Simulation of heat transfer performance using middle-deep coaxial borehole heat exchangers by FEFLOW |
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| Authors: | KANG Wen-kai LIU Feng YANG Fei-fan WANG Hua-jun |
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| Affiliation: | 1.School of Energy and Environment Engineering, Hebei University of Technology, Tianjin 300401, China2.Institute of Hydrogeology and Environment Geology, Chinese Academy of Geological Sciences, Shijiazhuang 050061, China3.Technology Innovation Center of Geothermal & Hot Dry Rock Exploration and Development, Ministry of Natural Resources, Shijiazhuang 050061, China4.China University of Geosciences (Beijing), Beijing 100000, China |
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| Abstract: | Due to its large heat transfer area and stable thermal performance,the middle-deep coaxial borehole heat exchanger (CBHE) has become one of the emerging technologies to extract geothermal energy. In this paper,a numerical modeling on a three-dimensional unsteady heat transfer model of a CBHE was conducted by using software FEFLOW,in which the model simulation was compared with the other studies and was validated with experimental data. On this basis,a further simulation was done in respect of assessing the influencing factors of thermal extraction performance and thermal influence radius of the CBHE. The results show that the outlet temperature of the heat exchanger decreases rapidly at the initial stage,and then tended to be stable; and the thermal influence radius increases with the increase of borehole depth. The heat extraction rate of the borehole increases linearly with the geothermal gradient. Rock heat capacity has limited impact on the heat extraction rate,but has a great influence on the thermal influence radius of the CBHE. When there is groundwater flow in the reservoir,the increase of groundwater velocity will result in the rise of both outlet temperature and heat extraction rate. The heat affected zone extends along with the groundwater flow direction; and its influence radius is increasing along with flow velocity. In addition,the material of the inner pipe has a significant effect on the heat loss in the pipe,so it is recommended that the material with low thermal conductivity should be used if possible. |
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| Keywords: | Geothermal energy Coaxial borehole heat exchanger Geothermal gradient Groundwater velocity Rock heat capacity Thermal influence radius |
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