# Research on Heat Exchange Law and Structural Design Optimization of Deep Buried Pipe Energy Piles

^{1}

^{2}

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## Abstract

**:**

## 1. Introduction

## 2. Test Overview

#### 2.1. Project Overview

#### 2.2. Test Plan

^{3}J/(kg·°C); $v$ is the circulating water volume flow (m

^{3}/h), $\rho $ is the density of the circulating fluid (kg/m), and the value is 1000 kg/m

^{3}; $\Delta T$ is between the outlet temperature (T

_{out}) and the inlet temperature (T

_{in}), and L is the length of the heat exchanger (m).

## 3. Finite Element Numerical Simulation

#### 3.1. Basic Assumptions

- (1)
- The fluid, heat exchange tube, concrete and soil are homogeneous, and their thermal performance is independent of temperature.
- (2)
- The self-weight of the fluid, the contact thermal resistance between the U-shaped pipe wall and pile foundation, the pile foundation and the surrounding soil are not considered.
- (3)
- Assuming that the initial temperatures of the soil and pile foundation are the same, the temperature at the far boundary of the soil remains unchanged.
- (4)
- The influence of groundwater on the heat exchange of energy pile is ignored.
- (5)
- The change of soil temperature along the depth direction is ignored.
- (6)
- The influence of environmental factors on shallow soil temperature is ignored.

#### 3.2. Basic Assumptions

## 4. Result Analysis and Discussion

#### 4.1. Well Depth

#### 4.2. Pile Length

#### 4.3. Inlet Water Temperature

#### 4.4. Flow Rate

^{3}/h, the temperature of the inlet pipe changes intensively with time and decreases greatly along the deep well. The flow rate in the early stage of operation is low, and the energy pile can carry out more heat exchange, resulting in no significant increase in the temperature of the inlet pipe; when the flow rate is 1.0 m

^{3}/h, the temperature of the inlet pipe has a significant rise, and the temperature change downward along the heat exchange pipe is small; the comparative test shows that when the flow is low, the early heat exchange effect of energy pile is more obvious, and tends to stabilize faster.

^{3}/h, the decrease in inlet pipe temperature increases with increasing well depth, but the slight decrease in the temperature of the heat exchange pipe at the bottom of the deep well cannot be ignored, because when the flow rate is low, the heat exchange effect of the pile foundation and the upper part of the deep well is significant, resulting in a temperature difference that is too small between the lower heat exchange pipe of the deep well and rock–soil, this leads to a decrease in the heat exchange effect, this phenomenon is more significant with increasing well depth, therefore, when the flow rate is low, excessively long well depth is avoided. At the same time, affected by the water inlet pipe, the temperature change of the outlet pipe is small and mainly concentrated in the lower part of the deep well, which is due to the strong thermal interference of the outlet pipe caused by the high temperature around the pile foundation and the upper deep well.

^{3}/h, the changes in water temperature and unit heat exchange at the inlet and outlet of pile #2 with time under different well depths are shown in Figure 16. In the field test 5, the temperature difference between the inlet and outlet of the energy pile is 5.7 °C, and 3.7, 5.1, 5.6 and 6.1 °C for the simulation test (tests 18–21), the results show that within a certain range, the lower the flow rate is, the greater the temperature difference between the inlet and outlet of the energy pile. By exploring the relationship between the unit heat exchange and the length of the deep well, the increase in well depth will work together with the flow rate to reduce the heat exchange effect of the energy pile, if only considering the summer to obtain a lower outlet water temperature, the heat exchange can be ignored, the demand for heat exchange in winter requires reasonable flow and well depth. By comparing the unit heat exchange with Figure 8 (tests 4 and 14–17), we explored the influence of different flow rates on the heat exchange effect of the energy pile, the analysis shows that when the energy pile operates at a low flow rate, a larger temperature difference between the inlet and outlet can be obtained, however, according to Formulas (1) and (2), the flow rate also affects the calculation of heat exchange, through calculation, it can be known that when the flow is within a certain range, the unit heat exchange increases with increasing flow.

