An Application Study of Ground Source Direct …system integrated with capillary radiant system is analyzed. The experimental results show the outlet temperature of the GSDC compound

4th International Conference On Building Energy, Environment

An Application Study of Ground Source Direct Cooling Compound

System

J.H. Chen1, J. Song1, Z.L. Wu1

1Department of Urban Construction and Environmental Engineering,

Chongqing University, Chongqing 400013, China

SUMMARY

Based on a small mountain tourism building in Chongqing

(China), combined with experiment and numerical simulation,

the reliability and adaptability of ground-sink direct cooling

(GSDC) and ground source heat pump (GSHP) compound

system integrated with capillary radiant system is analyzed. The

experimental results show the outlet temperature of the GSDC

compound system with fresh air in summer could satisfy the

indoor comfort during 3 weeks’ intermittent operation and the

diurnal average coefficient of performance (COP) of system is

up to 7.6 on the typical hot day. In winter, with switching control

of compressor running states, the GSHP could promise the soil

temperature in good recovery. A three-dimensional heat transfer

model of GHEs is built to analyze the operation of whole cooling

season. With the simulated building load, the whole year

unbalance rate of rejection and absorption heat was 7.1%,

which demonstrates the reasonability of the compound system

operating for continuous years.

INTRODUCTION

Ground source heat pump (GSHP) has become one of the

representative forms of energy saving, efficient and low

operation costs, and been thought of the most energy saving

promising by many famous academic researchers (Xu 2010;

Sanner et al. 2003). So far many researches concentrating on

the heat transfer mechanism, aid design software of ground

heat exchanger (GHE) and operation simulation (Aristodimos

and Marita 2001) have been studied.

Ground source direct sink (GSDC) system is a technology which

directly utilizes shallow geothermal resource and well connects

with the temperature and humidity independent control (THIC)

system and capillary radiant system. On the one hand, the soil

temperature in summer in most areas of China could satisfy the

capillary’s requirement of high temperature supply water and on

the other hand partial indoor sensible load could solve the

problems of unbalanced heating and cooling load.

Li et al (2009) built a GSDC system with fan coil in severe cold

district in China. The soil temperature rose 0.0167°C per day

during 45 days’ intermittent operation. Ni et al (2012) tested the

GSDC system in Shanghai for 20 days and achieved high COP

but poor dehumidifying ability due to the high outlet temperature

of single U buried pipe. Zhao (2014) proposed the GSDC

system with water storage and proved its economic benefits. Xin

et al (2012) established the soil sensible heat handled model

including total heat recovery from hot water unit and GSDC

system.

So far the researches on GSDC system still remains in the early

stage. Most experiments are aimed at cooling season but the

effects on the soil after a whole year operation is also of vital

importance. No experiments or projects were applied to

evaluate the feasibility of GSDC system with capillary. As a

result, based on a small mountain tourism building in Chongqing

(China), this paper combines experiment and numerical

simulation to analyze the reliability and adaptability of the GSDC

and GSHP integrated with capillary compound system.

METHOD

Experiment method

Fig. 1 Schematic of the ground sink direct cooling and heat

pump system

ISBN: 978-0-646-98213-7 COBEE2018-Paper243 page 723


The experiment was based on a small mountain tourism

building in Chongqing (China), with the above mentioned

compound system. By regulating valves, the system could

realize three kinds of operation: GSDC, GSHP cooling and

GSHP heating. Fig.1 shows the system schematic.

At design stage, the thermal response test (TRT) was made to

confirm the thermal properties of soil, the heat exchange

capacity for unit well depth, etc. On the basis of the TRT results,

4 test wells (90m in depth) with heat exchange capacity

39.46W/m2 and 1 monitor well with same depth were set.

Due to the special load of the mountain building, the intermittent

cooling tests (8:30 ~18:00) by GSDC system were made in

summer and the continuous heating test in three different kinds

of capillary supply water temperature conditions (30°C, 35°C,

40°C) of GSHP were made in winter.

Indoor horizontal temperature measuring points were evenly

arranged in diagonal shape and at each point the temperature

is measured at the height of 0.1m, 0.6m, 1.1m, 1.4m, and 1.7m

to tell the vertical temperature field. For soil temperature, two

points respectively on inlet and outlet pipes at every 10m in

depth added with 5m below the ground of 2 GHE wells and 1

monitor well were tested to analyze the heat transfer.

Simulation method

Because of the limited test time, a three-dimensional unsteady

heat transfer model based on the experiment system was built

by Gambit to analyze the system feasibility in whole year. The

main structure of GHEs was simplified into three parts: double

U buried pipes, backfill zone inside the borehole and soil outside

the borehole. The parameters of each part were set on the basis

of TRT and realistic situations.

