Study On Operating Characteristics And Condenser Performance Optimization Of Organic Rankine Cycle

As a widely used low grade energy recovery technology,organic Rankine cycle(ORC)has been proposed because of its high efficiency in utilizing low temperature heat sources,such as industrial waste heat,solar energy.biomass energy,geothermal energy and ocean thermal energy.With the development of this technique,a series of related scientific and technical problems are still needed to be solved.This paper highlights the key technologies of integration and optimization of ORC system,including the operation characteristics and performances of ORC system and its main components,the dynamic operation characteristics of control process,the effects of working fluid charge ratios,the recuperator ORC system performances and the installation optimization of condenser.Relevant experiments under different working conditions were carried out.An example integrated ORC power system with R245fa as working fluid was designed and established.A single screw expander was integrated in the system to drive the synchronous generator to generate electric power.The effects of cold and heat sources temperatures on ORC system performance were analyzed under different load and expander speed.The heat source temperature mainly affects the irreversible loss of evaporator and condenser.With the increased of heat source temperature,the expander inlet superheating as well as the irreversible loss of evaporator and condenser were increased,but electrical efficiency was decreased.However,due to the increasing pumping power,the system net efficiency was reduced.On the other hand,the cold source temperature mainly affects the irreversible loss of expander and working fluid pump.With the increased of cooling source temperature,the isentropic efficiency of expander and pump is reduced.Thus,the total irreversible loss of the system was increased,yielding the smaller electrical and net efficiencies.The maximum system electrical and net efficiencies were 7.02%and 5.62%,respectively.An adaptive ORC control system was developed.The PI control strategy was used,starting from calculation of the residual frequency between real-time measured frequency and desired frequency.The Industrial Personal Computer(IPC)sends the signal to adjust the pumping frequency to change the pumping flow rate of the organic fluid.Thus,the vapor pressure at the expander inlet was controlled to achieve the desired rotating speed of the expander.The system successfully followed the change of external loads,and self-adapts various thermal-hydraulic parameters to achieve insensitivity of electric energy quality parameters for the step change of heat source and cooling water parameters.The experimental finding shows that the step change of external loads does not influence the electric energy quality,but the steady oscillation of heat source parameters influences the electric energy quality.Due to the fluctuating temperature of heat source,the ORC system operating parameters were fluctuated slightly versus time.The main reason for the instability of the system is the fluctuating of condensing pressure.The effect of working fluid charge ratios on the ORC system performances was examined and the cavitation mechanisms and measures to avoid pump cavitation were analyzed.The ORC system can only successfully operate at working fluid charge ratios from 35%to 50%.For lower charge ratios,the working fluid could not flow back into the liquid tank,while the higher charge ratios led to the larger back pressure of the expander.Both the lower and higher working fluid charge ratios cause faults in the system operating.The best working fluid charge ratio exists,and the maximum system efficiencies can be obtained at the charge ratio of 42.5%.The maximum system thermal,electrical and net efficiencies were 7.74%,7.02%and 5.62%,respectively.When the working fluid charge ratio was less than 42.5%,the liquid level height of tank was relatively small,the pump could not be completely submerged.Owing to the lower boiling point of R245fa,the R245fa liquid in the pump was flashing during the pump piston reciprocating motion,which causes unstable flow and pump cavitation.Thus,the pumping power and the back pressure of expander were increased.The expander isentropic efficiency and the system efficiencies were reduced.When the working fluid charge ratio was higher than 42.5%,liquid flooding occurred in the condenser.Consequently,the condenser effective heat transfer area reduced,yielding the higher back pressure of expander.Meanwhile,the gravity pressure drop caused by the actual liquid height level difference between the liquid tank and the condenser also greatly increased back pressure of expander.The combined effects reduced the expander isentropic efficiency,yielding the larger R245fa mass flow rate.Thus,the pumping power was increased and the ORC system efficiencies were reduced.The operation characteristics and performances of a recuperator ORC system was investigated.Contrast tests were conducted on the ORC system with or without a recuperator integrated.The recuperator can greatly increase the inlet temperature of evaporator and reduce the inlet temperature of condenser,yielding the smaller thermal loads of the evaporator and condenser.Consequently.the irreversible losses in the heat transfer processing were reduced and the system efficiencies were increased.At the heat source temperature of 100℃,the maximum shaft power thermal efficiencies of the ORC system with or without a recuperator integrated were 7.15%and 6.43%,respectively.The heat source temperature had an insignificant influence on the recuperator ORC system performances.With the increasing conduction oil inlet temperature,the evaporator inlet temperature was increased but the condenser inlet temperature was kept constant.Thus,the heat transfer temperature differences in the evaporator and condenser were kept constant,which led to the unchanged thermal loads of evaporator and condenser.However,the irreversible losses in the heat transfer processing were increased.R245fa condensation heat transfer was investigated in a shell-tube condenser at slightly inclined angles.Slightly inclined angles covered a range from-30°(inclined downflow)to 30°(inclined upflow).The condensation heat transfer coefficients reached minimum values at the horizontal position.Both inclined upflow and downflow enhance condensation heat transfer.An optimal inclination angle of-30°existed at which heat transfer coefficients reached maximum.To reach better thermal performance,the shell-tube condenser of ORCs should be operated at the inclination angle of-30°.For inclined upflow,a negative gravity force component acts on the flow direction,which resists the inertia force induced motion,increasing the shear stress on the vapor-liquid interface due to enlarged velocity difference on the interface.This phenomenon triggers the vapor-liquid interface wave to generate unstable interface.Two mechanisms are competed with each other to determine the condensation heat transfer coefficients.First,the increased liquid layer thickness on the tube bottom deteriorates the condensation heat transfer for inclined upflow.Second,the unstable vapor-liquid interface enhances the condensation heat transfer due to the strong mass and momentum exchange across the vapor-liquid interface.The comprehensive effect of the two mechanisms yields the enhanced condensation heat transfer with inclined upflow than horizontal flow.For inclined downflow,the gravity force component on the flow direction is co-current to the inertia force direction,accelerating the liquid layer motion.The decreased liquid layer thickness on the tube bottom accounts for the condensation heat transfer enhancement for inclined downflow compared with horizontal flow.Condensation heat transfer coefficients were correlated based on the Froude number of vapor phase and inclination angles.The correlation well matched the experimental data.