DC/AC Coupled Design Practices for Battery Systems
Overview
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 causes 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.
Explanation
Grid-tied solar inverters are designed to drive as much power as possible into the utility grid. The grid can essentially absorb all power fed into it so this isn’t a problem when the grid is operational. However if the grid fails and the system is operating off-grid with batteries, there is a very limited ability to absorb excess power.
For example, assume the power is out and the system is running on batteries and solar. The AC coupled PV cancels out some load and the rest back-feeds the batteries UNREGULATED. This is detrimental to battery lifespan as batteries require a proper charging algorithm to remain healthy. This can also cause surging within the system, especially with micro-inverters, as they turn on and off. Since there is no float capability in a purely AC coupled system operating off-grid, the batteries are short-cycled causing sulphation and a reduction in overall cycle lifespan.
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. Even with the battery inverter’s ability to frequency shift and shut down the grid-tied inverter, the result is that the batteries are not charged properly. 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.
Best Practice
For larger systems we may choose to DC couple part of the PV so that the batteries receive a proper charge while the grid is down, as well as AC couple the remainder of the PV due to higher efficiencies of grid-tied inverters. 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. A system using lithium batteries will usually be DC coupled, depending on the capabilities of the pack’s BMS.
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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