Sunday, October 17, 2010

Solar Sundays Part VIII- getting closer!

*Warning* - this post is a lengthy, detailed description intended primarily for solar geeks. Proceed at your own risk.
The next step for the solar heating project was to install aluminum absorber plates to all the copper tubes in the system. When I built my prototype panel this spring, I stamped the plates myself. While this was a lot of fun to do once, the thought of pounding out a couple hundred more was not too appealing. Plus, I found a great source for high-performance pre-stamped plates so decided to buy them instead. These plates are made with an over sized groove which wraps around the copper tubing to maximize the contact surface and increase the heat transfer efficiency of the solar collector. They were also custom cut to my desired length, so all I had to do prior to assembly was paint the groove area with metal primer to prevent galvanic corrosion where the aluminum and copper are in contact. Time saved= lots. Normally, the absorber plates are fastened to the plywood backing of the collector framework, sandwiching the copper tubing in between. However, because I wanted a modular design that could be easily disassembled, I opted to build the fin-tube assembly separate from framework. I basically followed this design from the Build-It Solar website, with some modifications to work for my situation. I started by laying one of the copper tubing assemblies on a plywood work surface on the ground, then spacing it off the plywood with strips of 1" foam (I used the pieces of foam that were trimmed off when insulating the collector frameworks, but any thickness would work here).
Then a 3" wide strip of aluminum flashing was centered under the first tube, between the tube and foam strip.
Using a pair of modified Vice Grip 'clamps' (I had a friend weld these up for me, but they can also be purchased from the folks who made the absorber plates), the aluminum absorber plate is clamped tight to the copper tubing and held in position while a pair of sheet metal screws are attached at each end, through to the aluminum strip below. The foam strips underneath keep the screws from penetrating into the plywood work surface below. Next, the clamps are moved towards the center of the absorber plate and two more sets of screws are placed as shown. I found it beneficial to stand on the clamp with one foot to flatten out the absorber plate as much as possible before attaching the screws.
Then, repeat the process for three plates per copper riser... ...and a total of 24 plates per collector. Once the collector is finished, it could be lifted off the plywood work surface and the foam strips are easily peeled off the tips of the screws from behind. The result looks as follows- great contact between the aluminum and the copper! The finished assembly is light and easily movable. I also like that the fin-tube assembly is isolated from the plywood backer by the foam insulation in this design (less mass inside the solar collector is a good thing).
Two panels finished, seven more to go! Once all nine collectors were finished and connected back into the framework, I painted the whole system with Rustoleum High Heat black paint. The remainder of the bull work consisted of burying insulated water lines as well as power and communication wires between the solar array and the barn. I rented a Ditch Witch for a day to carve a trench about 150 feet across the yard. I was able to get about 30" deep with this machine- not below the frost line, but since the system will contain anti-freeze, it shouldn't be a problem. When the barn foundation was put in, I included a run of Thermopex under the slab and out into the yard about 20 feet. Now I needed to extend the insulated lines the remaining 60 or so feet to the solar array. Since Thermopex is around $12-$13 per foot, it was cost-prohibitive to use it again and I opted to make my own system (for about 1/4 the cost). I cut 1.5" XPS foam board into strips and glued them together using spray foam insulation...
...Then put a temporary board on top and clamped it while the foam cured.
The cured insulation assemblies (each 8 feet long) were positioned over the trench and 1" pex lines were placed inside before foam-gluing a cover piece over each one. I staggered the seams between the top and bottom pieces of foam for strength. Once the finished assembly was cured, I turned it on edge (it was too wide to fit in the 5" trench otherwise) and covered the top and sides with poly. The poly was taped around the insulation to hold it in place during installation. Finally, the wood cross members were removed and the pipes lowered to the bottom of the trench- it was a tight fit in places, but it worked!
I left a couple feet of pex on one end so the new run could be coupled to the existing Thermopex end using pex fittings. After the connection was made, I wrapped the bare pex with pipe insulation...
...and wrapped a slit piece of 4" corrugated pipe around that before covering the area with pea rock and landscaping fabric.
After pressure-testing the water lines, the trench was back filled with gravel to about the 4" depth, where installed a run of conduit containing a CAT5 wire and four T-stat wires.
FINALLY, the remainder of the trench was filled and pathways cleaned up- good as new!

1 comment:

mt solar said...

Nice work on the solar for sure! (and the woodworking and brickwork is incredible). Not sure if you have designed your solar pump station and whether it will AC or DC powered but a thought: with the PEX supply line it is a good idea have at least one DC pump (I like the Laing D5 Ecocirc) on the solar loop to make sure you never get complete stagnation in your collectors. I have experienced stagnation on a system with PEX lines and it can cause a major blow-out. Good luck and that system should crank the heat!