Simulation of PV converters for standalone site

[nicolas.uhl]

Hello,

I have a problem to simulate a standalone system (PV+battery+generator).

My simulation results give me a high level of "unused energy (battery full)".

I analysed the graphs, and the it is because the PV converters stop when battery SOC is 100% and restart when battery SOC is 75%.

So, solar power is lost and the load is fed from the battery (even if solar power is available), until the battery SOC reached the "restart" threshold.

This is not the way it works in reality.

In real life, when battery is full, the PV converters are still running and the PV energy is used to feed the load (thus keeping the battery fully charged).

How can I simulate this ?

Why do I need to define a stop and start threshold for the PV converters ?

Moreover PVsyst requires to define an hysteresis between the stop and start threshold.

Thanks in advance for your help.

Best regards

[André Mermoud]

Yes, most of the usual Solar converters work with a "Stop / Start" behaviour, based in the battery state, as it is done in PVsyst.

Some sophisticated converters use indeed a more complex charging strategy: when the battery is full (i.e. attains the charging OFF threshold), they don't cut, but reduce the voltage appplied to the battery, and maintain it at a value ensuring a little current (floating mode).

This has not yet been implemented in the simulation of PVsyst, it should be done soon.

This mode could indeed be used for ensuring your proposition, which is to maintain the PV production when it is sufficient for covering the user's needs. This will require a special operating mode of the controller, which is to exactly adjust the output power for ensuring this floating voltage. In the same way as when limiting the output power of grid inverters, this will be obtained by displacing the operating point on the I/V curve.

However if your battery is sufficiently sized (2-3 days of consumption), as far as you don't have to cut the load because it is discharged, the balance of the unused energy will be exactly the same. Because the Solar PV system cannot produce more energy than the user's needs !

If the battery remains charged during what would otherwise be the "battery cut", it will not need to be charged afterwards.

The only difference will be the battery wear due to cycling , and eventual battery efficiency losses.

You could indeed observe a (very small) advantage if you have to cut the user due to a complete discharge of the battery.

[nicolas.uhl]

Thanks for your feedback.

I better understand how PVsyst works.

But I can't understand your statement : " the balance of the unused energy will be exactly the same"

I can't see how it could be the same.

If my battery is getting fully charged during the day, my converters are still running to supply the load and the battery is maintained in Floating.

The solar power is used (not at the MPPT point) for the load.

So the energy is not fully lost.

So the total of unused energy can not be the same.

Could you clarify ?

Thanks and best regards

[André Mermoud]

Please think globally.

When keeping the battery charged and using the rest of the PVproduction, you have indeed less Unused energy at this time.

But during the next night you will discharge the battery to a lesser extent (higher DOD) than if it was discharged during this episod.

This means that during the next day you will reach the full charge earlier, and therefore have more unused energy.

Globally, over a long period and if you don't reach the Loss-of-load, you cannot produce more energy than what you consume (neglecting the losses in the battery).

This is the first principle of the thermodynamics: the conservation of the energy.