Impact of Flow Direction in Borehole Heat Exchangers on Heat Recovery Efficiency in BTES Systems: A Multi-Year Simulation Study

The European Union’s climate policy promotes waste incineration as an alternative to landfilling. However, the continuous nature of waste generation, combined with seasonal variability in energy demand, leads to a mismatch between heat production and consumption in waste-to-energy systems. Seasonal Borehole Thermal Energy Storage (BTES) systems are one option for mitigating this mismatch. The aim of this study was to develop a theoretical geological model of a BTES-type storage system located in the Carpathian Foredeep and to simulate its multi-year operation using the FEFLOW numerical modeling software. The model represents an initial assessment of system performance under assumed geological and operational conditions and has not yet been validated against field measurements. Two operating scenarios were analyzed. In the 1st scenario, both heat injection and heat extraction from the storage system occurred through Borehole Heat Exchangers (BHEs) located in the center of the storage system. In the 2nd scenario, heat extraction is initiated through heat exchangers located in the outermost zone of the BTES field. The analysis focused on the temperature of the circulating working fluid in the U-tubes and on variations in heat transfer rate during injection and extraction cycles. The energy performance of the system was evaluated for both configurations. The results showed that the reversed-flow operating strategy provided a higher thermal energy recovery ratio and more favorable long-term thermal performance than the center-to-center flow configuration, indicating the potential feasibility of BTES operation under the assumed Carpathian Foredeep conditions.