【作者】 王志奇；

【导师】 周乃君；

【作者基本信息】 中南大学 ， 动力工程及工程热物理， 2012， 博士

【摘要】 我国工业余热特别是低温烟气余热资源丰富,节能潜力巨大。由于缺乏有效的技术手段,低温烟气余热没有得到充分利用,使得我国的能源利用率偏低。有机朗肯循环(Organic Rankine Cycle,简称ORC)具有蒸发压力和冷凝压力较低、循环热效率高及设备相对简单等优点,是一种有效的低品位余热发电技术。围绕该技术开展相关的研究工作,对提高我国能源利用率,改善我国低温余热资源利用技术缺乏的现状具有重要的意义。本文在广泛查阅相关文献的基础上,建立了ORC系统的多目标优化数学模型,采用模拟退火算法对系统参数进行了优化计算,为系统选取了合适的工质；搭建低温烟气余热发电实验装置,对系统的稳态及动态性能进行了实验研究；建立了ORC系统的动态过程数学模型,对热源温度、工质泵频率发生阶跃变化时系统的动态特性进行了仿真分析。本文完成的主要工作与相关结论如下：1)有机朗肯循环的参数优化及工质选择。在对有机朗肯循环进行热力分析的基础上,以单位输出功率所需换热面积及系统热回收率为目标函数,建立ORC系统的多目标优化模型；以蒸发压力、冷凝压力、蒸发器及冷凝器管内的介质流速为设计变量,采用模拟退火优化算法,利用Matlab软件编写计算程序对循环参数进行了优化计算；在最优工况下,分析热源温度、蒸发器的最小传热温差、蒸发器面积富裕度以及工质物性参数对系统性能的影响；建立系统的经济分析模型,对比分析了最佳条件下系统的经济性。2)优化结果分析。结果表明：随着工质沸点的升高,循环的最佳蒸发压力逐渐减小；系统的最佳蒸发压力随着热源温度升高而增大,当蒸发压力接近工质的临界压力时,热回收率最大；当热源温度低于180℃时,在研究的工质中R123的综合目标函数值最小,发电成本最低,为最佳工质。热源温度高于180℃时,循环采用R141b更加适合；蒸发器内最小传热温差为15℃时,系统的综合性能更高；随着蒸发器面积富裕度的升高,单位输出功率所需换热面积线性增加,而热回收率变化很小。热源温度较低时,系统的发电成本与投资回收期随热源温度的升高而迅速降低。当烟气温度低于100℃,由于发电成本高、投资回收期长而不适合采用ORC技术。3)实验系统的设计与搭建。在确定实验设计方案的基础上,对主要实验设备进行了选型分析,并对热风炉、蒸发器、冷凝器进行了设计计算,完成了实验系统相关装置的选型,搭建了低温烟气有机朗肯循环发电实验系统。系统包括热源部分、工质循环回路、冷却水回路以及润滑油循环回路等四个部分,采用R123为循环工质,以热风炉产生的低温烟气为热源,采用整体翅片管式的蒸发器,冷凝器采用水冷却的方式,膨胀机则选用涡旋式膨胀机。4)系统稳态特性的实验研究。实验结果表明：实验条件下,涡旋膨胀机的最大效率为56%,最高转速为1240rpm;涡旋膨胀机效率随着膨胀比的增大而提高,而受工质过热度的影响较小；膨胀机转速随着膨胀比、工质流量的升高而增大,随着负载的增加而降低。当热源温度为215℃、蒸发压力在1.0MPa～1.08MPa时,循环的热效率最高为8.5%、输出功率最大,为645W；蒸发压力在0.56MPa～0.65MPa时,系统的热回收率最大,为22%。循环的热效率、系统输出功率以及(?)效率随着蒸发压力的升高而增大,系统热回收率则随着蒸发压力的升高而降低；随着热源温度的升高,系统输出功率、(?)效率以及热回收率增大；随着过热度的增大,系统输出功率、(?)效率以及热回收率降低,但循环热效率变化较小。5)对系统中热源条件变化、工质泵停/启以及工质泵频率变化时的系统性能进行了实验研究。结果表明：随着二次风阀门的开启,烟气的温度迅速下降,烟气的流量则迅速上升；膨胀机入口温度逐渐降低,并在阀门开启后2.7分钟时趋于稳定。此时工质为汽液两相状态,涡旋膨胀机的转速也逐渐升高；与初始状态相比,膨胀机转速提高了9.6%。在泵停／启过程中,停泵后蒸发压力的下降速度明显低于开泵过程中蒸发压力的上升速度；膨胀机转速及循环热效率随着泵的关闭而逐渐降低,并在泵启动后35s达到最小值；随后转速及热效率逐渐升高,逐步达到稳定状态。工质泵频率调整时,压力与工质流量在调整频率后2分钟时处于稳定状态,而膨胀机入口温度稳定所需的时间更长,约为4.5分钟。6)系统动态性能仿真模型的建立。以质量守恒方程、能量守恒方程为基础,将蒸发器分为过冷区、两相区以及过热区,建立了蒸发器的移动边界数学模型；针对管壳式冷凝器,将其分为过热区、冷凝区以及过冷区三个部分,采用控制容积法建立了数学模型；将涡旋膨胀机视为涡旋压缩机的反向运动,结合涡旋膨胀机的工作过程,建立了涡旋膨胀机的稳态数学模型；采用多项式拟合的方式,建立了多级离心泵的稳态数学模型。为各部件选择合适的状态参数,对系统热力参数的耦合关系进行分析,确定了各部件之间系统参数的相互关系；分析蒸发器与冷凝器的求解思路,确定了系统动态性能仿真时各部件的求解顺序。7)系统动态性能的仿真研究。在Simulink仿真平台上构建了ORC系统各部件的仿真模块,并根据系统参数之间的耦合关系,将各部件模型组合成系统仿真模型。结合实验测试数据,对系统仿真模型进行了检验,并对系统在热源温度以及工质泵频率发生阶跃变化时的动态特性进行了仿真。结果表明：仿真模型具有较高的精度,稳态仿真结果与实验结果的误差小于5%。当烟气温度从150℃阶跃到170℃时,蒸发压力、冷凝压力以及循环热效率基本不受热源温度的影响；蒸发器内过冷区与两相区的长度迅速减小,而过热区的长度则快速增大；冷凝器中的过热区不断增大、两相区先降低后升高,而过冷区则逐渐降低；膨胀机输出功率随着烟气温度的增大而不断增加,系统的响应时间为300s-400s。当工质泵频率正向阶跃10%时,蒸发压力迅速增大,蒸发器内过冷区、两相区所占的比例增加,蒸发器的出口焓值不断减小；冷凝器内的压力则呈现先降低后升高的趋势,冷凝器出口焓与冷凝压力的变化规律相同；膨胀机的输出功率迅速增大,而循环效率则呈现先降低后升高的趋势,整个过程的响应时间为200s-300s。本文的研究成果可以为工业规模低温烟气余热ORC发电系统的设计与研制提供理论依据。更多还原

【Abstract】 The industrial waste heat resources of our country, especially in low-temperature flue gas, are abundant. Therefore, there is a great potential of energy conservation. The waste heat of low-temperature flue gas has not been fully utilized for lack of valid method, which leads to the low energy efficiency. Organic Rankine Cycle (ORC) is considered to be an effective power generation technology due to its higher thermal efficiency, lower evaporation and condensation pressures and relatively simple system. The farther research about ORC is very important to improve the energy efficiency and the situation of technology shortage for low-temperature waste heat recovery.In the present dissertation, based on plenty of reviewed literatures, a multi-objective mathematical model for ORC was developed. The parameters were optimized by simulated annealing algorithm (SA) and the ideal working fluid was recommended for system. Then an experimental system for low-temperature flue gas heat recovery was constructed. According to the system, the steady and dynamic performance was tested. Further, a dynamic