## 3 Results and Discussion
### Dynamic Analysis
#### 3.2.2 Financial Analysis  
To evaluate the system from an economic point of view, the selling price of hydrogen was considered as an additional parameter, except for the net electricity load, and the TES tank volume. Different values of the hydrogen price were assumed to take into account different future market conditions. The fundamental financial indexes, which are the payback period and the net present value, are presented in Figures 10-15. At first, all these figures indicated that the payback period increased slightly with the increase in the storage tank volume, while the net present value had a minor decreasing rate. Moreover, Figure 10 shows that the payback period decreased when the net electricity load increased. On the other hand, the net present value had exactly the opposite trend, as it is shown in Figure 11. Figures 10 and 11 present the results when the hydrogen price was equal to 15 EUR/kg. The lowest payback period value was calculated at 6.98 years, while the highest net present value is EUR 908,998. Both values were defined when the net electricity load was equal to 175 kW, and the TES tank volume was at 10 m\({}^{3}\); therefore, it is important to mention that if the hydrogen price was assumed at this level, the greater the net electricity production, which would lead to lower hydrogen generation, which is more economically viable. In addition, when the hydrogen price was considered to be equal to 20 EUR/kg, the aforementioned trends tended to change. First of all, lower payback period values were calculated for the net electricity load values from 125 to 175 kW according to Figure 12. The lowest payback period, which was equal to 6.98, was defined when the net electricity load was equal to 175 kW, and the TES tank volume was at 10 m\({}^{3}\); however, larger levels of the net present value were determined when the net electricity load was varied from 0 to 50 kW, as it is illustrated in Figure 13. The highest net present value of EUR 947,740 was calculated when there was no net electricity load, and the TES tank volume was equal to 10 m\({}^{3}\). In parallel, the lowest financial performance, i.e., when the net present value was found at lower levels, and the payback period at higher levels, was determined for a 100-kW net electricity load. Thus, hydrogen generation appears to be more financially sustainable compared to net electricity production, in some cases. The value of 20 EUR/kg  
Figure 9: Annual hydrogen production for different values of net electricity load and TES tank volume. can be characterized as a \"critical\" one, as the previously defined tendency with which net electricity generation performs better financially is now overturned. Furthermore, Figures 14 and 15 depict clearly that hydrogen production was more economically viable than net electricity generation, as the hydrogen price was assumed at 25 EUR/kg. Figure 14 indicates that the payback had a decreasing rate if the net electricity load decreased, and the electricity input to the electrolyzer increased. The net present had an increasing rate with the decrease in the net electricity load, which led to an increase in hydrogen production, as it is shown in Figure 15. In this case, assuming the price of 25 EUR/kg, the highest net present value levels and the lowest payback period values were specified to be equal to about EUR 1,409,579 and 5.83 years, respectively if the net electricity load was equal to zero, and the TES tank volume at 10 m\({}^{3}\). Consequently, the increase in the price of hydrogen makes hydrogen production, and the overall configuration, more profitable. Figure 10: Payback period for different values of net electricity load and TES tank volume when the hydrogen price is equal to 15 EUR/kg. Figure 11: Net present value for different values of net electricity load and TES tank volume when the hydrogen price is equal to 15 EUR/kg. Figure 12: Payback period for different values of net electricity load and TES tank volume when the hydrogen price is equal to 20 EUR/kg.