ASHRAE Journal - January 2020 - 47

TECHNICAL FEATURE

return pipe side. The three curves represent the variation of temperature with the depth of the main hole (0
to 100 m (328 ft)) in summer, autumn, and winter of
2016. The temperature variations in the three seasons
were similar and generally higher in winter and lower in
summer, and tend to be between the two in autumn. At
the depth of 10 to 40 m (33 ft to 131 ft), the low temperature tended to be stable, because the thermal conductivity of weathered mudstone and sandy conglomerate
was larger, and as a result, the impact of temperature
was much signiﬁcant. Again, due to similar reasons, the
temperature ﬂuctuation after 40 m (131 ft) was comparatively larger. However, the inﬂuence of temperature
was much greater, as the temperature tended to be
stable after 90 m (295 ft), because of the small change of
temperature in the pipe and the change of soil layer into
mudstone.
1. In Figure 4A, the 4 m (13 ft) spacing temperature of the
main hole in summer oscillated between 7.6°C (45.7°F)
and 8.3°C (46.9°F); the winter temperature was higher,
ﬂuctuating between 7.8°C (46.0°F) and 9.9°C (49.8°F)
while the transition season varied within the interval.
At a depth of 10 to 40 m (33 to 131 ft), the temperature
tended to be stable at an average temperature of 7.6°C
(45.7°F) ±0.1°C (32.2°F). The main hole in winter reached
the highest temperature at 70 m (230 ft) deep, and the
lowest temperature in many places in summer was 7.6°C
(45.7°F). The average temperature in the three seasons
changed from 7.83°C (46.09°F) to 8.21°C (46.78°F).
2. Figure 4B presents a temperature variation trend
of 5 m (16 ft) spacing between main holes with a small
variation in summer, autumn, and winter. Values were
lower in summer, higher in winter and average in a
transition season, but less in three seasons. The average temperature of three seasons was 7.9°C (46.2°F);
the lowest temperature was recorded during summer
at 6.6°C (43.9°F) at 40 m (131 ft) deep while the highest temperature was 9°C (48°F) in winter at 10 m (33 ft)
depth. The average temperature in the three seasons
changed from 7.4°C (45.3°F) to 7.8°C (46.0°F).
3. In Figure 4C, the temperature variation trend of
the main hole (6 m, or 20 ft) in summer and autumn
was similar. The temperature was lower in summer,
the average temperature was 6.6°C (43.9°F), and the
lowest temperature was 6.2°C (43.2°F) in 10 m (33 ft).
Meanwhile, the temperature of autumn and winter were
roughly the same, at an average temperature of 8.5°C

(47.3°F). The highest temperature in winter 10 m (33 ft)
9.1°C (48.4°F). The average temperature in the three
seasons changed from 6.6°C (43.9°F) to 8.5°C (47.3°F).
In summary, the temperature difference of the main
hole in winter and summer was large at the interval of
6 m (20 ft), while the linear variation was uniform at the
intervals of 4 m (13 ft) and 5 m (16 ft), and the temperature difference between the main hole was not large.
Particularly, when the spacing between the buried holes
was 5 m (16 ft), the temperature difference remained
consistent with the range of the temperature, and the
unit could be adjusted uniformly.
However, when the spacing between the buried holes
was 4 m (13 ft) and 6 m (20 ft), the temperature difference changed greatly, and it was necessary to adjust
the unit well to maintain a relatively stable unit performance index.
It can be seen from Figure 5 that the soil was initially 9.3°C (48.7°F) at 10 to 40 m (33 to 131 ft) and
the data was measured after the ground source heat
pump was operated for a period of time. The 4 m (13

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JAN UARY 2020

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ASHRAE Journal - January 2020

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