Experimental and numerical research on the thermal performance of a vertical earth-to-air heat exchanger system

The Earth-to-Air Heat Exchanger (EAHE) system is an efficient and clean geothermal application technology that can be used for pre-cooling in summer and heating in winter. This paper proposes a novel Vertical Earth-to-Air Heat Exchanger (VEAHE) system that uses baffles to divide the vertical duct into two ventilation tunnels with a hollow area at the bottom for air circulation. This system occupies a small land area and has a relatively high geothermal energy utilization efficiency. This study evaluates the thermal performance of the system through experimental tests under various operating conditions. Additionally, a numerical model of the system was established to explore the influence of baffles length, thickness, and duct depth on its thermal performance. The experimental results show that the 2.5-meter deep VEAHE system achieves an average air pre-cooling temperature reduction of 5.42 °C, with a maximum temperature reduction of up to 7.58 °C. Below the 1.2-meter mark of the system, the cooling capacity of the descending pipe is 1.52 times that of the ascending pipe. The simulation showed a Maximum Absolute Relative Error (MARE) of 3.15 % compared to the experimental results. As the length and thickness of the baffles, duct length, and soil thermal conductivity increase, the average outlet air temperature gradually decreases, while the system's heat exchange capacity significantly improves, in contrast to the duct diameter. Among the influencing factors, the duct length has the greatest impact on the system. Under the recommended configuration, the system's maximum pre-cooling potential is 915.90 W, with the outlet air temperature ranging from 12.05 °C to 14.79 °C.