## 2 Power Plant Operation Strategy in Dynamic Simulation
Under high DNI conditions the specified conditions of HTF are achieved, and the surplus of HTF is transferred to the TSS to start the charge phase. When the TSS is completely charged, i.e., maximum TSS capacity of 1025 MW\({}_{\rm th}\) h is reached, some SCAs are faced to the ground (stow mode) in order to avoid a further increase in the collected thermal energy. Optionally, the nominal HTF mass flow can be reduced below 600 kg/s when clouds prevent the DNI from falling on a number of loops. In this case, the thermal storage is discharged, if possible, in order to maintain the target heat flow to the PB. The storage discharge period can be classified during the daylight into cases: first, if the thermal oil mass flow in the SF continues decreasing from 600 kg/s down to a minimum value of 312 kg/s, a part of the HTF coming from the PB is redirected to the thermal storage system and the rest of the thermal oil is sent to the solar field. If the DNI is still low, the operation strategy proceeds to maintain the thermal oil mass flow at a designated minimum value of 312 kg/s. Here, the power plant is operated by transferring all the useful thermal power absorbed within the SF to the PB and compensating the rest of HTF from the thermal storage system if a discharge mode is available. Once the HTF mass flow in the SF falls below a minimum value of 312 kg/s due to the decrease in solar radiation, the designated temperature of thermal oil at the PB inlet is changed to 377 \({}^{\circ}\)C instead of 393 \({}^{\circ}\)C, and the nominal mass flow of thermal oil remains unaltered at 600 kg/s. As a result, if the mass flow of thermal oil in the SF continues falling to zero, the TSS supplies the nominal thermal power (125.75 MW\({}_{\rm th}\)) similar to the night period, as long as the storage discharge is possible. In this mode, 600 kg/s of HTF are directed to the TSS heat exchangers, hence no HTF is circulated in the solar field. Whenever thermal storage discharge is not possible and there is a lack of DNI, the PB is stopped, and the thermal oil is circulated in the SF at 1 kg/s as in the first period. Hence, if the turbine is stopped several times during daytime hours because of cloudy periods, the HTF will be warmed up again which in turn leads to a second start-up process. Therefore, several start-ups can occur during the day depending on meteorological data. After the clouds pass over, the HTF can be routed to the solar field again and the process repeats starting from the third phase, as explained above. The fifth phase (transient period) starts when the HTF mass flow falls to a value of 312 kg/s during the sunset period. The designated HTF temperature at the SF outlet is altered from 393 \({}^{\circ}\)C to 377 \({}^{\circ}\)C and the PB is operated based on the stored energy until the complete depletion of the TSS. Fossil fuel is used in the real power plant when there is no energy stored in the storage system. However, the fossil fuel backup system has not been included in the APROS model for the results obtained in this study since the data of the real fossil fuel backup system were not available. Finally, this operation strategy is applied in the validated power plant model for the cloudy summer day selected in this study.