Integrated Ground-Source Heat Pump With Cooling Storage

8/3/2019 Integrated Ground-Source Heat Pump With Cooling Storage

1/13

A new integrated system with cooling storage in soiland ground-coupled heat pump

Yanshun Yu a,*, Zuiliang Ma b, Xianting Li c

a School of Power Engineering, Nanjing University of Science and Technology, Nanjing 210094, PR Chinab School of Municipal and Environmental Engineering, Harbin Institute of Technology, Harbin 150090, PR China

c Department of Building Science, Tsinghua University, Beijing 100084, PR China

Received 6 July 2006; accepted 11 September 2007Available online 25 September 2007

Abstract

For the purpose of decreasing the peak electricity, balancing the on and off-peak electric load and utilizing the renewable geothermalenergy, a new integrated system with cooling storage in soil and a ground-coupled heat pump is presented. In the integrated system, themoist soil acts as the material for cooling storage, and pipes serve as the cooling storage devices and geothermal heat exchangers simul-taneously. In the cooling season, the cooling energy is stored in soil during the off-peak period and is extracted for space cooling duringthe on-peak period. While in other seasons, the system works as a ground-coupled heat pump for heating or cooling. A mathematicalmodel which describes the charging and discharging processes of the integrated system has been developed and validated, and a computercode has been implemented to simulate the operational performance of cooling charging and discharging in soil. A parametric studyindicates that the charging inlet temperature, tube diameter, moisture content of soil and pipe distance are important factors in deter-mining the cyclic performance of the integrated system. 2007 Elsevier Ltd. All rights reserved.

Keywords: Integrated system; Cooling storage in soil; Ground-coupled heat pump; Underground heat exchanger; Cooling charging and discharging

1. Introduction

The studies on how to shift the on-peak electric load andto utilize the renewable energy are the common goal now-adays in the field of air conditioning. Ice storage andground-coupled heat pump (GCHP) are two main technol-ogies in such kinds of applications. While the former is

aimed to reduce the air conditioning on-peak electricaldemand and the latter is to utilize the renewable geother-mal energy.

The first known record using ground as a heat source fora heat pump was in a Swiss patent issued in 1912 to Hein-rich Zoelly [1], and later this kind of devices were called asGCHP. The GCHP systems offer an attractive alternative

for both residential and commercial heating and coolingapplications and they offer significant energy use anddemand reductions on the power grid [2,3]. Therefore, theyare recognized to be energy efficient heating and coolingsystems [4].

In recent years, most studies have concentrated onenhancing the GCHP system performance, improving the

accuracy of the design method and reducing the size ofthe borehole heat exchangers (BHE). In a first stage, theresearch mainly focused on the heat transfer model andsimulation method [58], thermal performance of BHEand ground or backfill materials [911]. Then many effortsaimed at the definition of a reliable software tool and asimpler and more accurate testing procedure to determineground properties in order to design the system correctly[12,13]. At present, the studies about the accurate model-ling [14,15], technoeconomic appraisal and comparison[16,17] are still being performed.

1359-4311/$ - see front matter 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.applthermaleng.2007.09.006

* Corresponding author. Tel.: +86 025 84315485.E-mail address: [email protected] (Y. Yu).

www.elsevier.com/locate/apthermeng

Available online at www.sciencedirect.com

Applied Thermal Engineering 28 (2008) 14501462
mailto:[email protected]:[email protected]


2/13

Due to the increase in energy costs and the developmentof GCHP, more and more GCHP projects are being imple-mented in the world. Lund and Freeston [18] reported thatthe worldwide application of GCHP systems have had aconsiderable growth of almost 59% since 1995, and at thebeginning of 2000, the installed capacity in 26 countrieswas around 5275 MW.

Although the GCHP technology has a popular develop-ment in the world, the large area required and the heatingor cooling capacity decrease for long-term operation havelimited further applications on a large scale. Especially incooling mode, the conductivity of the soil around the coilsdecreases notably as the soil is dried by the rejected heat[19]. This will reduce the cooling energy provided by theGCHP and may not satisfy the designed requirement forseveral years operation. For example, the first GCHP sys-tem installed in Shanghai in 1989 has met the difficulty inrejecting heat in cooling seasons [20].

On the other hand, cooling of commercial buildings inBeijing consume about 3050% of their total electricityconsumption, and the cooling load mostly occurs duringthe peak period, which results in high peak demand of elec-tricity. Ice storage is a technology where cooling energy isproduced and stored in a insulated storage tank at night,and then is extracted for cooling during on-peak period.Therefore, the ice storage technology is an alternative strat-egy to reduce the high peak-load demand for space cooling.

