System-theory Approach to the Operation of Heat Exchangers in District Heating Systems

In this study, the systematic analysis, design, and operation of district heating systems (DHS) are investigated. Particular emphasis is placed on the characterization of the heat transfer processes occurring between the primary and secondary circuits in indirect district heating configurations, with special attention given to the modeling of heat substations. The governing heat transfer equations applicable to heat substations are derived, and both the direct (fundamental) and inverse problem formulations are established. Based on the developed models, operational working points are defined in accordance with varying consumer-side heat demands. The fundamental heat balance equations are derived, input and output variables are identified, and solutions for the output variables are obtained, thereby determining the system's operating point under known boundary conditions. The analysis is limited to steady-state operation and steady-state operating points. Furthermore, an optimization framework is proposed, applicable from both the service provider's and the end-user's perspectives. The presented models are inherently adaptable to both viewpoints.