Experimental Study And Dynamic Performance Simulation For Low-temperature Waste Heat Generation System Based On Organic Rankine Cycle

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.