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Old 06-08-11, 10:58 PM   #1
blahname
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Default DIY Flat Plate/Concentrating Collector

Hi all, I've been contemplating building a few flat-plate solar heat collectors for a domestic hot water and in-floor heating system I hope to build quite soon. There's some directions over on builditsolar that show how to build something with either PEX or copper pipe bonded to 6" aluminum flashing roll with silicone, stapled to a plywood back-plate. Link: The $1000 Solar Water Heating System This all gets mounted in an insulated box, and glazed with polycarbonate sheeting.

Kudos on the design, however the silicone joining the collector plates/fins can't transmit heat very well, and a metal-to-metal bond would be better. Commercial $1000+ flat-plate collectors usually have a copper collector fin or sheet welded to a copper pipe somehow, thus achieving much better thermal conductivity.

Then there's the cost. A 6" x 50' roll of aluminum sheeting is $20-30, where the same thing in copper is more like $120-150. As for attaching the cheaper aluminum sheeting to the copper pipe, I've found a way to braze them together, but the process is *very* tricky and often results in melting through the thin aluminum sheet rather than getting a good bond. Also, the brazing rods retail for $5 for two 10" rods. Cha-ching!

All-copper? If I buy copper sheeting instead of the cheaper aluminum, I could just solder the sheets to the pipe and have a wonderfully conductive length of plate collector to paint black and install in my glazed box. I also wouldn't melt through anything and make holes and extra work for myself.

Concentrating collectors... After some research, a concentrating collector would be better for me, as I live in a cold winter climate (in Ontario, Canada). I'm trying come up with something in between a cheap flat-plate design and a more costly concentrating collector.

Here's a drawing of what I was thinking of:



Basically, I would use cut-in-half sections of 4" PVC pipe, lined with something reflective, with 1/2" copper pipe, painted black, suspended above the curved, reflective surface as in the drawing (end view). My hope would be that the pipe soaks up as much of the direct and reflected heat as possible, and gets much hotter than just a plain-old black plate would get, especially on colder days.

Any thoughts on my approach?

I think the design might work, but I'm not sure what to use for a reflective material. Custom-made glass/mirror seems absurd for a cheap DIY project. I thought about using polished stainless steel sheeting, but it's difficult to get, hard to keep shiny when you work with it (scratches easily) and costly, although almost mirror-like in reflectivity.

I almost think that the cheap 6" x 50' roll of unfinished aluminum might do the trick, if I can get it mounted cleanly to the inside of the 4" pipe (which is basically just a form) and maybe polish it up a little.

Is this crazy, or crazy enough to work?

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Old 06-08-11, 11:17 PM   #2
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Apparently a chunk of the cost in bonding aluminum fins to copper tubing is that copper and aluminum can have an electro chemical reaction if it's not a perfect bond, it will more or less eat away at the metal ruining your investment.
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Old 06-09-11, 07:25 AM   #3
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Interesting idea, but if you don't mind I'd like to poke some holes in it.

First off, Gary did do some testing to see the efficiency hit from going with copper-copper to copper-aluminum, and there was only a ~4% efficiency loss. For the price of a copper-aluminum panel, you can easily make one more panel to make up the difference in heat output and still be way under cost for copper-copper of the same output.

Second, you want a parabolic lense to properly reflect the sunlight at the pipe. A semi-circle will not do nearly as well.

Third, PVC will melt at the temps a solar hot water panel sees, especially during stagnation.
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Old 06-09-11, 08:03 AM   #4
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Thank you all - good points indeed. I agree that a parabolic reflector, especially one with a tracking mechanism, would be best, but the spirit of the design is to get some reflective benefit, while keeping the costs extremely low and the build relatively simple.

Regarding the PVC pipe, I'm thinking of using it for a form, but probably not including it in the final assembled panel now. Properly bent/assembled aluminum should hold itself together.

