## 5 System Performance Mapping  
The ORC system was simulated using a component-oriented modeling approach that refers to as functions the models of the different components discussed above. The system solver takes the system boundaries as inputs plus a set of assumptions that are relevant for the ORC system operation. The system boundaries are: (i) the hot fluid inlet temperature to the evaporator; (ii) the cold fluid inlet temperature to the condenser; (iii) the hot fluid pump speed; (iv) the cold fluid flow rate; (v) the expander shaft speed; and (vi) the ambient temperature. The assumptions are that: (i) the superheating degree in the evaporator is equal to 5 K, and (ii) the subcooling degree in the condenser is 5 K. The solver inputs and outputs are shown in Figure 22.  
Figure 21: Expander shaft power (**left**) and isentropic efficiency (**right**) with expander pressure ratio (PR) and shaft speed (\(T_{\text{su,exp}}=65^{\circ}\text{C}\), \(P_{\text{su,exp}}=15\text{ bar}\), \(T_{\text{amb}}=15^{\circ}\text{C}\)).  
Figure 22: Inputs and outputs of the ORC object-oriented model.  
The solver initiated using some guess values found in three iterative loops as shown in Figure 23. In the inner loop, the refrigerant pump speed was found to reach the preset superheating degree in the evaporator. In the next loop, the pump discharge pressure, which is directly associated with the expander suction pressure, was adjusted so that for the given pressure ratio the mass flow rate of the scroll expander was close to the mass flow rate produced by the refrigerant pump. Then, the condenser function was solved, and its discharge temperature was used to update the guess value of the pump suction temperature. In the outer loop, the expander discharge pressure, which determines the condenser pressure, was found to reach the preset subcooling degree in the condenser. The loops were terminated when the residuals of the objective parameters were small enough.  
The system performances were mapped in terms of net electric power and efficiency for different heat source temperatures and expander shaft speeds as presented in Figure 24. The range of the variable parameters was kept as in the experimental data to avoid ex  
Figure 23: Flow chart of the object-oriented solver of the ORC system.  
trapolation of the semi-empirical models. The system performances improve at higher heat source temperatures, which is associated with the available thermal load in the heat source. Moreover, the peaks of the net electric performances were obtained for shaft speeds significantly lower than the electric generator maximum shaft speed (1500 rpm). This is due to the conditions of the experimental data of the scroll expander, which were used to tune the semi-empirical model as discussed in the following.  
During the test campaign, high shaft speeds were achieved when the shaft torque was reduced resulting in a drop in the shaft power. In other words, if the electric generator produced enough power, the expander shaft power remained lower than the generator's nominal speed due to the generator brake torque as shown in Figure 25. Therefore, the expander model reflects this phenomenon in higher shaft speeds, leading to a lower net electric power and net electric efficiency.