Anti PID protection – is it effective?

 Photovoltaic panels manufacturers strongly focus on efficiency increase trying to lower energy losses in photovoltaic plants, including PID phenomenon.

PID – what is it?

Potential induced degradation in photovoltaics is a consequence of potential difference between aluminium frame, mount or solar glass (0 V) and cells that are located on string extremes (i.e. 600 V). High potential difference produces electrostatic field that helps ions to migrate from solar glass to frame, causing power losses (in extreme cases even up to 30%) through stray currents.

The problem was specified only few years ago (first detailed publications come from 2006). Apart from high voltage, this phenomenon is enforced by high temperature and high level of humidity. Happening only randomly, the process can be reversed but if it occurs on a continuous basis, it can provoke permanent degradation of p-n junction  (electrochemical corrosion).

Label of anti-PID protection

It is therefore worth checking if panels that we sell are tested for PID. And even if they have such labels, those should be treated with caution. Many producers give “anti-PID” label to their products and independent laboratory tests show that this phenomenon is almost never avoided in 100%.

There are many testing agencies for PID  (i.e. NREL, TUV, Fraunhofer). The IEC 62804 standard characterize modules that pass the test with power loss less than 5% and no major defects. Test conditions: module rated system voltage and polarities, chamber air temperature 60o + 2 oC, chamber relative humidity 85 + 5%, 98h.

Talesun recently introduced its new module labeled as “PID-free” meaning the degradation will never exceed 1%. It was obtained by using a new material for encapsulant (from Dow Chemical). The results were confirmed by Photovoltaik-Institut from Berlin and are going to be tested by TÜV Sud as well. This same lab has recently tested JA Solar panels from RIECIUM line that pass 500-hour test for PID in rough conditions (85% humidity, 1000 V, 85 oC), with degradation lower than 2%.

Ways to avoid PID

There are certain ways to avoid PID. As much as we cannot do a lot about environmental factors, we can verify the composition of photovoltaic systems. The choice of glass, mount and encapsulant have shown to have a huge impact on PID. Why? Ingredients that are suspected to provoke PID are for example : sodium (in solar glass), ARC (anti-reflection coating) structure and EVA (most popular encapsulant provoking also the “glass-corrosion”). There are also inverters that reduce or eliminate PID effects as well as separate PV boxes (i.e. PV Offset Box from SMA). Those devices earthen the negative pole of photovoltaic arrays shifting the potential to positive when the array voltage falls below defined threshold.

Source: Advanced Energy, SMA

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