With increasing pressure to reduce the fraction of energy coming from fossil fuels, there is an increased need for research into feasible and sustainable energy sources, such as solar energy. The problem with solar energy is the mismatch between supply and demand, and so the energy needs to be stored. This research paper is a part of the project titled "Tackling the Challenges of a Solar-Community Concept in High Latitudes", and aims in helping to design a thermal energy storage system for southern Finland that is economically feasible and has a high performance. For this purpose, a back-calculation of the underground thermal energy storage (UTES) of the Kerava Solar Village was performed. The primary objective was to calibrate the numerical models to be used in an optimisation by quantifying the thermal properties of the surrounding granite and soil. The UTES of the Kerava Solar Village consisted of a rock pit filled with water and two surrounding rings of boreholes. COMSOL Multiphysics 5.2® was used to create a model in which the temperature of the rock pit was used as the heat source and the heat propagation through the surrounding rock as the output to which the measured data was compared. The best replication of the temperature inside the rock near the surface was achieved with a conductivity of 2.8 and 1.0 W/(m•K) for granite and soil respectively. When looking at the deeper sections, the best fit was obtained for a conductivity of 5.5 and 1.0 W/(m•K) for granite and soil respectively. These results are conflicting, and outside the realistic range for granite. The strange behaviour of the measured isotherms could be explained by a presence of an additional heat source in the ground originating from a leakage of hot water from the tank into surrounding rocks. The critical issue identified in this study was the lack of data. All the parameters of the system, such as the geology, hydrology, and detailed technical drawings, but also the temperature distribution inside the heat source, and heat storage medium need to be known for back-calculation study to be successful.

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