| The Organic Rankine Cycle(ORC)has great application prospects in utilizing low-value residual heat and renewable energy.However,its unstable heat and cold source causes the system to operate in off-design conditions for a long time and leads to low efficiency.In this respect,it lacks in-depth research and therefore needs to comprehensively study the performance of the organic Rankine cycle and the control and adjustment strategies of the off-design conditions.For the variable working conditions(with variable heating source,variable cooling source),the capacity modulation of ORC with the pure working fluids is done by adjusting the mass flow rate,for ORC with the working-fluid mixtures,the capacity modulation of the variable heat and cold source is matched by changing the mixing ratio of the mixtures in addition to changing the mass flow rate.This paper mainly studies the Organic Rankine cycle performance under variable working conditions.The main work are carried out as follows:(1)Thermodynamic analysis and model of the designed 100 kW ORC system are made,and ORC simulation programs based on Matlab platform are written;(2)Performance of seven pure working fluids R134 a,R152a,R600 a,R245fa,R245 fa,R123,R365 mfc,R601 under variable working conditions is simulated,including heat load,output power and system efficiency,and analysis about the difference of thermodynamic performance of pure working fluids under variable working conditions are made so that provide reference for mixed working fluids simulation;(3)The simulation of the heat load capacity modulation,the output power improvement ability and the thermodynamic efficiency of two typical mixed working fluids is done;(4)Based on the conclusions of(3),simulate the operating characteristics of various mixed working fluids under the same working conditions,and compare and analyze the factors affecting the capacity modulation ability.The main results are as follows:(1)Simulation comparison of the characteristics of pure working fluids on the same working conditions: simulation result under the condition of a heat source temperature of 90 ° C,heat source flow rate of 3 kg /s,cold source temperature of 20 ° C,and cold source flow rate of 2 kg/s as an example.R134a(the fluid with lowest boiling point)has the highest heat load of 121.4 kW and efficiency of 4.57%;R601(the fluid with highest boiling point)has the lowest heat load of 36.3 kW and an efficiency of 0.18%.R152a(the fluid with boiling point only higher than R134a)has the largest output power of 7.3 kW;R601 has the lowest output power of 3.8 kW.Different working fluids ORC have different operating loads.Under the same working conditions,the lower the boiling point of pure working fluid,the greater the heat load,the greater the output work,but the lower the efficiency.Mixing low-boiling,high-load working fluids with high-boiling,low-load working fluids can provide good heat load regulation while maintaining good thermodynamic efficiency of the system.(2)Simulation of the characteristics of pure working fluids under variable conditions: taking R134 a and R601 as examples.For every 1 °C increase in heat source temperature,the heat load increases by 1983 W and 1689 W,respectively,and the output power is increased by 166 W and 256 W.For every 1°C increase in the temperature of the cold source,the heat load is reduced by 955 W and 218 W,respectively.The output power is reduced by 264 W and 89 W.For each additional 1kg/s,the heat load increased by 6694 W and 771 W respectively,and the output power increased by 562 W and 111 W.For every 1°C increase in the cold source flow,the heat load increased by 1254 W and 70 W,respectively,and the output power increased by 373 W and 30 W.The low-boiling and high-load working fluid has a larger range of heat load variation under variable operating conditions.Relatively speaking,the increase of the heat source temperature increases the output power of the high-boiling working fluids,while the cooling source temperature,the heat source flow rate,and the cold source flow rate increase the output work of the low-boiling working fluids more efficiently.(3)Simulation comparison of the characteristics of mixed working fluids on the same working condition: 1)under the conditions of heat source temperature of 100 ?,heat source flow rate of 3 kg/s,cold source temperature of 20 ° C,and cold source flow rate of 2 kg/s,R134a/R123(boiling point difference is 53.89 °C),the system heat load varies from 54.1 kW to 139.1 kW,the heat load adjustment range is 161.4% by adjusting circulating concentration;R600a/R601(boiling point difference is 47.81 °C)system heat load from 44.7 The kW change is 98.2 kW and the adjustment range is 121.5%.The mixed working fluid load regulation capacity of the same type of refrigerant is related to the boiling point difference of the working fluids pair.The larger the boiling point difference,the greater the heat load regulating ability.2)Based on the conclusion of 1),seven mixed working fluids and three groups of mixed working fluids mixed by compositions with similar boiling points but different type were further simulated under the conditions of heat source temperature of 100 ° C(373.15 K),heat source flow rate of 3 kg/s,cold source temperature of 20 ° C,and cold source flow rate of 1.5 kg/s.have found that the heat load adjustment ability of mixed working fluids composed of different types of refrigerants is greater than the heat load adjusting ability of mixed working materials composed of the same type of refrigerants.(4)Simulation of the characteristics of mixed working fluids under variable conditions: taking R134a/R123 as an example.When the heat source temperature is raised from 100 °C to 110 °C,the heat load of the R134a/R123 ORC system changes from 66.4 kW to 159.7 kW,the adjustment range is 140.6%,and the maximum output work is at a concentration ratio of 0.6/0.4,obtained as 10.63kW;when the heat source flow rate becomes 5kg/s,the cyclic concentration heat load changes from 55.8kW to 151.0kW,the variation range reaches 170.5%,and the maximum output power is 9.8kW at the concentration ratio is 0.7/0.3;The source temperature is increased from 20 °C to 35 °C,the heat load is changed from 50.3 kW to 126.1 kW,the variation range is 150.8%,and the maximum output power is 5.3 kW at a concentration ratio of 0.6:0.4;the cold source flow rate is changed to 2.5 kg/ In s,the cyclic concentration thermal load was changed from 54.4 kW to 143.6 kW,the variation range was 164.2%,and the maximum output power was 9.6 kW at a concentration ratio of 0.8:0.2.Increasing the heat source temperature,heat source flow rate,cold source flow rate and lowering the cold source temperature can improve the system heat load,heat load regulation capacity and output power lifting capacity.This paper innovatively studies the ORC variable load operation regulation technology under mixed working conditions,and simulates the load regulation of mixed working fluid ORC system under variable working conditions,quantitatively simulates the thermal load,thermal load variation range,output power and thermodynamic efficiency of the design system under different working conditions.This paper rich the theoretical research of ORC's operation under variable conditions,and play a certain guiding role in practical control and regulation. |
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