Even though they usually include an inverter, most battery-based PV systems are “DC-coupled”—the PV array sends DC electricity to the system through a charge controller to the battery bank. That DC power is then drawn from the batteries for the loads, including through a battery-based inverter for AC loads.
AC-coupled PV systems are gaining wider acceptance and support through several battery-based inverter manufacturers. In AC-coupled systems, the DC power from the array is first converted to AC by a batteryless inverter, to be used by the AC loads through an AC load panel. Any unused energy is used by a separate battery-based inverter that either converts the AC to DC to charge the batteries, or, if it is a grid-tied system, it can also pass through to additional AC loads and/or the grid.
A batteryless inverter would normally not turn on without the utility grid present, but many battery-based inverters create a quality sine wave that’s good enough for the batteryless inverter to synchronize with. In grid-tied systems, when the grid goes out, the battery-based inverter isolates both the load subpanel and the batteryless inverter from the utility grid via an internal transfer switch allowing the batteryless inverter to remain on without being connected to the grid.
For residential systems, a primary advantage of AC coupling over the traditional system design is that you can add battery backup to an existing batteryless grid-tied PV system without changing the existing system’s wiring. An AC-coupled system can also be more efficient than a typical battery backup system because the batteryless inverter is doing the majority of the power conversion. Efficiency is generally in the 96% to 98% range compared to 90% to 95% for a typical battery-based inverter.
Disadvantages are a more complicated system to design and program, and more expense, since you’ll need two inverters (or more) instead of a single inverter and a charge controller. In off-grid use, AC-coupled systems are not self-restarting if the battery-based inverters happen to shut down because of low battery voltage. If this happens, the batteryless inverter does not sense AC voltage, and thus does not turn on to send array energy to the batteries. A DC-coupled system can self-restart even if the inverter shuts down from low battery voltage, because the charge controller can still charge the batteries.
Different inverter designs handle AC-coupling differently—mostly in how they control the system in “off-grid” mode—when the batteries are at 100% state of charge (SOC). The batteryless inverter is designed to extract as much energy as possible from the array and send it into the “grid”—in this case, the battery-based inverter. When the batteries are fully charged and there are not sufficient loads, the batteries can overcharge since there is no way to slow the charge rate.
Normally, the charge controller in a DC-coupled system includes the function of reducing output from the array when the batteries are full. Most batteryless inverters do not include this function. When used with Sunny Island inverters, SMA America’s Sunny Boy inverters include it. (Note: Sunny Island inverters output 120 VAC, so either the Smartformer or a second Sunny Island inverter will be required to produce 240 VAC to match the output of a 240 VAC grid-tied inverter.) The battery-based inverter communicates with the batteryless inverter to restrict incoming power to only what is needed. Other models, such as those from Magnum Energy, OutBack Power, and Schneider Electric, support AC-coupled systems but use less-sophisticated control methods to force the inverter to go offline. This happens either by adjusting the frequency of the AC power, which indicates an “out-of-spec” waveform to the batteryless inverter, or by using a relay to disconnect. This on/off control of the array versus a multistage charge controlling is harder on the batteries. Because of this, some manufacturers support using their battery-based inverters only for grid-tied AC-coupled systems, where excess energy is sent to the grid—but not for off-grid systems, where charge control is important.
For more information, see “Choosing a Battery-Based PV Inverter” in this issue.
http://www.homepower.com/articles/solar-electricity/design-installation/ac-coupling-methods/page/0/1
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