Modelling and control of a district heating system

Abstract

The aim of this study was to investigate whether the supply temperature to a district heating system could be decreased if a dynamic model of the system is used to determine the supply temperature control point, and to what extent the decrease of the supply temperature would improve the electricity efficiency of the connected combined heat and power plant. This was done by a case study of the district heating system of Nyköping and its 58 MW heat and 35 MW electricity CHP plant. The district heating network was approximated as a point load with a variable time delay. Prediction models of the heat load, return temperature and transport time of the system were estimated from operational data of the heat plant. The heat load was modelled with an ARX model using the 24 hour difference of the outdoor temperature as input signal and the 24 hour difference of the load as output signal. A comparison between using a regular control curve and using the dynamic model for controlling the supply temperature indicated that with the same risk of heat deficit, the dynamic control strategy could increase the electricity production with 390 MWh per year, most of it during the winter months. This would correspond to an increased annual income of about 200 000 SEK for the owner.