Experimental Investigation On The Heat Exchangers And System Of Organic Rankine Cycle

As the heat to power conversion technology for low-medium heat source,organic Rankine cycle(ORC)has a great prospect in the field of renewable energy utilization(solar energy,geothermal energy and biomass energy)and waste heat recovery.Experimental investigation of ORC plays an important role in verification of theoretical model and popularization and application of ORC power system.In this paper,the operating performance of pure working fluid ORC system under heat source/sink design condition and off-design conditions was studied firstly based on the improving ORC test bench.And then,a series of experiments were carried out to explore the operating characteristics of heat exchanger and the influence of heat transfer area on the performance of ORC system under different working conditions.Finally,the operating characteristic of the ORC system with zeotropic mixture working fluid was analyzed under the design and off-design conditions,and these experiment data was compared with that of the ORC system with pure working fluid.The research focuses on the following three aspects.Firstly,the steady-state operating test was conducted baesd on the improving ORC test bench,aiming at studying heat exchangers and system operating characteristics of ORC with pure working fluid.Under the heat source/sink temperature of 130~oC and 20~oC,respectively,as well as the expander speed of 1500 rpm,the effect of working fluid mass flow rate on critical components and system performance of ORC was investigated experimentally by changing the working fluid mass flow rate(from 0.1 kg/s to 0.16 kg/s).The changing law of heat exchangers,expander,working fluid pump and system operating characteristics with working fluid mass flow rate were obtained from experiment data.Beside,under the off-design conditions(the heat source/sink temperature of 120~oC-140~oC and 15~oC-25~oC,respectively,as well as the working fluid mass flow rate of 0.1 kg/s-0.16 kg/s),the critical components operating characteristic and system performance of ORC with pure working fluid R245fa were investigated,obtaining the changing law of heat exchangers,expander,working fluid pump and system operating performance with different external conditions(heat source/sink temperature)and internal condition(working fluid mass flow rate).Results showed that with the increase of temperature difference between heat source and heat sink,net power output and system thermal efficiency rise.Net work and power output and system thermal efficiency peak at 2.05 k W and 4.67%,respectively when temperature difference between heat source and heat sink reaches to 125~oC.The growth of temperature difference between heat source and heat sink is conducive to improving the net power output and thermal efficiency,while the exergy efficinency will decrease.The results is helpful to guide ORC system with pure working fluid running under the optimal condition,improving the comprehensive operating performance of system during complete period.Secondly,in order to investigate the effect of heat exchange on ORC system performance,6heat exchanger combinations were utilized as the heat exchangers of ststem,studying the operating characteristic of heat exchanger with different area and comparing the operating performance of ORC system with different heat exchanger combinations under the heat source/sink design condition(heat source/sink temperature of 130~oC and 20~oC,respectively)and the heat source/sink off-design condition(heat source/sink temperature of 120~oC-140 ~oC and 15 ~oC-25~oC,respectively).The heat transfer and operating characteristics of heat exchangers with different area and the effect law of heat transfer area on operating performance of ORC system under design and off-design conditions.The results showed that equivalent total heat transfer coefficient decreases with the increase of heat transfer area,while net power output and thermal efficiency of the system changes to 9.44%and 7.91%respectively.The heat transfer area has significant effect on performance of ORC system.A larger heat transfer area will weaken the heat transfer performance of heat exchangers which declines the utilization rate of heat transfer area,although the system net power output and thermal efficiency could also be improved by making the heat transfer area larger..The results indicated the influence law of heat exchanger on ORC system performance,which is of guiding significance for system evaluation,design and selection of heat exchanger.Finally,in order to explore the heat exchanger and system operating characteristic of ORC with zeotropic mixture working fluid,R1234ze(E)/R245fa were used as working fluid.The operating characteristic of the ORC system with zeotropic mixture working fluid was studied under different conditions(the heat source/sink temperature of 100~oC-130~oC and 10~oC-25~oC,respectively,as well as the working fluid mass flow rate of 0.1 kg/s-0.16 kg/s),aiming at finding the changing law of heat exchanger,expander,working fluid pump and system performance with external conditions(heat source/sink temperature)and internal condition(working fluid mass flow rate).Meanwhile,the operating performance was compared between system with R245fa pure working fluid and that with zeotropic mixture working fluid R1234ze(E)/R245fa under design condition.The results showed that it is more suitable for ORC system with zeotropic mixture working fluid to operate under the low temperature of heat source/sink.Compared with the pure working fluid system,the net power output and system thermal efficiency of the ORC system with zeotropic mixture working fluid increased by 13.8%and 9.5%,respectively when the working fluid mass flow rate increased to 0.16 kg/s.Compared with the ORC system with pure working fluid,the ORC system with zeotropic mixture working fluid is more suitable to the conditions of low heat source/sink temperature and high working fluid mass flow rate.The results confirmed the applicable working conditions of the new working fluid R1234ze(E)/R245fa,which is helpful to guide the ORC system with zeotropic mixture working fluid to operate in the optimal state.