The Full Operating Performance Study Of Waste Heat Recovery Systems For Internal Combustion Engine In Distributed Energy System

Distributed energy system is a kind of small-scaled energy conversion utilization system installed near the user and can provide a variety of energy.Therein,the triple supply system CCHP(Combine Cooling,Heating and Power)is one of the most important forms.The waste heat recovery system is a very important part of CCHP,reflecting the cascade utilization of energy in the whole system.Reasonable configuration of waste heat recovery systems can make the primary energy efficiency of CCHP more than 80%.Owing to the frequent fluctuation of the user load in CCHP,in order to take full advantage of waste heat during the entire operation period,this paper conducted a study on the full working condition performance of ORC(Organic Rankine Cycle)and the cogeneration system coupling with ORC as the waste heat recovery system of internal combustion engine.Firstly,the steady-state design model and dynamic simulation model of ORC and absorption refrigeration system are established.At the same time,the influence of dynamic model and static model on the calculation accuracy of exhaust heat source side is studied.Based on the above work,this paper constructed the module library of the main components in thermodynamic systems and designed the user graphical interface for the module,which can be used to conveniently constitute the dynamic models of different thermodynamic systems,including complex cascade systems.The whole working condition model construction platform is the foundation of all research contents in this paper.With the steady-state design model,different forms of ORC(low temperature ORC,high temperature and back pressure ORC,high temperature and condensing type ORC and double-stage ORC)are designed as the waste heat recovery system of a 1000 kW natural gas internal combustion engine.These systems are compared from thermodynamic performance,technical difficulties and economic aspects under the rated working condition of the engine to reveal their advantages and disadvantages in different aspects.Based on this,the steady-state performance of three kinds of cascade systems under rated working condition(electricity-cooling cogeneration,double-stage ORC and double-effect absorption refrigeration system)are compared.The results indicate that the electricity-cooling cogeneration system has the highest performance.Therefore,it is improved to become a three-level system,and the performance is further improved.Based on the steady-state performance analysis,a variety of different ORC dynamic models are established with the customized module library to study the effects of the engine condition change,working fluids selection and system design parameters on the system dynamic response characteristics.The results show that the system state parameters,such as evaporation pressure and superheat become more sensitive to mass flow rate change as the decline of engine working condition.According to these rules,the controller in low working condition is adjusted,showing obvious control performance improvement.The design parameters such as evaporation pressure and condensation pressure do not have much effects on the system dynamic response,so systems with different design parameters can use the same controller.However,different working fluids show significantly different dynamic characteristics.In general,working fluids with high critical temperature tend to have slow response speed.Then,the dynamic model is used to compare the system performance under different engine working conditions,and the factors and general rules affecting the off-design performance are revealed.The results show that it is helpful to improve the off-design performance when the evaporating pressure is designed in the flat region where the output power just varies slightly with evaporating pressure.In addition,the temperature difference between cooling water and condensing temperature is also helpful to improve the system off-design performance.Due to the above reasons,when the engine working condition declines,the relative decrease speed of power and efficiency of double-stage ORC,high-temperature condensing ORC and back-pressure ORC are similar,and they all significantly slower than that of the low-temperature ORC.This also indicates that the system which matches well with heat source not only has good performance under the design condition,but also has nice off-design performance.At last,the dynamic model of the cooing-power cogeneration waste heat recovery system is established,and its off-design performance is compared with the doubleeffect absorption refrigeration system.At the same time,the dynamic model of the whole CCHP system using these two waste heat recovery systems is established separately.Under the condition of real user load and real time,the effects of these two waste heat recovery systems on the overall CCHP efficiency during a whole typical day is compared.The results show that the cooing-power cogeneration system has more potential of energy saving.