Thermal performance of a steady state physical pipe model for simulating district heating grids with variable flow

Modern district heating grids are typically operated using variable flow control. Challenges arise when modeling thermal interactions in variable flow networks as tracking of transport delays is required. In this work, a pipe model is constructed using a steady state heat transfer model combined with a variable transport delay model. The resulting model (i.e. the SS-VTD model) is validated experimentally using measured data from a district energy grid located in Saanich, Canada. Modeling error and computational intensity of the SS-VTD model are assessed numerically by comparing simulation outputs with those obtained using a transient model (i.e. the 1D-PDE model). Matlab®/Simulink® software is used to construct the models and conduct the simulations. Fifteen scenarios are simulated over a range of time step sizes and pipe segment lengths. In all scenarios, decreasing pipe segment length and/or time step size results in greater computational intensity. Varying pipe segment length is found to have a minimal impact on the error of the SS-VTD model, whereas varying time step size is found to have a significant impact. Results show that when compared with the 1D-PDE model at the same fixed time step of 1 s, the computational intensity of the SS-VTD model is approximately 4000 times less.