Theoretical And Experimental Explorations On Heat Transfer Performance Of Vertical Ground Heat Exchanger In Soils With Groundwater Advection

Ground source heat pump systems(GSHPS)can utilize shallow geothermal energy to provide cooling and heating for building air conditioning systems, which is efficient, energy saving and environmental friendly. The performance of heat transfer between ground heat exchanger(GHE) and the soil surrounding is one of the research focuses in the field of GSHPS related technologies. At present, the heat transfer models of GHE used in practical project designs are mainly based on pure heat conduction theories, but can’t consider the influence of groundwater advection on the heat transfer performance of GHE. This may cause the GHE to be oversized. As a result, the initial and operational costs of the systems improve. Therefore, it is necessary to establish a heat transfer model to accurately assess the heat transfer performance and heat transfer effectiveness, in which the influence of groundwater advection on the heat transfer of GHE can be taken into consideration.Heat transfer analytical models of GHE are widely used in GHE design calculation and heat transfer analysis due to the convenience of the calculations. On account of the fact that the existing GHE analytical model considering the influence of groundwater advection can’t solve the coupled heat transfer problem from the in-tube fluid to the soil with groundwater advection comprehensively, a coupled heat transfer analytical model was established to take the influence of groundwater advection on borehole heat exchanger(BHE) with single and double U-tube buried inside into consideration, with the temperature of the borehole wall coupling the moving finite line heat source model in porous medium outside the borehole with the quasi-three-dimensional steady state heat transfer model inside the borehole. And the solution to the analytical model can be obtained through iterative optimizing calculation making use of the software of Matlab. The reliability of the solution to the model was verified by a set of data obtained an in-situ thermal response test conducted in Sichuan province. Moreover, the heat transfer analytical model of multiple BHEs was established based on the single BHE model in terms of superposition principle. A sand box test bench in reduced scale was designed and constructed based on Darcy’s law. The temperature responses to heat emissions of single and multiple line heat sources in saturated sand and with groundwater advection were experimentally tested. And the analytical model solution was validated by the experimental data once more.Then, the tube outlet water temperature and the soil temperature response to the single borehole heat exchanger heat emissions under the influence of groundwater advection were studied. A new heat transfer evaluation index “heat transfer efficiency of GHE” was proposed to illustrate the extent of the actual heat exchange capacity of GHE approaching its maximum heat exchange capacity. And, the influence of soil physical properties, groundwater advection velocity, inlet water mass rate and inlet water temperature on the heat tranfer performances were quantitatively analyzed in terms of the heat transfer evaluation index of “heat transfer rate per unit borehole depth”. Moreover, the soil temperature response and to a continuous and an intermittent heat emissions of the multiple BHEs in soils with groundwater advection were studied and the soil recovery characteristics were analyzed. And, the thermal interference between the multiple BHEs was analyzed for different layoutsof the boreholes. It is foundthat the groundwater advection and intermittent operation can greatly alleviate thermal accumulation in soils enclosed by multiple BHEs, and slow down the temperature rise of the entire soil. It is suggested that he borehole spacing along the groundwater advection direction be enlarged by somewhat, while the borehole spacing vertical to the advection direction be shortened, or reduce the number of the borehole along the advection direction.In view of different soil properties of geological layers appearing along the vertical GHE depth, and groundwater advection only existing in a part of soil layers, this will affect the heat transfer performance of a GHE. An unsteady heat transfer analytical model was established for stratified soil layers outside a BHE based on the superposition principle, taking the influence of groundwater advection existing in a certain soil layer and soil physical property difference into account. The soil temperature response to the GHE in soils with and without the effect of groundwater advection were calculated by the stratified analytical model established and the results were compared with those from the homogeneous analytical model solutions.It is found that the soil temperature around GHE along axial direction was non-uniform distributed due to the soil stratification characteriatics, and the borehole wall temperature change was different because of the difference of physical properties of different soil layers. The thermal effect distance in the soil layer with groundwater advection increased and the rise in groundwater advection velocity may cause it to enlarge. Therefore, the stratefied model can be used to analyze the influence of groundwater advection on the heat transfer profermance of GHE in stratified soil. And the maximum thermal effect distance of those in different soil layers is suggested to be taken as the borehole spacing in designin the boleholes layout.