Critical Technologies For Geothermal Exchange System In Loess Area

Ground source heat pump systems are increasingly used in loess areas,because of its environmental protection,energy conservation and wide applicability.How to obtain accurate thermal parameters in loess areas? Which of the heat transfer models should be selected for calculation? How to choose the type of exchanger and backfill materials? How to guarantee the durability of ground source heat pump system? All these questions are urgent to be answered in loess region.So,this paper has carried out research work.Based on the heat transfer theory of loess,the heat transfer model of buried pipe heat exchanger is analyzed.By analyzing the measured data with the theory of line heat source model and cylinder heat source model,the theoretical calculation result of line heat source is closer to the measured value,the foundation for the model selection in loess areas is provided.On the basis of experimental study on thermal conductivity,the correlation between thermal conductivity,moisture content and dry density of loess is analyzed.The relationship between thermal conductivity and moisture content of undisturbed loess and paleosol is obtained.The relationship between thermal conductivity,dry density and moisture content of remolded loess and paleosol is obtained.Laboratory experiments on thermal conductivity of backfill materials in loess areas are carried out.The change rule of thermal conductivity of common backfilling materials with density or moisture content is obtained,and multiple backfill materials with different ratios are proposed.The preferred backfill material is proposed by the thermal parameter test comparison of various backfill materials and field verification.The temperature differences of different geomorphic units are analyzed through different geotechnical temperature measurement.The variable temperature layer thickness and geothermal gradient of each geomorphic unit are given.The estimation formula of rock and soil mass temperature in loess areas is obtained.The applicability of ground source heat pump system in each geomorphic unit is evaluated by analytic hierarchy process.The applicability ranking of the ground source heat pump system for the local units is proposed.Based on field experiments,the comparative analysis of heat transfer performance of heat exchangers with different heat transfer forms,different pipe diameters,different depths and different spacing of heat transfer holes was carried out.The heat transfer performance of double U shape heat exchanger is much better than that of single U shape.In the same form and the same depth of heat exchanger,the heat transfer performance can be improved with the increasing of the pipe diameter.When the depth of heat exchanger reaches a certain value,the heat transfer performance will decrease.The heat transfer of the underground pipe heat exchanger will get better with the increasing of the buried pipe spacing.The suitable form of heat exchanger and the suitable depth and the suitable spacing of heat exchange hole are given.It provides a reliable basis for the design of ground source heat pump heat exchanger in loess areas.Finally,utilizing the aforementioned results,the ground source heat pump system is designed.A complete annual monitoring of temperature and heat transfer effect are carried out.The formation temperature response law and magnitude of ground temperature change have been ascertained.The validities of design parameters such as calculation model,thermal parameters,preferred backfill materials,ground temperature estimation method,heat exchange form and hole spacing are proved.It provides theoretical support for long-term operation of ground source heat pump system in the loess areas.