In-situ Soil Thermal Response Test And Its Data Processing Methods Studies

Ground source heat pumps make use of geothermal energy in shallow soil as theheat and cold source of air condition system, this technology develops rapidly andwere widely applied in China. However, the analyzing model of rock-soil thermalresponse test given in the criterion at home is very simple. But there are differentmodels used in engineering practice, which may produce different results. Thus, thedesign accuracy and operational performance will be affected. The conclusion obtainedfrom the model suggested in the criterion based on the test data may variate from thereal value. Further studies needs to be carried out.In the paper, a set of ground thermal response test device with electrical heatingequipment was developed in terms of the measurement principle, An in-situ thermalresponse test were conducted on the soil in the place where a practical project will beimplemented. Firstly, the thermal conductivity, the volumetric heat capacity of the soiland the resistance of borehole interior were obtained by curve fitting method of theexperimental data and by double parameter estimation method respectively. theoptimizing parameters of the thermal conductivity and the volumetric specific heat werefound by double parameter estimation method based on the combination of onedimensional borehole interior model and infinite length cylindrical heat source model ofborehole exterior, the combination of two dimensional model of borehole interior andinfinite length line heat source model of borehole exterior, and the combination ofquasi-three dimensional model of borehole interior and finite length line heat sourcemodel of borehole exterior respectively, which make the sum of squared differencesbetween the average temperature of fluid in tube or tube outlet computed based on these models and the test value minimum. In addition, the influence of testing time, backfilledmaterial, the distance between two U type branch tubes and borehole diameter on thedata processing results were analyzed by using the double parameter estimation methodbased on these three models. It is found that abandoning the initial ten hours test data,which may contain fluctuations because of the instability at the initial stage, is good toimprove the stability of analyzed results. And it comes to the conclusion that the thermalresistance of borehole interior of quasi-three dimensional model is higher than thoseobtained from the two dimensional model.Then the test data was analyzed by using numerical simulation method. Thesimulation results of the outlet temperature based on these three models mentionedabove were obtained by the input of soil thermal physical parameters analyzed to theestablished three dimensional models (equals to the actual size). It is proposed that thecombination of quasi-three dimensional models of tube interior and two dimensionalfinite length line heat source model of tube exterior is better than the other twocombinations by comparing the simulation results with the test data.Finally, the influence of the inlet velocity and temperature and the initial soiltemperature on the heat transfer capacity of the buried tube, the buried tube heat transfercapacity in the ground of different parameters was computed, and the heat transferperformance was analyzed by considering the soil temperature distribution. The resultcan be referred for the design of the practical ground source heat pump systems.