In industrial energy systems, pronounced fluctuations in heat demand create the need for long term thermal energy storage, which enables more efficient use of renewable energy sources and reduces dependence on fossil fuels. Underground thermal energy storage systems represent one of the key solutions, with heat transfer fluids playing an essential role, as their thermodynamic properties determine the rate of heat transfer, operating temperatures, and the magnitude of thermal losses over seasonal time scales. The master’s thesis first presents a theoretical overview of underground thermal energy storage technologies and the properties of selected heat transfer fluids. Based on these theoretical foundations, a numerical model was developed in the TRNSYS simulation environment to analyse the influence of different heat transfer fluids, storage volumes, and solar collector field sizes on system performance. Simulations were carried out for seasonal operating conditions, and key performance indicators were calculated together with an analysis of the charging, discharging, and operating regimes. The results show that the choice of heat transfer fluid significantly affects the heat transfer rate and thermal losses, while the storage volume determines system stability and seasonal efficiency. It was found that an optimal combination of parameters reduces losses and improves the overall energy efficiency of the system in industrial applications.
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