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Physics help please - heat storage - Page 3

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Posted by Ecnerwal on February 22, 2009, 2:48 am

Effectively the same as stone or concrete. Figure about R1 per inch,
perhaps R 0.25 or R 0.125 for a reasonable wall thickness (1/4-1/8
inch). If you are thinking a large vessel, the walls need to get thicker
to handle normal handling stresses (ie, a 20 gallon crock with 1/4"
walls is going to be VERY delicate, while a 1 quart vessel with 1/4 inch
walls is a bit on the chunky side).

Compared to metals, which conduct heat better and can be a LOT thinner
as well, "a fairly poor conductor of heat". Compared to a lot of other
materials (such as plastics), "a fairly good conductor of heat."

Firing in a solar kiln - that might be a much bigger challenge than you
realize. Firing ceramics involves temperatures where radiation loss is
rather huge, and also requires control of the rate of change of
temperature so that the ceramics don't crack from thermal stresses.

A solar kiln is a BIG windmill to tilt at.

Start with a solar oven that actually bakes good bread, as opposed to
sort of being able to cook a casserole, which is as far as a lot of them
ever get.

------ back to your problem and solutions -----

Depending, always, on what you want or how you plan to fit things in,
and how costs work out - one option would be canned water - either
"seltzer" or plain (I don't recall seeing water in cans "normally" but
at least one of the big beer companies cans water for sending into
disaster areas as a source of packaged clean water) - canned seltzer
water, bought on sale/in bulk might be as cheap as any method you could
build, with nice, conductive, thin sealed aluminum cans...

Then again, you could collect old glass beer bottles (how expensive that
is depends on whether you are in a deposit state or not), preferably
avoiding the "twist-off" type and get bottle caps and a capper from a
home brewing supply store, clean them out, fill them with water and cap
them. Reusing and taking out of the waste stream (few places can find an
economical market for recycling glass these days).

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

Posted by Morris Dovey on February 22, 2009, 7:01 am
Ecnerwal wrote:

I guess that shouldn't be too surprising - and I'd guess that the bulk
of ceramics thermal R&D has been directed toward producing less
conductive materials, rather than more. Still, I'll add it to my
ever-growing list of "Things I Should Learn More About".

The only extrusion processes I've had a glimpse of were for aluminum and
concrete. I would guess that there might be some interesting
possibilities in that direction: whether might it be possible to extrude
helical fins on either inside or outside of a circular tube, or whether
there might be advantages to using stacked hexagonal tubes...

I'm not even sure how the tubes might be used. Filling them with water
and letting warmed air heat them is a possibility  but so is using them
as passages for air through an earth-filled box...

Yes, yes - pretty much everything turns out that way. I forgot where the
saying comes from, but: "the impossible just takes a bit longer." I
don't really have a deadline to meet.

Absolutely. The only kilns I've ever seen were the ones my ex used in
school (electric, about a yard in diameter and a yard deep - and there
were some spectacular failures)

I'm finding that out as I move from a 4'-wide trough to an 8'-wide
trough for another project. Actually, I think the kiln might be a really
interesting project (famous last words, I know.)

'S ok - casseroles are good food. AFAIK, ovens come in two flavors: the
kind you bake your bread /on/, and the kind you bake your bread /in/.
The first solar convection ovens may be a bit spendy, but so were the
first electric convection ovens.

Good idea - I hadn't thought about that. The good folks at A-B sent us
canned water during the '93 floods. I wonder what could be done to
extend can longevity without significantly degrading conductivity...

Probably OK in a DIY context - but I'm trying to work toward general
solutions that can be mass-produced. Perhaps this might be the
application to consume that recyclable glass.

Thanks - you've given me a lot to think about. :)

Morris Dovey
DeSoto Solar
DeSoto, Iowa USA

Posted by Morris Dovey on February 21, 2009, 6:12 pm
 Ecnerwal wrote:

I'm still mulling over the original calculation, and just wrote a simple
C program to do the (revised) calculation:

#include <stdio.h>
int main(void)
{  double area = 30 * 40;           /* Area heated                   */
    double d_air = 0.0767;           /* Density of air at sea level   */
    double h_air = 0.24;             /* Specific heat of air          */
    double v_air = area * 10;        /* Volume of air (area x height) */
    double m_air = v_air * d_air;    /* Mass of air                   */
    double d_concrete = 149.8;       /* Density of concrete           */
    double h_concrete = 0.18;        /* Specific heat of concrete     */
    double m_concrete = m_air * h_air / h_concrete;
    double v_concrete = m_concrete / d_concrete;
    double depth = v_concrete / area;
    printf("Depth of concrete required is %f inch\n",depth * 12);
    return 0;

Which produced: "Depth of concrete required is 0.081923 inch"

Sticking with my static context, I think this is telling me that if the
floor were a 6" slab (which the example is) and if it were sitting on a
perfect thermal insulator (which the example is not), then if the entire
slab were raised to whatever we choose to call "room temperature", it
would contain more than 73 times as much heat as is needed to raise the
air from "really cold" to that temperature.

I think that to take this further would require abandoning the static
problem context and dealing with the imperfect nature of the insulation
under the slab and in the structure walls, ceiling, windows, etc - but
it's a place where I can "hang my hat" for the moment.

Morris Dovey
DeSoto Solar
DeSoto, Iowa USA

Posted by nicksanspam on February 22, 2009, 2:18 pm

In my experience, cloudy plastic milk jugs crack after a while. Clear
plastic soda bottles don't creack, but over a year, enough water vapor
diffuses through their walls to make them floppy containers. o

Lately I lean towards thinwall PVC pipes tucked between basement ceiling
joists and shiny high-temp ceiling mass upstairs.


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