The pressing need to mitigate global warming and transition to sustainable energy solutions has accelerated the development of innovative energy systems. This thesis investigates the sizing and design of a photovoltaic thermal (PVT) system integrated with aquifer thermal energy storage (ATES) within a fifth-generation district heating network (5GDHN) for a case study in the Werfgebied district in Hilversum, Netherlands. The study focuses on the configuration, storage distribution, and optimisation of component sizing within the district heating network to minimise overall electrical power usage, thus reducing grid dependency and CO2 emissions. A Python model of the multi-energy carrier system is developed, embedding the physical principles underlying the thermal and electrical properties of the components. The research finds that an optimal configuration for the ATES and PVT combination involves a single ATES well rather than distributed thermal energy storage. The results indicate that the size of the aquifer significantly affects the overall operating temperature and its fluctuations. A larger ATES maintains a stable but relatively colder temperature. Optimal sizing is achieved at the maximum allowed operating temperatures of an ATES in these areas, resulting in the most favorable temperature for maximum COP in the heat pumps. This minimises grid exchange and CO2 emissions. The optimal ATES size is determined to be 380,000 m3, in combination with 800 PVT modules, leading to a total CO2 equivalent emission of 856 tonnes.