Research on the heat transfer characteristics of rock and soil under the effects of vertical double U-type buried-pipe heat exchanger

This study based on a vertical double U-type buried-pipe heat exchanger of ground source heat pump engineering project in Nanning through an in situ test monitored the temperature changes in horizontal and vertical directions of the buried-pipe surrounding rock layer and explored the geotechnical heat transfer characteristics. Our results showed that in the deepest measuring point of the monitored hole, the temperature progressively drops in the unit run-time, which accumulated a drop of 5.2 degrees C in 3 years. However, the temperature did not change during the running of the unit stop. This research also revealed that the change of temperature at other measuring points has a correlation with the operation of the unit. The measuring point temperature decreased during the unit run time, whereas the measuring point temperature increased but is still lower than the initial ground temperature when the unit was not operating. It is also found that the increasing degree of the temperature at each measuring point diminished with the increasing depth of each point. For example, the accumulation of temperature at 5 m deep, 1# hole, amounted to 2.4 degrees C in 2009, whereas at a depth of 15 m, it amounted to 0 degrees C. The research further showed that the temperature of each measuring point dropped every year and that the variation degree of temperature decreased with the increase of the depth of the measuring point, which suggests that the ground source heat pump has a cumulative effect on the ground temperature.