Dear Ahmad Mumtaz,
SAM's power tower models assume the receiver is made up of flat panels with the dimensions you specify on the Tower and Receiver input page. For a description of the type of receiver that SAM can model, please see the Help topic for that page.
For more technical descriptions of SAM's performance models, including the power tower models, please see the Performance Documentation page on the SAM website:
sam.nrel.gov/performance
Best regards,
Paul.