A PV backsheet is designed to protect solar PV modules however poorly performing backsheet materials are preventing some solar plants from achieving their projected lifespan. Increasingly, asset managers and O&Ms are discovering systemic solar PV module backsheet failures, with 12GW already known to be at risk. Asset owners are facing gradual systemic early module failures which over time results in financial losses. Systemic degenerative issues can take many forms and can occur at any time during a solar plants life. Fully digitising the asset down to the module record is key in reducing the cost and impact of systemic degenerative issues such as backsheet failure.
How important is a solar panel’s backsheet?
The solar panel backsheet is placed under repeated mechanical and environmental stress and so it must perform it’s purpose well to ensure the overall longevity of the entire panel. Typical photovoltaic (PV) modules have the layer structure shown in the image below. A backsheet, usually made of a polymer or a combination of polymers, is used to cover the back of solar PV modules. The main function of this layer is to provide electrical isolation of internal circuitry with the external environment. Therefore, damage may cause a serious safety hazard.
Most backsheets have multiple layers, but there are also single layer backsheets used in PV modules. Widely used PV backsheets have layers based on polymers such as Polyethylene terephthalate (PET), Polyvinyl fluoride (PVF), Polyvinylidene fluoride (PVDF) and Polyamide (PA) with thicknesses ranging from 30 to 270 µm. Solar panel backsheets play a significant role in protecting a PV module from stress and so the long term durability of a backsheet is crucial.
The left image shows the typical layer structure of a solar module. The right image shows the typical structure of a multi-layer backsheet.
(Left image credit: Trina Solar Right Image Credit: PVEL Labs)
What are the field failures and impacts?
In recent years, field failures of backsheets have increased at an alarming rate. These include cracking (inner or outer layer), delamination and yellowing. A recent 2019 study stated of all module defects that occur in the field, 14.3% are due to the backsheet. An increase of 48% in 2019 compared to the previous year. Field observations show rates of backsheet degradation increasing over time with all types of backsheet materials. Reports say that the field failures of the PA type of backsheet reach up to 95% in only six years of operation.
As backsheets are designed to work as an insulator, a defect or failure usually causes a detrimental impact on PV module performance. Ground faults due to reduced insulation resistance are the most common result of backsheet failure. Which often leads to inverter tripping, causing significant power loss. As well as performance loss, ground faults can also lead to safety issues, such as arcing, potentially creating a fire hazard for the whole system. Particularly dangerous for large PV systems, as the leakage current can go unnoticed for a long time due to the sensitivity range of fault detection components.
Cracking and delamination of the backsheet can also lead to corrosion of ribbon connections and busbars within the internal circuit. Corrosion will cause hot spots which means that energy is dissipated in the form of heat causing additional power loss.
Examples showing module field failures
Testing, inspection, and detection of backsheet degradation
The PV module certification standard IEC 61215 and the safety standard IEC 61730 are designed to test PV modules against early failures and do not simulate environmental stresses that PV modules may experience in the field. With the advances in reliability testing of PV modules and materials, backsheets can now be subjected to combined and sequential accelerated stress tests to reveal failures that are similar to the ones observed in the field.
The backsheet degradation and failures of PV modules in the field are usually identified through a visual inspection. Depending on the level of the degradation, a rubbing test using an insulating material for observation of chalking can be done. Before this is done, the string should be isolated from the electrical circuit. Thermal imaging can also be used to identify the hot spots that are caused by corrosion or delamination due to a backsheet defect. This can be used in conjunction with high-resolution imaging using drones to capture possible delamination visible from the front side of the PV module.
The type of backsheet used in a PV module and the degradation (if any) can also be identified using a Fourier Transform Infrared (FTIR) spectroscopy in the field, which is a technique used to determine the polymeric materials. It uses infrared light to scan the samples and detect the chemical properties of the material. Any change in the chemical composition due to degradation can be detected by comparing results with an unaged material.
Left to right – Thermal hotspot, rubbing test and mobile visual inspection.
How to address the problem of backsheet failure
Above’s intelligent inspection and software solutions can help you understand the severity of the issue, build robust PV module warranty claims and provide a digital solar plant for the rigorous monitoring of your modules. Get in touch to find out how our solutions can help you manage solar plant module health.
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