Grid turns on before battery reaches minimum depth of discharge

Hello,

I am trying to simulate a off-grid system using SAM. I have set the load profile and dispatch controller to meet the load in the morning using the battery first. However, the grid seems to turn on even before the battery reaches the set depth of discharge (30%). On Jan 16th for example, the battery state of charge is at 42% and the remaining load can be met by the battery. Is there a way to correct this issue?

Thank you,
Anurag

Hi Anurag,

There are a few things happening here:

The battery state of charge (SOC) is only a partial indicator of the energy available in the battery. Because of battery degradation, even if the SOC is above the minimum nominal capacity, the actual remaining capacity of the battery may be lower than the minimum.

The battery maximum discharge rate limits how much power the battery can deliver, so even if the battery SOC is above the minimum, if it cannot discharge enough power to meet the load (accounting for any losses), SAM may use the grid for supplemental power.

One potential issue that I do see is that in the last time step of a period where the battery is discharging to meet the load before a period when the PV system starts charging the battery, SAM uses grid power to supplement the battery even when the battery SOC is relatively high. I will ask one of our battery model developers to investigate.

I downloaded an hourly TMY weather file for New Delhi from the NREL NSRDB and interpolated it to 15 minutes. Unless you are using the same weather file I am different results than you, so I can't describe specific days, but I am seeing the pattern you describe.

Best regards,
Paul.

Hi Paul,

Thanks for looking into this and for the feedback. I am attaching my weather file if it makes it easier for the battery model developers and us to be on the same page. With this weather file, you should see the grid turn on January 16th briefly at 9:30 AM while the battery is at 42% state of charge.

The points you mentioned about actual remaining capacity and maximum rate of discharge should not be an issue here since I tried to rerun the analysis with lower "minimum state of charge" and higher "max c-rate of discharge" without a change in the behavior described above.

I did notice that if I stop the load after an hour and a half (9:30 AM) and then periodically turn it on at 15 minute intervals, I can get the battery to fully discharge to the minimum allowed state of charge.

Cheers,
Anurag

Hi Anurag,

Thank you for attaching your weather file.

For a lead-acid battery, SAM limits the discharge current according  to Equation 26 in DiOrio (2015) Technoeconomic Modeling of Battery Energy Storage in SAM available at sam.nrel.gov/battery-storage/battery-publications.html .

The information in SAM's Help system (Battery Storage, Behind the Meter, under "Battery Bank Sizing") also explains how the maximum discharge power is related to the size of the battery. Given the discharge rates required for your application, only about 50% of the capacity is available due to the bound charge and discharge equations. Either using a larger lead-acid battery or battery with a different chemistry should make it possible to avoid using grid power.

Best regards,
Paul.