclusion

It would be awesome if this Instructable was only the first step in Convection-Evaporation Cooling becoming a widespread and widely used technology.  This project was an experiment, and meant to exercise and test the concept I had imagined.  This would undoubtedly be much more effective if it were scaled up to be used in homes, where air conditioning costs a fortune to run all summer, normally on carbon-produced electricity.

I'd like to point out also, that if water were not added to the Unit, it could act as a heater, using sunlight to heat the air in the shaft, and simply funneling it into the home/car.  Definitely an awesome feature if this were to be considered as a device to be incorporated into homes.


At this point I'd like to sincerely thank you for reading my Instructable.  I'd like to encourage you to leave a comment, because I like reading them and it motivates me to actually document my projects as opposed to silently enjoying them.  If you wanted to rate this Instructable and Follow/Subscribe that would be awesome as well!

Good luck with your build!

Step 11: Videos!

The first video shows how the Unit is assembled and set up.

The second video explains how the Unit works.

About This Instructable




Bio: I am currently a mechanical engineering student at the University of Toledo, and the founder of the University of Toledo Maker Society. I have a ... More »
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