Yet Another Fin Construction

There has been some ongoing discussion on the best way to build and secure the absorber tins to the riser tubes in homemade solar collectors.

A bunch of the the methods people have used are covered here:

The method detailed just below is yet another way -- I used this on my new 100 sqft collector. I like it quite a bit, and I'll use it again if I make more collectors.

Click on pictures for full size

The fins I used are made by Tom -- they are 0.018 aluminum with a well formed groove to
fit the copper riser tube.
Any of the other techniques listed here can also be used to make the fins as long as
the resulting groove is a good fit to the riser pipes.

The fins need to be cleaned and then primed as shown in this picture to reduce any likelihood of galvanic corrosion between
copper and aluminum.

To build the absorber assembly, I put a flat strip of 0.018 aluminum under the riser tube, and layout two beads of silicone on the flat strip as shown.

I put a bead of silicone along the bottom of the groove in the main fin (the one with the groove in it -- right picture above).

All the reasons why I think the silicone is very important to the long term performance of the absorber are covered here...

Please read this carefully if you are thinking about leaving the silicone out.

Now push the grooved aluminum fin firmly over the riser tube. Make sure the 3 inch wide strip under the riser tube stays centered on the riser.

Then clamp the fin tightly onto the riser tube. The clamp shown here is the one that Tom will make for you, but you can use the one Kenneth worked out, or the wood workers special that I did -- you just need something that really squeezes the fin down onto the riser, and results in little or no (paper thin) gap between the riser and fin with all the the silicone squeezed out.

The very thin remaining layer of silicone conducts heat much better than an air gap would and also bonds the whole riser and fin together into a stable, 50 year life assembly.

Now, with the clamp or clamps still in place, screw the grooved fin to the underlying flat aluminum sheet with short, self tapping sheet metal screws.

Once the screws are in place, the camps can be removed.

Make sure you space the screws close enough together to prevent buckling between screws.

If you use self taping short sheet metal screws, this goes very fast.

I did this with an old piece of carpet under the absorber to give the screws something to go into that is easy to remove the absorber from.

These could be a bit shorter, but I had a huge unused box of this size -- quarter inch would be fine.

The screws hold the fin tightly compressed to the riser tube with minimal gap. Once the silicone cures, the screws are not really needed, but they do provide some extra long term insurance that the fin and riser will remain in tight contact.

Reasons I like this approach:

Once the 3 inch flat piece under the riser and the grooved absorber fin are clamped and then screwed together the grooved fin is held very tightly against the riser tube making for a very small (and thermally conductive) gap between the riser and the fin.
While the silicone (once cured) is the main thing holding the riser fin assembly together, the sheet metal screws (unlike staples) do provide some long term insurance against the fin and riser separating over time should the silicone not get completely evenly distributed.
The flat 3 inch piece doubles the thickness of the fin in the root area where the heat flow is greatest. It also slightly increases the contact area between the riser and fin. Both of these should improve performance a little.
The thermal mass of the plywood sheet that was used under the fins on some of the earlier designs is eliminated, and this improves collector performance a bit.
This is pretty much a goof proof way of putting things together. It requires a little more time and material, but is hard to do it wrong.
I like the idea that when built this way, the absorber becomes an independent component -- it can be dropped into a collector frame that is built in place on a wall or where ever. As in this picture -- the collector frame is built directly on the wall, and the completed absorber is then hung inside the frame. This allows the frame to be built larger than one could move -- this one is 12 ft wide.

I'd like to hear any thoughts on this method (pro or con), or any ideas for improving the construction or performance -- Gary...

This is the wood version of the fin clamp.

Gary June 17, 2010