## 3 Results and Discussion
### Exploitation of a Hypothetical Battery System in Combination with Other Use Cases  
To offer a more comprehensive comparison of the performance of the energy flexibility concept, we performed an economic viability analysis of a hypothetical battery system. Initially, we examined it as a standalone system, then in combination with the national electricity surplus utilised for aluminium smelting and a hypothetical 90 MWp PV system. The economic feasibility of the investment depends on the electricity surplus and aluminium prices, as well as aluminium production costs. For clarity, we utilised the higher electricity surplus price from Table 3 and analysed the same scenarios as in the previous subsection. For all scenarios, we first calculated the NPV for a standalone battery system with an energy capacity of 30 MWh and a rated power of 12.5 MW, following the methodology described in Section 2.4. The battery system was charged with electricity surplus at the national level within its capacity, with the stored energy later directed to aluminium smelting. The results of the calculation over a decade are shown in Figure 12. The results suggest that dedicating a battery system solely to aluminium smelting is not financially viable. Notably, due to the lack of surplus energy and ongoing operational costs, the NPV decreases in all scenarios during the initial years. In the more favourable Scenarios 2 and 4, the NPV begins to increase after a few years, whereas in the other scenarios, it continues to decline. \begin{table}
\begin{tabular}{l c c c} \hline
**MWp** & **Scenario** & **IRR (\%)** & **PP (Years)** \\ \hline \multirow{6}{*}{60 MWp} & Scenario 1 & \(-\)4.60 & / \\  & Scenario 2 & 2.99 & 9.25 \\  & Scenario 3 & 9.20 & 6.72 \\  & Scenario 4 & 12.31 & 5.84 \\  & Scenario 5 & \(-\)10.06 & / \\ \hline \multirow{6}{*}{90 MWp} & Scenario 1 & \(-\)5.26 & / \\  & Scenario 2 & 2.18 & 9.68 \\ \cline{1-1}  & Scenario 3 & 8.25 & 7.03 \\ \cline{1-1}  & Scenario 4 & 11.28 & 6.11 \\ \cline{1-1}  & Scenario 5 & \(-\)10.63 & / \\ \hline \end{tabular}
\end{table} 
Table 4: IRR and PP for the hypothetical 60 MWp (a) and 90 MWp (b) PV systems at varying aluminium prices and aluminium production costs. Figure 11: NPV for five scenarios with varying aluminium prices and aluminium production costs for the hypothetical 60 MWp (**a**) and 90 MWp (**b**) PV systems over a decade. The economic viability was then calculated for a combination using the national electricity surplus first for aluminium production up to the consumption limit of the large aluminium smelter, with the remaining electricity surplus being used to charge the battery system within its capacity and the charged energy later utilised for aluminium production. The results are shown in Figure 13 and Table 5. The results show that the investment is not financially viable within 10 years in any of the scenarios, although the most favourable Scenario 4 comes close to achieving profitability. flexibility concept), which can produce enough aluminium to save up to EUR 17 million over a 10-year period. From a low-risk perspective due to the lack of investment, it would be worthwhile to use these electricity surpluses in any case, even if a modest-sized PV system were constructed. We also carried out economic calculations for the case of an investment in a battery system, which, due to the high initial costs, is not yet recouped over a period of 10 years, using the battery system exclusively for the purpose of storing the peaks and consuming them. Certain limitations should be considered when interpreting the results. The extrapolations of future electricity consumption and production are subject to uncertainty, as they depend on current projections and do not account for possible changes in energy policy or market conditions. Furthermore, the lack of detailed data on local PV installations means that local self-consumption was not explicitly separated, potentially affecting the accuracy of consumption and surplus assessments.