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re: the beginnings
24 may 1999
bloody viking   wrote:
>although not impossible, solar heating in minnesota is challenging...

december is the worst-case month for solar heating in minneapolis. an 17.9 f
average day with daily high of 25.5 has 820 btu/ft^2 of sun on a square foot
of south wall, making 820/(70-17.9) = 15.7 btu/dd, vs 1000/(70-30.4) = 25.3
in january in philadelphia.

a square foot of double-glazed r2 south plastic film sunspace glazing with 80%
solar transmission in minneapolis gains about 650 btu/ft^2. with 80 f air, the
sunspace would lose 6h(80f-22f)1ft^2/r2 = 175 btu/ft^2 over a 6 hour december
solar collection day, for a net gain of 475 btu/ft^2 per day. a frozen pond or
snow to the south might reflect 30% more sun, raising the net gain to 670 btu. 

a double-glazed horizontal surface with no overhead reflector gains about
344 btu/ft^2, for a net gain of about 170 btu/ft^2 per day. 

a 16' cube with r20 insulation and a thermal conductance of 6x16'x16'/r20
= 77 btu/h-f needs about 24h(70-18)77 = 96k btu/day, which might come from 
96k/475 = 200 ft^2 of south sunspace glazing or a 16x16' bubbleroof and
110 ft^2 of sunspace. 

a 16'x8' south waterwall with a 6" bubblewall on each side might gain 83k
btu/day and lose 6h(t-22f)128ft^2/r2 from the south side during the day and
18h(t-10f)128ft^2/r20 from the south side at night and 24h(t-70f)128ft^2/r20
from the north side over an average december day, so

     83k = (384+115.2+153.6)t -8448 -1152 -1075, and t = 144 f, 
with this linear model. suppose reradiation limits the waterwall to 130 f.

keeping the cube warm for 5 days in a row requires 480k btu, and a cubic foot
of 130 f waterwall stores about 64btu/f(130f-80f) = 3.2k btu of useful heat,
making the waterwall 480k/3.2k = 150 ft^3, so it needs 150ft^3/128ft^2 = 1.17
feet of thickness, ie about 14", eg 9 compartments for 55 gallon plastic film
drum liners per 4'x8'x14" module, made with a few 2x4s and bolts and some
10 cent/ft^2 4' welded-wire fence.

larger cubes need proportionally smaller heat stores, as their losses grow
with the square and their volumes grow with the cube of their dimension.


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