AC Coupling vs DC Intermediate circuit

[tsarbiker]

Hi,

 

I have been comparing 2 systems.

They are the same except one has one hybrid inverter and the second one has an inverter + an battery inverter.

What I would expect is that the system with the DC coupling would be more efficient.

It doesn't have to do DC-AC AC-DC but just DC-AC to provide electricity for the load.

In practice they are extremely similar and I just don't understand how that is possible .

The losses for the battery is almost the same, I would expect double the amount for AC coupling.

How is this possible?

 

Thanks!

BatteryAC.pvprj BatteryDC.pvprj

[developer_mo]

Hi tsarbiker,

that is definitely an interesting question. Many thanks for the post.

Your assumption that DC coupling is more effective because there are fewer conversion losses makes sense. The fact that the losses are nevertheless very similar is due to the fact that a DC-coupled system often achieves low efficiency when only a small amount of power is required. This is because the energy still has to be converted into alternating current by the PV inverter in the DC coupling, but the PV inverter has a lower efficiency at low loads than the AC-side battery inverter, which is designed precisely for the battery.

For example, if only a small refrigerator needs to be supplied with energy at night and this draws power from the DC-coupled battery, the PV inverter operates in a poor efficiency range (red marked zone in the screenshot below). If the DC-coupled battery were to be charged exclusively with a high utilization rate, the system with the DC coupling would probably be more efficient than the one with the AC coupling.

Hope that helps explaining the similar results.

Kind regards

Mikio

 

[tsarbiker]

This is awesome , it makes sense now .

 

Thank you!