There are a number of research works reported on thetheoretical analysis and experimental study on ice storagesystem. Shin et al. [21] studied the water spray evaporationmethod for ice particle production with theoretical andexperimental approach. Eames and Adref [22] presented

an experimental study aimed at the characteristics of the

freezing and melting processes for water contained inspherical elements. The water solidification phenomenonfor ice-on-coil has been simulated by Soltan and Ardehali[23] and the amount of time needed for solidification ofwater around a circular cross-section coil has been deter-mined. Although there are a few applications of ice storagesystems [24], the larger volume and higher initial cost of ice

storage devices have restricted its wide applications.Therefore, how to overcome the disadvantages of the

GCHP and ice storage will be a challenge. For the purposeof balancing the on and off-peak electric load and utilizingthe renewable geothermal energy, a new integrated systemwith cooling storage in soil and ground-coupled heat pumpsystem is presented in this paper. Although there has beenresearch on cooling thermal energy storage in soil, these areseasonal energy storage [2527], and the system with cool-ing storage in soil under short time cycles and integratedwith a GCHP system aimed to cut the peak-load and utilizethe geothermal energy has not been reported.

In this paper, a mathematical model described thecharging and discharging processes of the integrated sys-tem is established, and the validation and parameter stud-ies are performed.

2. Configuration and principle of the integrated system

Fig. 1 shows a new integrated system with cooling stor-age and ground-coupled heat pump system. The system iscomposed of a duplex-mode heat pump unit, coolingtower, underground pipes, pumps. In the system, the bur-ied pipes serve as the thermal storage devices and geother-mal heat exchangers simultaneously, and in different

operational mode the different pipe distance is required.

Nomenclature

c specific heat (J kg1 C1)D diameter (m)fs solid phase factor (%)

H latent heat (J kg

1

)j flag, for inner pipe j= 0 and for outer pipe j= 1L leg distance of U-tube (m)Q cooling energy (J)r radial coordinate (m)rp half a borehole distance (m)T temperature (C)t time (s)u velocity (m s1)W moisture content by weight (%)x axis coordinate (m)

Greek letters

a convective heat transfer coefficient (W m2C1)

q density (kg m3)

k thermal conductivity (W m1 C1)h azimuth

Subscriptsei innereo outerf fluidfr frozenfi inletm mushyp pipes soilo initialufr unfrozend drydis discharged

ch charged

Y. Yu et al. / Applied Thermal Engineering 28 (2008) 14501462 1451


3/13

In summer, the cooling energy is stored in the moist soilduring the nighttime, during the daytime the coolingenergy is extracted for space cooling, so the buried pipesrequire a small pipe distance to reduce the cooling loss.While in other seasons, the system operates on conven-tional mode of GCHP, so a more larger pipe distance isrequired to ensure the comeback of soil energy.

According to the load of buildings and the operationstrategy of the system, the integrated system can operatein the following modes:

1. Cooling charging and discharging mode. In summer, theheat pump unit operates on low-temperature conditionand stores the cooling energy in the moist soil partiallyor fully during the off-peak period. During the daytime,the unit stops running and the cooling energy stored insoil is discharged to satisfy the cooling requirement ofbuildings. The flow diagrams of this mode are shownin Fig. 2. In this mode, the power consumption duringon-peak period is cut down significantly and the running

cost of the system is decreased.2. Conventional mode of the GCHP system. In winter and

transitional seasons, the integrated system operates inheating and/or cooling mode of the conventional GCHPsystem, the soil takes as the heat source or sink of theheat pump system. The schematic diagram of this modeis shown in Fig. 3.

3. Mathematical model of charging and discharging

processes

In the charging and discharging processes of the inte-

grated system, the heat transfer of soil around the pipes

is a cyclic phase change process with freezing and unfreez-ing. In the charging period, the low-temperature fluid(usually 4 to 6 C) is pumped to the pipes and storescooling energy in the moist soil, and the soil temperaturearound pipes drops and then begins to gradually freeze.While in discharging period, the returned warm fluid ofload side enters the pipes and extracts the cooling storedin soil, as a result the frozen soil absorbs the heat andbegins to unfreeze. In cooling season, the freezing andunfreezing of soil is cyclic with the charging and discharg-

ing processes.

1

2

3

4

12

8

5

9 10

7

11

6

Fig. 1. Schematic of the integrated system. (1) compressor, (2) condenser,(3) throttling device, (4) evaporator, (5) closed circuit cooling tower, (6, 8,12) pump, (7) load, (9, 10) header and (11) underground heat exchangers.

Open

Closed

1

2

34

12

8

5

9 10

7

11

6

2

3

4

12

8

5

9 10

7

11

6

1

Open

Closed

(a) Flow diagram of charging

(b) Flow diagram of discharging

Fig. 2. Flow diagram of cooling thermal charging and discharging of soil.

1452 Y. Yu et al. / Applied Thermal Engineering 28 (2008) 14501462


4/13

In the charging and discharging periods of the inte-grated system, cooling losses always exists, and the lossesis mainly due to the heat transfer between the outer bound-ary of pipes and the undisturbed soil. Therefore, to reducecooling losses, the duplex-functional underground heatexchangers should be arranged in bundles and with equalborehole distance. The schematic diagram of pipe bundlesand the single inner and outer pipe are shown in Figs. 4 and5, respectively.

