## 2 Material and Methods
### Financial Evaluation  
The investment cost of the cogeneration unit (C\({}_{0}\)) consists of the cost of the PTCs, the thermal energy storage tank, the organic Rankine cycle, the PEM water electrolyzer, the hydrogen compressor, the hydrogen storage tank, and the heat exchangers (water preheater and hydrogen cooler). It is expressed below:  
\[\text{C}_{0}=\text{C}_{\text{PTC}}+\text{C}_{\text{T}}+\text{C}_{\text{ORC}}+ \text{C}_{\text{WE}}+\text{C}_{\text{HC}}+\text{C}_{\text{HST}}+\text{C}_{ \text{HEX}} \tag{40}\]  
In addition, the annual cash flow (CF) is determined assuming the annual inflows and outflows. The annual inflows are defined as the income from electricity and green hydrogen production, while the annual outflows include the operation and maintenance costs.  
\[\text{CF}=\text{Y}_{\text{el}}\cdot\text{K}_{\text{el}}+\text{Y}_{\text{hydr }}\cdot\text{K}_{\text{hydr}}-\text{K}_{\text{O\&M}} \tag{41}\]  
In the previous equation, (Y\({}_{\text{el}}\)) is the yearly electricity production in kWh, (Y\({}_{\text{hydr}}\)) is the yearly hydrogen generation in kg, and (K\({}_{\text{hydr}}\)) is the selling cost of hydrogen in \(\text{\text{\text{\text{\text{\text{\text{\  
database is used for the thermodynamic properties of all the examined working media. The aforementioned developed models have been validated by the same research team considering experimental data in Ref. [35].  
The cogeneration unit is located in Athens, Greece. The solar field consists of 5 modules of the LS-2 PTC, so the collector aperture (\(\mathrm{A_{a}}\)) is equal to 195 m\({}^{2}\). Furthermore, the thermal energy storage tank volume (\(\mathrm{V_{T}}\)) is defined by the ratio of the collector aperture to the storage volume \(\left(\frac{\mathrm{A_{a}}}{\mathrm{V_{T}}}\right)\). This parameter is used in the literature and a typical value, that is assumed in this study, is equal to 80 [13; 32].  
The simulation is conducted by taking into account a solar irradiation value that is equal to 700 W/m\({}^{2}\), a solar incident angle of 30\({}^{\circ}\), and an ambient temperature of 25 \({}^{\circ}\)C. Moreover, the configuration operates only on sunny days, so for the location of Athens, the operating hours are assumed to be 2500 per year. According to Ref. [36], if the steady-state simulation is performed in the aforementioned conditions, the results are similar to the results of a detailed dynamic analysis, for a concentrated solar power plant in Athens. So, these values are used to estimate all the important outputs, including the financial indexes. In parallel, it is important to mention that no significant degradation of the PEM water electrolyzer is expected, as the system is assumed to operate 62,500 h during the 25-year lifetime. It is estimated that the electrolyzer efficiency decreases by almost 1.1% over 10,000 operating hours [37].  
At first, the analysis is focused on the organic working fluid selection based on the ORC exergy efficiency. Furthermore, a parametric analysis is performed and split into two sections, the thermodynamic and the financial one. The examined thermodynamic parameters are the share of ORC electricity production that powers the PEM water electrolyzer, as well as the solar beam irradiation value, which varies from 500 to 900 W/m\({}^{2}\), with a  
24.776 kW, while the overall energy and exergy efficiency is determined at 18.15% and 19.49%, respectively. So, MDM is selected as the most proper working fluid for this case study. This fluid is an appropriate choice for the examined heat source temperature level. The most important parameters and outputs are presented in Tables 4 and 5, respectively.  
Figure 4: ORC electricity production depending on the parameter (\(\alpha\)) for different working fluids.  
Figure 3: ORC energy efficiency depending on the parameter (\(\alpha\)) for different working fluids.