Experiment Study Of Waste Heat Driven ORC System And Simulation Analysis Of Heat Exchanger Equipment

Oganic Rankine Cycle has broad application prospects in low-medium temperature thermal energy（solar energy,geothermal energy,biomass energy and waste heat,etc.）.The exergy loss and cost of the heat exchange equipment dominate in the ORC system,but the current experimental study on ORC system and the study on the off-design operating characteristics of heat exchanger equipment are not deep enough.Accurately obtaining heat transfer,flow and irreversible loss distribution and migration characteristics in different areas of heat exchanger under variable operating conditions is of great significance for deeply understanding the performance of heat exchanger under variable operating conditions and optimizing heat exchanger to match ORC system.Based on logarithmic mean temperature difference method and three-region principle of heat exchanger,this paper establishes simulation model of heat exchanger for pure and zeotropic working fluid in ORC system and validates experimental data of ORC system under different operating conditions.Using MATLAB software,the model by input import parameter of the fluid in the heat exchange equipment,using the dichotomy assumes that the heat exchange equipment working fluid enthalpy value,iterative calculation until the heat exchange area and the actual heat exchange area difference<1%.The model of pure working fluid heat exchange equipment was verified by 18 groups of ORC system experimental data.The results show that 94%of the simulation results of evaporator and condenser heat exchange are within±3%.The model of zeotropic working fluid heat exchanger was verified by the experimental data of 34 groups of ORC system.The results show that the error of 97.1%and 88.2%of the simulation results of evaporator and condenser heat exchange is within±3%.The experimental results show that the simulation model of heat exchanger has good accuracy and reliability.The heat transfer characteristics of ORC system with pure working fluid were studied by experiment and simulation,and the influence of external conditions（temperature difference between heat sink and heat sources 95-125℃）and internal conditions（working fluid flow 0.1-0.16 kg/s）on heat transfer performance of ORC system with orthogonal design was explored.It is found that the heat transfer performance of the heat exchanger can be improved by increasing the mass flow rate of the working fluid.When the mass flow rate of the working fluid is from 0.10 to 0.16 kg/s,U_eq,eva is increased by 151.9%,the area of the evaporation area is increased by 192%,U_eq,con,is increased by 38.2%,and the area of the condensation area is increased by 33%.The increase of heat source temperature is limited to the increase of evaporation temperature,and the increase of cold source temperature will cause the increase of condensation temperature,which will lead to the decline of heat transfer performance of heat exchange equipment.The off-design characteristics of zeotropic working fluid ORC heat exchanger equipment and system were studied by experiment and simulation.The zeotropic working fluid R245fa/R1234ze（E）（mass ratio 0.25/0.75）was selected to study the effects of mass flow（0.1-0.14kg/s）,heat source temperature（100-130℃）and heat sink temperature（10-25℃）on heat transfer characteristics and system performance of heat exchanger.It is found that the increase of mass flow rate of working fluid is beneficial to the increase of evaporation temperature and the decrease of superheat.In the case of mass flow from 0.10 to 0.14 kg/s,the thermal efficiency of the system is increased by 105.3%,U_eq,eva is increased by 74.5%,the evaporation area is increased by 68.7%,U_eq,con is increased by 30%,and the condensation area is increased by 33%.However,the increase of heat source temperature and heat sink temperature is not conducive to the improvement of heat exchange performance of heat exchanger equipment and the thermal efficiency of the system.The differences between the zeotropic working fluid ORC system and the pure working fluid ORC system and the zeotropic working fluid ORC system with different component ratios at different flow rates（0.10-0.16 kg/s）under reference conditions were compared and studied.The results show that the ORC system efficiency and heat exchange performance of heat exchanger equipment increase with the increase of flow rate for all refrigerants,but the ORC system performance of pure working fluid R245fa is the best.The highest system thermal efficiency of zeotropic working fluids R245fa/R1234ze（E）（0.75/0.25）,R245fa/R1234ze（E）（0.5/0.5）and R245fa/R1234ze（E）（0.25/0.75）are 3.58%,4.38%and 2.41%respectively.The output and net work of zeotropic working fluid R245fa/R1234ze（E）（0.5/0.5）exceeds that of pure working fluid R245fa at 0.16 kg/s,but pure working fluid R245fa has the highest system thermal efficiency at all flow rates and the highest thermal efficiency at 0.12 kg/s（4.70%）.Off-design operation condition is an unavoidable problem in practical application of ORC system.Experiments and simulations have studied whether the adaptability of zeotropic working fluid R245fa/R1234ze（E）（0.75/0.25）ORC system under variable operating conditions can be improved by adjusting the expander speed（1200-1650 rpm）.The results show that when the expander speed is 1650-1200 rpm,the saturated liquid temperature of evaporator is increased by 8.8℃,the superheat is also reduced by 7.7℃.The thermal efficiency of the system is increased from 2.59 to 4.12（up to 59%）,U_eq,eva increases by 7.6%,the area of evaporation area increases by 14.1%,U_eq,con increases by 2.4%,and the area of condensation area increases by 5.8%.Therefore,adjusting the speed of expander can effectively improve the adaptability of ORC system under off design conditions.