Research On Modeling And Optimal Control Of Direct Air Cooling Unit

The direct air-cooling technology of thermal power plants has a very significant water-saving effect.It is of great significance to China's "three north" areas which are rich in coal and shortage in water.Therefore,the newly added thermal power units in these areas basically use air-cooling technology to solve the problem of water scarcity.However,under complex environmental and meteorological conditions,including hot air recirculation,high temperature in summer and the pumping effect of axial flow fans,the working efficiency of large-scale axial flow fans of air-cooled condensers(ACCs)is directly affected.Even at the action of transverse ambient wind,the air flow deformation at the inlet of the upwind air-cooling cells is serious,which easily causes the inlet temperature of the ACCs to rise,the heat transfer capacity of the ACCs to reduce and the power consumption of the fans to increase.At present,the traditional speed control of axial-flow fan in direct air-cooling system still maintains the uniform speed of all fans,and continuously provides cold air to ACCs for heat transfer.However,it is found that even if the rotational speed of axial-flow fan in all ACCs is the same,the heat transfer efficiency of each air-cooling cell varies greatly under the disturbance of adverse effects of ambient wind.The problem can not give full play to the expected role of the axial flow fan,resulting in excessive consumption of electric energy.Combining with the industrial operation characteristics of direct ACCs,this paper focuses on the theory and method between the back pressure of steam turbine and the speed regulation of axial-flow fan group,and studies how to realize the optimal control strategy of direct ACCs.Aiming at the problem that the rotational speed of the axial-flow fan of direct air-cooled unit is the same and the electric energy can not be saved effectively,starting from the temperature and velocity fields,the steam turbine back pressure and the speed of the axial-flow fan group are taken as the main parameters to control the operation of the air-cooled island.On the premise of meeting the economic back pressure,a calculation model of the air-cooled island array based on the temperature and velocity fields is proposed.The relationship between heat transfer efficiency of temperature field and velocity field of direct air cooling unit is determined by thermodynamic calculation under variable working conditions.The average heat transfer efficiency benchmark is also established.According to the real-time situation of each air-cooling unit,the speed of the axial-flow fan in high-efficiency air-cooling cell is increased and the speed of the axial-flow fan in low-efficiency air-cooling cell is reduced.To ensure the reasonable output control of the axial flow fan,let the high efficiency air cooling cell bear the load of the low efficiency air cooling cell,avoid all the same speed of the axial flow fan,resulting in the adverse effect of low heat transfer efficiency,so as to achieve the optimal control of the rotational speed of the axial flow fans in the direct air cooling system,so that the system can obtain the best operating efficiency and reduce power consumption.In view of the back pressure control strategy of direct air-cooled power generating unit,this paper tries to find an ideal divisional regulation strategy.It took the computational fluid dynamics(CFD)to numerically simulate the temperature field and velocity field of axial flow fan clusters in the direct air-cooled power generating units at working state.Then it divided the axial flow fan cluster into arrays through cluster analysis.By adjusting the speed of these ar-rays according to the back pressure of the steam turbines,it analysed the speed-adjustment influence of axial flow fan clusters under different arrays on the back pressure of the direct air-cooled units.The numerical simulation results show that the proposed pressure regulation strategy for reducing the rotational speed of axial-flow fans in specific zones can reduce the unit back pressure very well.Combining these zones and reducing the speed at the same time,the pressure regulation effect is better-Furthermore,it lays theoretical foundation for further research on the regulation strategy of steam turbine back pressure in direct air-cooled power generating units.In view of the direct air-cooled power generating unit under the action of the environment wind,fan cooling capacity of the ACCs would vary differently in different areas,when influenced by the exhaust recirculation and the coupling effects of axial fan cluster pumping.This variation would lead to higher power consumption and lower energy efficiency for the traditional back-pressure control strategy.Using the CFD method,the back pressure of the unit in the different fan speed would be calculated,then the grey differential incremental correlation method was proposed to analyze the influence on the back pressure of the unit when processed the speed regulation of air-cooled island wind turbine generators,and then took the working efficiency of single air cooling cell into consideration when researched the pressure regulation problem.The results show that,larger the fan ventilation rate will not necessarily cause higher the heat transfer efficiency of the finned tube bundle of the air-cooled unit.Rational usage of efficient air-cooled unit can effectively improve the energy efficiency of air cooling island,and achieve the purpose of energy saving.It will also be able to provide theoretical and experimental basis for further research on the optimization control of axial flow fan in air-cooled island.