Time convention for hour angle

Dear Paul,

I am creating the tool for generating solar radiation data and using SAM to check some of the calculations. My question is which hour I should use in order to calculate solar elevation angle (alpha) when converting the horizontal beam radiation Ibeam_hor into direct component (DNI), as shown below:

Ibeam_hor=Itotal_hor-Idiff_hor; DNI_calc(hour)=Ibeam_hor(hour)/cos(90-alpha(hour-??))

According to the manual if I understood correctly it should be alpha(hour-0.5), but in this case the results do not exactly match with DNI values from weather file (radiation data for Almeria is exported from SAM as .csv file). I get perfect match only if I take alpha(hour) instead.

Thank you for your help and time.

Best Regards,

Tahir

Dear Tahir,

I assume that you are using the EPW weather file for Almeira, Spain from this website:

EPW Weather Files for Spain

The source of that data is described here:

Spanish Weather for Energy Calculations (SWEC)

The EPW weather file format is described in the documents linked from this page:

Weather Data Format Definition

The time convention for SAM's solar performance models is described in SAM's Help system:

Weather Data Overview

Have you tried comparing the results of your code with data from one of the TMY2 files included with SAM? Those files follow the time convention described in Help. I am not sure if the SWEC data for Spain uses some other convention.

Best regards,
Paul.