Specify IRR target mode not available notice

Hello SAM Team,Thank you for the excellent software. I am usingSAM to model PV + Battery energy storage systems (Single owner) to investigate
the techno-economic aspects for a small island power system. To ensure acceptable performance over a PPA period of 20 years, I am specifying a certain percentage of battery
replacement in a specific year (replacement at specified schedule option).
However, when I attempt to use the solution mode “Specify IRR target”, I get
the following message: “Specify IRR target” solution mode is not available! Forprojects with battery replacement schedules, choose “Specify PPA price,” or
change the battery replacement option on the Battery Storage page to “No
replacements.”. I get the same notice when selecting the “replace at specifiedcapacity option”.Despite thismessage SAM allows simulation to proceed and apparently the simulation is
successful because the IRR which is displayed on the results page is the
target. The results are the same when I reach the target IRR by varying the PPA
price manually. Grateful if you could confirm whether the notice is correct or
not and if SAM is able to use the Specify IRR target mode with battery replacement schedule. I am using SAM Version 2020.11.29, 64 bit.

During simulation,I also get the following warning: “Calculated negative global horizontalirradiance of -1.03603 W/m2 at time [y:2014 m:12 d:31 h:19], set to zero.” Theweather file I am using is from re.jrc.ec.europa.eu/pvg_tools/en/tools.html#TMY .I would appreciate to know what causes this message and how to clear it.Finally, I wouldalso like to know what is the difference between the Levelized Cost of Energy
and Levelized Cost of Generation?Thank you,Regards,Nawaz

Hi Nawaz,

You can ignore the message about using the "Specify IRR target" option with battery replacements. Both solution modes work with battery replacements, and with O&M costs specified in specific years. We will remove the message in an update to SAM 2020.11.29: github.com/NREL/SAM/issues/516 .

You can also ignore the error message about negative GHI means if it is not for very many hours, and if it is for hours when the solar resource is low. In this case, the error is for Hour 19 on December 31 when I would not expect the array to be producing very much power anyway.

When you use a weather file with DNI and DHI data, SAM calculates the GHI to report in results for reference but does not use it in simulations. SAM calculates array output from DNI and DHI. It uses the equation GHI = DHI + DNI * cos(Z), where Z is the solar zenith angle. For some sun angles, that may result in a negative value.

Please see the Help topic for levelized cos of energy for a description of how SAM calculates the LCOE. You can compare that definition to your definition of levelized cost of generation.

Best regards,
Paul.

Hello Paul,

Thank you very much for the helpful explanations.

Best regards,
Nawaz