Numerical Study On The Main Influencing Factors Of U-shaped Vertical Ground Heat Exchanger Performance

Ground-source heat pump (GSHP) is a new technology of renewable energy and achievesthe seasonal utilization of resources. GSHP is widely used in engineering practice for itsenergy conservation and environmental protection, high energy efficiency coefficient, stableoperation and convenient management and other advantages. The GSHP system adopting avertical U-shaped ground heat exchanger is better suited our national conditions of small percapita area occupation. The comprehensive performance of GSHP system depends on the heattransfer performance of ground heat exchanger. To study the influencing factors of groundheat exchanger can strengthen the heat transfer between the pipe and soil, while improving thecomprehensive performance of GSHP system effectively. These factors directly affect theresults of the geotechnical thermal response test, and geotechnical thermal parameters are thebased foundation to design GSHP system which should be accurately measured.In view of this, the paper has discussed the factors of U-shaped vertical ground heatexchanger performance. The full-size three-dimensional model of the vertical U-shapedground heat exchanger is firstly established in this paper to couple the groundwater seepageand geotechnical layering in thermal physical properties. And fluent, the professionalnumerical simulation software, is applied to simulate the geotechnical thermal response testprocess. Not only is the temperature change of the fluid at inlet and outlet of the buried tubeand the soil around monitored and analyzed during the numerical simulation, but also theresults of the thermal response test and the numerical simulation are compared and analyzed,which verifies the rationality and accuracy of the model.Then, the paper has applied the numerical model to simulate the thermal response testunder the different conditions such as groundwater level changes, the same level ofgroundwater seepage under different speeds and geotechnical stratified of thermalconductivity to analyze the impact on the performance of heat exchanger. And throughanalytical calculation model, analysis the simulated data to get the geotechnical integratedthermal parameters, which are used to study the impact of these factors on the thermalresponse. In addition, the quantitative analysis is obtained aiming at the adverse effects ofinitial stage of GSHP system on the system performance to study the effect from differentstart running time, which provides theoretical basis and reference for practical engineeringdesigning. It turned out that when groundwater depth increases, which will undermine theheat transfer performance of ground heat exchanger. Groundwater flow can enhance the heattransfer capacity of ground heat exchanger, and the greater the speed, the more significant role. When thermal conductivity coefficient changes along the depth direction, the heattransfer performance of soil is not as good as in homogeneous soil under the same averagethermal conductivity coefficient. The results of thermal response test will change whengroundwater level changes, groundwater seepage speed changes and geotechnical stratified ofthermal conductivity varies. If the initial operating time is different, the integrate performanceof heat pump unit is influenced, It can be concluded that the COP of GSHP decreases byabout1.8%in hot season and1.3%in cold season. So the initial operating time should beselected reasonably.