Physics help please - heat storage
Morris Dovey
of air-heating solar panels, and I'm trying to develop an reasonable
understanding - and to grasp some of the physics (thermodynamics?) involved.
Jeff's question elsethread prompted me to try to work up some numbers
and methods - and I'd appreciate some critical review of my logic.
I've been thinking of temperature as a measure of energy per unit mass,
so I knew I wanted to be able to determine the mass of the air in a 30'
x 40' x 10' structure. I did a couple of web searches and found:
density of air at sea level = 0.0767 lb/ft^3 [1.229 kg/m^3]
and I calculated
volume of air = 30 ft * 40 ft * 10 ft = 12000 ft^3 [339.8 m^3]
which allows me to figure
mass of air = 12000 ft^3 * 0.0767 lb/ft^3 = 920.4 lb [417.486 kg]
(Yes, I know that a pound isn't a mass unit - but bear with me or
validate with the proper metric units. Is there a name for a "pound"
when we want to use it as a unit of mass?)
I was wondering what mass of concrete it would take (at the same
temperature) to store however much heat there was in the 12000 ft^3 of
air, and I seemed that it would take the exact same mass.
mass of concrete = mass of air
My next question was to what depth would I need to warm the concrete
slab in order to store that much heat? I recognized that unless I
restricted myself to a non-dynamic context I'd get lost, so let me
stipulate a purely static context.
I googled again to come up with
mass of concrete = 149.8 lb/ft^3 [2400 kg/m^3]
so I could calculate
volume of concrete = 920.4 lb / 149.8 lb/ft^3
= 6.144 ft^3 [0.174 m^3]
Since the area of the floor is 1200 ft^2, if the heat were distributed
evenly over the entire floor, then the depth of the concrete needed to
store that heat would be
depth = 6.144 ft^3 /1200 ft^2
= 0.005120 ft = 0.06144 inch = 1.56 mm
At this point I leaned back in my chair and asked myself if the numbers
seemed reasonable, and had to confess that I just don't know - and it
seems like a really good time to get some knowledgeable help?
If I screwed up, please point out where and how...
--
Morris Dovey
DeSoto Solar
DeSoto, Iowa USA
http://www.iedu.com/DeSoto/
Morris Dovey
> I googled again to come up with
>
> mass of concrete = 149.8 lb/ft^3 [2400 kg/m^3]
Oops - should have been
density of concrete = 149.8 lb/ft^3 [2400 kg/m^3]
Ecnerwal
You need a property/number called specific heat.
In english (mostly american these days) the mass unit is lbm, or the
godawful slug, btw - lbm is more common.
A btu is the amount of heat to raise one lb water 1 degree F.
Air is easier to heat than water - its specific heat is 0.24, per my
notes as incorporated into my spreadsheet where I figure out how much
heat I need. I have a slightly different (not hugely) mass figure of
13.9 cubic feet of air per pound in my notes, so I'd have 863 lbs of air
for your 12000 cubic feet.
Thus, to heat 863 lbs of air one degree F takes only 207 btu.
863 lbs of water would take 863 btu.
Concrete, per a quick web search, seems to be a specific heat of 0.18
(be sure to get the "Btu specific heat" - one of the metric measures is
wildly different, one seems to be about the same)
863 lbs of concrete takes (or gives, if storing) 155 btu.
So, you need more mass of concrete than mass of air for the same heat
storage/release.
If in reasonable containers, water is a nice thermal storage medium,
physical size to number of btus stored. Concrete, not so much. 1970's
tech was heavy into concrete, and did not work well.
Using the floor adds other problems _ it can't get too hot, or the place
is uninhabitable. It may not be well insulated, so you may lose lots of
heat. To a large extent, you need to think about "heat to raise the
whole slab so many degrees" / heat given off by the whole slab cooling
so many degrees" rather than heating a thin layer of it - which is
connected to the rest of it...
One of the best approaches to hot air heat storage is the thermal
capacitor which Nick Pine used to go on an on about - it's another guys
design/patent (probably expired by now) - Norman ...?
Essential idea is to have a insulated high temperature heat store that
is insulated from the living space, and run that up to 150-180 degrees,
if enough heat is available. When heat is called for in the space, run
air through the store if the collectors are not producing heat. Put your
hot water heater in it and get solar hot water form it as well...
