Old flat panel system performance ......or lack of !

[Bladegrabber]

Thanks for accepting me on the forum.
i live in southern Spain and inherited a 2 panel ( 2m x 1m) siphonic system with close coupled 200L storage tank above on a stand .
The system wasn’t heating the water and I stripped it and refilled the closed loop glycol water mix but it still wasn’t working with the coil inlet only getting upto 35c so I drained it again and filled the panels and tank coil circuit with vinegar and left it to soak hoping to remove any scale which had built up including in the NRV which seems to be working ok .
Having flushed it out and refilled the closed loop it is working better with the coil inlet reaching 48c and the return / outlet typically around 25c which after a days sun gives us hot water upto 40c . Looking at the 2 panels I am not convinced their working as well as they could and using my boroscope camera through a plug / port I noticed the internal pipes and attached plates are a dirty white/ grey colour . My question is should they be that colour as I thought flat Matt black was the best colour for heat absorption from the sun? Maybe their coated in something special or maybe the paint has just been bleached off them ? Look forward to hearing your thoughts on it and maybe any suggestions as to what to paint them with if it’s needed .

thanks
Roy

[LucMan]

Please upload some pictures.

[Bladegrabber]

E8DAEF89-6CC8-484F-BFAA-7BDEFA395ED7.jpeg

[J.P.M.]

Thanx for the flix.
Who made the panels ? Any nameplate/data shots ?
A guess on how old they are ?

I've not seen such a discoloration on solar thermal panels, but my practical experience in such matters is likely much less than Lucman's, so I'd pay more attention to what he has to say about that.

But, depending on how old the collectors are, and who made them, back in the day - late '70's to mid '80's maybe, there were a lot of what are called "selective surface" type coating(s) put on solar thermal collectors. The most common of the later period were (and still are) fit for purpose. I've got two on my roof. Some of the earlier products were less so.

Something else from your photo: Sometimes, over time, the bond (hopefully a continuous braze or solder between the tubes and the fins) will fail. That metal to metal joint is essential in getting and maintaining good heat transfer between the fin and the tube. Without it, or with reduced contact if the joint breaks contact in some spots and not others, performance will be reduced.

Having written that, I took a well made collector apart (Sunearth) that was being scrapped out for the copper value and to my surprise, about half of the continuous tube to fin joint had failed. I know for a fact that the collector was operating not too far off it's design parameters until removed from service. The degree of failed joints in the units came as a surprise to me, given my (limited) experience with the usual Sunearth (good to my experience) quality but mostly because the were doing as well as they were with the tube to fin bond(s) being as deteriorated as they had become.

Such may be the value of believing everything I read in solar energy text books.

While you are correct in stating black is beautiful for solar thermal, you're improvement in performance after cleaning/flushing suggests the fading you show in the picture may not be the only thing going on.

Usually, getting at an absorber behind a sealed glazing is not an easy task. But, if you do manage to pull it off (and it can be done - I've done it a couple of times), and if you're thinking of a simple flat black spray paint job, there's the little mater of paint outgassing that may well wind up coating the inside of the glazing with the off gas material and blocking a lot of the irradiance. Kind of self defeating. If it was me (and it sure ain't), and I still wanted solar hot water, I'd make sure what you have is still leaktight and think about adding a thermal collector in series after (downstream) of the existing legacy system. The tricky part, if this is a thermosiphon system, will be keeping/ensuring that the new flow pattern keeps everything going "uphill" in the heating portion of the thermosiphon loop.

[Bladegrabber]

Hi JPM , thanks indeed for your response and feedback, the system is pressure tight ( i used to work in oil& gas production offshore ) as that was one of my first checks. Its a system from circa 1992 and from what i can see with my boroscope the fins are all still welded to the pipework inside . I take your suggestion of another modern panel in series / parallel and had thought it may work out ok but will also look at a seperate water storeage tank and making it into a pumped system so will wait and work out whats best for the $ value . Roy

[LucMan]

By the looks of your picture the absorber seems to have lost the original black coating and the fins seem to have peeled slightly away from the tubes. Either the coating has been baked off over the years, or the coating, insulation, or glass seals have off gassed and coated the collector. Older panels had problems with the original coating longevity and the method of connecting the fins to the tubing. Several years ago AET, a US manufacturer had a limited issue with the seals under the glass that were out gassing and coating the inside of the glass. You can try and scrape the the fins to see if there is still a black coating under the current oxidized coating.
Either way you should be able to pull the glazing off the panel to either clean or re-coat the absorber as needed, don't forget to clean the back side of the glass, "Zep" industrial foaming glass cleaner works the best. Check the "Build it Solar" web site for re-coating recommendations if needed.

[Bladegrabber]

Lucman, thats great clear advice thanks, i will open one panel up first and see what i find and report back.

Cheers
Roy

[J.P.M.]

Hi JPM , thanks indeed for your response and feedback, the system is pressure tight ( i used to work in oil& gas production offshore )

You're welcome. FWIW, I spent a good part of an engineering career designing various pressure vessels/systems that might well have gone on rigs you worked on.

Good luck.