## 6 Results
### Heat Transfer Fluid Behaviour  
The simulation results and experimental data are compared on 2 July 2010, including (DNI), the HTF temperature at the exit of SF, the thermal energy transmitted to the PB and the thermal stored energy accumulated in the TSS. In Figure 5, DNI data were measured in several weather stations on a selected day at the location of Andasol II. The average values of this data were used as inputs to the power plant model for the chosen day. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Andasol II** & \multirow{2}{*}{**Units**} & **SF** & **TSS** & **HP Turbine** & **RH/LP Turbine** \\ \cline{2-6}
**Working Fluid** & & **HTF (Thermal Oil)** & **HTF (Thermal Oil)** & **Steam** & \\ \hline \multirow{3}{*}{Conditions at the inlet} & (bar) & P = 31.3 & P = 17.5 & P = 104 & P = 19.4 \\  & (\({}^{\circ}\)C) & T = 295 & T = 393 & T = 384 & T = 383 \\  & (kg/s) & \(\dot{m}\) = 1100 & \(\dot{m}\)= 500 & \(\dot{m}\)= 54 & \(\dot{m}\)= 44.3 \\ \hline \multirow{3}{*}{Conditions at the outlet} & (bar) & P = 18.2 & P = 12 & P = 20.5 & P = 0.05 \\  & (\({}^{\circ}\)C) & T =393 & T = 293 & T = 215 & T = 30 \\ \cline{1-1}  & (kg/s) & \(\dot{m}\) = 1100 & \(\dot{m}\)= 500 & \(\dot{m}\)= 47.7 & \(\dot{m}\)= 33.7 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Technical data of the real power plant. After the outlet of SF, the simulated thermal oil temperature on 2 July 2010 displayed a good match with the measurements, as illustrated in Figure 5. At t = 9:59, a technical fault occurred, causing a drop in the measured temperature of HTF to 0 \({}^{\circ}\)C. The warm-up periods in the SF include two phases. The first phase begins at sunrise. As a consequence, the temperature of HTF raises from 170 \({}^{\circ}\)C to the designated temperature at the inlet of the SF (295 \({}^{\circ}\)C). In the second of the warm-up phase, the simulated temperature of thermal oil continues to rise until the designated temperature of the SF outlet (393 \({}^{\circ}\)C) is at t = 11:13. Additional heating of the HTF temperature is avoided by controlling the HTF flow through the SF to maintain the temperature of HTF at the designated outlet temperature (393 \({}^{\circ}\)C). Between t = 11:13 and t = 14:18, the temperature is kept stable at 393 \({}^{\circ}\)C despite severely fluctuating DNI values. After t = 14:18, the HTF temperature drops and reaches 375 \({}^{\circ}\)C because of the absence of stored energy and low DNI. At t = 14:30, the temperature of thermal oil increases to the designated outlet temperature. Thereafter, the temperature of thermal oil remains constant until t = 16:59, and here the HTF temperature decreases to 377 \({}^{\circ}\)C at t = 17:10 because the power plant operates in thermal storage mode. At t = 18:04, the simulated HTF temperature decreases from 377 \({}^{\circ}\)C to 287 \({}^{\circ}\)C at t = 18:49 because of the absence of stored energy, and DNI is completely absent due to dense clouds. After t = 18:49, the power plant continues operating only with the thermal storage until the end of the day. After the clouds clear, the thermal oil temperature rises again to 355 \({}^{\circ}\)C at t = 19:22. Afterwards it decreases to 250 \({}^{\circ}\)C because of heavily cloudy skies. At t = 21:00, the temperature increases again to 293 \({}^{\circ}\)C because of the removal of clouds. Thereafter, the temperature drops because of the sunset. Thermal power transferred to the PB is calculated based on the HTF mass flow at the inlet of PB and the variation between the thermal oil temperature at the inlet and the outlet of SF. In the actual power plant between t = 8:49 and t = 9:59, thermal power is generated even though there is no DNI, and storage power is exhausted. This indicates that the power plant was running on the fuel system during this period. Hence, this difference affects the production of electricity, which is dependent on the thermal energy behaviour.