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re: solar pool heating
12 oct 2004
n. thornton  wrote:

>> >here's my thinking, maybe you can tell me where its wrong. the suns ir
>> >changes to temp rise on the outer surface of the pipe.
>> 
>> as does the rest of the spectrum. if full sun (250 btu/h-ft^2) falls on
>> a 1' long x 1" diameter black pipe, it collects about 20.8 btu/h. in 60 f
>> 4 mph air, with 80 f water inside, the 0.262 ft^2 of 80 f pipe surface
>> loses about (80-60)(2+4/2)0.262 = 20.9 btu/h, so the net gain is -0.1 btu/h.
>
>but the pipe surface is not at 80f during use, its more than 60c. over
>140f. i know that from using a hose/polythene panel, the water is hot
>enough to burn if it stagnates. we drew the water off it every 40
>minutes, and it was most hot.

that's shooting yourself in the foot to start with, efficiency-wise.
efficient collectors run cool, with moving water. 

>plus winds go well above 4mph some days... that translates to an
>amount of heat loss i wouldnt know how to calculate.

that doesn't interfere with your process of coming to conclusions :-)

>> a         vt       q         t     
>> 
>> 0.0833    59.8     -0.256    79.3   (1" poly over 1" pipe)
>> 0.1666    88.1     +6.617    96.8    2"        "
>> 0.3333   102.2    +12.394   111.5    4"        "
>> 0.5000   106.9    +14.951   118.0    6"        "
>> 1.0000   111.8    +17.964   125.6    1 foot    "
>> ...
>
>nick... i dont even know what a vt q and t mean. i havent studied
>thermalmatics.

what's thermalmatics? :-) a is the number of square feet of glazing
that covers a 12"x1" tube. q is the btu/h of sun the tube collects.
a = 1 (the most efficient of your designs) has a mere 100x17.9/250
= 7.2% solar collection efficiency in full sun, and it loses lots of
heat at night. piffle. solar heating is an exacting science. if you
want to experiment with less education, why don't you take up surgery?

>> polycarbonate has good longwave ir thermal resistance. it comes in 4'
>> rolls and costs about $1.50/ft^2 and lasts about 20 years. if a 300'
>> roll of 1" pipe costs $59.99 at true value hardware stores, what's
>> the most economical value of a?
>> 
>thats like asking a sheep farmer what the best alloy is for rocket
>engines. i dont even know what a is, sorry. on electronics, building
>or invention i could be more help.

you might think of this as basic electronics, if you'd ever heard of a
thevenin equivalent circuit :-) i suggest you look it up, and begin to
inform your oft-incorrect opinions with numbers. 

nick




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