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re: is solar viable for our new house?
16 feb 2001
george estep wrote:
>hi nick,
hi george,
> o.k. i ignored the freezing part and followed that with a math error.
hey, close enough for the net :-)
> can you explain why you used ln((32-20)/(150-32)) instead of
>ln((32-20)/(150-20))? methinks it is a typo.
it cools from 150 to 32 f when 32 = 20 + (150-20)exp(-t1/(rc)), right?
so 32-20 = (150-32)exp... and ln((32-20)/(150-20)) = -t1/(rc). yes, that
was a typo (really a thinko--i had it right the first time, but went back
and changed it while thinking about spreading 23 tons of 2a modified stone
in the rain, before calling to postpone delivery till monday. i was also
thinking about buying the stone by weight, when wet stone weighs more, so
my $205 might only buy 22 tons. still seems like a bargain, vs pvs... :-)
> also, can you define rv and ra for me? i am having trouble with the
>symbology for thermal resistances. the area-less unit is r-value.
right... ft^2-f-h/btu, ie per square foot of the surface perpendicular to
the heat flow. divide by area, and you have an absolute thermal resistance
that no longer depends on the area. r-values are stamped on insulation in
stores, except for radiant r-values, which are more confusing, since they
depend on absolute temps, direction of heatflow, air gaps, and absolute
temperatures. (altho they are still important, since 1" "r7.2" double-faced
foamboard becomes r15 with a few inches of air on each side and a 50 f
mean temp and horizontal heatflow.)
>is this value defined only for the direction of interest, i.e.
>perpendicular to the heat flow?
yes. you might write it in units like ft^2-f-h/btu-in, ie per inch of
thickness, in the direction of heatflow.
>you define thermal resistance as r-value/a in your solar thermal
>tutorial, circa 1999. is there a symbol for this?
just r, like g. europeans sometimes use g-values in w/m^2c.
>i'm just trying to get up to speed on these thermal things so i can work
>through the rest of my solar water heating project.
they are building a new radiotelescope in green bank, wv. maybe you
can buy an old one... line a 200' dish with foil, install it north
of the house, train it to point at the kitchen window...
>...the vendors don't seem to want to publish any data useful for
>engineering these things (collectors, tanks, heat exchangers, etc.).
it's usually available in some form. for instance, a curve of efficiency
vs (tc-ta)/i for collectors, where tc and ta are collector and air temps
and i is sun power in btu/h-ft^2, with some (large) specified water flow
rate. heat exchangers can have an "effectiveness" (efficiency) spec e at
specified flow rates, where tco = tci + e(thi-tho), and tc and th are hot
and cold fluid temps flowing in (i) and out (o) of the heat exchanger.
here's another spec: magicaire's $150 all-copper 2'x2' shw 2347 horizontal
duct heat exchanger transfers 45k btu/hour between (6 gpm?) of 125 f water
and 68 f air at 1400 cfm, with a 0.1" h20 (air) pressure drop.
nick
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