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re: air conditioner heater info sought
16 aug 2000
mitch dickson wrote:
>...i would like to pour a pad 8 feet wide and 24 foot long. the inside...
>would be built a solid concrete and firebrick rectangle 6 feet by 20 feet
>by 4 feet high.
concrete has about 25 btu/ft^3-f of specific heat by volume, so raising
the temperature of this shoebox 1 f would store 6x20x4x25 = 12,000 btu.
>...the outside of the pad would be a wall out of 6 inch block the same
>4 feet high. this space between the wall and the solid rectangle would
>be filled with styrofoam or similar insulation making a giant "cooler".
styrofoam has about r5 f-ft^2-h/btu per inch of thermal resistance, so
the sidewalls would have about 256ft^2/r30 = 7.5 btu/h-f of conductance.
the top and bottom would each add 192ft^2/r30 = 6.4, making the total
20.3 btu/h-f. you might (latex) paint the styrofoam and omit the block.
the shoebox would have a time constant rc = 12kbtu/f/20.3btu/h-f
= 591 hours or 3.5 weeks. good... starting at say, 150 f on a 30 f
day, with the lid closed and no external heat load, it will cool
to about 30+(150-30)exp(-24/591) = 145 f after 24 hours.
>the entire top of the solid rectangle would be painted black and a
>6 inch air space above that would be double pained glass...
double pained, eh? maybe us r2, with 80% solar transmission. otoh,
the concrete has about us r0.2 per inch, ie r9.6 for 48", which is
not a good thing, if we are heating the whole slab from the top.
>a hinged and well insulated top would be on top of this whole structure
>that would be opened and closed by a motor and pulley system operated
>by a photocell system. the bottom of this cover would be a mirror.
say 90% reflective? you might want reflective sides above the shoebox too,
so less morning and afternoon sun bounces off the reflective top and onto
the ground.
>what this would be is a giant solar stove or cooker reaching daytime
>temperatures of well over 170 degrees.
with no external heating load and about 120ft^2 of solar aperture and an
average of 1200 btu/ft^2 of sun on an average 30 f day (where i live in
january), the shoebox would collect about 0.9x0.8x120x1200 = 104k btu.
at t degrees f, it might lose 6h(t-30f)120ft^2/r2 = 360(t-30) btu over
6 hours. if gain equals loss, t = 319 f. lookin good... we can't make it
much more than about 150 f without a selective surface.
>...how long would this stored heat last?
say a small, well insulated house with a conductance of 200 btu/h-f needs
24h(70-30)200 = 192k btu on a 30 f cloudy day, and the shoebox has enough
airflow and air passage area so it can heat the house until it cools to
80 f. if it starts at 150 f, it can keep the house warm for d cloudy days,
where 192kd = (150-80)12k, so d = 4.4 days. not bad...
>...how big would this have to be to work?
bigger. it needs more sun-gathering surface, since it looks like it only
collects about 104k btu/day and the house needs 192k. you might consider
filling it with water instead of concrete, and heating the water with a
fixed parabolic solar reflector with a trough or transparent poly duct
along the ground near the north edge that's filled with (pumped) water
during the day and empty at night. the water storage tank might be more
compact, and alongside. this structure might also be a carport with a
curved roof and an open south side, if you often commute during the day.
nick
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