## 4 Conclusions  
The work is focused on a renewable energy plant based on solar energy, which provides two useful outputs, i.e., electricity from an ORC module, and green hydrogen via PEM water electrolysis. Additionally, two energy storage technologies are integrated, which are TES, using thermal oil, and hydrogen storage. The configuration, which is placed on the Greek island of Kythnos, provides electricity, and, in parallel, can store the excess electricity generation through the production of hydrogen, which can be exploited as a fuel in plenty of applications on the island. Thus, the installation was studied on dynamic conditions, through the weather and irradiation data of Kythnos, and optimized to be thermodynamically efficient and economically viable. The most important aspects of the present analysis are summed up below:  
1. Different combinations of the net (base) electrical load, and the TES tank volume, as well as various hydrogen price values, were examined.
2. In the optimum scenario, a collector aperture of 975 m\({}^{2}\), a storage tank volume of 10 m\({}^{3}\), and a net electricity load of 75 kW were determined.
3. The net electricity generation was equal to 210163 kWh per year, while the hydrogen production amount was found at 2356.5 kg per year.
4. The overall energy efficiency was calculated at 14.52%, and the exergy efficiency at 15.48%.
5. The payback period and the net present value of the investment were defined at 6.73 years and EUR 1,073,384, respectively.
6. The proposed system can meet the various demands of an island, or a remote area, in a low-carbon and sustainable way, with acceptable financial viability.  
Figure 20: Monthly energy efficiency, exergy efficiency, and hydrogen production amount in kg.  
**Author Contributions:** P.L.: Conceptualization, methodology, software, validation, investigation, formal analysis, writing--original draft preparation, writing--review and editing; E.B.: Conceptualization, methodology, investigation, writing--original draft preparation, writing--review and editing; G.C.: supervision, writing--original draft preparation, writing--review and editing; C.T.: Conceptualization, supervision, writing--original draft preparation, writing--review and editing. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This research received no external funding.  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** Data available after request.  
**Conflicts of Interest:** The authors declare no conflict of interest.  
**Nomenclature**  
A Surface, m\({}^{2}\)  
A\({}_{a}\) Collector aperture, m\({}^{2}\)  
C Component capital cost, EUR  
C\({}_{0}\) System capital cost, EUR  
c\({}_{p}\) Specific heat capacity under constant pressure, J/kg-K  
D Diameter, m  
ex Specific exergy, J/kg or J/mol  
Ex Energy rate, W  
F Optimization objective function, -  
G Gibb's free energy, J/mol  
G\({}_{bn}\) Solar beam irradiation, W/m\({}^{2}\)  
H Height, m  
h Specific enthalpy, J/kg or J/mol  
i Discount factor, %  
J Current density, A/m\({}^{2}\)  
K Cost, \(\xi\)  
K(\(\theta\)) Incident angle modifier, -  
m Mass flow rate, kg/s  
M Mass, kg  
N Polar flow rate, mol/s  
N Project lifetime, years  
NPV Net present value, years  
P Pressure, bar  
PBP Payback period, years  
P\({}_{el}\) Electricity rate, W  
Q Energy rate, W  
q Thermal losses per collector aperture, W/m\({}^{2}\)  
s Specific entropy, J/kg or J/mol  
T Temperature, \({}^{o}\)C or K  
t Time, s  
U Heat loss coefficient, W/m\({}^{2}\)K  
u Velocity, m/s  
V Volumetric flow rate, m\({}^{3}\)/s  
V Volume, m\({}^{3}\)  
V\({}_{cell}\) Cell potential, V  
W Heat rate, W  
Y Yearly energy value, kWh  
YE Yearly energy value, kWh