Several situations can affect a PV system’s output, and new troubleshooting tools are available to help identify system losses and take some of the guesswork out of pinpointing the problems.
Production-based incentives, which pay system owners based on the amount of energy (kWh) their systems generate, make keeping an eye on system performance even more important than for other incentive programs. Being aware of common array problems, knowing how to maintain the system, and understanding how to evaluate performance (and how to troubleshoot) are vital to keeping performance and incentive payouts at their peak.
Potential Array Problems
While PV systems have no moving parts (compared to wind and microhydro systems) and can be extremely reliable, it does not mean they do not have potential performance problems, which can stem from external and internal issues.
External issues, such as shade from growing trees and module soiling (dust or soot from local air pollution), are common problems that can reduce energy output significantly. Studies on module soiling show an average annual energy loss of 5% for arrays that are not periodically cleaned. These types of problems are usually easily solved by intermittently trimming vegetation and cleaning arrays.
Impact to PV systems from critters is another external issue, but one that takes a little more consideration to fix. Wires might be damaged by rodents chewing on them; modules soiled by birds pooping on them; or cells shaded by weeds sprouting between the module frames from dirt and/or bird “fertilizer” beneath the array.
The fix-it for stopping critters in their tracks is to install rodent barriers and/or bird spikes. Many installers are tackling this problem preemptively, including some kind of screen or wire that keeps critters out but allows air to flow beneath the array.
Internal problems, such as module/cell damage, can also reduce system output. Sometimes these problems are easy to spot, but often they are not.
Visually inspecting the PV array once a year is a good idea. Look for cracks in the glass, brown/burn spots on both the front and the back of the modules, burnt solder joints on the cell “grid,” and signs of delamination and cell damage.
System Evaluation & Troubleshooting Tools
If the modules pass a visual inspection, that’s a good first step. In many cases, though, module/cell damage is invisible, so it is important to know how to evaluate your system’s performance, which can alert you to problems that may be brewing.
System owners often use the inverter’s built-in meter as the primary indicator of system performance. During a clear, sunny day around noon, you can check the system’s output (in watts) and compare it to the array size, derated to account for system inefficiencies. System efficiency typically ranges from 70% to 80%. This derate accounts for power losses due to module heating, dust, inverter efficiency, wiring voltage drop, module production tolerance, and module mismatch. For example, a 4,000-watt PV array would be expected to generate 70% to 80% of that value, or 2,800 to 3,200 watts. What action do you need to take if the meter reading is significantly lower than expected?
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 under performing modules can be helpful in establishing a PV warranty claim.
Growing Pains & Array Performance
Much of the current photovoltaic industry has focused on reducing the cost per installed watt, which, over the last five years, has dropped—from $10 per watt to between $5 and $6 per watt. However, it is possible that the swift ramping up in PV module production to meet this demand may be the source of some of the internal module problems discussed here. The good news is that monitoring options can alert us to problems as they arise, helping system owners maintain their systems and allowing module manufacturers to see their products’ performance over a wide range of installations.
Whether you are interested in maximizing your economic return or you simply want to generate as much renewable energy as possible, plan to periodically check your system’s output and watch for potential problems. Catching these issues early can keep you from losing precious sun-generated kWh for years to come.