Optimization And Analysis Of An Organic Rankine Cycle Of Cascade Type System

As the problems of power shortage and environment pressure become increasingly prominent, people actively look for effective ways to deal with the energy problems. The heat resources worldwide are rich, especially the middle-low temperature waste heat which accounts for more than half of industrial waste heat resources. If this part of waste heat can be reasonable recycled, it will be of great significance to energy conservation and the relief of power shortage.Organic rankine cycle is an effective way for recovery of low temperature waste heat.At present, the research of organic rankine cycle is concentrated in the part of low temperature heat source, in order to recover the medium temperature waste heat resources of flue gas efficiently, organic rankine cycle of cascade type is researched, focusing on widely existed flue gas waste heat resources of 200-300 ? in industrial field. The system is analyzed in the aspects of systematic structure, refrigerant and operating parameters, the main research contents and conclusions are as follows:Firstly, for the sake of the recycle of flue gas waste heat, the thermodynamics model of organic rankine cycle of cascade type is established.In the model, the influence of refrigerant superheat degree of high level on system performance is analyzed, net output power and thermal efficiency of per unit quantity of the flue gas are set as the target function, and the comparation of organic rankine cycle of cascade type and organic rankine cycle of single stage are made.Then, according to the selection criteria of organic rankine cycle,12 kinds of organic refrigerant of high-temperature stage and four kinds of organic refrigerant of low-temperature stage are selected, the refrigerant performance is analyzed, the net output power,cycle heat efficiency and exergy efficiency of per unit quantity of flue gas are set as the target function. Within the range of 200-300? flue gas heat source, 24 pairs of refrigerant are discussed about their influence on the change of target function, then the best working pairs of high-temperature stage and low-temperature stage are selected. Results show that, within the range 200-300 ? flue gas heat source,among all the selected refrigerant of high-temperature stage,the thermal efficiency of benzene and toluene are at higher level, but the system net output power are lower than other refrigerant, the system using refrigerant of siloxane type has the largest net output power and good work ability. What is more, the thermal efficiency of R245 fa is higher than R123 of low-temperature stage.Finally, from the aspect of the first and the second laws of thermodynamic the influence of main parameters including evaporation temperature of high-temperature stage and condensation temperature on the system performance are analyzed. The results show that the working medium flow of high-temperature stage and low-temperature stage increases with the rise of evaporation temperature of high-temperature stage. When the evaporation temperature of high-temperature stage are higher, the thermal efficiency and total system thermal efficiency become higher. The high level of evaporation temperature, the higher thermal efficiency and total system thermal efficiency. When the evaporation temperature changes within the scope of investigation, the total system net output power has an optimum value, and at low evaporation temperature, the net output power of low-temperature stage is the main part of the total net output power. On the other hand, when the heat source temperature is constant, the effect of condensation temperature of high-temperature stage on total system thermal efficiency based on organic rankine cycle of cascade type is not obvious.