Study On Multi-boreholes Vertical U-tubes Heat Transfer Model

As a new HVAC technology, ground source heat pump (GSHP) has been morewidely used in China with the advantages of protecting environmental, saving energyand improving efficiency. Ground source heat exchanger (GSHE) is the coreequipment of ground source heat pump system, which is generally composed by aplurality of vertical U-tubes. Creating the heat transfer model of vertical U-tube is theprerequisite for designing the size of GSHE. Most of the heat transfer models considersingle borehole as the research object, and the heat transfer model of multipleboreholes is obtained with superposition. When the quantity of boreholes is large, thismethod not only can not achieve satisfactory calculation speed, but also failed inanalyzing the influence of borehole spacing to the efficiency of GSHE. Consideringthe multiple boreholes and the vertical U-tubes as a whole, this article analyzes theheat transfer mechanism of the two parts outside and inside the boreholes. Establishthe Heat transfer model of the two regions above, and has an experimental verification.Besides improving the model calculation speed, this model analyzes the influence ofborehole spacing to the heat transfer process, and provides a theoretical basis forspacing design. The specific research works include:1. Based on finite length heat source, considering the double boreholes and fourboreholes as examples, establish the multi-boreholes heat transfer model outsideboreholes for the whole region in the cylindrical coordinate system, and use the―area accumulation method‖to calculate soil temperature field.2. According to the definition of g-function, gain the general analyticalg-function solutions of the heat transfer model of double boreholes and four boreholes,and improve them with the method of averaging in the depth direction.3. Against the ambiguity of requirements for borehole spacing in―ground sourceheat pump systems engineering technical specifications‖, according to the heattransfer model of double boreholes and four boreholes, study on the spacing designbased on borehole wall representing temperature.4. Considering the fluid depth Infinitesimal as study object, by analyzing the heattransfer process between the fluid depth Infinitesimal and the borehole wall, establishthe heat transfer model inside boreholes of the single U-tube and double U-tubes thesetwo arrangements.5. Using Siemens S7-200PLC as next-bit machine, VB as host computer, develop the soil thermal conductivity testing and temperature acquisition system onthe basis of rock-soil thermal response tester, and gain the effective thermalconductivity of soil.6. Under the existing conditions, use the PDE toolbox and Fluent to simulate theheat transfer process outside and inside boreholes, and compare the result of modelcalculation with the simulate experiment to verify the accuracy of the proposedmodel.