Solar Space Heating Storage Tank Size [RE-wrenches]

[Travis Creswell, Ozark Solar]

Hello All,

I am designing a storage tank (atmospheric) for the radiant in floor heating
system for my new shop.  The office portion the shop will be constructed of
Insulated Concrete Forms (IFC's) with a concrete roof.  This will allow it
to be also be used as storm shelter.  Three tornados missed us by less than
5 miles last spring.

These same forms will make a great tank as they are plenty strong and well
insulated.  One wall of the storage tank can be shared with the office
reducing the number of forms needed for the tank and the concrete pump trunk
will already be on site for the office walls.  I've had an engineer look it
over and he has given it his blessing to build a tank up to 8' tall!

Of course I will have to line the tank, which is easy enough with EPDM.  A
reasonably tight insulated lid will need to be constructed as well.

I'll have 400 ft2 of liquid flat plates and a very efficient wood
gasification boiler rated at 140,000 Btu's output.  Does 600 Btu's/ft2/per
day sound right for the solar thermal array?  I'm thinking that is safe as
here in Southwest Missouri our average winter daytime temps are reasonably
mild.

My main questions are the size and shape of the tank.  I can build the tank
in any size or shape (tall and skinny vs. short and wide for example).  In
looking over some of my dated solar books, 2 gallons per sq ft seems to be
the maximum ratio of thermal collector to storage.  I assume that "rule of
thumb" was long before the widespread use of radiant in-floor heating.
Since radiant in-floor requires lower temps than baseboards or forced air is
it acceptable (or required) to use a larger storage tank?

I've built a very detailed spreadsheet that allows me to see how long it
will take to "recharge" various sizes of tanks with the solar thermal array
and the wood boiler.  The most efficient operation of the boiler is to be
fill it with wood then run wide open until the wood is gone.  If the tank is
not large enough then the boiler will shut down and smolder thus lowering
the efficiency of the burn cycle.  The spreadsheet shows me that one firing
of the boiler will easily charge up to a 1500 gallon tank.  But this size of
tank amounts to nearly 4 ft2 of thermal per gallon of storage.  One idea is
to build a tall skinny tank.  I could place the solar heat exchanger higher
in the tank so it has less water to heat.  This is assuming that
stratification will keep all that heated water up near the top.  Can anyone
confirm if this is a reasonable expectation?  The space heating tanks that
I've seen are very short but that might have been for structural reasons.

Does anyone have a source for the finned copper coils that were commonly
used in these systems?  I'd rather use them than make my own coils.

Any thought and suggestions are appreciated.  I'd be glad to share the
spreadsheet and the system diagram should anyone want it.

Thanks in advance!

Travis Creswell
Ozark Energy Services, Inc.

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