Re: SB: Re: SB appropriate thermal mass-heat storage

[Sherwood Botsford <sbotsford@sjsa.ab.ca>]

On Monday 23 September 2002 10:02 am, Chuck & Linda wrote:
: Greetings again,
:
: I recently just mentioned the deep heat storage system we are
: putting in place. I am building this in the next few days and
: I have a burning question that I am hoping some of you
: engineer types could sound off on.
:
: The solar designer has always used sand inside the heat
: box/floor. Well to fill this two feet deep will cost about
: $1800 in sand or rock. Meanwhile I am sitting with huge mounds
: of loam and wondering about incorporating the loam. Again the
: idea is to use part of Aug and Sept and October to charge up
: the floor and then slowly release the heat over the cold
: season. I am wondering about the thermal performance issues of
: loam versus sand. The solar designer has never done anything
: but.... I thought about using the loam in the first foot at
: the bottom where the tubes are buried and sand or rock up top.
: I don't want to do it if the performance would drop, but I am
: wondering if there would be any drop in performance?
:


Two issues come up:

1.  To what extend is heat transfer through the mass dependent 
on slow convection through the air spaces?  Loam would have 
smaller air spaces than sand.

2.  What sort of total volume are you using?  Are you serious 
about trying to store heat on an annual basis rather than a 
daily basis?  Specific heat of rock/sand is quite low.  Runs 
about 1/3 of the heat capacity of water on a volume basis. 
(About 1/8 on a weight basis.)  A cubic meter of water stores
1000 KCal/degree C = 4000KJ/degree C

If this is the floor of the building it will not store enough 
heat from day to day to make a seasonal difference, unless you 
are in a climate that hardly needs heat.

Domestic hot water runs a few gigajoules/month  A gigajoule 
would be 10 cubic meters of water changing temperature by 25
degrees C.    You have to tap the heat of a very large volume fo
rock or earth.

If this fill is being done in trenches spread tthrough the land 
then it may be feasible, although you will do better to have two 
separate systems if you want to do both heating and cooling.  
(In the summer hot air is removed from the attic to heat the 
heat sponge side.  Meanwhile, you draw air in from the cool side
to cool the place down.  In the winter you cool off one side, 
and draw heat from the summer side.)  Bear in mind that this is 
not very well insulated, so there is a fair amount of loss.  If 
the heat sink is big enough, you don't care.

In this case, sand vs loam should make no difference except to 
keep the piping stable.
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-- 
Sherwood Botsford				1-780-848-2881 Ext 134
St. John's School of Alberta			RR - 5 Stony Plain, Alberta TZ7 
1X5
Alternate address sgbotsford@yahoo . com

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