Evaluation Of The Ground Thermal Conductivity In Heze,Shandong Province Using DTRT Test

The thermal conductivity of rock and soil is an important parameter for the development and use of shallow geothermal energy.The Thermal Response Test(TRT)is a commonly used method to determine thermal parameters such as the thermal conductivity of underground rock and soil and the thermal resistance of the borehole.In this study,a distributed thermal response test(DTRT)was carried out by using distributed temperature sensing technology(DTS)in Heze,Shandong Province,to determine the thermal conductivity of the ground in this area.A U-shaped and a double U-shaped pipes were installed into ground via borehole.For each pipe,along its outer surface,an optic fiber was fixed to capture ground temperature during processes of heating and cooling,including the initial ground temperature.Based on the captured distributed temperature,the thermal conductivity of each stratum was obtained.By combing the DTS technology and the finite element method,the temperature disturbance range caused by the heat exchanger tube in the test was calculated.Based on the laboratory thermal response model tests,the effective thermal conductivity of four common saturated soils were measured,and the effect of seepage on the effective thermal conductivity of the soil layer was analyzed.In addition,heating and cooling conditions,heating time,types of U-tubes,optical fiber installation techniques were discussed.According to literature review and numerical simulation results,the effects of drilling backfill material and seepage are analyzed.The main results and conclusions are summarized as follows:(1)In order to overcome the shortages of conventional test method in TRT tests,the DTS technology is adopted in this study to achieve continuous borehole temperature profiles.(2)Due to the influence of air convection and water evaporation,the top silty-clay layer with a depth of 0-6.8 m has a larger thermal conductivity during the heating process,however,this phenomenon was not observed during cooling process.The sandy layer in the depth of 39 m-41 m is obviously affected by seepage flow,where the thermal conductivity is higher than that of other strata both in heating and cooling processes.(3)In the DTRT test,the thermal conductivity calculated form the double U-tube is closer to the true value than the single U-shaped tube,so the double U-tube is suggested to be used.(4)The numerical simulation results show that the temperature disturbance radius is 1.2 m.Thus,it is advised to set observation hole at a location with a distance of 0.5-1.2 m away from the heat exchange hole.(5)The thermal conductivity calculated by DTRT is closer to the actual value than using numerical simulation method,because the numerical model could not simulate the actual thermal resistance.(6)In the laboratory multi-layer soil thermal response model test,the DTS technology has obvious advantages because it can collect continuous effective thermal conductivity profile of multi-layered soil.It is found that seepage and soil type have significant impact on soil thermal conductivity.Both the DTS measured data and numerical simulation results indicate that the sand with seepage has the largest thermal conductivity,followed by sand without seepage,silt,clay and organic soil(7)In the DTRT test,both under the heating and cooling conditions,the temperature time history curves measured by DTS has a very good agreement with the average temperature rise of inlet and outlet.(8)Different test time and effective time period affect thermal conductivity calculation.Prolonging the heating time can reduce the thermal conductivity test error.For a general geothermal field test,the heating time of 48 h is sufficient(9)Under the same heating power,the temperature measured by double U-tube is lower than that of single U-shaped tube,because the double U-tube has a larger contact with surrounding strata.In addition,the double U-tube satisfies more hypothesizes of line source than single U-tube.Further,it is found that the temperature value measured by the optical cable installed inside the tube are slightly higher than that measured by the optical fiber cable installed outside wall of tubes,which indicates that the temperature measured by optical fiber on the outer wall is closer to the actual ground temperature.(10)The effect of hole radius and backfill material on the test results can be evaluated by thermal resistance.Literature review and numerical simulations show that the smaller the heat exchange borehole diameter is,the better the heat transfer is.The effect of seepage on the heat convection efficiency of the ground source heat pump is very significant,which has been demonstrated by previous literatures,the laboratory and filed tests.