## 3 Results and Discussion
### Comparative Study on Ground Heat Exchangers
#### 3.1.1 Analysis of Water Temperature in the Ground Side  
Benefitting from the high-temperature medium-depth geothermal energy, the water temperature and heat extraction in DBHEs are higher than that of GHEs in SD-GHPs. Figure 3 compares the inlet and outlet water temperature of the GHEs. It can be seen that the outlet water temperatures of DBHEs in MD-GHPs are all higher than 20 \({}^{\circ}\)C, being the maximum temperature 40 \({}^{\circ}\)C with an inlet water temperature of 23 \({}^{\circ}\)C under intermittent operation mode for office buildings in MG-8. Besides, the DBHEs in MG-3 are installed in abandoned wells which used to extract underground water with a depth of 2000 m, thus, leading to the greatly decreasing of ground temperature. Therefore, the outlet and inlet water temperature in MG-3 are the lowest among all MD-GHPs with values of 20 \({}^{\circ}\)C and 9.7 \({}^{\circ}\)C under continuous operation mode for residential buildings. As for the SD-GHPs, the outlet water temperatures of GHEs are all lower than 14 \({}^{\circ}\)C. Where the highest outlet water temperature reaches 13.7 \({}^{\circ}\)C with an inlet water temperature of 10.2 \({}^{\circ}\)C under intermittent operation mode for school in SG-3. In addition, the lowest outlet water temperature is 7.7 \({}^{\circ}\)C with an inlet water temperature of only 5.5 \({}^{\circ}\)C under continuous operation mode for residential buildings in SG-1. Figure 4 then presents the statistical analysis of the outlet and inlet water temperature. The values from top to bottom for a box stand for maximum, upper quartile, median, lower quartile and minimum, which are the same in subsequent figures. Results show that the median of outlet water temperature is 11 \({}^{\circ}\)C in SD-GHPs, and 29 \({}^{\circ}\)C in MD-GHPs, while the median of inlet water temperature is 8.2 \({}^{\circ}\)C in SD-GHPs, and 19.3 \({}^{\circ}\)C in MD-GHPs. The field test results demonstrate that the water temperature in the ground side of MD-GHPs are much higher than that of SD-GHPs; thus, the energy performance of MD-GHPs could be greatly improved. Figure 3: Field test results of the outlet and inlet water temperature in the ground side. In order to analyze the influence of depth of GHEs on water temperature, the field test results are clarified based on the depth of GHEs, and the results are depicted in Figure 5. The mean temperature of outlet and inlet water temperature are 10.6 \({}^{\circ}\)C and 8.2 \({}^{\circ}\)C with GHEs' depth of 100-120 m, 25 \({}^{\circ}\)C and 16.8 \({}^{\circ}\)C with GHEs' depth of 2000 m, 30.7 \({}^{\circ}\)C and 19.5 \({}^{\circ}\)C with GHEs' depth of 2500 m, 41 \({}^{\circ}\)C and 23 \({}^{\circ}\)C with GHEs' depth of 2800 m. It can be seen that with the increasing of GHEs' depth, the outlet and inlet water temperature of the ground side increase obviously, which shows the advantages of DBHEs in MD-GHPs in attaining high temperature heat source. Moreover, in order to make a thorough analysis of heat transfer performance of GHEs, the outlet and inlet water temperature in SG-1 and MG-5 are monitored for the whole heating season with the sample interval of 10 min. As shown in Figure 6, at the beginning of the heating season, the outlet and inlet water temperature of MD-GHPs reach 46.4 \({}^{\circ}\)C and 34.8 \({}^{\circ}\)C. Then they gradually decline as the continuous operation of heat pump systems. At the medium period, the outlet and inlet water temperature remain stable and reach more than 30 \({}^{\circ}\)C and 20 \({}^{\circ}\)C separately. At the end of the heating season, the inlet water temperature rises, since the heating load declines, thus, leading to the rising of outlet water temperature, which shows the heat storage capability of the medium-depth geothermal energy. The average outlet and inlet water temperature in the ground side can reach 32.6 \({}^{\circ}\)C and 23.4 \({}^{\circ}\)C among the heating season.