A numerical model of a vertical earth-to-air heat exchanger (VEAHE) system was developed for the first time. Compared to conventional EAHE system models, the developed model considered the vertical distribution of soil temperature and thermal conductivity. The model validation against experimental data showed a good agreement. A parametric study was then conducted to investigate the effects of tube parameters, insulation parameters, mass flow rates and soil types. Results indicate that a system with a smaller tube diameter provides more thermal capacity with a fixed air mass flow rate, while a larger tube diameter can enhance the system's COP. An increase in tube depths causes the outlet air temperatures to decrease/increase in summer/winter, with a smaller daily oscillation. Compared to PVC and PE, stainless steel can be recommended as the most appropriate tube material, taking into account both heat exchange and soil pressure. Polyurethane, rubber or rock wool as insulation materials can be used as thermal barriers to shield the undesirable heat gains from the shallow soil with the almost same insulation effects. A thickness of 30 mm and a length of from 4 to 5 m can be recommended as the most appropriate insulation parameters for this proposed system.@en

A vertical earth-to-air heat exchanger system (VEAHE) system was proposed in this study. Compared to conventional EAHE systems, it has smaller land use and higher geothermal energy utilization efficiency. To validate its thermal and economic feasibility, an experimental setup was established to examine the variation of its outlet air temperature, air temperature along the tube and soil temperature. Also, the energy metrics of the proposed system were calculated for its economic assessment. Results showed that the outlet air temperature of this system ranged from 22.4 °C to 24.4 °C in summer and from 16.0 °C to 18.0 °C in winter. A detailed air temperature profile along the whole tube revealed that, the air temperature difference under 5 m in deep soil was more than twice as many as that from the inlet to 5 m. For the soil temperature at different depths, its recovery rate approximately equaled to the corresponding changing rate at the same depth during operation. For the proposed system, its energy payback time was calculated as 8.2 years, and its CO2 emission mitigation potential and total carbon credits were calculated as 7170.42 kg and $203.43 respectively, given an economic lifespan of 20 years. In addition, its monetary payback period was calculated as 17.5 years. The above results demonstrate the feasibility and effectiveness of the proposed system in exploiting deep ground soil as a natural heat sink/source in summer/winter. Meanwhile, the energy metrics analysis indicates that the proposed system is economically viable.@en

Vertical earth-to-air heat exchanger (VEAHE) systems have smaller land occupation and more efficient usage of geothermal energy than conventional earth-to-air heat exchanger (EAHE) systems. However, a large fluctuation in its outlet air temperature subject to ambient air temperature variations may occur, preventing it from meeting the indoor thermal comfort requirements. To address this issue, a novel VEAHE system integrated with annular phase change material (PCM) was proposed. The PCM can reduce the outlet air temperature fluctuation due to its ability to store and release energy. To evaluate its thermal performance, a numerical model was developed and validated by experimental data. The results showed a good agreement between the simulated and monitored data, with a maximum absolute relative error of 1.59% and 1.34% for the outlet air and PCM temperature, respectively. Then the validated model was applied to investigate the effects of PCM and its parameters on the system's thermal performance. Results showed that the annular PCM can decrease the outlet air temperature fluctuation of the system without PCM by 31% and 29% under air velocities of 1 m/s and 2 m/s respectively, enabling more stable outlet air temperatures to be achieved. The effect of PCM thermal conductivity depended heavily on the PCM thickness. Improving the system's thermal performance by simply increasing PCM thermal conductivity may not be an effective method, particularly when the PCM thickness was relatively small. As the PCM thickness exceeded 5 mm, an increase in the PCM thickness had a relatively small effect on the variation of outlet air temperatures. The outlet air temperature range decreased with the increase of PCM lengths, while a downward trend in the fluctuation decrease can be observed. In addition, an economic analysis of the proposed system was conducted, indicating that its static payback period was 20.8 years.@en

In this study, a new vertical earth-to-air heat exchanger (VEAHE) system coupled with tubular phase change material (PCM) components was proposed. The proposed system has smaller occupied areas and higher geothermal energy use efficiency than traditional EAHE systems. Moreover, the tubular PCM component enables the system's air temperature at the outlet to be more stable, thereby enhancing the system's thermal performance. In particular, the tubular components' simple geometry, easy fabrication, convenient assembling-disassembling and low-cost facilitate their usage in practical applications. To explore the system's thermal performance, an experimental set-up was established and its numerical model was developed. Then the model was verified through a comparison between the system's simulated and monitored air temperatures at the outlet. The verification produced acceptable results with the maximum absolute relative error of 1.33%. Based on this model, the influences of the tubular PCM component, tube depth, PCM conductivity and container length on the proposed system's thermal performance were investigated. Results indicated that the tubular PCM components can effectively reduce the VEAHE system's air temperature peak and fluctuation at the outlet, and increase its average cooling capacity. For the proposed system, as the tube depths increase from 12 to 24 m, the air temperature peak and fluctuation at the outlet decrease from 25.74 °C and 3.59 °C to 21.01 °C and 0.62 °C, respectively. Different thermal conductivity of PCM has almost same influences on the system's air temperature at the outlet as the container diameter is 50 mm. Such influences, however, become apparent and require to be considered as the container diameter increases to 150 mm. The air temperature fluctuation at the outlet decreases as the container lengths increase, and 12 m can be considered as an appropriate length for the proposed system. Additionally, the system's static payback period was calculated as 18.03 years.@en