Modelling of Borehole Solar Energy Storage Concept in High Latitudes

Globally there is an increasing pressure to reduce the greenhouse gas emissions and increase the use of renewable sources of energy. The storage of solar heat is a crucial aspect for implementing the solar energy for heating purposes especially in high latitudes, as it is the case in Finland, with sun insolation high in summer and almost negligible in winter when the domestic heating demand is high. To use the solar heating during winter thermal energy storages are required. In this paper, equations representing the single U-tube heat exchanger are implemented in weak form edge elements in COMSOL Multiphysics to dramatically speed up the calculation process for modelling of a borehole storage layout. Borehole seasonal solar heat storage of 6425 m3 volume with 64 boreholes is successfully simulated. After 10 years of operation the simulated borehole pattern recovers 62.8% of the stored seasonal thermal energy and provides 380 MWh of thermal energy for heating in winter.