Study On The Optimal Buried Depth Of The Vertical Double U-tube Ground Heat Exchanger For A Ground-coupled Heat Pump System

The buried depth of the vertical U-tube ground heat exchanger(GHE) in aground-coupled heat pump(GCHP) system has great influence on the performance andthe construction cost of the GHE system as well as the reliability and the economy ofthe whole heating and cooling system. The paper established the heat transfer model forvertical U-tube GHEs which can set different original temperature and physicalparameters of the soil based on the real GCHP project. Meanwhile, the thermocoupletemperature sensors were pre-buried on the GHEs to monitor the heat exchange effectof the GHEs, and the research on the optimal buried depth of the vertical U-tube GHEswas conducted through the simulation and the experiment.The heat transfer process of the U-tube GHE with the soil was analyzed. Athree-dimensional pipe-group model of vertical double U-tube GHEs for heat transferwas established, the different original temperature and physical parameters of the soilcan be set vertically in this model, which was closers to the real situation. The essentialinput parameters of the model were explained.The experimental platform was established based on the real project, in order toprovide essential parameters for the simulation. The initial temperature distribution ofthe soil was acquired and the inlet and outlet water temperature of the GHE, the sourceside flow rate and the variation of the underground soil temperature were monitored.The acquired initial temperature distribution of the soil, the inlet water temperatureof the GHEs and the source side flow rate were used as the initial conditions andboundary conditions of the simulation. The variation of the soil temperature around theGHEs with the operation time under the heat rejection and heat extraction mode weresimulated respectively and compared with the measured data to validate the reliabilityof the model.The evolution of the water temperature in the GHEs and the heat flux of each pipesection under heat rejection and heat extraction mode were analyzed respectively. Theresults showed that the change of the underground soil properties causes the higher heattransfer rate in the pipe section of0-70meters.Based on the analysis, six of the buried depth schemes that are60meters,70meters,80meters,90meters,100meters and110meters were selected, thecorresponding models were established, the optimal inner water velocity and the heat transfer effect under different inlet water temperature of each buried depth scheme wereanalyzed through the simulation.At last, the research method of optimal buried depth of the vertical U-tube GHEswas introduced. The number of boreholes and the parameters of source side pumps foreach buried depth scheme were chose to calculate the initial investment of the system.The annual operation fee of the system under constant flow rate/variable temperaturedifference and variable flow rate/constant temperature difference condition werecalculated respectively to get the proper operation mode of the system. The optimalburied depth of the vertical U-tube GHEs for the GCHP system is80meters accordingto the comparison among the dynamic annual costs of these selected buried depthschemes.