# Investigation of a Solar-Driven Organic Rankine Cycle with Reheating
## 3 Results and Discussion
### Dynamic Comparison for a Typical Day  
The second stage in the present study is the dynamic analysis of the unit. A typical sunny day is selected to be examined in June (172 day of the year) and the results are presented below. Figure 5 depicts the solar collector efficiency, the ORC thermodynamic efficiency, and the system efficiency on a daily basis for different saturation temperature levels for the reheat system. The nominal electricity capacity is selected at 15 kW, while the heat rejection temperature is selected at 36 \({}^{\circ}\)C in order to be 5 K higher than the maximum daily temperature of the day [26]. According to Figure 5, a higher saturation temperature reduces the solar thermal efficiency because the system operates at higher temperatures and thus the solar field operates also in higher temperatures, a fact that leads to greater thermal losses and lower collector thermal efficiency. On the other hand, higher saturation temperature leads to higher thermodynamic efficiency, a result in accordance with Figure 3. The system efficiency is found to be maximized for a saturation temperature at 140 \({}^{\circ}\)C with 7.13%, while the ORC efficiency is 17.03% and collector efficiency 42.02%. The maximization of the system efficiency in an intermediate temperature is a result of the different behavior of the collector and the ORC efficiencies with the saturation temperature. In other words, a higher saturation temperature enhances the ORC efficiency and reduces the collector efficiency. System efficiency, which is proportional to the product of these parameters, is maximized in an intermediate value, a critical conclusion that indicates the need for a detailed optimization procedure. Figure 4: Ratio of the thermodynamic efficiency of the reheat case to the simple case (vertical axis is in a logarithmic scale). Figure 3: ORC thermodynamic efficiency for the heat and simple cycles. After an initial evaluation, the next step is the parametric analysis for different combinations of the saturation temperature and the nominal electricity capacity. It is useful to state that the nominal electricity capacity of the system is selected at 10, 15, 20, and 25 kW. Figure 6 depicts the daily solar efficiency for the different examined design scenarios of the reheating system. It is obvious that a higher saturation temperature reduces the daily collector efficiency. Moreover, in cases with lower electricity capacity, the collector efficiency is lower and its reduction is also more intense with the augmentation of the saturation temperature. For the scenario of 10 kW capacity, solar collector efficiency is significantly reduced; the fact proves that in this case, the system operates at high-temperature levels where there are high thermal losses. Practically, low electricity production means low heat extraction from the tank to the HRS, something that leads to great storage in the tank associated with high operating temperatures in solar collectors. Figure 7 depicts the results of the ORC thermodynamic efficiency. Higher saturation temperature enhances the ORC performance for the reheating system, a reasonable result according to the aforementioned analysis (see Figure 3). The interesting result is that the ORC is not highly influenced by the electricity capacity of the unit. However, it is useful to state that higher electricity capacity increases the thermodynamic efficiency of the system. Figure 8 demonstrates the system efficiency for the different designed cases of the reheating system. The system efficiency has higher values when the saturation temperature and electricity capacity increase. It is very interesting to state that the increase of the capacity is very important and more specifically the case of 10 kW leads to significantly low yield, while 20 kW leads to an adequate yield. Moreover, the saturation temperature is a very important parameter and has to be over 120 \({}^{\circ}\)C because, after this limit, its increase is not as effective in the system.