Figure 1 shows the conventional V-I system. Compressed vapor discharged by the compressor passes through the condenser and becomes a subcooled liquid; then, the condensed refrigerant follows one of two paths. One path is throttled by a sub-expansion valve, while the remaining path proceeds directly to the economizer. The expanded refrigerant cools the non-expanded refrigerant by running through the economizer.

Figure 1: 
V-I heat pump

The lower temperature liquid refrigerant is then expanded by a main expansion valve before being evaporated in an evaporator. By-passed and heated refrigerant is injected into the injection port of the compressor and merged with the refrigerant from the evaporator during the compression process. The system configurations are essentially the same, regardless of the system purpose, i.e., cooling or heating; however, areas of application differ.

Cooling and heating in the V-I system is performed via an evaporator and condenser, respectively.

Figure 2 depicts the V-I system with an applied re-cooler. The system operates in the same way as the conventional system shown in Figure 1, with the exception of there being an added component, i.e., the re-cooler. Refrigerant from the economizer absorbs heat from the evaporated refrigerant by passing through the re-cooler. Consequently, the suction line temperature and required amount of compression is reduced, thereby increasing the system COP.

Figure 2: 
V-I heat pump with an applied re-cooler for cooling

Figure 3 illustrates the V-I heat pump with the re-heater. The system employs the re-heater in order to increase the temperature of the by-passed refrigerant from the economizer. The re-heater exchanges heat between the by-passed refrigerant from the economizer and the discharged refrigerant from the compressor. Although it transfers heat between the inlet and outlet refrigerant of the compressor, the system is simulated, as it may yield desirable effects on the heating capacity.

Figure 3: 
V-I heat pump with applied re-heater for heating

Additionally, the solar panels have been installed in the re-heater-applied V-I system in order to enhance the heating capacity and system performance. Figures 4, 5, and 6 illustrate varying configurations of the V-I heat pump with an applied re-heater and solar panels. Figure 4 shows the solar panels installed on the suction line to increase the evaporating pressure and temperature. Although the heat from the sun does not directly affect system capacity, it can increase the evaporation pressure, which subsequently decreases the system pressure ratio. Figure 5 shows the solar panels installed outside of the system to pre-heat the water before it flows through the condenser. By installing the panels outside of the system, it is possible to provide direct heat to the water. Finally, Figure 6 shows the solar panels installed in a position that precedes that of the re-heater; this was purposed to provide additional heat to the compressor. The sun provides direct heat to the system, a feature that differs from the remaining systems. Each of the systems shown in Figures 4, 5, and 6 are expected to discharge high temperature refrigerants that consequently increase the heating capacity.

Figure 4: 
V-I heat pump with applied re-heater and solar panels for heating: Alignment 1

Figure 5: 
V-I heat pump with applied re-heater and solar panels for heating: Alignment 2

Figure 6: 
V-I heat pump with applied re-heater and solar panels for heating: Alignment 3

Aspen HYSYS version 8.6 [5] has been selected as the simulator program, and the Peng–Robinson equation of state [6] as the thermodynamic package. The package has been employed because it can calculate the thermodynamic processes of refrigeration systems with high accuracy.

Following are the assumptions applied in the system modeling.

1) The refrigerant in the compressor is compressed adiabatically.

2) Pressure drops and heat losses in the heat exchangers and pipes are negligible.

3) Refrigerants in the expansion valves yield isenthalpic processes.

4) The injection process is an isobaric process.

Table 1 and Table 2 list the respective analysis conditions for the cooling and heating applications of the V-I system analysis. Values marked by asterisks represent standard conditions.

Figures 7 to 11 demonstrate COP variations in the refrigeration system cooling as a function of the various parameters given in Table 1. Figure 7 illustrates COP as a function of condensing temperature. As the condensing temperature increased, the system COP decreased. This is because a higher condensing temperature generated a higher pressure ratio, which therefore mandated an increase in power consumption to generate the same cooling capacity. A system comparison revealed the following: the COP of the system equipped with a re-cooler is higher than that of the conventional V-I system. However, this difference in COP decreased from 1.73% to 1.38% as condensing temperature increased. More explicitly, re-cooler installation yielded optimized system performance at lower condensing temperatures.

Figure 7: 
COP as a function of condensing temperature

Figure 8: 
COP as a function of the degree of economizer subcooling

Figure 9: 
COP as a function of by-passed mass flow ratio

Figure 10: 
COP as a function of degree of re-cooler subcooling

Figure 11: 
COP as a function of intermediate pressure

Figure 8 demonstrates COP variation with respect to the degree of subcooling in the economizer. The system COP increased with increased economizer subcooling as a result of the increased enthalpy difference in the evaporator. Because of this increase, the mass flow rate in the evaporator and the total amount of compression were reduced. The system performance was also enhanced by the re-cooler. The re-cooled V-I system consistently yielded a 1.59% higher COP as compared to the conventional V-I system.

Figure 9 shows the COP as a function of the by-passed mass flow ratio. The by-passed mass flow ratio is defined as the ratio of the by-passed mass flow rate measured at the condenser exit to the mass flow rate in the condenser. Increase in the by-passed mass flow ratio caused the cooling capacity of the evaporator to increase because of an increased enthalpy difference. Because the mass flow rate of the injected refrigerant increased as well, the amount of compression also concurrently increased. Although the increase of cooling capacity initially yielded more significant effects on system performance than the increase in power consumption, the amount of compression became increasingly more significant as the by-passed mass flow ratio increased. Consequently, the system exhibited optimized performance when the mass flow ratio was approximately 0.25. As is shown in Figure 8, the V-I system with the re-cooler yielded a 1.53% higher COP than the conventional system on average.

Figure 10 plots the system performance as a function of re-cooler subcooling degree. The system COP increased with increased re-cooler cooling capacity; this is because the re-cooler reduced the amount of required compression.

Figure 11 illustrates COP variation with respect to intermediate pressure. The standard intermediate pressure was set as 1200 kPa; this value was calculated and simplified according to the following equation:

P_m, P_H and P_L represent the intermediate, condensing, and evaporating pressure, respectively. Although the system performance exhibited improvement with a medium pressure increase, system stability and economizer performance declined because the temperature difference between the inlet and outlet of the economizer had decreased. In practice, the HYSYS program presented an error, indicative of a temperature difference in the economizer owing to reduced stability above 1200 kPa of intermediate pressure. Hence, the V-I systems with 1200 kPa of medium pressure yield the highest COP among the realizable intermediate pressure conditions in Table 1.

A heat pump performance comparison is presented in Figure 12. Incongruent with the assumption mentioned in Section 2.1, the system performance declined even with an installed re-heater. The re-heater exchanged the heat between by-passed and discharged refrigerants, as mentioned above. Because of this heat transfer, the refrigerant temperature at the condenser inlet decreased; this reduction is more significant than the temperature increase of the discharged refrigerant resulting from an injection of relatively hotter vapor into the compressor; this process caused the heating capacity to decrease. Applying the solar panels resulted in a 1% and 0.41% marginal increase in system COP for Alignments 2 and 3, respectively. Conversely, Alignment 1 yielded a lower COP than the standard V-I heat pump. Consequently, the re-heater was found to yield undesirable effects on the V-I heat pump system in terms of system performance. In addition, the re-heater was found to yield adverse influence on the compressor because of an increased discharge temperature that is hypothesized to eventually lead to compressor breakdown with continuous use.

Figure 12: 
COP comparison among heat pump types