Study on heat transfer performance of geothermal pile-foundation heat exchanger in GSHP system

. In order to study the heat transfer performance of geothermal pile-foundation heat exchanger in ground source heat pump system (GSHP), the physical models of pile-foundation heat exchanger and heat exchanger group were established. The heat transfer processes of pile-foundation heat exchanger and heat transfer performance was analyzed both in cooling and heating mode. To carry out the simulation of heat transfer process for 3×3 energy piles, an office building located in Nanjing was introduced. The all-year dynamic building load was calculated with DeST, including the cooling period from June to September, the heating period from December to March and two recovery periods. After ten year’s running, the average soil temperature increases in non-equilibrium condition. Study results are approximate to the actual situation and can be used as theoretical basis for the design and application of pile-foundation heat exchanger in GSHP system.


INTRODUCTION
Nowadays, urban heating has been accounting for the largest proportion of the total building energy consumption in China [1]. By extracting heat into and injecting heat from soil, GSHP system can run stably by avoiding the impact of weather changes on system performance, meeting the requirements of sustainable development strategy. Without material exchanges with atmosphere, the pollution of waste heat, vapor and noise can be reduced. Moreover, GSHP system has a great advantage in investment and maintenance costs [2].
However, the disadvantages of GSHP system cannot be ignored neither. To invest a new system, a large area of land is needed and drilling holes costs additional investment. Heat transfer performance of heat exchangers with different configurations including single U-shaped, single W-shaped, double U-shaped, treble U-shaped and spiral forms were studied and analyzed [3] [4]. The structures of double U-shaped and treble U-shaped are more complex, and the latter one has the higher heat transfer efficiency obviously [5]. In practical engineering applications, stability and rigidity in pile-foundation heat exchangers were verified to be crucial to the efficient operation of system [6].
In this paper, the treble U-shaped form of geothermal pile-foundation heat exchangers in GSHP system was selected and studied by simulation. The simulation processes were carried out by Fluent based on the Finite Element Method for Nanjing area. Heat transfer processes of pile-foundation heat exchanger and heat exchanger group were studied in different conditions.

Physical models
The physical models and configurations of pile-  Table 2.

Solution and method
When simulation for heat transfer performance of pile-foundation heat exchanger was carried out, inlet velocity of water was set to be 0.6 m·s -1 and inlet temperature was specified about 308 K. The Reynold number of flow in pipes was more than 7000, indicating the turbulent water flow. Consequently, standard k-ε was opted for simulation as well as energy equation.

Pile-foundation heat exchanger
In order to observe and analyze simulation results,  The temperature distributions on line L inside the concrete pile with system running are depicted in the following Fig. 3. The position of concrete pile is between -0.3m and 0.3m. It can be concluded that, in cooling mode, the temperature in concrete pile was obviously higher than that in soil. Moreover, soil temperature decreased with the increasing of distance between soil and pile surface. In heating mode, thermal effect was opposite to that in cooling mode. In addition, the temperature in concrete pile and soil increased with system running, but the upward trend was reduced. After 10 days, the temperature tended to be stable.

Pile-foundation heat exchanger group
To carry out the simulation of heat transfer process for 3×3 energy piles, an office building located in Nanjing was introduced. The all-year dynamic building load was calculated with DeST, including the cooling period from June to September, the heating period from December to March and two recovery periods, as shown in Fig. 5.

Fig. 5. Building load
The distributions of average soil temperature with system running in non-equilibrium and equilibrium conditions were shown in Fig. 6. In non-equilibrium condition, heat injection into soil in cooling mode was obviously higher than heat extraction from soil in heating mode. After system running for one year, the value of average soil temperature increased by 0.42K inevitably.
After ten cycles, the value rose to 2.96K. Furthermore, it would increase ceaselessly with system running.
Contrastively, after one year's running in equilibrium condition, the average soil temperature was reduced by 0.16K, and after ten cycles, the value decreased to 0.61K.
With system running, the average soil temperature tended to be stable at the end of the following cycles. Obviously, the equilibrium condition of cooling and heating load is

CONCLUSION
In this paper, the treble U-shaped form of geothermal pile-foundation heat exchanger in GSHP system was selected and studied via numerical method. Heat transfer processes of the pile-foundation heat exchanger and heat exchanger group were studied in different condition. It was found that the higher thermal conductivity of pilefoundation heat exchanger contributed to the higher heat transfer efficiency than soil. In cooling mode, the temperature in concrete pile was obviously higher than that in soil. In heating mode, the trend was opposite. After ten year's running, the average soil temperature increased by 2.96K in non-equilibrium condition and decreased by 0.61K in equilibrium condition. The equilibrium condition of cooling and heating load was beneficial to the system running safely and efficiently. The study results in this paper can be used as theoretical basis for the design and application of pile-foundation heat exchanger in GSHP system.