Hybrid Car – More Fun with Less Gas

Re: cheap water tank needed ! - Page 6

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Posted by iain-3 on March 2, 2005, 11:17 pm
Ah, 100 psi walls.  Okay.  The other problem, of course, is that we
enough turbulence to mitigate film insulation, but we have almost no
pressure head to drive that turbulence.  HX design looks tough.

But maybe I can figure out a way of hanging coils of this tubing in my
chimneys, so that the chimney flow is perpendicular to the tubing.
the loops from string with rock weights at the bottom?

Iain> Let's look at the 500K BTU/hr pool HX...

Nick> Where did that come from?

The heat store has one heat input (the panels) and five heat outputs
(interior hydronic, interior fan coils, DHW, pool, and outside patio).
I'd like to have HX for just the pool and DHW, and run the other loops
directly off the heat store water.  The other loops are all far more
sensitive to temp drops.

Nick> You mentioned 250K Btu/h. I said one 300' piece of 1" pipe would
Nick> do that with a 10 F temp diff.

It won't, and you didn't say it would.  You said, quite reasonably,
that one 300' piece of 1" pipe would move 23K BTU/hr with a 10 F diff.
Only after that did I divulge that I have 800 ft^2 of panels, and am
willing to tolerate just a 5 F difference.  Also, I expect quite a bit
more than 100 BTU/hr/ft^2 from those 67 degree Heliodyne panels.  All
of which makes an HX from the panels to tank unreasonable.

Basically, the system sizing is driven by the winter heat demands of
the house, and putting one or two HXs between the panels and the
radiant floor makes the system much too inefficient and therefore big
and expensive.  If the prospect of rocks ends up scaring me too much,
I'll back off to a much smaller HDPE tank.  But I don't see HX's being
cost effective, except in the high-temp-differential portions of the
system (pool and DHW).  Even the DHW case, I think the HX is going to
be a real bummer in January, when the tank temp dips into the 90s.

One intermediate point I've been tossing around is the idea of burying
a couple of 1500 gallon HDPE cisterns in a much larger insulated rock
and water tank.  This suffers the temperature limitations of the HDPE
cisterns, but potentially has the relatively cheap BTU storage of the
big rock and water tank, without the problems of water flowing over
rocks then through panels or radiant.

Two 1500 gallon tanks might have a total area of 300 ft^2.  If the
HDPE tanks were 10 BTU/hr-F-ft^2 (1/10 inch thick walls), and
convection took care of getting the heat to the rest of the rock-and-
water tank, the HDPE would need 8.3 degrees F delta to move 600K BTUs
in one day.  That's almost reasonable.  Panel collection efficiency
would go down though, since the tank temp would spike during
collection.  And no effective stratification in the tank.  I think
this idea (HXs you can climb into) is a loser, but I'll post it for

Anyone have an idea how thick the walls of these cisterns are, or
better yet what their R or U values are?

And I wonder what the R value of a steel tank would be?

Posted by iain-3 on March 3, 2005, 8:15 am

Here's a 1400 gallon XLPE cistern: