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re: materials & sizing q
1 mar 2001
timla wrote:
>i can see seattle, so i'd guess that i have seattle's insolation and
>climate, but a little colder - not quite spokane, though.
seattle has an average outdoor temp of 40.5 f in december, with
420 btu/ft^2-day on a south wall, and 61.5% of that is beam sun.
i'll change the numbers below for seattle.
>> ...you might keep horses, with a 16' tall x 32' long x 16' deep
>> parabolic run-in shed that solar heats water... near the house,
maybe in the house, eg the attic...
>> with 512 ft^2 of solar aperture, and a 4'x8'x16' tank containing 32768
>> pounds of water with 448 ft^2 of r26 sides (6" of fiberglass plus 1.5"
>> of foam) in the basement, so rc = 32768btu/fx26ft^2-f-h/btu/448ft^2
>> = 1902 hours (79 days), semi-seasonal storage...
>would that be a frame & plywood box lined with plastic?
a single $200 20' wide x 32' long folded piece of epdm rubber.
this larger-than-usual "semi-seasonal storage" would let you use some of
november's solar heat in december and keep warm for more cloudy days
in a row. november is warmer and sunnier than december, at 45.5 f, with
530 btu/ft^2-day on a south wall. a 1-axis ew concentrator can collect
59% of that. with more sun and less needed house heat, you can put some
solar heat in the bank.
>btw, horses = work & expense, and i haven't ridden in 21 years...
>the building is an idea, though...
horses were just a thought. you couldn't keep much in the building,
because it would shade the target. howard reichmuth, pe (who lives
in hood river, or) designed the ecotope concentrating greenhouse
at pragtree farm in seattle, which works like this. i understand
it's still working well after 25 years or so. you might visit...
his greenhouse has glass on the south side, vs a run-in shed for
horses. that makes it warmer at night, but increases the cost and
reduces collected sun by about 10%. according to his measurements
about 50% of the sun that enters the greenhouse ends up heating
the water in the trough near the north wall.
on an average december day, the shed might collect 0.9^2x258x512ft^2
= 107k btu of beam sun in 200 f water. it might store 32768(200f-100f)
= 3.3 million btu of useful heat (and 4096 gallons of useful rainwater)
enough to keep a 100k btu/day house warm for 33 cloudy days in a row.
>i'm considering a small prototype to prove that a) i can build the thing,
>b) that it will indeed work, and c) provide enough heat to replace the heat
>pump. i agree with you, but need to prove to myself that this is feasible
>before commiting to it.
you might start with an 8' cube with a 2'x4'x8' tank above the reflector.
>...i found that the emergency heat (electrical resistance source) in the
>heat pump is 20kw, which is coincidentally about 68 kbtu, and when using
>it in that weather, the duty cycle was actually fairly low...
you might get an accurate measurement of the house thermal conductance
by using the electrical resistance heaters for a day and measuring the
indoor and outdoor temps and the duty cycle. that's an easy experiment.
keeping the house 65 f in december with 107k btu/day of solar heat
and 300 kwh/mo (34k btu/day) of electrical usage means lowering g
to 141kbtu/(24h(65f-40.5f)) = 240 btu/h-f, ie raising the average
wall and ceiling r-value to 2880/240 = 12.
nick
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