Research On Multi-energy Optimization Of Integrated Energy System Based On Energy Circuit Theory

With the development of economics,the integrated energy system(IES)has become the focus of academia and industry.The ever-increasing energy consumption will cause problems such as environmental pollution,scarcity of fossil fuels and rising fuel costs in the world.To solve the energy problems and realize the “dual carbon” goals,related research has been carried out on energy systems in many countries.The integrated energy system can improve energy efficiency and promote the utilization of renewable energy,but it contains multiple energy forms,complex operation modes and tightly coupled links,which bring challenges to the modeling and power flow calculation.The multi-energy networks of electricity,gas and heat are the main components of the integrated energy system.The energy circuit theory can derive the coupling matrix and model equation of the multi-energy network.The model equations are applied to the multi-energy optimization and operation of the integrated energy system and it can improve the comprehensive energy efficiency.Therefore,this paper mainly studies the dynamic performance of electric circuit models,thermal circuit models,gaseous circuit models,and multi-energy optimization methods for the integrated energy system.The main contents are as follows.(1)The application of the “smart grid” has been extended to the heating network,and the solution of the thermal circuit model is of great significance to the research of the integrated energy system.The calculation of the thermal model partial differential equation is large and the solution is complex.Not only the heat loss but also the delay of heat transfer must be considered.In this paper,a thermal circuit model is derived for the heating network from an electrical-analog perspective.Aiming at the problem of different thermal circuit models under different conditions,the fractional-order model of the thermal circuit is proposed.The Adomian polynomial decomposition method based on natural transformation is used to solve the model.The heat losses and transfer delays of the heating network are obtained.The results show that the thermal circuit analysis theory can describe the complex dynamic characteristics of the heating network and the calculation procedure is simple,fast and accurate.(2)Natural gas is more and more widely used,and pipeline transportation has become more complex.Due to the friction between the gas and the inner wall of the pipeline,as well as the heat transfer between the gas and the environment,the pressure and flow of gas change dynamically with location and time.The dynamic characteristics of the natural gas network bring great challenges to the modeling of integrated energy systems.According to the energy circuit theory,a simplified model of the natural gas pipeline is established in this paper.Laplace transform is applied to the simplified model and the solution of the partial differential equation is converted into a simple algebraic form.According to the solution of algebraic form,a two-port network model of the natural gas pipeline is proposed by using the concept of transfer matrix in electric circuit theory.The relationship between gas pressure and flow at the two ports can be represented by a transition matrix.There are four distribution parameters such as gaseous circuit capacitance,gaseous circuit inductance,gaseous circuit resistance and gaseous circuit guide in the transition matrix.According to the gaseous circuit two-port model,the solution of the gaseous circuit equation can be obtained quickly.The results show that the solution of the gaseous circuit model can not only describe the dynamic response of pressure and flow but also describe the influence of the inclination angle of the natural gas pipeline.(3)Energy hub(EH)has a large potential for realizing the objective of implementing complementarity coordination and optimization of IES.EH is a device with multiple inputs and multiple outputs.Its internal structure is complex,and the coupling matrix is difficult to describe.As the energy flow becomes more and more complex,models of the mapping relationship between inputs and output are difficult to automatically generate in the computer.To solve this problem,a standardized matrix modeling method based on an energy circuit diagram is proposed.Firstly,the energy hub is simplified into an energy circuit diagram,and then the energy circuit diagram is transformed into a signal flow diagram of the corresponding topology.On this basis,the energy constraint equations are established according to Kirchhoff’s extensive properties law.Based on the proposed EH model,a bi-level multi-energy programming model of electricity,gas and heat is constructed for maximizing profits and minimizing costs.By using Karush-Kuhn-Tucker(KKT)optimal condition,relaxation technique and linearization method,the model is transformed into a mixed-integer linear program(MILP).Finally,the mixed-integer linear programming model is solved in Matlab using the CPLEX solver.The results show that the multi-energy optimization method of the integrated energy system can effectively optimize the allocation of various energy sources and reduce the cost of energy consumption.(4)The microgrid is the end of the integrated energy system and the difficulty of modeling and controlling increases due to its flexible structure and changeable operation mode.Demand response(DR)can increase the flexibility of microgrid control,but the loads are usually difficult to predict,and the system energy management method corresponding to uncertain load demand response should be further researched.Firstly,an automatic modeling method for the microgrid system based on an energy circuit diagram is proposed.In the directed energy circuit diagram,the branches are represented by logic variables and state variables,and the energy constraint equations of the microgrid can be easily generated by inputting the parameters of nodes and branches.To solve the uncertainty problem of random load participating in the demand response,a demand response method based on probability is proposed.Finally,energy constraint equations and demand response methods are applied to a multi-objective energy management system(EMS)for the microgrid.The results show that the multi-objective energy management method including demand response can significantly reduce the start-up times of the diesel generator and the operation costs of the microgrid.The comparative analysis of electricity,heat and gas reveals that three heterogeneous energy sources have similar mathematical model forms.In two cases,the integrated energy system coupled with electricity,gas and heat,and the microgrid energy management method considering demand response,effectively verified the effectiveness of the multi-energy system optimization method.This paper can provide support for promoting the multi-energy optimization management of the integrated energy system.