## 3 Results and Discussion
### Comparative Study on Ground Heat Exchangers
#### 3.1.3 Analysis of Heat Extraction Performance of DBHE in MD-GHPs under Intermittent Operation  
The intermittent operation mode has a great influence on the heat transfer performance of DHBEs. As for MG-8, the depth of DBHE is 2800 m, while the diameter of the inner tube and outer tube are 100 mm and 210 mm. Thus, one DBHE contains nearly 96.7 m\({}^{3}\) of the ground side water, which could serve as heat storage in the ground side. Therefore, when the MD-GHPs turn off, the ground side water still extracts heat from the ground with temperature increasing continuously. Then when the MD-GHPs turn on again, the outlet water temperature and instantaneous \(Q_{g}\) per DBHE will be significantly higher. Figure 7 shows the outlet and inlet water temperature, as well as the \(Q_{g}\) per DBHE under intermittent mode in MG-8 from Monday to Friday. During the weekday, the MD-GHPs operate from 6:00 am to 5:00 pm and turn off at other time. Benefitting from the intermittent operation, the outlet and inlet water temperature could reach 44 \({}^{\circ}\)C and 23.4 \({}^{\circ}\)C with \(Q_{g}\) reaching over 800 kW when the MD-GHPs turn on. Then the values decline gradually with continuous operation. The average outlet water temperature could reach 41 \({}^{\circ}\)C with an average \(Q_{g}\) of 672 kW during working time, which is much higher than that of DBHEs under continuous operation mode. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Project**} & **Space Heating Area,** & **Number of** & **Space Occupation** & \\  & \(A_{H}\) **(m\({}^{3}\))** & **GHEs** & **of GHEs, \(A_{G}\) **(m\({}^{2}\))** & \\ \hline SG-1 & 43,000 & 450 & 1800 & 0.0419 \\ SG-2 & 18,500 & 196 & 784 & 0.0424 \\ SG-3 & 32,769 & 270 & 1080 & 0.0330 \\ SG-4 & 35,024 & 590 & 2360 & 0.0674 \\ SG-5 & 27,236 & 280 & 1120 & 0.0411 \\ SG-6 & 141,289 & 950 & 3800 & 0.0269 \\ SG-7 & 202,000 & 900 & 3600 & 0.0178 \\ SG-8 & 67,688 & 800 & 3200 & 0.0473 \\ SG-9 & 42,000 & 300 & 1200 & 0.0286 \\ MG-1 & 20,600 & 2 & 12 & 0.0020 \\ MG-2 & 43,500 & 3 & 18 & 0.0010 \\ MG-3 & 56,000 & 4 & 24 & 0.0005 \\ MG-4 & 37,800 & 3 & 18 & 0.0024 \\ MG-5 & 133,400 & 8 & 48 & 0.0008 \\ MG-6 & 185,100 & 10 & 60 & 0.0010 \\ MG-7 & 15,000 & 2 & 12 & 0.0008 \\ MG-8 & 33,160 & 2 & 12 & 0.0008 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Space occupation of GHEs. Figure 7: Operation of the ground side under intermittent mode. Therefore, the instantaneous \(Q_{g}\) of DBHE is not suitable to evaluate and compare the heat transfer performance of DBHEs between intermittent and continuous operation mode. Consequently, the accumulated heat extraction in 24 h (\(Q_{g,a}\)) is raised, and results are shown in Table 4. It can be seen that the value of \(Q_{g,a}\) per DBHE in MG-8 is nearly the same as the value in MG-3 and MG-4, which operate in continuous mode, but with a lower depth of DBHEs. That is to say, even though the intermittent operation could increase the instantaneous \(Q_{g}\) per DBHE, the \(Q_{g,a}\) per DBHE is decreased compared to the continuous operation mode. However, during practical operation, the inlet water temperature could be decreased, and the flow rate could be increased to increase the \(Q_{g,a}\) per DBHE to reach the same value under continuous operation. Consequently, the large depth of DBHEs makes it serve as heat storage in the ground side. Then equipped with heat storage in the user side, the MD-GHPs could form a double heat storage system. For commercial buildings which have intermittent space heating demand, the MD-GHPs could operate for heat storage in the user side during the nighttime with cheap electricity price. Then during the daytime when the electricity price is normally high, the MD-GHPs could turn off, and the heat storage in the user side can be released for space heating; thus, the operation cost can be greatly reduced. For residential buildings, the double heat storage system can also be applied to meet the continuous space heating demand.