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Will the boil point be higher

rt_2rt_2 Member Posts: 86
I have a 50 evac tube solar system for my domestic hot water. It is a closed loop system. The pressure in the loop is around 20 PSI. I know that water boils at 212 degrees F and I assume the glycol I have in the loop would also boil at that temperature. I was thinking, where the pressure in my loop runs about 20 PSI, would that raise the boiling point of the fluid like it does in an automobile? With a radiator pressure cap on a car radiator rated for let's say 10 PSI, the thermostat used would be around 225 to 230 Degrees. Just a thought. I was always concerned as to how high I should let the temperature go in my system. I generally let it go up to 170 degrees and then dump a lot of that heat/temperature during the night by the process called thermal syphoning.


  • Mark EathertonMark Eatherton Member Posts: 5,843
    Yes, the boiling point will be higher, but glycol doesn't have to be vaporized to break down. Evac tube collectors typically can see stagnation temperatures of 450F or higher, at the absorber.

    Here is a study done by NIST in 1985 which explains the breakdown process. (link at bottom of this page)

    BTW, by their nature, evac tube collectors have very little night sky re-radaition heat loss potential due to the vacuum in the tubes. Most evac systems I've been exposed to require some means of a heat dump (copper finned tube in the attic for example) to relieve excess overheating.

    On my flat plate system, I have incorporated a 2" copper annulus heat exchanger that I buried 15 feet deep and grouted in blue shale. If my storage tank hits 180 degrees F and there is still solar energy available, I open a 3 way diverting valve and send the excess energy into the Earth.

    There are MANY videos on line of how to drill an inexpensive vertical bore hole (don't call it a well unless you'd like a visit from your state water agency) for the purposes of gaining or dumping thermal energy. I made mine out of unistrut, used a HydroBoring tool, and powered it with a Hole Hawg drill head. Drilling in blue shale is a TOUGH proposition, even with a sharpened hydrobore bit.

    During initial testing, I was actually able to build a "bubble" of heat in the shale that lasted for a few days before depleting. My original idea was to consider the possibility of using this Earth heat exchanger to store excess seasonal energy for harvesting at a later date using GSHP technology. I can also use the cool earth to provide radiant cooling to my dwelling via the radiant ceiling. In actually monitoring the system performance, I've determined that at present, I really don't have a lot of excess solar energy to store (only 2 collectors currently on line) and the environment is such that I've never had to resort to cooling the structure other than the use of night time ventilation.

    Once I add the other 4 collectors to the array, I will most probably have to dump energy.

    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
  • rt_2rt_2 Member Posts: 86
    Thank you Mark for the info. I think I'm going to raise my maximum temperature to around 190 degrees F. As far as dumping heat, if my 80 gallons of stored water is at 160 degrees and I let it thermal syphon all night, the temperature will drop down to about 130 degrees.
  • hot_rodhot_rod Member Posts: 13,476
    Wow, that's a lot of collectors for a typical residential DHW system. Do you have a large DHW load?

    I agree with ME reply. I would urge caution with high operating temperatures. The potential to pop the T&P in the tank increases, especially if the tank sensor is low on the tank.

    I think a dump is a critical component for evac tube systems, especially if you vacation in the summer months.

    Or cover some of the array. There was a company building nice heavy duty canvas covers for evac tube arrays. Built like a boat cover.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
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