--
Cats, coffee, chocolate...vices to live by
daestrom
> and methods - and I'd appreciate some critical review of my logic.
>
> I've been thinking of temperature as a measure of energy per unit
> mass,
As long as your mass doesn't undergo a phase change (liquid->solid or
liquid->gas), this is a very good measure of the amount of heat stored in a
mass.
> so I knew I wanted to be able to determine the mass of the air in a
> 30' x 40' x 10' structure. I did a couple of web searches and found:
>
> density of air at sea level = 0.0767 lb/ft^3 [1.229 kg/m^3]
>
> and I calculated
>
> volume of air = 30 ft * 40 ft * 10 ft = 12000 ft^3 [339.8 m^3]
>
> which allows me to figure
>
> mass of air = 12000 ft^3 * 0.0767 lb/ft^3 = 920.4 lb [417.486 kg]
>
> (Yes, I know that a pound isn't a mass unit - but bear with me or
> validate with the proper metric units. Is there a name for a "pound"
> when we want to use it as a unit of mass?)
(Actually, in a lot of engineering, we use two 'pound' units. One is the
pound-mass (lbm) and the other is the pound-force (lbf). One pound-mass
resting on your scales in earth-normal gravity (32.2 ft/s^2), results in a
force of one pound-force. So Newton's equation is modified slightly to make
all the math work out F=m*A/g-subc where g-subc is a 'conversion factor'
between pound-force and pound-mass).
But that's besides the issue here :-)
>
> I was wondering what mass of concrete it would take (at the same
> temperature) to store however much heat there was in the 12000 ft^3 of
> air, and I seemed that it would take the exact same mass.
>
> mass of concrete = mass of air
Nope. Here you have missed one important point. The amount of heat it
takes to warm up one lbm of air by one degree is different than what it
takes to warm up one lbm of concrete by one degree.
Assuming you aren't containing the air in a fixed volume and causing the
pressure to rise as you heat it, the amount of heat it takes to warm one lbm
of air one degree F is about 0.24 BTU. The amount of heat to warm up a lbm
of concrete one degree is a bit less at about 0.18 BTU.
So to store the same amount of heat, you need
mass-of-concrete * 0.18 = mass-of-air * 0.24
mass-of-concreate = mass-of-air * 0.24 / 0.18
(the values 0.24 and 0.18 are referred to as 'specific heat capacity' for
the concrete and 'specific heat capacity at constant pressure' for air.
With gasses you have to specify whether it is at constant pressure or
constant volume).
>
> My next question was to what depth would I need to warm the concrete
> slab in order to store that much heat? I recognized that unless I
> restricted myself to a non-dynamic context I'd get lost, so let me
> stipulate a purely static context.
>
> I googled again to come up with
>
> mass of concrete = 149.8 lb/ft^3 [2400 kg/m^3]
>
> so I could calculate
>
> volume of concrete = 920.4 lb / 149.8 lb/ft^3
> = 6.144 ft^3 [0.174 m^3]
>
Adjusting that for the difference in specific heats, you would actually
need...
volume of concrete = 6.144 ft^3 * 0.24/0.18 = 8.19 ft^3
> Since the area of the floor is 1200 ft^2, if the heat were distributed
> evenly over the entire floor, then the depth of the concrete needed to
> store that heat would be
>
> depth = 6.144 ft^3 /1200 ft^2
> = 0.005120 ft = 0.06144 inch = 1.56 mm
>
> At this point I leaned back in my chair and asked myself if the
> numbers seemed reasonable, and had to confess that I just don't know
> - and it seems like a really good time to get some knowledgeable help?
>
> If I screwed up, please point out where and how...
Not off by much. This illustrates the fact that 'air' by itself is a lousy
way to store heat. If you had just 120 ft^3 of water (specific heat
capacity in these units for water is 1 BTU/lbm-F and density of about 62.2
lbm/ft^3) you could store...
heat-stored per degree F = 120 ft^3 * 62.2 lbm/ft^3 * 1 BTU/lbm-F = 7464 BTU
/ F
Compared with your volume of air (100 times larger)
heat-stored per degree F = 12000 ft^3 * 0.0767 lbm/ft^3 * 0.24 BTU/lbm-F =
220 BTU / F
It's the very low density of air that really kills you for heat storage.
