Module-Level Performance

Module-based microinverters, AC modules, and DC optimizers can help systems make the most of the solar resource by maximizing each PV module’s individual performance. But is the added expense and complexity worth it?

Microinverters, AC modules, and DC optimizers—module-level power electronics (MLPEs)—are gaining in popularity for their ability to squeeze the maximum energy out of a PV system, especially in sites with partial shading. Here’s what you need to know to determine if MLPEs are right for your system and situation.

Microinverters

Microinverters are small, self-contained inverters, ranging from 200 to 400 W AC, that are paired with a PV module to produce grid-tied AC. They mount on the PV module’s frame or the rack where the module is attached. The microinverters’ outputs are wired in parallel by their shared AC power cable.

• Microinverters accomplish their function using four basic circuits which:
• Change the PV module’s low-voltage DC to high-voltage DC (typically 250 to 450 VDC)
• Change the high-voltage DC to sine-wave AC
• Use MPPT to squeeze out the maximum power from the PV module
• Detect the presence of the utility grid before feeding power to the grid

A PV module must match the microinverter’s input specifications for voltage range and/or number of PV cells in the module (i.e., 60, 72, or 96 cells). Micros have some mounting flexibility, and may be attached to the PV frame or mounting rack.

AC Modules

An AC module is a PV module with a factory-attached microinverter—a close cousin of the microinverter, but with some important differences. AC modules are tested and certified to Underwriters Laboratories (UL) standards as a complete product. They bear three certifications—one for the PV module; one for the inverter; and a third for the pair as a complete product stating the required limitations (like maximum number of AC modules that can be paralleled). 

To remain in compliance with the product’s UL certification, failed AC modules must be replaced as a complete unit, regardless of whether the inverter or the PV module is at fault. Field replacement of either item alone invalidates the product’s UL safety certification.

Microinverters that are not sold as part of an AC module are required to incorporate a ground-fault detector and interrupter circuit to turn off the inverter should an unwanted current path develop within the PV module. AC modules are exempt from this requirement. TheNational Electrical Code (NEC) also differentiates AC modules from microinverters—Section 690.6 of the NEC states: “The requirements of Article 690 pertaining to PV source circuits (the DC side of the PV module) shall not apply to AC modules.” This simplifies and lowers the cost of installation of AC modules compared to microinverters.

Another advantage with AC modules is that because the inverter doesn’t have to be mounted separately, installation time is reduced. Additionally, there’s a single point of warranty contact for both the PV module and the inverter.

DC Optimizers

DC optimizers adjust the output from each PV module to match the other modules in the system. But unlike microinverters, they output DC—not AC. Subsequently, they work only with string-inverter-based systems, shifting the task of maximizing PV power from the string inverter to the unit connected to each PV module for module-level MPPT. The result boosts power production from a few percent up to 25% or more, depending on shading or other issues.

Since they adjust each PV module’s current to match other modules in the string, optimizers may simplify system design in the event of shading or mounting orientation differences, such as roof planes that face different directions. While the net system output power may be less than the maximum available, it will still be greater than if optimizers weren’t used. In cold climates, they can help regulate the PV modules’ output voltage, preventing it from exceeding the inverter’s maximum DC voltage input. This allows “extra” PV modules to be connected into a circuit without exceeding that maximum voltage. An array that was limited to 12 PV modules might be expanded to 14 modules without danger of excessively high voltage in the winter, while yielding 16% more energy output. As long as the inverter capacity is large enough, those two extra modules will offer that additional capacity year-round. Over the lifetime of the system, this can add up.

Optimizers attach to PV modules or the rack much in the manner of microinverters, or may be pre-attached to modules in place of PV junction boxes (Tigo Energy and SolarEdge). Like microinverters and AC modules, optimizers also offer module-level performance monitoring for tracking the performance of each PV module in your system.

The optimizers’ MPPT may not co-exist with a string inverter’s MPPT, in which case the string inverter MPPT should be disabled. String inverters from ABB (Power-One), Fronius, KACO, SolarEdge, and others are “optimizer-aware” and can either have their own MPPT disabled or changed to allow the optimizers to function. When optimizers are present on every PV module in a system, the string inverter’s MPPT isn’t needed. Under less-than-ideal PV conditions, optimizers will outperform the string inverter MPPT. Under perfect conditions of no shade and an ideal operating environment for the PV modules, however, the improvement with optimizers will be minimal.

MLPE Advantages

Maximum power from each module. MLPEs allow each module to operate at its maximum potential—regardless of its neighbors. String inverters typically require modules to be wired in strings of eight to 14 modules, and the weakest-performing module in the string limits each module. This could be a weak module from the factory, or one with shade or orientation problems.

Incremental design. Microinverter and AC module systems can be built with as few as one module at a time. This is helpful to deal with budget constraints, or if there’s not enough room for the number of modules required to power a string inverter. They also can be used to supplement a string-inverter system that’s electrically maxed out, but when there’s still roof space remaining for additional modules.

Easier system expansion. Integrating microinverters or AC modules into an existing larger system can usually be done by connecting them into the existing utility service with other inverter equipment. Additional breakers are required for each separate circuit.

Accommodates various module orientations. Microinverters and AC modules are very effective in systems where the PV modules can’t all be in the same plane.

Safer. High-voltage DC is eliminated in microinverter and AC module systems, increasing safety. The highest DC voltage in such systems is that of a single PV module. With no high-voltage wiring, and with DC cables from one PV module connected to one microinverter, the likelihood of DC-side “ground faults” and “arc faults” are reduced.

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.

MLPE Disadvantages

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).

To Go MLPE—or Not

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.