## 5 PTSPP Model Description
### Power Block (PB)  
The boiler in this power plant consists of a series of heat exchangers. Each line includes a superheater, evaporator, economiser and reheater. The steam-water path is designed based on two pressure sections, namely the main system pressure and reheat sections, as described in Figure 4.  
The water coming from the condenser enters a low-pressure feedwater circuit. Thereafter, it passes through five low-pressure preheaters to raise the temperature from 35 \({}^{\circ}\)C to 165 \({}^{\circ}\)C at the outlet of LP PH\({}_{5}\) (low pressure preheater) using the steam extracted from the LP turbine. The water is then collected in the deaerator, which is considered a type of open water heater. It is then pumped to the HP preheaters before entering the steam generator. In the deaerator, the water is purged of oxygen by HP steam flowing from the HP turbine to avoid corrosion. Both HP and LP preheaters work by the steam extracted from the turbine. These heat exchanger preheaters contain tube and shell sides where the extracted steam is condensed in the shell side while the water is heated in the tube side. Although the steam extracted from the turbine reduces the turbine power production, it also raises the feedwater temperature in the tubes, resulting in an improvement in the plant cycle efficiency. The steam exits the economiser and streams into the HP feedwater tank. The level of the HP tank is regulated by the HP feedwater main control valve (FW MCV\({}_{\text{HP}}\)). The circulation process between the evaporators and HP tank is carried out by the HP recirculation pump (HPRP) which generates saturated steam. Here, the HP tank works as a separator. While the water remains in the drum and mixes with water coming from the economisers, the saturated steam leaves the drum and enters to the HP superheaters, and here saturated steam absorbs additional heat from the thermal oil. Then, the HP superheated steam enters the HP turbine part. The steam temperature is kept from increasing above 384 \({}^{\circ}\)C at the turbine inlet by the HP attenperator, which is placed at the superheater outlet. The HP attenperator is supplied the feedwater by the HP feedwater pump. The pressure and temperature of steam entering the steam turbine are 106 bar and 384 \({}^{\circ}\)C, respectively. On the one hand, part of the steam is extracted to the high-pressure preheater (HP PH), while on the other hand, part of the steam is streamed to the reheaters in order to reheat the steam up to 383 \({}^{\circ}\)C. Thereafter, the reheated steam enters the LP turbine with a pressure of 19.4 bar. At the outlet of the LP turbine, the steam flows into thecondenser. After condensing the steam, the water is pumped using condenser pumps to the LP preheaters and then the cycle will continue in the power plant again. Figure 4: Steam generator circuit. Table 1 shows the properties of steam and HTF obtained from the Andasol II plant at the nominal load. To regulate the operating process of the power plant, it is essential to have control circuits. Here, the controllers for the LP/HP preheater level, the SH and RH attenperator, the drum level, the economiser water bypass and the steam bypass control circuits are implemented in this model. * Undesired steam is bypassed by HP and LP bypass controllers to the reheater and condenser before entering the HP and LP turbine, respectively. These controllers are used in the shutdown and start-up processes as well as for the steam turbine trip of any load. * The attenperators (SH Attemp CV and RH Attemp CV) adjust the steam temperature to (384 \({}^{\circ}\)C) at the turbine inlet. This cooling process is implemented by injecting some of the water from the boiler water pump to the attemperators. * In order to achieve the stability of flow in the evaporator, a small amount of water transferred from the economiser inlet is bypassed into the inlet of the drum.