| In the process of geothermal utilization,energy transfer and thermal interaction are the key factors that determine energy efficiency of Ground Heat Exchanger(GHE)and Extraction-Injection Well(EIW).Especially for groundwater-source energy utilization,steady and efficient operation of the system is up to what that relatively stable or dynamic equilibrium state occurs when the recharged cold and hot water reaches the pumping zone,which stabilizes the pumping temperature at a certain value.For geothermal utilization system of groundwater source,the extraction wells can extract the heat recharged in summer in winter and the cold recharged in winter in summer effectively,which thus increases the evaporating temperature and decreases the condensing temperature to achieves efficient seasonal storage and effective energy utilization.In fact,the cooling load and the heating load are out of balance in most districts of China.The underground tends to largely gather the cold or the heat,triggering severe thermal interaction.The system's performance is reduced due to growing thermal breakthrough of the pumping temperature.What's more,the system may have to be abandoned.Based on National Natural Science Foundation of China(No.51376080),the ground-source forms have been reformed and optimized to aim for multi-field collaborative control of temperature field,seepage field and hydraulic field to make the temperature distribution tend to be uniform and to maintain the minimum thermal breakthrough.One method is to investigate compound ground source structure of GHEs and EIWs,where EIWs bear major load and GHEs bear the rest small load.The other method is to propose an improved semi-filter well(SFW)in comparison with the traditional full-filter well(FFW)to gain active control of the groundwater's flow direction,which turns into the innovate part of the paper.The test rig of compound structure of GHE-EIW has been built in the paper to investigate geothermal heat transfer as well as validate the effectiveness of the simulation.After that,the experiments within limited size scale are extended to ones that are conducted under conditions of practical size scale,and the simulation models follow.By means of the numerical computation method,multi-cycle,cross-seasonal and wide-area simulation research and analysis on numerous variables have been performed in the static aquifer and in the aquifer where groundwater flows naturally,respectively.It is hoped to reveal the evolvement rule of thermal flow field and improve and perfect the mechanism of groundwater heat transfer and the control theory of energy transfer.The researches on experimental conditions for the compound structure show that adding GHEs in the well group produces limited effect on the pumping temperature and temperature field,thus the compound geothermal structure is reasonable and feasible.Under the action of groundwater transverse flow,the experiments on different flow velocities of groundwater and various extraction-injection rate of flow show that transverse flow can exercise its restraints upon increasing thermal breakthrough.The numerical confirmatory experiment shows that the maximum relative error is less than 6% and the maximum root mean square error is no more than 0.5.Based on engineering practice,this paper conducts simulation analysis on the compound geothermal forms.By means of model construction and computing method perfection,the paper has performed investigations on energy transfer and energy utilization of the compound geothermal form considering multi factors.The load distributions,arrangement forms of GHEs and arrangement forms of EIWs are chosen to be variables,and different levels are set to investigate heat transfer behavior in aquifers.When the ratio of GHEs to EIWs is 2:3 in winter and 0:1 in summer for load distribution mode,it can well prevent thermal breakthrough from deteriorating,and decrease aquifer thermal interaction in the pumping zone.When GHEs are centralized lengthways and EIWs are in aligned arrangement lengthways,the pumping temperature has minimum drop and thermal interaction is not severe,which contributes to geothermal recovery.In addition,reverse well mode and summer start-up mode bring about more efficient utilization of recharge energy,preceding fixed well mode and winter start-up mode.Under conditions of transverse flow of groundwater,considering adverse flow,down flow and cross flow,the paper focuses on changes of thermal flow field and seepage field respectively taking load distributions and arrangement forms of GHEs for example.Taking compound geothermal structure of lengthways centralized GHEs and aligned EIWs for example,the effect of transverse flow of groundwater has been investigated.The results show that adverse flow is the best,cross flow takes second place.Both the two flow states can restrain aggravating thermal breakthrough and optimize temperature field.The down flow state is the worst.To further conduct system optimization,the control flow form of filter tube has been optimized based on traditional filter tube form and the semi filter is first proposed.In heating dominant district and balanced heating and cooling district,energy flow characteristics between full flow mode and semi flow mode are studied considering different loads,extraction-injection well distances,flow rates and extraction-injection temperatures.The results reveal that the semi flow mode can decrease thermal breakthrough and fluctuation of the pumping temperature,and the energy loss due to thermal breakthrough is small as well.It guarantees that geothermal temperature can recover efficiently.In the end,four control flow modes are investigated.The results show that as long as the extraction well is the semi-filter well,it can keep lower thermal breakthrough,especially if the extraction well and the injection well are both the semi-filter well.Therefore,the directed semi-flow extreaction-injection form possesses practical engineering utilizability and provides a novel approach for efficient geothermal utilization. |
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