Practice And Optimization Of Thermal Response Test For Borehole Heat Exchanger

Shallow geothermal energy is a low-temperature and clean renewable geothermal resource hidden in shallow rock and soil and groundwater.It is rich in resources,widely distributed,and stable in temperature.Its development and utilization technology is mature,and it has certain reproducibility and storage.It also has significant benefits such as economic feasibility,energy saving,and beneficial environment.At present,ground-coupled heat pump(GCHP)systems using shallow geothermal energy as a source of cold and heat have been widely used.The design and advancement of such projects are inseparable from the thermal response test(TRT)for the evaluation of the thermal properties of the heat transfer area.With the popularization of GCHP systems and the uncertainty of the terrain climate,TRT may be conducted under extreme geomorphic climate conditions,and the type and size of borehole heat exchanger(BHE)are also uncertain.Complex geological conditions such as groundwater seepage and changes in test conditions such as power outages and variable heating power will affect the results of TRT.These problems need to be encountered in the face of the actual test before they can be analyzed and solved,and then in practice to optimize the thermal response test.and then TRT can be optimized in practice.In order to explore the feasibility of the development of GCHPs in desert areas,construction and testing process of a GCHP project in Shapotou area,Ningxia was followed.The construction and testing process of the project was designed for the unique geological landforms and climatic conditions in the desert area,which can provide a reference for the development of TRT in the desert area.In the cooling condition,the heat transfer rate was 47.65 W/m,and the heat transfer capacity is strong.In the heating condition,the heat transfer rate of the buried tube was 25.67W/m.In practical applications,an auxiliary heat source should be added for heating.According to this conclusion and the monitoring of the temperature difference between day and night in the actual test,a compound energy storage type desert heat pump system is proposed,which provides a solution to resolve the limited utilization of shallow geothermal energy in desert areas.In order to understand the effect of groundwater seepage on the TRT,it's considered to apply the geophysical survey method of groundwater survey to test well,of which the natural electric field frequency selection method is applied to the Gaoxian test well in Sichuan and the high-density electrical method is applied to Anhui Huaining test well.The results suggest that due to the influence of the test site and other factors,the high-density electrical method is not fully applicable.The test accuracy of the natural electric field frequency selection method needs to be improved,and it is necessary to find a test method that is more suitable for matching the TRT site.Subsequently,the results of TRT and groundwater prediction in Gaoxian were modeled by ANSYS,and the groundwater flow velocity in the test area was predicted.It was believed that the depth of the confined aquifer was about 70 m,which matched the local terrain.The groundwater velocity was about 2.5 m/d.The main function of groundwater seepage was to slow down the heat accumulation in the BHE area,but it has little effect on the thermal conductivity of the rock-soil.In the investigation of TRT analysis under power failure,the TRT project in Tongchuan area was relied on.four methods of superposition method,equivalent time method,indirect method and separation method were used to analyze the thermal data.The results show that the thermal conductivity was 1.21 W/(m·K),the volumetric heat capacity was 2413 k J/(m~3·K)and the thermal resistance was 0.41 m~2·K/W.At the same time,variable heating power TRT were explored.Under different heating powers,there were both a stable phase of heat transfer rate and a stable phase of average fluid temperature,and the stable heat transfer rate was approximately linearly related to the average fluid temperature.And in the process of variable power,the heat transfer rate is linearly shifted from the old balance to the new balance.Finally,based on the viewpoint of heat transfer balance,a new TRT method is proposed.Its thermal conductivity is 1.47 W/(m·K).The initial ground temperature is 16.06?,and the predicted volumetric heat capacity is 2122 k J/(m~3·K)or 2910 k J/(m·K),which was close to the obtained test results.Finally,in the TRT project in Huaining,Anhui,four variable heating power conditions of 1500 W,1000 W,500 W and 0 W were designed to verify the variable power TRT theory and method.Through the analysis of the fitting curve,the thermal conductivity obtained was 2.807 W/(m·K),which is approximately the same as the thermal conductivity of 2.837 W/(m·K)in the conventional TRT.This can reflect the application of variable heating power TRT in geotechnical thermophysical properties test was feasible.These subjects study the solution ideas and research methods of TRT in complex geological and geomorphic conditions,power failures,and variable heating power test to achieve the practice and optimization of TRT.Among them,TRT combined with hydrogeological survey and numerical simulation analysis can play a greater role in the analysis of geothermal properties and groundwater status.This also provides a new solution for the determination method of groundwater velocity.Variable heating power TRT also enriches the TRT method and provides more choices and experience for future engineering practice.