Research On Heat Transfer Enhancement Of The Vertical U-tube In Ground-Source Heat Pump System

The purpose of this essay is to use the renewable and clean geotherm energy effectively, enhance the comfort of habitable environment, and achieve environmental conservation. The energy efficiency of traditional air-conditioning system is low due to the influence of ambient temperature. There are some issues in water-source heat pump system, such as difficult injection, groundwater pollution, ect. Ground-source heat pump system is widely used in heating, refrigerating and hot water supply in abroad, but the research and application of domestic start late. The key to promote the development of ground-source heat pump is to solve heat transfer enhancement between ground pipe heat exchanger and the soil, and the higher initial investment.Taking the vertical U-tube heat exchanger as the research subject, the method numerical simulation is adapted to research on influencing factors of the heat efficiency in ground pipe heat exchanger. Innovations are described in the following.(1) Based on the actual structure size of ground pipe heat exchanger, three-dimensional models about single-U and double-U ground pipe have been established to analyze main influence factors about heat transfer by means of heat exchanging quantity and U-tube heat exchanger efficiency. The results show that the heat transfer is obviously influenced by the well depth, velocity, backfill material and branch distance, but not U-tube diameter and material.(2) From the analysis of heat transfer process of ground pipe heat exchanger, when the well depth is less than150m, the total heat exchanging quantity improves with the increase of well depth, but when more than250m, drilling costs rise as well. In addition, the total heat exchanging quantity changes slowly, the U-tube heat exchanger efficiency reduces and thermal interference aggravates with the further increase of well depth, so the scope of recommended depth is150m-250m. The improvement of flow velocity leads to an increase in total heat exchanging quantity and U-tube heat exchanger efficiency. When flow velocity is more than0.8m/s, the increase rate is low and the pressure drop increases rapidly, so the scope of recommended economic velocity is0.4m/s-0.8m/s.(3) In the scope of economic well depth and velocity, the heat exchanging quantity of parallel connection of double-U tube increases about40%with respect to serial connection, so the parallel connection is better. Comparision of thermal interference between single-U and double-U tube shows that double-U tube is more serious. In order to reduce the thermal interference, some methods have been proposed, such as adding thermal insulating board and insulating layer. The numerical results show that the heat exchanging quantity is increased more than8%by adding thermal insulating layer, which is superior to thermal insulating board. Based on the findings above, the thermal conductivity of backfill material has been studied, it is more suitable that the thermal conductivity of backfill material is close to the soil if there is no thermal insulating layer. When thermal insulating layer is applied, the optimal range of the thermal conductivity of backfill material is25-30W/m-K, and the total heat exchanging quantity increases20%-40%.(4) In order to cut down the initial investment of the system, multi-U-tube heat exchanger is analysed on the basis of previous models, and the layout in the drilling is optimized. The results show that the appropriate groups of U-tube in each drilling is three. When the three-U tube is used, a new kind of combined structure has been proposed, which is more convenient to construction. This structure is more compact, the outlet pipes are binded together and the thermal insulating layer is added external. In the scope of economic well depth and velocity, the total heat exchanging quantity increases30%compared with single-U tube after optimizating the layout and numbers of U-tube in each drilling. An appropriate increase of the thermal conductivity of backfill material can make the investment of underground part reduces46%compared with single-U tube.