ABSTRACT

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.

1. INTRODUCTION

The problem of the global climate change requires a considerable effort to reduce the greenhouse gas emissions by increasing the use of renewable sources of energy. The public awareness to use alternative energy sources is growing, and market possibilities are emerging. Besides the solar electricity produced by photovoltaic (PV) solar cells, one of the typical applications of renewable energy is the solar heat, where energy from the sun is used to heat up water and space in buildings. Although the price of PV cells is continuing to drop dramatically every year (Kurtz et al., 2017), the solar thermal energy is still considered to be simpler to storage for extended time periods. The seasonal storage is of particular importance in high latitudes, as it is the case in Finland, where solar insolation is highest in the summer when the heating demand is low and lowest in winter when the demand is high. In Tackling the Challenges of a Solar Community Concept in High Latitudes, Academy of Finland (AOF) project, the solutions for seasonal heat storage in high latitudes are sought.

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