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solar collector runaway?

Post in 'The Green Room' started by Ehouse, Jan 27, 2013.

  1. Ehouse

    Ehouse Minister of Fire

    Joined:
    Jul 22, 2011
    Messages:
    936
    Loc:
    Upstate NY
    What happens in this situation; Collector with tubes arranged so that the in feed is at the bottom, tubes are arranged to promote thermosyphon, outlet piped directly to inlet, all surfaces well insulated except for collecting surface, bright sun ,and say 20*F.

    a. Collector blows apart.

    b. Constant temperature of the system is reached and holds till sun fades.

    c. Collector freezes up.

    d. Other.

    Ehouse

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  2. Circus

    Circus Member

    Joined:
    Jan 11, 2013
    Messages:
    126
    Loc:
    EC Wisconsin
    Anything can be intentionally broken.
  3. GaryGary

    GaryGary Feeling the Heat

    Joined:
    Jul 12, 2010
    Messages:
    291
    Loc:
    SW Montana
    Hi,
    Not sure why you would want to do that, but you can use this calculator to get a rough idea:
    http://www.builditsolar.com/References/Calculators/Collector/ColEfic.htm

    Basically, you can take your choice of collectors -- i used the Heliodyne, put in 20F outdoor temperature, and keep the 317 BTU/sf-hr sun (full sun).
    You then need to just keep entering higher Average Collector Temperatures until the calculated collector efficiency comes out zero. When the efficiency is zero, it means the collector is losing as much heat out the glazing as is coming in, and that's the situation in your question.

    I get that the collector efficiency goes to zero with a collector temperature of 240F.
    If there is no pressure relief, then what happens depends on whether the collector can take the internal pressure associated with 240F -- I think that's about 25 psi -- so, probably nothing happens (assuming there is a little expansion space to accommodate the expansion of the liquid water as it heats up -- like a little air bubble at the top of the collector).


    Gary
  4. Cynnergy

    Cynnergy Feeling the Heat

    Joined:
    Oct 15, 2012
    Messages:
    291
    Loc:
    Coast, BC
    I worked at a nature centre for a while that had solar hot water collectors on the roof (evacuated tubes). It was a bit of a silly install - the pumps running the system worked on mains power (no electrical backup), so when the outdoor temps went down to -5-ish*C for a few days and it was beautiful and sunny, but then the power went out for a day or two... the pumps stopped working, the water stagnated and poof went one of the evacuated tubes to relieve the pressure.
  5. Ehouse

    Ehouse Minister of Fire

    Joined:
    Jul 22, 2011
    Messages:
    936
    Loc:
    Upstate NY
    Gary, that's exactly what I wanted to know. I'm just trying to do some thought experiments for unconventional applications. I hadn't considered pressure. probably a small air-hammer tube would work. If you'll indulge me, what happens when you add a small (say 10 gal.) well insulated reservoir to the system. How 'bout a conventional pressure tank. Will I still accumulate 240* or close to it? Add a mixing valve. Open system or closed loop in the tank. Aha! small indirect! Starting to sound like a conventional set-up after all.
  6. GaryGary

    GaryGary Feeling the Heat

    Joined:
    Jul 12, 2010
    Messages:
    291
    Loc:
    SW Montana
    If you locate the 10 gallon tank above the collector, you have a thermosyphon solar water heating system, which works fine.

    Gary

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