## 2 Experimental Setup and Test Procedure  
Figure 1 shows the schematic diagram of a heat pump system with a rated heating capacity of 3500 W and a power input of 1270 W, in accordance with the ISO 13256-2 [14]. The working fluid adopted was R32 (Sanowart, Zibo, China), which is a potential substitute for R410A. The system consists of a hermetic rotary compressor (LG, Seoul, Korea), two plate heat exchangers (Danfoss, Nordborgvej, Denmark) working as the condenser and the evaporator, respectively, and an electronic expansion valve (Sanhua, Hangzhou, China) with a step motor. Detailed specifications of the major components of the tested heat pump system are provided in Table 1.  
The condenser was cooled and the evaporator was heated by circulating water, which was kept at a given temperature by two thermostats. The entering water temperature of the indoor heat exchanger (EWT of ID HEX) (Danfoss, Nordborgvej, Denmark) was set at 40 \({}^{\circ}\)C, according to ISO 13256-2(14), and the water flow rate through the condenser and ethanol flow rate through the evaporator were measured by a turbine flow meter (Autoflow, Seoul, Korea) at a fixed rate of 0.6 m\({}^{3}\cdot\)h\({}^{-1}\). The test condition of the heat pump system is presented in Table 2.  
The experiment was first performed at a fixed superheat of 10 \({}^{\circ}\)C, which was controlled by the EEV (Electric expansion valve) with the PID controller (Sanhua, Hangzhou, China). Then, in order to obtain the wet refrigerant state at the compressor inlet, the EEV opening was manually increased from the EEV opening position, corresponding to the given superheat. During the test, the temperature, pressure, mass flow rate, and power input of the heat pump system were recorded and monitored through the data acquisition system for 40 min, with an interval of 2 s. For the validation of the energy balance between the water and the refrigerant side, the water heat capacity was calculated by the water flow rate and the water temperature difference between the inlet and the outlet, and the refrigerant heat capacity was calculated by the enthalpy difference between the inlet and the outlet of the condenser. Both were in good agreement, within \(\pm\)5%. The measurement accuracies and specifications of the sensors are summarized in Table 3. The uncertainties of the heating capacity and Coefficient of Performance (COP) were estimated by single-sample analysis, according to ASHRAE (American Society of Heating, Refrigerating, and Air-conditioning Engineers) Guideline 2 [15], and were considered to be ca. 6.3%. \begin{table}
\begin{tabular}{c c} \hline \hline
**Components** & **Specification** \\ \hline Indoor heat exchanger & Manufacturer: Danfoss \\ Model No.: B3-030-20-3.0-HQ \\ Outdoor heat exchanger & Type: Brazed plate type heat exchanger \\ \hline \multirow{4}{*}{Compressor} & Manufacturer: LG Model: GA102 MFB \\  & Type: Inverter Hermetic Rotary compressor \\ Displacement 10.2 CC & Displacement 10.2 CC \\  & Nominal cooling capacity: 3.5 kW \\ \hline \multirow{4}{*}{Electric Expansion Valve} & Manufacturer: Sanhua \\  & Model No. DPF(o) 1.4 \\ \cline{1-1}  & Step (pulse): 2000 \\ \cline{1-1}  & \multicolumn{1}{c}{Orifice diameter: 1.4 mm} \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Specification of the major components. Figure 1: Schematic representation of the test rig.