###### Abstract  
The refrigeration cycle of an air source heat pump system is an important information that reveals critical operating data, such as the cooling capacity, power consumption, and performance of a system during operation. Operating data, such as refrigerant pressure and enthalpy in situ, can be difficult to measure. Therefore, this study developed an in situ refrigeration cycle measurement method using the airside data of an air source heat pump. A method for measuring the refrigeration cycle is proposed using the characteristics of evaporation, compression, condensation, and expansion processes. The distance function was analyzed by normalizing the difference between the refrigerant pressure and enthalpy of the existing and proposed measurement methods. In addition, the distance function for the maximum error of the pressure and enthalpy measurement devices was analyzed and compared with the distance function of the entire data used in the evaluation. All the evaluation data had low distance function values with a maximum difference of 5%, confirming the reliability of the proposed refrigeration cycle measurement method. The power consumption and calculated COP were also evaluated using the proposed method. The Mean Bias Error (MBE) of power consumption and COP were 0.15% and 0.04%, and the coefficient of variation of root-mean-square error (CvRMSE) was 8.967% and 7.14%, respectively. air source heat pump (ASHP); refrigeration cycle; measurement method; in situ data +
Footnote †: journal: Applied  
doi: 10.3390/00001  
consumption and cooling load measurements are required to monitor the COP [16]. COP data are useful for decision making in operating and controlling air-source heat pump systems. Therefore, many studies have been conducted to measure and predict the COP [17; 18]. Generally, air-source heat pump systems do not monitor and store operational data separately. Therefore, to monitor the COP of an air-source heat pump system in operation, measured data, such as the enthalpy and pressure of the refrigerant and power consumption, are required. On the other hand, when measuring the pressure and enthalpy of the refrigerant in situ, there may be difficulties due to cutting and drilling the refrigerant pipe. Another way to measure the performance is to use the product-specific test characteristics of the system. [PERSON] [19] identified the problems with performance measurement methods that rely on product-specific test characteristics in the field. As an alternative, performance measurements were conducted using Compressor Set Energy Conservation (CSEC). The field applicability of cooling operating conditions was confirmed. [PERSON] [20] also examined the performance measurement of an air-source heat pump system using the CSEC method under heating conditions. Errors occurred when the compressor behavior was changed, such as starting and stopping, but the effect was insignificant for long-term measurements. The inlet and outlet refrigerant temperature and pressure data of the evaporator and condenser are required to collect the main operating data of the system in operation. Depending on the site, it may be challenging to collect the required data. [PERSON] [21] proposed to collect data through sensors embedded in the system. In the field, however, only limited information can be obtained, such as the surface temperature of the refrigerant and the inlet and outlet air temperatures of the evaporator and condenser. Moreover, it is difficult for users to access the manufacturer's monitoring data, which are usually not publicly available. Several studies have been conducted on prediction methods to address the difficulties in measuring the COP of air-source heat pump systems. [PERSON] [22] also stated that it is important to understand the COP of heat pumps in the field. Statistical and machine learning models were developed to predict the COP of air-source heat pump systems. For the data-driven analysis, the operational data and COP of 12 heat pumps in a renovated house were collected.