Hi Giovani,
The heat sink component in the steam Linear Fresnel model requires that the field outlet temperature meets or exceeds a temperature close to the design outlet temperature. At field outlet temperatures colder than this threshold temperature, instead of sending the field outlet to the heat sink, the model instead recirculates the field outlet back to its inlet. So when the heat sink is not operating, the field is either warming up or cooling down, and the field inlet and outlet temperatures are the same.
Ty