Simultaneous optimization of system structure and working fluid for the three-stage condensation Rankine cycle utilizing LNG cold energy

For the power generation systems utilizing liquefied natural gas (LNG) cold energy, most researches paid attention to enhance the heat exchange process to improve the performance, but the compression and expansion process are less considered. The arrangements for compression and expansion process can affect the working conditions of turbines and pumps, respectively, thus affecting the system performance. Therefore, this paper optimizes the arrangements for compression and expansion process on the basis of the three-stage condensation Rankine cycle proposed in our previous work. For nine cycles with different structures, this paper proposes a superstructure that contains these possible cycle structures to improve the efficiency of the optimization. Firstly, the reliability of the superstructure optimization method is verified. Then, the effect of the gasified pressure of LNG on the optimum cycle structure is studied. Finally, the cycle parameters, structures and working fluids are simultaneously optimized through the proposed superstructure cycle. Results show that the arrangements for compression process have little effect on the cycle performance, while those for expansion process have a relatively significant effect. Furthermore, the optimum cycle structure is not affected by the gasified pressure of LNG and only depends on the used working fluid.