Analytical investigation on the optimal tube length of Earth-to-Air Heat Exchanger (EAHE)

Abstract

The Earth-to-Air Heat Exchanger (EAHE) is a promising passive technology that utilizes the earth's thermal energy for cooling/heating applications in buildings. The tube length is one of the crucial parameters affecting the EAHE system's thermal performance. Using this type of heat exchanger with an optimal tube length can help save materials, reduce operational costs, installation expenses, and required space. Selecting the optimal tube length is a major challenge for designers. In the present paper, a novel analytical study has been conducted to determine the optimal tube length of EAHE based on the Coefficient Of Performance (COP). The results of the optimal tube length in transient conditions reveal that it is affected by operation time. It is found that after 6 days of operation, thermal saturation in the soil around the tube takes place, and the ultimate optimal length is achieved which is about 37.9 % longer than the initial optimal length. Ignoring the ultimate soil thermal saturation may cause a false reduction in the estimated optimal tube length by about 57.9 % and an overestimation of the thermal performance of EAHE.