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Artificial thermal mass

Post in 'The Green Room' started by Circus, May 12, 2013.

  1. Circus

    Circus Member

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    I've been looking for a thermomass scheme for my solar hot air collectors. There aren't any good phase change 70::F materials, water takes up to much space and the ::DTT sucks.
    Contemplating using a heat pump melting wax that cools a normally 85::F house during the day then the hardening wax warms a normally 60::F house during the night. Would such a scheme really save energy?

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  2. woodgeek

    woodgeek Minister of Fire

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    Off the top o my head, if the day/night temp swing outside is 20°F, your COP difference might be at most 0.2, i.e. 2.2 versus 2.4, so might save 10% or so.....you might give lot of that back in additional blower energy use.

    From a technical point of view, how do you package the wax for good air heat exchange? A jumble of mason jars?
  3. Circus

    Circus Member

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    No outside air is used. The HPs condenser would be within 40ish gallons of paraffin. The ::DTt between the room and 135 ::F paraffin would need less surface area for heating.
  4. woodgeek

    woodgeek Minister of Fire

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    Still confused. I figured you were going to pump heat into the paraffin during the day (when it is warm outside) and then transfer the heat with forced air into the conditioned space at night. Ideally, the wax would melt at normal delivery temps for an ASHP, like 90-110°F.

    Alternatively, if you had a phase change at 40°F, you could 'charge it' during a brief warm period in the afternoon with a large volume of forced air, and then pump it into the house at night? I get COP of ~3 at 40°F, and 2.2 at 30°F outside.
  5. Circus

    Circus Member

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    Your replies are equally confusing. Maybe the word "heat pump" has unintended meaning. I'd like to buffer the temperature swings common from solar hot air collectors in the winter. The refrigerant loop would draw heat from the 85 F room melting the lowest temperature phase change paraffin I know of. In the early morning the solidifying paraffin would radiate its stored heat to the cool room.
  6. GaryGary

    GaryGary Feeling the Heat

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    Hi,
    Why not take the 120F ish air directly from the solar hot air collector and run that through a higher temperature phase change material, then send the somewhat cooled air stream from the collectors on to the house, then at night use the paraffin to heat the house?

    Gary
  7. woodgeek

    woodgeek Minister of Fire

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    I hear ya now. And I second Gary's question. Moving and transferring heat with hotter air is always easiest, below 10°F difference, hard to do much.
  8. Circus

    Circus Member

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    Not hot enough. System design runs cool minimizing reradiation.
  9. GaryGary

    GaryGary Feeling the Heat

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    Hi,
    Its pretty hard to keep the temperature rise for an air collector in full sun less than 50F, which gives you 120F output. If you run about 3 cfrm per sqft of collector through a decently efficient collector, the rise will be be about 50F -- 3cfm per sqft is quite a bit of airflow -- most people have a hard time getting to an honest 3 cfm/sf.

    You might also look at the "deep mesh" collector, which will (hoepfully) provide high temperature outlet air and good efficiency:
    http://www.builditsolar.com/Experimental/DeepMeshCol/120116Test.htm

    Gary
  10. Circus

    Circus Member

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    Gary, very impressive documentation. Very different collector design philosophy.
    I'll just have to build my "active heat battery" and monitor the electricity consumed next winter. My collectors will never melt paraffin without restricting air flow. Not happening.
  11. woodgeek

    woodgeek Minister of Fire

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    Could have two collectors...one low temp stored in the space, one higher temp dumped to the paraffin store.

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