Performance Evaluation And Optimization Of ORC Power Generation System Recovering Low-grade Waste Heat

Energy sources are the power to push development and progress of human society forward. And China is a major energy consumer. However, due to the lack of science and technology in China, the energy utilization ratio is not high and a great deal of low-grade waste heat is pumped directly into the atmosphere, which cause enormous loss of energy and bring great destruction to our environment. Low-grade waste heat power generation system based on organic Rankine-cycle(ORC system), by which the low-grade waste heat can be transformed into high-grade electric energy, has proved to be an effective way to improve energy utilization efficiency and reduce environmental pollution. And, with its unique advantage and wide market application prospect, the ORC power generation technology has become one of the hot topics in research field of energy saving.On the basis of the previous researches on ORC power generation technology, a model of ORC power generation system recovering low-grade waste heat has been established in this paper. Then, the programming software Engineering Equation Solver has been used in the simulations of ORC system, and some research works have been conducted in this paper. The details are as follows:1. Performance analysis and thermodynamic optimization of ORC system. With the thoughts of high-efficiency, economy, thermodynamics perfection and environment protection, four performance parameters are selected as the four key indicators which can reflect the characteristics of ORC system. And then, a comprehensive evaluation method of ORC system performance based on the multi-objective optimization is put forward in this paper. After that, this method is used to study the impact of the main thermodynamic cycle parameters on the ORC system performance. According to researches, the results gained by using a single-objective optimization method with only one performance parameter as the indicator, cannot often take a global view of the ORC system performance and sometimes may lead to a deviation. Researches also show that the evaporation temperature has a big effect on the comprehensive evaluating indicator of ORC system performance, and the comprehensive evaluating indicator increases firstly and then decreases with the increment of the evaporation temperature, while other thermal parameters such as condensation temperature, pinch point temperature difference of heat exchanger, degree of superheat and degree of supercooling have a wide range to let the comprehensive evaluating indicator be almost unaffected by them. Besides, an iterative calculating method is proposed to be used in the multi-objective optimization of ORC system. And by this calculating method, the optimal thermodynamic cycle parameters can be got to make the ORC system perform best under a fixed heat source.2. Study of the influence of waste-heat temperature fluctuation and regenerator on ORC system performance. Due to practical demand for production the waste-heat temperature fluctuates in some ranges regularly, hence the effect of waste-heat temperature fluctuation on ORC system is studied in this paper. The result shows that, the higher the grade of waste heat, the better the heat recovery efficiency of ORC system is, and thus the waste heat with a higher grade should be recovered first. It also can be found that, the energy recovery result of ORC system under the optimum operating condition is better than that under the actual operating condition. And the system performance difference between the two operating conditions is becoming bigger with the expansion of waste-heat temperature fluctuates.In order to explore the effect of regenerator on the ORC system performance, a related research work has been carried out. Results suggest that, some of ORC system performance, not all, is improved when a regenerator is installed additionally in the ORC system. In the precondition of the same thermal efficiency, the evaporation pressure is reduced when a regenerator is installed in the ORC system, which reduces the requirements of intensity and airtightness for ORC system.However, after weighing the benefits against requirements from high-efficiency, economy, thermodynamic perfection and environment protection, it has been found that the comprehensive performance of ORC system decreases slightly when a regenerator is installed. So it is not strictly necessary to install a regenerator in the performance optimization of ORC system.3. Thermo-economic study of evaporator in ORC system. The evaporator is a key device of ORC system, and carrying out the thermo-economic study of it is very significant to the improvement of whole ORC system. Based on the perspective of exergy recovery, considering the costs of the evaporator in whole life cycle, time-value of fund and the price difference between the heat-exergy and power-exergy, a thermo-economic evaluation model with the annual net profit NPV as an important criterion is established for the evaporator. It is found that, a contradiction is produced when using a single-objective optimization method with the exergy recovery as the only indicator, and under the condition the project of waste heat recovery is technologically feasible but isn’t economically viable. And the greatest benefit of ORC system recovering waste heat can be achieved when the best match of heat transfer area, mass-flow of working fluid and flow ratio is realized. To avoid the sharp fall when the working condition fluctuates, the flow ratio should be retained in the specific range which is a little larger than the optimal flow ratio. Besides, research also shows that, the ORC system with larger heat transfer area does not always get better energy recovery. In some time, to reduce the investment a smaller heat transfer area is acceptable, and under this situation the loss of the annual net profit is just a little.Above all, studies and discussions on ORC system have been carried out from several points of view. A comprehensive evaluation method of ORC system performance based on the multi-objective optimization and a thermo-economic evaluation model for the evaporator are put forward in this paper, which provide reference and basis for engineering designing, follow-up study and investment decisions of ORC system.