Experiment Research Of The Direct Expansion Ground-source Heat Pump System And The Analysis Of Geothermal Heat Transfer

The ground-source heat pump (GSHP), which is a renewable energy technology, has attracted great attention for its advantages including energy-efficiency, environmentally clean, and high-efficiency. The technology is wildly used in the developed countries. The study of GSHP technology is attached more and more attentions in China. The application of the system keeps on growing at high speed in recent years. The direct-expansion heat pump (DXHP) is a special kind of ground-source heat pump which uses a buried copper piping network through which refrigerant is circulated.The direct expansion heat pump system has some specific advantages. Firstly, the efficiency is better than the conventional GCHP for copper rather than PE tubes is adopted as the materials of geothermal heat exchanger (GHE). Secondly, the power input is reduced and the COP is higher as there is no the secondary heat exchange loop and no water pump. It also has disadvantages. On one hand, The lubricating oil may be accumulated in the bottom of the U-pipes in the heating mode if the buried copper is too long. On the other hand, the refrigerant circulates naturally in the cooling mode if the buried copper is too long.A DXHP system is designed and then the system's performance test is performed in a cooperative corporation. A further analyzation is carried out at last. During the experiment, the temperature data of the copper of GHE in different depth is acquired either in the heating mode or in the cooling mode and the temperature distributions around the water well is also acquired. In the experiment, the continuous and intermittent operation are all performed in the cooling mode, but only the intermittent operation is performed in the heating mode as there is an oil return problem. The factors referring to lubricating oil return are analyzed. According to the testing data, the calculated COP of the system is very high.As the refrigerant's phase changes when it flows through the GHE, some simulations are performed in this paper with ANSYS using the cylindrical source theory. It is included that the simulations are well accordant with the testing data. The heat transfer simulation of DXHP's GHE is also performed with other backfill materials. The temperature distribution and impacting factors are analyzed. Furthermore, the influence of different intermittent rates (the hours when the GSHP is on to when it is off) on the temperature distribution is also analyzed. The conclusion shows that the GHE's heat transfer efficiency with single well of the DXHP system is higher than the conventional GCHP system by 47%. Finally, the temperature distributions around several inclined boreholes are gained. In the experiment, the refrigerant can circulate naturally in the cooling mode, but there is an oil return problem of the compressor in the heating mode. Some suggestions are put forward to solve the oil return problem. IIn this paper, the principle of the GSHP system is developed, and the characters of the GSHP system is introduced. The works done in this paper is helpful for further research and application of GSHP system.