## 2 Material and Methods  
### Instruments and Equipment  
The experimental setup of the variable-speed rolling piston refrigeration system is shown in Figure 1. The system is composed of various components, including a rolling piston compressor 1, a condenser 2, an electronic expansion valve 3, an evaporator 4, a subcooling device 5, and a refrigerant flow measurement device 6, as well as a cooling water and chilled water circulation system 18 and 19 and other relevant elements.  
Figure 1: Schematic diagram of the experimental setup.  
The compressor in this experimental setup is a variable-speed rolling piston compressor from Shanghai Hitachi, equipped with an internal gas-liquid separator. The compressor frequency can be set by a universal variable-frequency drive, with an adjustable frequency (\(f\)) range of 16.6 Hz to 120 Hz. The theoretical displacement volume V is 10.2 mL, and the compressor power consumption W is measured by a digital power meter. Both the evaporator and the condenser adopt compact brazed plate heat exchangers, where the heat exchange occurs between the refrigerant and the water. The refrigerant is R32, and the flow rate of the refrigerant, \(m_{r}\), is measured by a Coriolis flow meter. The temperature \(T_{v}\) and pressure \(P_{v}\) of the refrigerant before the electronic expansion valve can also be measured. The subcooling degree of the refrigerant is controlled by the constant-temperature water tank 5. The opening value of the electronic expansion valve is manually controlled by a stepper motor. The condensation and evaporation temperatures are controlled by automatically adjusting the heating power of the water bath in the cooling water and the chilled water circulation system. The volume flow rate of chilled water, \(q_{v,w}\), is measured by float flow meter 17, and the flow state of the refrigerant can be observed in sight glass 12.  
T and P represent the temperature and pressure measurement points. The water inlet and outlet temperatures of the evaporator are denoted as \(T_{w,i}\), and \(T_{w,o}\), respectively, while the suction and discharge temperatures of the compressor are denoted as \(T_{suc}\) and \(T_{dis}\), respectively; these are all measured using built-in platinum resistors. The refrigerant outlet pressures of the evaporator and condenser are denoted as \(P_{e}\) and \(P_{c}\), respectively, and are measured by pressure transmitters. The physical diagram of the experimental setup is shown in Figure 2. Table 1 provides details on the sensors and their corresponding uncertainties. The system operating parameters are collected by a Siemens S7-300 PLC programmable controller(SIEMENS, Berlin, Germany), and the cooling water and chilled water temperatures are controlled by PID. The data collected by the PLC are transmitted via data lines to the PC, monitored in real time, and reported on the human-machine interface based on the Sunway (ForceControlV6.0) configuration software development.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Items** & **Products and Model** & **Manufacture** & **Parameter Range** & **Uncertainties** \\ \hline Volume flow (L/min) & Rotameter (Z-4003) & USA General Electric Company (Boston, MA, USA) & 4–36 & \(\pm\)4\% \\ Pressure (kPa) & Pressure transmitter & Danfoss China-Danfoss (Shanghai, China) & 0–3500 & \(\pm\)0.075\% \\ Temperature (°C) & Thermal resistance & Shanghai Instrumentation \& Automation Co. & –10 to 125 & \(\pm\)0.2 °C \\  & (P100) & (Shanghai, China) & & \\ Mass flow (kg/min) & Rheonik mass flowmeter & China yuyao Zhenxing Flow Meter Factory & 0.25–5 & \(\pm\)0.2\% \\  & (RHEB0A1 NNN) & (Ningbo, China) & & \\ Power (W) & Power meter & Shanghai Hongyp Electronic Technology Co. & 0–1000 & \(\pm\)0.5\% \\ \hline \hline \end{tabular}
\end{table}
Table 1: The experimental sensors and uncertainties.  
Figure 2: Actual diagram of the experimental setup.