## 2 Materials and Methods
### Methodology
The resultant mass of hydrogen to be compressed for sale is then determined using Equation (3):  
\[\mathrm{Hydrogen}_{\mathrm{mass}}[\mathrm{T}]=\frac{\mathrm{IND}\cdot\mathrm{ H}2[\mathrm{T}]}{\mathrm{LHV}\;\mathrm{H}_{2}} \tag{3}\]  
Losses are attributed solely to the hydrogen system. Losses associated with electricity production and transmission from RES and the conventional system are considered negligible due to their minimal impact on the overall efficiency. Additionally, the transmission losses through the interconnection are regarded as practically nonexistent, given the high efficiency involved in the interconnection infrastructure. This assumption simplifies the analysis by focusing on the most significant source of energy loss, the hydrogen system. Figure 18 provides a quantification of the losses linked to the operation of the electrolyzer and/or fuel cell system. The limited hydrogen storage, export capacity of the interconnection, and maximum hourly capacity of the electrolyzer system imply the possibility of rejection of RES production in case of surplus. The annual rejections of the system are considered an important metric of the design's efficiency. High rates of rejections imply RES installation that neither contributes to the island's electricity demand nor is exported through the interconnection, therefore failing to contribute financially. Combined with their high cost of installation and maintenance, they could potentially burden the expenses of the investment needlessly. Figure 14: Imports from the Peloponnese–Crete interconnection. The flowchart in Figure 19 details the methodology used to estimate the system's rejected power per operating hour, ensuring power balance for all hours of the simulation:  
After computing the hourly values to simulate the system's operation, the sustainability metrics utilized encompass the assessment of capital expenditure and net present value. This evaluation leverages the prices associated with the necessary components for investment and consumable costs, as detailed in Table 10. Using as reference [42; 43], Table 10 lists inverters necessary for converting DC current generated by PV, WT, and the fuel cell system into AC current, as well as a compressor for compressing electrolysis-produced green hydrogen and storing it in suitable hydrogen tanks. Figure 15: Power exported to the mainland through the interconnection (1 st half). Figure 16: Power exported to the mainland through the interconnection (2 nd half). \begin{table}
\begin{tabular}{c c} \hline \hline
**Component/Consumable** & **Price** \\ \hline Hydrogen Price: & 3.50 EUR/kg \\ Hydrogen Tank Cost: & 6 EUR/kWh \\ PV Cost: & 600 EUR/kW \\ WT Cost: & 1000 EUR/kW \\ Electrolayer Cost: & 1500 EUR/kW \\ Fuel Cell Cost: & 1500 EUR/kW \\ Compressor Cost: & 50 EUR/kW \\ Inverter Cost: & 100 EUR/kW \\ Desel Price: & 200 EUR/k \\ Mazut price: & 995.35 EUR/lt \\ RES OGM cost rate: & 2\% \\ Total OGM cost rate: & 5\% \\ Annual Discount Rate: & 3\% \\ \hline \hline \end{tabular}
\end{table} 
Table 10: Investment-related costs. The interconnection price is determined to be equivalent to the HEnEx Market Clearing Price (MCP) published in the HEnEx platform for each hour of the year, denoted in EUR/MWh, to which Crete, like all market participants, is subject. The expected lifespans of the fuel cell and electrolyzer system are estimated to be 10 years, whereas those of PV and WT are projected to be 20 years. Using Table 10, the NPV is calculated using Equation (4):  
\[\text{NPV}=\sum_{\text{i=1}}^{\text{t}}\frac{\text{Cashflows(i)}}{\left(1+ \text{r}\right)^{\text{i}}}-\text{CAPEX} \tag{4}\]  
The CAPEX comprises the costs associated with the PV and WT installation, coupled with inverters and the hydrogen system, which includes the electrolysis system, fuel cell system, hydrogen tank, inverter connected to the fuel cell, and compressor.