## 2 Methodology
### Refrigeration Cycle Measurement Method  
There are two methods to calculate the COP: by measuring the cooling capacity and power consumption and using Equation (1), and by measuring the pressure and enthalpy  
Figure 1: Refrigeration cycle of an air-source heat pump. of the refrigerant and using Equation (2). Both methods require information, such as the refrigerant enthalpy, pressure, and mass flow rate of the operating air-source heat pump system, but there are difficulties in installing pressure gauges and flow meters because they require processes, such as drilling and cutting refrigerant pipes. Therefore, this study proposed a method for measuring the refrigeration cycle of an air-source heat pump system using data that are easy to measure from the indoor side without using data that are difficult to measure, such as refrigerant enthalpy, pressure, mass flow rate, and power consumption. The proposed refrigeration cycle measurement method of the air-source heat pump system considers the characteristics of heat exchange between refrigerant and air during evaporation and condensation under an isobaric process, and takes the characteristics of the refrigerant changing under isentropic and isenthalpic processed into account during compression and expansion. Figure 2 presents the conceptual diagram of developing the air-source heat pump system refrigeration cycle measurement method based on in situ data. The refrigerant in the evaporator and condenser of an air-source heat pump system evaporates and condenses under isobaric pressure. The refrigerant passing through the compressor and expansion valve compresses and expands in an isentropic process and an isenthalpic process, respectively. During cooling operation of an air-source heat pump system, the refrigerant in the evaporator absorbs heat from the indoor air, and the refrigerant in the condenser releases the absorbed heat to the outdoor air. Heat exchange between the refrigerant and air occurs in the evaporator and condenser. When no heat loss occurs during the heat exchange of refrigerant and air, according to the first law of thermodynamics, the  
Figure 2: Conceptual diagram of the refrigeration cycle measurement method. heat change in the refrigerant and air in the evaporator and condenser can be expressed as Equation (3). At this time, the amount of heat can be calculated using the mass flow rate, specific heat, temperature, and enthalpy change in the fluid, which can be expressed as Equation (4). In the condenser of an air-source heat pump system, only sensible heat exchange occurs when air absorbs heat from the refrigerant. In the evaporator, on the other hand, the refrigerant absorbs heat from the air. In this case, the heat in the air is absorbed as both sensible and latent heat. Therefore, when calculating the heat content of an air-source heat pump system, only sensible heat exchange is considered in the condenser and both sensible and latent heat exchange are considered in the evaporator. \[Q_{ref}=Q_{air} \tag{3}\]  
\[Q=\dot{m}_{air}C_{p}(t_{out}-t_{in})=\dot{m}_{air}(h_{out}-h_{in}) \tag{4}\]  
where \(Q_{ref}\) is the heat transfer rate of the refrigerate in kW, \(Q_{air}\) is the heat transfer rate of air in kW, \(\dot{m}_{air}\) is the mass flow rate of air in kg/s, \(t_{in}\) is the temperature of the fluid entering the control volume in \({}^{\circ}\)C, \(t_{out}\) is the temperature of the fluid leaving the control volume in \({}^{\circ}\)C, \(h_{in}\) is the enthalpy of the fluid entering the control volume in kJ/kg, \(h_{out}\) is the enthalpy of the fluid leaving the control volume in kJ/kg. The enthalpy of the air entering and leaving the evaporator is needed to calculate the heat content of the evaporator, where both sensible and latent heat exchange occurs. The enthalpy of the wet air can be calculated using Equation (5). The temperature and absolute humidity of the air are required to calculate the enthalpy.