Dear Ahmed;
Please have a look at the files at the following links
Concentrating Solar Power Systems
sam.nrel.gov/case-studies
Modeling SEGS VI by physical trough model
sam.nrel.gov/node/73601
You can start your study with similar values in those files.
I am working on a long term research study. I find and cite to some thesis like "Simulation and Performance Evaluation of Parabolic Trough Solar Power Plants by ANGELA M. PATNODE".
You can find some thesis and publications in that scope. They present some general information about the limitations of the state of the industry. For the state of the art or state of the science or the state of the technology you should find the latest R&D studies and their corresponding publications. They will present the R&D materials capabilities.
Please take into consideration that your SAM study will be an iterative process. You will start with some initial values and finalize your study with your final values.
Have a nice day
Best Regards
Burak Omer Saracoglu, PhD, MSc, BSc.
www.frontiersin.org/people/BurakSaracoglu/104045/publications
www.researcherid.com/rid/G-5621-2013
www.researchgate.net/profile/Burak_Saracoglu