Hi Bernardo,
Please see this post for a discussion of field and loop timestep-averaged temperature:
sam.nrel.gov/forum/forum-general/1910.
Over a given time step, if the solar field is producing more power than the TES can absorb, SAM defocuses collectors in the field to attempt to ensure that the TES is full at the end of the time step. Because calculating the field defocus fraction is an iterative process, the solution may not always be perfect.
SAM considers the TES to be full when it cannot accept additional mass of HTF, so if the hot tank temperature is colder than design and the tank is full, then the charge state will be less than capacity, but there is no room for additional charging.
If you enable dispatch optimization on the System Control page, the dispatch model may decide to leave some room TES for a future time step (although this should be rare).
The field can also defocus if it can’t absorb all the flux without exceeding mass flow or temperature constraints. Types of defocus are covered in this post:
sam.nrel.gov/forum/forum-general/3841
As for the results in your screenshot, we could not replicate them exactly because we do not have the weather file you are using. The mass flow and field fraction plots show that whenever the field is defocused, the mass flow is always 6. That suggests that the field is
is defocusing because of receiver mass flow / temperature constraints. One exception is hour 17 on July 19 where it looks like TES is full. Note from the description in the post I linked above that the time series output “Field optical focus fraction” includes only “plant control” defocus (collectors are defocused to avoid exceeding power cycle design limits) while “Field fraction of focused SCA” includes both plant control defocus and receiver control defocus (collectors defocus to avoid exceeding receiver design limits) signals.
Best regards,
Paul.