Ground Source Heat Pumps (GSHP) and Underground Thermal Energy Storage (UTES) - Key Vectors to a Future Energy Transition

Climate change, exacerbated by the increasing greenhouse gas emissions (GHG), has abruptly altered the life on Earth during the last few decades. Extreme weather, scarcity of fuels and natural resources, proliferating social inequalities and conflicts, are symptoms that the situation is getting out of hand. In this context, our energy systems, still dominated by the utilization of fossil fuels, are responsible for high emissions and air pollution, especially in cities. The decarbonization of heating and cooling networks is a priority, and ground-source heat pumps (GSHP) combined with underground thermal energy storage (UTES) offer an attractive technology to match supply and demand, allowing efficient integration of renewable energy sources and waste heat recycling. This dissertation analyses in the first place the integration of GSHP and aquifer thermal energy storage (ATES) in both district heating and cooling networks, in terms of technoeconomic feasibility, efficiency, and impact on the aquifer. A holistic integration and a mathematical modeling of GSHP operation and energy system management are proposed and demonstrated throughout two case studies in Finland. Hydrogeological and geographic data from different Finnish data sources are retrieved for calibrating and validating the groundwater models, used to simulate the long-term impact of GSHP-ATES operation. Another Finnish case study and large-scale GSHP / borehole thermal energy storage (BTES) application - Aalto New Campus Complex - is also investigated in this research. The specifically developed methodology for management of measured data is considered essential due to its capability to handle data with high uncertainty (thermal meters) by using highly accurate data regarding GSHP power demand. Operational data and relevant GSHP performance indicators are presented and analyzed, and a variety of measures for improving system operation are proposed. Additionally, several methods are developed for modeling the effective thermal resistance of groundwater-filled boreholes, deploying a working algorithm coupled with BTES simulation tool. It is observed that in real operation the effective thermal resistance can vary significantly, concluding that its update is crucial for a reliable long-term simulation of the BTES field. The overall argument of this dissertation is that, even with limited and uncertain data, it is possible to assess the ATES integration for district heating and cooling with reasonable accuracy. By dispatching heating and cooling loads in a single operation, GSHP-ATES integration is technically viable and economically feasible, causing a limited long-term impact on the aquifer. Furthermore, the dissertation also highlights the importance of accurate monitoring and modeling of operating GSHP–BTES energy systems, including detailed modeling of their groundwater-filled boreholes - for efficient, reliable and sustainable long-term operation.