The Impact Of Boundary Conditions Of Vertical Borehole Ground-coupled Heat Pump On Heat Transfer Efficiency

The Ground Heat Exchanger Technology of Ground—coupled Heat Pump not only retains the advantages of groundwater source heat pump, which consist of energy-saving and protection of the atmospheric environment, but also avoids the highly dependence on large amount of groundwater. What is more, it will not lead to the waste of groundwater resources and the pollution to groundwater resources. So it will become the dominant form of Ground—coupled Heat Pump. The reasonable design of ground heat exchanger, which is based on thermal properties of formation, can improve the heat-transfer efficiency of system and make it operate reliably and economically.Heat transfer principles and heat transfer enhancement are introduced detailedly in this paper, thus we can get a further understanding of heat-transfer process of the ground heat exchanger system, and educe ways to increase the heat transfer efficiency. The concepts of thermal properties and boundary conditions of heat transfer system are firstly proposed. The thermal properties of system means that thermal properties of rock and soil, thermal properties of the filler and the thermal properties of heat exchanger; And specific boundary conditions, include the form of drilling hole, diameter of the hole, the ratio of the filler, heat transfer medium, the type of heat exchanger and material conditions of heat exchanger. The relation between different boundary conditions of heat transfer system and transfer efficiency of system will be analysed and forecasted theoretically. And the most important work is that further verification by tests are also made. Because of, nowadays, there is no experimental study which researching on the influence of the type of drilling hole to the changes of thermal properties of rock and soil. We measured the thermal conductivity of undisturbed samples and squeezed samples by QTM-D2. By comparing the experimental results, it has been shown that after compaction, the thermal conductivity of the hole wall is greater, increase 0.1 W/(m ?℃), achieve to 1/14 to 1/16 of thermal conductivity of undisturbed soil sample, which means drilling the hole by compaction will get higher heat transfer efficiency than conventional rotary drilling, especially for the ground whose bulk density is small and water content is not quite large. This can provide construction and design of ground heat exchanger a valuable reference.