A New Steam Medium Model for Fast and Accurate Simulation of District Heating Systems

Abstract

In U.S. district heating (DH) systems, steam is the most common heat transport medium. Simulation of large steam DH systems requires a computationally efficient and accurate steam model; however, the commonly adopted IF97 water model is not fast enough for district-scale simulations, and its discontinuous thermodynamic property functions have shown to cause simulation problems and sometimes failure. To address these issues, this work introduces a new steam medium model for heating applications. Further, we avoid the common numerical challenges at the phase change boundary by adopting a novel split-medium approach. We implemented the model in the equation-based Modelica language and evaluated the accuracy and numerical performance across multiple scales: from fundamental thermodynamic properties to complete heating districts of several sizes. The new implementation produces similar accuracy as the IF97 model, but at a speed that is 5.6 – 9.3 times faster for complete DH simulations. Moreover, for the DH system with the IF97 model, the dimension of the nonlinear system of equations increases linearly with the number of buildings, but it stays constant with the new model; this is critically important for large scale system simulations. The new steam medium model is available open-source in the Modelica IBPSA and Buildings Libraries.