## 3 Results and Discussion
### Thermal Mattress Effect  
As explained in the heat mechanism analysis of [PERSON] et al. [57] because the heat exchange in the winter case can easily approach the thermal equilibrium into the heat transfer saturated state while in summer case the heat transfers from the circulated fluid to the huge, ubiquitous surrounding soil, the thermal gradient can always be kept in a continuous gap to keep a long-term unsaturated heat transfer (Figure 12). In Shanghai, the duration for cooling in summer is almost twice the time of heating needed in winter. The cooling load is much greater than the heating load during the running of GSHP system. At the same time, according to the thermal balance factor comparisons on the summer and winter cases and the heat transfer mechanism process, even under a same external loading (no loading), the heat stored in the muddy clay is higher than the heat released. That is to say, the thermal balance problem can hardly be avoided during the alternation of the summer and winter circulations in Shanghai if no reasonable management and planning are conducted. In the second law of thermodynamics, no process is possible in which the only result is the heat transfer from a body of lower temperature to a body of higher temperature. Spontaneously, heat cannot flow from cold regions to hot regions without the external work being performed on the system, which is evident from the ordinary experience of refrigerant. It means that the heat transfer is not a reversible process. On the operation of GSHP system, dissipating excess heat into the ground is much easier than extracting  
Figure 11: The determination of the temperature rising duration. heat from the soil. The heat exchange amount in the summer case is not consistent with that in the winter case during one single fluid circulation. The COP (coefficient of performance) of cooling in summer is greater than the heating in winter (It is generally \(>\)5.5 in summer and approximately 4.2 in winter [12; 33]). Even if a well-designed GSHP system can make the ideal external thermal balance calculation during alternation of the summer and winter cases, without considering the properties of the muddy clay soil, the temperature increment and the recovery time are both limited to the recovery circumstance. The muddy clay soil has a strong capacity of heat storage, but a relatively poor heat release ability, i.e., for the same heating and cooling load, the GSHP system was unable to approach a thermal balance without complementing another energy source due to the intrinsic properties of Shanghai muddy clay. Running of the GSHP system in the absence of considering the appropriate operation time in each case, the heat dissipated into the ground will be gradually accumulated and much more massive than the heat extracted out from the soil. The recovery process typically shows an asymptotic behavior, being fast at the start and then slowing down subsequently, and theoretically taking an infinite amount of time to reach its original state. However, practical intermittent utilization of GSHP system (e.g., 95%) will occur much earlier, generally on time scales of the same order as seasonal alternation [14]. Testing data did indicate that the temperature of the soil surrounding the GSHP system is always in the trend of increasing after the recovery time, no matter four cases in summer circulation or four cases in winter circulation. Even though the amount of a circle operation of one summer and one winter is not so much (0.5 \({}^{\circ}\)C), the accumulative effect could not be ignored. What is worse is the long-term accumulative effect. It is defined as a thermal mattress effect in the underground soil, within which the temperature is relatively higher than ambient, and the heat could not release easily. Firstly, the sustainable usage of the GSHP system will be influenced accordingly.