Ageing Photo Voltaics

2 03 2013

It’s a little known fact on the part of the now thousands of people who have bought PVs for their homes, that the panels degrade with age.  They are usually warranted for some 25 years to perform as specified for the warranty period.

To achieve this, new panels normally produce considerably more power than they are rated at, often as much as 20% more.  So a 250W panel may in fact produce as much as 300W when brand spanking new.  Over the first 12 months, this may drop to 275W, then the power curve asymptotically reaches 250W over the next 24 years…….  after which one would expect the panels to continue producing power for a long time, just less and less every year.

The US64 panels in our first array are now over ten years old, although I think they have only been on the roof for eight.  They are part of the system which feeds power to the grid and charges our emergency back up batteries.

As the panels degrade as explained above, the output voltage drops.  I distinctly remember how when the panels first arrived I excitedly unpacked one of them in the full sun to measure the Volts out with my multimeter, only to discover that my [nominally] 12V panel was actually putting out 24!  That was then….. today I’m lucky to get 19V [open circuit – for those who understand these things]

You also need to realise that on overcast days, that voltage is much lower – at night it’s actually zero after all!  And we’ve had lots of very rainy weather for the past month, which means that outside of the peak hours between 10AM and 2PM, the voltage wasn’t even 12V, thus not charging the batteries.  This can go on for a few days, but after a whole month of it, I was starting to worry about my batteries.US64's

When the system went in, I assumed it was an entirely nominal 48V system – 48V battery bank, 48V inverter, and strings of four panels to make 48V (five of them in fact, 20 panels).  What my mate Dave who did the commissioning never told me was that the inverter could actually accept 60V, and I never questioned him, after all he had an identical system at his place and he recommended it.

For the past eight years, it worked just fine, but that dreaded degradation was slowly catching up with me, until last year Dave informed we should have made strings of five, and we needed to rewire the panels.  Procrastination set in……  then another mate of mine, an electrical engineer called Doug who also had a similar system with failing batteries (for different reasons, let’s not go there right now) pre empted any attempts by me, and rewired his system into strings of five panels.  I visited him a couple of weeks ago, and he told me the transformation was well worth the trouble, his batteries are now just fine.

So to cut to the chase, a week ago today (and it’s pissing down rain again as I type, and we had another minor flood last week) Doug and I rewired the panels to 60V.  And I am so glad we did.  In weather like this it’s not unusual to lose power, and the last thing I want are dead batteries!

It turned out it wasn’t that hard.  We just de-wired the row of four panels at the right hand end of the array, broke the four looms from the other four strings, one at a time, and ran one wire through the existing conduits to one panel at a time from the now separated string.  And I got to do blokey stuff with my mate Doug……

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