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re: heating workshop
24 feb 2002
sydnesb  wrote:

>i have a 12'x24' shed which i use to house my ae system.
>the shed is also used as a workshop...

so you are a tinkerer :-)

>last year i completely insulated the shed...

r20 insulation with a very good vapor barrier (say 30 cfm of air leakage)
and 2 3'x4' r2 windows might make a thermal conductance of 24ft^2/r2 = 12
btu/h-f for the windows plus about 288/20 = 14.4 for the ceiling plus 27.6
for the walls plus about 30 for air leaks, a total of 84 btu/h-f. 

nrel data indicate that january is the worst-case month for solar house
heating in boulder, with 1370 btu/ft^2 of sun that falls on a south wall
and 750 btu/ft^2 on a horizontal (level :-) surface on an average 29.7 f
day. your shed might need 24h(65f-29.7f)84btu/h-f = 71.2k btu of heat on
that day, or less, if it's only heated for a few hours a day. battery
charging and inverter losses might contribute some of that heat, or the
heat loss from a small generator. 

>and would like to begin working on... a passive solar heating source...
>i have 24' of southern exposed roof on the shed with two angles to choose.
>its a gabel style of roof, one angle is about 60 deg and the other is 30...

i have a 12'x16' shed with a gambrel roof like that. the lower part of the
roof is 3'x16', and the upper part is 4.7'x16', with a 17 degree slope...
i've been thinking about how to solar heat it for a couple of years :-)

>i am considering four 2x2 skylights on the 60 deg angle roof...

a square foot of skylight with 80% solar transmittance would collect about
0.8x1370 = 1096 btu of sun on an average january day. with r2 (u0.5) thermal
resistance, it would lose about 24h(65-29.7)1ft^2/r2 = 424 btu, for a net
gain of 672 btu, so 16 ft^2 of skylights would gain 10.8k btu/day, making
the indoor temp 29.7+10.8k/24h/84btu/h-f = 35 f on an average january day. 

>and installing ceramic tile flooring to absorb the heat during the winter
>months.

ceramic tiles are kind of thin, without much heat capacity, and storing
the heat near the floor means it keeps the shed warm all day, whether you
want that or not. thermal mass above the ceiling might store more heat,
with a higher temperature swing and upper temperature limit...

>someone else recommended installing a few more solar panels and using
>the extra energy to power a standard electric heater.

would that be steven ("i am in love with photovoltaics") strong? :-)

>there is no means of shading the skylights during the summer months.

you might devise one...

>in order to prevent to much heat to build-up in the summer i installed
>several ventilators.

with enough thermal mass and surface (battery electrolyte comes to mind)
and airflow, you might open the vents at night and close them during the
day. this might happen automatically, with some passive plastic film
one-way dampers (dry cleaner bags hanging over chicken-wire-covered holes
in the wall.)

>for the skylights i am planning on installing an inner door which will
>have a light difusing panel which i hope to block most of the heat and
>still allow the light to pass.
 
full sun is about 10k footcandles, vs 50 for a well-lit room. you might
put 2 layers of 80% greenhouse shadecloth under the skylights, and vent
the warm air above outdoors in summertime and indoors in wintertime. 

>looking for any suggestions on heating and cooling the shed.

it looks like you need more solar heat, ie more solar glazing, about
100 square feet or more. and those skylights lose heat at night. and
what do you do on a cloudy day?

you might remove the shingles from the whole south roof and cover it
with 2 layers of greenhouse poly film inflated with air during the day
and tiny (1/16" or so) soap bubbles at night. not passive, but low-power,
and not too hard to arrange: put a 24' x 2" pvc pipe full of holes in
a trough formed from the the lower fold of the poly films and fill the
trough with a 10% sodium laurel sulfate (detergent) solution, and maybe
some sort of antifreeze.

inflate the films with air from a small blower above the water level
during the day, as greenhouses do, or just leave the films collapsed
together. at night, turn on a shop vac to make bubbles with about 1000x
the soap solution volume. put a return air path near the ridge with
a 2" hole covered with insect screen. as the film cavity fills with
bubbles from the bottom, air moves freely through the screen at the
top. when the bubbles hit the screen, they deform it, and a microswitch
turns off the shop vac. with a 1 hour bubble lifetime (plateau kept
bubbles alive for years in bell jars) the shop vac might turn on for
10 seconds a dozen times per night to replenish the foam.

with a 4'x24' south aperture and 80% solar transmission, you'd collect 
about 0.8x4'x24'x1370 = 105.2k btu of south wall sun plus 0.8x6'x24'x750
= 86.4k of horizontal sun, a total of 191.6k btu on an average day. if
71k of that goes to warming the lower part of the shed, the rest (120k)
could keep the part under the roof warmer. if 120k = 6h(t-29.7)6'x24'/r1
+ 18h(t-29.7)6'x24'/r20, t = 150 f, theoretically-speaking. bubbles are
better insulators at low temps (about r3 with 1/16" diameter and mean
50 f temp), and poly film is 80% transparent to longwave ir...

to store heat, we might put some water above the rafters in a tray painted
black, with a layer of clear flat polycarbonate above that, and allow room
air to fill the space above the polycarbonate. we might use an angled 4'x24'
layer of foil-faced foamboard to divide the attic lengthwise into a solar
south half and a storage north half. a person under the water tray would
receive radiant heat. we might warm the room air with a slow ceiling fan. 

the shed needs about 350k btu for 5 cloudy days in a row. that might
come from 350k/(130-80) = 7k pounds of water cooling from 130 to 80 f
in a 4'x20'x16" plywood box lined with a single piece of epdm rubber,
or a shallower box filled with a 50% 2:1 cacl2/licl solution, as we
distill water out on average days and add it back on cloudy days. 

nick




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