| Exploiting and recovering low grade energy can supply precious energy tohuman and be propitious to protecting environment. In a word, it is significant tosustainable development of human society. Organic Rankine cycle (ORC) technologyis advantaged in conversion and usage of low grade energy. Radial flow turbine isfeasible in ORC system with low grade energy. In order to give reference onoptimization design of ORC power generation system with low grade energy andoptimization regulation in varies conditions, this dissertation investigated onoptimization of ORC working fluids and cycle parameters and performance analysison radial flow turbine with paying attention to performance optimization of ORCsystem.Based on90°C low grade energy source with type of hot water, with condensingtemperature of30°C, with specific net power (net power with unit mass flow rate ofhot water,P n et/m water,H) as objective function, using cycle performance analysismethod with specified pinch point temperature difference, optimization of workingfluids and cycle parameters for subcritical ORC, near-critical ORC (with evaporatingpressure or supercritical heating pressure being closed to critical pressure) andtranscritical ORC was executed.33organic fluids were considered. With attention tothat pinch point position in evaporator shifts to the front of bubble point fornear-critical ORC, pinch point position was specified by calculating the twotemperature curves of hot water and working fluid. The results showed that:(1) cycleparameters were of importance to cycle performance and there were optimal cycleparameters for maximum specific net power for each specified cycle and workingfluid;(2) fluid property was of importance to cycle performance and there wasoptimal working fluid for maximum specific net power for specified cycle;(3)selection of cycle was of importance to cycle performance, subcritical ORC gave theleast specific net power (the optimal fluid is HFC32with the maximum value of8.83kJ/kg), transcritical ORC gave the largest specific net power (the optimal fluid isHFC41with the maximum value of9.98kJ/kg) and near-critical ORC gave mediumcycle performance between subcritical ORC and transcritical ORC. Near-critical ORC gave larger specific net power than subcritical ORC and hadlower cycle pressure than transcritical ORC, on the other side fluid property variedstrongly in near-critical region, which may influence the system stability. Withattention to the advantage and disadvantage, performance and stability of near-criticalORC were investigated on. Firstly, for HFC125, HFC143a and FC218which could beused in both subcritical and transcritical ORC, the transition process from subcriticalORC to transcritical ORC was studied on. The results showed that: specific net power,mass flow rate of working fluid and thermal efficiency varies continually fromsubcritical ORC and transcritical ORC; the maximum specific net power appeared insubcritical condition or in transcritical condition, which is specified by fluid (HFC125and FC218gave the maximum values of9.76kJ/kg and9.20kJ/kg in transcriticalcondition while HFC143a gave the maximum value of8.96kJ/kg in subcriticalcondition). Secondly, based on area of evaporator or supercritical heater, stability ofnear-critical ORC system was studied on. The results showed that sensitivity ofevaporator (or supercritical heater) area increased with evaporating (or supercriticalheating) pressure approaching to critical pressure. Therefore, in order to keep systemstable, near-critical ORC was not suggested in system.A radial flow turbine was optimally thermal designed for generation system withlow grade energy. Based on specified geometry dimension of the optimal radial flowturbine, according to one dimension flow theory and fluid expansion law,performance of radial flow turbine in variable conditions and regulation methods forvariable conditions were studied on. The results showed that: variable rotational speedmethod and changeable nozzle method could be used to regulate radial flow turbinewhen back pressure varied; patial admission method and variable nozzle angle couldbe used to regulate radial flow turbine when mass flow rate varied; variable rotationalspeed method and changeable nozzle method could be used to regulate radial flowturbine when inlet pressure varied.Considering the design defect of the radial flow turbine in our research group,several new nozzles were designed and produced with the aim of regulatingconditions of turbine. HFC245fa and HCFC123were selected as working fluid tostudy performance of tapered nozzle turbine and Laval nozzle turbine. The resultsshowed that:(1) there was maximum isentropic efficiency of radial flow turbine,(forexample, with HFC245fa as working fluid, turbine inlet pressure of0.3MPa, turbineinlet temperature of85°C, turbine outlet pressure of0.165MPa, the optimal rotational speed was about5000rpm);(2) tapered nozzle turbine was advantaged with lowturbine inlet pressure while Laval nozzle turbine was advantaged with high turbineinlet pressure; HCFC123gave higher turbine isentropic efficiency than HFC245fa.Based on existing ORC power generation system with low grade energy, powergeneration part based on radial flow turbine was established to study systemperformance. Based on90°C low grade energy, with condensing temperature of30°C,with specific electric power (electric power with unit mass flow rate of hot water,P g e/m water,H) as objective function, optimization of evaporating temperature wasexecuted. The results showed that with tapered nozzle turbine and HFC245fa, optimalevaporating temperature was69.16°C with specific electric power of1.278kJ/kg.Based on90°C low grade energy, with evaporating temperature of75°C, with specificelectric power as objective function, optimization of condensing temperature wasexecuted. The results showed that with Laval nozzle turbine and HFC245fa, optimalcondensing temperature was29.07°C with specific electric power of0.893kJ/kg.HFC245fa gave better cycle performance than HCFC123. Based on analysis ofexperimental data, variation of turbine rotational speed and generator efficiency withload resistance was studied on. The results showed that: turbine rotational speedincreased with load resistance; there was maximum comprehensive efficiency oftransmission and generation with generator rotational speed; generator efficiencydecreased with the increase of load resistance. |
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