Study On Soil Temperature Of Ground Source Heat Pump Long-term Operation

Ground source heat pump has been more widely used because of its high efficiency, environmental protection and energy saving. Underground soil is used for absorbing and Releasing heat in ground source heat pump system, so the change of soil temperature has a direct impact on the performance of the system. Therefore, this paper has a study on the distribution of soil temperature around ground heat exchanger during 10 years of continuous operation of the pump system. The main content is as follows.Firstly, the heat transfer model of double U-ground heat exchanger and underground soil is established. Thermal response test and soil temperature heating and recovery test are also finished. Through comparing the measured values with the simulation results of the buried pipe outlet water temperature and hole wall temperature, it shows the average error is 4.2% and 3.5%, which verifies the accuracy of heat transfer model.Secondly, the ground source heat pump system test-bed is built which is supported by the Qingdao science and technology bureau. the all-year hourly load of the test-bed is simulated through the Dest. Besides, the ground source heat pump system model is composed by the heat pump model and the ground heat exchanger model. According to the summer design conditions of average meter of heat exchange in 30 W/m, the change of underground soil temperature is simulated during 10 years of running of the heat pump system in the sandstone. It shows that average soil temperature increases linearly year by year within the 5m soil around the ground heat exchanger. The extent of heat accumulation of soil around the buried pipe increases year by year.Finally, it is separately analyzed the effect of different heat transfer conditions and different soil on the distribution of soil temperature around the ground heat exchanger. The greater the heat per meter depth of the hole is, the higher the average temperature of the soil around the pipe and the higher the degree of heat accumulation of the soil around the pipe. Under the same heat transfer intensity, the larger the specific heat and density of rock is, the higher the degree of heat accumulation of the soil around the pipe. The smaller the rock thermal conductivity is, the greater the soil temperature gradient. The variation of borehole wall temperature is also analyzed under different heat transfer intensity and different soil conditions during running of the pump system. The greater the heat transfer intensity, the greater the variation of borehole wall temperature. The smaller the rock thermal conductivity is, the greater the fluctuation of borehole wall temperature.This paper quantitatively compares the heat accumulation degree of underground soil under different heat transfer intensity and different soil condition,so that providing certain reference data for designing of ground-source heat pump systems.