At natural gas pressure reducing stations, gas first enters a water bath heater to boost its temperature slightly, and after leaving the heater, it enters the throttle valve to reduce its pressure. This method has two major drawbacks, which are the consumption of natural gas in the heater and the excessive loss of exergy in the throttle valve. The pressure of natural gas can be reduced by using a vortex tube and converting it into hot and cold currents, and then generating electricity by directing these currents to thermoelectric generators (TEGs). Investigation of energy, exergy and environmental performance of the hybrid vortex tube-TEG system and optimization of its performance for the working condition of Kermanshah pressure reducing station is the subject of the present study. The cold mass fraction (CF) and efficiency of vortex tube are considered as decision variables. Moreover, annual average first-law efficiency (ηI TEG), annual average second-law efficiency (ηII TEG) and annual total electric power generated by the TEGs (Pout) are considered as optimization target functions. The results of genetic algorithm based three-objective optimization revealed that the specifications of the optimal system are: vortex tube efficiency = 0.36967, CF=0.95598, ηI TEG=0.03442, ηII TEG=0.04544, Pout=42770.58 kW and amount of CO2 mitigation per annum = 8.55 tons.
