Full-scale Heat Transfer Analysis Of Ground Heat Exchangers With The Effect Of Groundwater Flow

Underground heat transfer is the key to further develop the technology of ground source heat pump,and it is the basis for the optimal operation of the system.In the traditional analysis,the heat transfer procedure around boreholes are separated into two regions.In order to simply the heat transfer analysis,there are specific assumptions are made in these separated regions,which would lead to deviations on the prediction of fluid temperature in the U-tube under different time scales.Although some scholars have proposed some analytical models that are applicable for the entire time scale,the impact of groundwater flow are not considered.Therefore,it is very necessary to develop a full-scale heat transfer model which can include the impact of groundwater flow.Based on the theories of composite medium line-source and the moving line-source,a full time-scale model has been derived by combing the methods of the matched asymptotic expansion and the superposition principle.The proposed full-scale model is able to predict the fluid temperature by considering both impacts of the groundwater flow and the axial heat transfer of the borehole.The model has been validated by the experimental data and the numerical results.By using the full-scale model,the impacts that related to the temperature response of the fluid under different time scales are further investigated.In most engineering projects,the ground-coupled heat exchangers are composed of a group of boreholes,and are normally operated with varied heating or cooling loads.Therefore,by following the superposition principle and the step load hypothesis,the full-scale model is improved to predict the fluid temperature response in a group of boreholes under one-year fluctuated building loads.Due to the fact that the soil thermal property is the key parameter that directly affect the underground heat transfer,the impacts of the meso-structure of soil on the effective thermal conductivity is experimentally studied by the transient measurements.Results indicate that the proposed full-scale model is able to predict the thermal response of the fluid in the U-tube from several minutes to decades,and within short time scale,it improve the temperature deviations that existed in the traditional line-source models,which are normally neglect the transient heat transfer in the borehole.With the existence of groundwater flow,the thermal response of the fluid in the U-tube behaves differently within different time scales.In the short and middle time scale,the impact of the axial heat transfer of the borehole can be neglected,but in the long time scale,it depends on the velocity of the groundwater flow.For a group of boreholes with impact of groundwater flow,it has been found that water flow can reduce the difference of heat transfer performance among boreholes located in different positions.The variations on the boundary conditions cannot be neglected in the long time scale.Finally,it is found that the effective thermal conductivity of porous soil varies with porosity,water content,fractal dimension and size ratio.Both fractal dimension and size ratio would affect the contact resistances among solid particles,and further lead to fluctuations on the values of the effective thermal conductivity.