The Optimization Of The Compresssion And Expansion Configuration In The Multi-Condensation Rankine Cycles

In the industry chain of liquefied natural gas（LNG）,LNG cold energy is commonly recovered through power generation cycles.Organic Rankine cycle（ORC）is widely used in recent years because of its simple structure and mature technology,but there is still great room for improvement in its power generation performance.In order to improve cycle performance,most studies focus on increasing the energy integration and the reducing irreversible loss of the cycle,ignoring the effects of the compression and expansion layouts.Therefore,different compression and expansion layouts are proposed based on the multi-condensation Rankine cycle.The study is carried out to determin the optimal cycle configuration.Firstly,two compression layouts and two expansion layouts are proposed for the two-stage condensation Rankine cycle（2C-ORC）,and four different configurations of 2C-ORC are obtained.The sensitivity of the system performance to pump and turbine efficiency is analyzed with the net output power as the objective function and the actual efficiency of the turbine is predicted.It is found through the study that whether under the constant or non-constant turbine efficiency,there exists the optimal two condensation temperatures to maximize the net output power in the four cycles.The compression layout has no significant effect on the cycle performance（<1%）,while the serial expansion layout is better than the parallel expansion layout（increased by 17.29%under non-constant turbine efficiency）.Through the optimization of the working fluid,the best pure working fluid is R245ca and R123 under the constant and non-constant turbine efficiency,respectively.Whether the turbine efficiency is constant or not has a significant effect on the optimal working fluid and the performance order of the candidates.Secondly,three compression layouts and three expansion layouts are proposed for the three-stage condensation Rankine cycle（3C-ORC）,and a novel synchronous optimization method is constructed with the models of superstructure and working fluid selection coefficients,in which the optimal cycle parameters,the best configuration and the most suitable working fluid can be obtained in only one step.Through the verification of the method,it is found that its results are reliable and time-saving.Through the study of different cycle configurations,it is found that for the same working fluid,the compression layout has little effect on cycle performance（<1%）,while the effect of the expansion layout is relatively obvious（the maximum difference is about 3.4%under constant turbine efficiency）.Through the pure working fluid selection process,it is found that the optimal cycle configuration is not affected by the LNG gasification pressure when the working fluid is spercified and for different working fluids,the optimal compression layouts of them are always the serial layout,but the optimal expansion layouts are different.After that,the optimum cycle parameters,configuration and pure working fluid are determined by the synchronous method under different LNG gasification pressures.Finally,the compression and expansion layout of 2C-ORC and 3C-ORC are optimized based on mixture working fluid.After improving the working fluid selection coefficients,the synchronization method is also suitable for mixture working fluid.It is found that under different LNG gasification pressures,the performance of the 3C-ORC with serial compression layout and serial expansion layout is the best when the maximum net output power is taken as the target,and the 2C-ORC with parallel compression layout and the parallel expansion layout are recommended when the minimum total investment cost is taken as the target.According to the multi-objective evaluation and the analysis of the optimum design parameters,the 2C-ORC with serial compression layout and serial expansion layout is recommended under different LNG gasification pressures.As for the working fluid selection,the binary working fluid mainly composed by C₂H₄ or C₂H₆ is recommended in both single-objective and multi-objective cases.