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re: solar shop heating question
22 dec 2002
gary  wrote:

>i use my two car attached garage as a shop...

me too, altho it's somebody else's garage.

>i would like to figure out some simple scheme to provide solar heating
>during the day.  it has a large, south facing door that gets good sun
>exposure.   the door is the type consists of 4 horizontal panels that
>run on tracks to open.

mine too. i added some uv greenhouse polyethylene film on the inside of
the doorframe, but it isn't well-sealed at the bottom or edges, and it
needs to open once in a while for large objects, which doesn't help.
i like the light from the open door when i'm working...
>has anyone seen a scheme (or have ideas)  for using the door area to provide
>solar heating?   i would consider replacing the door with a new door that
>has some form of collection built in, or adding something in front of the
>current door to collect heat, or modifying the current door.

if i can get permission, my next step would be to lay a 2x4 flat on the
outer edge of the concrete pad 6" south of the door and staple some film to
the top of the outside of the door frame and the 2x4 and attach the film to
the vertical edges of the door frame with aluminum greenhouse extrusions with
zigzag wires that can be undone easily to open the door. permission may seem
like a small thing, but the owner has a rarified sense of aesthetics and
a hideous but justified fear that i may cover her house with plastic film. 

i live near phila, with 1000 btu/ft^2 on a south wall on an average 30 f
january day. my garage is 12x22x8' tall, with one long side sharing the
house wall and a 9'x7' door with r5 foam insulation and an r30 ceiling
and r15 walls and a 3'x7' r2 door and a 3'x4' r2 window. with the big door
open, the garage conductance is 7'x9'/r1 = 63 btu/h-f for the film plus
12/2 = 6 for the window plus 21/2 = 10 for the small door plus 264/30 = 9
for the ceiling plus 239/15 = 16 for the north and east walls, a total of
104 btu/h-f. 

with 90% transmission, the film over the garage doorframe would collect
0.9x7x9x1000 = 57.6k btu/day. an average indoor temp t over 8 hours with
57.6k = 8h(t-30)104 t = 30+57.6k/(8x104) = 98 f. warm enough :-)

>i live in bozeman, mt -- daytime highs this time of year in the 20'sf.

nrel data indicate that december is the worst-case month for solar heating
in helena, with 810 btu/ft^2 on a south wall on an average 21.2 f day with
an average daily high of 31.3. with an average (21.2+31.3)/2 = 26.3 daytime
temp, my garage might be 26.3+0.9x7x9x810/(8x104) = 81.5 f on an average dec
day in helena. if you covered the whole south wall with film and added some
dark window screen between the door and the film or painted it a dark color,
the garage might be 26.3+0.9x8x12x810/(8x104) = 110 f :-)

you might keep it warm at night as well by leaving the door open 6" at the
bottom and adding a one-way plastic film damper on the outside, hinged at
the top, to allow indoor air to flow into the space between the door and
the film during the day and return through the slot that forms at the top
of the door when it's open. a bottom slot grill could stop the film and
prevent reverse airflow at night. an indoor latch could discourage burglars. 

the south wall would then gain 0.9x8x12x810 = 70k btu over 6 hours and lose
about 6h(t-26.3)8x12/r1 during the day and 18h(t-21.2)8x12/r6 at night. the
rest of the garage with a nighttime thermal conductance of 41 btu/h-f would
lose 24h(t-21.2)41 over an average day. what's t, if the energy that flows
into the garage equals the energy that flows out, over an average day?

>some means to prevent overheating in the summer is also probably necessary.
open all the doors and windows. summer sun has a higher angle, so less will
enter the door, and the airflow will remove heat. 


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