Thursday, April 22, 2010

Test of 2 Thermosyphon Solar Water Heating Systems Using Homemade Collectors

I've been doing some simple testing of thermosyphon solar heating systems that make use of our two homemade collector designs.   The idea being to see how these collectors would perform in a thermosyphon system and just to learn more about thermosyphon systems.

For thermosyphon systems, the heated water storage tank is placed above the collector.  The inlet of the collector is connected to the bottom of the tank, and the outlet of the collector is connected to the top of the tank.

As sun heats the water in the collector, it becomes less dense and rises out the top of the
collector and into the tank.  The water leaving the collector is replaced by cold water from the bottom of the storage tank.   As long as the sun continues to heat the water in the collector, a circulation loop is setup in which water flows continuously out the bottom of the tank, through the collector, and then back into the top of the tank. 

When the sun goes off the collector, the water in the collector cools, and circulation no longer
occurs since the water in the collector is more dense than the tank water and naturally stays in the collector.

So, in this system, the circulation is all by natural forces -- no pumps needed.  And, the control is automatic -- no controllers needed. 
These systems are very simple and are widely used in some parts of the world.

The two collector configurations I tested include the collector that uses PEX tubing with aluminum heat transfer fins, and the configuration that uses copper tubing with aluminum heat transfer fins.  Here is the performance log for the copper tube collector (click for full size):


All the details on the test are here:

Please read all the details from the links above if you have an interest in this, and maybe you can help answer some of the outstanding questions.

But, the tentative bottom line appears to be:
- The collector that uses the PEX tubes is probably a bad choice for this thermosyphon application.  The collector temperatures observed would likely result in a short life for the PEX tubing.

- The copper tube collector with the top/bottom manifolds and vertical risers worked well, and would likely be a good candidate for a homemade collector in a thermosyphon collector.  There were some anomalies that showed up in the performance logging -- maybe you have an idea what is causing these?

April 22, 2010


  1. Hey Gary,
    Interesting test. Love the picture of "Big Sky Country". That was one of the things I always loved about Montana. When we going to see an update on the DHW system work you did this late winter?

  2. Hi Gary,

    I'm not 100% sure about the source of your bubbles, but here is my experience:

    I immediately recognized the pattern as air bubbles before i read your note that it correlated with bubbling coming from the collector. In a coroplast test panel i built, i saw similar behavior when it ran low on water (it was kind of leaky :)

    In another test setup i had, i found that with 'fresh' tapwater in a 5gal bucket running through clear tubing, and being pumped with a little fountain pump, bubbles were CONSTANTLY forming in the tubing and radiator, and i had to reorient it to allow all the air to escape. So it may just be a case of lots of dissolved air in your water, which may eventually sort itself out.

    Not sure if that is helpful

  3. Thanks Nuf..

    That's interesting on the dissolved air in fresh water -- had not thought about there being that much.

    The problem pretty well went away on the copper collector once I got that little dip out of the outlet pipe caused by the elbow fitting where it left the collector.

    I may be watching this to closely -- if I did not have the loggers on the system I'm not sure I'd every realized there was a "problem" :)


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