process mathematical model of ORC system was developed and the dynamic performance simulation was performed for system with a step change of heat source temperature and pump frequency.The main contents and conclusions for the present study are as follows:1) Based on the thermodynamic analysis for ORC system, a multi-objective mathematical model was developed. The ratio of heat transfer area to net power and heat recovery efficiency were used as the objective function and was optimized using the SA. Evaporation and condensation pressures, working fluids and cooling water velocities were varied in the process of optimization. The optimization procedure was conducted with a simulation program written in Matlab. Under the optimal conditions, the effects of waste heat temperature, pinch temperature difference and area rich degree on the system performance were int. And the economic performance was compared with an economic model for ORC system.2) The optimization results show that optimal evaporation pressure decreases as the boiling temperature of working fluids increases. And it increases with the increase of heat source temperature. When evaporation pressure closes to the critical pressure of working fluids, heat recovery efficiency reaches to maximum. Compared with other working fluids, R123is the best choice for the temperature range of100℃～180℃and R141b shows better performance when the temperature higher than180℃. In order to get higher performance for ORC system, the suitable pinch temperature in evaporator is about15℃for the exhaust temperature range from100℃to220℃. With the increase of excess area of evaporator, the area for per unit power increases. However, the change of heat recovery efficiency can be ignored. Economic characteristic of system decreases rapidly with heat source temperature. ORC system is uneconomical for the temperature of exhaust lower than100℃.3) Based on the design conditions, the type of equipments in experiment system was selected. Then the design calculation for stove, evaporator and condenser was performed. According to the designed and selected equipments, the system was established. The system consisted of four part:the heat source section, the circuit for working fluid, cooling water and lubricant circuit. R123was selected as the working fluid of the experimental system, and low-temperature flue gas produced by stove was regarded as heat source. Integral fin tubes were applied to the evaporator and the condenser was cooled by water. The expander was originally a scroll compressor, adapted to operate in reverse.4) The steady performance of ORC system was tested. The results show that the greater pressure ratio, the higher efficiency of scroll expander. However, the superheating of working fluid has little effect on it. The rotating speed increases with the pressure ratio and mass flow rate of working fluid, and it decreases with the increase of loads. The maximum efficiency of scroll expander is56%, and the maximum rotating speed is1240rpm. When heat source temperature is215℃and evaporation pressure is1.0MPa-1.08MPa, ORC system gets the highest cycle efficiency and output power. And the corresponding result is8.5%and645W, respectively. In the experiment process, the maximum heat recovery efficiency is22%. The corresponding heat source temperature is215℃and evaporation pressure is0.56MPa-0.65MPa. With the increase of evaporation pressure, cycle efficiency, power generated by expander and exergy efficiency increases. However, the heat recovery efficiency decreases with it. For the same evaporation pressure, as the heat source temperature rises, power generated by expander, exergy efficiency and heat recovery increases. But