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re: conserving electricity
20 oct 2000
anthony matonak wrote:
>...explore the wonderful world of solar cooking. if you wish to spend
>the money you can even build a solar stove/oven combination which can
>cook 24 hours a day without consuming any fuel.
not easy or small... we might start with something like this:
the most serious mistake was making the outer container of the receiver
of plywood. we thought that the plywood would be sufficiently insulated
from the copper panel which was the receiver proper, that it would not
get too hot. the copper panel was separated from the plywood by 4" of
fiberglass insulation. nevertheless, the plywood caught fire and the unit
was completely destroyed. we suppose this is a success, of sorts...
from "a solar collector with no convection losses," (a downward-facing
receiver over a 4:1 concentrating parabolic mirror) written by
h. hinterberger and j. o'meara of fermilab, ca 1976
say a thermal mass above the mirror maintains 400 f for 5 30 f cloudy
days, and the mass is initially 600 f... 400 = 30+(600-30)exp(-120/rc),
so rc = -120/ln(370/570) = 278 hours. the mass/cooking surface might be
a 4'x4'x1' thick collection of 80% steel and 10% copper and 10% concrete
glue storing heat like water by volume: 16ft^3x64btu/f-ft^3 = 1024 btu/f.
rc = 277 hours, so r = 277h/1024btu/f = 0.27 h-f/btu ("ohms.") a 1' oven
above the mass gives the exterior walls about 80 ft^2 of surface, so
they need rv/80 = 0.27, ie us r22, about 6" of fiberglass insulation.
if the oven loses 24h(600f-30f)/0.27 = 51k btu/day, and a square foot
of sun-facing surface might gather 0.9x0.9x1000 = 810 btu/day, so we need
51k/810 = 62 ft^2 of surface, say an 8x8' heliostat on the north side
(south in oz) with a fixed secondary mirror under the oven.
we might regulate the oven temp with a bimetallic demon that opens
a hole over an air channel inside the mass or lifts one edge of an
insulating pad that covers it.
>a one (1) square foot skylight is probably equal to a few hundred
>watts of incandescent lighting.
sure. a well-lit room has 50 footcandles, vs 10,000 for outdoor sun,
so it might daylit with 1/200th of the floorspace as windows, vs 8%
per the boca code. for instance, a 10'x20' room might have a 1 ft^2
south window near a white ceiling with a lightshelf below to bounce
the sun up onto the ceiling. we might stuff 2" foamboard into the
rest of the window frames in wintertime to reduce heat loss, with
a very tight fit to avoid condensation.
nick
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