In SAM, we use the therm "auxiliary heater" to refer to the fossil backup system, which uses a gas boiler to provide heat to the HTF at the power block to supplement solar energy. The timing and quantity of fossil backup energy depends on the parameters you specify under Thermal Storage Dispatch Control on the Thermal Storage page. For the physical trough model, the "Aux heater outlet set temp" on the Thermal Storage page is for the fossil backup system, not the storage freeze protection system as suggested in Help. (I have corrected that mistake for the Fall 2012 version due for release in a few weeks.)
The hot/cold heater set point, tank heater capacity and efficiency are for the storage freeze protection system, which SAM assumes uses electricity to heat the fluid.
In SAM, freeze protection is provided by electricity. The auxiliary boiler is part of the fossil backup system and does not provide freeze protection.
Please see the diagram on p 80 of the physical trough reference manual. You can download the manual from the
Performance Models page
on the SAM website. I will put a copy of the diagram in the Help system.
Best regards,
Paul.