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Community and Q&A

Solar thermal stagnation issues

user-757117 | Posted in Energy Efficiency and Durability on

Does anyone have any experience with a closed-loop (ie: non-drainback) solar thermal system that circulates with a DC pump wired directly to a PV panel?
My concern is that there may be days where there is enough sun to heat the glycol but not enough to run the pump – or maybe it’s a sunny day but the PV panel has snow on it. In either situation, I’d hate to cook the glycol.

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  1. Peter Powell | | #1

    If your thermal panel is warm but your pv panel is not activating the pump, then the pv panel is probably undersized. Typically, if snow covers both panels, the thermal panel will melt off first and can get hot with no circulation of the glycol, however the glycol can take it, especially in the winter. My system has two pv panels- the other one runs the secondary (heat exchanger) loop. Many times I've had both pv panels snow covered but no ill effect to the glycol, which I test annually. Stagnation temperatures in the summer are a bigger concern if no hot water is used for a few days.

  2. GBA Editor
    Martin Holladay | | #2

    I have shared your concern in the past. It's a good idea to watch your roof in winter during the morning, so you know where the sun hits first, and which parts of your roof tend to hold snow. Be sure that your PV panel is in a sunny part of your roof, where the sun hits first, and not tucked into a corner where the sun arrives late.

    Like Peter, I haven't had any problems. As he said, this is a winter issue, and your glycol is going to be cold in winter anyway.

    The steeper the angle of any solar module or panel, the less snow it will hold. You might consider using a bigger PV module, and installing it at a steeper angle. As long as it generates enough power to run your pump, even when the sun is high in the sky during the summer, a steeper angle will help you in the winter.

    Finally, remember that the opposite situation -- one in which the PV module receives sun while the solar thermal panels are still snow-covered -- is also undesirable. You don't want your DC pump to be moving glycol through cold panels that are still shaded by snow. Fortunately, in most cases the snow should melt (or slide) off the solar thermal panels at about the same time as it does from the PV panel.

  3. kevin_in_denver | | #3

    A recent improvement in glycol system design, the "steamback system", can prevent your glycol from getting cooked, ever.

    Everything about steamback design can be learned at my thread at HeatingHelp:

    Or this webinar:

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