Actually sitting here talking to my son about your project.
I drew him something last night asking if he'd be able to do it. Forming a bunch of nearly full circles in the copper plate through which the copper pipe would have to pushed. The idea being to put the copper plate in as much contact with the tubing as possible. I don't think simply soldering the pipe to a flat sheet is as efficient as you can get. Too little contact with the copper sheet.
What he said was that it'd be better to press half-circle shapes into two sheets of thin copper and sandwich the pipe grid between them. Full contact all around the pipe. And a heckuva lot easier to do. Then just screw the sheets of copper together so the assembly could be taken apart if necessary later.
I suggested a 4x8x1/2" "tank" but he doesn't like that idea because the amount of water touching the heated surface is such a small fraction of the total volume.
He just showed me a drawing it'd be hard to describe in writing. But I'll try. A rectangular box like we're talking about, but more like 4' tall and about 6" wide and deep. Inside the box, a curved sheet of polished aluminum. Bent nearly into a semi-circle. Then a piece of round stock of any material that doesn't transfer heat well (so it doesn't suck out much heat), then copper tubing tightly wrapped around the stock. I'd assume wound so that there are no spaces between coils. This would be suspended above the curved aluminum "mirror" so that the light coming in would hit ALL of the coil. And hooking up an array of these.
I like this idea.
He said he'll consult his physics professor to see what shape the mirror should be and the optimum positioning of the coil, which he's currently assuming is the center of the imaginary circle of which the mirror is part.
He just clarified. The stock around which the tubing is wound is stainless steel not only because it wouldn't suck the heat out of the coils but also so it'll reflect some light onto parts of the coil not directly facing the sun. Also, he said the coil would not be wound tightly. There'd be about 1/4" gap between winds so the inside stock has some surface to reflect light onto.
The idea for the mirror isn't only to reflect and focus light all over the coil but to increase the number of hours during which it's capturing the most heat energy it can without repositioning it.
I told him that if trying to capture the most energy across the largest amount of sun movement would be a compromise, it'd be better to assume perfect alignment and capture as much heat as possible in that one position and I can take care of keeping the alignment right throughout the day.
He also suggested a thermocouple, which would turn the pump on and off automatically as the sun comes up and goes down so you're not cooling the water at night. Seems to me that'd work or a photoelectric sensor, which can easily be bought at Home Depot.
Anyway, he's intellectually invested in this project now, so I'll be curious to see what he comes up with. I'm going to try to have him and me build it soon so it can be tested as a way to put some heat into the workshop, though in that case it'd be like spitting in the ocean. But would test the concept.
Oh, I asked him, and his major is "Manufacturing Engineering", but said my description of "making stuff" isn't far off, since that's the aspect that fascinates him the most. Grabbing ideas out of the heads of people like me and turning them into real objects he can touch and observe working as intended.
He said he can probably have me a working prototype in a couple of weeks. His professors are real generous about letting him spend class time (and nights) with access to everything the school's got so he can make stuff.
Since he's got such a love and knack for making things, he's already getting started making marketable stuff (like my wonderful 4x10 welding table that cost him $350 to make but would sell for a multiple of that) and is always looking for more stuff to make/sell because what intrigues him most upon graduation is self-employment (that apple didn't fall as far from the tree as I initially thought) just making and selling things. Which dovetails nicely with my plans because I don't have a fraction of his talent for actual creation of stuff, but have a seemingly endless supply of ideas and the ability to finance his work and share in the profits. I think we could end up with quite a partnership.
Oh, and we're keeping our eyes open for a totalled Civic Hybrid because both of us are dying to tear it down and not only put the electric motor to more effective use, but see how the heck the tranny works in it.
I'd love so much to use that motor and tranny in a car but with far taller gearing, lots more battery capacity, and a very small turbo-charged diesel engine of low horsepower but similar torque to the Honda's gas engine. And with the diesel, it might be possible to make a 158-volt generator to mount on it along with the standard 12-volt, to do more charging of the electric motor's battery.
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