Battery charging&discharging logic

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lx9
Battery charging&discharging logic

Hello SAM support team,

Thanks for your dedicated efforts in designing and upgrading this software. I set up a simple PV-battery model and used storage dispatch controller to regulate battery behaviours. However, I am confused by the operational logic of battery in SAM, even after I read several use manuals mentioned in your website. There are three cases in my attached model:

Case1: Peak-shaving 1 day look behind. In the screenshot of case 1, I found between Aug1 and Aug10, battery system is not working even with sufficient SOC

Case2: based on case1, I upscaled the battery size&power to 50kWh&10kW. Strangely, battery is ineffective during first 11 months.

Case3: I chose automated grid power target. I set power from grid to a fixed 3kW during all year. I identified maloperation of battery and showed them in red circles of case3's screenshot.

No financial factors are applied to my model. If possible, could you give me some hints on tuning the model? Sorry for my limited understanding in battery design, all battery models I used default parameters.

Besides, one extra question, I know the Parametrics function in SAM. I want to use it to identify optimal combinations of kWh&kW of battery. Could you tell me which criteria for optimal sizing? One staff from your team suggested in the video is to use Net Present Value. But I don't want to add financial parameters in my model.

Look forward to your reply
Many thanks

Tags: 
Paul Gilman

Hello,

In order to answer your questions, I will need a copy of the .epw weather file you used so that I can generate results that are consistent with yours. Would you mind either attaching that file to your original post above, or emailing it to me?

As for a metric for your parametric studies, you could use any output you like. For example, you might optimize to maximize battery efficiency, or to maximize annual energy discharged from battery, depending on what you are trying to accomplish. You can explore available metrics after you run a simulation on the Results page, Data Tables tab, under "Single Values" -- those variables would also be suitable to use as output variables for a parametric simulation.

Best regards,
Paul.

Paul Gilman

Hello,

Thank you for uploading the weather file.

To answer your questions about Case1 and Case2: For the peak shaving options, SAM is trying to dispatch the battery to minimize the demand charge for each month. At the beginning of each month, it does not discharge the battery until it reaches full charge to ensure that the battery is fully charged at least once a month. That's why for some months, you will see that the battery state of charge gradually increases until it reaches the full charge capacity (an input on the Battery Storage page that you can change -- it is 95% by default for Lithium-ion batteries). It uses power from the PV array to charge the battery, so it can take a few days to fully charge the battery, depending on the size of the array.

For Case3, I will need to investigate and get back to you.

As for a metric for your optimization, SAM does not generate one that indicates the amount of fluctuation. You could write an LK script to develop your own metric. For an introduction to LK scripting, please see this webinar recording:

https://sam.nrel.gov/node/69358

Best regards,
Paul.

ndiorio
ndiorio's picture

Hello,

I will also take a look at these and see if the battery model is operating correctly. At first glance there are some possible discrepancies in what is occurring.

Best,
Nick

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