Most renewable energy systems are DC-coupled, which means power sources, such as a PV array, wind generator, and battery bank, are joined together on the DC side of the system. An inverter is used to convert DC to AC power for any AC loads present.
AC coupling joins the various power sources on the AC side. With a PV system, this technique requires using more than one inverter since DC sources of power are still present and each power source must be converted to AC before they are joined.
The Anderson-Von Mertens off-grid system consists of a PV array that feeds through an SMA America Sunny Boy string inverter. The output of the Sunny Boy inverter is then routed to the AC load center—just as in a batteryless, grid-tied system. This system also uses two SMA Sunny Island inverters to convert 48 VDC from batteries into 120/240 VAC power that also feeds the AC load center.
With this AC coupling setup, the Sunny Island inverters provide AC voltage to the AC load center, effectively “tricking” the Sunny Boy inverter into thinking grid power is available and enabling it to send out power from the array. This power can then be used for powering AC loads that are running and/or be pulled back through the Sunny Island inverters to charge the batteries.
The last power source in the system is the Kohler generator that exports power to both AC inputs of the Sunny Island inverters for battery charging when the PV array isn’t producing sufficient power. It also sends power to the AC load center.
Finally, a manual transfer switch allows the AC load center to be fed by the Sunny Boy and the Sunny Island inverters or the backup generator if the Sunny Island inverters fail.
A primary advantage of using an AC-coupled setup in an off-grid system is that high-voltage DC strings of modules can be used, reducing wire-size requirements and, therefore, array wiring costs. Plus, through the use of a string inverter, AC-coupled systems offer improved system efficiency since the array is not directly tied to the battery bank.