The Studies Of The Model For Vertical U-type Geothermal Heat Exchangers

With the increasing concern over the environmental problems,sustainable policy has been becoming a household word across all over the world. Owing the features of energy saving and environment friendly, ground-source heat pump (GSHP) systems for HVAC have aroused greater and greater interest in China in recent years. GSHP, which has been widely used in developed countries in Europe and American, is an instrument that utilizes the vast geothermal energy stored under the earth for the heating and air conditioning in commercial and residential buildings. GSHP features in its geothermal heat exchanger (GHE), which has been buried underground. Thus, the fundamental task of the research and application of GSHP is to grasp the mechanism of heat transfer and to establish model with high accuracy to describe the heat transfer procedure of GHE from/to its ambient soil.Based on the realities and practical experiences of geothermal heat exchanger, this paper divides the heat transfer process into two spatial sectors with the borehole wall as the division between them, and then makes further studies respectively.The first sector is the solid soil/rock outside the borehole. As for this sector, a finite line-source model has been initiated and its analytical solution has been obtained. The complete discrepancy between the finite line-source model and infinite line-line source model initiated by Kelvin, which has been widely used in sizing GHE, has been deeply discussed. According to this new model the temperature field around a borehole has been analyzed and two different typical borehole wall temperatures have been put forward and completely discussed. Then a convenient method to determinate the typical borehole wall temperature, which yields acceptable error in practical engineering, has been recommended.The other sector is that inside the borehole. As for the conduction inside this sector, based on previous two-dimensional model and considering the circulating fluid temperature response along the depth of the borehole, a quasi-three-dimensional model has been initially developed, which is more close to the case of practical engineering, and its analytical expression has been worked out. Based on this quasi-three-dimensional model, the impact of thermal"short-circuit", or thermal interference, between the U-tube legs on the performance of the ground exchanger can be revealed and has been discussed.