Influence of dynamic turbine efficiency on performance of organic Rankine cycle system

doi:10.11949/j.issn.0438-1157.20180928

Abstract

Abstract:

The centripetal turbine efficiency varies greatly with the change of operating parameters and the type of working fluid, and a one-dimensional analysis model of radial-inflow turbine is introduced. The effects of evaporation and condensation temperature on the turbine efficiency were investigated, and a comparative analysis on thermodynamic and economic performances of the organic Rankine cycle (ORC) system with constant turbine efficiency and dynamic turbine efficiency was presented. NSGA-Ⅱ is employed to conduct multi-objective optimization of ORC system, which was to select the optimal working fluid and determine the optimal evaporation and condensation temperature. Meanwhile, the optimal operating parameters of ORC system with constant and dynamic turbine efficiency were compared, and the variation of turbine efficiency with heat source temperature was studied. The results show that the turbine efficiency increases with the decrement of evaporation temperature or the increment of condensation temperature. After introducing dynamic turbine efficiency, the increment of net power output with increasing evaporation temperature slows down, and the sequence order of some working fluids changed. The optimal working fluid and the optimal operating parameters are different between ORC system with constant and dynamic turbine efficiency, which indicates that constant turbine efficiency will cause errors in selection of optimal working fluids and determination of operating parameters. As the heat source inlet temperature raises, the difference of optimal evaporation temperature and net power output between the ORC system with constant and dynamic turbine efficiency increases. The higher heat source inlet temperature is, the greater error caused by adopting constant turbine efficiency will be.

Key words: organic Rankine cycle, constant turbine efficiency, dynamic turbine efficiency, multi-objective optimization

摘要：

向心透平效率随运行参数的变化及工质种类的不同有较大差别，引入向心透平一维分析模型来计算透平效率，分析蒸发温度与冷凝温度对透平效率的影响，比较固定透平效率与动态透平效率有机朗肯循环（ORC）系统的热力性能与经济性能。采用非支配解排序遗传算法（NSGA-Ⅱ）优化ORC系统筛选出最优工质，确定最佳蒸发温度与冷凝温度。同时比较了不同热源温度下固定透平效率和动态透平效率ORC系统的最佳运行参数，分析了透平效率随热源温度的变化。结果表明：透平效率随蒸发温度的降低或者冷凝温度的升高而增大，采用动态透平效率后，系统净输出功随蒸发温度升高而增加趋势减缓，工质排序也发生了变化；对于固定透平效率与动态透平效率ORC系统，经多目标筛选后所确定的最优工质及最佳蒸发温度和冷凝温度均有一定差异，表明若采用固定透平效率会对工质筛选及参数优化造成一定误差；随着热源温度的升高，固定透平效率与动态透平效率ORC系统之间最佳蒸发温度与净输出功差异逐渐增大，说明热源温度越高，采用固定透平效率引起的误差越大。

关键词: 有机朗肯循环, 固定透平效率, 动态透平效率, 多目标优化

CLC Number:

TK 123

Peng LI, Zhonghe HAN, Xiaoqiang JIA, Zhongkai MEI, Xu HAN. Influence of dynamic turbine efficiency on performance of organic Rankine cycle system[J]. CIESC Journal, 2019, 70(4): 1532-1541.

李鹏, 韩中合, 贾晓强, 梅中恺, 韩旭. 动态透平效率对有机朗肯循环系统性能的影响[J]. 化工学报, 2019, 70(4): 1532-1541.

Figures/Tables 18

Fig.1 Schematic diagram of basic ORC system

Fig.2 T-s diagram of basic ORC system

Fig.3 h-s diagram of radial-inflow turbine

Fig.4 Velocity triangles of radial-flow turbine

Fig.5 Velocity ratio-degree of reaction curves of radial-flow turbine

Table 1 Initial parameters for radial-inflow turbine

Parameter	Symbol	Value
nozzle velocity coefficient	?	0.95
rotor blade velocity coefficient	ψ	0.85
ratio of wheel diameter	D _r	0.5
absolute velocity angle at rotor inlet	α ₁	15
relative velocity angle at rotor outlet	β ₂	30

Fig.6 Flowchart of NSGA-Ⅱ

Table 2 Cycle and economic parameters for simulation of ORC system

Parameter	Value
heat source inlet temperature/K	433.15
heat source outlet temperature/K	363.15
amount of waste heat/MW	1
ambient temperature/K	293.15
ambient pressure/MPa	1.01
pump isentropic efficiency/%	80
interest rate/%	10
plant economic life/a	20
annual plat operation time/h	7000

Table 3 Properties of working fluid candidates

Working fluid	Molar mass/(g·mol^-1)	Normal boiling point/K	Critical pressure/MPa	Critical temperature/K
R236ea	152.039	279.34	3.502	412.44
R114	170.921	276.741	3.257	418.83
R245fa	134.048	288.29	3.651	427.16
R245ca	134.049	298.28	3.925	447.57
R123	152.931	300.973	3.662	456.831
isopentane	72.149	300.98	3.378	460.35
pentane	72.149	309.21	3.37	469.7
cyclohexane	84.161	353.886	4.075	553.64

Fig.7 Comparison of predicted turbine efficiency and experimental data

Fig.8 Variation of turbine efficiency with evaporation temperature

Fig.9 Variation of turbine efficiency with condensation temperature

Fig.10 Variation of net power output with evaporation temperature

Fig.11 Pareto frontier results of different working fluids for ORC system with constant turbine efficiency

Fig.12 Pareto frontier results of different working fluids for ORC system with dynamic turbine efficiency

Fig.13 Variation of optimal evaporation and condensation temperature with heat source inlet temperature

Fig.14 Variation of turbine efficiency with heat source inlet temperature

Fig.15 Variation of net power output with heat source inlet temperature

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