Performance comparison of single and double U-tube borehole heat exchangers in ground source heat pump systems
Bibliographic record
Abstract
As global energy demand for heating and cooling continues to rise, largely driven by increasing temperatures and reliance on air conditioning, sustainable alternatives such as ground source heat pump (GSHP) systems are critical to reducing dependence on fossil fuels and mitigating grid instability. This study develops a numerical model to evaluate the thermal performance of single and double U-tube borehole heat exchangers (BHEs), namely sBHE and dBHE, integrated with a heat pump. The model examines the impact of borehole depth, mass flow rate of the circulating fluid, and heating and cooling loads of a building on system efficiency. Three different cases of dBHE consistently outperform the base case of a 75-m sBHE with a total mass flow rate of 0.2 kg/s and a constant building demand of 3.52 kW (1 ton) for heating and cooling. These results are obtained under identical soil, grout, pipe and working fluid thermal properties for all cases, including the base case, as well as identical borehole size, U-tube size and shank spacing.In Case 1, a 75-m dBHE with a 50% reduced mass flow rate (0.1 kg/s in total) improves entrance water temperature (EWT) to the heat pump from -0.13°C to 3.22°C in heating mode and from 30.5°C to 25.7°C in cooling mode, while increasing heating coefficient of performance (COP) by 2.8% and cooling COP by 10.9%. In Case 2, when subjected to higher building loads (4.22 kW (1.2 tons) instead of 3.52 kW), a dBHE can still sustain system efficiency for both heating and cooling modes, while increasing heat extraction by 25.2% and heat rejection by 21.6%. In Case 3, a 60-m dBHE can achieve comparable performance to the 75-m sBHE, with a minor 0.6% increase in heating COP and less than a 2% decrease in cooling COP, suggesting that the depth of a dBHE can be reduced by up to 20% without sacrificing performance. These findings highlight the superior efficiency, adaptability, and cost-saving potential of dBHE for GSHP applications, enabling optimized performance across various design constraints.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.001 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.001 | 0.001 |
| Open science | 0.001 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.002 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".