## 3 Results and Discussion
### Thermal Balance Parameter Analysis  
To consider the thermal balance, a circle of one summer case and one winter case should be compared in the GSHP running. In this research program, two cases of 35 \({}^{\circ}\)C from summer and 0 \({}^{\circ}\)C from winter were selected for analysis because the temperature differences in these two cases between the U-tube and surrounding soils were both around 17 \({}^{\circ}\)C. It means that this is the same thermal gradient compared to the beginning of tests. From Figure 10, it showed the temperature variations during muddy clay (the location of No.3 TS). It can be found that the time of temperature increase in the summer case is longer than the temperature decrease in the winter case. In other words, the time of heat storage and heat equilibrium in the summer case is longer than that in the winter case in Shanghai muddy clay. Since each group test was conducted under a same heat flux rate and a constant temperature and humidity circumstance, correspondingly the amount of heat storage is larger than the heat release in the muddy clay layer after one alternative cycle of GSHP operation. Figure 10: Time-dependent temperature curves of No.2 TSs: (**a**) 35 \({}^{\circ}\)C; (**b**) 0 \({}^{\circ}\)C.  
Figure 9: Time-dependent temperature curves of TSs of different distances to UT along each row in the fluid circulation of \(-\)10 \({}^{\circ}\)C: (**a**) Array 1 TSs; (**b**) Array 2 TSs; (**c**) Array 3 TSs; (**d**) Array 4 TSs. To quantitatively analyze the thermal balance circumstance during the operation of GSHP system in our research program, two specific parameters were defined and deduced according to the monitoring data, which evaluated the thermal balance from the two aspects of temperature amount and running time. First is the temperature variation ratio \(\alpha\). This parameter is defined to evaluate the degree of temperature changes by circulation based on the initial temperature difference between circulation fluid and surrounding soils. As shown in Equation (1), \(i_{T}\) is referred to the temperature difference between the circulation fluid and the initial ground soils. The initial soil temperature was regarded as 17 \({}^{\circ}\)C; hence the \(i_{T}\) evaluated as the temperature difference is respectively 18 \({}^{\circ}\)C, 23 \({}^{\circ}\)C, 28 \({}^{\circ}\)C, 33 \({}^{\circ}\)C in summer cases and 12 \({}^{\circ}\)C, 17 \({}^{\circ}\)C, 22 \({}^{\circ}\)C, 27 \({}^{\circ}\)C in winter cases. \(\Delta T\) is the final stable temperature change in each TS after the circulation ends. \[\alpha=\frac{\Delta T}{i_{T}}, \tag{1}\]  
Shown as the results in Table 5, temperature variation ratios of No.3 TSs in the muddy clay layer (i.e., TS 2-3, TS 3-3, and TS 4-3) are all calculated. It is found that in the previous three cases either of summer circulations or winter circulations, the temperature variation ratios are almost the same. That is to say, the temperature variation (such as in TS 2-3 during the summer cases, they are 7.44 \({}^{\circ}\)C, 9.61 \({}^{\circ}\)C, 11.04 \({}^{\circ}\)C; correspondingly 2.58 \({}^{\circ}\)C, 3.69 \({}^{\circ}\)C, 4.49 \({}^{\circ}\)C during the winter cases) increases as the initial temperature difference elevated. When enlarging \(i_{T}\) by either increasing the circulation temperature in the summer cases or lowering it in the winter cases, the temperature variation became larger and the temperature variation ratios in the summer cases of 35 \({}^{\circ}\)C, 40 \({}^{\circ}\)C, and 45 \({}^{\circ}\)C or winter cases of 5 \({}^{\circ}\)C, 0 \({}^{\circ}\)C, and \(-\)5 \({}^{\circ}\)C, just slightly changed and kept at the same level.