Performance Simulation And Experimental Research On Vertical U-tube Heat Exchanger For Ground Source Heat Pump With Different Operation Mode

Ground source heat pump (GSHP) system have been increasingly applied in architectural air-condition as these systems can make significant contributions to reductions in electrical energy usage, using renewable resource efficiently, and have been recognized as an environmental-friendly alternative to conventional unitary system. Vertical U-tube heat exchanger have been widely popular because of many notable advantages it has, such as fewer area requirement, extensive application, running steadily, high performance, lower costs on system maintenance etc. So this paper has focused on the U-tube heat exchanger, the relevant research and results are as follows:Firstly, the paper introduced amply the sort, principle, advantage, and developed condition of GSHP system. For the sake of better analyze the heat transfer process of the underground heat exchanger, the heat transfer model of exchanger in the paper was separated into two parts by the borehole wall: a transient one-dimensional line-source model outside borehole and quasi-three-dimensional steady-state model inside borehole. Then combined with temperature response theory conducted by pulse heat under continuous and intermittent operation mode, a more integrated and practical model was established, this model considered many influencing factors, such the system operation mode, fluid velocity, borehole depth, tube type, thermal property of grout and soil, etc.Secondly, the paper presented a whole summer experimental results for cooling about 60 meters borehole in depth installed a vertical U-tube heat exchanger in Shanghai. In order to study on the heat transfer performance of U-tube, several different flow velocity and operation mode were applied for experimental testing, according to the different operation mode, three phases of experimental testing had been divided, they were all-day continuous operation, daytime operation only, and all-day intermittent operation. All-day continuous operation mode was separated into three parts based on the flow velocity of 0.34m/s, 0.37m/s, and 0.42m/s. The results showed the best average quality of heat transfer per meter of borehole is 73.66W/m at the 0.42m/s flow velocity, it exceeded 5.12W/m compared to 0.34m/s, and it exceeded 3.51W/m compared to 0.37m/s. So 0.42m/s was chosen as the excellent flow velocity of the experimental testing, in term of different ratio of on-off time, daytime operation only, and all-day intermittent operation were deployed. It found that, under the same condition, the system performance of intermittent operation is better than the all-day continuous operation, the daytime operation only was the best among them, the average quality of heat transfer per meter of borehole and coefficient of system performance were 75.14W/m.Finally, the heat transfer model established by the paper was verified by the experimental date. When input stated parameters, the inlet & outlet water temperature of U-tube exchanger could be simulated. Compared the simulated values with experimental results, average absolute error was 0.6446℃, and average relative error is 1.84%. This comparison suggested the model has high accuracy and rationality. In addition, the U-tube heat exchanger's heat transfer performance under continuous and intermittent operation mode were simulated and calculated by the heat transfer model, some influencing factors, such as fluid velocity, tube type, thermal property of grout and soil, different operation modes were analyzed, several conclusions were listed in order to guide the GSHP engineering designs.