## 4 Results and Discussion
### Combined ORC-Single Evaporator VCR Cycle Results
Rise in \(\eta_{\text{ORC}}\) increases \(\text{COPVsys}\). Increase in thermal efficiency of ORC also causes rise in \(\eta_{\text{ex,ORC}}\) according to Equation (7). So, \(\eta_{\text{ex,sys}}\) goes up for all fluids as depicted in Figure 6b. Critical temperatures of R134a and R227 ea are 101.1 \({}^{\circ}\)C and 101.8 \({}^{\circ}\)C, respectively. Vapor at 100 \({}^{\circ}\)C will have low enthalpy and so expansion will end in two-phase region. This is dangerous for turbine and decreases efficiency as seen in Figure 6b. The total mass flow rate decreases with increasing boiler temperature for all fluids as given in Figure 6c. The reason for that is decrease of \(\dot{\text{m}}_{\text{ORC}}\) as mentioned in explanation of Figure 6a. Decline in \(\dot{\text{m}}_{\text{ORC}}\) effects negatively total mass flow rate. Figure 7 shows the effect of temperature of condenser in ORC on system performance. As condenser temperature increases, exit enthalpy of turbine increases. \(\dot{\text{m}}_{\text{ORC}}\) must increase to ensure constant turbine power since there is no change in refrigeration cycle. Heat input to the boiler goes up with increasing \(\dot{\text{m}}_{\text{ORC}}\) and \(\dot{\text{m}}_{\text{ORC}}\) decreases. For this reason, \(\text{COPVsys}\) declines for all working fluids as can be seen from Figure 7a. Decrease in thermal efficiency of ORC as a result of increase in condenser temperature also causes falling in \(\eta_{\text{ex,ORC}}\). So, \(\eta_{\text{ex,sys}}\) goes down for all working fluids as shown in Figure 7b. Total mass flow rate rises with increase of condenser temperature from 30 \({}^{\circ}\)C to 50 \({}^{\circ}\)C as presented in Figure 7c. \(\dot{\text{m}}_{\text{ORC}}\) goes up because of reasons mentioned in interpretation of Figure 7a. As a result, increase in \(\dot{\text{m}}_{\text{ORC}}\) leads to a rise in total mass flow rate of the system. Figure 8 explains how system performance is influenced with turbine isentropic efficiency. Increase of turbine isentropic efficiency tends to rise \(\text{COPVsys}\) as shown in Figure 8a. Specific turbine work increases as isentropic efficiency goes up. \(\dot{\text{m}}_{\text{ORC}}\) needs to decrease to maintain constant turbine power. So, \(\dot{\text{Q}}_{\text{boil}}\) declines and thereby \(\eta_{\text{ORC}}\) increases. \(\text{COPVsys}\) goes up concerning increase in thermal efficiency of ORC. \(\eta_{\text{ex,ORC}}\) is positively affected with increase of \(\eta_{\text{ORC}}\) according to Equation (7). Because there is no change in VCR cycle, \(\eta_{\text{ex,VCR}}\) remains constant. Increase in \(\eta_{\text{ex,ORC}}\) leads to rise in \(\eta_{\text{ex,sys}}\) according to Equation (10) and this can be seen for all working fluids from Figure 8b. Figure 6: Effect of \(\rm T_{\rm boll}\) on \(\rm COP_{\rm sys}\) (**a**), \(\rm\eta_{\rm ex,sys}\) (**b**), and \(\rm\dot{m}_{\rm total}\) (**c**) in combined ORC-single evaporator VCR cycle. Figure 7: Effect of \(\mathrm{T}_{\mathrm{cond,ORC}}\) on \(\mathrm{COP}_{\mathrm{sys}}\) (**a**), \(\mathrm{\eta}_{\mathrm{ex,sys}}\) (**b**), and \(\mathrm{\dot{m}}_{\mathrm{total}}\) (**c**) in combined ORC-single evaporator VCR cycle. Figure 9 indicates effect of ambient temperature on coefficient of performance of system. While investigating influence of change of ambient temperature, we assumed to be 5 \({}^{\circ}\)C temperature difference between ambient and condensers. This temperature difference was only utilized while drawing this graph. Previous Figures 3-8 accepted constant ambient temperature. It can be seen from this graph that increasing ambient temperature leads to decrease in CO\({}_{\text{sys}}\). The reason of this situation is rise in compressor power. Graphs of the combined ORC-single evaporator VCR cycle show complete accordance with previous studies [2; 3; 10; 11; 13; 15]. It can be clearly stated from these figures that R600a achieves best system performance and can be selected as the most efficient working fluid among proposed four fluids.