A Study Of Influencing Factors Of Heat Exchange Capacity Of Vertical Ground Heat Exchanger

On the basis of consulting a large number of data and combining with the exploration of shallow geothermal energy in different regions of the country,this paper studies the influencing factors of the heat exchange capacity of a vertical ground heat exchanger for engineering practice.The influencing factors can be divided into three categories: geological factors,engineering factors and operation factors,including the initial average ground temperature of rock-soil,geological structure,thermal properties of rock-soil,hydrogeological conditions,depth of heat source wells,types of heat exchangers,backfilling materials and flow rate of heat exchange medium.It is found that the temperature difference between the initial average ground temperature of rock-soil and the heat exchange medium is positively correlated with the heat exchange capacity of the heat exchanger.The average ground temperature of rock-soil is linearly related to the average atmospheric temperature,and the heat exchange efficiency of vertical buried pipes can be qualitatively judged by the average atmospheric temperature,which is directly proportional to the two.The thermal properties of rocks are generally larger than those of loose accumulation layers.In rock formations,the comprehensive thermal conductivity of the vertical ground heat exchanger is positively correlated with the thermal diffusivity and: sedimentary rocks(sandstone and limestone)> metamorphic rocks(Shi Ying sandstone)> igneous rocks(basalt and andesite).Under the constraint of porosity,the water content is positively correlated with the heat exchange capacity of the vertical ground heat exchanger.The influence of groundwater seepage on the vertical ground heat exchanger is difficult to quantify at present,but can be used as an important parameter to assist in judging the suitability of a ground heat pump in developing and utilizing shallow geothermal energy.Rock-soil thermophysical properties are the key factors in the design of the ground heat exchangers.The thermophysical properties such as thermal conductivity,specific heat capacity and thermal diffusivity only objectively reflect the heat transfer or storage capacity of rock-soil,and they do not directly affect the heat exchange efficiency of heat exchangers,but reflect the heat exchange capacity of heat exchangers.The heat exchange capacity of the heat source well increases with the increasing depth,but the unit heat exchange efficiency will decrease and the cost will also increase,so the selection of the depth of the heat source should be comprehensively determined in a combination with the project situation.The depth of 120 m of a vertical buried pipe in the Tianjin area can not only obtain a larger unit heat exchange efficiency,but also a larger single-well heat exchange amount.The heat extraction rate of the double U heat exchanger with the same depth is 9.91%-37.75% higher than that of the single U,and the heat removal rate is 0.9%-35.6% higher than that of the single U.The selection of single or double U for actual projects should be determined comprehensively in a combination with such factors as project cost and construction site.In the area dominated by unconsolidated accumulation strata from a depth of 200 m to shallow,the 1:1:3 ratio of bentonite,cement and sand is a better choice for backfill,which can obtain the thermal conductivity of about 1.37W/m? and also has good stability and permeability.In addition,the heat source wells backfilled with virgin pulp and medium coarse sand can also obtain better heat exchange efficiency,and the material cost and construction cost are lower.The heat exchange capacity of a double U vertical ground heat exchanger is tested under the flow rates of 0.31,0.42,0.52,0.63 and 0.79 m/s.The temperature difference between the inlet and outlet of the heat exchanger is negatively related to the velocity of the medium.Under heat removal conditions,the heat exchange capacity of the vertical ground heat exchanger is directly proportional to the medium flow rate,and the heat exchange efficiency of the heat exchanger with medium flow rate greater than 0.52 m/s increases slowly.Under the heating condition,the heat exchange capacity of the heat exchanger peaks when the medium flow rate is 0.52 m/s.Intermittent operation is theoretically beneficial to improve the heat exchange capability of vertical ground heat exchangers,but higher power heat pump units need to be configured,and the initial investment of the ground source heat pump system is bound to increase.