# Performance Analysis of Slinky Horizontal Ground Heat Exchangers for a Ground Source Heat Pump System

^{1}

^{2}

^{3}

^{4}

^{*}

## Abstract

**:**

^{3}) than reclined HGHE (1.58 m

^{3}), which has higher thermal conductivity than site soil. For mass flow rate of 1 L/min with inlet water temperature 7 °C, the 4-day average heat extraction rates increased 45.3% and 127.3%, respectively, when the initial average ground temperatures at 1.5 m depth around reclined HGHE increased from 10.4 °C to 11.7 °C and 10.4 °C to 13.7 °C. In the case of intermittent operation, which boosted the thermal performance, a short time interval of intermittent operation is better than a long time interval of intermittent operation. Furthermore, from the viewpoint of power consumption by the circulating pump, the intermittent operation is more efficient than continuous operation.

## 1. Introduction

## 2. Description of the Experimental Set-Up

#### 2.1. Material Selection and Ground Soil Characteristics

#### 2.2. Details of Experimental Set-Up

#### 2.3. Experimental Data Analysis

_{p}is the specific heat of water (J/(kg·K)), T

_{i}and T

_{o}are the inlet and outlet temperatures of water, respectively.

^{2}·°C)), A is the heat transfer area (m

^{2}) of GHE, ΔT

_{LM}is the logarithmic mean temperature difference (°C).

_{LM}proposed by Naili et al. [8] was adopted as:

_{g}is the ground soil temperature (°C).

#### 2.4. Uncertainty Analysis

_{i}) is the uncertaninty of independent N set of mesurements in any result represented by R = R(x

_{1}, x

_{2}, …, x

_{N}), then the uncertainity of the result can be calcutated by:

## 3. Results and Discussion

#### 3.1. Daily Average Ground Thermal Behavior (Undisturbed Ground Temperature)

#### 3.2. Comparison of Performance of Standing and Reclined Slinky Horizontal Heat Exchangers

#### 3.2.1. Inlet and Outlet Temperatures of Circulating Water

#### 3.2.2. Heat Exchange Rate

^{3}trench volume was backfilled by typical Japanese sand. On the other hand, for the reclined GHE, 0.225 m × 1 m × 7 m = 1.58 m

^{3}volume of trench was backfilled by the same type of sand. The remaining upper parts of the trenches of both GHEs were backfilled by site soil. Hamdhan and Clarke [29] confirmed that the thermal conductivity varies with material condition and soil’s thermal conductivity and was significantly influenced by its saturation and dry density. The thermal conductivity of clay and sand with 20% water content is 1.17 and 1.76, and at a water content of 40%, the conductivity values are 1.59 and 2.18 W/(m·K), respectively [30]. Since the backfill volume of sand is higher for the standing GHE (4.20 m

^{3}) than for the reclined GHE (1.58 m

^{3}) and the thermal conductivity of sand is higher than that of soil, this may be another reason for the higher heat exchange rate of the standing GHE than the reclined GHE. The installation of slinky HGHEs consists of only piping and excavation work. Though piping of both GHEs is the same, the excavation work differs, as 1 × 1.5 × 7 = 10.5 m

^{3}is required for reclined orientation and 0.5 × 2 × 7 = 7.0 m

^{3}for standing orientation. In contrast to the excavation work, the standing GHE is cost effective.

#### 3.3. Effect of Heat Extraction on Ground Temperature around GHE with the Reclined Slinky HGHE

#### 3.4. Effect of Variation of Ground Temperature on Thermal Performance of the Reclined Slinky HGHE

_{case-2}= 1.4 °C, ΔT

_{case-3}= 1.9 °C and ΔT

_{case-4}= 2.8 °C, respectively. Since ΔT

_{case-4}> ΔT

_{case-3}> ΔT

_{case-2}, higher heat exchange was experienced in case 4. The ${\mathrm{T}}_{1},{\text{}\mathrm{T}}_{4}{\text{}\mathrm{and}\text{}\mathrm{T}}_{7}$ at 1.0 m depth are affected by this higher heat exchange in case 4. As the higher heat extraction affects a longer distance around the GHE, attention should be paid to maintain an optimum distance between GHEs for the installation of multiple slinky HGHEs. The behaviors of the drop in ${\mathrm{T}}_{1},{\text{}\mathrm{T}}_{4}{\text{}\mathrm{and}\text{}\mathrm{T}}_{7}$ at 1.5 m depth for cases 2, 3 and 4 are similar to the discussion pointed out for Figure 6.

#### 3.5. Intermittent Operation of Reclined HGHE

_{g}is the undisturbed ground temperature at 1.5 m depth and T

_{o}is the outlet water temperature of the GHE. This parameter can be called the overall heat transfer response with respect to the change in the temperature difference between undisturbed ground and outlet water. Figure 9 shows the time variation of $\overline{Q}/\u2206T$ values for different intermittent operations. It is seen that the $\overline{Q}/\u2206T$ for all intermittent operations is significantly higher than for the continuous operations. In the intermittent operation, the off period reduces the effect of heat degradation of ground soil around the GHE; thus, the ground is allowed to recuperate its thermal condition during this off period. The heat regeneration in the off time period contributed significantly to the increase in the heat exchange rate.

