Long-Term Module Replacement and Serviceability

When you have been in business installing PV systems for a few years, you begin to see the challenges in keeping systems running in a costeffective manner for the long term. One major O&M issue is how to handle broken or malfunctioning modules. System owners purchase your products expecting reliable energy production for 25 years or more, and the industry must think in terms of these long life cycles.

Sometimes the glass on a PV module shatters. This may be due to hail, vandalism, thermal expansion or reasons unknown. Regardless of the root cause, module manufacturers do not typically warrant post-installation module breakage. So the cost of replacing the broken module is borne by the system owner; and the installation is facilitated by the O&M provider, which is often but not always the system installer.

Post-installation module replacement can present any number of challenges, most notably the availability of parts. Module manufacturers are generally engaged in a process of continuous, incremental product improvement. The model you are purchasing today will not be available indefinitely. While it would be ideal for system installers and O&M providers if manufacturers maintained a back stock of older products, manufacturers and distributors are often motivated to reduce and eliminate stranded inventory.

Whenever an identical replacement part is unavailable, the following recommendations can be helpful.

Same product line, different batch. Absent a direct replacement, the next best thing is to track down a module from the same product line, ideally one with a higher power rating. Most manufacturers batch cells and modules so that multiple power ratings are available within the same product line. Regardless of any differences in rated power, the mechanical characteristics within the product line should be identical.

If a higher-rated module from the same product line is installed as a replacement, the existing source circuit and array should continue to operate as originally intended. As Jim Dunlop explains in chapter 5 of Photovoltaic Systems (American Technical Publishers, 2007), “The current output for a circuit of dissimilar devices in series is limited to the current of the lowest-current output device in the entire string.” As long as the rated current of the replacement module meets or exceeds that of the existing devices, system operation should be unimpeded. The key is to not introduce a bottleneck in the circuit.

Note that the operating characteristics within a specific product line can change slightly from year to year, as can frame profiles and dimensions. In addition, keep in mind that products operating on a roof for several years exhibit predictable degradation. If you have to replace a 180 W monocrystalline PV module that has been in the field for 3 years with a new 175 W module from the same product line, that is likely acceptable and should present a minimal impact on performance.

Similar electrical characteristics and dimensions. In some cases, necessity may dictate that you use a replacement module from a different product line or manufacturer. If so, be aware that this is not as straightforward as simply replacing a 175 W module with any other 175 W module. It is important to identify a module with similar current characteristics. Ideally, the voltage characteristics and temperature coefficients should also be a reasonably close match.

It is also important to find a module with similar mechanical dimensions so that the replacement fits within the old module’s footprint. Aesthetic issues, like matching the frame color, may or may not be an issue, depending on the application.

Reduce string sizes. If you cannot replace the broken module, it may be possible to remove one module from each of the remaining source circuits. This would require a careful analysis of the resulting low array voltage to ensure that the operating voltage remains within the inverter MPPT window during the summer months.

The obvious downside to this approach is that it reduces the array capacity, which tends to limit this option to one- or two-string residential installations. Array aesthetics may also be compromised if unsightly gaps are visible from the ground. However, this solution does not require replacement parts.

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