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I'm brainstorming a shop building with built-in solar panels (ala the Solar Shed). I see most header design panels the tubes run longitudinal (up and down), as opposed to thermosyphon collector's lateral tubes. I'm not designing a thermosyphon system. But is there a valid reason for longitudinal over lateral collector tubes? I would imagine the expansion of the tubes to be a design factor, but I would think the reduction in Tee joints would be an asset in design. Let's hear your theories and facts.
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A vacuum insulated tube (most commonly seen with headers) absolutely must have a vertical component to its orientation. They are designed as heat pipes. The inner tube contains a fluid (alcohol, water, acetone) which evaporates in the body of the tube, condenses at the top when cooled at the upper header, and runs back to the bottom where it is vaporized again.
SunnyBoy 3000 US, 18 BP Solar 175B panels. -
That's interesting in all, but I'm not interested in a Vacuum (Evacuated) Tube collector. My questions pertain strictly to a flat plate (Harp style) collector designs with bottom pipe, risers, and top collection pipe (header). Sorry for any confusion.Comment
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For flat plate design for use in a pumped system, it's common for the parallel tubing (the "risers") to be vertical with the inlet at the lower end of the collector on one side, and the outlet at the top on the other side. However, that arrangement is not essential. It is thought that such an arrangement does make things easier for drainage, but again, it's not essential. Having the inlet/outlet on opposite sides of a panel improves the flow distribution and is often easier to plumb, but not always, and helps to avoid or reduce the effect of (but does not eliminate) "dead" or hot spots caused by maldistribution of flow. Try to keep opposite side inlet/outlet in any case.
Orienting a collector so that the risers are mostly horizontal is perfectly acceptable as long as there is enough slope for drainage, and the in one side - out the other is maintained. The same goes for the vertical riser orientation with respect to adequate slope for horizontal header drainage (as well as ensuring nothing gets left in the lines after drainage to freeze). A rise (slope) over the length of a horizontal run equal to the horizontal line's diameter is usually sufficient - so a 1" line section is ~ 1" higher at its start than at its finish.
The differential thermal expansion of the tubes with respect to what they are attached to - including the absorber plate(s) is a very big consideration, especially when trying to design the tube to absorber fixation method. Simple clamping leads to poor heat transfer and poor performance, not unlike poor performance of electrical contacts not firmly affixed. The best thermal joints are welded, soldered, brazed, etc., depending on materials. If you simply clamp the tubing to the absorbers, you'll be wasting materials, and thermal efficiency of the panel will be penalized something like 15 -30 % or so, maybe more. if copper tubes/headers and Al absorbers are used, the aluminum will be cathodic to the copper and corrosion problems will probably, sooner rather than later, impair performance further. Stick with all copper and solder the tubing to the absorbers.
If you're interested, see Duffie and Beckman. Their explanation of flat plate thermal design is considered by many to be the best.
Short tip: Consider 1/2 " risers, 3/4" or 1" headers, ~ .007" or so copper sheet/strip, 6" wide, spot soldered ~ every 3"-4 " or so. Insulate the back/sides of the collector w/2" or more of outgassed fiberglass (not a plug, but see Knauf insulation for outgassing considerations). Low iron glass would be nice, but hard to find for DIY.
See builditsolar for ideas and some maybe some of what not to do.Comment
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Thank you. 'Many interesting points. I just bought Duffie and Beckman's, Solar Energy Thermal Processes.Comment
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You are most welcome. Education will help keep you out of trouble - but no guarantees.
BTW, that title is actually the 1st ed., copyrighted 1974 (ISBN # 0-471-22371-9).The information is still good, but the newer editions (1st ed., 1980, ISBN # 0-471-05066 or 2d ed., 1991, ISBN # 0-471-51056-4), and subsequent eds. have a slightly different title: "Solar Engineering of Thermal Processes", and a lot more information. But again, the original title still has most of what you seem to be looking for. There are later eds. after the 2d as well. as well.
Depending a bit on how familiar you are with the subject, and not suggesting you put the cart before the horse, but Chap. 7 in the old edition (and Chap 6 in the newer eds.) has most of the meat on flat plate design you're probably looking for. FWIW, I've verified more than some of what's there through construction and measurement, or at least enough to trust most of the rest.
Holler back with any questions, or if I can be of any interpretive help.
Good luck.Comment
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Thanks again. I figured it was an older edition. New was around $125+, after that I reasoned I was due to visit the local library. The used copy I ordered was $6
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You're welcome. Sounds like a wise choice to me. The info with respect to the basics of flat plate design are valid as presented in any of those eds.
Except for heat exchanger penalty factors, the basics of flat plate design haven't changed probably since the early '50's or before.
Note the short paragraph on P.142 between eq. 7.5.15 and 7.5.16 dealing with the importance of absorber to tube bond conductance (Cb), dealing with why simple clamping is basically ineffective , and also how Cb affects F' and ultimately collector efficiency through eq. 7.5.17.
You can always spend the extra $$'s on newer versions if/as the need arises. The 1st ed. will keep you out of trouble.
Regards ,Comment
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