The goal of this study is to investigate the possibility of replacing the water bath heater and pressure relief valve with a hybrid vortex tube-photovoltaic/thermal system in pressure reduction stations like city gate stations. By using this system, both the natural gas pressure can be reduced to the desired level (from 5-7 MPa to 1.5e2 MPa) and electricity can be generated. The effect of cold mass fraction of vortex tube (0.01e0.1), air mass flow rate (0.569e5.69 kg/s) as well as the length (1e10 m), width (0.5e5 m) and depth (0.1e0.5 m) of the photovoltaic/thermal system on the energy and exergy performance of the hybrid vortex tube-photovoltaic/thermal system is investigated. Then, the genetic algorithm based twoobjective optimization is used to find the appropriate value of these parameters to maximize the annual average first-law efficiency (hI) and second-law efficiency (hII) of the hybrid system. It was found that the hI and hII of the optimal system are respectively 74.05% and 19.46%. Moreover, it was observed that the use of optimal system causes a reduction in the CO2 emission by 27.7-tons per year.
