Performance Evaluation And Experimental Study Of Organic Rankine Cycle For Low-grade Waste Heat Recovery

From the perspective of energy consumption structure,global energy consumption is still dominated by primary energy.Although China’s total energy resources are abun-dant,it is highly dependent on coal resources.Most of the energy waste in the form of waste heat.Recovering low-grade waste heat for power generation has a great signifi-cance for improving energy efficiency and solving environmental problems.In the low-temperature heat-electricity conversion technologies,organic Rankine cycle（ORC）has the advantages of simple structure,high recovery rate,high flexibility and low op-erating cost,and is an effective technology for power generation using low-grade waste heat.Based on comprehensive literatures review of the organic Rankine cycles,the thermoeconomic performance of subcritical ORC systems with different heat exchanger structures was compared.The influence of superheat and internal heat exchanger（IHX）on the the thermoeconomic performance of three ORC system structures were studied.The energetic analysis,exergetic analysis,economic analysis and environmental impact analysis（4E analysis）model of supercritical ORC system were established,and a mul-ti-factor evaluation method based on multi-objective was proposed.The original test bench of micro ORC for power generation system was reconstructed,and the steady and dynamic experimental studies were carried out based on the improved experimental system.The main research contents and conclusions are as follows:（1）Plate heat exchanger（P）,shell-and-tube heat exchanger（S）and fin-and-tube heat exchanger（F）were combined to form different evaporator and condenser combina-tions,and four ORC system configurations were established:ORC-PP,ORC-SS,ORC-FP and ORC-FS.The thermoeconomic comparison study,parameter optimization and sensitivity analysis of subcritical ORC systems with different heat exchanger struc-tures were carried out by using electricity production cost（EPC）as evaluation index.The results showed that the EPC of ORC-PP was significantly higher than that of ORC-SS,ORC-FP and ORC-FS,and among them,the ORC-FS showed the best eco-nomic benefit.The dynamic payback period（pp_d）and EPC had the same trend with the evaporation pressure.When heat source temperature being 160°C and using different working fluids,the EPCs of ORC-PP were about 8.0-8.4 years,and these of ORC-SS,ORC-FP and ORC-FS were 5.5-7.0 years.（2）Simple ORC（S-ORC）,superheated ORC（SH-ORC）and superheated ORC with IHX（IHX-ORC）were established.Based on different heat source conditions and working fluids,the effects of superheating and regeneration on the thermoeconomic performance of ORC systems were studied,and a comprehensive index（a）for evaluat-ing the thermoeconomic performance of IHX-ORC was defined.The indicators consid-ered the heat source temperature and load,and the type,critical temperature,latent heat and specific heat capacity of the working fluid.The results showed that with the in-crease of heat source temperature and heating load,there was a critical threshold index that makes the economic benefit of IHX-ORC superior to that of S-ORC.In addition,the critical index was greatly affected by the type parameter（x）of working fluid.Based on that reason,this paper proposes a criterion[a>1.90625+0.4258x]to evaluate the economic feasibility of working fluids in the IHX-ORC system.（3）The influences of working fluid type and heat source conditions on the ther-moeconomic performance of subcritical ORC system were analyzed.For all working fluids,the thermal efficiency of and exergy efficiency of IHX-ORC were 10-17%higher and 5-10%higher than those of S-ORC respectively,when the heat source temperature and heating load increased and the EPC of IHX-ORC was close to or equal to that of S-ORC.For all wet working fluids R161,R152a and cyclopropane,the thermal effi-ciency and exergy efficiency of SH-ORC were always higher than those of S-ORC,but the EPC of IHX-ORC was always lower than that of S-ORC.The thermoeconomic per-formance of the IHX-ORC system using dry working fluids was better than wet work-ing fluid when the heat source temperature and load rised to a higher level.（4）For the 4E analysis and multi-factor evaluation method based on mul-ti-objective,the results showed that R134a has the best thermoeconomic performance among all working fluids,followed by R32,R290,R1270 and R1234yf.When the weight of the environmental impact criterion is 0.8,the feasibility index level（FL）of R290 is the largest（FL=3.54）,followed by R1270（FL=3.33）,R32（FL=2.90）,R134a（FL=2.79）and R1234yf（FL=2.39）.Under the given conditions,the comprehensive performance of R1234yf were the worst compared with other working fluids.（5）The irreversible loss of supercritical ORC and Equivalent CO₂ emissions of components were calculated.The irreversible loss of the system was found in the ex-pander and heater.For all working fluids,the sum of the irreversible losses of the two devices accounted for more than 75%.Equivalent CO₂ emissions（ECE）associated with components were mainly derived from the manufacture and recycling of expander and heater.The main source of ECE for working fluids R134a,R32,R1270 and R290 was the leakage of working fluid,accounting for 68-83%.For R1234yf,the equivalent CO₂emission was equivalent by the working fluid filling and working fluid leakage.（6）The original 300W micro experimental ORC system was improved and exper-imental studies were carried out on the improved experimental system.The results of steady state experiment showed that the rated power of load had a great influence on the outlet pressure of the evaporator,the mass flow rate of working fluid,and the speed of the expander.The adiabatic efficiency of the expander and the growth rate of power output decreased with the increase of rated power of load.The maximum adiabatic effi-ciency of the expander was only 43%,and the maximum power generation was 120W.In the dynamic experiment,when the rated power of load increased to 480W,the system started to destabilize.At that time,the expander inlet pressure increased by 19%,the condenser inlet temperature,the mass flow rate of working fluid,the expander outlet pressure and the expander outlet temperature decreased by 4.6%,30%,20%and 6%,respectively.The changes in the mass flow rate of the working fluid and the inlet and outlet pressure of the expander were most obvious.The experimental system had lim-ited balance ability for disturbance caused by the changes of rated power of load.How-ever,when the disturbance is too large and the system is unstable,the system could not be restored to a stable state by using simple load reduction operation.