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. © All Right Reserved.