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re: thermal mass
27 jun 1996
 wrote:
>nick@vu-vlsi.ee.vill.edu (nick pine) wrote:
>> mike dickson  wrote:
>> 
>> >i was wondering what was the best material to make a thermal mass out of,
>> 
>> water is good. about 3 times more heat capacity (62 btu/f-ft^3) than rocks
>> or concrete by volume, with less thermal and airflow resistance, in sealed
>> containers. iron or steel are almost as good, at 54 btu/f-ft^3, but more 
>> expensive...
>
>right. water.

it's nice we agree on that. 

>> >i plan to run pipes through it and heat with hot water from a
>> >solar collector. any ideas.
>> 
>> sounds complicated and expensive.
>
>balderdash!

i'll see your balderdash, and raise one balderdash :-) does $30/ft^2 seem
cheap to you, not counting roof mounting brackets and installation and
insulated pipes and tanks and antifreeze and heat exchangers and pumps
and the electrical power needed to run the pumps, if that square foot of
"solar collector" only collects the heat equivalent of a gallon or two
of oil per year, and that's all it does, vs a sunspace or a transparent
south roof or solar siding on a house?

>nick, we keep going over the same material. 

true. you should give up. or use numbers.

>air is a pitiful heat transfer medium.

true, compared to water.

>the only thing that is has is fluidity.

no, it also has heat capacity, and it doesn't freeze, which is nice, and
it requires no plumbing or heat exchangers, and sometimes no pumps or fans,
and leaks are not a serious business. it wasn't long ago that one of the
alternative energy experts around here said "air has no mass." :-)

>how umpteen many more times heat capacity has water got than air

about 4000 umpteen times, by volume, 1000 times less density and 1/4 the
specific heat by weight. is this a problem? can planes fly? should we
abandon them all for boats? 

>(the range of air's heat capacity that you're going to give me will
>relate directly to how much water vapor is in that air)?

wrong again. why don't you come up with that range? water vapor does
make a small difference. care to guess which direction? :-)

>water can be pumped thru 1/2" pipes that are easily insulated, taking up
>much less indoor (read valuable) real estate,

it may surprise you to learn that air may not need pipes, that one can move
air through whole rooms and floors and walls and ceilings sometimes, with
absolutely no loss of floorspace.

>how much more efficient a heat exchange would that be than blowing warm air
>around a room with 55 gal drums in it?

i give up. how much? you tell me. where are your numbers? why does this matter?

>add to that the longevity of copper pipe

do we also add the longevity of wood or drywall or fiberglass insulation 
or plastic 55 gallon drums? :-) 

>and the ability to take heat off from this storage tank with more
>copper pipe to feed dhw,

i'd use about 20' of fin-tube pipe for that, in a thermosyphon loop.

>or your spa, or your pool, or any one of your space heat zones without
>heating rooms above or adjacent to the storage room or the ducting

insulation... another matter of numbers. where are your numbers, dan?
  
>and the cost and complexity become minor.

perhaps, if you are rich as croesus, or enjoy complexity.
did you say you were in the copper piping business, dan? :-)
 
>>>i may just blow air over it instead of running water pipes through it
>>>if this turns out to be much easier or better.
 
>>good idea. using free 55 gallon plastic drums or recycled 5 gallon paint
>>pails, with lids? or $25 waterbed mattresses holding 300 gallons, or big
>>cheap circular swimming pools, or plywood boxes or trenches made with a
>>single 28 cent/ft^2 piece of 20' wide epdm rubber roofing material folded up
>>like a chinese restaurant takeout box?
 
>if, like most fluid based systems... the storage temps consistantly approach
>150f plastic drums, $25 waterbed mattresses, and cheap swimming pools are a
>disaster waiting to happen.

so are airplanes, especially lately. engineers specialize in disasters
waiting to happen. the better ones avert disasters for a very long time.

>your trench idea will work up to a point. the ground will become a heat
>sink and cool your storage off...

this is a matter of degree, of course. numbers again, and design. looked at
figure 6 on page 22.13 of your ashrae hof lately? it shows fine grain soils
eg clay (there's a lot of that where i live) have a thermal conductivity
that approaches 0 as they become very dry, in the region with less than 5%
moisture. at 8%, fig 6 shows their thermal conductivity as 5 btu-in/h-ft^2-f,
and the clear exponential curve extrapolates along a line to 0 from there,
so one might well expect that such a soil at 1% moisture would have a thermal
conductivity of 5/8 btu-in/h-ft^2-f, ie about r20 for a foot of soil. not bad,
i'd say. how do we keep the soil dry? put a vapor barrier all around it, with
a few air holes at the top, and something warm in the middle. i suppose they
do this with underground district heating pipes in sweden and siberia. 

>paint cans aren't even worth discussing.

ok. let's not discuss them. except, i might add, i just filled up a 5 gallon 
plastic paint pail with 150 f water and stood on top of it. even bounced a bit,
in the interest of science. perhaps you will try this with a plastic 55 gallon
drum. i wouldn't expect temps much over 130 f without a selective surface.

>i would suggest the largest electric water heaters that you can find, or unused
>oil tanks, or strong fiberglass tanks, or reinforced block walls forming a
>tank that then is lined with edpm, or recycled milk tanks (stainless, of
>200-300gal size, from farm supply), even steel 55 gal drums.

i would suggest that that will work, but it's a waste of money. perhaps a
plywood box with some 2x4's around it, lined with epdm rubber, as widely used
for off-peak electric heat storage.

thanks for making my morning, dan! this is like shooting fish in a barrel.

may you remain ever ignorant. 

and silent.

cheers,

nick



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