Simulation fails without DNI data

Hello,

I have 1minute weather file with columns Year,Month,Day,Hour,Minute,GHI,DHI,DNI,TDRY,WSPD,WDIR and I can run PV system simulation succesfully. But when DNI column is not present in the file, my simulation does not want to run and results in error:



According to www.nrel.gov/docs/fy18osti/67399.pdf (page 12), SAM should be able to work with GHI+DHI data, without DNI.

Am I doing something wrong?

Thank you

Hello,

Be sure to choose the appropriate "Weather File Irradiance Data" option under "Albedo - Sky DIffuse Model - Irradiance Data (Advanced)" on the Location and Resource page:



Best regards,
Paul.

yes, that was the case. Now simulation is running, and only several warnings appears:
SAM calculated negative direct normal irradiance -732.982 W/m2 at time [y:1990 m:12 d:5 h:18], set to zero.
But I think it is just a result of calculation of DNI, some kind of minor issue (maybe due to solar geometry, different model, ..?).

thank you very much for quick and accurate support.

Best regards
Branislav

Hi Branislav,

Yes, if there are only a few of those errors out of the 8760 hours in one year, you can ignore them.

Best regards,
Paul.

Hi Paul,
errors are in count about 20 per month (average). They occur in morning or evening, typically one or two in hour. For example, this is a list for October:

SAM calculated negative direct normal irradiance -356.634 W/m2 at time [y:1990 m:10 d:1 h:5], set to zero.
SAM calculated negative direct normal irradiance -834.828 W/m2 at time [y:1990 m:10 d:1 h:5], set to zero.
SAM calculated negative direct normal irradiance -3418.52 W/m2 at time [y:1990 m:10 d:1 h:5], set to zero.
SAM calculated negative direct normal irradiance -281.697 W/m2 at time [y:1990 m:10 d:2 h:5], set to zero.
SAM calculated negative direct normal irradiance -861.727 W/m2 at time [y:1990 m:10 d:2 h:5], set to zero.
SAM calculated negative direct normal irradiance -244.557 W/m2 at time [y:1990 m:10 d:24 h:4], set to zero.
SAM calculated negative direct normal irradiance -428.493 W/m2 at time [y:1990 m:10 d:24 h:4], set to zero.
SAM calculated negative direct normal irradiance -1732.08 W/m2 at time [y:1990 m:10 d:24 h:4], set to zero.
SAM calculated negative direct normal irradiance -256.498 W/m2 at time [y:1990 m:10 d:25 h:4], set to zero.
SAM calculated negative direct normal irradiance -698.732 W/m2 at time [y:1990 m:10 d:25 h:4], set to zero.
SAM calculated negative direct normal irradiance -3810.47 W/m2 at time [y:1990 m:10 d:25 h:4], set to zero.
SAM calculated negative direct normal irradiance -329103 W/m2 at time [y:1990 m:10 d:25 h:18], set to zero.
SAM calculated negative direct normal irradiance -281.736 W/m2 at time [y:1990 m:10 d:28 h:4], set to zero.
SAM calculated negative direct normal irradiance -15593.1 W/m2 at time [y:1990 m:10 d:28 h:4], set to zero.
SAM calculated negative direct normal irradiance -291.927 W/m2 at time [y:1990 m:10 d:30 h:4], set to zero.
SAM calculated negative direct normal irradiance -886.767 W/m2 at time [y:1990 m:10 d:30 h:4], set to zero.
SAM calculated negative direct normal irradiance -145.701 W/m2 at time [y:1990 m:10 d:31 h:4], set to zero.
SAM calculated negative direct normal irradiance -587.71 W/m2 at time [y:1990 m:10 d:31 h:4], set to zero.

Errors never occur around noon time, only morning or evening. So I think this is due to calculation on the edge of horizon (sun under/above horizon) and mostly due to very fine 1-minute time interval of data. Or do you think there would be another problem?

thank you and best regards
Branislav

Hi Branislav,

I don't think there is another problem. The code for these calculations is here: github.com/NREL/ssc/blob/728c55c9cef1781af1ee946d76542efead86f0fe/shared/lib_irradproc.cpp#L1059 .

Best regards,
Paul.