Solar Collector -- Small Panel Test -- Conductive Silicone

This is another in the series of tests using small collectors to measure the effect of design changes on DIY solar water heating collectors.  This test is aimed at determining if it is worthwhile to use a silicone caulk with a high thermal conductivity when sealing the small gap between the copper riser tubes and the aluminum fins.   This issue of what is best to use between the copper tubes and the aluminum fins is discussed in detail here...

The original set of small panel collector tests is here...

The conductive silicone I used is made by Silicone Solutions -- this is the data sheet:

silicone solutions conductive silicoen
The specification sheet for the high conductivity silicone.  The thermal conductivity is about 3 times greater than regular silicone caulk.

conductive silicone caulk tube
This shows the absorber with the conductive silicone caulk before it was painted black.

siliconecaulk and fin
This shows the copper tube to aluminum fin joint with the silicone fill.  The critical thing is to keep the gap paper thin.
It does not show in the picture, but there is a 3 inch wide strip of flat aluminum just under the tube and aluminum fin.  To assemble the fin, a small bead of the silicone is placed in the fin groove, then the fin is clamped very tightly onto the copper tube, then while still clamped, the grooved aluminum fin is screwed to the underlying 3 inch wide flat piece of aluminum.  See here for details on the assembly...

collector test setup
This shows the 3 collectors each heating its own reservoir.

The two collectors are as identical as I can make them except for the differences being tested  -- in this case one with regular silicone caulk between fin and tube and the other with conductive silicone caulk between tube and fin.  The third collector is the baseline copper tube soldered to copper fin collector that is included in all tests.

The water reservoirs are of identical size and construction, and the water charge is weighted into each of the reservoirs to insure that they start with equal amounts of water.  The temperature of each reservoir is logged, and the performance of each collector is in direct proportion to the amount it is able to heat its reservoir during the test.


Conductive silicone vs Regular Silicone

The plot shows the results for a 3 way side by side by side test of:

1 -Black solid line: A reference collector with copper risers soldered full length to copper fins.

2 -Red dash line: A collector made with copper risers and aluminum fins with regular silicone between fin and riser

3- Green long dash line: A collector made with copper risers and aluminum fins with a conductive silicone between fin and riser

The conductive silicone shows a temperature gain that is 0.5% better than the regular silicone.  This is a pretty small change and may well be just experimental variation.

So, based on this, I would say that its not worth going to much of any extra trouble to find a conductive silicone caulk. 


Gary May 26, 2011