Study On Operating Performance Of Working Fluid Pumps And Matching Characteristics With The Organic Rankine Cycle System Used For Vehicle Engine

The organic Rankine cycle systems which convert low grade waste heat and renewable energy to useful power,have aroused a widespread concern of scholars and been widely studied due to their high efficiency,simple structure and economic benefit.Many scholars have conducted a great number of researches about expander and heat exchanger used in the ORC system,but few have paid enough attention to the working fluid pump.A working fluid pump determines the mass flow rate and the evaporation pressure of the ORC system,and to increase the evaporation pressure can be helpful for improving the performance of the whole system and each component.Meanwhile,a working fluid pump is also a consumed power component,whose operating conditions have a significant influence on net power output and thermal efficiency.Recently,the research on working fluid pumps mainly focuses on assuming an intended evaporation pressure to match a required flow rate to study the ORC system performance.However,the two parameters should be determined by the actual operating conditions of the working fluid pump,rather than being matched arbitrarily.The electric power input of the working fluid pump is mainly calculated by the flow rate of working fluid and the enthalpy difference between the inlet and outlet of the pump or even be ignored,which will cause it seriously lower than the actual electric power input.For the research on working fluid pump used in the ORC system,the operating parameters of working fluid pump should vary with different operating conditions.The hypothetical values which have not been verified by experiments cannot reflect the actual performance variation of ORC system,and conceal the effects of working fluid pump on system performance.Therefore,it is of great importance and significance in exploring the coordinated variation of operating parameters of the working fluid pump under variable conditionsIn order to address this problem that a pump seriously affects the low grade waste heat recovery and utilization,the paper is for sake of improving the net power output of the ORC system.The working fluid pump performance test platform was built under a simulated ORC condition.The characteristic curve of working fluid pumps is obtained to investigate the coordinated variation of operating parameters of pumps under variable conditions though experiments on the four typical working fluid pumps(multistage centrifugal pump,hydraulic diaphragm metering pump,vortex pump,roto-jet pump).The effect and matching characteristics of the key parameters of working fluid pumps including flow rate,pressure,and power input,on the ORC system were revealed theoretically under variable conditions.Further combined with the relevant experimental data,the neural network model of a pump applied in the ORC system was built,and the optimum value of the pump was gained so as to obtain the optimal operating conditions of the pump to meet the requirement of the variable working conditions of the ORC system.Based on a thermodynamic model of the simple ORC system,the effects of the thermodynamic parameters(condensation temperature and supercooling degree)on the pump power and back work ratio were analyzed theoretically.As can be found from the research: the electric power input shows a rising trend with condensation temperature decreasing,but corresponding BWR decreased,which makes the overall system performance improved;A higher supercooling degree makes the electric power input and BWR increased.This case disadvantages the improvement of the system performance.Additionally,based on the experimental results,the influence of the pump outlet pressure on the mass flow rate,specific speed,electric power input and actual operating efficiency at different pump speeds was studied.The control method of reducing the electric power input under variable operating conditions was explored.We can conclude that the specific speed of the four kinds of working fluid pumps decreases with the increase of the outlet pressure,and increasing the pump speed can raise the specific speed and mass flow rate;Among the studied pumps,the hydraulic diaphragm metering pump is more efficient on the aspects of the electric power input,changing in the range of 0.12-0.36 kW,and reducing the pump speed and outlet pressure can decrease the power input.The actual operating efficiency of the multistage centrifugal pump is higher,and the maximum value can reach up to 59.96%.The actual operating efficiency of the four working fluid pumps varies with different operating conditions.The output pressure and speed of the pump can be controlled harmoniously to meet the demand of the system to reduce the electric power input of the pump.Based on the results of performance analysis,the variation of operating parameters of the ORC system with operating conditions of pumps was analyzed theoretically,such as back work ratio,exergy loss rate,net power output,thermal efficiency,heat absorption rate.It is found that the exergy loss rate of the four pumps increases with pump speed.The BWR using multistage centrifugal pump is only 0.07 on average,and the value of other pumps can reach 0.12.In practice,we should try to avoid a higher BWR.The net power output of the system using the hydraulic diaphragm metering pump increases with outlet pressure and pump speed,compared with others first increasing and then decreasing.The hydraulic diaphragm metering pump shows a potential for efficient recovery of lower heat capacity.The multistage centrifugal pump has the potential to efficiently recover higher heat capacity.In practical application,the outlet pressure and pump speed should be coordinated to maximize the net power output of the system and avoid effective energy waste.