Light affects PV panels output. An insight to LID and LeTID.

Diletta Darrigo
29 July, 2020

 

Light affects PV panels output. An insight to LID and LeTID.

 

 

The fact: Light acts on the materials and affects the yield form the crystalline silicon cells of photovoltaic modules, contributing to two types of degradation, the well-known LID and the mere recently discussed LeTID (Light and Elevated Temperature Induced Degradation).

The LID effect.

 

 

Well-known in the industry for more than 20 years, Light Induced Degradation (LID) is measurable from the first hours of exposure of solar panels to sunlight, due to traces of oxygen included in the molten silicon during the Czochralski process for creating silicon crystals, hence the silicon wafer.

The positively charged particles of O₂ (oxygen) can spread throughout the cell and create complexes with boron, reducing the designed panel’s yield compared to the final flash tests carried out at the production plant. Although this is an unavoidable phenomenon, not all solar panels in the market are equally resistant to the LID effect. aleo modules have an average LID of only 0.6%, which is very low, and is a guarantee of the quality silicon used.

As a result of high quality, aleo guarantees 98% of the initial power (the one measured at the factory, in the end of production) for the first 2 years of operation of the module.

The fact: 
aleo modules offer the highest and longest guarantee in the industry.

The advantage: 
The PV-System owner jumps right into pole position, by way of minimizing losses, maximum solar energy production from day one.

 

The LeTID effect.

 

 

Known as Light and Elevated Temperature Induced Degradation (LeTID) it is a relatively recently discovered phenomenon, not yet as thoroughly studied and contrary to LID, this kind of degradation can occur years later after high operating temperatures have affected the panels.

On the spec sheets we refer to NMOT values, (Nominal Module Operating Temperature) and state the nominal operating conditions: 800W/m², 20°C, 1.5AM and with wind 1 m/s. Under these conditions, the temperature of the panels can vary depending on type i.e. 44.5°C for aleo premium modules (X63) or +1°C for aleo full black modules (X83). Recent literature depict that problems with LeTID can arise when temperatures on panels exceed 50°C or more, which is not uncommon in summer when more than 70°C have been measured.

PERC cells, and in particular multi-crystalline are quite sensitive to this degradation, because their passive back is rich in hydrogen. Hydrogen could be at the origin of this phenomenon, but relevant studies are still at such an early stage, that any hypothesis is pure speculation.

Last December PV-Tech.org estimated that 80% of the cells produced globally in 2020 would be PERC-type and that demand for this technology would reach 158GW by 2022. It is therefore legitimate to wonder what moves cell manufacturers to adopt this technology and to bet on its evolution.

There are essentially 2 reasons: Low production cost and the ability to maintain yields over time.

In the case of LeTID there is not yet an official test procedure, but there are some indications that will be adopted in the next draft of IEC 61215 -1: Ed.2.0. To pass the test, cells should not degrade more than 5%.

aleo has approached this by testing of real operating conditions simulation.

486 hours in a climatic chamber at 75°C, to a current value resulting from the difference between Isc and Impp, aleo panels sailed through this test with a maximum power loss of 1,52%.

That is confidence of a stable technology for the entire expected life of a quality system.

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