Study on Heat Transfer Factors and Optimal Design of Ground Heat Exchangers Under Groundwater Seepage
|School||Taiyuan University of Technology|
|Keywords||ground-coupled heat pump system ground heat exchanger groundwater seepage coupled thermal conduction and groundwateradvection COMSOL Multiphysics optimization design|
Since the global energy crisis is increasingly serious, energy strategy has been changed from alternative energy to environmentally sound, circular clean and renewable energy in the worldwide, it is vital for the sustainable development of society and economy to explore low carbon economy and renewable energy. Ground-coupled heat pump (GCHP), which takes advantage of shallow ground temperature energy and with good stability and environmental protection, is recommend by governments and the industry associations and its application scope extends continuously. According to some statistics, the energy used by GCHP was up to214782billion J in2010, was more than2.45times in2005, and the annual average growth rate was19.7%. In China, the application of GCHP develops quickly and becomes one of the hot topics with the help of the government. At present, the studies of GCHP at home and abroad are centralized on increasing the heat exchange efficiency of ground heat exchanger, by means of improving the model of ground heat exchanger, determining rationally the optimum size, pipe arrangement, and running mode.For this reason, the influences of groundwater seepage on heat transfer performance of ground heat exchanger are studied, based on porous media flow and heat transfer theory. A coupled thermal conduction and groundwater advection model under groundwater seepage is developed, the soil temperature field around ground heat exchanger influenced by groundwater seepage and some soil parameters such as density, heat conductivity coefficient, specific heat and porosity are analyzed using the software of COMSOL Multiphysics. The results show that the soil temperature field around the ground heat exchanger with seepage is different from that of without seepage; the temperature field with seepage is asymmetrically distributed, and skews along the seepage flow direction, the offset and the area of the influence become bigger as the groundwater velocity increase. When the seepage velocity varies from le-6m/s to5e-6m/s, the heat-affected zone increases from1m to1.8m; the seepage strengthened the heat convection in soil, sped up the heat diffusion, and the heat exchange amount of ground heat exchanger is clearly more than that of without seepage. The influences of soil parameters on temperature field are as follows:the denser soil has smaller thermal diffusivity and smaller area of influence, and it is bad to the heat transfer and easy to produce heat accumulation; when the density of soil varies from1200kg/m3to2400kg/m3, the heat-affected zone decreases from1.8m to1.6m. The soil with higher thermal conductivity has larger thermal diffusivity and larger area of influence, it is good for the heat transfer around ground heat exchanger; when the thermal conductivity ranges from0.9W/m-k to3.6W/m-k, the heat-affected zone increases from1.5m to1.7m. The soil with higher specific heat has smaller thermal diffusivity and smaller area of influence, and it is bad to the heat transfer, and easy to produce heat accumulation; when the specific heat capacity of soil ranges from1200kg/m3to2400kg/m3, the heat-affected zone decreases from1.6m to1.2m. The soil with more pores has larger area of influence, when the porosity ranges from0.1to0.4, the heat-affected zone decreases from1.7m to1.6m.The spacing and arrangement of ground heat exchanger of the GCHP system is designed optimally in Xijiayuan District, using the coupled thermal conduction and groundwater advection model and the COMSOL Multiphysics software. By analyzing the weather, geology and hydrogeological conditions in the project region, site testing the thermal response, simulating and comparing the cold and heat accumulation and the areas of the influence induced by the ground heat exchanger under different pipe arrangements, the optimum design scheme is determined:580pipe holes with the depth of120m, high density polyethylene single U-shape pipe with the diameter of32mm, order arrangement with spacing of4m.