Modeling district heating pipelines using a hybrid dynamic thermal network approach

A novel numerical method is presented for fast and accurate simulation of the dynamic thermal behaviour of buried pipelines such as those in district heating systems. The model is based on a combination of a conduction response factor method, known as the dynamic thermal network (DTN) method and a one-dimensional discretized heat transfer fluid flow model, the so-called plug flow N-continuously stirred tanks (PFST) model. This combination enables the model to effectively take into account the short timescale dynamic effects of pipelines including longitudinal dispersion of turbulent fluid and its thermal capacity and also transient ground heat transfer. The combined DTN-PFST model is validated by reference to experimental data from both the lab-scale representation of a district heating system and monitoring data from a full-scale operational system. The comparisons between simulation results and experimental data demonstrate a good level of accuracy of the proposed model in predicting the dynamic thermal behaviour of pipelines. The model has also been found to be several orders of magnitude more computationally efficient than corresponding 3D numerical models. Both the accuracy and computational efficiency of the proposed model make it well-suited to the design and analysis of district heating distribution networks. The model is also expected to be well-suited to the modeling of horizontal ground heat exchange pipe systems.