Borehole thermal energy storage (BTES) systems offer a sustainable approach to the storage of thermal energy in the subsurface, facilitating effective heating and cooling for buildings. These systems have been effectively implemented in multiple countries that have adapted the technology by accommodating their distinct geological and climatic conditions to support heating and cooling demands or to reduce dependence on fossil fuels. This study incorporates a real load profile from a multi-building thermal energy network in Vernal City, Utah (USA) and, to the best of our knowledge, is the first to examine the performance of parallel and serial connections of borehole heat exchangers (BHEs) within BTES systems. The analysis evaluates recovery efficiency, heat transfer rate, and net present cost. The BTES model was developed using FEFLOW software and simulated for 10 years. A comparative analysis of the impact of BHE connections demonstrated a consistent and efficient recovery rate of 97%. The parallel connection delivered a spatially uniform heat transfer rate of 10.23 W/m per unit length, with a heat flux of 1,085.19 W/m2 due to consistent flow distribution, supported by high effective thermal resistance of 0.0513 K/(W m) and a low pressure drop of 2.17 kPa. Serial connections exhibited spatially variable heat transfer rate ranging from 8.24 to 12.84 W/m, driven by sequential flow and enhanced local thermal gradients, with a higher peak flux of 1,231.05 W/m2 and significant pressure drop of 48.30 kPa. Annual borehole wall temperature drift reveals critical long-term differences. Parallel connections undergo linear cooling to − 3.0 <InlineEquation ID="IEq1"> <EquationSource Format="TEX">$ ^{\circ }\text {C}$</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mmultiscripts> <mrow /> <mrow /> <mo>∘</mo> </mmultiscripts> <mtext>C</mtext> </mrow> </math></EquationSource> </InlineEquation> by year 9. Serial connections exhibit <InlineEquation ID="IEq2"> <EquationSource Format="TEX">$\sim $</EquationSource> <EquationSource Format="MATHML"><math> <mo>∼</mo> </math></EquationSource> </InlineEquation> 50% less cumulative drift due to enhanced fluid mixing and axial heat redistribution, significantly improving subsurface thermal stability. A 20-year life-cycle cost analysis at a 5% discount rate revealed that parallel systems are more cost-effective, with a net present cost of $576,071 compared to $608,842 for serial systems, due to low pressure drop, reduced pumping power, and simpler piping infrastructure. Although serial connections offer advantages in applications that require large temperature differences, parallel systems provide superior hydraulic efficiency, operational reliability, and economic performance. The negligible impact of the type of connection on recovery efficiency allows engineers to prioritize scalability, installation constraints, and total cost in system design. These findings support parallel BHE connections as the preferred choice for most BTES applications, with serial connections reserved for specialized high-gradient scenarios.

Impact of Borehole Connections on Thermal Recovery and Economic Performance in Borehole Thermal Energy Storage Systems

期刊论文
发表日期：

2026年3月26日

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Michael Osei-Boateng,
Liangping Li,
Matthew Minnick,
Haiyan Zhou,
Zhi Ye

摘要

Borehole thermal energy storage (BTES) systems offer a sustainable approach to the storage of thermal energy in the subsurface, facilitating effective heating and cooling for buildings. These systems have been effectively implemented in multiple countries that have adapted the technology by accommodating their distinct geological and climatic conditions to support heating and cooling demands or to reduce dependence on fossil fuels. This study incorporates a real load profile from a multi-building thermal energy network in Vernal City, Utah (USA) and, to the best of our knowledge, is the first to examine the performance of parallel and serial connections of borehole heat exchangers (BHEs) within BTES systems. The analysis evaluates recovery efficiency, heat transfer rate, and net present cost. The BTES model was developed using FEFLOW software and simulated for 10 years. A comparative analysis of the impact of BHE connections demonstrated a consistent and efficient recovery rate of 97%. The parallel connection delivered a spatially uniform heat transfer rate of 10.23 W/m per unit length, with a heat flux of 1,085.19 W/m² due to consistent flow distribution, supported by high effective thermal resistance of 0.0513 K/(W m) and a low pressure drop of 2.17 kPa. Serial connections exhibited spatially variable heat transfer rate ranging from 8.24 to 12.84 W/m, driven by sequential flow and enhanced local thermal gradients, with a higher peak flux of 1,231.05 W/m² and significant pressure drop of 48.30 kPa. Annual borehole wall temperature drift reveals critical long-term differences. Parallel connections undergo linear cooling to − 3.0 $ ^{\circ }\text {C}$ $^{\circ} C$ by year 9. Serial connections exhibit $\sim $ $\sim$ 50% less cumulative drift due to enhanced fluid mixing and axial heat redistribution, significantly improving subsurface thermal stability. A 20-year life-cycle cost analysis at a 5% discount rate revealed that parallel systems are more cost-effective, with a net present cost of $576,071 compared to $608,842 for serial systems, due to low pressure drop, reduced pumping power, and simpler piping infrastructure. Although serial connections offer advantages in applications that require large temperature differences, parallel systems provide superior hydraulic efficiency, operational reliability, and economic performance. The negligible impact of the type of connection on recovery efficiency allows engineers to prioritize scalability, installation constraints, and total cost in system design. These findings support parallel BHE connections as the preferred choice for most BTES applications, with serial connections reserved for specialized high-gradient scenarios.