| The energy consumption by human activities keeps increasing every year.While the fossil energy,as the main source of energy,has its non-renewable limitations,and it has become the consensus of all countries in the world to seek the sustainable development of renewable energy.Geothermal energy is a low-carbon,environmentally friendly and green renewable energy.Energy pile system is one of the main ways of utilizing geothermal energy.According to different modes of buring pipe,the common configuration of energy pile system can be divided into U-type,W-type,parallel double-U-type,parallel multi-U-type and spiral type.However,these methods of buring pipe have the shortcomings of weak heat transfer ability and obvious thermal interference and so on.On this basis,a energy pile system incorporating deeply buried pipes(denoted as D-U-type energy pile herein)is proposed in this thesis,which is simulated by using finite element software to analyze the influence of different parameters in order to further improve its heat transfer capacity.The main research work is as follows:(1)The numerical model for a single energy pile is built using COMSOL software and is validated by comparison with model test in the literature.The comparative analysis of D-U-type,U-type and W-type energy pile models shows that the heat transfer capacity of D-U-type is higher than that of U-type and W-type.The parametric study finds that when the flow rate increases from 0.342 m~3/h to 1.368 m~3/h,the outlet water temperature increases from 30.50~°C to 33.74~°C;when inlet water temperature changes from 35~°C to 50~°C,the temperature difference between the inlet and outlet increases by 4.06~°C;the diameter of the heat exchange tube has little influence on the heat exchange capacity,and 20 mm diameter can be selected considering economic factors;the higher the ratio of pipe depth to pile length gives lower outlet water temperature,but there are limits.(2)The numerical model for energy pile group is constructed using COMSOL and validated by comparison with model test in the literature.The simulations of pile group finds that the outlet water temperature of the central pile is higher than that of the single pile model at 0.53~°C due to thermal interference.The parametric study finds that setting the pile spacing above 4 m or setting the separation pile can effectively reduce the thermal interference;the heat transfer capacity of the quincunx pattern is slightly better than that of the square pattern;when the pile diameter changes from 400 mm to 1000 mm,the outlet water temperature of the centeral pile drops from 31.82~°C to 31.32~°C;the outlet water temperature of the centeral pile decreases successively in clay,sand,and rock;the thermal conductivity and specific heat capacity of soil have a great influence on the heat transfer capacity,whereas the density has very little influence.(3)The intermittent operation of D-U-type energy pile group is investigated.The simulation results shows that when the system enters the intermittent state from the running state,the temperature of the pile begins to decline and presents restorability,whereas the temperature of the soil continues to rise.The heat exchange efficiency of the intermittent operation mode is higher than that during the continuous operation mode,and the temperature of soil between piles and surrounding soil is significantly lower than that during the continuous operation mode,which means that intermittent operation mode is better for the long-term heat transfer of D-U-type energy pile group. |
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