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Glass questions.

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  • Glass questions.

    I scored three pieces of double-pane glass today and I'm hoping it will be useful for a solar project of some sort. I've been thinking of making a solar water heater, but I'm also considering a solarium off my backdoor or even a small seperate greenhouse or greenhouse bed. I suspect the glass is tempered, so I doubt it could be cut. I'm wondering if it's possible to seperate the panes so I have twice as much, or would it brake in the process. One piece is just shy of 24x36", one is 36x80" and one is a long wedge which I didn't measure. Is this the type of glass to use on a solar project. Link to photo.

  • #2
    It's hard to say if it's the type of glass to be used on a solar project. That depends on what exactly you are trying to do and what coating the glass has. For plant growth, full spectrum light is probably best (no coating), but many greenhouse applications benefit from "low E" coatings which result in higher R-values, better insulation for greenhouse temperature regulation at the expense of cutting off some of the red light that plants need. There are many types of glass coatings ranging from almost no effect to severely cutting off the ends of the spectrum. The high-end residential materials try to pass most of the visible light while cutting off the long wavelength infrared, thus not allowing much heating. For hot water heating, your best bet would be a glass with no coating so-as to let all the infrared through. Without knowing what coating is on your glass it's a crap shoot. I'll bet you can separate the dual pane glass without breaking but sounds like a pita.


    • #3
      That's good info, Dave. Based on the house that it came from, I'm guessing that mine may be the high-end residential type that doesn't allow much heat through. Is there any way a lay person can determine if there's a coating on the glass. All I can think of to do is a test; make a small box heater with the smaller piece to see how well it heats the space.


      • #4
        See this: for some introductory info on low e coatings.

        Generally, low e coatings will reduce energy transmission through a glazing by increasing reflectivity in somewhat selective wavelength bands, usually in the infrared ranges.

        If the primary goal is to heat something via solar radiation rather than prevent heat loss through, say. a window/glazing, low e glass may not be the best choice, since the goal is usually to get as much energy to the use (heating/PV/whatever) as possible. Uncoated glass already does a pretty good job of that. That, and the fact that, aside from the breakage issue, glass is pretty impervious to chemical attack and has an indefinitely long service life, has made it the historical material of choice for solar applications.

        If the primary goal is to reduce heat/energy transmission (gain or loss) through a glazing and solar gain is a secondary, or even an unwanted situation, low e glazing is what's wanted - to help prevent overheating by increased solar reflectivity (lowering solar gain) or lowering heat transmission (either gain or loss) through shaded or non equator facing windows for example.

        If the goal is to reduce heat loss through a window rather than maximizing solar heat gain (that is, minimizing heat loss, say, at night), low e coatings can have the advantage of higher reflectivity for longer wavelength energy (the stuff thermal radiant energy is mostly composed of, and what's trying to get through a window and escape into the nite, never to be seen/felt again) that are useful, probably cost effective and sometimes quite noticeable.

        But, for example, if you want to heat water with a flat plate solar collector, low e glass is about the last thing you want to use because the low e coatings, in reflecting infrared, are not that selective, with the result that they can also block other portions of the nearby spectrum, some of which are also parts of the incoming solar energy spectrum, and in so doing, reduce the solar gain (and thus collector efficiency) more than the reduction of heat loss from the collector. Long story. Short answer: for solar thermal, and PV, don't use low e glass.

        For sunspaces and passive solar applications, when dealing with low e options for glazing, some careful and informed design work is usually necessary to balance solar gains, which will usually be reduced w/low e coatings, and heat losses which will also be reduced with low e coatings. FWIW, if one of the goals of an otherwise unheated sunspace or solar tempered addition/application is to provide heat to a dwelling, I'd use uncoated glazing if possible (and BTW, mostly not possible in CA due to energy mandates), and use other, variable, moveable/adjustable and time controllable methods to reduce unwanted heat gain/loss.

        BTW, all the commercial low e coatings, while somewhat similar in some ways, are not the same as, and have a different purpose than anti reflective coatings that most quality PV panels use.

        I also wonder, but have no knowledge of what effect changing the wavelength distribution of transmitted light will have on plant growth or health.

        Add: a butane cigarette lighter flame will look different through low e vs. uncoated glass. Also, for some older coatings, coated and uncoated glazing sometimes look different when viewed through polarized lenses. For either, you'll need an uncoated lite of glass for comparison. Also, coated glass often looks different when viewed from the inside vs. out side a dwelling, depending on viewing angle and how the sun is hitting the window.
        Last edited by J.P.M.; 04-02-2017, 01:04 PM.


        • #5
          JPM, thanks for the reply. The practical question is; how does one determine whether they have low E glass, or glass with other coatings, when they are obtaining used glass.
          BTW, I found this regarding window coatings and solar heat gain coefficient (SHGC). About 1/3 down the page.


          • #6
            Originally posted by LarryJ View Post
            JPM, thanks for the reply. The practical question is; how does one determine whether they have low E glass, or glass with other coatings, when they are obtaining used glass.
            BTW, I found this regarding window coatings and solar heat gain coefficient (SHGC). About 1/3 down the page.
            You're welcome. See latest edit. I try to remember to reread the post I'm responding to before I pull the trigger on a response. Missed it this time.