Since the heat transfer between borehole heat exchangers and

their surrounding soil was influenced by many unstable variable,

the following available assumptions are made to simplify the

model. Thermal resistances caused by the contact between

backfill materials and soil or borehole heat exchangers are

negligible. The gravity and solar radiant are ignored and the fluid

velocity remains unchanged. Heat transfer in the bending of U-

tube, which is relatively small compared to the whole heat

transfer, is negligible. The soil in different depths is considered

as the same, with the integrated thermophysical parameters

and average soil temperature. In addition, thermal parameters

of buried pipes, soil and fluid are thought unchanged in the

transfer process.

The heat transfer model is composed of two parts: one is

transferred between soil and backfill and the other is between

the fluid and pipe wall. The soil heat transfer, as an unsteady

process, adopts the 3D unsteady heat conductivity model

without interior heat source, whose general form is

𝜕(𝜌𝑇)

𝜕𝑡=

𝜕

𝜕𝑥(

𝜆

𝐶𝑝·

𝜕𝑇

𝜕𝑥) +

𝜕

𝜕𝑦(

𝜆

𝐶𝑝·

𝜕𝑇

𝜕𝑦) +

𝜕

𝜕𝑧(

𝜆

𝐶𝑝·

𝜕𝑇

𝜕𝑧) （1）

Where 𝐶𝑝 is specific heat, J/ (kg·K); 𝜆 is thermal conductivity,

W/ (m·K); and T is temperature, °C.

The fluid in GHEs is seen as turbulent and the Re-normalized

group (RNG) k–ε model is general accepted when calculating

the fluid flow and its heat transfer. The standard k–ε equation,

along with the continuity equation, momentum equation and

energy equation composes the governing equation, which could

be simplified further for the fluid water is re-circulating,

incompressible, constant viscosity and not concerned with

gravity.

Considering the turbulence state of circulating water, the

boundary flow field of inner pipe wall adopts the refined grid.

Due to the complicated and irregular structure, the backfill zone

is uniformly meshed every 0.5 meters in the axial direction and

the grid spacing of soil is increased by the factor 1.05 to improve

the accuracy. The mesh is displayed in Fig.2.

Fig. 2 The display of mesh

RESULTS

Performance of GSDC system in summer

The GSDC with ceiling capillary and dedicated outdoor air

system (DOAS) test procedure was operated for 3 weeks (15th

Jul~4th Aug). The day with peak load (26th Jul) is chosen as the

typical hot day to illustrate the cooling effect.



Fig.2 shows that the ambient temperature remains stable in the

range of 26~27°C and matches the indoor design temperature

(27°C) with the fluctuation of outdoor temperature and it is

26.56°C when the outdoor temperature peaks at 36.8°C. As for

the space temperature distribution, the maximum temperature

difference as sitting position and as standing position appears

0.4°C and 0.6°C respectively, which indicates a good mixing of

room air. The ceiling capillary cooling system with DOAS could

keep the indoor temperature field uniform.

Fig.3 The daily variations of indoor temperature on Jul 26th

The THIC system has great difficulty in dewing which happens

easily if the radiant surface temperature or fresh air temperature

is lower than dew point temperature. With the indoor load

increasing, the outlet temperature of GHEs increases while the

dehumidification of fresh air unit goes down, so the radiant

surface temperature and the fresh air temperature increases

gradually. As is illustrated in Fig. 4, with flow rate 350 m3/h, there

is no difference between fresh air temperature and dew point

temperature with the average 20.1°C and 20°C respectively. On

the contrary, the average radiant surface temperature (23°C) is

obviously 2~3°C higher than dew point temperature. So there is

no dew formation on the ceiling but tiny dew on the fresh air inlet.

By calculation, the indoor load and sensible part of it are 1.04kW

and 0.65kW respectively.

Fig. 4 Fresh air temperature, radiant surface temperature

and dew point temperature

Before the system operation, the natural soil temperature is

recorded via the monitor well and the highest appears in the

depth of 5m below the ground, where is easily influenced by the

solar radiant, and the temperatures below 10 meters

underground fluctuate slightly within 0.5°C with average

17.68°C, which proves that the soil is perfect natural cooling

source for capillary.

Fig. 5 (a) shows the variation of soil temperature on 26th Jul. The

soil temperature increases apparently from 18.30°C in the first

hour and then rises constantly by 1.51°C when the system stops

at 18:00 after which the soil recovered slowly. It is back to

18.44°C after 5 hours and 18.32°C at next 8:00. After

continuous running for 15 days, the initial soil temperature rises

from 18.09°C to 18.40°C, corresponding to 0.021°C/d (see Fig.

5 (b)). It concluded that the intermittent operation of running for

9.5h every day could guarantee the system in high cooling

capacity and soil in good recovery.

