Thermo-economic analysis and optimization of a cascade transcritical carbon dioxide cycle driven by the waste heat of gas turbine and cold energy of liquefied natural gas

Abstract

• A cascade transcritical CO 2 cycle is proposed to recover waste heat and cold energy. • The cascade transcritical CO 2 cycle is condensed by LNG at different pressures. • The effects of thermodynamic parameters on the system performance are studied. • The maximum exergy efficiency is 49.24% according to the optimization result. In this paper, a cascade transcritical carbon dioxide cycle is proposed to utilize the waste heat of gas turbine and the cold energy of liquefied natural gas (LNG). In the proposed system, the carbon dioxide of the bottom cycle and the top cycle are condensed at different pressures by LNG. Meanwhile, the LNG absorbs heat from carbon dioxide at different temperatures, leading to smaller temperature difference. The detailed models are established and the simulation results are discussed from the viewpoints of thermodynamics and economics. Under the design condition, the exergy destruction of the low-pressure condenser is the largest, followed by the high-pressure condenser. The investment cost of the high-temperature turbine is the highest. The exergy efficiency of the proposed cascade transcritical CO 2 cycle first rises then drops as the split ratio and the pressure of high-pressure condenser increase. Moreover, the system exergy efficiency would increase as the inlet temperature of high-temperature turbine rises and decrease as the pressure of low-pressure condenser rises. Finally, the optimization results indicate that the maximum exergy efficiency of the proposed cascade transcritical carbon dioxide cycle is 49.24%. The net power output of the optimal case is 15.53 MW, which is 6.14 MW higher than the reference system.