Hi Bernardo,
SAM's CSP models assume that the power cycle HTF is the same as the solar field HTF, so the specific heat Cp value in that equation is from the solar field HTF property table.
For models with thermal energy storage (TES), the TES storage medium may be different from the solar field HTF. In that case, during discharge of heat from the TES, the heat is transferred from the storage medium to the solar field HTF and then to the power cycle.
Best regards,
Paul.