Dear Tian,
SAM's solar water heating model is designed to model rooftop solar water heating systems that reduce the load of a conventional electric water heater. The financial model assumes that the value of solar energy is equal to the reduction in the building's electricity bill.
However, there is no limitation on the size of the system or load, so you may be able to use it for your district heating application.
For information about visiting NREL please see these websites:
www.nrel.gov/about/visiting-nrel.html
www.nrel.gov/careers/internships.html
Best regards,
Paul.