Re: Storing the summer heat
From: bw (bwegher_at_hotmail.com)
Date: 11/21/04
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Date: Sun, 21 Nov 2004 02:02:42 -0600
"The Ghost In The Machine" <ewill@sirius.athghost7038suus.net> wrote in
message news:v5f572-72u.ln1@sirius.athghost7038suus.net...
> In sci.physics, habshi
> <habshi@anony.com>
> wrote
> on Sun, 21 Nov 2004 01:42:03 GMT
> <419ff198.1014588@news.clara.net>:
>> What is wrong with the idea of storing summer heat
>> underground?
>
> Nothing. How long and how much?
>
> A good coffee thermos might store a liter or so of coffee, initially
> at near-boiling (100C), for 2 days and it might be still warm (40 C).
> These are admittedly estimates, but the heat leakage of said thermos
> is exponential; therefore plug into the equation
>
> energyLeft = energyStart * exp(-time * constant)
>
> bearing in mind that energyLeft is relative to room temperature (about
> 25C); we can estimate a liter of coffee at 100C to have, therefore,
> 75 Cal or 313,500 J and 40C coffee to have 15 Cal or 62,700 J,
> or
>
> 15 = 75 * exp(-172800 * K)
>
> or K = (log(75) -log(15)) / 172800 = 9.31387 * 10^-6.
> Call it 10^-5.
>
> There are a few issues with K; one obvious one is that
> it's surface-area dependent, all other things being the same
> (which they probably aren't). We'll assume a cubical coffee
> urn (0.1m x 0.1m x 0.1m) for simplicity; the walls would have
> a total area of 0.06 m^2.
>
> So let's assume SimplePlanet has 180 days hot weather (30 C),
> and 180 days freezing cold weather (-10C). Dig an underground
> thermos vault 10m x 10m x 10m and fill it with 1,000 metric tonnes
> of 30C water. Total usable heat storage capacity would be 10^10 cal
> or 41.80 teraJoules, since we want 20 degrees C water at least
> to come out.
>
> The walls here would be 600 m^2 total. This means K for this vessel
> would be on the order of 10^-5 * 10000 / 10^9 = 10^-10, and
> assuming the Simpletons didn't touch this nice heat vesicle,
> the temperature of the water just before summer starts again
> would be
>
> -10 + 40 * exp(-180 * 86400 * 10^-10) = 29.938
>
> Not bad. However, there are a few other problems, the most obvious
> one being the conveyance of the heat to the housing units; the
> piping adds surface area. One also has to worry about ensuring
> the water doesn't get clogged with algae and such.
>
> http://www.gsenet.org/library/07eng/insulate.php
>
> suggests a computation for the heat loss through a house but the
> units are slightly clumsy; one might be able to convert it thus:
>
> joules lost = area / (Rfactor * 0.17611) * time * deltatemp
>
> where 0.17611 has the units sec-m^2-K/J, note that we're
> assuming *constant* temperature here. If a block house has
> 6 R19 walls, 4 of 10m x 4m and 2 of 10m x 10m (this gives one
> 100 m^2 or 1076 ft^2 of living space, more than enough for about
> 3 bedrooms), one gets a total area of 360 m^2. Into that area we
> might cut two R2 doors (2m x 1m) and four R2 windows (1m x 1m).
> Assuming 20C internal temperature and -10C outside temperature,
> the heat lossage through the walls, ceiling, and floor would
> be 352 * 30 / (19 * 0.17611) = 3156 W for the walls, and
> 8 * 30 / (2 * 0.17611) = 681 W for the door and windows --
> and that's assuming the Simplekids don't leave a door or window open.
>
> Total power consumption: 3837 W. Over the SimpleWinter that
> translates into 59.67 gigaJoules. (This might be partially
> offset by the Simplekids themselves; a human body radiates
> about 125 W or so at most of heat. However, that would
> require quite a few kids...)
Simple rule of thumb for adult male metabolism over a day is about 100
watts. Skeletal muscle eff. is about 33 percent. Basal metabolism (sleeping)
is actually around 80 watts. Females about 70. Teenagers actually have
higher specific metabolism, but size is less than adult, so average is 100
watts.
> An interesting idea, and I can now see why some buildings have
> "heat reservoirs". However, one would have to manage the heat
> flow carefully; one false move and the house gets cold... :-)
>
> Also, Earth's winters tend to be more complex. A very cold
> cold snap or a pipe breakage might make things interesting.
>
>> Put it into a container with a high energy capacity but which is
>> perfectly insulated with something like space shutel tiles .
>> In the winter we can get energy out with a heat pump.
>> A heat pump will give four units of energy for each
>> electricity energy unit used and the same when getting it out ie
>> getting eight units per unit used .
>
> A theoretical heat pump's efficiency is highly dependent on the
> temperature between source and sink, and still requires energy
> to actually pump the heat.
>
> --
> #191, ewill3@earthlink.net
> It's still legal to go .sigless.
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