## 3 Results and Discussion
### Simple Parametric Investigation  
The next step in the present investigation regards the parametric evaluation of the examined unit. Four critical parameters are investigated and more specifically the turbine inlet temperature, the turbine inlet pressure, the cycle low-temperature level in the condenser and the solar irradiation level. Figures 5-8 depict the impact of the aforementioned parameters on system efficiency, collector efficiency, cycle efficiency and electricity production. Figure 4: Pressure—specific enthalpy diagram of the thermodynamic cycle in the default scenario. Figure 3: Temperature—specific entropy diagram of the thermodynamic cycle in the default scenario. Figure 5: Electricity, system efficiency, cycle efficiency and collector efficiency for different temperature levels in the turbine inlet. Figure 6: Electricity, system efficiency, cycle efficiency and collector efficiency for different pressure levels in the turbine inlet. Figure 5 shows the influence of the TTT on the results. Higher TTT leads to lower collector efficiency because the higher operating temperature in the collector leads to smaller thermal efficiency due to the thermal loss increase. Higher TTT leads to higher cycle efficiency due to the higher potential for work production, something that is in accordance with the Carnot cycle. The unit efficiency is the product of the other efficiencies and so the different impact of the TTT on the collector and the cycle efficiency makes the system efficiency have a combined behavior. Therefore, there is an optimal TTT that maximizes the system efficiency and consequently the electricity production. The next formula shows clearly the aforementioned analysis:  
\[\eta_{\text{sys}}=\eta_{\text{collector}}\cdot\eta_{\text{cycle}} \tag{52}\]  
Figure 8: Electricity, system efficiency, cycle efficiency and collector efficiency for different solar beam irradiation levels. Figure 7: Electricity, system efficiency, cycle efficiency and collector efficiency for different low cycle temperature levels. The optimum TIT for the examined case is found for 800 K. In this case, the collector efficiency is 73.62%, the cycle efficiency is 32.96%, the system efficiency is 24.27% and the electricity production is 44.14 kW. Figure 6 illustrates the impact of the high pressure on the examined parameters. Higher values of the pressure in the turbine inlet lead to an increase in all the efficiency indexes and electricity production. The increase of the pressure makes higher the cycle efficiency, something that is usual in Brayton cycles. The increase of the pressure level in the collector efficiency is a result that has been also found in Ref. [30]. More specifically, higher pressure leads to higher density and consequently higher mass flow rate inside the solar collector. The higher mass flow rate makes the heat transfer from the absorber to the fluid to be more effective due to the increase of the heat convection coefficient and thus the collector efficiency improves. The increase in the system and electrical efficiency is a result of the increases in the other efficiencies. Figure 7 depicts the influence of the low cycle temperature on the results. Generally, lower values of this parameter are beneficial for all the examined parameters. The lower condenser temperature makes the system operate to lower temperatures and so the collector operates in lower temperatures and the smaller thermal losses lead to higher collector efficiency. Moreover, lower condenser temperature reduces the low pressure of the system and so the turbine has a higher margin to produce work due to the higher expansion ratio. Therefore, the cycle efficiency, system efficiency and electricity are improved with the lower heat rejection temperature. At this point, it has to be commented that the system can operate during transcritical mode during the summer when there is a geothermal heat sink or a water-cooled condenser (e.g., from a river or from the sea).