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Dry stagnation temps in drainback thermal can be damaging? (11 Posts)
Dry stagnation temps in drainback thermal can be damaging?Over the past few weeks I've been reconditioning some old Solaron collectors. The collectors were originally installed atop a university building in Kansas, I believe in the 1970s. I don't know all the details of the system but it was 200+ collectors. It never worked properly and was removed a couple years ago when the roof needed repair. I assume the collectors have been allowed to stagnate for years. Some of them showed signs of previous leak repairs, others I had to repair due to physical damage. The absorbers are copper tube on copper sheet, soldered where the tubes lay in formed channels on the sheet. When I had them apart I noticed the solder looked like it had melted and actually dripped in some places. It was ugly but I thought they must have been manufactured that way. There still seemed to be a decent mechanical and thermal bond so I didn't think much of it.
Last week I got some of the collectors up on the roof. Within an hour or two they were at 380 degrees and still climbing. I didn't expect they would ever get that hot. I took a sample of the solder from a spare collector and it seems to be normal 60/40 with a melting point of 370. So now I know why the solder on the absorbers looked like it had been dripping, it was. At this point there's not much I can do. I had been under the impression that dry collectors would not be damaged by overheating, one of the advantages of a drainback system. I sized the system for my winter space heating load, assuming that the collectors could sit dry for most of the summer without damage.
I guess I'm just looking for any comments or suggestions. Any damage to the collectors has already been done, and was surely done years ago. It just doesn't seem right to have the solder in the collectors becoming liquid every day when the sun comes up.
stagnationhere is the math for potential collector stagnation temperatures, using SRCC test data numbersl. Full sun day, at 85F ambient.
Newer collectors will have the absorber welded to the tube. Forge weld, ultrasonic or laser are the 3 most common methods used today.
Soft solder is on the borderline, if you are actually seeing that temperature. Perhaps a 95/5 with an advertized melt point of 430- 450F or so.
SilPhos would be best if you need to re-do over 1000F depending on the alloy.
...What is "I" in that formula. I assume it's insolation but where does
that number come from? I have the specs for single glazed Solaron
collectors, mine are double glazed, not sure how much that's going to
affect the end result here. Using I=317 as provided in the sample, with
the single glazed Solaron numbers I get 374 degrees stagnant. That's
real close to the 380 degrees I measured on the thermistor. I have
thermistors in every collector, clamped to the internal header pipe near
the outlet. I'm going to hook up a data logger this week and record a
full days temps along with ambient.
It's too late to re-solder
the absorbers, and it probably couldn't be done without destroying the
black chrome plating anyway. They're already mounted and they're not
coming down in my lifetime. If the system is a complete failure I'll
leave them up there as a daily reminder of my mistake.
I is forInsolation or the intensity of the solar radiation striking the collector absorber. It s usually expressed as KWh/ meter sq. on SRCC data sheets.
317 is often refered to as an ideal "full sun" condition. 317 BTU/hr./ sq. ft. That number constantly changes and the weather conditions, positioning of the collector, etc will dictate what actually strikes the absorber.
Here is on site to get that info specific to your area. http://www.solmetric.com/annualinsolation-us.html
I suppose those collectors will still perform, if the tube is in a tight crimp to the absorber that is a plus. Plenty of homemade collectors with bare copper tube in a wood frame, no absorbers. There are all levels of construction for collectors :)
To get accurate, actual data for that system you could install a BTU meter and data logger.
Most of the new solar controllers, Caleffi included :) have a OHQM "option heat quantity measurement" function. Enter the flow rate from a visual meter reading, add a sensor to the return piping, now you have the info, delta T and flow rate, to get a fairly accurate number expressed in KWh on the control screen.
I've found this to be real close to what the more expensive BTU meters will provide and it is built into the controller, just enable the function.
Ahh,they are looking for insolation per square foot. Most charts I'm seeing are in square meters, that threw me off. It's probably been two or three years since I did all the calculations for this project, and I never looked at stagnation temps.
I have a Tekmar 157 variable speed controller with the BTU measuring feature. It will be quite a while until I have it running, lots of piping and wiring to do yet.
Thanks for your input.
pressurized DBis the way to go if you have a large array and a varying, and low, summer load.
Some installers run a 30% glycol in the DB in case the entire collector does not drain. Not my favorite method, but still better then a pressurized glycol that stagnates all summer long.
There are some nice DB functions in the solar controllers now, they help eliminate short cycling on first start up. The Grundfos 15-100 is a nice DB pump, high head, low flow.
It there a way....to run it in the summer and dump it into a pool or hot tub as a dump load?
I'dhave to buy a tub or pool first..... not really interested in either one. Manufacturer's docs recommend an outdoor fan coil for long periods of stagnation, or drain down for short periods. A fan coil seems like a big waste of energy. I think I'm just going to go ahead as planned and let them stagnate.
Not likely to be 60-40 solderIt was well known even back then not to use tin-lead solder inside a collector box or anywhere near the collector. The Solaron/Radco collector wasn't the best quality, but I'm 90% sure that 95-5 was used to solder on the fin. In 1983, Solaron started using an ultrasonically welded fin assembly.
Hey, they look really good in the photo. Glass, copper, and aluminum can be relied on to hold up over the years, eh?
Drainback was a good choice.Superinsulated Passive solar house, Buderus in floor backup heat by Mark Eatherton, 3KW grid-tied PV system, various solar thermal experimentsThis post was edited by an admin on September 14, 2013 1:44 AM.
...Yes, the panels seem to have aged pretty well. I do worry about the absorbers. I don't know how much erosion may have taken place, or internal corrosion from acidified glycol, and the copper seems extra brittle. They should work well for now, but I fear future failures. I painted the frames black, I didn't like the 1970's extruded aluminum look.
I don't really have any more pictures, there isn't much more completed yet. I have a 750 gallon insulated open-top fiberglass tank, I can't decide if I should put it underground or inside the building. I'm leaning toward underground, with a concrete lid, and an underground pump well next to it.
Pix of...the panels look good. Got any more of the rest of the system?