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A Stirling generator is a device that converts acoustic waves generated by thermal energy into mechanical vibrations and subsequently into electrical energy using a linear alternator. The conversion process can be described using the Stirling thermodynamic cycle, as shown in Fig. 1. Theoretically, the Stirling generator can achieve efficiencies close to the Carnot limit, indicating its high potential for practical applications once the technology is fully developed. In particular, when designed as a Free-Piston Stirling Engine (FPSE), the generator can be completely hermetically sealed while still operating as an external combustion engine, thereby providing significant advantages for deployment in extreme environments. Therefore, FPSE systems have been developed and applied in the United States and China for space- and defense-related power generation, and similar efforts are ongoing at the Korea Atomic Energy Research Institute (KAERI). The prototype Stirling generator used in this study was developed by KAERI and reported by Jeong et al. [10]. Under the conditions of 63.7 Hz operating frequency, 15.4 atm helium pressure, heater temperature of 600℃, and 5 V amplitude, the prototype achieved a maximum output of 3.05 W, corresponding to a reported efficiency of 4.15%.

Fig. 1. 
Principle of the Stirling generator.

The operation of a Stirling generator requires a sufficient temperature gradient (ΔT). Consequently, such systems are typically integrated with waste-heat recovery units [11] or solar concentrators [4] capable of providing a large ΔT. In the experimental environment proposed in this study, the hot end was controlled using an electric resistance heater, and the cold end was maintained using a water-cooled chiller system to achieve precise ΔT control. In addition, the type and pressure of the working fluid significantly influence the performance; therefore, an internal pressure control system consisting of a vacuum pump, pressure sensors, and an electronic regulator was implemented for accurate adjustment.

The test bench for the Stirling engine is broadly divided into two subsystems: (1) an environmental condition setup for engine operation (Fig. 2(a, b), and (2) a monitoring system for engine performance during operation (Fig. 2(c)).

Fig. 2. 
Schematic of Stirling generator hardware; (a) Stirling generator, (b) Internal liquid control system, (c) Electrical power generating system.

The environmental setup comprises two key functions: helium charging and temperature gradient formation. Before starting the engine, helium as the working fluid must be charged into the engine at the target pressure, and two requirements must be satisfied: (1) no other gases should remain inside the engine, and (2) the charging pressure must be precisely controlled. To satisfy these requirements, an internal pressure control system was designed, as shown in Fig. 2(b), which connects the engine to three ports. The first port is linked to a vacuum pump to evacuate the chamber before helium charging. The second port is a helium line equipped with an electronic regulator (CHP-HFE-5GI, Clippard, USA), which enables accurate pressurization of the engine. The third port connects to the atmosphere and is used for exhaust during depressurization.

In practice, the system is first evacuated for approximately 30 min using a vacuum pump, after which helium is introduced from a high-pressure cylinder through a regulator. Pressure variations during this process are monitored using two types of sensors: one covers a wide pressure range, enabling monitoring throughout the entire charging process, and the other provides a sufficiently fast response to track pressure oscillations at the operating frequency of the engine, thereby capturing transient variations during engine operation.

A temperature gradient was established after helium charging. The hot end was heated using a custom-made Joule heater powered by a DC power supply (N6700C, Keithley, USA), and the cold end was cooled using a coolant circulator (RW3-1035, JEIO Tech, Republic of Korea).

The engine-monitoring system consists of an AC power supply for startup and stable operation, a DAQ setup for measurement, and an external load for power consumption (Fig. 2(c)). Once the ΔT is established, the AC power supply initiates engine operation, and the engine thereafter produces continuous power. During laboratory experiments, the generated power was dissipated through a resistive electrical load in actual applications, this load is replaced by power storage devices such as batteries or capacitors.

Impedance matching within a circuit is critical for ensuring stable operation. The high-frequency AC voltage and current signals from the AC power supply, Stirling engine, and electrical load were monitored to calculate the effective power output and efficiency. In addition, multiple parameters, including the heater temperature, coolant inlet and outlet temperatures, internal helium pressure, AC voltages, AC currents, and electronic load state, were simultaneously monitored using the DAQ chassis (cDAQ-9189, National Instruments, USA). Specifically, an NI-9210 (National Instruments, USA) was used for temperature monitoring (8 CH), NI-9253 (National Instruments, USA) for DC current and pressure monitoring, NI-9225 (National Instruments, USA) for AC voltage monitoring (3 CH), and NI-9247 (National Instruments, USA) for AC current monitoring (3 CH). Furthermore, NI-9263 (National Instruments, USA) provided a DC voltage output (3 CH), whereas NI-9215 (National Instruments, USA) enabled DC voltage monitoring (4 CH), allowing regulator operation monitoring and future sensor integration.

Owing to the intrinsic dynamics of the Stirling engine, which are characterized by rapid piston motion, oscillatory pressure variations, and high-frequency voltage/current responses, the software system must be capable of synchronizing multiple hardware components while maintaining high monitoring frequencies. In this study, all hardware modules were integrated into a single LabView-based program. The software was organized into two main interfaces: one for system control and the other for detailed monitoring.

The system control interface (Fig. 3(a)) allows for the regulation of the helium charging pressure, AC power supply settings, and heater power input. The left display panel provides a schematic overview of the external environment of the Stirling engine, showing the hot- and cold-end temperatures, internal engine pressure, and pressure in the control system (Fig. 3(a-i)). The regulator can be operated through a dedicated controller (Fig. 3(a-ii)), whereas the Joule heater is controlled via a DC input panel that also displays the voltage, power, and current hot-end temperature (Fig. 3(a-iii)). Once the normal conditions are confirmed, the AC control panel initiates engine operation, with the driving frequency and amplitude defined as the operating parameters. The output voltage amplitude and power, both presented as RMS values, are monitored in the AC monitoring panel, whereas the Stirling monitoring section provides real-time data on the generated power (RMS value) and power factor.

Fig. 3. 
Software interface for the Stirling generator; (a) System control interface, (b) System monitoring interface.

The monitoring interface (Fig. 3(b)) provides more detailed measurements of the system parameters. Hot-end, coolant inlet, and outlet temperatures are recorded at 10 Hz to calculate ΔT (Fig. 3(b-i)), and the internal engine pressure is also tracked at 10 Hz (Fig. 3(b-ii)). The electrical power input/output of the Stirling generator, AC power supply, load, and power factor, are monitored at 10 Hz (Fig. 3(b-iii)). For higher-resolution analysis of the electrical circuit, the voltage and current signals are sampled at 2 kHz, which is approximately 15 times higher than the piston oscillation frequency (60–70 Hz), exceeding the Nyquist requirement to ensure precise waveform and power-transfer characterization (Fig. 3(b-iv)).