Parametric effects of ring-shaped phase change module on the temperature regulation of ultra-low carbon shallow geothermal ventilation system

As one of the most representative geothermal systems, shallow geothermal ventilation (SGV) system has been widely used with the advantages of simple configuration and low operation cost. However, this system is characterized by relatively large temperature fluctuations at the outlet in many application scenarios, which will reduce the thermal comfortable of the delivery outlet air temperature. The existing model development of horizontal buried-pipe SGV system tends to ignore the vertical buried-pipe part of the air inlet and outlet, and only considers the heat transfer model of horizontal buried-pipe, which may affect the accuracy of the developed model to a certain extent. Based on these, this study proposes a SGV system integrating the ring-shaped phase change material (RSPCM) module, and develops the dynamic heat transfer model considering the vertical part of the air inlet and outlet of the system. In this study, firstly, a relevant field experimental testbed is built and the developed dynamic model is verified with good agreement based on the experimental data, which has a maximum relative error of 4.83 %. In addition, the effects of RSPCM and its different parameters on the thermal performance of SGV system are investigated based on the developed model. The results show that RSPCM arranged at the air outlet can regulate the outlet temperature fluctuation of the SGV system and improve the comfort of the air supply temperature to a certain extent. When the inlet air velocity is 1 m/s, the temperature fluctuation at the outlet is 1.73 °C for the system comprising RSPCM, while the corresponding temperature fluctuation at the outlet is 2.10 °C for the system without RSPCM. The effect of thermal conductivity on the PCM's temperature regulation ability is not obvious enough for the proposed system application. Therefore, in practice, by increasing the thermal conductivity of RSPCM is not an optimal and feasible way when considering the economic cost. The RSPCM thickness is increased to a certain level and then this measure will not have much effect to improve the system's thermal performance. For this system structure and operating conditions, a more suitable RSPCM thickness should be about 20 mm, and the RSPCM with a phase-change temperature of 24 °C has a relatively good temperature regulation capability. The RSPCM has a better temperature regulation capability for the system outlet temperature when it is located at the outlet position. This study provid