(a)

(b)

Fig.5 Distribution of the soil temperature on 26th Jul (a) and in

15 days (b)

On the typical hot day, the whole cooling system still keeps in

normal and stable running. The water flow rate of tested room

is stable at 0.7 m3/h and that of system is 1.75m3/h. Fig. 6

reflects the water temperature and cooling capacity of capillary

and system. As is shown, the efficiency of GSDC system is so

high that capillary supply temperature is nearly equated to

GHEs outlet temperature. The radiant surface heat flux is



measured by the sensor with averaged 20.3 W/m2. Multiplied by

heating area (40 m2), the average effective heat release of

capillary is 0.81kW, accounting for 89.2% of the capillary

average cooling capacity (0.91kW). Considered with indoor

sensible load by fresh air (0.65kW mentioned before), it is only

55% of sensible load that is taken by capillary. Unlike the GSHP

system, all energy consumption of GSDC system comes from

water pump (0.55kW) and all the heat exchanged from soil is

directly provided for room. So connected with cooling capacity

(4.18kW), the average COP of GSDC system on Jul 26th is 7.60

and as high as 8.18 when the indoor load peaks.

Fig.6 Water temperature and cooling capacity of capillary and

system on 26th Jul

Performance of GSHP system in winter

The continuous tests in winter are made to compare the effects

of three different supply water temperature of floor capillary. The

results of three conditions in stabilization are listed in Tab.1. It is

shown that ambient temperature of 35°C condition is lower than

the designed value (18°C) while the indoor comfort of both 40°C

and 45°C condition could satisfy the requirements in ASHRAE

(2004) and ISO (2005).

Tab. 1 The tested results of three conditions in winter

Conditions 35°C 40°C 45°C

Outdoor temperature /°C 7.55 7.45 6.93

Ambient temperature /°C 16.08 18.08 19.12

Maximum vertical difference /°C 0.22 0.31 0.82

Capillary supply temperature /°C 35.13 40.25 45.73

Capillary return temperature /°C 33.25 35.97 41.67

Capillary temperature difference /°C 1.88 4.29 4.06

Flow rate /m3/h 0.62

Seen in Tab. 2, the increase of floor surface temperature is only

1.1°C when the supply temperature changing from 40°C to 45°C

and the effective heat ratio of 45°C condition is no more than

70%. As a conclusion, the 40°C condition is the perfect for to

achieve the comfort with low energy consumption and high

efficiency operation.

Tab. 2 The heating capacity analysis of 40°C,45°C conditions

As is seen in Fig. 6, in the beginning of system operation, the

unit works in the double compressor mode and the soil

temperature changes dramatically. When ambient temperature

stable, the unit switches to the single compressor or stop mode,

where the soil could provide less heat and get short recovery.

Seen from the curves, the duration of every two switch is nearly

30min, when the soil temperature recovers by around 1°C.

Fig.7 Variations of soil temperature at different depth in 40°C

condition

Although the unit consumption of single mode is less than that

of double mode, lower cooling load decreases its unit COP (3.65

and 3.49 for single and double mode respectively). Added to the

application of fixed frequency pump, the COP of GSHP system

is as low as 2.61 and 2.85 respectively for the two modes.

Model validation

The test data from 17th Jul to 27th Jul is chosen to validate the

effectiveness of model. The initial inlet temperature is set as the

initial soil temperature (18.12°C). Influenced by the numeric

diffusion of unsteady state phase, there is a difference between

simulated and tested data at the beginning. Gradually, the

simulated outlet temperature basically matches the

Conditions 40°C 45°C

Radiant surface temperature /°C 24.00 25.10

Heat capacity /kW 3.34 3.96

Heat capacity unit area W/m² 83.17 100.65

Heat flux W/m² 60.74 69.66

Effective heat ratio /% 73.03 69.21

Lost heat ratio /% 26.97 30.79



experimental one and the relative error of two stabilizes within

1% (see Fig.8 (a)). Because the effect of solar radiant on fluid is

ignored in simulation, the tested soil temperature is always

higher than the simulated. But the difference decreases with the

simulation time passing and concentrates on the recovery stage

(Fig.8(b)). The average is 0.28°C in difference and 1.7% in

relative error. So the numerical model is enough accurate to

analyze the GSCD system performance during the whole

cooling season.

(a)

(b)

Fig. 8 Comparison of simulated and tested outlet water

temperature (a) and soil temperature (b)

Simulation results

To simulate a worse situation, the cooling hours in summer is

set as 13h and the rest to recover for all rooms when the annual

dynamic load calculated. Based on the proportions (55% and

89.2%) that concluded before and the simulated building hourly

sensible cooling load by software Dest, the heat released to the

ground by GSDC system could be calculated and used as the

fundamental data to simulate the outlet and soil temperature of

the whole cooling season.

