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re: hair brained solar cistern???
13 dec 2002
alan stankevitz   wrote:

>i'm really flattered that my house has taken up so much of your time 
>here on the newsgroup.

a lot of us have nothing better to do :-)

>nick, my r-value calculations are probably off...

we seem to have figured that out now. better data, and adding conductances
vs resistances for resistors in parallel... finding out that wood is 2-3x
more conductive for heatflow parallel to the grain reinforces my hunch that
cordwood walls are not great insulators, even with no thru log-ties, even
if the logs don't shrink or rot or absorb moisture much.

>the walls have yet to be completed. they do not contain sawdust between
>two walls. the second, inner wall has yet to be built. so far there is
>one exterior wall comprised of 8" cedar and red pine logs and mortar
>that contains 30% newspaper...

and a woodstove.

>...the rubble trench foundation was used to keep my costs low and
>reduce the quantity of concrete required for a foundation...

sounds good to me.

>and regarding the statement of not having any control over the system...

there are at least two control issues: uncontrolled heatflow out of the
floor, which can mean opening windows and wasting heat on warm days, and
possibly dumping heat from the floor to the outdoors via collectors,
without a differential thermostat to turn on the collector pump. (i like
low-temp inexpensive efficient trickle collectors combined with low-temp
heat distribution, with a separate higher-temp cloudy-day heat store.)

>i wanted to keep the system simple. it runs itself and the pumps are
>properly balanced with pv panels to prevent the pumps from running
>when solar conditions are less than ideal.

how can you do that, exactly? seems to me your system might heat the sand
up to 200 f on a warm sunny tuesday, then pump 200 f water into collectors
and 120 f water back into the sand on a cold sunny wednesday. 

>yes, the system is less than perfect. criticize it to your hearts content...

a lot depends on our initial expectations. when toby (not you) claimed your
sand bed could keep your house 70 f on a -4 f day, based on his crackpot
heat velocity theories, my bullshit detector went off. with moist sand (at
least 6%, i'd say, altho it can hold about 40%--when does it become too
soupy to hold up a slab?), and milder days, this seems more credible, altho
we have concerns about the max water temp required for charging the bed
and the pipe-to-sand resistance. 

we might be looking at something like this model now:

                     t1        t2        t3
    0.00068   0.0011 |  0.0011 |  0.0011 | 0.00083
tw ---www------www---*---www---*---www---*---www--- 70 f
                     |         |         |       
                    --- 7200  --- 7200  --- 7200    i btu/h --->
                    ---       ---       ---
                     |         |         |
                     -         -         -

ignoring the sand resistance close to the pipe, the wall conductance
might be 10 btu/h-f-ft^2... 750' of 3/4" pipe with 147 ft^2 of surface
might have a 1470 btu/h-f conductance or a 0.000608 conductance...
8xr0.083/600 = 0.0011, 20x600 = 7200, and 0.5/600 = 0.00083.   

we might simplify it this way...

    0.00171 | 0.00303 
tw ---www---*---www--- 70 f  i btu/h --->
      -->   |        
      ip   --- 
if i btu/h flows into house air, on average, t = 70+0.00303i. with 200 f
water for 6 hours, the peak current ip = (200-t)/0.00171 = 4i, with a 25% 
duty cycle, so i = 13.2 k btu/h, t3 = 80.9, t2 = 95.4, and t1 = 109.9 f. 

it looks like the house needs ventilation with outdoor air when the outdoor
temp is greater than 70-13.2k/278 = 22.5 f. if the house has to stay 70 f
with lower outdoor temps and no other form of heat, we might say the sand
stores no useful heat, since the house temp drops below 70 as soon as the
sand starts to cool off. 

if we allow i to decrease to 0 as you fire up your woodstove over a few
cloudy days, we might say the top sand layer stores (80.9-70)7200 = 78.4k
of heat, the middle stores (95.4-70)7200 = 182.9k, and the bottom stores 
(109.9-70)7200 = 287.3k (with infinite insulation below), a total of 548.6k
btu, the equivalent of 548.6k/25k = 22 hours of house heat at 25k btu/h. 

meanwhile, toby continues to simulate heat gain over months.

may he learn common sense.


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