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Efficiency with wider-spacing plex risers in solar water DIY panel

 
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I want to mount a solar panel with plex risers against a house wall that is already insulated.  

I am in NZ, and so the cheapest tee fittings will be $4 each but I can get the cheap box timber from the mill for $1 per metre (and have 200m sitting in  my paddock now), and have heaps of second hand greenhouse plastic to line it with.  
I also have heaps of sun-facing area to put them on.  
Being near the coast, our minimum winter temperature at night is -3C, and average winter maximum is 13C, so a single layer of plastic does bring quite a bit of warmth.


My question, then, is how wide can the risers be to make the whole thing cost effective?


https://www.builditsolar.com/Projects/WaterHeating/CPVCCollector/CPVCCollectorTest.htm

Looking at the bottom of this article, doubling the risers only gives 10% improved efficiency return, as does adding aluminium fins.  My guess is that having a quarter of the risers in the same space would result in about a 50% loss of efficiency, but I was wondering if any has tried this before I spend the time and money experimenting.
 
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Location: South Central Kansas
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Perhaps you could build a small scale model to experiment with?

https://www.engineeringtoolbox.com/thermal-conductivity-metals-d_858.html

 
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