By Default/Design, are Solar Panels Great Radiant Barriers?

I was talking to a co-worker the other day and explaining my plans to cover a test building with solar panels on the roof that the sun hits most. I was explaining how they would also shade the rooftop when I realized that theoretically... they will be doing much more than just shading. By converting some percentage of the energy to electricity, they will be effectively creating a radiation barrier. How well does that actually work in practice?

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  • BB.BB. Posts: 27,904Super Moderators, Administrators admin
    Re: By Default/Design, are Solar Panels Great Radiant Barriers?

    My attic is substantially cooler under sunlight now that my solar PV panels are up there... However the ~12-14% or electrical conversion still leaves ~86% or so of the energy behind--plus the black solar panels absorb more heat than my light colored roof.

    So--for me, I think it is simply shading that is reducing my attic temperature.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • jcgee88jcgee88 Posts: 154Solar Expert ✭✭
    Re: By Default/Design, are Solar Panels Great Radiant Barriers?
    retrodog wrote: »
    I was talking to a co-worker the other day and explaining my plans to cover a test building with solar panels on the roof that the sun hits most. I was explaining how they would also shade the rooftop when I realized that theoretically... they will be doing much more than just shading. By converting some percentage of the energy to electricity, they will be effectively creating a radiation barrier. How well does that actually work in practice?

    When I first started thinking about solar, I thought about
    this shading as being one of the subtle benefits, too. When
    I asked my contractor about it, he could not give me any
    estimate of the effect. My specific question was, "what
    would the panel shading effect be, in terms of equivalent
    R-factor?"

    Later I spoke with a house inspector. He had me think of
    it from a different light. Yes, he said, it would have some
    effect, especially since it was in the most sunny part of
    the roof. But, as a percentage of the roof, the number
    is small. In my case, I have a 1200 sq foot floor plan.
    My roof is the traditionally angled architecture, so let's
    ballpark that as 1800 sq feet (my Pythagoras is too rusty
    to get that exactly). My PV array is about 208 sq feet.
    So, I am shading on the order of 10% of the total area.
    If this shading reduced the radiant heat into the roof
    by, say, 40%, then the total effect is only 4% reduction.
    That falls into "noise level" as they say. So, it's not zero,
    but it is not much, either.

    John
  • dreesdrees Posts: 481Solar Expert ✭✭✭
    Re: By Default/Design, are Solar Panels Great Radiant Barriers?

    My experience:

    I have panels installed on my nearly flat roof 2nd story. Insulation varies between old R13-R19+ - the R19 plus having been installed when I've need to do work in the area.

    The ceiling under the panels stay at indoor temps in the summer.

    The ceiling not under the panels is warmer - seems to be about 2-5*+ warmer on a summer day depending on how thick the insulation is at that point.

    Had a 4'x2'spot with no insulation the ceiling there would read 30* over indoor temps - read over 110*F there. Fixed

    Temps taken using an infrared gun.

    Would be nice to have one of those thermal infrared cameras to identify all the hot spots on a summer day to easily identify the problem areas.

    So yes - solar panels do seem to work very well at keeping the effects of solar irradiation out of the house. Makes sense - I now have a large shaded area of my roof with a 6" air gap between the panels and roof. Up on the roof, the roof surface seems to stay at ambient under the panels. Dramatic difference just using your hand to feel temperatures.
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