DC coupling battery

[Patrik]

Hello,

I want to ask about CD coupling system, I created BYD battery with DC coupling but there is an issue that the battery is not working properly.

PV system should be working fine, but after calculating there is no use of battery for my application of grid connect battery system with own consumption.

As you can see on the graphs above there is only battery charging but no use of battery storage to feed the load.

Is there something that i did wrong or it is just your system issue ? I also tried a DC generator coupling but nothing changed.

I also enclose project in attachments and the parameters below.

I also want to regulate the feed-in the grid but i was not able to achieve this without operative battery system.

Thank you for your help with this problem. It took me a lot of time solving this, but with no satisfying result.

Patrik

PVSOL_1.pvprj

 

[developer_mh]

Hi Patrik,

thanks a lot for your question and for providing the project file, that always helps a lot. In your case there is actually no error, the battery is working well. It is just that in die diagrams the yellow bars are representing PV energy AND battery energy together (see the text in the legend). For DC coupled systems, the PV and battery energy are fed into the system on one common point, so the energies are displayed together. In the table view for the simulation results, you can see, how much energy is stored into and taken from the batteries:

 

Hope that helps, kind regards,

Martin

[Patrik]

Hi, Martin !

Yes, so battery is working but if I compared the AC/DC coupling for project it not working right, the usage of DC coupling battery is so low and there is a huge feed-in to grid.

graph.1 : Without battery system. There is grid Feed-in 1243 kWh/y.

graph.2 : PV system DC coupling battery system. There is grid Feed-in 1120kWh/y.

So  battery is working but it is so limited. There is enough capacity to store more energy and reduce the grid feed-in.

graph.3 : PV system AC coupling battery system. There is grid Feed-in 199 kWh/y.

As you can see when I use AC coupling PV system, there is a huge difference compared to DC coupling. And also no energy from grid is used to charge batteries.

If i compare both systems there should be nearly same grid feed-in, if batteries are not charged from grid.

Maybe there is something that i forget about.

Thank you for helping me.

Patrik

Enclosed project in attachments.

 

PVSOL_1.pvprj

[developer_mh]

Hi Patrik,

I understand your doubts. There are two differences of DC coupled systems that you should have in mind when you compare AC and DC coupled systems:

1. The DC coupled storage is only connected to one of the PV inverters. That implies that the other inverters are never participating in charging the batteries and all PV energy of the other inverters is fed into the grid if it cannot be used to cover the load in that moment
2. The charging power of DC coupled systems is subject to the available PV power of the inverter. Only if the PV power of that inverter is higher than the load, the excessive PV power is used to charge the batteries.

In your case the systems behaves like in the graph below very often:

• During the day, between 08:00 and 16:00, the PV power of inverter 1 (dark grey) is sufficient to cover the load (black)
• Please note that the dark grey line is PV and battery power together (as it is DC coupled)
• Only the (little) excessive energy of inverter 1 is used to charge the batteries (blue)
• The energy of inverter 2 (light grey) is nearly completely fed into the grid (red) between 09:00 and 17:00, as the load is already covered by inverter 1
• In the evening, the batteries are discharged to help cover the load (yellow), but as there is only a small amount of stored energy in them, they are rapidly empty - also compare the SOC of the batteries (purple):

 

This leads to a very small amount of battery energy used over the year.

Hope that clarifies the matter a bit. If you have further questions, please don't hesitate to ask!

Kind regards,

Martin