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  • Solar air collector - covering part of the window?

    I built a collector that I want to put inside my very large window facing south-west. The room naturally warms up when it is sunny, so I wanted to capture more of that sun exposure and produce pure heat. I'm not heating during the day, so the temperature goes down to 15 degrees celcius. The air coming out of my collector is about 24-25 degrees. It seems effective. However, I just stumbled across this site: https://rimstar.org/renewnrg/window_...er_furnace.htm and basically, there would be a net zero benefit to capturing the sun's energy through a window.

    I imagine that the benefits are much smaller because the glass does reflect a certain amount of heat outside. However, I would assume that the difference in temperature with the collector does somewhat prove it is effective. What do you guys think?

    Thanks for the help!

  • #2
    Originally posted by Marquis View Post
    I built a collector that I want to put inside my very large window facing south-west. The room naturally warms up when it is sunny, so I wanted to capture more of that sun exposure and produce pure heat. I'm not heating during the day, so the temperature goes down to 15 degrees celcius. The air coming out of my collector is about 24-25 degrees. It seems effective. However, I just stumbled across this site: https://rimstar.org/renewnrg/window_...er_furnace.htm and basically, there would be a net zero benefit to capturing the sun's energy through a window.

    I imagine that the benefits are much smaller because the glass does reflect a certain amount of heat outside. However, I would assume that the difference in temperature with the collector does somewhat prove it is effective. What do you guys think?

    Thanks for the help!
    First off, the website is probably B.S. and peddling crap to the solar ignorant, or you misunderstood what was being conveyed. Most modern windows that are not facing mostly away from the equator (more northerly in the northern hemisphere) will do quite well in all except very cold climates.

    As for putting a collector in a place where a window currently is, don't do it.

    You're confusing heat quality with heat quantity. A window with standard double glazing will supply more heat to a dwelling than an air cooled collector with double glazing that covers the window.

    Reason: The solar energy passing through the window will enter the dwelling and heat the interior either directly via thermal radiation heat transfer or by multiple reflections around the dwelling, heating everything the reflections hit. The collector will intercept the same amount of solar energy as the window it's crammed into, but will heat air going through it to a higher temperature than the interior of the dwelling, making it quite warm relative to the window glazing temp. The (now) warm collector will therefore lose more heat to the outside by virtue of, and in proportion to, the temperature difference between the collector and the outside environment. Sincee the window and the collector will intercept approx. the same amount of solar energy, the one that loses the most will deliver the lesser amount of heat to the dwelling. The collector, losing more heat by virtue of its higher temperature, will deliver less heat to the dwelling than the window.

    See a website called builditsolar.com for a lot of ideas. Just be careful and think safety. Long story, but many years ago an early collector design I built caught fire while I was checking it for stagnation temp.

    Always think safety first. Expect the unexpected. It will happen. Honest.

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    • #3
      Thanks for the reply. Ok, I need to clarify that my window is maybe 9 feet by 6 feet, and my collector occupies maybe 2x2 feet. The collector sits about 1 feet away from the window. The collector has aluminum downspout, flat black. So while I agree that the collector will intercept same energy as 2x2 space out of 9x6 total window space, by putting aluminum downspout, I added mass, which results in higher temperature than solar rays simply hitting my light colored floors (wood).

      Isn't it?

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      • #4
        Originally posted by Marquis View Post
        Thanks for the reply. Ok, I need to clarify that my window is maybe 9 feet by 6 feet, and my collector occupies maybe 2x2 feet. The collector sits about 1 feet away from the window. The collector has aluminum downspout, flat black. So while I agree that the collector will intercept same energy as 2x2 space out of 9x6 total window space, by putting aluminum downspout, I added mass, which results in higher temperature than solar rays simply hitting my light colored floors (wood).

        Isn't it?
        No. Do what you want but know you do not understand what is going on.

        If you want to gain the most energy from the 6' X 9' window do not block any incident solar energy that's hitting it.

        Reread what I wrote and know that if you want to maximize heat gain from the window, do not reduce the amount of solar energy that hits the window.

        BTW, if you put the collector inside the house, exposed to the sun, the net gain in solar energy to the dwelling will be zero or less by doing so. Less because by doing so, you may increase the interior temp. temporarily, and so, increase the building heat loss slightly (by adding darker materials ) and in doing so also lose some ability to manage the interior temp.

        If you want to put the collector outside the house while not blocking the window, find a way to duct air from the house, to the collector and back, moving it with probably a squirrel cage blower, but know that while what you'll have done can be a fun and learning experience, the amount of heat gained and $$ saved will probably not be worth the cost in time, materials or the cost of the electricity to power the blower. Also watch out that you don't run afoul of local building codes.

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        • #5
          That article seems fairly accurate.

          JPM - it's basically stating the same as what you stated in your posts..

          WWW

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          • #6
            Originally posted by Wy_White_Wolf View Post
            That article seems fairly accurate.

            JPM - it's basically stating the same as what you stated in your posts..

            WWW
            You're correct, it's not complete B.S., just subtly pushing window solar collectors while not mentioning some of the drawbacks.

            Not to veer off the subject too far or deflect criticism of my opinion, but window solar collectors have other drawbacks not mentioned in the included piece such as making the window less air tight by vibration if the collector is powered, or increasing infiltration if/when the window to ducting sealing loosens up. Also, and as I suggested, the economics of small collectors is usually or often unfavorable, although I fully appreciate that cost effectiveness considerations usually do not apply to hobby type learning experiences.

            Thank you for the correction.

            J.P.M.

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