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re: is my near horizontal collector that bad?
1 dec 2005
it's bad enough to convince many people that "solar heating won't work."

gosolar23  wrote:

>600 sf of trickle collector, around 25 deg sloped south.  polycarbonate
>covering.  aluminum siding just like harry thomasson wanted.

ge's lab tests say the polycarbonate may only last 2 years. why not call
and ask for a graph of their results? one layer will have significant heat
loss via evaporation from the siding and condensation under the glazing.
 
>using day long insolation for january, i get around 1000 btu/ft2. 

nrel says 680 btu/ft^2 falls on the ground and 1050 falls on a south 
wall on an average 48.7 f january day with a 55.6 daily max in sf, so
the average daytime temp is about 52. the average windspeed is 10.4 mph,
which makes the outer glazing loss about 2+10.4/2 = 7.2 btu/h-f-ft^2.

a square foot of 25 degree south tilted collector has 0.906 ft^2 of
horizontal surface and 0.423 ft^2 of south wall surface, so it would
collect 0.906x680+0.423x1050 = 1060 btu/day, of which 954 might pass
through a single polycarb layer. what do you plan for an average day
water temp?

with tw (r) water in the collector and glazing temp tg (r), 100(pw-pa)
= (tg-(52+460))7.2 btu/h-f-ft^2 might flow through the glazing, with
pw = e^(17.863-9621/tw) and pa = e^(17.863-9621/tg) "hg, so... with tw
= 560 r (100 f) and pw = 1.935 "hg. tg = 538.8-13.89e^(17.863-9621/tg).
tg = 530 f (70 f) on the right makes tg = 528 on the left, then 529.0,
528.8, and 528.9 r (68.6 f), so (68.8-55)7.4 = 102 btu/h would flow out
from the glazing, leaving about 954/6h-102 = 56.6 btu/h of useful heat,
about 600x56.6 = 34k btu/day from 600 ft^2 of glazing. there's no useful
heat output at 119 f.

>i guess for suny days that is around 600,000 btu. maybe 40% efficiency
>ok around 240,000 btu's. maybe 12 days of clear days a month?

you might do a tmy2 simulation. or look into cogen. you might run the 
exhaust from a grid-tied honda eu2000 into a natural gas water heater. 

nick




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