Hard shading damages panels?

System

I have had a 3kw grid tied system on my house for 8 years. Two parallel strings of 9 panels on either side of a dormer to a Sunnyboy inverter. The installer decided to use two strings to deal with a dormer on the center of the roof.

Recently, one string (west) has stopped producing power because of a panel with burn marks and a local technician theorizes that hard shading from a dormer has caused this failure. Has anyone heard of hard shading causing either a cell or diode to burn up and open the circuit? The only proposed fix is to rewire the whole system, mostly with Enphase inverters for about 9 or 10 grand. Any thoughts on this burn out problem? The local technician said that the other 7 panels that have hard shading for a few hours per day will eventually burn out also.

Any thoughts would be much appreciated!

Thanks.

BB.

Re: Hard shading damages panels?

The answer is it should not matter--but it probably does:

Kyocera KC130 output issue...diodes?

That thread has not discussed shading yet--but I do wonder if that is why the panel diodes failed.

Solar panels are hot (in direct sun most of the time) and the diodes they use are not heat sinked to metal boxes but, for the most part seem to be just leaded devices in plastic junction boxes. Not a great environment for any power electronics.

Add that some vendors appear to parallel diodes (also a questionable practice) which may not increase reliability.

You may also have an unrelated problem too... I had solar panels that failed after around 6 years... The failed panels had light browning above the electrical junction boxes. And, for the most part, they were not subjected to any shading issues.

-Bill

mike95490

Re: Hard shading damages panels?

Shade forces the power through the bypass diodes, but many of the diodes don't appear to have proper heatsink capacity. So something burns out. Shade is to be avoided.

If you have 2 strings, maybe they can be re-wired to have one unshaded, and the other gets shade.

RCinFLA

Re: Hard shading damages panels?

All PV cells have manufacturing defects. The concern is the shunt resistance and particularly concentrated points of low resistance within a cell. The best quality monocrystaline cell may have a shunt resistance of several thousand ohms. Thin film cell generally the have the worst shunt resistance, lower then 1 ohm is some cases but thin film shunt resistance is usally evenly distributed across cell. Polycrystaline cells are prone to point defects at the poly-silicon grain boundaries.

The low shunt resistance does not matter much in normal illuminated operation as only a small percentage of the illumination generated current is wasted down the shunt resistance defect when cell is running with 0.5 vdc of forward bias condition. 1 ohm with 0.52 vdc MPP forward diode voltage means only 0.5 amps loss on a 5 to 8 amp producing cell. It is a different situation when a shaded cell drops out and becomes reversed bias with a higher voltage caused by other cells in series that are still illuminated. The high reverse bias causes hot spots in the cell. 10 volts of reverse bias on a cell with 1 ohm shunt resistance is now dissipating the full illumation current times 10 volts. For an 8 amp capable cells that would be 80 watts of potential heating on the cell. In good quality panels, generally cells with less then 10 ohms shunt resistance are rejected and not used in fabrication of panel. Again, a second test needs to look for concentrated defects. The only way to do this is with IR heat sensing to look for hot spots in the cell wafer. UL certification actually steps through each cell in the test panel shading a single cell at a time looking for hot spots with an IR camera. It is one the most important safety test by UL. Of course the panels submitted to UL by the manufacturer can be 'cherry picked', only containing high shunt resistance cells.

Cells should be screened for extreme point defects by IR imagining with reverse bias by the manufacturers. (many of the reject cells show up on E-bay for DIY panel builders). With the collapse of PV panel prices the manufacturers are likely digging deeper into the bottom of the barrel to keep the cost of yield loss as low as possible.

It is okay to use cells with poor shunt resistance if the number of cells covered by a bypass diode is reduced. This lowers the maximum reverse bias due to a shaded cell. Generally you want no more then 24 cells in series covered by a bypass diode. That would yield a maximum reverse bias of about 10 vdc when there is partial shading of the series string of cells.

Hot spots can further damage the cell and make the heat build up get worse over time.

http://www.pveducation.org/pvcdrom/modules/hot-spot-heating

http://www.pveducation.org/pvcdrom/modules/bypass-diodes