Thermodynamic and exergoeconomic assessment of a novel self-sustaining pressure regulation system with decoupled vortex tube-ejector for natural gas city gate stations

The dissipation of pressure energy and the associated Joule-Thomson cooling during natural gas depressurization represent significant inefficiencies in city gate stations. To valorize this waste energy, this study proposes a novel self-sustaining pressure regulation system coupling a vortex tube with an ejector. Distinct from conventional layouts, the vortex tube is positioned upstream to decouple energy separation from pressure recovery, thereby maximizing the conversion efficiency of the high-pressure head. In this configuration, the cold stream generated by the vortex tube actively harvests low-grade ambient heat—achieving zero-carbon enthalpy compensation—before being repressurized and remixed by the ejector. Comprehensive thermodynamic and exergoeconomic analyses demonstrate that under baseline conditions (3 MPa inlet pressure), the system achieves a thermal efficiency of 29.36% and an exergy efficiency of 3.37%. Crucially, the system maintains autonomous operation without auxiliary heating at ambient temperatures above −20 °C, reducing the Levelized Cost of Energy (LCOE) by $0.0019/m3 compared to conventional throttling valves. Sensitivity analysis identifies an optimal cold mass fraction of 0.5 and a critical pressure ratio limit of 8. Furthermore, the physical deployment and installation of a pilot-scale prototype have been completed at a station in Dalian, China. This achievement validates the feasibility of the design regarding compactness and compatibility with station facilities, demonstrating its readiness for future field applications.