Optimizing The Layout Of Shallow Geothermal Energy Exploitation Areas In Xi’an Using A Coupled Groundwater-thermal Model

Shallow geothermal energy,as a low-carbon,environmentally friendly clean energy source,is widely distributed throughout China.The development and utilization of shallow geothermal energy in the Guanzhong area is of great significance to the realization of the "double carbon" goal of the country and the promotion of green economic and social development.However,many projects using ground source heat pumps to exploit shallow geothermal energy have disrupted the thermal balance of the geothermal field due to insufficient preliminary research,which affects the sustainable use of shallow geothermal energy.Therefore,a three-dimensional groundwater-thermal coupling model is established in this paper using the temperature data of a borehole heat exchanger system in Xi’an.This paper investigated the response characteristics of the underground water-thermal system to the borehole heat exchanger system,and analyzed the effects of different hydrogeological conditions,thermal physical parameters and human disturbance conditions on the ground temperature field in the heat exchange area and the operation efficiency of the system.Finally,the optimized layout scheme of the system is discussed on the basis of the above study.Research shows that:(1)Long-term operation of the heat exchange system under actual conditions will produce "cold buildup".When the aquifer seepage velocity increases from 0.0015 m/d to 0.1 m/d,it can significantly alleviate the cold buildup in the heat exchange zone and promote the cold plume to move downstream.And the increase of seepage velocity will also enhance the exchange efficiency of the borehole heat exchanger.(2)The increase of porosity and volumetric specific heat capacity will also alleviate the cold buildup in the heat transfer zone and reduce the spread of cold plume.Increasing the porosity from 0.2 to 0.5 will slightly reduce the exchange efficiency of borehole heat exchanger,while increasing the volumetric specific heat capacity can improve the efficiency.Thermal conductivity increases from 1.5 to 2.0 W/(m·K)process,it will intensify the cold accumulation and promote the cold plume outward movement,but at the same time,it can also enhance the efficiency of the borehole heat exchanger.(3)The presence of pumping well downstream of the heat exchange zone can effectively reduce the drop of ground temperature in the zone,inhibit the movement of cold plume downstream and improve the heat exchange efficiency,but the opposite effect is achieved by the recharge well.Comparing the single and multiple pumping and recharge well combination scheme,it can be found that the scheme of deploying multiple pumping and recharge wells downstream of the heat exchange zone can not only alleviate the cold buildup but also inhibit the movement of the cold plume downstream.(4)It can effectively relieve the cold buildup under the original working conditions when the operation period and operating power of the heat exchange system are changed so that the heat exchange in winter and summer is the same.Within the available construction site,increasing the spacing between the geothermal heat exchange boreholes can also alleviate the problem of cold and heat accumulation.Cross-arranging the cooling holes with the heating holes has the same effect,but does not prevent the cold plume on the outside from being moved downstream.The results of the study can provide an important reference for solutions to mitigate the accumulation of ground cooling and heat in the process of developing shallow geothermal energy using borehole heat exchangers,and to suppress the downstream ground temperature disturbance by the heat exchange system.