Influence of the diameter of the cylindrical phase change material on the performance of the Earth-Air Heat Exchanger

The earth-air heat exchanger (EAHE) can significantly reduce electricity con- sumption associated with building heating or cooling. However, its thermal performance deteriorates under continuous operation due to thermal satura- tion of the soil near the buried duct. Phase change materials (PCM) have been integrated into EAHE systems to mitigate this issue, leading to im- proved thermal performance. Nevertheless, the influence of PCM geometry on system efficiency remains to be investigated. In this research, a 3D numer- ical model of an EAHE coupled with cylindrical PCM was developed using the climate and soil conditions of Viamão, Brazil, in ANSYS Fluent, a soft- ware based on the finite volume method (FVM). Validation and verification were carried out through comparisons with previous studies under similar cli- matic conditions. Results showed that the annual average thermal potential improves with increasing PCM container diameter, and consequently with the amount of PCM, following a quadratic trend. An enhancement of up to 150.84% and 96.40% was achieved for air heating and cooling, respectively. Although the fan power (W ̇ ) increased exponentially from 0.000512 W to 0.262 W, it remained orders of magnitude smaller than |Q ̇ |. As a result, the net power gain also increases with PCM volume, reaching approximately 174 W in heating and 193 W in cooling for the best case, demonstrating that the additional energy captured through enhanced thermal exchange outweighs the increase in fan power.