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a sacramento shoebox
3 nov 2001
j enquires: 
 
>...how many feet of solar closet would be necessary to heat
>1200 sq.ft. of living space.

that depends on the shape and amount of insulation and windows,
and you mean "sunspace," vs solar closet.

>dec. -45.3 ave.temp.,  820 btu/^2  330 difuse ?

yes, nrel's numbers for sacramento, with a 52.7 f average daily max.
the diffuse part isn't important for a sunspace. it is for
a concentrating greenhouse or solar attic.

>20x60x8' building- south wall 60x8'.  some room for light entry and door.

i'm not sure what you mean by "room for light entry and door." windows?
a vestibule? why not make it a simple shoebox? or maybe closer to square.
that loses less heat and is cheaper to build, for the same floorspace. 

you might make a 24x48x8' building with 12 8'x24'x12" sips and 96 ft^2 of
r4 windows and a conductance of 96ft^2/r4 = 24 btu/h-f for windows plus
1152ft^2/r48 = 24 for ceiling plus 1056/48 = 22 for walls. the average
ground temp is sacramento is 60.8, so you wouldn't lose much heat through
a slab with perimeter insulation, and it could heat a 55 f house if the
ground underneath were moist (you might make some provision to moisten the
central part, under the vapor barrier.) adding 0.2 air changes per hour 
(31 cfm) makes the total about 100 btu/h-f, so this house only needs
24h(65-45.3)100 = 47.3k btu on an average december day.

a frugal 300 kwh/mo of electrical energy use inside the house would supply
34k btu of that, leaving 13.1k btu/day or 65.9k for 5 cloudy days in a row.

with 50% solar transmission, 64 ft^2 of south windows and 8 ft^2 e/w and
16 n will provide 0.5(64x820+8x340+8x370+16x190) = 30.6k btu on an average
december day. you don't need a sunspace, just some way to store heat for
a cloudy week. i'd paint the slab dark near the south windows and keep the
house from overheating by ventilation on average days, in a t-shirt, and
close the windows and wear a sweater after a few cloudy days. a 24x48x4"
slab cooling 10 f releases about 24x48x4/12x25x10 = 96k btu. 

night ventilation might work for cooling, with an average 75.7 f in july
and an average daily high of 93.2 and 58.1 night low, and humidity ratio
w = 0.0087, which makes pa = 29.921/(1+0.62198/w) = 0.413 "hg. you might
make the house 68 f by dawn, with a couple of ceiling fans to wash the
slab and open well-shaded windows, and close the house up during the day
and let it gain about 12h(84-68)100 = 19.7k btu through the skin plus
17k from electrical usage, raising the slab temp 4 f to make the house
72 f with rh = 100x0.413/exp(17.863-9621/(460+72)) = 52%. comfy. 

how about a flat roof, with a layer of epdm on top (8'x24'x12" sips
supported by perimeter 2x12s will hold up 70 psf) and a few inches of
mulch and vegetation, and a roofpond with an insulating cover above,
hinged on the north and reflective inside, with a counterweight and
a zomeworks "teeter-totter" to open the lid to heat water?

an oversized cover open to a 60 degree elevation could reflect 90% of
the 988 btu/ft^2 per day of sun that arrives on that plane in december
into a clear pond cover that passes 80% of that, so the pond gains
1440 btu/ft^2, and loses about 6h(120-50)/r1 = 420 btu/day, for a net
gain of 1000 btu/ft^2, so we need about 50 ft^2 of pond to make 50k
btu/day of 120 f water in december. we might use a 4'x16'x1' deep pond,
and make a few holes in the sips below it for skylights. 

good luck,

nick




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