This would be an actual, rather than an ideal, thermal store - Neither
sphere, nor cylinder, nor cube - an old bulk milk tank which is roughly
10 feet long, 4 feet wide, and 2.5 feet deep for a net of 750-800
gallons (was claimed as 800, and I have not measured the interior more
precisely to account for the V in the bottom to see if it could really
be 800, or not - haven't spotted a size label on it yet.)
Running some planning numbers on it, guesstimate 6400 lbs water
(accounting for expansion if it's hot and assuming it's 800 gallons
capacity) toying with swinging it between 125F and 175 or 195F (300K BTU
to 447K BTU useful swing, which certainly makes the higher temperature
more attractive by 147K BTU if I can do it.) With radiant floor heat,
there's also some cushion there below 125F that could still be of use,
but 125 seems like a good lower bound for planning.
Crude pessimistic numbers say if I can insulate it to R49, it would only
lose 14-16K BTU per day (-20F day, I said it was pessimistic) just
sitting there (not accounting for losses out pipes). One question is
"insulate with what?" Various typical household insulations have issues
at temperatures much above typical household temperatures - which ones
reliably work well essentially at boiling (212F/100C)? The tank has 2"
thick walls which appear to be insulated with rock wool - I'm basically
planning to put it in a box with a lot more insulation and pretty much
ignore the walls when it comes to estimating overall insulation - a
certain degree of such pessimism overcomes any inadvertent optimism
elsewhere in making projects work more-or-less as designed.
One plausible approach in a 7000 heating degree day climate is to put
the bulk store inside the building being heated, so its waste heat is
not wasted, but I'm not planning to do that in this case because of
space required and also because summer still calls for hot water uses,
and I don't want the thing bleeding heat into the house at that point.
It's going in a shed with insulated pipes, per the current plan. That
also makes it easier to have the cellulose-stored-solar backup
(wood-fired water heater - a clean one, not a smoldering pollution
creator) not in the primary building for fire-safety paranoia reasons -
said shed is leaning toward concrete construction for the same reasons
right now. There will be solar inputs (long-term storage is needed to
make those effective here) and also contributions from generators
(cooling system hooked in) during power outages.
Exclusive of domestic hot water use, heating loads are 30K btu/hr worst
case and 8.5K BTU/hr "on average". A larger store would be nice,
obviously, but will require more tankage, which is not always easy to
come by both cheaply and in a reliable form that will take high
Cats, coffee, chocolate...vices to live by
On Mon, 06 Dec 2010 23:49:13 -0500
You might use foamglass under the tank, with something to spread
the load (a couple of sheets of 1" ply perhaps) it will take the weight of
the tank (IIRC the compressive strength is something like 90PSI) so you
won't need to compromise the insulation with supports. Then fill the rest
of the box with rockwool.
Steve O'Hara-Smith | Directable Mirror Arrays
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If you are going to build a shed, why not build it as an addition to the house
with a well insulated wall/door between it and the house? Keep the door open in
the winter, closed in the summer and you can have a seperate DHW system. Seems
like it would be easier to insulate the shed properly than the tank.