## 2 Material and Methods
### Simulation Methodology  
The configuration was placed on Kythnos island, Greece. Due to the limited available space on the island, a solar field of about 975 m\({}^{2}\), which included 25 PTC modules, was selected. The first step of the analysis was the conduction of a preliminary analysis, which was carried out in steady-state conditions through the EES software [42]. The preliminary analysis included the investigation of the operation of the main sub-devices, which were the solar collector, the ORC system, and the electrolyzer; thus, the collector thermal efficiency according to Equation (6), the ORC efficiency, and the hydrogen production rate were defined. Afterward, the work was focused on the dynamic performance of the system, studying its performance during the day and the year. The dynamic model was developed in Matlab. At first, in order to carry out the dynamic simulation, typical meteorological and irradiation data for this location were used from the Photovoltaic Geographical Information System (PVGIS) interactive tool, which is a large database developed by the European Commission [43]. In addition, the steady-state analysis results that defined the performance of the PTC, the ORC, and the electrolyzer, were provided as inputs to the dynamic model. Subsequently, the storage tank differential equations were solved for each time step, for every single day of the year. Each day was solved multiple times until the numerical simulation met the convergence criteria. The simulation ended if the absolute errors of the temperature values \(T_{c,in}\), \(T_{c,out}\), \(T_{s,in}\), and \(T_{s,out,2}\) between the beginning and the end of the day, were lower than \(10^{-4}\); therefore, the operation during the days of the year was determined. The main system outputs, which were the net electricity and the hydrogen production, were strongly dependent on the ORC electricity production. This value relies on the ORC heat input, which comes from the available solar energy and the ORC thermal efficiency. The cycle efficiency was assumed to be determined through the evaporator saturation temperature and the ambient temperature, which affected the condenser temperature and was known for each time step from the meteorological data. Nevertheless, the evaporator temperature was not known a priori; therefore, different values of this temperature were examined, to find the one that led to the maximum daily electricity production. At this level, the examined thermodynamic parameters were the net (base) electricity load absorbed by the local grid, which ranged from 0 to 175 kW, and the TES tank volume, which varied from 10 to 50 m\({}^{3}\). These two parameters represented the two different storage methods integrated into this unit. Apart from the energetic and exergetic analysis, the system was also evaluated economically. In the financial study, another parameter was defined, which was the price of hydrogen. This value ranged from 15 to 25 EUR/kg [44], as it was assumed the hydrogen price was going to increase in the following years, due to the rising demand. Therefore, through the transient simulations, the main thermodynamic and financial indexes were determined for each combination of the aforementioned parameters. \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameters** & **Values** \\ \hline Cost of electricity (\(K_{\text{el}}\)) & 0.30 EUR/kWh \\ Specific cost of PTC & 230 EUR/m\({}^{2}\)[13] \\ Specific cost of the TES tank & 1000 EUR/m\({}^{3}\)[30] \\ Specific cost of the ORC & 1800 EUR/k\({}_{\text{el}}\)[13] \\ Specific cost of the hydrogen compressor & 1600 EUR/k\({}_{\text{el}}\)[38] \\ Specific cost of the hydrogen storage tank & 600 EUR/kg [40] \\ Specific cost of the PEM water electrolyzer & 800 EUR/kWh [41] \\ Specific cost of the heat exchangers & 100 EUR/kW [30] \\ Project lifetime (\(N\)) & 25 years [30] \\ Discount factor (\(i\)) & 4\% [30] \\ Operation and maintenance costs (\(K_{\text{ORM}}\)) & 2\% of the capital cost [13] \\ \hline \hline \end{tabular}
\end{table}