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re: solar collector question
5 nov 2001
tat-2 wrote:
>i think you need to spend the extra $$ on copper pipe for the collector, or
>at least use some sort of metallic pipe in it...
i'd say the main problems with lynn's collector are a) not enough direct
collecting surface (the 6x1.5" pipes only occupy 38% of the 2' cross section
of the collecting box) and b) too much thermal mass (it looks like the pipes
store about 5 lb of water overnight, which cools off and has to be reheated
the next morning. seattle is 40.5 in december, and 420 btu/ft^2 of sun falls
on a south wall on an average day, of which only 380 will enter a box with
a cover with 90% solar transmittance. if the pipes cool to 45 f, heating
them to 95 f takes 250 btu, which wastes at least 250/380, ie 66% of the
solar input every day, before they begin to produce useful heat. a draindown
system would be more efficient.
metallic pipe would help a little here, but this collector really needs
a metallic absorber plate. maybe 0.005" copper foil on foamboard, with
small copper tubes soldered to the foil every few inches.
>to set up a collector that starts out with an extremely poor heat transfer
>source would limit the final output to maybe 10-20 warmer then the input
>water.
plastic is not a great heat conductor (about r1 per inch, like wood), but
the pipe wall is thin, so it has little thermal resistance, say r0.2, so
in direct sun, the outside wall of the pipe would be about 250x0.2 = 50 f
warmer than the water.
>with the plastic pipe your going to need all of the stored heat you
>can get.
storing heat in a collector kills the efficiency...
>it is going to take 4-5 hours of exposure to pass the heat energy
>through the pvc pipe.
no. the heat energy passes into the water almost instantly. the water
has thermal capacity and the pipe wall has thermal resistance, so you
might begin to calculate (like toby) how long it would take the sun to
raise some water inside the pipe so many degrees, but that doesn't
matter, since the water inside the pipe is moving. it doesn't just sit
there being heated. it is continuously being replaced by more water at
about the same temperature.
>didn't you ever take a garden hose that was in the sun for 4-5 hours and
>test it and find out it is only slightly warmer then body temp.
never :-)
>if this warmed water is being circulated through the concrete, the heat
>loss/ absorption of the concrete may consume 70-80% of it before it ever
>starts to warm the concrete.
sounds like nonsense to me.
>"lynn coffelt" wrote:
>> line collector box with reflective or absorptive material?????
>>
>> physical size of water heating (warming, anyway) of my collector is fixed
>> at 10' long by 2' high and 1' deep... the "box" will be insulated
>> plywood with probably single glass glazing material. the collecting will
>> be done by circulating water (or anti-freeze) through a lengthwise, series
>> connected array of 6, nearly 10' lengths of 1 1/2" abs dwv plastic pipe...
>> since much of the area inside the box facing the sun... will be covered
>> with the black pipe...
looks like only 38%.
>> i'm tempted to line the rectangular cross section box with reflective
>> material. (the pipe will be suspended approximately half way between
>> the glazing and the back of the box) this should, i reason, direct
>> reflected light on the back, top and bottom of the pipe. i'm hoping
>> that this results in more of the energy being absorbed by the pipe rather
>> than merely heating the air, and dealing with more loss back out through
>> the glazing.
sounds reasonable. you could calculate how well this would work, or
look it up in duffie and beckman's 1991 solar engineering of thermal
processes. a back reflector would return about (1-0.38)th of the sun
to the pipes, which would catch 38% of that. otoh, a black back and
sides would turn all the sun into hot air, which would also heat the
pipes, all over their surface, with an airfilm conductance of about
1.5 btu/h-f-ft^2. which is better, numerically?
>> i would greatly appreciate thoughts on this design. the new greenhouse
>> will use what heat i can glean to warm the just completed insulated
>> concrete slab floor. (two 5' by 6' slabs, 6" thick, with lots of copper
>> tube imbedded)
i'd forget the floor, and fill the space between 2 layers of poly film
with tiny cold soap bubbles at night, and let the sun heat some dark
colored water drums near the south wall during the day.
nick
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