Numerical predictions for underground thermal energy storage experiment in the Otaniemi research tunnel

Seasonal storage of solar thermal energy is an attractive way to utilise the underground space to increase the share of renewables and tackle the global challenge of climate change. One of the methods to store the solar energy is the borehole thermal energy storage (BTES), where the thermal energy is stored in the rock mass using borehole heat exchangers. This study presents preliminary results of numerical predictions for an in situ experiment of underground thermal energy storage in the research tunnel under Otaniemi campus. The in-situ experiment site consists of two horizontal boreholes of 5 m length drilled into granitic rock. One borehole is equipped with a single U-tube heat exchanger, and the hot water is circulated through it to heat up the rocks, while the second hole is used for temperature measurement of the rock. The in situ experiment set-up is modelled numerically using finite element method to investigate the influencing factors and predict its long-term thermal performance. The three-dimensional problem is solved with the transient heat conduction equations and the temperature distribution in the subsurface is obtained during 21 days of operation. A parametric study is performed to find the optimal operating conditions. The results of the numerical predictions are used for a detailed plan of the experiment. The simulated results will be later compared to the measured values obtained in the experiment.