With the support of this MSC project, we proposed a free-piston closed-cycle Joule engine generator (see the attached figure). To understand the operating features of the closed-cycle linear Joule engine generator (LJEG), we developed an accurate numerical model for the free-piston closed-cycle Joule engine generator with helium as working fluid in MATLAB/Simulink environment (Energy Convers Manag: X 2025;26:100909), and the numerical model is validated with experimental results of previous free-piston linear Joule cycle engine generator prototype developed by the same research group. The simulation results indicate that the model agrees well with the test data for piston motion, accurately reflecting the system's operating frequency and related characteristics. Although slight discrepancies are observed in the pressure fluctuations of the expander and compressor cylinders, the overall trends remain consistent, demonstrating the model's reliability within an acceptable range.
On the basis of the validated numerical model, we studied the dynamic characteristics and output performance of the system at different system pressures and compared it with an open-cycle LJEG with air as working fluid. The expander indicated power, the generator output power, and the system efficiency increase with rising pressure. In the case of the closed-cycle helium system, as the pressure rises from 5.0 to 9.0 bar, the system's output electric power increases from 1390 to 5405 W. Although the closed-cycle system exhibits a reduction in output power compared to the open-cycle air system, its efficiency improves by approximately 40% compared to that of the open-cycle air system, primarily owing to the favourable thermodynamic properties of helium. The findings suggest that, under the condition that the system’s sealing and the pressure resistance of pipes and critical components are adequately ensured, increasing the system pressure can effectively enhance its efficiency. Moreover, the adoption of working fluids with superior thermodynamic properties in closed-cycle systems further contributes to improving overall system performance.
Then, we developed a new free-piston engine generator (FPEG) prototype with a dual-piston configuration at Durham University (Energy Proceedings 2025;54:1-4). The new prototype incorporates two internal combustion free-piston engines with higher output power than the former prototype. In comparison with the former prototype, the opening and closing of the overhead intake and exhaust valves of each free-piston engine are activated by two cam motors to meet the high-frequency operation requirements. In addition, a flat-plate linear generator is deployed at the bottom of the two free-piston engines. The linear generator is employed to convert the mechanical power into electric power, and its mover is connected with the two pistons but not kept in line with them: the linear generator mover is parallel to the piston rod. Such a configuration can shorten the axial length of the FPEG prototype greatly.
We performed preliminary tests on the free-piston engine generator prototype. When the FPEG prototype operates in a two-stroke model, the supply mass flow rate of propane is 0.057 kg/s, the stroke of the piston is set to be 34 mm, and the operating frequency of the linear motor is 4 Hz. According to the in-cylinder pressure development, piston position and control signals for the two free-piston engines during a complete cycle, the right-hand (RH) cylinder injection starts following the start of the compression stroke. Before the RH piston reaches its top dead centre, the RH cylinder ignition is triggered. The maximum in-cylinder pressure achieved by the RH free-piston engine is around 10 bar. The operation process of the left-hand (LH) free-piston engine is similar and there is no big difference in the maximum in-cylinder pressure achieved by the two cylinders.
Finally, we performed techno-economic assessment on the free-piston closed-cycle Joule engine generator, with the application target of residential combined heat and power (CHP) provision and liquid hydrogen cold energy recovery for liquid hydrogen (LH2) fuel cell trucks. A Cryogenic Closed-cycle Free-piston Linear Joule Engine Generator (CCFLJEG) integrated with a fuel cell system of a heavy-duty truck, is designed to recover cold energy during the regasification of LH2. When using liquid green hydrogen as the fuel for the fuel cell truck, the installation of the proposed CCFLJEG would result in a net present value of £23,355, and the payback time is 0.7 years, which is attractive. When using grey hydrogen, the net present value drops to £10,908 and the payback time is increased to 1.4 years, which is still attractive considering a 10-year service life of the CCFLJEG.
