## 4 Modeling of the System Components
### Evaporator and Condenser
#### 4.3.2 Thermal Modeling
The obtained coefficients for the CHTC models are reported in Table 10.  
The results of the selected model are shown in Figure 19, neglecting a few points (seven points have been excluded for the condenser modeling results since the very low temperature pinch required a different set of empirical coefficients for the CHTC model compared to that able to predict the remaining data with good accuracy). The predicted performance of the condenser shows the good agreement of the model with the experimental data. The outlet temperature of the refrigerant is close to the inlet temperature of the water due to the relatively high mass flow rate of water and the large size of the condenser. Moreover, the high CHTC calculated in the two-phase zone can be recognized, with a noticeable relatively large temperature change in the water stream. * Evaporator:  
Similar to the condenser modeling, four single-phase and three boiling CHTC models were considered. The single-phase correlations were the same as for the condenser, and the boiling correlations were [PERSON] et al. [30], [PERSON] et al. [23], and [PERSON] et al. [31]. [PERSON]'s correlation was suggested for R410 boiling in PHEs, [PERSON]'s correlation for R245 fa and R1233 ZD in PHEs, and [PERSON]'s for R134a in PHEs. Due to the presence of more variables affecting the performance of the evaporator and the wider range of the working conditions, the coefficient of the boiling CHTC correlation was found for each single data point, then it was correlated to the Re and Pr numbers of the diathermic oil and R134a streams using the regression data fitting method. The best combination of the models was found using the [PERSON] and Crawford model for liquid and gas single-phase CHTC of R134a using coefficients of the original model, the Bogaert-Bolcs model for single-phase oil flow using coefficients of the original model, and [PERSON]'s model for boiling CHTC with one coefficient correlated to the flow characteristics of the oil and the refrigerant streams. The original coefficients of the aforesaid single-phase correlations and the correlated coefficient of [PERSON]'s model are reported in Table 13. The results of the obtained model are shown in Figure 20. The predicted performance of the evaporator shows very good agreement between the model and the experimental data. Figure 19: Results of the condenser hydrothermal model vs. experimental data: refrigerant discharge temperature (**top-left**), water discharge temperature (**top-right**), thermal load in the refrigerant side (**bottom-left**), and temperature profile for one of the experimental points (**bottom-right**). The lower accuracy of the evaporator model compared to the condenser one is due to the higher ambient heat losses from the evaporator compared to the condenser because of the significantly higher temperature of the evaporator. The heat losses also occur in the hot fluid pipe since the temperature sensors had a distance from the evaporator inlet and outlet ports. Another reason for the lower accuracy goes back to the higher fluctuations of the evaporator inlet and outlet pressures due to the direct impact of the plunger pump pulses and variations of the heat source temperature as discussed in the hydraulic modeling subsection. Nonetheless, the accuracy of the model for both the evaporator and condenser of the ORC unit can be considered satisfactory for rating their performances properly.