Thank you for the suggestion.
The Introduction to SAM Webinar (Feb 7, 2011) includes an introduction to modeling PV and parabolic trough systems.
The parabolic trough section starts at 1:08:20. I would recommend downloading the video by right clicking the "Download WMV" link rather than trying to stream the video. You can find a link to the webinar page here:
sam.nrel.gov/content/video-tutorial
Best regards,
Paul.