#### 4.5. Comparison and Optimization

## 5. Conclusions

- (1)
- An increase in well depth can weaken the influence of pile length on the heat exchange effect of energy piles, so the pile well ratio is an important factor affecting the heat exchange effect of energy piles. Through analysis, it is found that the best benefit can be obtained when the pile-to-well ratio is approximately 0.3–0.4.
- (2)
- The inlet water temperature is the most significant factor affecting the heat exchange effect of energy piles. When the inlet water temperature is low, the heat exchange tube temperature rises evenly, and the time to reach the stable state is short. When the inlet water temperature is high, it shows the opposite trend; at the same time, the change in inlet water temperature has little effect on the heat exchange radius of the energy pile.
- (3)
- The flow rate has a significant impact on the heat exchange effect of the energy pile, but the pile-to-well ratio should be given priority when determining the operating parameters of the energy pile, and then the flow should be set reasonably. If only the lower outlet water temperature is considered in summer, the pile-to-well ratio can be reduced.
- (4)
- By exploring the heat exchange effect of deep buried pipe energy piles under different influencing factors, it is found that the influence of inlet water temperature, well depth, flow and pile length on the heat exchange efficiency of energy piles gradually weakens.
- (5)
- The long length of deep well and the spacing of heat exchange tubes will aggravate the thermal interference of pile foundation and the upper part of deep well, based on the pile well ratio and the selection of backfill materials, the thermal interference phenomenon can be appropriately reduced.

## Author Contributions

## Funding

## Conflicts of Interest

## References

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**Figure 4.**Temperature distribution of #1 pile heat exchange pipe along the depth direction (5.5 kW, 1.0 m

^{3}/h).

**Figure 6.**The variation curve of outlet temperature and unit heat exchange of #1 pile under different well depth conditions (5.5 kW).

**Figure 7.**Temperature distribution of #2 pile heat exchange pipe along the depth direction (5.5 kW, 1.0 m

^{3}/h).

**Figure 8.**The variation curve of outlet temperature and unit heat exchange of #2 pile under different well depth conditions (5.5 kW).

**Figure 10.**Variation of the temperature of #1 pile inlet pipe with time under different heating powers.

**Figure 11.**Temperature distribution of #1 pile heat exchange pipe along the depth direction (3.5 kW, 1.0 m

^{3}/h).

**Figure 12.**The variation curve of outlet temperature and unit heat exchange of #1 pile under different well depth conditions (3.5 kW).

**Figure 13.**Variation of the temperature of the #2 pile inlet pipe with time under different flow rates.

**Figure 14.**Temperature distribution of #2 pile heat exchange pipe along the depth direction (5.5 kW, 0.6 m

^{3}/h).

**Figure 15.**Cloud diagram of temperature distribution at pile bottom and deep well bottom when #2 pile well is 100 m deep.

**Figure 16.**The variation curve of outlet temperature and unit heat exchange of #2 pile under different well depth conditions (5.5 kW, 0.6 m

^{3}/h).

Test Number | Pile Length (m) | Drilling Depth (m) | Heating Power (kW) | Flow Velocity (m^{3}/h) |
---|---|---|---|---|

1 | 23 | 100 | 3.5 | 1.0 |

2 | 5.5 | 1.0 | ||

3 | 5.5 | 0.6 | ||

4 | 18 | 100 | 5.5 | 1.0 |

5 | 0.6 |

Material | Thermal Conductivity (W/(m·°C)) | Thermal Capacity (J/(kg·°C)) | Density (kg/m^{3}) |
---|---|---|---|

Heat exchange pipe | 0.45 | 2300 | 950 |

Concrete | 2.2 | 970 | 2500 |

Rock-soil mass | 1.98 | 2240 | 1970 |

Circulation medium | 0.6 | 4200 | 998 |

Backfill material | 0.58 | 966 | 2650 |

Test Number | Pile Length (m) | Drilling Depth (m) | Heating Power (kW) | Flow Velocity (m^{3}/h) |
---|---|---|---|---|

6 | 23 | 50 | 30.2 | 1.0 |

7 | 75 | |||

8 | 100 | |||

9 | 125 | |||

10 | 50 | 27.2 | 1.0 | |

11 | 75 | |||

12 | 100 | |||

13 | 125 | |||

14 | 18 | 50 | 30.3 | 1.0 |

15 | 75 | |||

16 | 100 | |||

17 | 125 | |||

18 | 50 | 30.2 | 0.6 | |

19 | 75 | |||

20 | 100 | |||

21 | 125 |

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## Share and Cite

**MDPI and ACS Style**

Chen, Z.; Wang, B.; Zheng, L.; Xiao, H.; Wang, J.
Research on Heat Exchange Law and Structural Design Optimization of Deep Buried Pipe Energy Piles. *Energies* **2021**, *14*, 6449.
https://doi.org/10.3390/en14206449

**AMA Style**

Chen Z, Wang B, Zheng L, Xiao H, Wang J.
Research on Heat Exchange Law and Structural Design Optimization of Deep Buried Pipe Energy Piles. *Energies*. 2021; 14(20):6449.
https://doi.org/10.3390/en14206449

**Chicago/Turabian Style**

Chen, Zhi, Bo Wang, Lifei Zheng, Henglin Xiao, and Jingquan Wang.
2021. "Research on Heat Exchange Law and Structural Design Optimization of Deep Buried Pipe Energy Piles" *Energies* 14, no. 20: 6449.
https://doi.org/10.3390/en14206449