| Energy is critical to human survival and social development.In recent years,technological progress,economic development and population growth have led to an exponential increase in global energy consumption.Currently,the energy mix of the world and China is still dominated by fossil energy,which can lead to a fossil energy depletion crisis and global warming caused by CO2 emissions.For this reason,countries around the world have started to implement new energy strategies to gradually shift their energy supply partially or fully to renewable energy sources.Geothermal energy is a non-intermittent,potentially inexhaustible and highly promising renewable energy source,the development of which has attracted widespread attention.The distribution of hydrothermal geothermal energy resources in China has obvious regularity and zonality,and the overall distribution is uneven,and there is a problem of recharge in the process of utilization;the development of dry heat rock resources is limited by the technical means of survey,mining process equipment and other issues,and the investment cost is high;while shallow geothermal energy has the advantages of wide distribution,huge reserves,stable and sustainable,clean and environmentally friendly,and easy to develop and utilize,and the technology is relatively mature.Thermal extraction through buried tube heat exchang system in the way of"taking heat but not water"can achieve both efficient development of resources and effective protection of the environment.The buried pipe heat exchange system is a comprehensive shallow geothermal energy development technology,including resource investigation,site survey,engineering design,drilling and construction,heat pump heat exchange,operation and maintenance,and geological environment monitoring and other aspects.In order to ensure the efficient,economic and sustainable development and utilization of shallow geothermal energy,each link needs to be properly arranged,and reasonably connected and effectively integrated.Therefore,it is of theoretical and practical significance to investigate and monitor shallow geothermal energy and to study the heat transfer characteristics and heating capacity of buried pipes.The cold region refers to the region with the coldest monthly average temperature≤-10℃or the daily average≤5℃for more than 145 days.Jilin Province and Heilongjiang Province in Northeast China belong to this climate zone.In this paper,the research background of developing shallow geothermal energy in cold region by buried pipe heat exchange is used to carry out relevant research by means of theoretical analysis,outdoor tests,indoor experiments,and numerical simulations,etc.The main research contents and findings of the paper are summarized as follows:Firstly,based on site investigation and data collection,the lithology,geological structure,hydrogeology and shallow geothermal conditions of the study area are analyzed and summarized,and the necessity and suitability of adopting geothermal resources for heating in the study area are analyzed from multiple perspectives,and modeling data are provided for the simulation of geothermal pipe heat transfer performance.The investigation and statistical results show that:the study area is located at the eastern edge of the southeast uplift of the Songliao Basin in the cold region,with cold and long winters,high heating demand,and the necessity of geothermal heating;the regional tectonics is active,and the tectonic features are mainly fractures;the stratigraphy is mainly composed of Quaternary loose rock types and Cretaceous bedrock,and the developed lithologies are mainly clay,mudstone,sandstone,etc.;the temperature at 100 m underground ranges from 8.1 to 10.4℃,and the ground temperature gradient ranges from 2.0 to 4.0℃/100 m.The study area has excellent geothermal resource conditions at the four levels of"source,storage,cover and access"due to the coupling effect of geological structure and stratigraphy,and has the suitability of geothermal energy for heating.Secondly,traditional thermal response test and improved combined thermal response test studies were conducted in two study areas in Changchun City,in which distributed temperature measurement fibers and temperature sensors were used to monitor the full depth stratigraphic temperature and inlet and outlet fluid temperatures in real time,and to analyze and discuss the shallow ground temperature field distribution,integrated and stratified thermal conductivity,the effects of test conditions,and the temperature change characteristics during the stratigraphic heating and thermal recovery periods.The results show that the inlet and outlet fluid temperature evolve with time in three stages:a rapid increase stage（0-5 h）,a slow increase stage（5-27 h）and a quasi-thermal equilibrium stage（27-48 h）.Increasing the heating power will increase the temperature difference between the inlet and outlet fluid,but it has less influence on the test results;the test needs a certain time to stabilize,and the test time after reaching quasi-steady state has less influence on the test results.The underground geotechnical body temperature evolves nonlinearly during the heating and thermal recovery of the system,with the same trend as the temperature change of the inlet and outlet fluid,and the temperature recovery time is negatively correlated with the heating power.Considering the test cost and test accuracy,a heating power of 8 k W and a heating test and thermal recovery time of 36 h are recommended for the study area.The comprehensive thermal conductivity of 1.880 W/（m?K）was obtained from the conventional thermal response test,and the improved combined thermal response test obtained the stratified thermal conductivity:1.631 W/（m?K）for the pulverized clay layer,1.888 W/（m?K）for the mudstone layer,1.862 W/（m?K）for the muddy siltstone layer and 2.144 W/（m?K）for the pulverized sandy mudstone layer.Then,the basic physical properties and thermal physical parameters of the geotechnical samples were measured to analyze the causes and influencing factors of the differences between the thermal conductivity of laboratory and field tests.Based on the artificial neural network algorithm,a prediction model was established to predict the field thermal conductivity by the basic physical properties of the samples,and the model was used to supplement the stratified thermal conductivity of two test holes in Changchun and Harbin.The results show that the thermal conductivity of both field and indoor tests show an increasing trend along the depth and are stratified.The main reasons for the small indoor test results are the effects of moisture content and structural changes during sample transport and specimen size.The water content,pore ratio and density have a large influence on the thermal conductivity of the geotechnical body in this study area,and there is no covariance problem.The thermal conductivity is positively correlated with water content,negatively correlated with pore ratio and positively correlated with density.The error of the results of the established prediction model is mainly within±5%,with reliability and accuracy.Using the prediction model to supplement the lack of stratified thermal conductivity of two test holes in Changchun CY01 and Harbin M06,the predicted results converted to the integrated thermal conductivity have minimal relative differences with the field test results.Finally,based on the field geological and test data,numerical models of unitary and group pipe buried pipe heat transfer are established to study the evolution of heat transfer performance of buried pipe system and the temperature response of surrounding geotechnical body under the conditions of surface air temperature fluctuation,groundwater seepage,geotechnical stratification and gradient geothermal temperature.The results show that the heat exchange of buried pipes is positively correlated with the air temperature.When seepage is not considered,the temperature around the single pipe is uniformly distributed;when there is groundwater seepage,the seepage carries the cold volume to migrate and diffuse to form a plume area,which can alleviate the cold accumulation,and the long-term heat exchange of the single pipe is increased by about one time,and the heat exchange increases non-linearly with the increase of seepage rate.The heat transfer performance can be improved by appropriately reducing the temperature and velocity of the injected fluid.Selecting backfill materials with higher thermal conductivity than the ground layer can improve the heat transfer performance by 20%.When there is no groundwater seepage,the heat transfer performance of rectangular and plum-shaped arrangement of group pipes is basically the same,and the temperature of the geotechnical body around and inside the group pipes decreases after the system operation,and the temperature of the internal geotechnical body is lower than that of the surrounding geotechnical body,forming a superimposed plume zone of cold,and the heat transfer performance of the middle position pipe decreases about 11.5%compared with that of the single pipe;When considering the seepage conditions,the temperature field of the group pipe is shifted by the seepage of groundwater,and the cold migration forms a superimposed plume area,and the heat exchange of the group pipe in plum-shaped arrangement is increased by about 21%compared with the rectangular arrangement.The research results of this paper can provide technical support for geothermal survey,field testing and heat exchanger performance prediction in the process of shallow geothermal energy development in cold areas. |
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