# Investigation of Different Storage Systems for Solar-Driven Organic Rankine Cycle
## 4 Conclusions  
The objective of this work is the energy and financial comparison of three different storage systems in a solar-driven ORC. Parabolic trough collectors coupled to a storage system fed the ORC, which operated with toluene and was a regenerative cycle. The examined storage systems were of sensible and latent heat storage ways. More specifically, the conventional sensible tank with pure thermal oil was compared with the use of thermal oil-rocks and the use of PCM inside the tank. An analysis was conducted for the weather data of Athens with a developed dynamic model. The most important conclusions of the present study are summarized in the following bullets:  
* Among the examined working fluids for the ORC, toluene was found to be the best candidate. The saturation working fluid temperature in the heat recovery system that optimizes the system efficiency was found to be at 279 \({}^{\circ}\)C. Moreover, the investigation of different rock types for the storage system proved that the use of ceramic rocks was the best choice, according to both energy efficiency and net present value criteria.
* The parametric analysis for different collecting areas proved that the use of rocks inside the tank was always energetically beneficial, compared to the pure thermal oil design.
* The single-objective optimization proved that the optimum storage technique was with thermal oil-ceramic rocks, according to all the examined criteria. The PCM storage was the second-best technique, while pure thermal oil storage was the less attractive choice.
* The multi-objective evaluation methodology proved that there were five Pareto front points that were optimum cases (see Table 8). These cases regarded three designs with ceramic rocks and two designs with PCM. Generally, the use of ceramic rocks was a better choice financially, while the PCM was a more efficient choice, especially for higher collecting areas.
* For the design [\(A_{c}=180\) m\({}^{2}-V=8\) m\({}^{3}\)], the ceramic rock design led to 13.89% system efficiency and NPV to 129.73 k\(\acute{\text{e}}\); the PCM to 13.97% and 128.66 k\(\acute{\text{e}}\), respectively, while the pure thermal oil led to 12.48% and 105.32 k\(\acute{\text{e}}\), respectively. The efficiency enhancement was found to be 11.3% with ceramic rocks and 11.9% with PCM, compared to the pure thermal oil case.  
In the future, there is a need for investigating extra storage techniques such as chemical storage and storage with molten salt in order to perform a deeper and more detailed analysis. About the chemical storage, there are options for sorption processes and processes with reactions. The energy and financial indicators will have to be calculated in every case in order to find the best global storage scenario.  
Conceptualization, methodology, writing--review and editing, writing--original draft preparation, E.B.; conceptualization, methodology, investigation. I.S.; supervision, writing--review and editing, writing--original draft preparation, C.T. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
The authors declare no conflict of interest.