## 2 Power Plant Operation Strategy in Dynamic Simulation  
The operation strategy performed in the dynamic simulation of the parabolic trough power plant for a single day can be defined based on six operation periods, as explained below and depicted in Figure 1:  
* The first period is the period between the time of thermal storage depletion and sunrise. * The second period represents the first warm-up period of HTF, when the HTF temperature in the SF reaches the designated temperature at the inlet (295 \({}^{\circ}\)C). * The third period corresponds to the second warm-up period of HTF, where the temperature of HTF in the SF achieves the designated temperature at the outlet (393 \({}^{\circ}\)C). * The fourth period represents a full-load operation period during the daytime. * The fifth period represents the transient period that precedes the sunset. * The sixth period is the evening period, where the thermal storage is discharged until it is completely depleted. This operation strategy carried out in the dynamic simulation model represents the best approach comparing with the operator decisions implemented in the Andasol II. During the first and the sixth periods (night periods), the dynamic model checks firstly whether the thermal storage discharge is available, taking into consideration the boundary conditions of the power plant. If the discharge mode is available, the power plant produces electrical power (48 MWh). On the other hand, the HTF is not circulated in the solar field. Thermal storage continues discharging during night hours until stored energy is depleted. Figure 1: Schematic diagram of the operation strategy during a single day for a PTSPP model. Conversely, when the thermal storage discharge is not available, electrical power is not produced by the steam turbine. The HTF is still circulated by the recirculation pumps within the SF at 1 kg/s for one loop as long as the design inlet temperature (295 \({}^{\circ}\)C) is not achieved. When the thermal oil temperature at the outlet of each loop decreases lower than 70 \({}^{\circ}\)C (minimum input value when there is no solar radiation or stored energy), the HTF protection system is activated to prevent the thermal oil temperature in the SF from dropping below a specified minimum temperature. The second period (first HTF warm-up period) starts when the sun rises and continues until the designated temperature at the inlet (295 \({}^{\circ}\)C) is reached. After sunrise, the thermal oil in the SF is circulated with mass flow of 390 kg/s until the designate inlet temperature (295 \({}^{\circ}\)C) is reached. During this period the thermal oil is not transferred to the PB as the useful thermal power in the SF is still equal to zero. Subsequently, the third period (second HTF warm-up period) begins when the design inlet temperature (295 \({}^{\circ}\)C) is achieved. Thereafter, the temperature and mass flow of HTF will increase together, where the design outlet temperature (393 \({}^{\circ}\)C) is first achieved and then the nominal value of mass flow (600 kg/s). This amount of thermal oil mass flow sent to the PB must not exceed its limit value (600 kg/s) in order to avoid the production of more steam than required. It is worth mentioning that the useful thermal energy is sent to the PB on the HTF in this period. Coinciding with the third period, the start-up period of steam turbine starts until the steam production reaches its nominal value (55 kg/s). The assumptions implemented in the dynamic model of PTSPP are made to simulate the measured data curves for a 50 MW\({}_{\rm el}\) PTSPP, located in Spain. The simulation indicates an excellent agreement compared to the experimental data collected from Andasol II. The fourth period (full-load operation period) begins after the second warm-up period of HTF, where the design conditions of HTF (600 kg/s and 393 \({}^{\circ}\)C) are reached at the power block inlet. This period continues during daytime until sunset.