Saturday, July 28, 2012

Comparing the Performance of Two DIY Solar Water Heating Collectors -- CPVC vs Copper

Scott Davis has come up with an innovative and simple design for a solar water heating collector that uses CPVC pipes for the risers -- its easy to build and performs well.

Scott and his prototype CPVC collector.
The collector uses closely spaced CPVC riser pipes which are silicone bonded to a flat aluminum absorber sheet.    This makes for and easy and fast build.

I did a side by side performance test of Scott's design compared to the established DIY collector design that uses copper riser tubes spaced about 6 inches apart with thicker grooved aluminum fins to transfer heat into the riser tubes.  In this test, the CPVC design performed within 5% of the copper/alum collector, which I think is very good.

The side by side test of the two collectors.
There are a number of pluses and minus associated with the CPVC design compared to the copper/alum design -- things like cost, ease of build, performance, durability, and tolerance for stagnation temperatures are all important.  Which design works out best will depend on your situation -- I've tried to provide a little data on how the collectors compare in all these areas.  

I would be most interested in hearing your thoughts on the comparison and which collector you think would work out for your situation.

Full details on the performance and stagnation tests, construction, IR pictures, etc. for the two collectors...



Closer view of the test CPVC Collector.


IR image of the CPVC collector in operation
If you go ahead and build a CPVC collector, please let me know how it works out for you.

Gary





3 comments:

  1. Nice side by side rig. I think the objections to the CPVC as UV sensitive are addressable by putting the aluminum flashing on the sunside of the tube/metal array. That will positively shade the CPVC from UV (and make possible the use of a UV transmissive front glass to get more energy in and greater durability) Yes at greater cost. Also worth considering is a panel at the top and bottom of the collector frame that drops away/open when temperatures exceed the safe operating temp. That could vent heat quickly and inexpensively. Like a radiator's cap that pops, this would vent the entire top and bottom of the collector and let air flow up and cool it. I've seen heat activated louver actuators for greenhouse windows. Maybe they could be dragged into service. here is one but it activates too early: http://www.farmwholesale.com/greenhouse-supplies/cooling-and-ventilation/solar-vent-opener.html that page explains the working... sounds like phase change for parafin. This can be manipulated by picking different parafin formulations. Maybe not a DIY project at that point... Still food for thought. No?

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  2. The CPVC UV problem is addressed by your design as is, I should say, but my Aluminum first approach does it with some additional benefits... But getting ALL the heat into the CPVC would like some high quality contact with the aluminum... That is where I think I'd put some effort into maybe undulating the aluminum and riveting it to straps horizontal to the CPVC to get some clamping force to snuggle them up. That or bending jig I could drive over to create a rounded groove to positively grab the aluminum. But that would jackup the aluminum content... Still good on ya for sure.

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    Replies
    1. Hi Paul,
      I think that as you say, the problem with having the alum face the sun you would need to have much better thermal contact between the alum and the tubes. This goes against what Scott was trying to achieve, which was to see if spacing the tubes very close together would eliminate the need for grooved alum with the tubes fitting in the grooves.
      A lot of DIY collectors are built with homemade or premade alum fins with a groove that fits the tube tightly for good thermal contact -- for example: http://www.builditsolar.com/Projects/SpaceHeating/DHWplusSpace/Main.htm
      Scott was looking for something that is very easy to build and getting rid of the need for the grooved alum fins was part of getting there.

      Gary

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