Dynamic Thermal Resistance-Capacity Modeling and Thermal Short-Circuit Analysis: A Study on Natural Convection in a Direct-Expansion CO2 Downhole Heat Exchanger

This study addresses the challenge of thermal accumulation and low efficiency in conventional ground heat exchangers for building heating and cooling applications. A novel direct-expansion CO2 borehole heat exchanger (BHE) backfilled with well water is proposed to enhance heat transfer and mitigate soil thermal imbalance. A dynamic thermal resistance-capacity model (TRCM) coupling CO2 phase change with natural convection in well water is developed and validated against full-scale field experiments (135 m depth), with prediction errors below 5% under cooling conditions (MAPE 2.29%, RMSE 2.49%). Quantitative analysis reveals that natural convection in well water enhances overall heat transfer by 14.9% compared to soil-backfilled systems, despite intensifying thermal short-circuiting. Two practical enhancement strategies for building energy efficiency are proposed: (1) adding insulation to the rising pipe, which increases the heat transfer rate by up to 35.1%; and (2) implementing artificial well-water circulation, which achieves up to 50.5% enhancement, with an equivalent coefficient of performance (COP) reaching 52.5 under intermittent operation. The proposed system and the parametric analysis of these strategies offer effective solutions for improving the energy performance of ground-source heat pumps in buildings, contributing to reduced operational energy consumption and enhanced system reliability.