Strengthening Power Generation Efficiency Utilizing LNG Cold Energy By A Novel Multi-Stage Condensation Process

LNG cold energy is a kind of excellent clean energy,so it’s necessary to make full use of LNG cold energy,which can both save energy and improve the energy utilization.One of the most important ways to use LNG cold energy is power generation.The traditional methods of power generation using LNG cold energy including the direct expansion,organic Rankine cycle and the combined cycle.For the traditional methods of LNG cold power generation,the thermodynamic cycle condensation process is single-stage condensation,and the isothermal phase transition process of pure working fluids cannot match well with the LNG gasification process,resulting in a large irreversible loss of the condenser and the low efficiency of LNG cold energy power generation.The efficiency of LNG cold energy power generation can be improved by configuration improvement and working fluid optimization.Therefore,in order to improve the efficiency of the traditional methods of power generation,the idea of multi-stage condensation process is presented in this paper,which mainly from two aspects of cycle configuration and working fluid to improve the efficiency of LNG cold energy power generation.Firstly,the conventional LNG cold energy generation method is improved from the perspective of cycle configuration.In order to prove the correctness of the idea of multi-stage condensation process,a two-stage condensation Rankine cycle is proposed.The two-stage condensation Rankine cycle and the traditional LNG cold power generation methods are optimized by genetic algorithm.When the system performance is optimal,the net power output,thermal efficiency and exergy efficiency of the two-stage condensation Rankine cycle are increased by 39.05%,35.17% and 39.59%,respectively,compared with the combined cycle which has the best performance in the traditional method.The results show that there is a first-stage and second-stage condensation temperature that makes the net power output,thermal efficiency and energy efficiency of the two-stage condensation Rankine cycle optimal.And the second-stage condensation temperature has more obvious influence on the UA than that of the first-stage condensation temperature.By analyzing the vaporization pressure of different LNG and found that the vaporization pressure of LNG has an obvious effect on the optimal first-stage and second-stage condensation temperature range.Secondly,the effect of stage number of condensing process on the power performance and economy of the LNG cold energy power generation system is studied.Taking the direct expansion,one-stage,two-stage and three-stage condensation Rankine cycles,as well as the one-stage,two-stage and three-stage combined cycles as an example,these seven systems are optimized by genetic algorithm.With the net power output as the objective function,when the stage number of condensation process increases,the net power output increases.When chose the EPC as the objective function,the EPC of the combined cycle system is always lower than that of the Rankine cycle system with the same stage number of condensation process.If the net annual income is chosen as the objective function,when the stage number of condensation process increases,the net annual income increases.But if considering the net annual income increase rate and the control difficult degree of system,the two-stage combined cycle is recommended.Finally,in order to further improve the system performance of the two-stage combined cycle,the working fluid is designed.When binary mixtures are selected as working fluid,there is an optimal composition for mixed working fluids that make the net power output of the system is maximized.The largest net power output of each binary mixture is obviously better than that of corresponding pure working fluid.When mixtures of hydrocarbons are selected as working fluids,the overall performance of the two-stage condensation combined cycle is better than that of two other types of mixtures.With the increase of the component number of mixed working fluids,the net power output of the two-stage condensation combined cycle is increased,but the increase rate is gradually reduced.If consider the increase rate of the net power output and the difficulty of filling the working fluid comprehensively.The optimum component number of the hydrocarbon mixtures is three.