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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?

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.
Posts: 268
Location: South Central Kansas
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Perhaps you could build a small scale model to experiment with?

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