Thermal implications of geometric compactness in vertical earth–air heat exchangers: Experimental analysis of localized transient soil thermal accumulation and system thermal performance

This study experimentally investigates the thermal implications of geometric compactness in vertical earth–air heat exchangers (EAHEs) under space-constrained conditions. Two vertical U-type EAHE systems connected in series and differing only in internal pipe spacing (25 and 15 cm) were evaluated under warm–humid climatic conditions. Reducing pipe spacing intensified localized transient soil thermal accumulation near the inlet section, producing soil temperature differences between configurations of 0.7–1.2 °C at 5 cm from the pipe. At 15 cm, these differences decreased to 0–0.3 °C, while the radial attenuation of the soil thermal disturbance within individual configurations ranged from 61.5–88.9%. Heat transfer was spatially non-uniform, with approximately 53–63% of the total inlet-to-outlet temperature reduction occurring within the first well. Statistical analysis of daily mean values confirmed higher local soil temperatures in the compact configuration, whereas paired two one-sided tests demonstrated practical equivalence of outlet air temperatures within a predefined ±0.5 °C margin. The compactness-induced thermal differences were progressively attenuated both radially through the surrounding soil and longitudinally along the airflow path, resulting in comparable global thermal responses despite measurable local effects. This behavior provides experimental evidence of thermal decoupling between localized subsurface thermal dynamics and the outlet response under the evaluated operating conditions. The findings provide experimental evidence of the spatial attenuation mechanisms governing compact vertical EAHEs and contribute to the development of practical design guidelines for applications with limited installation space.