蒸发器换热过程对ORC系统混合工质选择和运行工况的影响

在有机朗肯循环（ORC）系统其他部件的效率均相同的条件下,蒸发器的换热量越大,不可逆损失越小,系统的做功潜力就越大。由于混合工质的相变温度滑移与热源的温度变化具有较好的“匹配性”,采用混合工质为循环工质,并使用流耗散率来描述混合工质与热源流体换热过程中的不可逆损失。结合 T-Q 图来分析蒸发器中混合工质与热源流体换热过程,发现混合工质与热源流体换热曲线围成的面积为流耗散率,并且分别得出了蒸发器换热量最大与不可逆损失最小的条件,以此指导系统最佳混合工质和运行条件的选择。针对确定的热源条件,在蒸发器换热量达到最大的条件下,以不可逆损失最小为目标,建立了基于蒸发器性能的系统最佳混合工质和运行条件的选择方法。采用参考文献中热源条件,筛选出最佳混合工质为R600a/R134a(0.2/0.8),在最佳蒸发器出口温度为365.75 K的运行条件下,换热量是文献中最佳混合工质的3.3倍,在系统其他部件的参数选择均相同的条件下,系统的净输出功率是文献中最佳混合工质的2.4倍。

Effect of evaporator heat transfer process on selection of mixture and operating condition in ORC system

Under the condition of the same efficiency of other parts of organic Rankine cycle (ORC) system, the greater the heat exchange capacity of the evaporator, the smaller the irreversible loss, and the greater the potential of the system. Because of the phase transition temperature glide of mixture has good “matching” with the temperature change of heat source, this paper uses mixture as working fluid, and uses entransy flow dissipation rate to represent the irreversible loss of heat transfer process between mixture and heat source fluid. Combining T-Q diagram, the heat transfer process of mixture and heat source fluid in evaporator is analyzed. It is found that the area enclosed by the heat transfer curves of mixture and heat source fluid is the entransy flow dissipation rate, and the conditions of maximum heat transfer quantity and minimum irreversible loss of evaporator are obtained respectively, to guide the selecting of the optimal mixture and operation condition. According to the determined heat source, under the condition of maximum heat transfer quantity, a method for selecting the optimal mixture and optimal operating condition of the system based on the performance of evaporator is established. Adopting the heat source listed in reference paper, the optimum mixture and optimum outlet temperature are R600a/R134a(0.2/0.8) and 365.75 K, respectively. The heat transfer quantity of the evaporator is 3.3 times, under the condition that the parameters of other parts of the system are the same, the net output power of the system is 2.4 times than the optimum mixture in the literature.