Now, for more heat storage in a small space, look for phase-changing
systems. Some salts can absorb quite a bit of heat changing from solid to
liquid. Glauber's salt is supposed to absorb something like 99 BTU/lbm at
80 F as it changes from solid to liquid (and give it back up when it
'freezes' back to a solid). (But Glauber's salt has one drawback, it 'wears
out' and won't change from solid-liquid as easily the more you cycle it)
daestrom
Morris Dovey
> Morris,
>
> You need a property/number called specific heat.
>
> In english (mostly american these days) the mass unit is lbm, or the
> godawful slug, btw - lbm is more common.
Thank you - lack of vocabulary can be a problem for me. I'll note the
"slug" and /use/ "lbm" in the future. :)
> A btu is the amount of heat to raise one lb water 1 degree F.
Yuppers. I was kinda hoping to sidestep around energy units, but see
that a nod in that direction is necessary to ensure that I'm working
with only one "flavor" of specific heat.
> Air is easier to heat than water - its specific heat is 0.24, per my
> notes as incorporated into my spreadsheet where I figure out how much
> heat I need. I have a slightly different (not hugely) mass figure of
> 13.9 cubic feet of air per pound in my notes, so I'd have 863 lbs of air
> for your 12000 cubic feet.
Your number may be better - I just grabbed the first I found, and ran
with it.
> Thus, to heat 863 lbs of air one degree F takes only 207 btu.
>
> 863 lbs of water would take 863 btu.
>
> Concrete, per a quick web search, seems to be a specific heat of 0.18
> (be sure to get the "Btu specific heat" - one of the metric measures is
> wildly different, one seems to be about the same)
>
> 863 lbs of concrete takes (or gives, if storing) 155 btu.
>
> So, you need more mass of concrete than mass of air for the same heat
> storage/release.
Aha! Then I care about the /ratio/ of the materials' specific heat
values. That was absent from my calculation (but won't be in the
future!) Thank you!
> If in reasonable containers, water is a nice thermal storage medium,
> physical size to number of btus stored. Concrete, not so much. 1970's
> tech was heavy into concrete, and did not work well.
>
> Using the floor adds other problems _ it can't get too hot, or the place
> is uninhabitable. It may not be well insulated, so you may lose lots of
> heat. To a large extent, you need to think about "heat to raise the
> whole slab so many degrees" / heat given off by the whole slab cooling
> so many degrees" rather than heating a thin layer of it - which is
> connected to the rest of it...
Right. This is the dynamic aspect that I wanted to overlook until I got
a better handle on the static one. As a non-physicist I feel somewhat
constrained to "baby steps" (and I'm procrastinating on my DiffEq review
- it's been a /long/ time...)
> One of the best approaches to hot air heat storage is the thermal
> capacitor which Nick Pine used to go on an on about - it's another guys
> design/patent (probably expired by now) - Norman ...?
>
> Essential idea is to have a insulated high temperature heat store that
> is insulated from the living space, and run that up to 150-180 degrees,
> if enough heat is available. When heat is called for in the space, run
> air through the store if the collectors are not producing heat. Put your
> hot water heater in it and get solar hot water form it as well...
I've been working on a passive water-based storage system (much as you
described), but it's not ready for "prime time" yet - resources are
already stretched to the limit.
Nick and I seem to come at this stuff from opposite directions. He seems
to focus on calculation, with (perhaps) practical results down the road;
and I focus on practical results and end up wondering why things work so
well. If we could ever stand to work together we'd probably be
dangerous. :-D
Morris Dovey
>> I was wondering what mass of concrete it would take (at the same
>> temperature) to store however much heat there was in the 12000 ft^3 of
>> air, and I seemed that it would take the exact same mass.
>>
>> mass of concrete = mass of air
>
> Nope. Here you have missed one important point. The amount of heat it
> takes to warm up one lbm of air by one degree is different than what it
> takes to warm up one lbm of concrete by one degree.
I sure did - and don't plan to repeat that error. :)
> Assuming you aren't containing the air in a fixed volume and causing the
> pressure to rise as you heat it, the amount of heat it takes to warm one
> lbm of air one degree F is about 0.24 BTU. The amount of heat to warm
> up a lbm of concrete one degree is a bit less at about 0.18 BTU.
