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.
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.