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re: temperature expectations from solar hot water panel
21 may 2005
doug wrote:

>i live outside of ottawa...

my 'ol nrc solarium workbook says 1603 wh/m^2 of sun falls on the ground
and 2943 falls on a south wall on an average -10 c january day in ottawa,
vs 1214 and 2257 on an average -8 c december day. a solar collector with
optimal tilt would get sqrt(1603^2+2943^2) = 3351 in january and 2563 in
december. the best tilt would be atn(2943/1603) = 61.4 degrees in january
and 61.7 in december.

>my main concern is not solar "domestic" hot water but solar heated
>water for my radiant floor... i was hoping that two 60 gal water tanks
>would be a practical size to substantially reduce my usage of propane...

two 60 gal tanks would store about 1000 btu/f, vs 4"/12"x25x1000 = 8333
(1 gallon per square foot :-) for a 4" x 1000 ft^2 concrete floorslab. 
those tiny tanks might not add much, unless the water is very hot on an
average day. how about a large unpressurized tank? could a 24'x4' deep
113k btu/f pool store heat for 5 cloudy days in a row? it wouldn't need
a big collector, if a sunspace warmed your house on an average day.

>how many btus can i collect in ~120 gallons of water with a minimum
>temperature of 90 degrees f on a typical day in dec or jan in ottawa
>using a flat panel collector.

january has 3351/(32-(-10)) = 80 wh/dd, vs 2563/(32-(-8)) = 64 in december,
so december is the worst-case month. a square meter of collector with a us
r1 (metric u5.68 w/m^2c) cover with 90% solar transmission and a large 90 f
water flow and a large 90 f water tank perfect back insulation would gather
0.9x2563 = 2307 wh and lose about 6h(32-(-8))1m^2xu5.68 = 1363 wh, for a net
gain of about 944 wh/day, ie 3220 btu, or 299 btu/ft^2. you might raise that
with a reflector or some extra glazing (a parasitic air heater) below the
collector. a 2-cover collector might gain 1869 and lose 6h(32-(-8))1x5.68/2
= 682, for a net gain of 1187 wh/day.

energie solaire (a swiss company) sells unglazed stainless steel collectors
with a selective surface for about $12/ft^2. they might work well in this
low-temp application. the efficiency equation on their web site indicates
100(0.959-8.91(0.375)-0.047(0.375)^2) = 62% with an average 30 c collector
and 0 c outdoor temp in full sun (800 w/m^2) in december in toronto.

nick




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