I guess the big question to answer is will the semi-circles reflect enough/any sunlight on the back and sides of the suspended copper pipe to make it worthwhile. If they do, it's possible that this rig would produce hotter water at lower ambient temperatures that plates alone would.
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Old 06-09-11, 08:55 AM   #5
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If I had to guess, I'd think that a full black matt is going to absorb more sunlight than a bunch of semi-circles. The reasoning being that a lot of sunlight is going to be reflected right back out of the collector. If they were actual parabolic lenses I would be inclined to think that it might capture more energy.
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Old 06-09-11, 10:06 PM   #6
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Default Parabolic Plug

Here I put in a plug for just a bit of math: designing a parabolic reflector really isn't too demanding.

The Creative Science Centre - by Dr Jonathan P. Hare
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Old 06-12-11, 10:05 PM   #7
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Hi,
One thing to think about on this design is to what extent a parabolic reflector will focus energy on the copper tube as the sun moves over the collector during the day. Generally, parabolic reflectors have to be tracked to keep there focus on a small receiver (the tube). As in George's collector: George's Workshop Solar

There are CPC (Compound Parabolic Concentrator) reflectors that will concentrate heat on a receiver for a longer period without tracking -- if you Google CPC reflectors there is a lot out there on them.

Gary
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Old 06-13-11, 03:15 AM   #8
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Hi
I have made many types of solar hot water collectors over the past 5 years or so, generally I have found its easier to use thin copper foil (0.25mm thick) fins soldered to the risers than using aluminum. Even though AL. is cheaper its long term reliability in this application is suspect (20 years +), ie the different rates of expansion between the differing metals will eventually work the fins loose, or cause corrosion failure if they ever come in direct contact. Copper foil is not that expensive, $26 per Kg here in NZ if purchased in a 50 Kg roll, will make many panels, go halves with someone else.

Cheers
Mike (NZ)
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Old 06-13-11, 10:19 AM   #9
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Quote:
Originally Posted by Solar Mike View Post
Hi
I have made many types of solar hot water collectors over the past 5 years or so, generally I have found its easier to use thin copper foil (0.25mm thick) fins soldered to the risers than using aluminum. Even though AL. is cheaper its long term reliability in this application is suspect (20 years +), ie the different rates of expansion between the differing metals will eventually work the fins loose, or cause corrosion failure if they ever come in direct contact. Copper foil is not that expensive, $26 per Kg here in NZ if purchased in a 50 Kg roll, will make many panels, go halves with someone else.

Cheers
Mike (NZ)
Hi Mike,
The thing that discouraged me from the all copper collector with fins soldered to copper tubes was the time involved in the soldering. I did one small test collector, and it seemed like it was going to be a whole lot of work to do a full collector.
I did try to gather all the info I could on what the likely life of these aluminum fin collectors might be and put it all here:
Galvanic Corrosion with Solar Collectors Using Copper and Aluminu
I was encouraged enough by what I found to go ahead and make all my collectors in this way, and have talked a lot of others into doing the same -- so I certainly hope they hold up Everything I was able to find from the ones made in the Maine Solar Workshops and the ones made by SunRay seems to point to very few problems.

I did take apart my 2 year old PEX and aluminum fin collector when I went with the new system. 2 years is not a long time, but every thing looked very pristene -- it was quite difficult to break the silicone bond between the PEX and the alumium. I cut right through the fin/PEX assembly in half a dozen places to see if there were any voids, and found none.

Gary
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Old 06-13-11, 11:14 PM   #10
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Hi Gary

Your collectors will be fine, the silicon, acting as a thin flexible bonding agent once set will still allow some differential expansion whilst keeping the fins stuck to the risers, filling the voids and effectively sealing out any moisture ingress; especially if copper risers are used.

I found copper fins easier to work with than AL. easier to bend, very fast to solder after clamping and using a gas torch and a bar of stick solder; but possibly requires more specialist tools. Also thin soft aluminum coil stock is not readily available in NZ, unless purchased in 250 Kg rolls, the alternative is a harder alloy and more difficult to work with.

Cheers
Mike

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