Fig.9 describes the variation of simulated outlet temperature of

the whole cooling season. The outlet temperature fluctuates

within the range of 18°C~20°C and peaks at 21°C in the middle

of cooling season. Since the fact that the building load

decreases by the late cooling season, the soil could remove the

day’s heat at night, leading to the similar fluctuation of outlet

temperature every day. To verify the indoor comfort at the

condition of 21°C outlet temperature, the air source heat pump

is used to replace the GSDC system, for whose outlet

temperature is influenced by building load and difficult to change.

As is seen in Fig.10, with the help of the fresh air, when the

outdoor temperature peaks (35.8°C), the ambient temperature

(27°C) and ceiling surface temperature (24°C) could remain

stable and satisfy the thermal comfort requirements.

Fig. 9 Hourly variations of the outlet temperatures of the GHE

Fig. 10 The hourly variations of environment parameter in the

condition of 21°C supply temperature

It is only partial indoor load that the GSDC system deals with,

the maximal inlet temperature of GHEs is only 23.8°C and the

heat released to the soil is far less than that of ordinary GSHP

system. Compared with initial soil temperature before operation,

the soil temperature rises from 17.69°C to 18.56°C (0.87°C in

total). Additionally, no short circuit and heat accumulation

happens around the GHEs (see Fig. 11).

Fig. 11 Hourly variations of the soil temperatures



So far, it has proved the cooling ability of GHEs in the first

cooling season. According to the Dest results, accumulative

indoor sensible cooling load is 7754kW and accumulative indoor

heating load without fresh air in winter, all solved by GSHP

system is 7149kW. Thus, via the proportion and equation (2),

the heat to the soil in summer is 4781kW and absorbed from the

soil in winter is 5146kW. The unbalance rate of soil heat is 7.1%.

𝑄1 = 𝑄ZR × (1 − 1/𝐶𝑂𝑃𝑅) (2)

where 𝑄1 is the heat absorbed from the soil in winter, 𝑄ZR is

the accumulated heating load by GSHP system and 𝐶𝑂𝑃𝑅 is

the coefficient of unit performance.

DISCUSSION

The fact that the GSDC system only solves partial sensible heat

makes the transferred heat between building and soil far less

than the normal GSHP system in summer. The experiment and

simulation results prove that GSDC system with capillary could

provide a stable long-term heating and cooling process and the

change of soil temperature could promise good indoor comfort.

When it comes to the energy saving, the energy consumption of

fresh air will be of crucial importance.

CONCLUSION

Connected with the simulation and experimental results, the

GSDC with heat pump and capillary compound system is

reasonable and feasible for cooling mountain building in

Chongqing (China). Three main conclusions are followed:

In the first cooling season, with the help of DOAS the outlet

temperature of GHEs could stasfy the capillarys’ requirements

and make comfortable indoor environment. Meantime, the

system keeps high COP.

In winter, 40°C is determined to be the perfect supply

temperature for capillary in terms of indoor environment and

heat capacity. The switch of GSHP unit running modes are good

for soil in recovery.

By simulation, the compound system could keep in good

operations in the whole cooling season. There is no soil heat

accumulation and the outlet temperature could suit the indoor

comfort. The soil unbalanced rate of absorption and rejection

heat is very low, which determined the feasibility of the GSDC

with heat pump and capillary compound system.

ACKNOWLEDGEMENTS

Great thanks to people for financing and building the

experimental system. Also thanks to those always helpful and

co-operative operational personnel in our group.

REFERENCES

Aristodimos J P, Marita L B. Influence of debonding in ground

heat exchangers used with geothermal heat pumps.

Geothermic. 2001, (3): 527-542.

Li Z J, Zheng R Y, Lei B W, Zhao L Q. Experimental study on

ground temperature variation characteristics of a ground-sink

direct cooling system [J]. Journal of Harbin institute of

technology, 2009, 06: 72-77.

Liu J Y, Xie X N, Qin F H, Song S J, Lv D L. A case study of

ground source direct cooling system integrated with water

storage tank system [J]. Building simulation, 2016, 9: 659-668.

Ni X, Tan H W, Lei Y. Experimental study of a free cooling

system using soil as heat sink in Shanghai [J]. Building energy

consumption, 2012, 10: 19-23.

Sanner B, Karytsas C, Mendrinos D, et al. Current status of

source heat pumps and underground thermal energy storage

in Europe[J]. Geothermics. 2003, 32: 579-588.

Xin Y G. Research and application of ground-source system of

sensible treatment in temperature and humidity independent

control system [D]. Hefei institute of technology, 2012.

Xu W. The development report for ground source heat pump in

China [J]. Construction Science and Technology, 2010, 18: 14-

18.

Zhao F Q. The application research on the compound air

conditioning system of ground source direct cooling system

and water storage system [D]. Shandong Jianzhu University,

2014.


Documents

An Application Study of Ground Source Direct …system integrated with capillary radiant system is analyzed. The experimental results show the outlet temperature of the GSDC compound