The problem, put simply, is that plastic solar thermal panels, if built like traditional solar thermal panels, cannot turn themselves off if they start to overheat. You cant just paint a plastic panel black and slap some glazing on it. If you stop removing the heat from the panel, and the panel is built from plastic, it will soften or melt eventually. This was recognized by Rob in his instructable:
"because the whole collector is made of plastic, it is important that the temperature doesn’t get too high or it will soften and possibly spring a leak. 80 degrees C (176 degrees F) is about the limit. Don’t think it can get that hot? Think again. ...Therefore this may not be a practical design for residential installation"
The problem is figuring out how to make a solar panel turn off. If you can figure this out, then solar thermal panels could be built of plastic and the cost reduced to a point where it is affordable to everybody. (i.e. $1000 per panel to $200) Why is this important? Because heating our homes (air and water) accounts for the majority of energy we consume in our homes. If solar thermal panels were cheap, we could heat our house with them and save billions every year.
So this is where I am coming from. I want to find a way to build a solar thermal panel from commonly available materials and have it compete with commercial systems. The design I have come up with may be the answer, or it may not, but if the instructable community would work on this problem with me, the benfits to our society could be huge.
check out the blog I created for this idea for more info and for updates as I have them.
Step 1: Use Polypropylene, Not Polycarbonate
Don't use polycarbonate!
Turns out that this plastic degrades in long exposure to hot water. Following up on some user comments, I checked out Coroplast, which is actually polypropylene plastic. long story short, coroplast is MUCH cheaper, MUCH more water-resistant (its used for plumbing) and easily recyclable. Not even any question about what is the better option.
However, you will have to heat-weld the manifold because polypropylene does not glue easily. Read about it at Wikipedia. Until I have the time to test this, and unless you want to give it a try, I recommend just thinking about this idea and coming up with more ways to make it better. There is obviously a lot we can do!
Step 2: Get Your Materials and Tools...
You will need:
>double-wall polycarbonate sheet. I used the 5/8'' but I think it would be better if you used the 1/4'' sheet. its cheaper and will probably work out better. I found a local supplier for Macrolux panels. You will have to find a local supplier. They come in 4'X8' sheets.
>65 micron silicon carbonate particles. The particles need to be about 65 microns, black, and as non-reactive with water or glycol as possible. Finding these particles was a bit challenging because I needed them to be pre-sifted down to a very small size. Silicon Carbonate is used in the abrasives industry and is available in 25lb bags. You will only need a few cups, so contact me if you want to buy some of what I have left over. I bought them here.
>40 micron metal mesh. Again, the minimum order here is far more then what you will need. Read this instructable and you will see how much you need, which will depend on how think your panel is. Contact me if you are interested and I can try to add some strips to Ebay.
>Acrylic plastic sheet or polycarbonate. Again, read this instructable to figure out how much you need, as it will depend on thickness of panel.
>table or skill saw (not shown)
>metal sheet cutter
>a little funnel (or just make one)
DON'T EVEN BOTHER WITH SILICON SEALANT. IT WILL NOT WORK.
Step 3: Cut the Polycarbonate Sheet to Size
Step 4: Cut the Wire Mesh
Step 5: Bend the Wire Strip Over the Top of the Panel
Step 6: Melt the Wire Mesh to One End of the Panel
The idea here is to melt the wire mesh into the top of the panel. I will describe this as best as I can, in reference to the diagram. First, insure the wire mesh is bend around the panel (1). Prop the panel up so you do not have to hold it. Turn on the heat gun and let it get hot. While pointing the heatgun at the the metal mesh (2), use the screwdriver to push the mesh into the panel by moving the screwdriver back and forth (3). You will actually see the plastic permeate the mesh. I strongly recommend you practice this on a few test pieces first. However, if you do screw up, you can always saw off the end of the panel and try again. If you apply too much heat, the panel will bend inward a little bit. This is OK as long as its not too severe because the manifold will wrap around the end of the panel.
Once the metal has cooled on a test piece, try to peel the metal off. It should be really hard to rip it off. If it is, you know you did it right.
Step 7: Add the Particles...
First, use about 2 teaspoons if your panel is 4' tall. I used 1 tablespoon and it was too much.
Second, be sure to measure it fairly carefully so that all channels get the same amount of particles.
Step 8: Repeat Steps 3-5 for the Top of the Panel, Sealing the Particles In.
Step 9: Design Your Manifold
What will follow are the steps I took to do this, but in the process I realized a way better way. I would strongly recommend you not do what I did and skip to step 12 where I show you how to make a manifold in one piece. If you are going to build this...innovate it!
Since I did not fully appreciate that I could just bend the acrylic, I cut strips of .2'' thick sheets and glued them together into a "C" clamp shape, as you can see from the magnified side view in the diagram.
Step 10: Cut the Acrylic Sheet Into Strips and Glue Them Together
Step 11: Install Inlet and Outlet Ports
Step 12: Glue and Seal the Side Pieces
Step 13: Make a Better Manifold Then I Did!
From what I have look into, you can buy a plastic builder or just make your own. You have to build this wire heating element. I found it for much less here.
Rather then seal the end with a glue gun like I did, I think it may be better to use a polyester resin. However, I am not a plastics expert. That's just what I am going to do for the next version.
Step 14: Admire Your New Panel
Step 15: Test the Panel
There is a video of me testing my first panel here.