DC/AC Coupled Design Practices for Battery Systems
AC Coupling: The Solar PV feeds a standard grid-tied inverter which then feeds power into the output of a battery based hybrid inverter. The output of the battery inverter tricks the grid-tied inverter to power up and feed into the system when the grid fails.
DC Coupling: The Solar PV feeds dedicated charge controllers, which convert and condition the power for battery charging and use within the system. The battery inverter converts this to AC for the loads and optionally pushes excess back to grid.
Should I AC or DC Couple the System?
It depends on what you’re trying to accomplish:
AC Coupling is most appropriate for:
1. Time of use shifting (not applicable for Residential properties in this area)
2. Peak demand reduction (generally for commercial properties only)
3. Self consumption without backup (where net-metering isn’t permitted, like Hawaii)
DC Coupling is most appropriate for:
1. Battery backup during outages
2. Off-grid systems
3. Maximum flexibility and battery lifespan
How Grid-Tied PV Inverters Normally Work
Grid-tied solar inverters are designed to drive as much power as possible into the utility grid. They operate by sensing the utility grid’s voltage and frequency, and then add power to that existing AC waveform.
The grid can essentially absorb all power fed into it so this isn’t a problem when the grid is operational, and is optimal if the goal is purely to lower the electric bill via net-metering. However if the grid fails and the system is operating off-grid with batteries, there is a very limited ability to absorb excess power.
How Grid-Tied PV Inverters Work “Off-Grid”
The battery supplies power via its own inverter, essentially tricking the grid-tied inverter into believing the grid is operational, so they fire up and start pushing power into the AC side of the system.
Why Not AC Couple for Backup or Off-grid?
The problems begin when the utility grid isn’t present to absorb that force-fed power from the PV inverter. Let’s assume that the power is out and the system is running on batteries and solar, or the system is off-grid. The AC coupled PV cancels out some load and the rest back-feeds the batteries largely unregulated. This is detrimental to battery lifespan as batteries require a proper charging algorithm to remain healthy. Lead-acid batteries are especially vulnerable since there is no absorb or float stage in a purely AC coupled system operating off-grid. The batteries are short-cycled causing sulphation and a reduction in overall cycle lifespan. Lithium batteries are capable of handling these micro-cycles much better, but there is still a major downside.
If the batteries are full and the loads are minimal, the batteries can be overcharged from the unregulated power being back-fed thru the inverter from the AC coupled PV. The crude way to prevent this is to frequency shift and shut down the grid-tied inverter. This can also cause surging within the system and wreck havoc with certain appliances, such as computer UPSs and clocks. In the controls industry this is called bang-bang control; which is hard on equipment because of the repeated on/off cycles.
Some PV inverters including those by SolarEdge and SMA will curtail their output via the frequency shift from the battery inverter, which greatly improves the situation, however you’re still feeding your appliances an AC waveform that’s not quite 60Hz and can’t properly charge a lead acid battery. Most micro-inverters can only be controlled by forcing the AC power out of spec and forcing them shut down, though some newer models can curtail as well.
Regardless, if the batteries become exhausted, the battery inverter shuts down and so does the grid-tied inverter. The system can not self recover in this situation since there is no longer an AC waveform to latch onto. This last reason is why AC coupling is rarely appropriate for backup or off-grid applications.
For larger grid-tied backup systems we may DC couple part of the PV so that the batteries receive a proper charge while the grid is down, as well as AC couple a smaller part of the PV due to higher efficiencies of grid-tied inverters while the grid is operational. This combination approach may be slightly more complicated to implement, but is extremely flexible and results in a balance of battery lifespan as well as overall efficiency.
Modern best practice for smaller systems is to DC couple all PV when batteries are present. For off-grid systems we always suggest a purely DC coupled approach to maximize battery lifespan and prevent unexpected downtime or failure.
What About SolarEdge?
SolarEdge is unique in that the grid-tied inverter is also capable of operating in backup mode (StorEdge). StorEdge happens to be the most efficient implementation possible; a DC coupled battery inverter and grid-tied PV inverter combined into one unit. They accomplish this because there are power regulating optimizers between the array and battery, acting as a sort of charge controller. When the grid is operational, the inverter pushes out as much power as possible to lower the electric bill. When the grid goes down, the inverter switches into a battery and solar powered backup inverter to run critical loads, automatically.
Mountain View Solar has extensive experience and training with design and implementation of combination systems, as well as off-grid DC coupled systems. Contact us today to learn how you can be energy independent!
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