| At present,geothermal energy resources are abundant in China,but their exploration rate and utilization degree are quite low,and their potential of exploitation and utilization is great.Compared with the shallow buried heat exchanger,the medium-depth geothermal heat exchanger has the advantages of high heat absorption efficiency,large heat transfer capacity and small area occupation.In particular,the medium-depth single-well geothermal heat exchangers of the heat-conduction type and pumping-recharging type,with the main working mode of"flow into the annular zone and out from the inner pipe"and compact coaxial structures to obtain the deep geothermal energy resources for direct utilization,have gradually been recognized by the geothermal industry and formed large-scale engineering applications.The main differences of heat-conduction type and pumping-recharging type geothermal heat exchangers are whether the borehole wall is a closed structure or uncased hole and whether the circulating fluid is groundwater or not.In this paper,the medium-depth geothermal heat exchangers of heat-conduction type and pumping-recharging type were selected as the research objects,and the theoretical model construction,physical parameter inversion,heat exchanger design,field test verification and operation characteristic analysis were carried out.Firstly,based on the research approach of"decoupling modeling of every partition and coupling calculation",the theoretical models of the single-well geothermal heat exchangers suitable for different heat-conduction and pumping-recharging types were constructed.By establishing the partition models of the unsteady heat conduction processes in the cylinder soil,groundwater seepage and heat transfer processes in the aquifers as well as the forced convection heat transfer processes in the coaxial pipes,the theoretical models of three kinds of heat-conduction downhole coaxial heat exchangers in the closed vertical geothermal wells,L-shaped geothermal wells and directional wells can be solved,and the theoretical models of two downhole coaxial heat exchangers in the single-well pumping-recharging geothermal system and downhole coaxial open loop geothermal system can be solved.Secondly,in order to improve the design accuracy of geothermal heat exchangers,the geotechnical sampling tests and thermal response tests were carried out in this paper,and the simulation model and parameter inversion method of the thermal response test based on geotechnical sampling test results were put forward.The simulation model of the thermal response test can reflect the characteristics of stratigraphic physical properties,environmental disturbance,flowrate fluctuation and so on.The simulation model was proved to be precise by the comprehensive evaluation of six indicators,such as soil thermal conductivity and borehole thermal resistance under multiple working conditions.It should be noted that these thermal response tests and simulation model were based on the geothermal heat exchangers in the shallow ground,but they offered feasible methods for economic inverse calculation of soil parameters in the deep layers and precise engineering design of the medium-depth geothermal heat exchangers.After that,this paper introduced the design method and field experiments of a pumping-recharging geothermal system with the total depth of 1764 m,summarized the engineering design processes of the underground downhole system,and completed the field experiments of groundwater pumping,heat exchange,and groundwater reinjection.During the operation,the outlet groundwater temperature could be stabilized at 56℃with the 100%groundwater reinjection rate,and the heat absorption rate at the source side was 1020 k W,with the heat transfer rate per unit power consumption of 16.9 k W_t/k W_e.The field experiments proved the technology feasibilities of the well-sealing and the geothermal energy extraction by pumping-recharging geothermal systems.After that,the theoretical calculations were conducted for heat-conduction type and pumping-recharging type geothermal heat exchangers.For a vertical closed heat exchanger with the depth of 880 m,the theoretical heat transfer rate and outlet fluid temperature in the stable operation stage were 83.21 k W and 44.95℃respectively,and the relative errors were 7.67%and 1.86%compared with the experimental results of90.12 k W and 45.80℃.The theoretical calculations of the single-well pumping-recharging geothermal system showed that at the end of operation,the hydraulic affected radius and thermal-affected radius of the recharging aquifer were 29 m and4.04 m respectively.The theoretical analysis indicated that the actual heat absorption rate was 1027 k W,and the theoretical maximum value was 1460 k W.For the value of1027 k W,the contributions of the heat conduction zone and the pumping recharging zone were 292 k W and 735 k W respectively.During the operation,the heat losses caused by the groundwater leakage through the aquitard and the heat short circuit cross the inner pipe wall were 32 k W and 401 k W respectively.Afterwards,the theoretical calculation of an downhole coaxial open loop geothermal system with the depth of 2070m was conducted.According to the theoretical analysis,the equivalent pumping recharge flowrate was 1.5 m~3/h and the heat absorption rate was 279 k W.Finally,the heat transfer enhancement method based on the reducing pipe ends was proposed for the downhole coaxial open loop geothermal system.By changing the diameter of the end pipe in the well bottom,the circulating groundwater through the annular region between the inner pipe and the borehole wall could be reduced,and replaced by the groundwater from the pumping aquifer,and thus heat transfer rate could be improved.The influence studies of several key parameters showed that higher geothermal temperature gradients and lower inlet fluid temperatures improved the heat transfer linearly for the closed vertical geothermal heat exchangers;when the Reynolds number of circulating fluid was greater than 1.12×10~4,meaning vigorous turbulence in the well,the heat transfer enhancement effect of increasing flow rate is no longer obvious.In the pumping recharging geothermal system,the heat absorption rate increased obviously by increasing the groundwater flowrate,but higher groundwater flowrate would speed up the groundwater leakage and the vertical heat transfixion of the aquitard;the lower permeability coefficient of the aquitard could alleviate the groundwater leakage and delay the occurrence of the vertical heat transfixion of the aquitard;the lower thermal conductivity of the inner pipe had some positive effects on the pumping temperature and heat transfer,but had little effect on the groundwater leakage and the vertical heat transfixion of the aquitard. |
