Operation optimization of district heating network under typical modes for improving the economic and flexibility performances of integrated energy system

The thermoelectric couple relation of combined heat and power unit and unstable renewable energy limit the flexible operation of integrated energy system, resulting in low renewable energy accommodation. The district heating network within integrated energy system contains massive pipelines, indicating the passive heat storage capacity and flexibility adjustment potential. This paper combines the dynamic performance of district heating network under four typical modes with system optimization to explore the network's heat accumulation and the flexibility improvement on integrated energy system. The convexification method is adopted to solve non-convex models contained in the dynamic models of district heating network by analyzing the characteristics of different control modes. Then, the flexibility index system involving flexibility, economy, safety, and reliability and evaluation method based on the analytic hierarchy process and entropy weighting method are proposed to compare and quantify the flexibility improvement of four typical control modes of district heating network. Finally, the optimization model is implemented into a case study to discuss the progress of network's heat accumulation on system's operational performance. The results show that the flexibility index evaluation system can comprehensively quantify the effectiveness of district heating network in promoting system flexibility. The flexibility score under temperature-flow-control mode is highest, 0.886. And the economy and wind power accommodation are increased by more than 1.38% and 2.57%, respectively.