# Investigation of Different Storage Systems for Solar-Driven Organic Rankine Cycle
## 3 Results and Discussion
### Parametric Investigations of the Storage with Pure Thermal Oil and with Thermal Oil-Rocks  
Firstly, the storage system with thermal oil-rocks is studied in this section. For the optimized system with toluene, different rocks were examined for the configuration with a collecting area at  
Figure 3: Investigation of different organic fluids for the ORC as a function of different pressure levels in the turbine inlet.  
160 m\({}^{2}\) and storage tank volume at 10 m\({}^{3}\). In all the examined rocks, the void fraction was 40%. Figure 6 shows that the highest system efficiency was found for ceramic material with 14.63%, while basalt and bricks were the next candidates with 14.59%, quartzite with 14.56%, and concrete with 14.36%. So, energetically the ceramic rocks were the best materials for the storage system. Figure 7 illustrates the NPV of these cases. The ceramic material led to 119.17 k\(\epsilon\), the bricks to 118.57 k\(\epsilon\), the quartzite to 118.16 k\(\epsilon\), the basalt to 118.04 k\(\epsilon\) and to 115.86 k\(\epsilon\). So, the financial analysis indicated that the ceramic rocks led to the maximum NPV. The financial and economic results made clear that the ceramic rocks have to be selected as the most appropriate candidate.  
Figure 4: Investigation of the ORC and the system efficiency with pure thermal oil [\(A_{c}=160\) m\({}^{2}\) and \(V=10\) m\({}^{3}\)] for different toluene saturation temperatures.  
Figure 5: Temperature–specific entropy depiction of the optimum design for the ORC.  
The next part regarded a parametric analysis for different collecting areas and storage tank volume at 10 m\({}^{3}\) with pure thermal oil. Figure 8 shows that the optimum system efficiency was close to 140-160 m\({}^{2}\). High collecting areas reduced the ability of the system to exploit efficiently all the useful heat product because the electricity production was set at 10 kW. Moreover, the low values of the collecting area made the system not able to reach easily the proper temperature limits in order to start the ORC operation, and so the efficiency was not maximized in the smallest examined collecting areas. Moreover, Figure 8 shows that the system with thermal oil-ceramic rocks presented higher system efficiency than the respective cases of the pure thermal oil for all the collecting areas. The efficiency enhancement was higher in higher collecting areas and it ranged from 0.28% at 100 m\({}^{2}\) to 5.86% at 300 m\({}^{2}\).  
Figure 6: System efficiency for different rocks in the storage tank for \(A_{c}=160\) m\({}^{2}\) and \(V=10\) m\({}^{3}\).  
Figure 7: Net present value for different rocks in the storage tank for \(A_{c}=160\) m\({}^{2}\) and \(V=10\) m\({}^{3}\).  
Figure 9 exhibits the electricity product for different collecting areas with pure thermal oil. It is clear that a higher collecting area led to greater electricity yield, but the increasing rate had a reducing trend. This fact was justified by the reduction in the system efficiency after the 160 m\({}^{2}\). Moreover, the yearly operating time is depicted in the same figure and its curve has the same trend as electricity production. The yearly operating time ranged from 2693 h up to 5226 h, which meant that the system operating capacity also ranged from 30.7% up to 59.7%.  
Figure 8: System efficiency with pure thermal oil for different collecting areas and the respective enhancement in the thermal oil–rocks case.  
Figure 9: Electricity production and yearly operating time for different collecting areas with pure thermal oil.  
Lastly, Figure 10 shows the NPV and the payback period for different collecting areas. The NPV I maximized in the range of 180 to 220 m\({}^{2}\), while the payback period was minimized close to 160 m\({}^{2}\). The maximum NPV was about 120 k\(\epsilon\), while the minimum payback period was about eight years.