Performance enhancement of two-stage condensation combined cycle for LNG cold energy recovery using zeotropic mixtures

The isothermal phase transition process of pure working fluids cannot effectively match the liquefied natural gas (LNG) gasification process, resulting in low efficiency of LNG cold energy power generation systems. In order to improve the temperature matching characteristics, mixed working fluids with a temperature glide during the phase change process can be adopted. In our previous study, the two-stage condensing process also effectively improved the temperature matching characteristics; thus, this paper presents a two-stage condensation combined cycle using zeotropic mixtures, and the effects of the type and number of components for mixed refrigerants on the two-stage condensing combined cycle system performance are studied. With the net power output as the objective function, the evaporation temperature, condensing temperatures, LNG expander inlet temperature, and working fluid mole fractions are optimised by the genetic algorithm. The results demonstrate that the net power output of n-C5H12 is the largest among the studied pure fluids. The net power output of the binary mixed working fluid at the optimum mole fractions is obviously superior to that of pure working fluids. The system performance is improved when the hydrocarbon mixture is selected as a working fluid, and the optimum number of components is three.