Potential induced degradation (PID) is an issue that all solar stakeholders, especially owners and O&Ms should be taking seriously even if it only affects a small minority of modules. As there is currently no proven method to reliably prevent PID, the best approach is to develop a preventative maintenance plan to detect and mitigate it in its early stages. Given the substantial time and expense associated with testing individual modules or strings using IV-curve tracing or electroluminescence testing, operators can use drone thermography to identify modules potentially affected by PID in a timely and cost-effective manner. This enables owners and O&Ms to target their resources to addressing PID before it manifests as significant production losses.
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What Is Potential Induced Degradation?
Potential induced degradation (PID) is a persistent issue affecting the output of crystalline-silicon and thin-film type PV modules. While its exact causes are still not fully understood at the microscopic level, on a macroscopic level it is known to be the result of leakage currents from the PV cells through the encapsulation material and on through either the protective surface glass or module backsheet and ultimately to the module frame. Different theories exist about the root cause(s) of these leakage currents; most hypothesize either a chemically induced breakdown in the resistivity of these components or the formation of microscopic shunting faults across them. Either way, it can cause a decrease in efficiency for the entire DC string of which the module is a part.
What's included in YOUR GUIDE...
Techniques for Exposing Warrantiable Items on Your PV Sites.
A Proven Roadmap for Successfully Implementing Drones in Solar.
Strategies for Identifying DC Losses and Avoiding Downtime.
Though PID only affects a small minority of PV modules, in severe cases, losses can exceed 30% of an affected module or sting’s production. For example, a 2 year study by British scientists of a 1.2 MW PV-plant in Spain found that, within just 4 months of the plant becoming operational, 4 separate DC strings were already showing early signs of PID. After just 12 months, the performance ratios of these four strings had declined by 28% to 39% as compared to a healthy DC string in the same plant.
Naturally, as the potential difference between the PV cells and the module’s external components increases, so does the risk of PID. This is why PID is most commonly (though not exclusively) observed towards the negative end of a module string. It also means that as utility-scale solar moves towards higher and higher DC voltages, the risk of PID is expected to escalate, underscoring the importance for system owners and operators to address this threat seriously.
What Causes PID on PV Modules?
PID does not have a single root cause and can affect any crystalline-silicon or thin-film module, regardless of age, manufacture, operating conditions, or installation. However, factors known to increase the likelihood of PID include hot and humid operating environments, improper grounding of the module frames or inverters, and damage from impact events or module mishandling during shipping or installation. Thus the threat of PID is yet an additional reason for EPCs and owners to pay close attention to these installation quality details even if it means extra upfront construction costs.
Similarly, there is no foolproof method to prevent PID. Some system operators have reported good results in mitigating its effects with “float controllers” or “anti-PID boxes.” These devices work by creating a connection overnight between a PV string’s negative electrode and ground in parallel with the inverter, thus allowing the differences in electric potential to dissipate. While these technologies may hold promise in the future; they are still relatively novel and lack a proven track record. They also increase installation complexity and cost, consume power to operate, are a potential point of failure, and are not necessarily compatible with all inverters or system configurations. Therefore, they do not yet eliminate the need for owners and O&M contractors to develop a preventative maintenance plan to detect and mitigate PID throughout a system’s operational lifespan.
Drone Thermal Image of PV Cells Experiencing PID
Using Drone Thermography To Identify PID
To grasp the value of using drone thermography for detecting PID, it’s crucial to contrast it with traditional methods, starting with the least effective: relying on production monitoring figures to detect PID’s effects.Generally speaking, PID manifests relatively slowly at first, with early stage production losses not being easily distinguishable from ordinary variations in module production. However, production losses increase dramatically as PID moves into more advanced stages and begins cascading to other modules in a string. So, significant production losses may already be baked in by the time that PID is identified using simple production monitoring.
Two other methods of detecting PID are reverse current electroluminescence (EL) testing and I-V curve tracing. While both of these methods are extremely reliable for detecting PID, even in its early stages, they are both extremely time and labor intensive, requiring individual PV modules to be disconnected and tested by highly qualified specialists under conditions that can be difficult to replicate in the field. As a result, it is generally cost prohibitive to apply these testing methods to more than a small sample of modules within a PV plant, which brings us to drone thermography.
As most PV plant operators already know, many kinds of electrical malfunctions (including early stage PID) produce a thermal signature that may be apparent to an experienced thermographer without the need for invasive testing, system downtime, or exposure of technicians to hazardous voltages. Using drone thermography also allows data to be collected for entire sections of a solar plant simultaneously unlike handheld thermography, which requires module-by-module inspection.
Of course, drone detection of PID does have challenges. Since PID is itself imperfectly understood, there are only general patterns, no universal rules, for how it manifests on a thermographic image. For example, many instances of PID show up as thermal anomalies on the scale of individual cells or cause string end heating patterns.
While drone thermography may not always provide a definitive diagnosis of PID, it remains a valuable tool for swiftly scanning entire sites and identifying potential areas of concern. Given that other inspection methods can be labor-intensive and may not always detect PID, drone thermography offers a much more cost-effective means to locate modules possibly in the early stages of PID. By pinpointing these modules, operators can then proceed with further inspection using traditional methods such as EL testing and I-V curve tracing. This also means initiating manufacturer warranty claims, if necessary, becomes more feasible. This approach presents a more efficient alternative to randomly testing modules or passively awaiting advanced PID to manifest as production revenue losses.
Getting Started: Aerial Thermography PID Testing Services
PID represents a real lurking threat that all PV manufacturers, EPCs, system owners, and O&M contractors should be taking seriously. As of today, the best strategy remains a proactive preventative maintenance plan to detect and mitigate PID in its early stages. Drone thermography has proven to be a vital tool to allow plant operators early identification of modules that may be suffering from PID and concentrate their limited O&M resources where they are most valuable.
At The Drone Life, we provide comprehensive and tailored aerial solutions for asset owners, EPC’s, and O&M’s. Our team of experts has extensive experience in all aspects of drone operation and thermography, allowing for the detection of PID more efficiently. Our pilots are FAA licensed and use the most advanced drone technology available for each project. Ensure your system isn’t suffering from PID by scheduling a free consultation with one of our experts today.
What's included in YOUR GUIDE...
Techniques for Exposing Warrantiable Items on Your PV Sites.
A Proven Roadmap for Successfully Implementing Drones in Solar.
Strategies for Identifying DC Losses and Avoiding Downtime.
