Flow Controller Assumptions in Physical Models of Geothermal District Heating Systems

Abstract

Geothermal district heating systems have been modelled in order to evaluate the influence of the outdoor temperature on the system flow. The district heating system was lumped into a single building, and the models were based on knowledge of its thermodynamics. The results were compared with data from the Municipal District Heating Service, Hitaveita The building thermodynamics can not describe the flow controller element, which defines the relation between the indoor temperature and the district heating water flow. The flow controller is a combination of the radiator control system and the behaviour of the residents. Three flow controllers were studied. The ideal controller, giving the correct flow to keep the indoor temperature at the set point, results in a steady state model for the system consumption. The P-controller (proportional) results in a dynamic model with one state, the system flow. The PI-controller (proportional and integrating) results in a dynamic model with two states, the indoor temperature and the system flow. A linear model based on the PI-controller gave the best results of the models tested.