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Physics help please - heat storage - Page 2

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Posted by Morris Dovey on February 20, 2009, 1:14 am
daestrom wrote:

I sure did - and don't plan to repeat that error. :)

Hmm - ok - I'll proceed under the assumption that the structures I'll be
heating will be leaky enough to be at constant pressure.

Understood - air /does/ have the advantages of low cost and that it
doesn't require space-consuming storage vessels. In the sample scenario,
the concrete was also conveniently present and did not "eat into" the
usable space. While I had expected that a third 8x6 panel would be
needed, it was not - and the ceiling fan and concrete slab approach
appears to provide more effective storage than I had expected - which,
along with Jeff's post, motivated /this/ exercise.

I did look at a couple of phase change storage possibilities (quite a
while back), but it appeared that the cost trade-offs favor more panel
area with less efficient storage.

I'm still looking at water for passive heat storage; but I have a hard
time working up much enthusiasm for anything that requires significant
maintenance, recurring expense, and/or introduction of additional
failure modes. Don't want much do I? :)

Thanks for your (valuable) input. You and Lawrence have helped a lot.

Morris Dovey
DeSoto Solar
DeSoto, Iowa USA

Posted by Ecnerwal on February 20, 2009, 3:07 am

You're quite welcome. In the episodes between rounds of spam, we
occasionally recall what newsgroups used to be like, and it was good ;-)

Plus, I for one find that the fairly basic, and really not needing much
in the way of Diff Eqs heating data/calculations are not out in the open
enough for my taste. I got them while taking an Agricultural Engineering
course at college, and much that had been opaque became clearer. Folks
that have trouble balancing their checkbooks may run screaming from it,
but it's pretty much high school math for the most part, and knowing
what some of these obscure units or numbers are/mean. When it's all
cranked through to come out in $/year, it makes planning a heck of a lot
easier, and seeing what makes a difference easier as well.

The thing to do with simple water storage is to select sealed containers
with a long lifespan, and not pump the water around - just let it sit
there and have air moved over it. No maintenance, little if any
recurring expense, and one failure mode (leakage of container) which
would lead to a recurring expense (new containers, preferably getting
the "long lifespan" right this time). IIRC, Nick thought that plastic
soda bottles were going to be good (lots of surface area to transfer
heat in and out) but found out after a few years that the thin plastic
gave up after a few years use.

Cats, coffee, chocolate...vices to live by

Posted by Morris Dovey on February 20, 2009, 4:28 am
 Ecnerwal wrote:

I think you're right. I do like to play the "what if" game so as to be
able to (more or less) confidently answer questions for folks who're
thinking about solar heating. Most of them don't want to hear a lot of
geekish stuff - but they do expect me to speak with some reasonable
degree of confidence, and they definitely want what I tell 'em to be
absolutely true and reasonably accurate. This exercise prepares me to
provide them with the much more general (less technical) answers they want.

We're definitely on the same page here. My notion of an acceptable
container material is stainless steel - and I'm of the opinion that the
system should (at a minimum) outlast the customer.

Morris Dovey
DeSoto Solar
DeSoto, Iowa USA

Posted by daestrom on February 21, 2009, 1:43 am
 Morris Dovey wrote:


ISTR reading about someone using PVC piping with suitable end caps.
Schedule 40 PVC pipe is readily available, thicker walls than soda bottles
though, so need lots of surface area to compensate.

Steel or copper is better at heat transfer, but $$$.

Some simple baffles around/between the pipes to force the air to criss-cross
around the pipes as often as possible would really help a lot.


Posted by Morris Dovey on February 21, 2009, 2:18 pm
 daestrom wrote:

I like stainless steel, but agree that any kind of metal is expensive,
but understand that PCV and plastics in general are petroleum-derived -
do you suppose that some type of clay/ceramic tubes could be used instead?

I'm visualizing a process where (capped) extruded clay tubes are being
fired in tubular kilns at the focus of trough-type solar concentrators.

Do you have any handle on the heat transfer characteristics of porcelain
or clay? (I'm pretty much clueless, but I'd be willing to bet there's a
wealth of knowledge available somewhere in Japan or China...)

Morris Dovey
DeSoto Solar
DeSoto, Iowa USA

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