AC Coupling in Utility-Interactive and Stand-Alone Applications: Page 5 of 16
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Microinverters and AC Coupling
Microinverters, and more recently ac modules, have redefined the industry’s notion of modularity over the past few years. The modular and expandable platform ac coupling provides makes it seem like a natural fit with the growing number of microinverters and ac modules entering the market. However, these products are almost entirely absent from the ac-coupled application landscape. SolarPro editors asked several microinverter and ac module manufacturers, as well as two companies that have microinverter products scheduled for release in 2012, about the use of their products in ac-coupled systems. While the responses were not always definitive, they do serve to clarify the manufacturers’ positions.
Enphase Energy is the leading microinverter manufacturer in the US and is currently expanding its presence in the European market. As such, questions from integrators who are interested in developing microinverter-based ac-coupled solutions often revolve around using Enphase products. While Enphase does informally permit the use of its microinverters in ac-coupled systems, it has not devoted internal resources to testing or developing support services for its products in ac-coupled applications. Considering the comparable sizes of the grid-direct and ac-coupled markets, Enphase understandably has a deep focus on other priorities. As a result, while this is not explicitly stated, its warranty terms currently exclude the use of its products in ac-coupled systems.
Enecsys, a European microinverter manufacturer with listed and CEC-eligible products for North America, has a similar position to Enphase. The company does not have any immediate concerns about using its products in ac-coupled systems that operate within normal grid specifications, but its warranty coverage does not currently include this use.
Exeltech recently released one of the first fully integrated ac modules in the US market. The company does not have any warranty restrictions regarding the use of its AC Module product in ac-coupled systems, provided that those systems meet several conditions. Its position could change in the future if Exeltech finds that the ac-coupled system topology is detrimental to its product. General prerequisites it currently notes include the following: A means must exist to disconnect the system from the utility in the event of grid failure, allowing the battery-based inverter to continue providing power to selected loads; the battery-based inverter must be designed to work with grid-direct inverters in a manner that is not detrimental to either device; and the battery-based inverter must employ a universal, nondestructive method such as momentary frequency shift to disconnect the ac modules when the batteries are fully charged and power production exceeds the requirements of the system’s critical loads.
Both Power-One and SMA America have microinverter launches scheduled for 2012. Power-One’s AURORA Micro 250 and 300 products (250 W and 300 W, respectively) are in final evaluation and beta-site implementation. The expected production ship date of these products is July 2012. At this point, Power-One’s microinverters have not been tested with battery-based inverters in ac-coupled applications. SMA has scheduled its Sunny Boy 240 (240 W) microinverter for release in early fall. Deploying the Sunny Boy 240 in ac-coupled systems will not void the warranty. The product is designed to be compatible with SMA’s Sunny Island system with special considerations, but the details of these are not yet available.