Numerical and Experimental Study on Performance of Ground Source Heat Pump and the Underground Heat Exchangers

Previous studies indicated that the imbalance of heat extracted from earth by the U-tube heat exchangers in winter and ejected into it in summer will affect the long term performance of GSHP and the soil temperature distribution around the underground heat exchangers. This paper presents a dynamic coupling model which can simulate the performance of GSHP system and soil temperature variation around the underground heat exchangers by considering the influence between them. The dynamic coupling model was established on the basis of one dimensional variable heat flow line source model and the dynamic heat pump model. Meanwhile a GSHP experimental system was built and the performance of the heat pump system and the soil temperature variation around its underground heat exchanger were recorded during one month operation. The simulation results of the experiment system were validated by the experimental results, and the margin of error of soil temperature was ±0.4°C, while inlet and outlet water temperature of the buried pipe was ±0.5°C. The comparison showed that the coupling simulation model is precise enough to analyze the long term performance of the GSHP system. The soil temperature variation and heat pump system performance in fifteen years in territories with a cold winter and a hot summer such as Nanjing in China was simulated by the dynamic coupling model. The results showed that the soil temperature will increase by 1.68°C～6.78°C due to the surplus heat ejected into the earth in summer, which will lead to the rise of the cooling water temperature, therefore the COP of GSHP will decrease in summer, whereas increase in winter. Research results show that the dynamic coupling model can reflect the heat pump operation conditions and soil temperature variation. The size of borehole heat exchangers’ cluster has a significant influence on the performance of GSHP during long time operation in territories with a cold winter and a hot summer, eg. the middle and downstream of Yangtze River.