Inverter built-in meter readings can help users discover bigger issues, such as an entire module string being offline or an extremely dirty array, but they aren’t accurate enough to uncover less-obvious problems.
Module-Level Monitoring. Until recently, PV system checking has been limited to evaluating the performance of the entire system. However, products that offer individual-module-level monitoring are now available and gaining popularity. Examples include microinverters and several DC-to-DC converter units, which allow users to view over a computer network each module’s output.
For example, if a module is shaded during part of the day, the report or visual display will reflect a much lower power output (or lifetime energy output) for that particular module. And on a sunny day, with a completely unshaded array, problem modules—such as those with damaged cells, solder bonds, or diodes—are easy to spot.
Before module-level monitoring, identifying a 33% reduction in output from one module (from one bad diode, when a module has three) would typically go undetected for the lifetime of the system. This is because the partial energy loss from an individual module only represents a small loss to the system. For example, in an array with twelve 250-watt modules, the loss of one diode in one module would cause less than a 3% loss to the system. Compared to a general derate of 20% to 30%, the loss due to the bad diode would be undetectable. While this loss may seem small, the lost energy production over the system’s lifetime adds up (see “Tools: Module-Level Monitoring & IR Cameras” sidebar).
PV Analyzer. For arrays without module-level monitoring, installers can check up on PV system health by using Solmetric’s PV Analyzer, which graphs a module or string’s current-voltage (I-V) and power-voltage (P-V) curves. Because the measurements are very sensitive, they reveal the effects of many types of array damage and degradation, which have “signature curves” that installers can look for. If a problem is detected, the Analyzer can be used on individual modules to pinpoint the culprit. (see “Gear” in this issue). While neither module-level monitoring or the PV Analyzer will tell you exactly what the problem is, they can prompt a closer look at individual modules in the array.
Infrared Cameras. Should a module issue be detected, finding the root of the problem can be tricky. Module bypass diodes are often not accessible for testing or servicing, and PV cell damage is often not visible. Because of these limitations, infrared (IR) cameras can be handy in module evaluation.
The heat generated by a damaged PV cell or solder bond (even an active diode) will show up as bright spots on the photos, pinpointing the source of power loss. Historically, these cameras have been too expensive for most installers to consider, and while many models are still very expensive (up to $40,000), less expensive units are now available ($1,200 to $8,000, depending on detection resolution and features). For installers—especially those working with large-scale systems—IR cameras are becoming a more common tool. Being able to provide IR photos of underperforming modules can be helpful in establishing a PV warranty claim. Other uses include inspecting homes for leaks in building envelopes (see “Easy Efficiency Improvements Pay Off” in HP142).