ASHRAE Journal - January 2020 - 42

TECHNICAL FEATURE

120 in total, were arranged in the surrounding area of
the building and inside the courtyard. A single hole
was 100 m (328 ft) deep, an aperture of Φ150 mm and
4 Φ32 heat exchange tubes were placed in each hole.
Considering the actual engineering requirements and
research needs, the spacing of the buried pipe arrays
were set to 4 m, 5 m, and 6 m (13 ft, 16 ft, and 20 ft),
respectively in this project, as shown in Figure 2 (Page 40).
The embedding methods were all buried vertically while
the other thermal properties affecting factors such as
pipe and backfill materials remained the same. The heat
transfer of buried pipes in soil was three-dimensional
unsteady heat transfer, and its heat transfer factor was
complex. According to the empirical formula, Equation 1,
the heat transfer between the fluid in the pipeline and
the inner wall of the pipeline:
Rf =

1
πdi h

(1)

Description of the Monitoring System Layout

U-shaped buried pipe wall thermal resistance,
Equation 2;
R pe =

de
1
ln[
]
2πk p
d e − (d o − di )

(2)

The thermal resistance of drilling and sealing materials, Equation 3;
Rb =

d
1
ln( b )
2πkb
de

(3)

Formation thermal resistance of a geothermal heat
exchanger consisting of N parallel boreholes, Equation 4:
Rs =

1
2πks

N


rb
x
(
)
I
+
I ( i )
∑

2 ατ 
i=2
 2 ατ

(4)

Additional thermal resistance due to short-term continuous pulse loading, Equation 5:
Rsp =

r
1
I( b )
2πks 2 ατ p

(5)

The total thermal resistance is given in Equation 6:
R = ∑ Ri = R f + R p e + Rb + Rs + F + Rsp × (1 − F )

(6)

where
di = inner diameter of the U-tube buried pipe, m;
Do = U-shaped tube buried pipe outer diameter, m
42

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JAN UARY 2020

De = U-tube underground pipe equivalent diameter,
de=√2do
Db = diameter of the hole, m
h = convective heat transfer coefficient of the fluid
to the inner wall of the pipe, 0.608 W/(m2·K)
Kp = thermal conductivity of U-shaped tube, 0.35 W/
(m2·K)
Kb = thermal conductivity of backfill material, 1.5 W/
(m2·K)
Ks = the average thermal conductivity of the formation, 1.2 W/(m2·K)
rb = drilling radius, 0.15 m
α = thermal diffusivity, 1.5×10-6m2/s
τ = operation time, s
τp = short-term pulse load running time, s
F = equipment running time in a working condition, h
The total heat resistance of in-situ buried pipe and soil
layer was 2.573 W/(m·K).

The data monitoring system had the function of automatic measurement and analysis of data. The sensor
measuring point arrangement of the analysis acquisition system is shown in Figure 1.
With the aim of studying the real-time variation of
soil temperature field at different well spacing, four
monitoring areas were set up in different well spacing
in the array well area, and one monitoring area was
set up in the L-type well area. The distribution of five
groups of monitoring wells is shown in Figure 2. Each
group of monitoring wells was provided with one main
monitoring well and two auxiliary monitoring wells.
The detailed layout of the monitoring wells is shown in
Figure 1. The main monitoring well was 100 m (328 ft)
deep, in total 11 temperature-measuring points were
set, of which the shallowest layer was 2.5 m (8.2 ft),
and the others were set at 10 m (33 ft) every interval.
The two auxiliary monitoring wells were placed 40 m
(131 ft) deep on both sides of the main monitoring well
at intervals of 1 m (3 ft) and 2 m (7 ft). Within the range
of underground 10 m (33 ft) to 40 m (131 ft), one temperature measurement point was set at intervals of 10 m
(33 ft). The temperature sensor layout in the monitoring
well is shown in Figure 2. The auxiliary monitoring wells
were 40 m (131 ft) deep, set at a distance of 1 m (3 ft) and
2 m (7 ft) from the main monitoring well, and the test
points were arranged at a depth of underground 10 m
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ASHRAE Journal - January 2020

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