Study On Dynamic Characteristics Of Organic Rankine Cycle System Using Mixtures As Working Fluid

There is abundant industrial waste heat in our country and there is a great potential for waste heat recovery.The Organic Rankine Cycle(ORC)technology is a rather promising method to generate electricity with low-temperature waste heat.It plays a key role in improving energy efficiency,saving energy and reducing pollution.However,the ORC system with waste heat recovery is susceptible to upstream heat sources.The system is highly coupled and nonlinear,which is a challenge for operation and control.In this work,the dynamic models of subcritical and supercritical ORC with mixtures are established,and the dynamic behaviors under different distributions of heat sources are analyzed.An improved dynamic model of ORC system with zeotropic mixture is firstly developed,which can be better adapted to the phase change process of working fluid.Accordingly,dynamic behavior of the ORC and dynamic distribution of evaporation stages are investigated.The dynamic response characteristics and corresponding static performance of mixtures with different components are compared.The influence of mixture with different components on system performance is analyzed and corresponding control strategies are proposed.The results show that the dynamic response is closely related to the mass flow,heat transfer and physical properties of the working fluid.The working fluid with larger flow and smaller latent heat has greater thermal inertia and longer response time.The heat exchange of the evaporator is mainly concentrated in the two-phase region,the heat exchange area of two-phase region accounting for 61.43% of the entire evaporator.In the two-phase zone,for the same dryness differences of working fluid,the required area of the cell near evaporation starting point is lager,and the fluctuation of the parameters in that cell is more significant.Thus more attention should be paid to the heat transfer and dynamic characteristic in the evaporation starting region.An improved dynamic model of supercritical ORC using zeotropic mixture as working fluid is developed.The effects of system heat transfer intensity and different heat source disturbances on system response were studied.When a step interference is introduced to heat source temperature,some system parameters fluctuate greatly.As the temperature of the heat source increases,the position of the pseudo critical point approaches the inlet of the heat exchanger.Enhancing heat transfer of the system can effectively reduce the system response time and required heat exchanger area.The corresponding time of the system pressure has an approximately exponential relationship with the comprehensive heat transfer coefficient of the heater.The large inertia system can filter the high frequency distribution so that high frequency fluctuation interference has little effect on the large inertia system.A rough estimation of the system thermal inertia with function prediction can provide a reference for system design and control in different applications.