Battery Efficiencies

Hello and thank you for all the detailed information on this forum. There are still a few questions I have not been able to find the answer to. 

I am trying to model AC-connected batteries with different roundtrip efficiencies (from technical specifications), but SAM requires charging and discharging power conversion efficiencies (AC to DC and DC to AC). Am I correct in assuming that the product of these two efficiencies that SAM requires is the same as the roundtrip power conversion efficiency (excluding auxiliary system losses)? I understand that SAM computes a total roundtrip efficiency when it runs the simulations, so I want to make sure that I am not double-counting any losses. Any tips on how to translate technical specifications into SAM inputs would be greatly appreciated. 

Also, I noticed that all Li-ion batteries have 96% as the default power conversion efficiency, each way. Is there a reason for this, given that in reality LFP is less efficient than NMC, for example? 

Lastly, is there any way to include the auxiliary system losses as a % loss instead of as a set kW loss? Would there be unintended consequences if the power conversion efficiencies were adjusted to include these system losses? 

Thank you, 
Nayeli 

Dear Nayeli,

The battery roundtrip efficiency in SAM is an output rather than an input to the model. You can use it as an indication of how much the battery is used, and how well the system is designed. You do not need to provide a value of the battery roundtrip efficency as an input. You could compare the SAM output value to the manufacturer's value as an indication of whether the battery as modeled is performing within the manufacturer's specifications.

The conversion efficiency inputs in the Power Converters group on the Battery Storage page are intended to account for losses in power electronics in the battery storage system, not for losses in the battery cells themselves. SAM does not have a detailed model of the DC/AC, AC/DC, and DC/DC conversion process. For example, to model AC to DC conversion, it simply multiplies the AC power value by the conversion efficiency.

The default values of the conversion efficiency inputs do not depend on the battery chemistry. You can change them from the default values to reflect the expected performance of electronics in your system design.

The battery ancillary equipment losses cannot be specified as percentages. Remember that there are also loss inputs on the Losses page that you could use to account for losses in the entire system as a percentage of incident irradiance, DC power output of the PV array, or AC power output of the inverter.

Best regards,
Paul.

Hi Paul,

Understood. Thank you for your quick reply. I will probably use the default power conversion efficiencies in SAM for the simulations and compare the roundtrip efficiency output from SAM with the battery data I have. 

Regards, 
Nayeli 

Hello again, 

I am now trying to model a vanadium redox flow battery in the generic system model and the results show that it discharges more energy than it charges, and an efficiency of 100%. I did not set any auxiliary system losses, but I did set power conversion losses and these do show in the lifetime hourly data. However, this same data and the annual energy flows show that the energy discharged > energy charged. Is there something wrong in the VRF model? I am attaching the SAM file for you review.  

Thank you, 
Nayeli 

Hi once more, 

I am modeling a vanadium flow battery with both power conversion losses and ancillary equipment losses and it seems SAM is not taking them into account. 

• While the lifetime hourly data does show power conversion losses, these do not show up in the annual data. 
• Battery system losses do not show up in either hourly nor annual data. 
• The efficiency is computed as 100% 
• There is more energy discharged than charged in both the hourly and annual data.  

Moreover, the issue with the auxiliary equipment losses is also present with LFP. 

I am attaching models with both vanadium and LFP, both including very large auxiliary system losses that are not showing in the results. Is there something I can do to make sure the simulations take these losses into account? 

Thank you, 
Nayeli 

Hi Nayeli,

There are some issues with the flow battery and battery ancillary loss models that we are addressing:

github.com/NREL/ssc/issues/404

github.com/NREL/ssc/issues/379

We plan to release Revision 3 to SAM 2020.2.29, which will address these and a few other issues.

Best regards,
Paul.