they decrease with the increase of superheating. The superheating has little effect on cycle efficiency.5) Dynamic performance during the process of changing temperature of heat source, pump stopping/starting and frequency of pump was tested. The experimental results show that temperature of heat source decreased and flow rate increased rapidly while opening the valve of secondary air. The expander inlet temperature decreases and it becomes stable2.7minutes later. At that moment, working fluid is at the state of gas-liquid, which leads to a increase of expander rotating speed. And it improves9.6%compared with the initial speed. The decreasing rate of evaporation pressure in stopping is obviously lower than that of increasing rate in starting. The rotating speed and cycle efficiency decreases when pump stops running. Although the pump starts again, the speed and efficiency did not increased immediately and there is a delay of35s. Then, rotating speed and cycle efficiency increases gradually. When the pump frequency changes abruptly, pressure and flow rate of working fluid reaches a steady state after2minutes. Expander inlet temperature takes4.5minutes to reach the steady value.6) A dynamic mathematic model for ORC system was developed. The evaporator consisted of three regions, including the sub-cooled region, the two-phase region and the superheated region. Based on the equations of mass continuity and energy conservation, a generalized moving-boundary model was developed to describe the transient behavior of evaporator. Each state of the refrigerant in the condenser was formulated by a control volume. The selected control volumes in the refrigerant region were as follows: superheating zone, condensing one and sub-cooled region. Compared to the evaporator, the dynamics of expander and pump was negligible. Therefore, steady models were presented. The scroll expander mode was obtained by operating a scroll compressor in reverse. And the model on multi-stage centrifugal pump was presented using the polynomial fitting method. Then the relationship between different components was analyzed and the solving way for system model was presented.7) A dynamic simulation model was established using Simulink according to the relationship between different models. Numerical solution was validated with the experimental results. Compared with the experimental results, the relative error of simulation solution was less than5%. This accuracy was believed to be sufficient for most engineering applications. Based on the validated model, the dynamic simulation was performed. The results show that the evaporating pressure, condensing pressure and cycle efficiency is about the same for a step change of heat source temperature. The length of sub-cooled and two-phase region in evaporator decreases rapidly. Instead, the superheated increases quickly. In the condenser, the variety of different zone is different. The superheating zone in condenser increases, and the condensing one decreases firstly and then increases. The sub-cooled zone gradually reduces. Power generated by expander improves obviously. The response time for system is about300s-400s. When pump frequency increases10%suddenly, the pressure, proportion of sub-cooled and two-phase region in evaporation increases. As a result, the outlet enthalpy of evaporator reduces gradually. The condensing pressure decreases firstly and then increases, and the change of enthalpy at the condenser outlet is similar. The response time for a step change of pump frequency is about200s～300s.The results are useful for the industrial application and design of low-temperature waste heat generation system based on organic Rankine cycle.更多还原