Solar cells degrade over time

"P" and "N" type solar cells
PERC and LeTID

 

I can see you glazing over right now but fear not, I am going to keep things simple.

 

Your solar panels have 60 cells inside (usually) and sometimes those cells have been lasered in half to make 120 half cells, but the point is that there are a lot of solar cells inside a solar panel.

 

The cells are made of silicon and there are two types, "P"s which are cheaper to make and found in almost ALL panels, and "N"s which are much more expensive and only found in the premium panels.

 

Usually, panels with "P" type cells will degrade by about 2.5% in their first year and then settle down to a steady 0.6% a year, meaning that in 25 years time they will be making about 17% less power if tested under the same temperature and light conditions that they were originally tested at in the factory.

 

It's called LID (Light Induced Degredation).

 

"N" type cells will degrade at half that rate so in 25 years they will be making about 9% less power.

 

In real terms, if we assume that a 6.6kW installation of panels in Perth starts off making 11,000 kWh then it works out that over 25 years you will get about 10,000 kWh MORE power out of "N" type cells. That's why those people playing the long game often opt for the premium panels.

 

PERC and LeTID

Around 2016 we started seeing panels proclaiming that they had a new and improved form of "P" type cell called PERC, and this brought about a significant jump in efficiency. Overnight we seemed to go from a standard of 250W to 300W per panel. 20% better. There has never been a sudden jump in efficiency of that magnitude before. Solar sales reps were in a state of excitement as they warned their customers off regular panels and convinced them that PERC was the way to go.

 

Pretty soon all the manufacturers had moved to PERC so that's all you could buy anyway at the lower cost end of town.

 

But then...

"PERC celled panels degrade at 8-10% over 3 years" say boffins.

That's on TOP of the existing known LID losses.

Jeez, you might as well not bother putting solar panels up if they are going to degrade like that.

Feathers flew I can tell you. Panic swept in as sellers of PERC panels realised that they would have to replace tens of thousands of panels under the "Performance Warranty", and the sellers of both the absolute cheapest "pre PERC" panels and also premium 'N' type celled solar panels neither of which use PERC technology lapped it up all the way to the bank.

 

The enemy was a thing called LeTID which is the same as LID described above, but with the additional ingredient of high temperature.

 

The trouble that we had with this LeTID thing, is that we just aren't seeing it in our World, the real World, here in Perth. We can see the annual losses from LID are reasonably accurate. More like 0.4% than 0.6% a year but you would expect that because a manufacturer is always going to err on the side of caution when offering a long term warranty.

 

Part of the reason perhaps for this discrepancy is that we don't measure like they measure.

We measure the end result, the actual AC electricity produced by real installations in Perth while the panels are still operating up on the roof. We don't take the panels back to a cold room and drop the cell temperature down to 25 degrees C and shine a very bright light, exactly 1000 W per square metre of fake sun at the panel as they do when they test or re-test a panel in the factory or test lab.

 

Perhaps that is why we see that there is NO difference at all, in fact, the opposite.
Our customers with PERC celled are seeing slightly elevated results...more output than has been typical.

One example is shown below but it is by no means out of the ordinary apart from them being 60km North of the CBD.

 

 

We are somewhat uniquely placed to know this. Since we started up in the solar business we have asked our customers to email in readings from their inverter several times a year. This allowed us to build up a huge database of real World (Perth) information. As far as we know, there's isn't another solar company who has consistently done the same.

 

So whilst we can not deny that others are convinced that this is a real thing and new testing standards are being introduced in 2020 to ensure that all PERC panels sold in Australia are LeTID free, it all seems a bit of a storm in a teacup to us, because, we just aren't seeing it in the real World.

 




 

 
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