Heat Transfer Analysis And Optimization Of Ground Heat Exchanger For Heat Storage Based On The Principle Of Temperature Field Uniformity

When ground source heat pump(GSHP)systems are applied in regions with imbalanced heating and cooling demand for a long time,the nonuniformity of underground temperature field will increase,which will lead to the decrease of heat transfer efficiency of ground heat exchanger(GHE).Based on the principle of temperature field uniformity,it is of great theoretical and practical value to solve the problem of GHEs' layout and optimal load for improving their efficiency.Based on heat transfer analysis and the principle of the uniformity of underground temperature field,the heat transfer model of multi-borehole GHE field is built up,and the multiple-population genetic algorithm(MPGA)is proposed for the uniformity of underground temperature field of multi-boreholes GHEs.The layout optimization of buried pipes and the simulation of the underground temperature field were carried out.By using the established MPGA,the layout optimization of 16 buried pipes and 25 buried pipes is studied and the temperature field simulation of multi-borehole GHEs is carried out.The optimization results show an approximately symmetrical layout of tubes in regions with imbalanced heating and cooling demand,with fewer tubes located in the central area and more tubes in the surrounding area;the distance between the tubes near the corners is smaller and the distance between the tubes in the center of the boundary is larger.The simulation results of temperature field show that the optimized layouts of tubes help reduce the gradient value of the underground temperature field,improve the uniformity of the temperature field,and alleviate the accumulation of cooling/heat in underground.And the more the number of tubes are,the more uniform the temperature field will be.The layout optimization of the buried pipes(36 boreholes)which is suitable for practical engineering is studied and analyzed.On the basis of the same occupied area as the traditional layout,three position zoning designs of buried pipes are proposed and the simulation results of temperature field are compared with the traditional designs with equal spacing between boreholes.The results show that the three kinds of zoning layouts can alleviate the cooling/heat accumulations to some extend compared with the traditional layout.And among the three optimized layout,the best improvement on the uniformity of underground temperature field can be obtained under the layout 3,which has no tubes arranged in the central area.Moreover,the excess temperature changes at the surrounding boundaries of the three zoning layouts are not significant in contrast to the traditional layout.At the same time,the influence of the parameters,such as thermal diffusivity,cooling/heating load ratio,operation duration and the numbers of buried pipes on the improvement of temperature field uniformity is analyzed.The results show that with the decrease of soil thermal diffusivity and the increase of the numbers of buried pipes,cooling/heating load ratio and the running duration,the advantage of heat transfer improvement is larger than that of traditional layout.The load optimization of the buried pipes(36 boreholes)which is suitable for practical engineering is studied and analyzed.The heat transfer variation of the underground temperature field between the traditional load design and the load zoning design are simulated and analyzed.The principle of the load zoning design is that the load of the buried pipes in the central area is small and that in the surrounding area is large.The simulation results show that the heat accumulation of the load zoning strategy in the underground is reduced compared with the traditional load strategy,the less load we put into the buried pipes which are located in the central area,the less heat accumulated in the underground.However,when the load ratio of the central/surrounding buried pipes reaches a certain value,the alleviation of heat accumulation tends to be stable.With the decrease of soil thermal diffusivity and the increase of the cooling/heating load ratio and the numbers of buried pipes,the advantage of heat transfer improvement of the load zoning strategy is larger than that of traditional load strategy.In the early operation stage of the system,the optimization effect of the load zoning strategy is relatively more obvious and it is gradually stabilized at the later stage.The load optimization of the buried pipes(64 boreholes)with seepage flow is studied and analyzed.According to the heat transfer characteristics of underground buried pipes with seepage flow,the load zoning principle is that the load intensity of the upstream buried pipes is large and that of the downstream pipes is small,and the underground temperature field of the zoning load strategy is compared with that of the non-zoning load strategy.The following conclusions can be drawn: along the seepage flow direction,increasing the load intensity of the downstream pipes can alleviate the heat accumulation in the underground.With the decrease of soil thermal diffusivity and the increase of the cooling/heating load ratio,the advantage of heat transfer improvement of the load zoning strategy is larger than that of traditional load strategy;and with the increase of seepage flow rate,the two kinds of the load strategies can both reduce the heat accumulation in the underground,but compared with the traditional non-zoning strategy,the improvement of heat transfer efficiency of the load zoning strategy is reduced.