Dear Fernando,
SAM's Empirical Trough model was originally developed as a spreadsheet model, and its results were compared to measured performance data for the SEGS VI system. Please see the Empirical Trough model paper referenced on the SAM website's performance model documentation page for more details:
sam.nrel.gov/performance
In general, SAM's Physical Trough model is better for systems that have different configurations. The Empirical Trough model works best for systems that have configurations similar to the SEGS systems. Please see the descriptions of the Physical Trough model and Empirical Trough models in SAM's help system for a discussion for more about the two models.
Best regards,
Paul.