Towards Open-Source Modelica Models for Steam-Based District Heating Systems

This paper introduces new models of the Modelica Buildings Library for thermo-fluid simulation of steam-based district heating systems in support of design, operation, and energy analysis. Steam represents a prominent and indispensable form of energy, providing energy for 97% of district heating and upwards of 84% for some manufacturing industries in the United States. Our primary contribution is to enable modeling and simulation of complete steam heating districts that was not previously possible at large scales for industry practices. We implemented open-source models using the Modelica standard, with models ranging from base classes through complete systems. In this paper, we present the newly developed models, including their main assumptions and physical relations, and demonstrate their application for complete district heating systems featuring N ∈ [10, 200] number of buildings. Compared to district models with the commonly-adopted IF97 water/steam model and equipment models from the Modelica Standard Library, the new implementation eliminates costly nonlinear systems of equations, significantly improving the scaling rate for large districts from O(N3.5) to O(N1.7). For an annual simulation with 180 buildings, this translates to a computing time reduction from 3.4 hours to 3.6 minutes. These results are critically important for thermo-fluid simulations of large steam heating systems.