# Dynamic Investigation of a Solar-Driven Brayton Cycle with Supercritical CO2
## 3 Results and Discussion
### Central Receiver's Results  
Figure 4 shows the yearly temperature variation of the central receiver's surface. This temperature seems to increase when the DNI density increases. This density increases throughout the summer months. The maximum temperature of the central receiver's surface appears to be 541.6 \({}^{\circ}\)C on the 26 th of July. The melting point of the chosen material for the central receiver's surface is 643 \({}^{\circ}\)C [43].  
Figure 3: Annual DNI’s variation for Athens.  
Figure 2: Annual ambient temperature variation for Athens, Greece.  
Figure 5 shows the heat fluxes of the solar thermal system. The maximum heat flux of the system must be the \(\rm Q_{sol}\). The \(\rm Q_{u}\) is zero for several consecutive operational hours throughout the year. This result is due to the control system applied to avoid negative values of \(\rm Q_{u}\) during the simulation. The higher the \(\rm Q_{u}\) value, the higher the instant solar thermal efficiency. The maximum \(\rm Q_{sol}\) is on the \(\rm 1^{st}\) of February, with a value of 73.3 \(\rm MW_{th}\), but the maximum density of \(\rm Q_{sol}\) appears during the summer months. The maximum \(\rm Q_{u}\) is found to be 47.5 \(\rm MW_{th}\) on the same day. As a result, the maximum daily solar thermal efficiency is found to be 64.8%.  
Figure 6 depicts the total thermal losses in comparison to the absorbed heat by the central receiver. The total thermal losses are never equal to zero, not even through the night,  
Figure 4: Annual central receiver’s outer surface temperature.  
Figure 5: Annual heat fluxes of total solar irradiance (\(\rm Q_{sol}\)), total heat absorbed (\(\rm Q_{abs}\)) and useful heat (\(\rm Q_{u}\)) for the solar thermal system.  
although the solar irradiance is zero. The reason for this is that, although the Q\({}_{\rm sol}\) is zero, the central receiver still has a higher surface temperature than the ambient. As a result, there are thermal losses due to convection with the ambient and radiation throughout the year. The maximum total thermal losses are during the summer months. This is because, although the ambient temperature rises, the temperature on the central receiver's surface rises at a higher rate, leading to an increase in total thermal losses.  
The total thermal losses are the sum of thermal losses due to convection with the ambient air and radiation. Figure 7 depicts the two parameters of thermal losses. Thermal losses due to convection are higher than those due to radiation. During the summer months, the percentage the total thermal losses that are convection losses has a maximum value of 83%, and the corresponding percentage of the radiation losses is 17%.  
Figure 6: Annual total thermal losses (Q\({}_{\rm loss}\)) of the solar thermal system in comparison with the total absorbed heat (Q\({}_{\rm abs}\)).  
Figure 7: Annual total thermal losses (Q\({}_{\rm loss}\)), thermal losses due to convection (Q\({}_{\rm conv}\)) and thermal losses due to radiation (Q\({}_{\rm rad}\)) for the solar thermal system.  
Figure 8 depicts the temperature variation of the molten salt stored in the high-temperature TES. This temperature must be within the temperature boundaries of the chosen molten salt. The lower limit is the melting point, while the upper limit is the maximum temperature at which the molten salt maintains stability. The maximum temperature of the molten salt is 507.9 \({}^{\circ}\)C, whereas the minimum temperature found in the cold TES, located before the solar tower, is 170.4 \({}^{\circ}\)C. The volume capacity of the hot TES reaches 65.9%, and the minimum density is found to be 1.72 g/cm\({}^{3}\). Figure 9 depicts the level of the volume capacity for the hot TES. This level expresses the ratio of the molten salt's volume to the tank's volume.  
Figure 8: Annual temperature variation of molten salt (T\({}_{\text{st}}\)) in the hot TES.  
Figure 9: Annual variation in the ratio of molten salt’s volume to tank’s volume for the hot TES.