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re: heat transfer in building material question
9 nov 2001
jerry ellinghuysen  wrote:

>hello group.

hi jerry,

>it appears that the most desirable structure, material-wise, is a low- (or
>controlled-) infiltration, highly-insulated material combined with high
>massed, high-density interior... sips seem like a great concept, but lack
>a high-mass contribution.

that might come from 2 layers of 1/2" drywall (1 btu/f-ft^2) easily applied
to the inside before assembly, or an insulated hydronic or plain slab, or
a few 12" vertical pvc pipes full of water, or a single block wall or a
room divider. "high mass" in living space stores less heat than mass with
a larger temp swing. and you need less mass, with lots of insulation.

>also, i live in colorado and want to build in what might be forest-fire
>country, so any additional protective contribution needs to be considered.

hardiplank siding ($4.58 for a 6.25"x12' lap board (1.5" dissappears
in the lap) or $24.81 for a 4x8' stucco board)? gunite? build a moat?
clear the brush for 100' and set up a wall/roof fire sprinkler system? 

>as colorado has an almost ideal mean temperature but can have some
>pretty extreme temperature swings, the loss of heat through a heat
>transfer mechanism is a concern.

sure. january looks like the worst-case month in denver. nrel says
1450 btu/ft^2 of sun (360 diffuse) falls on a south wall on an average
28.8 f day. you might screw together an $8/ft^2 48x72' house with 28
$1k 8'x24'x12" r48 sips with 288 ft^2 of r4 windows and 288ft^2/r4 =
72 btu/h-f of window thermal conductance plus 1632/48 = 34 for walls
plus 3456/48 = 72 for the ceiling, with 0.1 air changes per hour, ie
69 cfm of ventilation, for a total conductance of about 250 btu/h-f. 

the house would need 24(65-28.8)250 = 217k btu on an average january
day. consuming 600 kwh/mo of internal electrical energy could provide
72k/day of that, leaving 145k, or 726k for 5 cloudy days in a row. nrel
says east and west and north walls get 580, 580, and 200 btu/ft^2-day,
so 144 ft^2 of s windows and 64 ft^2 e and w and 16 ft^2 of n windows
with 50% transmission would gather 0.5(144x1450+2x64x580+16x200) = 143k
btu/day, which takes care of the house on an average day.  

you might make hot water in a sip shoebox on the roof for showers and
cloudy days. you could make a 4:1 concentrating parabolic trough reflector
with 80% reflectance by greasing mylar to kerfed foil-faced foamboard,
with a single layer of polycarbonate for a south wall. with a polycarb
cover over the shoebox collector, you could gather (1450-360)0.8x0.9
= 795 btu/ft^2-day of solar aperture and lose about 6h(180-30)0.25/r1 =
225 btu/ft^2 of shoebox, a net gain of 570 btu/ft^2 of aperture, so
you need about 50k/570 = 87 ft^2 of aperture or 22 ft^2 of shoebox
collector to heat 50k btu/day of water for showers.

you might store 726k btu for cloudy days in 900 gallons or 112 ft^3 of
water cooling from 180 80 f. the shoebox might be 16' long x 4' wide
x 2' deep, under a 16'x12' x12' tall solar trough with a 4' focus and
reflective endwalls.

nick




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