Analysis To The Dynamic Performance With Variable Conditions And Simulation To Organic Rankine Cycle

Organic Rankine cycle(ORC)is widely used in medium and low temperature heat recovery.However,most of the heat is fluctuating,such as the solar energy and the waste heat of industrial gas.It can be caused many problems when ORC is driven by variable condition.For example,absorbing too much heat in evaporator when temperature of heat source get higher can lead the working fluid get decomposed because of the higher temperature of working fluid;or the working fluid will be two phase state at evaporator outlet and cause liquid impact in expander because of the insufficient heat absorption when the temperature of heat source changes lower.Therefore,it is necessary to study the dynamic performance of ORC system under the variable condition.A combination of experimental research and simulation study was adopted to analysis the dynamic performance under the variable heat source.The experimental research mainly concentrated on the variation of heat source flowrate and adjustment to pump frequency.Meanwhile,a dynamic simulation library was set on Dymola? including heat exchangers,pumps,and expanders,etc.Based on the library,simulation models of ORC system driven by solar energy and waste heat of internal engine were established to study the system responses.What's more,a PID control strategy is proposed to adjust the system to working normally under the variable condition.The experiment showed that the flowrates of pump and the heat source significantly affected both the evaporation pressure of the system and the temperature of working fluid at evaporator outlet.In addition,the evaporation pressure increased and the superheat decreased with the increase of the pump flowrate.And the rise of flowrate of heat source led evaporation pressure higher.In order to further study the dynamic performances of ORC system under variable working conditions,the simulation has been done and gave some results.(1)For the solar ORC system,the maximum value of output power appeared near 14:00 p.m.,which is due to the small collector efficiency caused by the solar incident angle.Besides,superheat of evaporator can be considered as a resistance to cloud disturbance,and the higher superheat can resistant the longer cloud blockage.(2)For the ORC system driven by waste heat gas of diesel engine,the system with oil storage was more adaptable to the heat source fluctuation because of its greater thermal inertia.Moreover,the PID control strategy was adopted to adjust the rotation speed of pump and expander,so as to reduce the extents of variation for both evaporation pressure and superheat and improve the output power of the system.The results showed that for the solar ORC system,the output power of the system with control was increased by 24% compared to the unused one,and the variation range was also reduced from 6.3bar to 3.8bar.For the ORC system driven by waste heat of diesel engine,the variation range was reduced from 8.69 bar to 1.43 bar after adding the control strategy,and the reduction of superheat to zero was avoided.