In March and early April of 2022, I volunteered on an offgrid Fablab in the Algarve region of Portugal. My main project was to build a solar thermal system.
I didn’t finish the entire system, but I did finish the build of the solar thermal panel.
The Idea
The Treehouse bathroom shower is heated with an instant gas hot water heater. It’d be better if it were heated for free by the sun.
Solar thermal panels are simple devices. Build a box, run copper tubing in it, put a piece of glass on one side and point it at the sun. Solar radiation goes right through glass. Thermal radiation is stopped by glass.
So energy from the sun goes in, and then gets trapped inside the box with nowhere to go except into the water that is flowing through the tubes.
Thermosiphon systems are interesting in that the flow of water is driven not by a pump, but by the thermosiphon effect. Cold water is denser than hot water, so it sinks in a given volume of water. You can also say that hot water is more buoyant than cold water, so it rises in a given volume of water.
If you connect the hot water side of the solar panel to the top of the storage tank, and the bottom of the tank to the cold water side of the solar panel, water will naturally and passively flow through the solar panel when the sun is shining on it. No sensors, switches, moving parts, or even electricity required.
Thermodynamics is awesome.
Refurbishing the hot water tank
Philipp on the right angle, looking for holes.
Lucio (the workaway host) had an old gas tank style hot water heater hiding in a tree. I brought it up to the workshop, pulled off the wretched old insulation, scraped off as much rust as I could, and coated the whole thing in rust converter. Twice.
Yep, there’s yer problem right there.
Then we checked it for leaks. There were several at the bottom. Philipp, a German workawayer, welded those up, but still found three small microleaks.
We weren't able to fix these before I left. The remaining work to do with the tank is to:
Weld up the microleaks
Paint the whole tank to protect it from further rust
Insulate it with rockwool, and use thicker stuff than the old thin crap
Add the protective shell on to it with a patch because the circumference will be greater
Add a top hat to it so it doesn’t get rained on any more
Building the solar collector box
My inclination was to build the box using strips of scrap wood milled to size on the table saw. Lucio saw what I was going to do and said ‘No no no, just CNC it!’
Of course CNCing something as simple as a box is a little silly… but it is way funner, and having fun is at least half the point of all this.
So I modeled the box in Blender after taking careful measurements of the piece of glass we were to use.
After I was satisfied with my design, I link-duplicated the components, laid them out flat, pointed Blender’s camera at them, and used an SVG exporter to get a vector image file of the pieces. These files I cleaned up in LibreCAD and then gave DXFs to Joao, another workawayer.
Joao used Artcam to generate g-code files that the CNC machine could read, and then spent an afternoon babysitting a robot. At the end of the day, we had all the pieces for the box.
Assembly was easy. Luis screwed it all together and then coated everything with some sort of impermeabilize paint intended for keeping posts from rotting when you put them in the ground. In retrospect I’m not sure this stuff was the best choice for a device that’s meant to be in the sun as much as possible.
Bending the pipe
Lucio had a roll of flexible 12mm copper pipe. Bending such pipe to the tight radius we needed was not easy - if you try to do it by hand you’ll kink the pipe for sure. We needed a Tool.
I told Lucio that pipe benders were €176 on Amazon. He said “forget that. We’ll print one.”
We wound up CNCing the radius out of a chunk of hard wood of some sort, and then routering the pipe groove in it with a jig. Some rusty bits of metal and an old roller later and we had our pipe bending radius tool. Total cost: idk, a few cents for the electricity to run the CNC for ten minutes. Everything else was scrap.
The tool took a little getting used to to make nice bends, but by the end we had it figured out.
I drilled holes in the side, slid the two layers of copper tubing in the box, and fastened it down with wire from the scrapyard and salvaged screws.
Testing
I set up this test rig to make sure the thermosiphon action actually worked. It was a mild and cloudy day, but every time the sun came out for even a minute, the tubes heated up and warm water started visibly flowing through the jug. Success!
The Stand
I banged together the stand from pallets on a Friday morning. This represents real spiritual growth for me. Even a month ago, I wouldn’t have been able to just grab a pile of stuff and make something from it. My instinct is to model something perfect in 3d, go get the precisely dimensioned materials I want, and follow the build instructions I made.
That’s not how you build using salvage and scrap, though. You get together a pile of the materials you have to work with, and look at it, and sort of lean things together and just sort of play with it until the shape is about right, and then you figure out how to make it sturdy with lots of screws.
What’s Next
Once the tank is fixed up, it’s just a matter of plumbing up the connections between the solar collector and the tank, and patching in to the treehouse bathroom system. I’m bummed I couldn’t finish the whole system myself, but I’m glad to have finished the panel.
Hopefully another workawayer will finish it soon and Lucio has promised to send me pictures of it. I’ll update when I can.
Critical Thoughts
The panel is condensing quite a lot. I’m not sure what to do about it. It might just be drying out for a few days and then it’ll be fine.
The paint appears to be bubbling in the sun and heat. I think the impermeabilize stuff might not be appropriate for high heat and high UV exposure builds.