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re: seattle solar air collector under design/construction
6 jul 2001
"tim pierson" wrote in message
>i have decided to design and build a 96 sq. ft. air heater for my house
>near seattle...
nrel says the average december temp in seattle is 40.5 f, and a south
wall receives 1510 btu/ft^2 on a clear day (vs 420 on an average day.)
gary lindgren wrote:
>what about storage of the energy? how about a thernal mass to absorb
>excess energy and then use this at night to get heat to house.
on a _clear_ december day (less likely than an average day), the heater
might collect 0.9x1510x96 = 130.4k btu and lose about 8h(70-40)96ft^2/r1
= 23k btu, gaining 107.4k net, enough to heat a house with conductance g
for 24 hours, where 24h(70-40)g = 107.4k, or g = 149 btu/h-f. that's a
small house, eg a 24' cube with r19 walls and no windows or air leaks,
or a larger house with some heat gain from electrical usage.
mid-winter heat storage can be uneconomical if the air heater gain is
small compared to the house heating need. all the solar heat is used as
it is collected, with no wasted heat, and the furnace runs less often.
but, we might make an indoor thermal mass tower in an existing house to
store daytime heat for the night. say we have lots of airflow and mass
and mass surface, eg a collection of 8x8x16" 30 pound hollow concrete
blocks with about 5 btu/f and 6 ft^2 per block, and we thread 2 4" water-
filled pvc sewer pipes through the holes in each block, adding about
7.5 btu/f and 1.4 ft^2. the combination has 12.5 btu/f and 7.4 ft^2,
with a slow-moving airfilm conductance of 11 btu/h-f and rc = 12.5/11
= 1.1 h. it would fully charge to 70 f over 8 hours. say the mass temp
drops to tmin at dawn, and the house air temp drops to 60 f.
the house needs about 107.4kx8/24 = 71.6k btu for the 16 hours from dusk
to dawn, so we need n blocks, where 12.5n(70-tmin) = 71.6k. at dawn, it
needs (60-40)149 = 2980 btu/h, so tmin = 60+2980/(11n), and n = 600, eg
an 8' tall x 7' square tower. kinda big for a living room. it might work
better in a crawl space. it could be smaller if it received warmer air
from the air heater during the day, vs 70 f room air...
nick
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