Causes for PV module degradation
In general, photovoltaic panels can be used for 25 years. As the years pass by, the electricity produced is less and in turn, energy production is reduced gradually which is called module degradation. Degradation in other words, is the loss of power relative to the rated power. Due to this, the production warranty changes. The panel manufacturers guarantees almost 90% of the rated production for first ten years. Later, due to aging factor the production decreases. A gradual degradation in module performance can be caused by:
- Increase in series resistance (RS) due to decreased adherence of contacts or corrosion usually caused by water vapour
- Decrease in shunt resistance (RSH) due to metal migration through the p-n junction
- Anti-reflection coating deterioration.
The annual energy production values of a site are as follows:
Annual Energy Production (KWh)
Fig. Annual energy degradation over years
This annual energy production graph helps to calculate the degradation percentage. As seen, in the Year 1 the production is 809,600 kWh while in Year 2, it is 799,350 KWh. To calculate the degradation percentage for Year 1 and Year 2.
Degradation percentage = (production in Year 1 – production in Year 2) / production in Year 1
Year 2 degradation % = (809,600 – 799,350) / 809,600
= 1.266 %
There are different types of degradation in solar panels. They are, Light Induced Degradation and Potential Induced Degradation, Light and elevated Temperature Induced Degradation.
Light Induced Degradation (LID) is a loss of performances arising in the very first hours of exposition to the sun, with crystalline modules. It may namely affect the real performance with respect to the final factory flash tests data delivered by some PV module providers. It is unclear how it affects the performances with respect to the specified STC values. If the modules are sorted according to their final factory flash test for determining their Nominal Power class, the LID will indeed represent a loss with respect to STC.
The LID loss is related to the quality of the wafer manufacturing, and may be of the order of 1% to 3%. It is due to traces of Oxygen included in the molten Silicon during the Czochralski process. Under the light exposition effect, these positive-charged O2 dimers may diffuse across the silicon lattice, and create complexes with boron dopant acceptors. The boron-oxygen complexes create their own energy levels in the silicon lattice, and can capture electrons and holes which are lost for the PV effect.
Potential Induced Degradation (PID) can significantly impact PV system output and be a real headache for system owners. PID is a defect experienced by some PV modules when installed such that the PV cells are operated at a negative voltage with respect to ground (module frames are electrically grounded). The greater the negative potential difference relative to ground, the more severe the PID effect. However, not all module designs are inclined to PID and not all system designs trigger the failure mechanism.
Fig. Thermal image (yellow hot) of two strings showing signs of PID as seen by aerial IR inspection
LeTID is a form of solar cell degradation seen in the field and is accelerated by high irradiance at higher temperatures after hundreds of hours of light exposure. Indeed, a drop in efficiency of up to 10% can be seen if steps are not taken to prevent it. As the name suggests, LeTID is made to go worsen by higher operating temperatures and higher light intensity.