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re: warm flat plate collectors??
22 jul 2002
n. thornton  wrote:

>i keep reading that single glazed flat plate collectors will get nice
>and warm on winter days. i havent tried it, but find it hard to believe.

in the sun, they have no choice. ohm's law for heatflow says a dark plate
with lots of insulation behind it and an r1 cover over the front with 90%
solar transmission in 250 btu/h-ft^2 sun will be 0.9x250/1ft^2/r1 = 225 f
warmer than the outdoors, eg 255 f on a 30 f day. an oven thermometer on
the ground with an old window on top may show something like that. if not,
pile on some more windows. 

with, say, 0.1 gpm of 120 f water as a coolant, the heat capacity flow
c = 0.1x8x60 = 48 btu/h-f, the plate loses about (120-30)1ft^2/r1 = 90
btu/h to the outdoors, and the water gains p = 0.9x250-90 = 135 btu/h,
emerging at about 120+p/c = 123 f.

>i'm in britain. you always wanted to know after all :)

rich komp says the best solar locations in britain are about the same as
average us solar locations. 

nick




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