Selection of top working fluids for the two-stage cascade coupling cycle utilizing solar energy and LNG cold energy
Optimum selection of top working fluids for the two-stage cascade coupling cycle coupling utilizing solar energy and LNG cold energy was carried out under different coupling temperatures(Tihe)with a fixed bottom working fluid.The selec-tion principles of initial working fluids were established and the initial working fluids were obtained.Fixed working conditions and changes of evaporation pressure for the bottom cycle and the top cycle were specified.The thermal efficiency,the exergy efficien-cy and the area per unit work(APW)are the evaluation criterions for the working fluid.The results show that the optimum work-ing fluid varies with the variation of Tihe.Ethylene was the optimal working fluid with the Tihe of 163.15 K,while the optimal working fluid changes into sulfur dioxide with the Tihe of 263.15 K.Expect for four kinds of working fluids,the thermal efficiency and the exergy efficiency of all other working fluids increase firstly and then decrease with the increasing Tihe.However,theAPW presents an opposite trend.Every working fluid has an optimum Tihe and the optimum Tihe of different working fluid is different to each other.Carbon sulfide has the best thermodynamic performance under the optimum Tihe.
Solar energyLNGCycle couplingOptimum working fluidIntermediate heat exchange temperature
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