Performance Analysis And Multi-objective Optimization Of A Coupled System With Positive And Reverse Cycles Based On Segmented Closed Utilization Of Steam Energy

Energy is the material basis for human survival and development.With the continuous development of economy and society and the improvement of people’s living standards,the demand for energy is growing.A large amount of low-temperature waste heat is often generated in the process of energy utilization.Limited by low temperature,this part of energy is usually released into the environment,resulting in the energy waste.Thus,the efficient utilization of low-temperature waste heat plays an important role in improving energy conversion efficiency.Organic Rankine cycle（ORC）is a way of effective utilization of medium-low temperature heat into power.However,there are many researches on waste heat with variable temperature at present,while few researches on steam containing simultaneously variable temperature and constant temperature heat source.Compared with the variable temperature heat source,the main feature of steam is that the energy release process includes variable temperature heat release（superheated section and supercooled section）and constant temperature heat release section（condensed section）.Therefore,steam energy utilization should not refer to the overall utilization way of variable temperature heat source,and should be considered in accordance with its energy release characteristics.According to the energy release characteristics of steam,the idea of energy segmented closed utilization is proposed,and the new energy utilization method is systematically studied from the perspectives of thermal performance,economy and multi-objective optimization in this thesis.The main research contents and conclusions of this thesis are as follows:Firstly,according to the characteristics of temperature evolution in the process of steam energy release,the idea of segmental utilization of steam energy is put forward,and a dual-stage organic Rankine cycle（DORC）matching the steam energy release characters is constructed,in which a HT-ORC（high-temperature ORC）with binary non-azeotropic mixture as working medium is employed to match with variable temperature exothermic section of steam and a LT-ORC（low-temperature ORC）with pure organic medium as working medium is employed to match with constant temperature exothermic section of steam to improve effective energy input from the energy input source.The mathematical models for energy,exergy,economic and environmental protection analyses of Single ORC（SORC）with traditional overall utilization mode and DORC are established.The established DORC model is verified under theoretical and experimental conditions,respectively,and the minimum error is 0.04%and the maximum error is 3.05%,which verified the correctness of the established model.Secondly,the thermal,economic and environmental performances of DORC are analyzed based on energy segmented utilization,and the influences of key parameters,such as the type of working medium,R141b mass fraction and low-temperature ORC evaporation temperature,on the system performance are revealed.As the mass fraction of R141b is increased,the net output power,thermal efficiency and exergy efficiency of DORC are firstly increased and then decreased.Compared with conventional energy integral utilization,the net output power,thermal efficiency and exergy efficiency are improved by 5.65-27.18k W,0.24%-1.16%and 1.50%-5.06%,respectively,and CO₂ emission is decreased by 231.38-315.18 tons per year in the energy segmented utilization mode.The results show that the improvement of cycle performance is mainly caused by irreversible loss reduction of heat transfer between the steam and organic working fluid at the energy input end.The levelized cost of exergy（LCOE）and dynamic payback period（DPP）of DORC are gradually decreased with the increase of evaporation temperature,while the net present value（NPV）and multiple of investment cost（MOIC）are increased.Thirdly,as the effective energy that can be used to do work in the medium-low temperature heat accounts for a small proportion,most energy can only be discharged to the environment in the form of low temperature waste heat,resulting in a large amount of energy loss.The absorption heat pump（AHP）is employed as the bottom cycle to recover all exhaust steam waste heat of the improved DORC（IDORC）to supply heat,which forms an IDORC-AHP cogeneration system to realize the segmented closed utilization of steam energy.Compared with SORC and IDORC,the thermal efficiency of IDORC-AHP cogeneration system is increased by 83.41%-86.48%and82.56%-84.40%,and exergy efficiency is improved by 7.91%-15.83%and5.11%-8.36%,respectively.The ratio of heat source flow rate between AHP and IDORC is firstly decreased and then increased with the increase of steam saturation temperature,which indicates that there is an optimal steam saturation temperature so that AHP can completely recover the condensate waste heat of IDORC with the lowest heat source flow ratio.Compared with SORC and IDORC,the DPP of IDORC-AHP cogeneration system is shortened by 1.12-2.44 years and 1.97-3.03 years,and the MOIC is increased by 1.22-5.07 and 1.75-5.44,respectively.Finally,aiming at the conflict between economic performance and thermodynamic performance of IDORC-AHP cogeneration system,response surface method is adopted.Exergy efficiency of the system and LCOE of products are taken as objective functions,and steam superheat,steam saturation temperature and heat network return water temperature are taken as influencing factors for design experiments.The results are combined with the non-dominant sorting genetic algorithm with elite strategy and the multi-objective gray wolf algorithm to optimize the cogeneration system.The TOPSIS decision processing pareto optimal solution set obtained by multi-objective optimization,and determine the optimal design parameters of the system.The optimal exergy efficiency and LCOE of multi-objective genetic algorithm are 64.60%and 0.0546$/k Wh respectively.The optimal exergy efficiency and LCOE of multi-objective gray wolf algorithm are 64.60%and 0.0545$/k Wh respectively.Compared with the multi-objective genetic algorithm,the LCOE of the multi-objective gray wolf optimization algorithm is decreased by 0.0001$/k Wh,while exergy efficiency is consistent.The results indicate that the multi-objective gray wolf algorithm is more suitable for the new cogeneration system and has more prominent optimization ability.Finally,aiming at the conflict between economic performance and thermodynamic performance of IDORC-AHP cogeneration system,the response surface method is adopted.The exergy efficiency and LCOE are taken as objective functions,and steam superheat degree,steam saturation temperature and heat network return water temperature are taken as influencing factors for design experiments.The experimental results are combined with the non-dominant sorting genetic algorithm with elite strategy and the multi-objective gray wolf algorithm to optimize the cogeneration system.The TOPSIS decision is adopted to deal with obtained Pareto optimal solution set to determine the optimal design parameters of the system.The optimal exergy efficiency and LCOE obtained by multi-objective genetic algorithm are 64.60%and 0.0546$/k Wh,while the optimal exergy efficiency and LCOE obtained by multi-objective gray wolf algorithm are 64.60%and0.0545$/k Wh,respectively.Compared with the multi-objective genetic algorithm,the LCOE obtained by the multi-objective gray wolf optimization algorithm is decreased by 0.0001$/k Wh,while exergy efficiency is consistent.The results indicate that the multi-objective gray wolf algorithm is more appropriate for the new cogeneration system and has more prominent optimization ability.