SAM PV Battery System

Hi,

I am trying to model a residential hybrid PV system in SAM and am just struggling to get the battery part of the simulation to give the results I expect. I am modeling a 15kW system with a 14.8 kWh 48V LiPO4 battery. When I input 14.8 kWh as my battery size, it seems to me that the battery is actually way below this capacity and charges/discharges far quicker than it should and as a result the battery hardly gets used at night. I was just messing around and instead used a battery capacity of 1474 kWh and that gives me results more in line with what I expect. I have attached my project file to reference, any help would be appreciated.

Hi Craig,

The array and inverter in your file seem to be mismatched. The diagram on the Losses tab of the Results page shows a large "Inverter MPPT clipping" loss, which suggests there may be a problem with the array design. On the System Design page, under "Subarray 1" and "Subarray 2" you will see that the String Vmp (string voltage at maximum power rating) for Subarrays 1 and 2 is 241.8 V and 483.6 V respectively. The inverter minimum MPPT voltage is 500 VDC, which means that it is likely the subarray operating voltages will never be high enough for the inverter to work. You can investigate that in the results by looking at the "Subarray [1/2] Operating DC voltage" and "Subarray [1/2] DC power gross" variables: It turns out that the operating voltage for Subarray 1  never reaches 500 V, so that subarray does not contribute to charging the battery. Subarray 2 does manage to reach the minimum MPPT voltage, but it can only charge the battery up to a maximum of about 47% state of charge.

One way to fix the problem is to adjust the number of modules per string in each subarray. I tried 13 modules per string for both Subarras 1 and 2, and set the number of strings in parallel for Subarray 1 to 1 and for Subarray 2 to 2. This results in an array size of 16 DC kW and avoids the clipping loss. With this array configuration, the battery results for the smaller battery look more reasonable to me.

Best regards,
Paul.

Hi Paul,

Thanks for getting back to me. I picked up the inverter mismatch after I posted and changed the panel layout and inverter so that I had no losses coming from inverter clipping. The problem remained and the think I found the problem in the "Battery Cell and System" settings. In the original I had set the nominal cell voltage to 51.2 V, when I changed this to 3.4V, which was more in line with the electro-mechanical properties in the "Battery Voltage" section, I got results more in line with what I expected. Below are two plots made using the original model I uploaded (so inverter is still clipping and I am wasting a lot of power), the first plot with the initial battery settings and the second one with the new battery settings. As you can see, the battery is working in the second plot more or less how you would expect it to - I am not sure if what I have done is correct but it seems to be working.

Hi Craig,

The cell voltage is for a single battery cell, so should be something on the order of 3.6 for a Lithium-ion LFP battery.

Best regards,
Paul.

Thanks Paul.