Coupling various forms of energy, such as electricity, gas, cold/heat and other forms of energy, the integrated energy system has realized the cascade utilization of energy, enhanced the comprehensive utilization of energy and the flexibility of energy supply through the energy conversion and resource allocation between its sub-systems. However, the energy flows in different types of energy systems vary in their transmission characteristics, which hinders the unified modeling and overall analysis of the integrated energy system, so it is difficult to achieve and bring into play the overall coordination of the integrated energy system. In this paper, based on the theory of energy network and the axiomatic energy theory, the electrical equivalent model of the transmission line in thermal network and the fluid network are established. Combined with the energy conversion equipment model, the unified modeling of the multi-energy flow systems is completed. Then, taking the minimum total energy loss of the system as the goal, a regional integrated energy system operation optimization model is established, in which the particle swarm optimization algorithm is used to optimize the internal control variables of the system. Finally, an integrated energy system of an island with 32 thermal nodes is modeled and optimized, which verifies the validity of the proposed model and optimization method.

With the ever-increasing capacity of renewable energy, wind and solar energy curtailment has become prominent, which could limit further development of renewable energy. To solve this problem in distributed networks, the output characteristics of renewable energy and the sequential variation characteristics of load are analyzed. Based on that, a combined heating and power system, which consists of renewable energy, electric boiler, gas boiler and heat exchanger, is presented. As a coefficient is introduced to measure the curtailment of wind and solar energy, the optimal operation model of the combined heating and power system is established with minimizing the system running cost as the objective function, considering energy balance, limits of renewable energy curtailment, output of units and the operation requirement of distributed system. The optimization model based on Matlab calling Cplex solver is applied to case IEEE 33 to obtain the optimal curtailment of renewable energy, electric boiler output, gas consumption and operation cost. And then results are compared with separation supply of heat and power. Results show that in typical winter days combined heating and power system is effective in cutting operation cost by using the wind and solar energy to generate heat, which also promotes the accommodation of renewable energy and contributes to less natural gas consumption.

The heat storage characteristics of buildings have great potential in promoting the absorption of renewable energy and improving the flexibility and economical efficiency of the combined heat and power system. In this paper, a building with double-tube hot water heating system in a distributed combined cooling, heating, and power (CCHP) system is studied. Based on the analysis of the performance of radiators and the heat consumption of buildings, the heat storage model of buildings is established, which determines the room temperature in real time from the temperatures of supplied water and outdoor environment. Sensitivity analysis of several variable parameters is conducted to investigate the influence of minor disturbance of a parameter on indoor temperature. On this basis, this paper further establishes an operational optimization model of a distributed CCHP system considering the heat storage characteristics of buildings, taking the minimum daily operating cost as the objective function, and the indoor temperature limits, the supply and return temperature limits as constraints. The simulation results show that the heat storage characteristics of buildings can transfer the load, charge the heat at low electricity price and discharge at high, effectively improving system's economical efficiency and flexibility.