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Check my physics? Water heat storage

Post in 'The Green Room' started by dougstove, Sep 12, 2013.

  1. dougstove

    dougstove Member

    Joined:
    Aug 7, 2009
    Messages:
    221
    Loc:
    New Brunswick, Canada
    Hi;
    I am thinking about putting some tanks of water in my big room that contains my stove, as temperature moderators.
    I am working towards whether a water tank of reasonable volume can store and release enough heat to be a worthwhile moderator?

    From first principles:
    Heat capacity of water, 25C, 4.1796 J cm-3 K-1
    Heat capacity of room air, 25C, 0.00121 J cm-3 K-1
    Ratio: 3454
    So, 1 ml (= 1 cm3) of water, dropping 1C (= 1 K), releases enough heat to raise 3454 ml of air, by 1 C?
    So, 100,000 ml = 100 l, dropping 10C, releases enough heat to raise 345221500 l of air by 10C.
    My room has an air volume of ~ 128000 l.

    This looks like a 100 l water tank stores alot of heat in a 10C temperature rise, compared to the heat capacity of the air.

    I am sure the engineers on here have tables for this kind of thing.
    Am I way off base?
    cheers, Doug

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  2. jharkin

    jharkin Minister of Fire

    Joined:
    Oct 21, 2009
    Messages:
    3,317
    Loc:
    Holliston, MA USA
    You are correct that on a volume basis water has a LOT more mass and stores a LOT of heat relative to air.

    The problem you are going to run into is transferring the heat from the stove to the water and water back to the air. If you just put a big plastic barrel of water next to the stove its going to warm very very slowly and then cool very slowly and may not have the moderating effect you are hoping for.

    Without doing the math, to make this work how you expect I believe you will need a lot of surface area close to the stove. which might mean piping the water through coils or a radiator of some sort. Probably not worth the effort.
  3. cygnus

    cygnus Feeling the Heat

    Joined:
    Oct 23, 2010
    Messages:
    336
    Loc:
    Central, NJ
    +1. You would need a heat exchanger to get the heat in and out of the water when needed.
  4. woodgeek

    woodgeek Minister of Fire

    Joined:
    Jan 27, 2008
    Messages:
    2,711
    Loc:
    SE PA
    Gen'l rule of thumb is that you need a 10 degree temperature differential to get any worthwhile heat flow. IOW, unless your room temp is swinging ±10° or more, the heat flow in and out will be tiny, unless the area is huge.

    When I looked at this, I discovered that the heat capacity of all the air in my house, and the mass of the house was huge enough to absorb all the heat with a small rise in temp. IOW, if I could spread the heat of a burn over the entire house, the air and drywall would be a perfect 'heat battery' (right capacity, large area, heat where I want it). So you already **have** the storage you want (outside of the stove room, rest of the house).

    So, work on getting a nice big quiet fan, blowing tons of cool air from the rest of the house toward the stove. And voila you will be happy. I'd rec an actual fan, not that you try a ducted fan or anything fancy. The latter almost always have too low flow to get the job done.

    In my layout I have a ordinary sized door on the stove room, and then a open plan on the other side. If I get the heat through that door with a fan, nature does the rest.
  5. Circus

    Circus Member

    Joined:
    Jan 11, 2013
    Messages:
    126
    Loc:
    EC Wisconsin
    Using my 40 year old Machinery's handbook I got a ratio of 3248/1. Unless your temperature swings are great and durations long, your going to need a lot of surface area. A thousand free 2 liter pop bottles stacked against a wall works suprisingly well for daily swings.
    Warning: Your wife will leave and your children will kill themselves climbing the bottles.
  6. dougstove

    dougstove Member

    Joined:
    Aug 7, 2009
    Messages:
    221
    Loc:
    New Brunswick, Canada
    Thanks!
    I have the fan systems going already, bringing cold air from a lower level of the house. It works well.
    Kids are on side with drinking 2000 l of pop... wife is astonishingly tolerant ;)
  7. semipro

    semipro Minister of Fire

    Joined:
    Jan 12, 2009
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    2,346
    Loc:
    SW Virginia
    By far one of my favorite reference books though I think mine is only about 25 years old.
    Its amazing the stuff in there.
    A nice addition to any bathroom.

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