Several operational optimization problems occur in the current situation related to the district heating system (DHS) of Utrecht. The present control strategy is not optimized for the current operation of the DHS. This leads to energy losses, exergy losses and higher operational cost.
The goal of this research is to find improved control strategies for the DHS of Utrecht. This is obtained by the construction of a model of a DHS that enables the minimization of exergy losses as a method to reduce operational cost. Heat production and distribution from the heat production unit towards the city centre of Utrecht are modelled in Excel. The model includes the following model sub systems: the heat recovery unit in the production plant, the primary grid, the heat transfer station and the secondary grid. Fourdifferent control strategy scenarios are modelled next to the current control strategy, taking different primary heating trend lines into account.
Model results of the current control strategy show that the exergy losses are highest in the heat transfer station compared to the other components. Exergy losses in the pipelines and heat recovery unit are generally lower, but in the same order of magnitude. The exergy losses due to pressure drop are lower than 6% of the total exergy losses. Exergy losses in the pumps are very small, lower than 1% of the total exergy losses.
Decreasing the primary supply temperature improves a control strategy; this leads to smaller overall exergy losses as compared to the current control strategy. Operation of the heat recovery unit improves, operation of the preheater and low-pressure condenser increases and operation of the higher-pressure condenser(s) decreases. Exergy losses in the heat transfer station and in the primary pipelines are smaller as well. The primary mass flow increases, so exergy losses due to pressure drop and pump exergy losses increase as well, however these are still relatively low. Due to a maximum velocity constraint and unexpected weather conditions, the improved control strategy should increase the primary supply temperature during peak moments. Overall exergy losses for an improved control strategy are 7.8% up to 18.5% lower as compared to the current control strategy.