Experimental Assessment and Multi-Regional Climatological Screening of Earth-to-Air Heat Exchanger (EAHE) Thermal Potential

Earth-to-Air Heat Exchangers (EAHEs) use shallow-ground thermal inertia to pre-condition ventilation air. This study combines summer monitoring, segmented NTU modelling, climatological contextualisation, and multi-regional screening for a compact square-helical EAHE in Covilhã, Portugal. Five summer datasets (2025–2026; 30 and 50 m3 h−1) were analysed. Calibration used the two 2025 datasets; in 2026, only the deepest monitored point (E) and the physical outlet were valid. Maximum reductions up to point E were 16.40–22.40 °C, and median daily range attenuation was 89.3–95.8% at point E and 69–72% at the outlet, owing to daytime heat gain in the outlet riser. The 3U model gave hold-out RMSE of 0.94–1.22 °C and within-2025 transfer RMSE of 1.32–1.80 °C; in an exploratory 2026 test (1.09–1.98 °C), it had no skill over the ground-only baseline. With the installed 24 W fan, the sensible COP was 2.3–5.9 at point E and 1.8–2.7 at the outlet, where the net saving was negative; 9–45 refers to a hypothetical circuit-sized fan. Only sensible heat was evaluated; condensation could not be excluded. Across 9094 locations, 95.59% had a positive summer cooling indicator under a +5 °C surface offset (country-balanced: 83.97%; USA: 78.9% of the archive). The indicators support pre-feasibility screening only.