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re: small scale solar hot water heating
23 dec 1998
tssinc@nospam.idt.net (jerry chase) writes:
>copper flashing should also be available...
copper is such a good thermal conductor that some people have built good
solar collectors with thin copper foil attached or soldered to small
copper pipes on a fairly close spacing, with a foamboard backing. the
closer the pipe spacing, the thinner the foil can be. aluminum is half
as good a conductor as copper, and 4 times better than steel...
>...every metal has a different rate of thermal transfer and a different
>rate of expansion when heated... those that transfer energy more quickly
>will increase the efficiency of the unit. metals with different rates
>of expansion will break your solder joints over time.
how about clamping some thin metal around a copper pipe? and arranging the
mechanical "preload" so that when the assembly is hot, it is still in good
thermal contact, using the expansion rates and elastic properties of each
metal, bolt sizes, and so on? this works for automobile engines...
amckegne@cgocablenospam.net adds:
>making a good conductive connection between copper tube and an
>aluminum fin is a trick.
fin-tube radiator pipe is interesting. as the copper pipe warms and
expands, the fit to the aluminum fins gets better. i guess this would
work the opposite way if we tried to use it as a solar collector. as
the fins got hotter than the copper pipe, the fit would loosen...
>...the parts should be close together to begin with - ie a roll-formed
>"clip-on" fin. you need mass-production to justify the cost of the
>tooling. it has been done several times.
one application with tooling is big fins, dark-painted aluminum extrusions
with c shapes on the back that snap on to hard copper pipes. they have no
glazing or insulation of their own. they can make solar hot water when used
in a sunspace with a warm-water convection loop through an insulated tank
above, which might contain an electric heating element that rarely turns on.
can that tank be an ordinary $200 water heater? i wonder if the fittings
are large enough and the internal geometry is appropriate for natural
warm-water convection. and i've heard some water heaters have plastic pipe
for the cold water supply, and supplying hot water to the cold input can
hurt the feed pipe and void the warranty.
this water heating system seems simple. no pumps or electricity or heat
exchangers or antifreeze or insulation or big boxes to haul up to the roof,
and the fins can provide overhead summer shading for a sunspace with a
transparent roof. just solder together the "frame" (the pipes) and snap
on the fins. i saw a very attractive installation near princeton. i wonder
why they haven't caught on more. zomeworks still makes them, but they seem
rather expensive for what you get, at something like $10/ft^2.
i'm wondering about the performance of cheap imitation big fins, something
like brown-painted thick aluminum coil stock, ie roofing material (approx.
50 cents/ft^2 in 50' coils, a foot wide and 0.020" thick), bent into a u
around some 3/4" hard copper pipe, then sandwiched and bolted below the pipe
with some drywall l-bead on each side, then spread out flat again, with the
pipe as a bump in the middle, facing the sun, and the brown side of the
aluminum towards the sun.
these might be protected from freezing by filling a sunspace glazing cavity
with bubbles at night, or leaking enough heat from the house to keep the
sunspace from freezing (especially if it contains plants.) or using a couple
of electric strip heaters attached to the lowest fins with a couple of
freezestats on the highest, to make another water convection loop. or using
a couple of heat tapes with thermostats, or a couple of solenoid valves with
freezestats that open to allow water to flow out of the fins when they get
close to freezing.
hmmm. ice floats, right?
how about something like this?
. .
s . . . .
aluminum flashing ssbss . . 1/2" . .
.........................o... . copper . .
.........................l... . pipe . .
sstss . . . .
sssss is 1/2" galvanized s . . . .
drywall l bead --- . . .
.
<-- south | .
.
...then open it up like this: .
. . .
.s . . . .
home depot sells dark- .ssbss . . . .
brown-painted aluminum ~5" ...o... . copper . .
roof flashing for $46.50 ...l... . pipe . .
for a 50'x2'x0.027" roll. .sstss . . . .
.s . . . .
. . .
[use courier font.] .
| . this solar collector might be
. 60% efficient heating 120 f
--- . water inside an 80 f sunspace.
10 ts=25'sunspace temp (c)
20 tw=50'water temp (c)
30 i=1000*.9'peak insolation (w/m^2)
40 fw=.138'fin width (m)
50 fl=1'fin length (m)
60 ein=fw*fl*i/2'peak sun power into half-fin (watts)
70 sigfa=8.5'still air film thermal conductivity (w/c-m^2)
80 gfa=sigfa*fw*fl/2'half-fin-->air conductance (w/c)
90 tt=ts+ein/gfa'thevenin equivalent temperature (c)
100 thkae=.027'aluminum thickness (in)
110 thka=.0254*thkae'aluminum thickness (m)
120 siga=211'aluminum thermal conductivity (w/mc)
130 ga=fl*thka/(fw/2)*siga'half-fin al thermal conductance (w/c)
140 rf=1/gfa+1/ga'half-fin thermal resistance (c/w)
150 u=(tt-tw)/rf'useful heat into water (w)
160 eff=100*u/ein'solar collection efficiency (%)
170 print "sunspace temperature (c):", ts
180 print "hot water temperature (c):", tw
190 print "half-fin peak sun power (w):", ein
200 print "thevenin temperature (c):", tt
210 print "half-fin --> air conductance (w/c):", gfa
230 print "solar collection efficiency (%)", eff
run
sunspace temperature (c): 25
hot water temperature (c): 50
half-fin peak sun power (w): 62.1
thevenin temperature (c): 130.8824
half-fin --> air conductance (w/c): .5865
half-fin --> pipe conductance (w/c): 2.097157
sunspace temperature (c): 25
hot water temperature (c): 50
half-fin peak sun power (w): 62.1
thevenin temperature (c): 130.8824
half-fin --> air conductance (w/c): .5865
solar collection efficiency (%) 59.69448
the efficiency might be increased with the flat side facing the sun,
and a layer of foil on the back.
nick
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