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Doug's Simple Thermosyphon Solar Water Heater

Low Cost Passive Solar Hot Water  

It's been  over 17 years since I installed a PV pumped solar hot water system on my house,  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,  I was called to remove a system from a neighbors house. It was an example of the systems from the early eighty's -- overly complex, used grid electricity to run two pumps, and this system was aluminum. I was paid to remove two 4x8' aluminum panels, and the pumps, heat exchanger (HE) and water tank. One panel has 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 $1100. The pump was necessary because the HE tank is in the attic below the roof mounted panel. Hot liquid will rise on it's own, but must be pumped downward. However in the shop, I could have the panel down low, and the Heat Exchanger 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 Heat Exchanger. 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' 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 as 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 supplies, I just had some around.

Wrap around heat exchanger

Tank installed on shelf with insulation. Click on pictures to enlarge

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 6' 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 it's heat to the water tank, cools, and descends back down to the bottom of the panel.

I then super insulated the tank with 2" thick foam panels built to surround it. I used about 20' 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 I so far the heater hose allows enough movement to prevent too much pressure rise.   It has been thru several long, hot summers, and has not had any problems with expansion or contraction of the working fluid. I have changed to cheaper, more readily available automotive type ethylene glycol, as the double walled HE eliminates any contamination concerns.

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 of spent the $160 for the electric heater anyway,. The electric heater is wired up as 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 thru the HE and tank wall.

Doug

Jan 4, 2008

You can reach Doug at:  dougkalmer  AT gmail DOT com         (replace AT with @, and DOT with a period)

Doug's Other Projects

Doug has contributed a number of projects to Build-It-Solar covering a wide range of solar and renewable energy areas -- see them all...

Thanks very much to Doug for providing this material!