Research On Low-Grade Thermal Energy Utilization And Thermodynamic Characteristics Of Cogeneration Systems

Cogeneration based on energy cascade utilization has been recognized as an important energy-saving mode due to the advantages of high comprehensive energy utilization efficiency and low pollution emission around the world.A lot of waste heat is usually discharged directly or indirectly into the atmosphere due to lower energy quality in the cogeneration system,which causes huge amounts of energy loss and serious heat pollution to the atmosphere.The appropriate utilization of low-grade heat source is still an important factor to improve the energy utilization efficiency of the cogeneration.The core problem is the lack of in-depth study on internal thermodynamic characteristics.The thermodynamic characteristics analysis and optimization research of waste heat recovery schemes have been mainly carried out by recovering waste heat of exhausted steam from a steam turbine of a coal-fired cogeneration system and a gas-steam combined cycle based on absorption heat pump?AHP?as well as solar-aided coal-fired power generation system in the dissertation.The thermodynamic performance has been analyzed through modeling,programming and simulating.The primary research work and conclusions obtained are as follows:Firstly,the models of important components of coal-fired power plant have been established according to the first and second thermodynamic law.The effect of generation temperature,condensation pressure,evaporation temperature on the thermal characteristics of AHP has been detailedly studied through modeling and simulating the cycle of AHP in which aqueous lithium bromide?H₂O-Li Br?is used as working medium.The results indicate that the coefficient of performance?COP?of AHP increases with the increasing of the generation temperature,but the circulation ratio of dilute solution constantly decreases.However,the COP decreases and circulation ratio of dilute solution increases with the increase of condensation pressure.Secondly,the optimized scheme in which waste heat from exhausted steam of the steam turbines is recovered by absorption heat pump in coal-fired thermal power plants was simulated focusing on the CZK135-13.24/0.245/535/535 unit.Meanwhile,a comparative method was adopted to discuss the difference between the two heating systems,namely conventional heating system and waste heat recovery system with the AHP.The change laws of the performance and exergy indexes of the system have been revealed before and after modification under different unit loads and heating capacities.In addition,output power increment,gross coal consumption rate decrement,exergy loss decrement were also quantitatively revealed under different unit loads and heating loads as well as the increment of total thermal and exergy efficiency after the modification,which indicated the availability and applicability of the optimized scheme.Thirdly,a new cogeneration system which simultaneously integrates solar energy and absorption heat pump into a coal-fired power plant for waste heat recovery of exhausted steam from a steam turbine was presented based on the research above in the dissertation.First of all,a detailed energy and exergy analysis has been carried out for solar-assisted absorption heat pump scheme in which driving heat source is replaced by solar energy.The following cogeneration systems are considered:conventional cogeneration system?CCS?,cogeneration system with AHP driven by extracted steam?CSAES?and cogeneration system with AHP driven by solar?CSAS?.The thermodynamic performance variation was emphatically discussed after the solar energy was introduced into the system.The result indicates that output power increases by6.48MW,and coal consumption rate reduces by 14.29g/k Wh at 100%THA load.Besides,reduced coal consumption rate and increased power both increase with increasing heating capacity.Then,the comparative analysis has been implemented on different substitution options for each extracted steam of a steam turbine?extracted steam for AHP,heater of heating network,regenerative system except the deaerator?,which obtains the saving potential of each substitution scheme.The double effects of waste heat recovery and renewable energy utilization can be simultaneously achieved in optimization schemes.Finally,a new air-cooled gas-steam combined cycle cogeneration system with absorption heat pump for recovering waste heat from exhausted steam of the steam turbine to achieve double effects of energy and water saving was proposed based on a conventional water-cooled gas-steam combined cycle cogeneration system.First of all,the waste heat recovery scheme was simulated by recovering waste heat of exhausted steam from a steam turbine of a water-cooled gas-steam combined cycle?GSCC?cogeneration based on absorption heat pump.The thermodynamic characteristics variation was analyzed under different heating loads before and after modification.The result indicates that the net generated power,net total thermal efficiency and exergy efficiency obviously increase,and equivalent standard coal consumption reduces.The appropriate parameters of heating extraction steam were analyzed and optimized,and the evaluation criteria of appropriate parameters of heating extraction steam were given.In addition,the thermodynamic characteristics of the four heating modes have been discussed,namely,water-cooled GSCC cogeneration system with direct extraction steam for heating,water-cooled GSCC cogeneration system with AHP system for recovering waste heat,air-cooled GSCC cogeneration system with direct extraction steam for heating and air-cooled GSCC cogeneration system with AHP system for recovering waste heat.The results show that the thermodynamic performance of air-cooled waste heat recovery scheme is better than that of conventional water-cooled direct extraction steam for heating scheme.Therefore,the conventional water-cooled GSCC cogeneration can be modified into air-cooled GSCC cogeneration system with AHP system for recovering waste heat in coal-rich and water-deficient regions,which can not only save a lot of water but also obviously improve the thermodynamic performance of the system.Moreover,the detailed exergy analyses of the four systems have been carried out.The research results obtained can provide theoretical guidance for the upgrade and reconstruction of the combined cycle.