## 3 Results and Discussion
### Thermodynamic Analysis of the Heat Pump  
The examined heat pump was investigated with a created model in Engineering Equation Solver. The developed model was validated by using data from the Bitzer software [27]. The validation was performed for different combinations of evaporator and condenser temperature levels. Figure 4 depicts that the points are close to the \"y = x\" line, which is a fact that proves the high accuracy of the developed model. Moreover, the mean COP deviation was calculated at 0.94%, while the maximum deviation was calculated at 3.01%. The aforementioned deviation values were relatively low, and, thus, the created thermodynamic model is an accurate one.  
The next step in the thermodynamic analysis was the presentation of the isentropic efficiency for different operating conditions. Figure 5 illustrates that the isentropic efficiency was found in the range of 50.6% to 67.5%. For every condenser temperature, there is a specific evaporator temperature that makes the compressor operate with maximum isentropic efficiency. The global maximum isentropic efficiency was found for [T\({}_{\text{cond}}\) = 80 \({}^{\circ}\)C, T\({}_{\text{evap}}\) = 20 \({}^{\circ}\)C] and it had the value \(\eta_{\text{is}}\) = 67.5%. For low evaporator temperatures, the isentropic efficiency had higher values for lower condenser temperatures. On the other hand, for higher evaporator temperatures, the isentropic efficiency was maximized for higher condenser temperatures. These results indicate that the system presented optimum compressor operation for a temperature lift of around 60 K, which is a characteristic of the examined compressor.  
Figure 4: Comparison of the calculated COP with the developed model of the COP by [PERSON].  
Figure 6 illustrates the COP variation for different combinations of evaporator and condenser temperatures. It is clear that higher evaporator temperature increases the COP, while lower condenser temperatures increased the COP. These are reasonable results because the COP needs a small temperature lift in order to have higher values. According to the results, the COP range was found to be from 1.6 to 8.0 for all the examined cases. It is remarkable that, in Figures 5 and 6, the lines did not have the same range, which is something that was justified by constraints in the compressor operation. More specifically, for T\({}_{\text{cond}}\) = 40 \({}^{\circ}\)C, the evaporator temperature can be up to 20 \({}^{\circ}\)C, while for T\({}_{\text{cond}}\) = 50 \({}^{\circ}\)C the evaporator temperature can be up to 30 \({}^{\circ}\)C. This fact indicates that a minimum 20 K lift can be achieved with the present heat pump.  
At the end of this section, the heat pump thermodynamic charts are presented for a typical case with ambient temperature at T\({}_{\text{am}}\) = 10 \({}^{\circ}\)C, which means that the evaporator temperature is set at T\({}_{\text{evap}}\) = 3 \({}^{\circ}\)C, while T\({}_{\text{cond}}\) = 80 \({}^{\circ}\)C. Figure 7 depicts the temperature-specific entropy (T-s) depiction, while Figure 8 shows the pressure-specific enthalpy (p-h) depiction. Process 1 \(\to\) 2 is the compression in the compressor, process 2 \(\to\) 3 is the heat rejection under constant pressure from the condenser to the building, process 3 \(\to\) 4 is the isenthalpic expansion in the throttling valve, and process 4 \(\to\) 1 is the heat absorption from the ambient environment under constant pressure.  
Figure 5: Variation in the compressor isentropic efficiency for different evaporator and condenser temperature levels.  
Figure 6: Variation in the heat pump COP for different evaporator and condenser temperature levels.  
Figure 7: Temperature–specific entropy depiction (T–s) of the heat pump cycle for the case with ambient temperature at 10 \({}^{\circ}\)C.