Is the yield of your solar panels dropping? Then they might be the subject of potential induced degradation (PID).
PID occurs due to large negative voltages of series-connected solar modules. Those tensions ensure that charge carriers (electrons or positively charged ions) will shift, causing leakage currents and short circuits to arise in solar cells. This way the efficiency of solar panels will reduce.
Only very little is known about PID. “We know that the problem is initially caused by high negative voltages, which are a result of the series circuit”, says Jef Poortmans, director PV-technology at IMEC. “But we have to guess what precisely causes PID, which types of PID exist and which solar cells are affected. There is not even a unique test to detect PID. Via electroluminescence measurements we can measure which solar cells no longer function optimally, but it can have other causes.”
In any case, according to Poortmans the problem isn’t as big as some claim. “Otherwise manufacturers of photovoltaic cells had already started a research program on PID. What we do know is that PID occurs more often or faster in humid conditions and with a high salt content in the air.” Therefore one would expect solar panels in coastal areas would degrade faster than their inland counterparts. “This is what you would expect, but if it was the case we would have already heard something about it.”
However the Belgian company Ilumen claims to have found a smart solution to the PID ‘problem’. By installing a so-called PIDbox, individuals would again be able to regain their old yields, without a single module to be replaced.
The PIDbox is switched to the power outlet between the solar panels and the inverter (which transforms produced direct current to alternating current and supplies it to the power grid). At night, when the solar panels no longer generate electricity, the PIDbox reverses the current direction and the degenerated solar panels are ‘rejuvenated’. “Actually, we reverse the degradation process”, says Ismaël Ben-Al-Lal of Ilumen. “With the difference that the regenerative process, which occurs at night, passes by much faster.”
But what does this PIDbox do precisely? Ben-Al-Lal: “The bottom line is that we strengthen the pn-junction (the core of the semiconductor technology in each solar cell) at night by pushing a high current through it. Because of the high voltages involved, the solar panels are temporary disconnected from the inverter.” According to Poortmans this explanation (partly) makes sense. “It’s true that by reversing the current, the charge carriers can be put again in their proper place. However, I wouldn’t dare to call this the missing link. Moreover Ilumen overestimates the PID problem tremendously.”
In press releases Ilumen talks about efficiency losses of 80% after two years, all caused by PID. According to Poortmans other solutions exist. “You could connect an inverter to each module in order to abolish the high negative voltages. But then there should be a significant loss of efficiency.”
SOURCE : EOS – 22/04/2014