Field-programmable. Except for ReneSola and Samil Power, all other microinverters and AC Module products listed here can be remotely programmed to meet utility requirements. This can be important for grid-tied systems in certain regions. For example, the state of Hawaii requires that grid-tied inverters operate over wider voltage and frequency ranges than the UL1741 Standard stipulates. In Hawaii, nearly 15% of the utility power is produced by solar sources, with the rest supplied by diesel generators. If inverters in grid-tied systems in Hawaii were held to the values in the UL1741 Standard, during instances of abnormally low line frequency (which are more common with diesel generators), they would turn off, removing all of the solar-electric power from the utility grid.
Module-level monitoring. Module-level monitoring allows Web-based viewing of how each MLPE combination is performing. Since it presents a real-time, side-by-side comparison of each module/inverter pair, if something fails, it will show up on the monitor. The software can pinpoint exactly where the failure has occurred.
But variations in output power are normal in any system—two otherwise-identical units can have different power outputs. For example, one may show 210 W and another 194 W. Is the unit at 194 W malfunctioning? Probably not, yet the difference may cause a customer to worry—and possibly even contact the installer for a fix where there is none required.
If the difference in performance between two MLPE units is more than 25% and there is no obvious cause, keep an eye on it—even though it likely is temporary. If an output is 50% or more than other units, then take steps to get it corrected. While it could be a malfunctioning module, it also could simply indicate leaves or other debris on the array.
Replacement. Though many microinverter companies and AC module manufacturers offer reimbursement for warranty service call labor, it still takes time and effort to deal with them, and the reimbursement may not fully cover time and travel.
Failure within an AC module can require more effort to replace than a microinverter failure. To remain in full compliance with UL safety standards, the entire PV/inverter unit must be replaced.
Depending on the installation site and where the failed microinverter or AC module is within the array, it can be difficult and time-consuming to replace; multiple PV modules may have to be removed to reach the failed unit. On the other hand, if there are 20 PV modules in the system, and one microinverter fails, it represents an output loss of only 5%. If a string inverter fails, the output reduction is 100% until the inverter can be repaired or replaced.
Connectors. There are no “universal” connectors for PV modules. Microinverters and optimizers must be “connector-matched” to the PV module to which they’ll connect.
Exposure. PV-mounted electronics are outside where they experience wider extremes of heat and cold, and may fail sooner than components that are weather-protected. But units made with quality parts and manufacturing conditions should mitigate this. “Accelerated-life testing” of MLPEs has shown they can function properly over the life of the PV modules.
Improperly installed microinverters that come into contact with the backsheet or are placed in areas without adequate airflow may run hotter than intended, which will shorten their life.
New NEC requirement. The 2014 NEC requires AC arc-fault protection for wiring between microinverters and AC modules (see “Code Corner” in this issue).
The amount of power production improvement from microinverters, AC modules, or optimizers depends on many variables. According to studies by the National Renewable Energy Laboratory (NREL), if a completely unshaded PV system receives full sunshine year-round, the differences in power production between MLPE and string-inverter systems is negligible.
Systems that experience even a little shade may benefit considerably from the use of MLPEs. NREL tests reported up to a 13% power production gain under laboratory-controlled conditions in string-inverter systems that used optimizers.
Quality microinverters and AC modules used within their specifications can provide many years of trouble-free operation. Calculations and extensive testing show they’ll work for 25 years or longer.
Optimizers may be excellent add-ons for older PV modules, which NREL testing has shown degrade at slightly differing rates. As age-induced imbalance increases, so will the advantage of optimizers. However, PV modules made in the past 10 years show slower degradation, with less difference between modules, so adding optimizers may not provide as much of a boost compared to using them with older systems.
String-inverter systems tend to be less expensive than those with microinverters or AC modules, which is their main advantage. With microinverters and optimizers, there is no economy of scale, since each must be mounted, increasing installation costs. With string inverters, jumping from one size to the next often only incurs a minimal cost increase. However, as the minimum power range for string inverters creeps upward, small systems can make microinverters or AC modules increasingly attractive.
Is there a best product type to use? There’s no “yes or no” answer. It depends on the system, and where and how it’s installed. A need exists for each of these technologies in the renewable energy industry. There’s no one clear choice to fit all cases.
Web Extra: “Partial Shade Evaluation of Distributed Power Electronics for Photovoltaic Systems” • nrel.gov/docs/fy12osti/54039.pdf