DIARY ENTRY for 11 May 2019
PV Panel Repair Seminar
| ||On Saturday I spent the afternoon visiting Pierre Bolduc at his home southwest of Calgary, Alberta. More than a dozen of us had descended on his secluded house in a birchwood forest to look at a tremendous array of solar panels, and hear an interesting idea about repairing and reusing defective ones.|
|After the tour of Pierre's system, he introduced us to the kinds of defects that he finds in solar panels. Having installed more than 200 on his house, he had plenty of experience in repairing modules. In his opinion, a high rate of manufacturer's defects still exists in solar panels that can usually be found in the interconnection box on the back. While many folks believe that the main vulnerability of a solar panel would be the shattering or flexing of the glass, Pierre contends that no amount of care in shipping and handling of the panels will prevent the defects that are built-in at the factory, or arise from errors in installation. When these happen, there are "weak spots" in the design of solar panels that he has found to be at fault so often that it's not worth looking elsewhere first. When connecting his system's panels, he found many such defects in panels that looked perfectly intact.|
Pierre demonstrated the behaviour of a solar panel with failed connections in its junction box for us before diving in to attempt repairs. Discovering the defective panel was readily done - he connected a volt-meter to the power leads, pointed the panel at the sun, and showed us a disappointing 23 Volts from a panel that should have been delivering at least 35. To drive the point home, he connected the panel to a large resistor bank, so that while the current flowed he could again measure the voltage and current at the same time. Then the voltage dropped below 22 Volts (well below the target 26 Volts) and the current was only 4 Amps, well below the 8.7 Ampere rating of the panel. Tuning the resistance of his resistor bank to find a better power setting was fruitless. This panel could barely deliver 100 Watts when its data plate rates it at 250W.
Before taking the panel down to discuss repairing it, Pierre explained the connections of the cells in the panels. The one he was demonstrating with had 3 strings of cells. He then challenged us to put some shade over some of the cells to see the effect. When we did this, we could easily see that by covering just a part of some cells, the output of this panel dropped - even worse than it already was. Some cells could be shaded with no effect at all. At that point it was clear to everyone that the fault was found in the string that didn't respond to shading. A whole string would be knocked out, but other strings would still work. The working strings of cells are the ones still affected by shading. We were able to identify the left-hand string to be the defective one in this way. This proved to be useful when repairing the panel later.
|At this point, I was already eager to tear into the panel's junction box to find the culprit, but Pierre set the pace to make sure that everyone in the audience appreciated what needed to be done. We brought the panel into the garage, face down to work on the junction box. He had already opened it up to allow us to see where the problems lay. Pierre then explained how the parallel strings are bypassed by diodes, and that repair of the diodes and wire tab connections would be needed to repair this panel.|
Inside the junction box, each string of cell is connected in parallel with a diode. This is a "bypass" diode, which normally permits one string of cells to be shaded without sacrificing the output of the other strings. When one of these bypass diodes fails, however, it prevents that string from working at all.
Before you can get to them, however, you have to cut your way through a silicone seal known as "potting". De-potting electronics is a fact of life for electronic tinkerers like myself, but for the uninitiated it can be daunting. "What kind of damage will I do if I cut into it?" Well, rest assured that if the components inside are already defective, there's no great loss if you break something else while getting inside anyway. This is a case where breaking the case open is the right thing to do. It was a relief for me to finally start cutting into one, since I had been on tenter-hooks for almost an hour waiting to break in.
|Pierre equipped us with an impressive array of dentist's tools to cut and poke our way through the silicone "brick". We let the youngsters Luke and Nicole loose on them first. They made their way through, exposing the diodes and connection wires that joined the 3 strings together, as well as the connections to the external leads. Since a failed diode looks just like a working diode, none of us saw any obvious signs of problems. Again, Pierre pointed us to something we hadn't yet noticed, and that was a wire-tape lead coming into the junction box that had peeled up from its connection. He assured us that this was not due to our picking and cleaning of the silicone, but that he had expected exactly this to be the cause of this panel's bad output, based on the test we had just done in the sun.|
He brought out his solder iron and a few more tools to get us going, and we set Luke and Nicole loose upon it, putting the connection back together. For good measure, Pierre also demonstrated the process of replacing diodes. He showed us where to cut the leads, and where to solder the leads of a new one back on.
With that done, we carried the panel back out into the sun and hooked it up again. To cheers and back-slaps the panel delivered 34.6 Volts right away, and when hooked up again to the resistors to pull some current they delivered 8.5 Amps. Accounting for voltage drop, it delivered 238 Watts, proving that it had been restored to its nameplate capacity, at only the cost of some silicone, a bit of solder, and 3 schottkey diodes (less than 2 dollars each).
Then everyone else got a turn, since Pierre had collected plenty more defective panels to repair. I settled down over a panel that would only deliver 24Volts in the sun. Without considering which of the diodes could be the problem, first, I set about excavating the silicone. Getting started was easy, but taking out the silicone from around the terminals and diodes became finer and finer work. I persisted with the dentist's tools, until Pierre invited me to carry it around the side of the house. With a twinkle in his eye, he started up his pressure washer and handed me the hose. Catching on to what he meant, I directed the spray into the junction box and discovered that he high-pressure water was very effective at removing the remaining silicone. It also make a lot of back-spray into my face! My glasses protected my eyes from impinging gobs of silicone, but this really should be done wearing safety goggles!
With the space inside the junction box all clear, I got trigger-happy with the solder iron and replaced all of the diodes first. Another guest named Calvin came over to help, and we carried the panel into the sun to test it.
|The open-circuit voltage was up to about 34 Volts but only 4 Amps came out, not 8. We brought it back in and it didn't take me long to remember that other connection tabs could be broken. Sure enough, one was easily peeled up and perhaps only had silicone holding it down, not solder. If I had not been so eager to chop out the diodes, I might have noticed this tab first. Once I soldered this back down, the panel performed well in the sun. By the time I was finished, the sun was passing into the trees and with all of the cells working and tilting the panel this way and that, I got nearly 7 amps. I was still convinced that all strings were working together, because Calvin could cause the output to drop by shading any of the cells.|
With my panel done, I offered a little help to others, where I met Nicole, in grade 10. Her first solder joints (ever) looked good, so we carried her panel into the sun. It had a problem just like my panel had. We brought it back into the garage and checked the wire tabs. They seemed to be fine in Nicole's panel, but then I "stress tested" the soldered leads of her diodes, finding a weak one. We wetted the solder from it to the tab, and after that the panel worked splendidly. In all, 4 PV panels were repaired by the group. I'm certain that with practice and focus, a panel could be disassembled, tested, repaired, and sealed up again in an hour.
| ||Pierre's desire is to repair panels and donate them to schools and educational programs which can give young people hands-on experience with electronics, energy, and using tools for themselves. Another goal was discussed by the group, wherein some panels could be used for aid and charitable donations to third-world countries for electrification where there is none. There are also citizen science uses where the panels could be used to power devices like weather stations, gardens, or water in streams.|
11 May 2019