Hello Luis,
The preferred approach for heat loss modeling in the Linear Fresnel tool is to specify heat loss as a polynomial function dependent on temperature and (possibly) wind velocity. The evacuated tube model was originally developed for liquid HTF's such as Therminol or molten salt, and while it can model liquid water fairly well, it struggles to accurately model the internal convection in the 2-phase boiling regime and for superheated steam.
I would recommend using your detailed model to generate a heat loss correlation that adjusts the thermal loss to reflect the decreasing heat transfer coefficient of the receiver as water changes phase to steam. Once you do this, you can increase the discretization of the solar field if desired. SAM calculates the internal steam temperature as the flow increases in enthalpy throughout the loop, so any correlation you supply will be applied within the loop depending on the local fluid temperature.
As you probably noticed, separate correlations can be applied for the boiler and superheater sections. The nonlinearity of the heat transfer as it changes phase leads me to believe you may want to develop separate correlations for the boiler and superheater sections.
I've reviewed the methodology that we assumed for the evacuated tube model and don't believe it provides accurate enough results for boiler modules with high vapor fractions, and it likely underestimates the heat loss in the superheater. So again, I'd recommend using the polynomial heat loss model until we can improve the evacuated tube model for boiling/steam flow.
Best,
Mike Wagner
NREL
