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re: cheap hot water
15 nov 1998
jcollett   wrote:

>hey everybody. first time poster here.

hey j, welcome to the froup :-)

>i'm just getting into this solar water heating idea... i figure 30
>each 20' sections of 2 inch copper pipe(painted black) (in series)
>will hold abt 46 gallons.

i figure about 105 gallons. maybe you could use fewer or larger pipes,
and you might not want to paint them, since copper oxide is a somewhat
selective surface, ie it radiates heat less easily than paint. 

>what i haven't figured out is how to enclose/insulate the roof-mounted
>collector pipes cheaply.

you might put them in an insulated box with a solar air heater below, say
a single layer of r1 polycarbonate replex plastic with 90% transmission,
and a layer of dark window screen inside, 8' tall x 12' wide, to gather
0.9x8x12x1200 = 104k btu of solar heat on an average december day in
philadelphia and lose 6hours(120-30)8x16ft^2/r1 = 52k, for a net gain
of 52k btu/day. it would lose less heat if located inside a warm sunspace.

this could work efficiently if the pipes have a total surface on the order
of 960 ft^2, about 10x the glazed area, but 30 20' 2" pipes only have
about 300 ft^2, and they won't fit into a 12' box. hmmm. how about 500
10' 3/4" pipes, or 35 10' 3" pipes and a pv-powered fan to move 500 cfm
of air past them at 10 mph?

>considering some type of home-made parabolic reflectors, but no good
>ideas yet.

putting a linear crude fresnel parabolic reflector under the shoebox full
of pipes might be interesting. made from 1/2" foil-faced foambord with 
slots on the back and a clear cover to protect the foil from the weather.
you could end up with something like this :-)

   the most serious mistake was making the outer container of the receiver
   of plywood. we thought that the plywood would be sufficiently insulated
   from the copper panel which was the receiver proper, that it would not
   get too hot. the copper panel was separated from the plywood by 4" of
   fiberglass insulation. nevertheless, the plywood caught fire and the unit
   was completely destroyed. we suppose this is a success, of sorts...

   from "a solar collector with no convection losses," (a downward-facing 
         receiver over a 4:1 concentrating parabolic mirror) written by
	 h. hinterberger and j. o'meara of fermilab, ca 1976

>the collector pipes could be lowered slightly on one end so that they
>could be drained easily if needed thru a valve. (we get abt 15 nights a
>year below freezing here.) 

you might wrap one of the lower pipes with heat tape with a thermostat,
or forget the pipes and use an old auto radiator with an electric fan to
collect warm water and store it in a warmer compact tank in the basement.

have fun :-)

nick




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