## 2 Methodology  
### Performance of the Air-Source Heat Pump System  
The performance of an air-source heat pump system can be represented by its COP. The COP can be used as a criterion to judge the thermal efficiency of an air-source heat pump system, which is the ratio of the cooling capacity of a heat pump to its power consumption, as shown in Equation (1) [29]. \[COP=\frac{Q}{W} \tag{1}\]  
where _COP_ is the coefficient of performance, \(Q\) is the cooling capacity in kW, \(W\) is the power consumption in kW. Measurements of the COP of an air-source heat pump system require measurements of the cooling capacity and power consumption. The cooling capacity refers to the amount of heat absorbed by the refrigerant from the room-side air in the evaporator of an air-source heat pump system during cooling operation. A low COP means a higher power consumption for the same cooling capacity and a lower cooling capacity for the same power consumption. Another way to calculate the COP is to use the refrigeration cycle. Using a refrigeration cycle on a pressure-enthalpy diagram, the inlet, and outlet refrigerant enthalpies of the evaporator and condenser can be obtained. Equation (2) shows the COP calculation method using the pressure-enthalpy diagram. \[COP=\frac{h_{1}-h_{4}}{h_{2}-h_{1}} \tag{2}\]  
where \(h\)1 is the evaporator outlet refrigerant enthalpy in kJ/kg, \(h\)2 is the condenser inlet refrigerant enthalpy in kJ/kg, and \(h\)4 is the evaporator inlet refrigerant enthalpy in kJ/kg. The refrigeration cycle provides information on the refrigerant pressure and enthalpy during the compression, condensation, expansion, and evaporation of an air-source heat pump system. Figure 1 illustrates the concept of the refrigeration cycle of an air-source heat pump system on a pressure-enthalpy diagram. Points 1, 2, 3, and 4 in Figure 1 are the evaporator outlet refrigerant point, condenser inlet refrigerant point, condenser outlet refrigerant point, and evaporator inlet refrigerant point, respectively. They represent the main points of the refrigeration cycle. The lines connecting each point represent the compression, condensation, expansion, and evaporation processes. The characteristics of each process in the refrigeration cycle are as follows. The ideal compression process is an isentropic process that draws refrigerant vapor evaporated from an evaporator into a compressor as superheated vapor and compresses it to facilitate liquefaction at high temperatures. The enthalpy change between the inlet and outlet of the compressor is equal to the work of the compressor. In the condensation process, the refrigerant state at the inlet of the condenser is superheated vapor at a high temperature and pressure, which is cooled by the cooling fins to alter its state from gas to liquid and release the latent heat of condensation. The condensation of the refrigerant in the condenser occurs under constant pressure, which is called an isobaric process. On the pressure-enthalpy diagram, the isobaric change is represented by the horizontal line, where the enthalpy difference is equal to the calorific value of the latent heat of condensation released by the condenser. In the expansion process, the refrigerant at high pressure, which has been subcooled through the condenser, is depressurized through throttling action as it passes through the expansion valve and becomes a saturated liquid at a lower temperature. The change in state at the expansion valve is called throttling expansion, an isentropic process. The enthalpy of the refrigerant before and after the expansion valve does not change. Finally, in the evaporation process of the air-source heat pump system, the refrigerant in the saturated liquid state after passing through the expansion valve, absorbs the latent heat of evaporation of indoor air in the evaporator and changes into saturated vapor and then into superheated vapor.