Batteries can only store energy in its direct current (DC) form so the very essences of energy storage is DC. The reason that DC lends itself to being storable is because it has a unidirectional flow. A graphical representation of this would be a straight horizontal line. However, energy that is expressed in alternating currents (AC) has an electric charge flow that changes direction. This is depicted as a sine wave in a graph. Trying to store energy in its AC form would be like trying to capture a wave–it’s impossible.
Knowing that the battery is limited to DC energy and that solar modules produce in DC, it becomes obvious that the rest of the storage hardware should also be in DC. This is because each time that there is a conversion from AC to DC or vice versa, there is some amount of energy loss. But when PV power is stored directly in the battery in its DC form, there are no additional conversions from AC to DC and then back again to AC for use in the home or export to the grid. This means that a DC-coupled solution allows for higher system efficiency because there will only be one total conversion.
Beside minimizing the initial energy losses, there are other benefits to a DC-coupled storage system. First of all, a DC-coupled system can be implemented with only one inverter, which initially means simpler installation. Also, having one inverter managing the system makes it easier to coordinate advanced functionalities that are required in some locations, rather than trying to synchronize and coordinate these functions between two different inverters in an AC-coupled solution.
A DC-coupled solution also allows using PV power above the inverter rating, while an AC coupled system will not. For instance, in some locations there is a limitiation on the size of the inverter. So, if the inverter size is limited to 8 kW, with an AC-coupled system this would include both the PV inverter and the inverter for the battery. This means that an 8-kW, AC-coupled system would only be able to accomodate a 4-kW inverter for PV and a 4-kW inverter for storage–thus limiting the PV system to only 4 kW. But with a DC-coupled system, the 8-kW limitation would be allocated to only one inverter, thus allowing a larger PV system and greater battery charge/discharge capability.
Additionally, a DC-coupled system is able to route more energy to the battery because the energy flow is not limited by the inverter capacity. This is because the energy is routed directly to the battery without needing to go through any conversion. However, in an AC system, the inverter acts as a bottleneck for energy flow. As an example, in a PV system that has a 10-kW production but a 7.6-kW inverter, the energy to the grid and the AC-coupled battery would be limited to 7.6 kW. This means that potential energy would simply be lost. However, with a DC-coupled system the 7.6 kW would be routed through the inverter to the grid and the additional 2.4 kW would be sent directly the battery, without needing to pass through the inverter. Because the inverter does not limit power in an DC-coupled system, system owners are able to increase energy production, which leads to improved ROI.
DC-coupled solutions are also beneficial for backup storage systems. When an inverter is in backup mode because the grid is temporarily down, the inverter may try to power the backed-up loads with energy from both the battery and PV. When this is done in DC-coupled solution, the energy only needs to be drawn from a single inverter. But with an AC coupled system, the two or more inverters will require complicated syncronization. While at first look, it might seem that an AC-coupled system would split the burden up between the two inverters and potentially lighten the load of each, but it actually makes the energy management to be more complicated and can cause the energy production and storage to be decreased.
Lior Handelsman
By Lior Handelsman, VP of marketing & product strategy and founder of SolarEdge