Simulation of heat transfer performance using middle-deep coaxial borehole heat exchangers by FEFLOW
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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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Table 1. Parameters of the borehole and CBHE
Parameter Unit Value Borehole diameter m 0.25 Borehole depth m 2 000 Inlet pipe diameter m 0.177 8 Inlet pipe wall thickness m 0.008 05 Inlet pipe thermal conductivity W/(m·K) 45 Outlet pipe diameter m 0.1 Outlet pipe wall thickness m 0.01 Outlet pipe thermal conductivity W/(m·K) 0.42 Grout thermal conductivity W/(m·K) 3 
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Table 2. Major parameters used for model validation
Parameter Unit Value Borehole diameter m 0.311 Borehole depth m 2 000 Inlet pipe diameter m 0.177 8 Inlet pipe wall thickness m 0.009 19 Inlet pipe thermal conductivity W/(m·K) 45 Outlet pipe diameter m 0.11 Outlet pipe wall thickness m 0.01 Outlet pipe thermal conductivity W/(m·K) 0.45 Grout thermal conductivity W/(m·K) 4
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Table 3. Parameters used in the sensitivity analysis
Parameter Unit Lower limit Base value Upper limit Inlet temperature ℃ - 4 - Discharge rate of fluid m3/h - 15 - Porosity - - 0.3 - Thermal conductivity of rock W/(m·K) - 3 - Geothermal gradient ℃/m 0.02 0.03 0.05 Rock heat capacity 106 J/(m3·K) 1.5 2.0 3.5 Groundwater velocity m/s 1.16×10-7 0 1.16×10-5
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