District heating networks (DHN) in the Netherlands have the potential to supply sustainable heat but are currently designed to deliver around 80 \% of the heat from a sustainable source. The peak demand is delivered with a gas boiler because of flexibility and costs. With the share of DHNs increasing in the Netherlands and the goal to be CO$_2$ neutral by 2050, a replacement for gas boilers in DHNs has to be found. This research evaluates the potential of seasonable thermal energy storage to supply the peak load in small-size DHNs in the Netherlands.

The research consists of 3 steps. First, the key performance indicators (KPIs) of the implementation of STES in DHNs are determined and the technologies with the most potential per seasonal thermal energy storage (STES) category (sensible heat storage (SHS), latent heat storage (LHS) and thermo-chemical energy storage (TCES)) are determined. Then a Matlab model is developed to determine the volume, storage losses and total efficiency of the three selected technologies. Multiple scenarios are defined to evaluate the technical and financial potential of STES in different situations and configurations. Lastly, the STES are assessed on their potential to be integrated into a DHN to supply peak demand by analyzing investment costs, operating costs, volume, storage losses, and total efficiency. Throughout the research, a gas boiler is used as the reference scenario.

The models developed in the research are validated and can be used in specific case studies to determine the necessary volume of the STES, temperature profile throughout the year in the STES, mass flows and temperature of the supply HTF to the DHN, and the total losses of the STES, using the heat demand of the DHN, the HTF, the PCM, the reactant, the shape of the tank, the insulation thickness an material and the heat supply of the source.
With the outputs of the developed models, the costs can be calculated. The output of the model and the costs can be used to select the optimal STES to be implemented in the DHN of that specific case study.

This research shows that SHS is most feasible to be integrated into a small-size district heating network to deliver winter peak load in the existing built environment regarding volume, CO2 emissions, and costs.\\
A buried tank with water as the storage medium and heat transfer fluid (HTF) is selected as the technology with the most potential within SHS. For LHS, Paraffin is selected as phase change material (PCM) and water as HTF. For TCES, potassium carbonate is selected as the reactant.