| Shallow geothermal energy is one of the renewable energy sources with great potential for development,and its main utilization method is ground source heat pumps.The geotechnical conditions,groundwater conditions and construction conditions greatly affect the operational efficiency of ground source heat pumps.In order to achieve a more scientific development of shallow geothermal energy,it is necessary to investigate the shallow geothermal energy endowment conditions,including geological topography,hydrogeological conditions and soil thermophysical properties.These factors determine the suitability of shallow geothermal energy.Evaluating the suitability of shallow geothermal energy will provide reference values in engineering construction and urban planning.The current research on evaluating shallow geothermal energy mostly used Analytic Hierarchy Process（AHP）,which quantifies expert experience and has been applied in many fields.However,there are errors caused by subjective factors in this method.In addition,the most important parameter when designing a ground source heat pump system is the in-situ thermal conductivity of the soil.This parameter is obtained from the in-situ thermal response test,which is time-consuming and expensive.Moreover,only the in-situ thermal conductivity of the location can be obtained,which makes it more difficult to obtain an accurate regional distribution and causes large errors in the subsequent evaluation of shallow geothermal energy.In order to solve the above problems,the paper reviewed the current status of research on in situ thermal conductivity of soil and evaluation of suitability of shallow geothermal energy.With the Hongdao Economic Zone of Qingdao as an example,the paper conducted a detailed investigation of the test area through geological investigation,hydrogeological investigation,recharge test,soil sample collection and analysis,and in situ thermal response test.To obtain the distribution of thermal conductivity more accurately,this study used an artificial neural network model to predict the distribution of in situ thermal conductivity in the study area.The results were used to calculate and evaluate the suitability class of local shallow geothermal energy.To reasonably and comprehensively evaluate the resource potential of the study area,this study establishes the suitability evaluation systems for groundwater-type ground source heat pump system and borehole-type ground source heat pump system respectively.The combined evaluation method,which integrates subjective and objective evaluation methods,is used to divide the suitability zones in the test area through geo-information system（GIS）.Finally,the suitability level of the two types of ground source heat pumps and the properties of soil are considered comprehensively to calculate the shallow geothermal energy potential,and development and utilization suggestions are made based on the calculation results.The main research results are as follows.（1）The main rock types in the study area are granite,basalt,andesite,sandstone and conglomerate,etc.The conditions of shallow geothermal energy endowment are good.（2）An artificial neural network model is used to predict the in-situ thermal conductivity of soil.The training database contains geotechnical properties and groundwater conditions.The model is proven to be effective for predicting the thermal conductivity in Qingdao.（3）The evaluation system of two types of ground source heat pumps was established based on geological conditions,hydrogeological conditions,thermophysical conditions,hydrochemical conditions,hydrodynamic conditions and construction conditions,with a total of 15 indicators.After evaluation,the groundwater-type ground source heat pump system is not suitable for the whole study area,while the area of suitable area for borehole-type ground source heat pump system is 110.04 km2;the area of medium suitable area is 65.02 km2,and the area of unsuitable area is 88.19 km2 the area of suitable area for b is 110.04 km2.（4）The static storage capacity of shallow geothermal energy in the study area is about 3.80×1014kJ,equivalent to 1.30×103 million tons of standard coal.When considering the land use coefficient,the shallow geothermal energy resource potential of the whole region is medium level.The annual thermal energy after considering the thermal efficiency conversion is 14.92×1012kJ,which translates into a coal saving of5.09×105tons a year.The artificial neural network model proposed in the study can be used to predict the in-situ thermal conductivity of other areas in Qingdao and reduce the cost of thermal response tests;the shallow geothermal energy evaluation system and combined method will provide reference for suitability of shallow geothermal energy in other area. |
