## 2 Materials and Methods
### Methodology
The specified priority chain is summarized in Figure 6:  
The proposed approach for managing excess RES production exceeding the total hourly load and minimum conventional unit operation requirements employs a multi-step strategy. Firstly, the system prioritizes conversion of any surplus into hydrogen for storage in available tanks through electrolysis. The electrolysis process efficiency is estimated to be around 80% [37]. This approach capitalizes on hydrogen as a viable storage solution, enabling the use of excess RES during low-demand periods. Following hydrogen storage prioritization, any remaining excess beyond the technical minimum and storage capacity is exported to the main grid. This export is limited to a pre-determined value. The export of surplus electric energy through the interconnector is subject to a pre-established nominal value. This value is contingent upon either the interconnector's maximum load capacity or negotiated power purchase agreements with the relevant electricity provider. Figure 6: Priority axis of load satisfaction. Finally, if a surplus persists after storage and export, the strategy suggests the additional option of hydrogen production for sale, primarily targeting the industrial sector up to the full capacity of the electrolysis system. Any remaining excess production beyond this point is, thus, rejected by the system. The specified process is summarized in Figure 7:  
On an hourly basis, the simulation calculates the hourly RES production. This includes the RES production recorded for each hour T of the year 2023, CRETE RES[T] (sourced from HEnEx data) and the additional production from the RES system expansion. The production from the expansion is determined by multiplying the installed capacities of PV and WT systems (denoted as PV\({}_{\text{cap}}\) and WT\({}_{\text{cap}}\)) by the normalized output of a photovoltaic and wind park located in Crete, using Equation (1):  
\[\text{RES}[\text{T}]=\text{CRETE RES}[\text{T}]+\text{PV}_{\text{cap}}\times \text{PV}_{\text{norm}}+\text{WT}_{\text{cap}}\times\text{WT}_{\text{norm}} \tag{1}\]  
Direct RES refers to the renewable energy production that directly meets demand, as calculated in Figure 8.  
The determination of the hydrogen storage system's state of charge is delineated across two figures: Figure 9, illustrating the charging instances, and Figure 10, depicting  
Figure 8: Computation of RES production successfully allocated for demand satisfaction. Figure 7: RES production allocated following the proposed priority. the discharging instances. The hydrogen system discharges hydrogen to contribute to load through the fuel cell system until satisfaction or total release. The tank is filled with green hydrogen produced through electrolysis by an amount attributed to the surplus of RES energy production or until full capacity. Figure 9: Hydrogen storage’s state of charge in case of required discharge. If hydrogen storage is ample, Figure 11 computes the contribution of the fuel cell system to the hourly demand. The fuel cell system's priority is higher than the power plants', which is so in order to decrease the carbon footprint associated with the emissions originating from the plants' fuel consumption. The contribution of the fuel cell system is quantified through the monitoring of the hydrogen tank's state of charge for each hour. Figure 12 calculates the amount of surplus energy effectively stored in the hydrogen tank for each hour. In order to estimate the current annual CO\({}_{2}\) emissions derived from diesel and mazut, the data were used in [38] and presented in Table 7. Since the data concern the year 2021, the assumption being made is that the energy mix has not deviated significantly in the span of 2 years. Figure 6:0 Hydrogen storage’s state of charge in case of electrolysis process after load satisfaction. Using Table 7, the shares of the two fuels of the total conventional production are estimated. This is achieved by estimating the efficiency of the two fuels, which is calculated using Table 8 and [39].