## 2 Methodology
### Refrigeration Cycle Measurement Method
\[h=C_{p}t+x\big{(}h_{g0}+C_{pw}t\big{)} \tag{5}\]  
where \(h\) is the enthalpy of moist air in kJ/kg, \(C_{pa}\) is the specific heat of dry air at constant pressure in kJ/kg\({}_{a}\)\({}^{\circ}\)C, \(t\) is the temperature in \({}^{\circ}\)C, \(x\) is the humidity ratio in kg\({}_{w}\)/kg\({}_{a}\), \(h_{g0}\) is the enthalpy of saturated water vapor at 0 \({}^{\circ}\)C in kJ/kg, and \(C_{pw}\) is the specific heat of water vapor at constant pressure in kJ/kg\({}_{w}\)\({}^{\circ}\)C.  
The absolute humidity was calculated using the relative humidity, which is easy to measure, and the enthalpy of wet air was calculated. Equation (6) can be used to calculate the saturated vapor pressure as a function of temperature. Equation (7) can be used to calculate absolute humidity from the relative humidity. \[P_{s}=0.611e^{\frac{17.27}{293.+t}} \tag{6}\]  
\[x=0.622\bigg{(}\frac{\varphi P_{s}}{P_{0}-\varphi P_{s}}\bigg{)} \tag{7}\]  
where \(P_{s}\) is the pressure of saturation vapor in kPa, \(\varphi\) is the relative humidity in %, \(P_{0}\) is the standard atmospheric pressure in kPa. The mass flow rate of air is required to calculate the air calorific value of the evaporator and condenser. The airflow rate can be calculated using Equation (8), which can be obtained by measuring the air velocity or airflow rate. \[\dot{m}_{air}=q_{air}\gamma_{air} \tag{8}\]  
where \(q_{air}\) is the airflow rate in m\({}^{3}\)/s, \(\gamma_{air}\) is the specific weight of air in kg/m\({}^{3}\). The enthalpy change in the refrigerant in and out of the evaporator and condenser is required to measure the refrigeration cycle of an air-source heat pump system. The refrigeration cycle can be determined without measuring the existing enthalpy of refrigerant input and output by calculating the enthalpy change in air input and output of the evaporator and condenser. Using the discharge air volume and air enthalpy of the evaporator and condenser, the enthalpy change in the refrigerant input and output of the evaporator and condenser can be expressed, as shown in Equation (9). \[\Delta h_{ref}=\frac{\dot{m}_{air}\Delta h_{air}}{\dot{m}_{ref}} \tag{9}\]  
where \(\Delta h_{ref}\) is the enthalpy change in refrigerant in kJ/kg, \(\Delta h_{air}\) is the enthalpy change in air in kJ/kg, \(\dot{m}_{ref}\) is the mass flow rate of refrigerant in kg/s. The refrigerant in the evaporator and condenser of an air-source heat pump system evaporates and condenses in an isobaric process, and the refrigerant flowing through the compressor and expansion valve compresses and expands in an isentropic process and an isenthalpic process, respectively. The refrigeration cycle measurement method using these characteristics can be categorized into five steps, as shown in Figure 3.  
* _Step1._ Check the refrigerant used by the air source heat pump system. Different refrigerants have different physical properties, resulting in different pressure-enthalpy diagrams. Specifically, it is important to know that the saturated liquid line and saturated vapor line are different for different refrigerants. Generally, air source heat pump systems use R401A refrigerant. In this study, a refrigeration cycle measurement method was developed and evaluated based on an air-source heat pump system using R410A refrigerant. * _Step 2._ Show the evaporation process on a pressure-enthalpy diagram. In an air source heat pump system, the evaporation is an isobaric process. Measure the surface temperature of the refrigerant pipe on the evaporator side and mark the evaporator discharge refrigerant point (point 1) that is above the saturated vapor line. Calculate the enthalpy change in the evaporator-side refrigerant using Equation (9) using the measurement data on the air side of the evaporator. The point of evaporator suction refrigerant (point 4) is found by subtracting the calculated enthalpy change from the enthalpy at point 1. The evaporation process of an air source heat pump system can be shown on a pressure-enthalpy diagram by connecting points 4 and 1.