Abstract
Integrated thermal energy systems provide heating and cooling for several commercial and residential buildings. These systems usually have both short-term and seasonal thermal energy storages. High cooling demands lead to a big amount of excess heat. This heat could be exported to a district heating system. Low temperature district heating can increase the integration of renewable and waste energy sources and may significantly contribute to the overall efficiency of future energy systems. In this study, the interaction of an integrated thermal energy system in Norway with district heating was investigated. The main parts of the energy system were heat pumps with 1 MW total cooling capacity, solar thermal collectors as well as water tanks and boreholes for thermal energy storage. It was assumed that heat from the solar collector tank could be exported to the district heating supply line, while the condenser heat from the heat pump was considered to be exported to the return line. Dynamic simulations were performed using a Modelica model of the energy system. An important result of the system simulations was the energy balance of the borehole thermal energy storage. Without heat export, the storage was charged more during summer than it was discharged during winter. This imbalance could lead to a ground temperature increase. To ensure feasible long-term operation of the energy system, the average annual ground temperature should remain constant. By exporting heat to the district heating system, borehole heat balance could be achieved and operating costs could be reduced.