| Heat transfer and field temperature distribution of large scale U-tube ground heat exchanger (GHE) is analyzed by the finite line source model and the heat pump unit model is established by DOE-2 model. By these two models, the GHE outlet circulation outlet water temperature, the heat pump unit COP, the longest running period of the heat pump, the unit refrigeration power and other parameters are obtained. Take the case of heat release to the ground summer is larger than the heat extraction from the ground in winter, the impact of GHE zoning running on the long-term operation of GSHP system has been analyzed.To alleviate the ground heat accumulation after lone term running of GSHPs, the GHE zoning operation mode is recommended, i.e. all buried pipes are put into running in the summer load and only the pipes in the central area are used in winter.At a given building load, the two operation modes of zoning running or full running are compared for GHEs with different number of pipes. In the case of zoning operation, the GSHP systems can operate normally in 20 years or even longer when the summer and winter load ratio ranges in 1:5-2:1. On the contrary, in the case of full running mode, the GSHP cannot run normally during its whole service life because of the over high water temperature of the buried pipes outlets in the last several years. At the give GHE load, the analysis also indicates that, for cases with unbalanced summer and winter loads, the GSHP systems of zoning operation mode has longer running duration than those of full running mode.The results show that for cases with apparently unbalanced summer and winter loads, the GSHPs can extend the operation life by using the method of GHE zoning operation without extra initial investment. However, GHE zoning operation can also used to modify the existed GSHP system which do not match the operation requirement due to ground thermal accumulation. |
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