Validation and conceptualization of thermal zones: A new method of efficiently utilizing borehole thermal energy storage systems

• Experimental and numerical comparison of single-zone and two-zone BTES field. • Zonal operation requires less high-quality heat while maintaining an increased thermal efficiency. • Reduced thermal losses from the sides of the BTES volume with zonal operation. • Exergy efficiency improvements with zonal operation. This study presents the results of an investigation into the thermal losses from borehole thermal energy storage fields. The objective of this work is to explore a novel two zone borehole thermal energy storage field and evaluate the thermal losses and both energy and exergy efficiencies in comparison to the typical single-zone series arrangement. The methodology includes experimental and numerical investigation, with experimental results used to validate three dimensional models of the borehole field. The numerical models are used to further explore the concept of multi-zone borehole fields, quantifying energy and exergy losses through the top, sides, and bottom boundaries. The results showed that operating a two-zone borehole field, with a concentric radial second zone operating at a lower inlet temperature, maintains the quality of the injected heat in the center zone. The two-zone operation also required less high-quality heat while maintaining an increased thermal efficiency. The energy efficiency of the two-zone field increased to 81 % whereas the typical single zone series arrangement borehole field saw an efficiency of 60 %. Advantages were also seen for the two-zone borehole field with an exergy efficiency of 13 %, compared to the 6 % exergy efficiency of the typical borehole field. These implications of these findings highlight the reduction in required high quality heat when charging a zonal borehole field. The results also highlight the need to explore independent operation of concentric zones within borehole thermal energy storage fields with focus on maintaining the storage temperature quality.