Working fluid selection and multi-objective optimization of organic Rankine cycle with variable turbine efficiency

doi:10.11949/j.issn.0438-1157.20171349

Abstract

Abstract:

To harness heat of 523.15 K high temperature flue gas, pentane, hexane, heptane, cyclohexane, MM (hexamethyldisiloxane), benzene and toluene were selected as working fluid candidates. With selection of one-dimensional radial-inflow turbine efficiency prediction model to replace constant turbine efficiency model, and net power output and exergy efficiency as target functions, organic Rankine cycle (ORC) system was simulated for multiple output variables by using non-dominated sorting genetic algorithm (NSGA-Ⅱ). Optimal solution of each working fluid was determined from Pareto frontiers by ideal point estimation. The results show a strong correlation between turbine efficiency and volumetric flow ratio (VFR) of working fluids in a way that turbine efficiency curve trends oppositely to VFR curve. At fixed heat source conditions, benzene is the optimal working fluid whereas toluene and cyclohexane are sub-optimal. Exergy efficiency accelerates in a downward trend at evaporation temperature above 400 K, but net power output slows down in a rise trend at evaporation temperature above 410 K. Optimization with constant turbine efficiency model somewhat affects screening results of optimal parameters and best working fluid, which are deviated from actual outcomes. However, optimization with variable turbine efficiency model can reduce such error and results are much closer to engineering practice.

Key words: organic Rankine cycle, multi-objective optimization, one-dimensional radial-inflow turbine efficiency prediction model, thermodynamic, economic, volumetric flow ratio

摘要：

针对523.15 K的中高温余热烟气热源，选取戊烷、己烷、庚烷、环己烷、MM（六甲基二硅氧烷）、苯和甲苯为候选工质，引入一维向心透平效率预测模型取代固定透平效率，以（火用）效率和系统净功为目标函数，基于NSGA-Ⅱ算法对ORC系统进行多目标求解，采用理想点辅助法对各工质Pareto前沿进行决策寻优。得出以下结论：工质的透平效率与透平等熵膨胀比（VFR）存在较强的相关性，透平效率曲线与VFR曲线的变化趋势相反；在给定热源条件下，苯是最优工质，甲苯和环己烷是次优工质；当蒸发温度超过400 K时，（火用）效率下降趋势加快，当蒸发温度超过410 K时，系统净功上升趋势放缓；定透平效率寻优会对最佳参数与最优工质筛选结果造成一定影响，与实际存在偏差；变透平效率寻优可以减少误差，更接近工程实际。

关键词: 有机朗肯循环, 多目标优化, 一维向心透平效率预测模型, 热力学, 经济, 透平等熵膨胀比

CLC Number:

TK123

HAN Zhonghe, MEI Zhongkai, LI Peng. Working fluid selection and multi-objective optimization of organic Rankine cycle with variable turbine efficiency[J]. CIESC Journal, 2018, 69(6): 2603-2611.

韩中合, 梅中恺, 李鹏. 变透平效率有机朗肯循环工质筛选及多目标优化[J]. 化工学报, 2018, 69(6): 2603-2611.

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