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re: thermo-siphon (from re: solar hot water question)
29 jul 2001
david buchner wrote:
>...the backing in the box is a piece of pretty heavy-gauge sheet steel,
steel isn't a great heat conductor. for good solar collection efficiency,
you might (arbitrarily) try making the back plate thick enough that the
thermal resistance from the plate to the air in the box is ten times the
resistance between the plate and a tube, measured from the line halfway
between the tubes.
if the resistance from plate to still air is us r0.7, with a perfect
thermal connection between the tubes and the plate and a 1' tube spacing,
the plate needs r0.07 max across a 6" width with a 1' length and a d inch
thickness, ie a d/12 ft^2 cross sectional area. steel has a 26 btu/h-f-ft
thermal conductance, ie r0.0032 per inch, ie r0.019 for a square foot 6"
thick, so we need 0.07 = 0.019/a or a = 0.27 ft^2 = d/12, ie d = 3.25",
ie a steel plate 3.25" thick...
remember that thermal mass problem? steel has about the same specific
heat by volume as water. reheating a square foot of steel 3" thick from
40 to 120 f every morning takes about 1x3/12ft^3x64btu/f-ft^3(120-40)
= 1280 btu, ie about 5 hours of full sun before the collector starts
to supply useful heat.
copper has a 226 btu/h-f-ft conductance, so a copper plate would only
have to be 26/226x3.25 = 0.37" thick.
of course you could decrease the tube spacing.
>i used a bunch of those little copper strap doohickeys and rivets
>to hold the pipe down to it fairly tight. there's still space, though,
>so i don't know how effective it is. i wonder if there's some kind
>of goo i could glurp into those gaps...
solder it down? or maybe use some of that hard black high temp furnace
cement, vs the kind with fibers. or maybe some mortar, or sand and "water
glass" (sodium silicate.) anything's better than air.
or make your collector a flat shallow water trough under a mylar film
concentrating parabolic reflector :-)
nick
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