>
> So to store the same amount of heat, you need
>
> mass-of-concrete * 0.18 = mass-of-air * 0.24
>
> mass-of-concreate = mass-of-air * 0.24 / 0.18
>
> (the values 0.24 and 0.18 are referred to as 'specific heat capacity'
> for the concrete and 'specific heat capacity at constant pressure' for
> air. With gasses you have to specify whether it is at constant pressure
> or constant volume).
Hmm - ok - I'll proceed under the assumption that the structures I'll be
heating will be leaky enough to be at constant pressure.
>> My next question was to what depth would I need to warm the concrete
>> slab in order to store that much heat? I recognized that unless I
>> restricted myself to a non-dynamic context I'd get lost, so let me
>> stipulate a purely static context.
>>
>> I googled again to come up with
>>
>> density of concrete = 149.8 lb/ft^3 [2400 kg/m^3]
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.
> Now, for more heat storage in a small space, look for phase-changing
> systems. Some salts can absorb quite a bit of heat changing from solid
> to liquid. Glauber's salt is supposed to absorb something like 99
> BTU/lbm at 80 F as it changes from solid to liquid (and give it back up
> when it 'freezes' back to a solid). (But Glauber's salt has one
> drawback, it 'wears out' and won't change from solid-liquid as easily
> the more you cycle it)
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.
Ecnerwal
Morris Dovey <mrd...@iedu.com> wrote:
> 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.
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.
Morris Dovey
> 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.
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.
> 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.
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.
daestrom
> Ecnerwal wrote:
>
<snip>
>> 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.
>
> 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.
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.
daestrom
Morris Dovey
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
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.
David Williams
-> 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
If only because they speak English, I would try enquiring from one of
the famous pottery companies in England - Wedgwood, etc.. Try googling
under "Stoke on Trent".
dow
Ecnerwal
Morris Dovey <mrd...@iedu.com> wrote:
> Do you have any handle on the heat transfer characteristics of porcelain
> or clay?
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).
Morris Dovey
> In article <49a00cb0$0$89872$815e...@news.qwest.net>,
> Morris Dovey <mrd...@iedu.com> wrote:
>
>> Do you have any handle on the heat transfer characteristics of porcelain
>> or clay?
>
> 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."
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...
> Firing in a solar kiln - that might be a much bigger challenge than you
> realize.
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.
> 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.
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)
> A solar kiln is a BIG windmill to tilt at.
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.)
> 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.
'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.
> ------ 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...
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...
> 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).
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
Yuppers - I knew as soon as I hit send that my list was woefully
incomplete. I didn't mean to slight anyone. Even NASA, ESA, and CERN may
have a crumb or two in their pantries. :)
nicks...@ece.villanova.edu
>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.
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.
Nick
David Williams
-> 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).
Here in Ontario, Canada, beer bottles are reused. Most bottles go
around the system more than 20 times before they are broken or
otherwise lost. There's a deposit of 10 cents a bottle to encourage
people to return them for reuse.
dow
Nelson
"David Williams" <david.w...@bayman.org> wrote in message
news:1235321885.9...@bayman.org...
Guys, I'm following this discussion more or less as an interested
lurker... but how practical would it be to use a large container full of
broken glass as a storage medium, and pass air through that both to
"charge" and recover the heat? Probably cheaper even than clean gravel.
Nelson
Robert Scott
>Guys, I'm following this discussion more or less as an interested
>lurker... but how practical would it be to use a large container full of
>broken glass as a storage medium, and pass air through that both to
>"charge" and recover the heat? Probably cheaper even than clean gravel.
The specific heat of glass is about 20% that of water. So for the same weight
of material, water will hold 5 times as much heat. But then consider that glass
is about 2.6 time as dense as water. So for a given volume, water will hold not
5 times, but 1.92 times as much heat as water.
I know you were asking about the comparison to clean gravel, but I don't have
those stats handy. But one thing that all these types of heat storage
technologies have in common is that they take a lot of energy to move the air
through them. You can easily make your system a net waster of energy if you are
not careful to turn the system off when the delta temperature falls below a
certain level.
Robert Scott
Ypsilanti, Michigan