CSP models and NSRDB data time convention

When I choose a NSRDB weather data file for my parabolic trough model, I see the following text in red:

CSP models use a different time convention than NREL NSRDB data.  See Help for details.

But when I look through help I don't find anything that addresses this.  Can you explain what this means and what I need to do to my NSRDB data files to allow the parabolic trough model to use it correctly?  When I use as-is I find that maximum DNI doesn't match with minimum zenith angle which is what I would expect.

Thanks,  Matthew

Hi Matthew,

There is a short explanation in the "Time and Sun Position" topic of Help: samrepo.nrelcloud.org/help/weather_time_convention.htm

Let me know if you need any additional clarification.

Best regards,
Paul.

Paul - from the link you provided, it looks like the difference is that the NSRDB data starts at 0:30am (and then every hour after that) whereas SAM expects 0:00am?

The warning in SAM seems to suggest that the issue with NSRDB data is unique to CSP, but the link that you sent seems to refer to all fo SAM - is that correct?

So what's the suggested correction for NSRDB files?  Manually alter the time on all rows?  Can you get away with subtracting a 30 minute offset the timezone?

Thanks,
Matthew

Hi Matthew,

There is no need to make a correction.

The hourly irradiance data in the NSRDB represents the solar irradiance in W/m2 available at the midpoint of the hour.

SAM's concentrating solar power (CSP) models assume that the hourly DNI value represents the average energy available over the hour, and calculate sun angles at the midpoint of the hour.

For hourly solar resource data, the two assumptions are equivalent.

Best regards,
Paul.

If that's the case, then why is there a warning message in red saying that they use a different time convention (see attached screenshot).

As well, when I run my model, my maximum DNI doesn't occur at the time of minimum zenith consistently (see attached as well).  This seems to be a problem with interpreting the solar resource file as that's the only thing that would affect those two quantities.

 

Hi Matthew,

The red warning is to let you know about the difference. If you are using sub-hourly solar resource data, it might be a consideration.

The coincidence of beam irradiance and sun zenith angle is easier to see in the graph if you change the model to "stepped lines" (click the wrench icon to change the mode). You can also use the data tables tab to compare the values.

 

Best regards,
Palu.