backing plate thickness for DHW collector

[strider3700]

I'm starting to think about putting together a few solar collectors to help with the heating of my hotwater.  I'll be running glycol in the collectors due to the possibility of winter freezing.   What I had been thinking of was a simply glass covered box with copper tubing running through it.  The tubing would be soldered to a backing plate to increase the surface area being hit by sun.   Your pretty typical homemade collector.  

Anyways I was worried about corrosion taking place if I attached the copper pipe to a steel backer and then remembered that I have a roll of copper left over from insulating an MRI machine.  The issue with the copper is it's thin,  think double the thickness of your standard household tin foil.   I'm not sure if this helps or hurts in the long run.   I think that being thin it should get hot fast but I'm not sure how it works when it comes to transfering the temperature to the tubing/glycol.  

Basically do I want a big thick plate to hold lots of heat  or do I want a quickly heating thin one?

[jimjjnn]

Thin is good for quick heat transfer as it gives up heat fasy. Thick would hold heat longer and give it up slower. Both would work tho.Others can add to this as I just abbreviated my thoughts on this.

[rpcancun]

why do you need to heat hot water?

[strider3700]

Because the more solar generated heat that goes into the water the less electric generated heat is needed.  Since my electric hotwater tank is about 40% of my power bill these days I'd like that to go down.

[GaryGary]

Hi,

Some of the commercial absorber plates use copper backing that is 0.007 inch thick.  That might give you an idea.  

The purpose of the copper backing is to absorb the solar radiation, and then to transfer it by conduction along the copper to the tube.  The thinner copper should absorb the solar radiation just fine, but the thinner the copper backing, the higher its resistance and the more temperature differential is needed to transfer the heat along the fin to the tube.  The higher fin temperatures make for more losses out the glazing.  If your copper is thinner than the 0.007, you could space the tubes closer together, or just take the (probably small) decrease in efficiency.  

Basically, you want the fin to be thick enough to do a good job of transfering the heat along the fin to the tube -- its not a good idea to make it so thick it stores heat -- storing heat IN the collector is almost always a bad idea.

If you can provide the copper thickness, I have a method from the "Active Solar Collectors" book to estimate fin efficiency.

Gary

[Ungrounded Lightning Rod]

Thicker is better - though too thick is a waste.

Thin has more thermal resistance from the place where the heat is collected to the place it's delivered, slowing the transfer.  This means the part farthest from the tubes gets hotter - possibly enough to warp things or reduce the amount of heat collected by letting more reradiate back through the glass cover.

If not for that last (and raised thermal losses in the plate->air->housing->surroundings path) the raise in temperature would essentiall make up for the higher thermal resistance and things would stabilize at the same heat collection rate.  Net result of that effect is that thickness donesn't matter a whole lot.

I'd use something about the thickness of flashing if I had it, but since you have this handy give it a try.  Having it there is a LOT better than not, and if it's so thin it overheats in the middle enough to be a problem you can always use more tubes spaced closer together.

[strider3700]

  It's 3 ounce copper   I think that works out to 0.0032 inches?  Like I said  thin

Would it be worth doubling the sheets up or would the air gap make it a waste of copper.  I have a big roll but really really don't want to be using a lot of it for this if it's not worth it. I'd rather drop the money and pick up something else.

Thanks