Nowadays, large scale district heating networks (DHNs) in the Netherlands do not reach their potential as a renewable heat supplier, as the thermal energy is often produced by fossil-fueled installations. By 2030, the share of DHNs in the total thermal energy supply will increase and the thermal energy will mainly be supplied by renewable sources. The transformation toward renewable heat supplied DHNs is accompanied with a decrease in flexibility. Seasonal thermal energy storage (STES) systems are to a certain extend able to bring back flexibility.
The research question has been answered in four steps. First, the seasonal thermal energy storage technique with the highest potential for implementation in a large scale district heating network has been studied. Secondly, the most relevant characteristics of the STES were analyzed and used to create a model in Matlab of the thermal performance. Thirdly, the desired dimensions of the STES and its impact on a DHN were determined. A Matlab model was used to analyze the performance of the STES. This model also determined the optimal strategy to supply the thermal energy to the DHN. Finally, an analysis provided the economic performance of the optimal STES. Throughout the thesis, the district heating network of Utrecht is evaluated as a reference. This research confirms that it is economically and technically feasible to implement STES systems in an existing large scale DHNs. The most suitable STES technique is high temperature aquifer thermal energy storage (HT-ATES), as the investment cost and the required surface area are relatively low. The results show that multiple suitable locations are identified for HT-ATES in the DHN of Utrecht. A decentralized HT-ATES operating in a 'Total Coverage Winter' mode promises to have the best economical performance and can be economically competitive with other heat sources in the DHN. These results are based on the current prospect of the characteristics of the DHN of Utrecht (price of surplus heat, available sources, desired thermal power, supply temperature). The economic performance of the HT-ATES depends strongly on the specification of the DHN. A higher economic performance can be achieved with different characteristics for the DHN. The HT-ATES is expected to have a lower LCOH if either the storage volume is increased, the supply temperature is decreased or the thermal power is optimized with the stored volume. When these adjustments are satisfied the HT-ATES can out perform the current gas peak boilers (14 €/GJ).