Optimization And Analyses Of Multiple-stage Rankine Cycle Systems With LNG Cold Energy Utilization

With the growth of the world’s population and the development of the new economy entities,the global energy demand is rising with the imperative transformation of the energy structure.As one type of the important clean energies,LNG(Liquefied Natural Gas)has gained increasing attention both from the international and domestic energy markets.Efficient LNG cold energy recovery becomes one of the significant energy issues to be discussed correspondingly.This paper first introduced the actual engineering applications of the natural gas and LNG home and aboard.Secondly,a detailed investigation and contrastive analyses based on cryogenic power generation cycles using LNG cold energy were carried on in view of the academic field.Based on the survey above,the cryogenic multiple-stage Rankine cycle systems with LNG cold energy utilization were discussed,which mainly revolved about:(1)Peng-Robinson equation was used to calculate the LNG physical properties and the effects of methane composition ratio and vaporizing pressure of LNG on the LNG cold exergy and cold energy were discussed respectively,and the segmentation rules of LNG vaporizing curves under subcritical and supercritical pressures were analyzed;(2)A LNG cascade two-stage Rankine cycle system was proposed and designed according to the LNG vaporizing curves and the selection and comparison of the single-composition working fluids were carried on under the LNG vaporizing pressure at 0.6 MPa,2.5 MPa,4.5 MPa and 7 MPa,and the criteria of candidate working fluids included the single-level thermal efficiency and exergy efficiency,the thermal efficiency and exergy efficiency of the whole system,the equipment exergy losses and the system exergy distribution and the pinch heat curve analyses;(3)A LNG parallel two-stage Rankine cycle system and a LNG three-stage Rankine cycle system were proposed and designed,applying different types of working fluids including single-composition,binary and ternary organic working fluids,and the thermal efficiency and exergy efficiency of the whole system,the equipment exergy losses,the system exergy distribution and the pinch heat curve analyses were discussed in different cases and the composition optimization of mixed working fluids based on the genetic algorithm was implemented to realize a good match between cold and hot fluids and reach a better net power output.