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re: long term heat storage for unusual and inexpensive houses
15 oct 1996
nick pine  wrote:
 
>how about building a house on top of a strawbale pond foundation, inside a
>$1/ft^2 commercial plastic film greenhouse? the ponds might be 2 bales deep,
>about 32" tall, inside a standard 30' wide greenhouse like this: 
>
>     ---                  .
> sun               .             .           
>               .                     .          
>     15'     .                         .     
>           .                             .            [64 feet long]
>          .                               .   
>         .                                 . ----
>        .b      water     b     water      b. 32" 
>     ....b................b................b.................
>        |                30'                |
 
this would collect about 30% more sun in december, where i live, if it
had a frozen reflecting pool outdoors, to the south.

>as to thermal performance, strawbales are about r50, so we have (roughly)
>rc = r50/2000 ft^2x32"/12x2x13x62x64 = 6877 hours, or 287 days or 41 weeks
>or 9.4 months. not quite passive annual heat storage, but this stucture has
>a lot of glazing, and it could have a ceiling fan blowing some warm air down
>under the length of the floor.

as another alternative, the house might have a floating floor.

some ferrocement barges, perhaps, or some 8' x 16' platforms, bolted together
and wrapped below with a single folded piece of epdm rubber that comes up to
the top of their 1' sides, with strawbales for insulation, and no air gap
underneath, and solar heat moving into the water via some some air-water heat
exchangers near the top of the greenhouse. if the strawbales had a woodframe
or ties to hold them together, and some chicken wire and concrete on top, the
"platform" below might just be an epdm rubber bag. 

the heat exchangers could be some fin-tube pipe with a thermal conductivity
of about 5 btu/hr-f per linear foot, or some used auto radiators with their
electric fans, which might lose the heat equivalent of 1 gallon of gas per
hour, idling on a hot day at 100k btu/hr/(200f-100f) = 1,000 btu/hr-f (?),
or some fan-coil units like magic-aire's $150 all-copper 2' x 2' shw 2347
duct heat exchangers, bolted onto window fans, transferring 45k btu/hour
between 125 f water and 68 f air, ie about 800 btu/hr-f at 1400 cfm, with
a 0.1" h20 pressure drop.

how efficient would this be, as a solar collector? the solar heat, about
300 btu/ft^2/hr, max, wants to leave to the colder outside world via the
glazing thermal resistance, which could be r1/960ft^2 in this case, but it
also wants to flow from warmer air into cooler water via the air-water heat
exchangers, so we have something like this thermal circuit, for a square foot
of glazing, where i live, in december, with an average daytime high of 43 f: 

                ------------               raw
   43 f--------| 300 btu/hr |->------------www----->---------tw, water temp
           |    ------------    |  heat exchanger resistance
           |         r1         |
            ----<---www------------ta, air temp near ceiling
	     glazing resistance

in a circuit like this, current divides according to the conductances.
so if raw = 1/3, say, and tw = 43 f, 200 btu/hr would go into the water
and 100 btu/hr would go back out through the glazing. if the water were
warmer or raw higher, less heat would flow into the water.

if the water temp were 100 f, and raw = 1/8, eg 1 auto radiator for each
8'x16' section of glazing, we would have

                ------------    a          1/8
   43 f--------| 300 btu/hr |->------------www----->---------tw = 80 f
           |    ------------    |  heat exchanger resistance
           |         r1         |
            ----<---www------------ta, air temp near ceiling
	     glazing resistance

so 300 btu/hr go into point a, of which (ta-100)/(1/8) flow into the water,
and (ta-43)/1 flow into the air. so 300 = 8(ta-100) + ta - 43, or 
257 = 8 ta - 800, or ta = 1057/8 = 132 f, and about (132-100)/(1/8) = 256
btu/hr flow into the water, so the efficiency is 256/300 = 85%. using 2 layers
of plastic would reduce the sun by 10% or so, but allow conventional air
inflation of poly film pillows to reduce plastic wind fatigue. it would raise
the efficiency, but double the glazing cost, to 10 cents per square foot.

nick



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