| Shallow geothermal energy is a clean energy buried deep below the surface of the 200m within the scope of. Because of its wide distribution, large reserves, high efficiency and no pollution, is more and more widely used in city building heating and refrigeration.. However, their utilizations inconsistent with the enormous amount of energy available underneath the surface of the earth. Projects of this nature are not getting the public support they deserve because of the uncertainties associated with them, and this can primarily be attributed to the lack of a comprehensive acquaintance with the heat transfer process between the Borehole Exchanger (BHE) and the surrounding soil. This shortage cause an arbitrary design for the BHEs, which would lead to the serious drop in performance of BHE. The uncertainties of the performance of the BHE would bring great barriers of the promotion of the new technology. For this energy field to have a better competitive position in the renewable energy market, it is vital that engineers acquire a proper understanding about the Ground Source Heat Pump (GSHP). This article aims at obtaining a deeper understanding about the process of the heat transfer between the BHE and surrounding soil which is under the behavior of the groundwater seepage and suitability evaluation on development and utilization of shallow geothermal energy from the regional scale and field scale.,Based on the thermal response test principle, heat transfer in the hole and heat transfer out of the hole, in situ test in Fuzhou field and a conditions and the model of the borehole exchanger under conduction manners and heat infiltrates coupling manners was established with FELOW. The energy efficiency, heat transfer endurance and heat transfer in the unit depth were introduced to quantify the energy efficient and the endurance period. The performance of a BHE in soil with and without groundwater seepage was analyzed of heat transfer process between the soil and the working fluid. Basing on the model, the varied regularity of energy efficiency performance an heat transfer endurance with the conditions including the different configuration of the BHE, the soil properties, thermal load characteristic were discussed. Discussion for multi-layer soil, study the heat transfer characteristics under the conditions of groundwater seepage,and compare it with the single layer soil’s heat transfer characteristics under the conditions of groundwater seepage the And an investigation about thermal dispersivity was also analyzed its influence of on heat transfer performance. The article finally launched the shallow geothermal energy suitability evaluation, Evaluation of shallow geothermal energy for regional scale by using the analytic hierarchy process, Evaluation the practical working condition of shallow geothermal energy by simulation.Ultimately, through the above study obtained the following conclusions:1.On the condition of non-seepage, heat diffuse radially in the shape of circle which is easy to cause the accumulation of heat. It takes long time for Borehole exchangers to reach local thermal equilibrium (over 10 years). On the condition of seepage(1×10-6m/s), heat transfer through the upstream of groundwater would be inhibited, but heat transfer through the downstream can reach a longer distance which can weaken the negative effect caused by heat accumulation and shorten the time the borehole exchangers reach the local thermal equilibrium. Meanwhile, the groundwater flow can improve the transfer efficiency of the Ground Source Heat Pump(GSHP).When the borehole exchangers run continuously ten years, its heat transfer power for per unit depth will increase 68%(0-60m exists seepage flow)comparing with the condition of non-seepage.2.0n the condition of seepage and non-seepage, the changing of initial temperature of rock and soil mass has no effect of heat transfer efficiency. However, in terms of refrigerate condition, the lower the initial temperature, the higher the temperature difference between the import and export water of the borehole exchanger. And the time the borehole exchanger run at high level will be lasting for a longer time. Lower soil temperature will be better for the running of borehole exchangers in the area where refrigeration is needed.3.If the length of borehole exchanger increased, the heat transfer power will be enhanced and the time the borehole exchanger running at high level will be longer, but it does not mean that the ability of heat transfer can be reinforced. Per unit of the heat transfer power is determined by the total amount of heat transfer and the depth of borehole exchanger. Before designing, we should consider the amount and the depth of the BHE.4.The velocity of circulating fluid in the buried pipe must be maintained in turbulence flow. Under the premise the heat pump working in order, the quicker flow velocity, the bigger the capacity of heat transfer. But the BHE which have a large flow velocity demands more powerful circulating pumps. So it is necessary for us to make a balance between the capacity and the cost, the circulating liquid rate of flow should be controlled between 35 m/d to 45m/d (The inner diameter of pipe is 0.031m)5.By the thermal condition BHE’s operation sustainability research, We found that in winter and summer heat cold heat exchanger unbalanced condition, there will be soil temperature imbalance, BHE heat capacity decreased gradually so that shallow geothermal energy is not sustainability, we should regrade the shallow geothermal energy as "Energy storage " to develop. Determine the operation strategy of hot and cold balanced, sustainable development of shallow ground temperature.6. Set of buried BHE for the same operation strategy, comparison of groundwater seepage conditions and groundwater seepage conditions group BHE operating characteristics and the change of soil temperature field. Results show that in the presence of groundwater seepage conditions group pipe heat exchanger exchanger efficiency coefficient is better than no groundwater seepage conditions of heat exchanger efficiency coefficient. In the three kinds of operation strategy, seepage type group BHE of surrounding rock and soil temperature can maintain a relatively stable for many years, so the ground heat exchanger should be regional settings and groundwater runoff conditions are good, and even can take engineering measures to increase the velocity of groundwater flow around the heat exchanger, to improve the heat transfer heat transfer efficiency and heat capacity for sustainable development.7.The borehole exchanger which is buried in multilayered rock and soil mass, usually only exists groundwater seepage in one layer. The quicker seepage flow velocity the better for heat transferring. The modifiable Peclet number was summarized to judge the intensity of heat transfer by convection. When the modifiable Peclet number is 0.0041, the amount of heat transfer power will increase 5% comparing with non-seepage case. And when the modifiable Peclet number is 0.1291, the amount of heat transfer power will increase 30% comparing with non-seepage case.8.According to the Thermal Response Test (TRT) with forced groundwater seepage, a contrast was given out between the circulating temperature of the borehole exchanger and the emulation temperature value. And the area where the seepage flow exist was also investigated. The comparison result shows that the error between the emulation value and the actual value can be accepted within engineering permissible error range. The final result proves that the model of heat infiltrates coupling model established in this context is reasonable, which can be applied to engineering design.9. According to the area of shallow geothermal energy assessment, the Fuzhou area suitable for the development and utilization of shallow geothermal area mainly concentrated in the quaternary marine plain and residual slope area, the area are easy drilling, conductivity of aquifer, groundwater run off alternating strong. The total area of assessment area is 714.8 km2. The suitable area is 133.6 km2, the not so suitable area is 210.5 km2, not suitable for the area is 370.7 km2, accounted for the total area of assessment area,29.45%,51.86% and 18.69%.10 in the Fuzhou area on the site scale suitability evaluation, from two aspects of low power of office space and high power medium hotel evaluation, evaluation results show:to make heat exchange system more sustainable operation, does not appear temperature continues to rise, accumulation of heat conduction phenomenon, cooling and heating load imbalance rate should be suitable less than 45%. |
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