## 5 Results and Discussion
### Technical Performance
#### 5.1.2 Impact of Surplus Electricity: Scenarios II and IV  
The inclusion of electrical surpluses in Scenarios II and IV increases electricity production. Moreover, this integration would prevent a significant amount of CO\({}_{2}\)-equivalent emissions that would otherwise be generated by the non-renewable share of the Spanish electricity mix from being released into the atmosphere. Assuming an average emission factor of 0.21 tCO\({}_{2}\)-eq/MWh\({}_{\text{e}}\) for the period 2016-2021 [71], it is estimated that between 687 and 3153 tonnes of CO\({}_{2}\)-eq could be avoided through the effective use of surplus electricity. Even in years with minimal surpluses, such as 2018, the use of particles enhances electricity generation by up to 3 percentage points compared to scenarios without surpluses. Figure 7 provides a detailed comparison with other cases, illustrating the variation in electricity production relative to Scenario I, as well as the quantity of electrical surpluses stored in each scenario.  
In terms of overall conversion efficiency (sun-to-electricity) and CF, the highest values are observed in Scenario II for the year 2021, reaching 18.8% and 65.8%, respectively. Scenario IV, however, surpasses these figures in the same year, with gains of 1 and 4 percentage points in efficiency and CF, respectively, highlighting its enhanced energy recovery strategy. Table 11 provides a consolidated overview of these metrics, along with a side-by-side comparison of Scenarios I and III.  
Regarding energy density, no changes were observed as the values remain consistent with those reported at the end of previous section.  
One of the key performance indicators for battery systems is round-trip efficiency [72]. This metric is defined as the ratio between the energy produced using the battery and the electricity absorbed during charging. In this study, as a CSP plant is also included, the efficiency is adjusted using the following equations:  
\[E_{disch}=E_{II/IV}-E_{I/III} \tag{11}\]  
\[\eta_{rt}=\frac{E_{disch}}{E_{load}} \tag{12}\]where E\({}_{\text{II/IV}}\) refers to the total energy produced in Scenarios II/IV, which account for electrical surpluses, and E\({}_{\text{I/III}}\) refers to the energy produced in Scenarios I/III, which exclude electrical surpluses.  
Applying (12), an average round-trip efficiency of 25.56% is obtained for the period 2016-2021, while Scenario IV achieves a slightly higher efficiency of 27.27%.  
These values fall below the range reported in the literature for this type of storage system, typically between 45 and 65% [72]. However, in real-world projects, these values often decrease to 25-40% [48]. The lower efficiency observed in this study is primarily due to the storage strategy being optimized for solar irradiation rather than for electrical surplus storage. In addition, the lack of temporal alignment between surplus availability and storage demand further limits the overall conversion efficiency. Future improvements could involve dynamic dispatch strategies that better match surplus profiles.  
Lastly, regarding the plant's energy efficiency, as discussed in the previous section, an increase is observed for all years in the study period, ranging from 1% to 9%. This improvement is more pronounced in scenarios using particles compared to molten salts, aligning with the observations made at the end of previous section.