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re: help!! woth solar energy
9 nov 1998
chris peppler   wrote:

> hello my name is chris peppler and am currently in grade9 or 3
>secondary in canada and am seeking help of anyone. the help i need
>is an idea or a plan on how to make a solar heated dog house stirctly
>running on solar energy. the disign must be cheap and practicle...
>it has to be able to raise the internal temperature 5 degrees celcious
>the fastest thanks. oh and the only sun it gest are two heat lamps 
>50 centimeters away.

how about a 4' cubical doghouse made with hollow 8x8x16" concrete blocks
and foamboard? putting a 5 pound brick inside each of 3 block holes gives
a wall with a thermal capacity of about 8 btu/f-ft^2 and a surface of about
8 ft^2/ft^2, ie about 128 btu/f and 128 ft^2 for a 4'x4' wall. we might
make the north and south walls and floor this way. 

january is the most difficult month for doghouse heating in philadelphia,
with a 30-year average monthly outdoor temperature of about 30 f and 1,000
btu/ft^2 per day of sun on a south wall. let's assume the dog is happy from
50 to 80 f (he might go outdoors or prop the swing door open with his nose
if it's warmer inside), and we have at most 5 cloudy 30 f days in a row.
let's assume the average steady-state indoor temperature is 80 f in january. 

suppose the doghouse has a shallow sunspace (ie, a solar air heater) with
a 4x4' flat polycarbonate r1 south wall with 90% solar transmission, and
the sunspace temperature is 90 f during 6 hours of solar collection on
an average day, so it gains 16ft^2x1,000btu/ft^2x0.9 = 14,400 btu of sun
and loses 6h(90f-30f)16ft^2/r1 = 5,760 btu, for a net gain of 8,640 btu.

the dog might contribute another 100 btu/h, ie 2.4k btu/day of heat for
the structure, for a total of about 11k btu/day, ie 460 btu/h, so to make
80 f indoors on an average day, we need 80f-30f = 460xrv/96ft^2, ie a
minimum exterior wall r-value of about 10. let's use 2 layers of 1.5"
styrofoam, making the wall r-value 15 and making the doghouse potentially
warmer inside. the outer layer will last a long time, with latex paint.

making the doghouse 50 f after 5 days (ignoring the dog's heat) requires
50 = 30+(80-30)exp(-120h/rc), where the time constant rc = -120/ln(0.4)
= 131 hours. with r = r15/96ft^2, we need c = 838 btu/f, or more. with
384 btu/f in bricks and blocks, we might add 15 9x9x13" tall recycled 4
gallon plastic tubs with lids (eg ropak tubs, made in fullerton, ca, used
for soap, and dried fruit from the cherry central cooperative in traverse
city, mi 48684) full of water stacked 3-high along the north wall, raising
c to 864 btu/f. but why not make the walls themselves out of 48 of these
tubs, with horizontal 1x3s between for attaching the foamboard? they are
free, and very strong and stable when stacked, with 6.5 times more thermal
capacity by volume than concrete blocks, and lightweight and easy to
transport when empty, since they nest. that makes c = 1,536 btu/f...

this doghouse thermal conductance is 96ft^2/r15 = 6.4 btu/h-f, so it
loses 24h(80f-30f)6.4 = 7,680 btu of heat on the first cloudy 30 f day,
cooling to 80-7,680/1,536 = 75 f. on the second, it loses 24h(75f-30f)6.4 
and cools to 70.5 f. rc = 15ft^2-f-h/btu/96ft^2x1,536 btu/f = 240 h, so
50 = 30+(80-30)exp(-t/240), and it cools to 50 f after t = -240 ln(20/50)
= 219 hours or 9 cloudy days in a row.

it might look like this, from the top (use courier font):

                             4'
             -------------------------------
            |     |                         | 
            |  s  |  t    t    t    t    t  |
            |   v1|v2v3                     | 
            |  s  |  t                   t  |         t are 4-gallon tubs.
            |     |                         | 
   south    |  s  |  t                   t  | 4'      h is a bottom hole.
            |     |                         | 
            |  s  |  t              b    t  |         b is a bowl for
            |     h                         |         rainwater from
            |  s  |  t    t    t    t    t  |         the roof.
            |     |                         | 
             -------------------------------

the sunspace might be about 6" thick (at least 1/15th of the height),
with a layer of dark window screen stretched from the north top edge to
the south bottom edge. v1, v2 and v3 might be $12 leslie-locke automatic
foundation vents in series, near the top of the sunspace,with their
bimetallic springs adjusted to open louvers on temperature rise at 70 f,
open on temperature fall to 70 f, and close on temperature rise to 80 f.

this doghouse would take a long time to warm 5 c, perhaps forever, with
mostly longwave ir from the heat lamps, blocked by the polycarbonate.
a faster-heating superinsulated doghouse might have no solar heat, just
70f-30f = 100rv/96ft^2, so rv = 38, eg 8" styrofoam walls. a 100 btu/f
dog inside could warm the 64 ft^3 of air at a rate of about 86 f/h, ie
6.3 minutes for a 5 c temperature rise.

nick




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