Assessing Borehole Thermal Energy Storage Patterns for Renewable Heating Solutions

As the transition to low-carbon energy systems accelerates, the heating sector remains a major challenge due to renewable heat supply and demand that are often mismatched in time. Borehole Thermal Energy Storage (BTES) offers a practical solution for seasonal heat storage, but its performance depends strongly on borehole arrangement, thermal interactions, and charging strategy. This study investigates hybrid BTES field configurations that combine coaxial and U-tube boreholes to improve heat storage effectiveness and spatial temperature distribution. A three-dimensional numerical model was developed in COMSOL Multiphysics to simulate heat injection and subsurface thermal evolution for three hybrid borehole patterns. The model considers realistic soil and fluid properties and applies safe thermal loading limits for each borehole type. Waste heat from the Kaunas CHP plant was used as the representative heat source, and charging durations were determined based on allowable borehole heat injection rates. The results show clear differences among the layouts investigated. The baseline configuration (Pattern A) generates the highest local temperatures near the central coaxial borehole, producing a strong thermal core but steeper gradients across the field. Pattern B broadens heat dispersion but remains more center-focused. Pattern C provides the most uniform thermal distribution, with smoother lateral gradients and more balanced temperature growth at the monitored locations. These findings indicate that hybrid BTES layouts can be tailored to different design priorities, such as maximizing peak temperature or improving spatial uniformity and long-term storage utilization. Overall, the study demonstrates that borehole pattern optimization is a key factor in hybrid BTES performance and provides practical guidance for designing large-scale underground thermal storage systems for renewable heating applications.