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Questions on Flat Plate Collectors

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Posted by zeb on July 25, 2007, 7:18 am
Hi all,

I have been contemplating diying a flat plate solar collector for
domestic water heating.

The various designs that I have seen on the net, are based on tubing
which has been pressed/embeded  into metal sheets, which have either
square (or semi-circular) channels made in them. These sheets form the
solar collecting surfaces for solar radiation/energy.

Having given these some thoughts I have the following questions that
require answers.

Q1. Efficient transfer of heat from the solar collecting surfaces to
the liquid in the pipe would depend upon a good thermal contact
between them. Can the arrangement above assure such a contact without
soldering/brazing between the pipe and the sheet metal.

Q2. Water entering from one side of the pipe would progressively
collect heat, and consequently its temperature would rise. Thus heat
transfer at the exit end would suffer. Would it not be more effcient
in terms of heat collection, if cold water entered from both sides and
exited from both sides. This will mean having a cold and a hot water
header pipe on both sides, rather than the existing practise of one
cold and one hot on each side.

Following from Q1 above, I propose a system based upon a collecting
pipe with sheet metal soldered/brazed radially as heat collecting
surface, like this  " --O--".

This would ensure better thermal contact with the pipe thus improving
heat transfer.

Comments would be greatly appreciated.


Posted by Robert Scott on July 25, 2007, 11:12 am

Soldering or brazing may be more efficient, but at what cost?  It limits you as
to your choice of materials.  What if you could achieve the same increase in
efficiency by placing more tubes slightly closer together?  You might get just
as much of an improvement in efficiency with less cost.

Again the issue is cost-effectiveness.  If the flow rate is increased, the delta
T between input and output can be reduced and thus the problem you mentioned
would not be so bad.  Having input and output connections on both sides sounds
like it takes a lot more plumbing to integrate into a system.

Robert Scott
Ypsilanti, Michigan

Posted by Mike Scaife on July 26, 2007, 8:56 am
Have you priced copper pipe recently, If you use less pipe and soft solder
thin copper foil across the riser tubes makes very efficient heat transfer.
I use 0.25mm soft copper foil, available in rolls of 300mm width, just push
it by hand across the tubing to achieve the partial wrap to get good heat
transfer when soldered. Its quicker than pop riveting heat collector fins
with hundreds of rivets.

Use a reasonable water flow and the delta input to output is only 8 degrees
across 7 sq meters of panels in noon summer sun.

Mike Scaife (NZ)

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