## 4. Conclusions

## Acknowledgments

## Author Contributions

## Conflicts of Interest

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**Figure 2.**(

**a**) Outer surface LDPE-coated copper tube; (

**b**) Installation of slinky-coil ground heat exchangers.

**Figure 3.**Measured daily average ground temperature and ambient temperature from 1 February 2016 to 31 March 2017.

**Figure 4.**Time variation of inlet and outlet water temperature and ambient temperature in heating mode during cases 1 and 2.

**Figure 5.**Comparison of heat exchange rate between standing and reclined oriented GHEs for cases 1 and 2.

**Figure 6.**Variation of ground temperature around the reclined GHE with operation time in different loops and undisturbed ground temperature during cases 1 and 2: (

**a**) at 0.5 m depth and ambient; (

**b**) at 1.0 m depth; (

**c**) at 1.5 m depth.

**Figure 8.**Variation in ground temperature from 4 March to 20 April 2016 around the reclined GHE and undisturbed ground: (

**a**) at 0.5 m depth and ambient; (

**b**) at 1.0 m depth; (

**c**) at 1.5 m depth and water temperature difference between the GHE outlet and inlet.

**Figure 9.**Time variation of $\overline{Q}/\u2206T$ value in different intermittent operations: (

**a**) 6 h and 12 h interval (

**b**) 2 h interval.

**Figure 10.**Percentage increases in $\overline{Q}/\u2206T$ (12 h average) for 12 h and 6 h interval operations based on continuous operation.

Material | Inner Diameter (mm) | Outer Diameter (mm) | Density (kg/m^{3}) | Specific Heat (J/(kg·K)) | Thermal Conductivity (W/(m·K)) |
---|---|---|---|---|---|

Copper (inner) | 14.6 | 15.9 | 8978 | 381 | 387.6 |

LDPE (outer) | 15.9 | 17.08 | 920 | 3400 | 0.34 |

Ground: clay [27] | - | - | 1700 | 1800 | 1.2 |

Item | Name and Technical Specifications of the Measured Equipment | Uncertainty |
---|---|---|

The water temperatures at the inlet and outlet of standing and reclined GHEs | Pt100 Temperature range: −200 to 600 °C Sensor type: Class A, 4 wire | ±0.15 °C |

Soil temperature in the ground | T-type thermocouple Temperature range: −200 to 200 °C | ±0.5 °C |

Ambient air temperature | Lutron SD Card Data Logger Model: HT-3007SD Range: 0–50 °C Resolution: 0.1 °C | ±0.8 °C |

Mass flow rate of water in standing and reclined GHE | TOFCO flow meter Model: FLC-605 Range: 0.5–5 L/min | ±5% |

Heat exchange rate (W/m) | - | ±5.8% |

Overall heat transfer coefficient, UA-value (W/°C) | - | ±5.5% |

Case | Operation Period | Inlet Water Temperature (°C) | Flow Rate (L/min) | Average Initial Ground Temperature around GHE at 1.5 m Depth (°C) | Reynolds Number |
---|---|---|---|---|---|

1 | 23–26 February 2016 | 7 | 2 | 10.4 | 3268 |

2 | 4–7 March 2016 | 7 | 1 | 10.4 | 1634 |

3 | 12–21 March 2016 | 7 | 1 | 11.3 | 1634 |

4 | 17–20 April 2016 | 7 | 1 | 13.7 | 1634 |

Operation | Flow Rate (L/min) | Average Heat Exchange Rate (W/m) | Average UA-Value (W/°C) | ||
---|---|---|---|---|---|

Standing | Reclined | Standing | Reclined | ||

Case 1 | 2 | 3.17 | 2.66 | 42.66 | 35.58 |

Case 2 | 1 | 2.61 | 2.25 | 36.12 | 30.75 |

Cycle Operation Time | Operation | Period of Integral | S (W·h/(m·°C)) |
---|---|---|---|

120 h | Continuous | Over the whole period | $\overline{)126.2}$ |

Over every 6 h interval | $\overline{)\begin{array}{c}66.2\\ 67.5\\ \overline{)104.1}\\ \overline{)88.8}\end{array}}$ | ||

Over every 12 h interval | |||

Intermittent | 6 h interval | ||

12 h interval | |||

24 h | Continuous | Over the whole period | $\overline{)37.0}$ |

Over every 2 h interval | $\overline{)\begin{array}{c}19.6\\ \overline{)28.2}\end{array}}$ | ||

Intermittent | 2 h interval |

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**MDPI and ACS Style**

Ali, M.H.; Kariya, K.; Miyara, A.
Performance Analysis of Slinky Horizontal Ground Heat Exchangers for a Ground Source Heat Pump System. *Resources* **2017**, *6*, 56.
https://doi.org/10.3390/resources6040056

**AMA Style**

Ali MH, Kariya K, Miyara A.
Performance Analysis of Slinky Horizontal Ground Heat Exchangers for a Ground Source Heat Pump System. *Resources*. 2017; 6(4):56.
https://doi.org/10.3390/resources6040056

**Chicago/Turabian Style**

Ali, Md. Hasan, Keishi Kariya, and Akio Miyara.
2017. "Performance Analysis of Slinky Horizontal Ground Heat Exchangers for a Ground Source Heat Pump System" *Resources* 6, no. 4: 56.
https://doi.org/10.3390/resources6040056