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Build a low-cost solar water heater. (Alternative energy).

It's been over 10 years since I installed a PV pumped solar hot water system on my house (COUNTRYSIDE Vol 85/1), and that success made me want to design a system for my shop/studio outbuilding. Because people saw the panel on the roof of my home, word got out that I knew something about solar and I was called to remove a system from a neighbor's house. It was an example of the systems from the early '80s--overly complex, used grid electricity to run two pumps, and this system was aluminum. I was paid to remove two 4'x8' aluminum panels, and the pumps, heat exchanger (HE) anal water tank. One panel had a leak, but the other checked out fine.

While the system on my house works well, I did have to invest in a 10 watt solar electric panel to run a small circulating pump, and have a stainless steel heat exchanger tank built. The total for the house solar system was $1,100. The pump was necessary because the HE tank is in the attic below the roof mounted panel. Hot liquid will rise on its own, but must be pumped downward. However, in the shop, I could have the panel down low, and the HE tank up high, so I hoped to not need the expensive pump and PV panel. Fortunately for my purposes, the aluminum hot water panel had a serpentine pattern, so the liquid enters on one end down low, goes the length of the panel, rises a few inches and comes back, repeating the pattern. This works well for a thermosyphon system, as the heated liquid naturally rises, pulling cooler liquid in below. Technically, the cooler denser liquid is heavier, so it is pulled down by gravity, displacing the lighter, hotter liquid, but for simplification, heat rises. Now I just needed to figure out how to build a low cost HE.

I bought a new 38-gallon electric water heater, the shorter style. I removed the metal ends and shell, promptly voiding the warranty, I'm sure. I used a wood chisel to remove two strips of foam insulation below and above the thermostat. I bought a 50-foot coil of 5/8" copper tubing, and wound it around the tank as tightly as I could, using strap clamps to pull it into direct contact with the metal tank walls. The idea is to keep the coils low on the tank and in close contact with the exposed steel. I wrapped the coils in copper flashing and twisted copper wire around the flashing to hold the coils in as tightly as I could. Leaving the two tubing ends sticking out, I insulated the tank with radiant barrier, which is reinforced aluminum foil, then a layer of Reflectix, which is double bubble pack covered with Mylar. Both are available at building supply stores but I had some laying around.

I reinforced a heavy duty shelf in my shop and used the front end loader to put the tank on it. The bottom of the tank is about six feet above the top of the collector panel. I then used 5/8" automotive heater hose to connect the top of the panel (hot out) with the top of the HE coil around the tank, and the bottom of the coil to the bottom of the panel. This way, whenever the sun shines, the non-toxic antifreeze (propylene glycol) heats up, rises to the top of the HE coil, gives up its heat to the water tank, cools, and descends back down to the bottom of the panel. I then super insulated the tank with two-inch thick foam panels built to surround it. I used about 20 feet of heater hose, and at first I thought I'd have to use an expansion tank to accommodate the expansion and contraction of the fluid with temperature swings, but so far the heater hose allows enough movement to prevent too much pressure rise. I have not been through a hot summer with it yet, but I have the collector panel at a steeper winter angle, so I believe it will be okay.

I put a tire valve at the highest point of the HE tubing so I could bleed air from the system, and check pressure. I have about $60 in the solar end of the system. I would have spent the $160 for the electric heater anyway. The electric heater is wired for use as a back-up. The system works, but the weakness is the smaller size of the tubing (5/8") and the lack of a pump means the temperature difference has to be fairly great to circulate much heat. I am considering going back in to add cement around the HE coils, in direct contact with the tank, to improve the heat flow through the HE and tank wall.

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Publication:Countryside & Small Stock Journal
Date:May 1, 2003
Previous Article:A year-round solar greenhouse: Part III. (The Solar Cowboy).
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