The mathematical model of charging and dischargingprocesses is based on the energy balance subject to the fol-

lowing assumptions:

(1) The moist soil is homogeneous.(2) The effect of coupled heat and moisture transfer dur-

ing freezing and unfreezing is ignored.(3) The phase change of soil water occurs in a small tem-

perature range, and the thermal properties in frozenand unfrozen soil regions are constant but not neces-sarily equal.

(4) Using the equivalent diameter method to deal withthe U-tube, and the equivalent diameter of the U-tube is Deo

ffiffiffiffiffiffiffiffiffiffiffiffiffiffi2Do L

p, where Do and L are outer

diameter and leg distance of U-tube, respectively,

[11].(5) The effect of surface temperature is ignored, and the

boundary between two pipes is adiabatic. For innerpipes, the problem is axially symmetric and only theradial direction is considered. While for outer pipes,the radial and azimuth direction are consideredtogether.

(6) The temperature and velocity of fluid are uniform atany coil cross-section and the initial moisture contentand temperature in soil at any point are also uniform.

With these assumptions, the following equations can be

derived for the charging and discharging periods of theintegrated system.

(a) Heat flux in the fluid within the pipe and pipe innerwall

oTf

ot u oTf

ox 2kpqcf rei

oTp

or

rrei

; r< rei: 1

(b) Energy balance in the pipe wall

qcpoTp

ot kp

r

o

orroTp

or

j kp

r ooh

1

r

oTp

or

;

2

Open

Closed

(a) Heating mode

(b) Cooling mode

1

2

34

12

8

5

9 10

7

11

6

Open

Closed

1

2

34

12

8

5

9 10

7

11

6

Fig. 3. Flow diagram of GCHP operation mode.

Outer pipes

Inner pipes

Isothermal outer boundary

Fig. 4. Schematic diagram of pipes bundle.



5/13

where j is a flag, for the inner pipes j= 0, and for theouter pipes j= 1.

(c) Frozen region of soil

qcfroTfr

ot kfr

r

o

orroTfr

or

j kfr

r ooh

1

r

oTfr

oh

:

3

(d) Unfrozen region of soil

qcufroTufr

ot kfur

r

o

orroTufr

or

j kufr

r ooh

1

r

oTufr

oh

:

4(e) Mushy region of soil

qcmoTm

ot 1

r

o

orrkm

oTm

or

j 1

r

o

oh

km

r

oTm

oh

qd W H ofs

ot: 5

(f) Boundary conditions At the interface of fluid and

tube inner wall

aTp Tfrrei kp

oTp

or

rrei

: 6

At the interface of soil and tube outer wall

kpoTp

or

rreo

ksoTsorrreo ;

Tpjrreo Tsjrreo :

8


6/13

tube. In order to measure the temperature distributionaround the tube, 13 thermometers with Pt100 (#2#14)were located at three different sections, and in every sectionthe thermometers distributed every 50 mm from center ofthe U-tube, as shown in Fig. 6b.

The experiment began with two-day pre-storage of cool-ing and then operated seven days with the cycle of 12 hcharging, 8 h discharging and 4 h stopping. The fluid waswaterethylene glycol (30% by weight) mixture, and theflow rate during the cooling pre-storage, charging and dis-charging periods were 0.314, 0.311 and 0.388 m3/h, respec-tively. Accordingly, the fluid velocities in the pipe were0.77, 0.76 and 0.95 m/s. The initial temperature of the bur-ied pipe, fluid and soil were uniform and equal to 10 C.

In the experiment, the sand temperature distribution,inlet and outlet temperature of fluid were recorded hourly.The parameters of pipe and sand are shown in Table 1.

The input data of the model are:

(1) the physical properties and geometrical parameters ofthe pipe materials (density, specific heat, thermal con-ductivity, length, diameter and pipe distance);

(2) density, specific heat and thermal conductivity forboth frozen and unfrozen states, and moisture con-tent by weight of soil;

(3) fluid density, specific heat, thermal conductivity, flowrate and inlet temperature.

The outputs of the model are the fluid and coil temper-atures along the coil, the soil temperature distributionaround the coil, the energy exchange between fluid and coiland energy flow from soil to coil or inverse.

Fig. 8 shows the comparison of calculated and experi-mental results on hourly fluid outlet temperature and tem-

(a) Experimental rig

(b) Layout of the temperature sensors

3500 6500 5650 800

50

5

0

50

50

5

0

50

50

5

0

50

50

2#

3#

4#

5#

6#

7#

8#

9#

10#

11#

12#

13#

14#

Fig. 6. Schematic diagram of the experiment rig.



7/13

perature difference between inlet and outlet of pipe fluidduring the pre-storage period. It can be seen that the calcu-lated results are coincident with the test results, with theerror of more about 0.1 to 0.2 C in the outlet temper-ature between the measured and calculated data. Fig. 9shows the deviation of the comparison between the testand the simulated charging cooling energy, the errors arewithin 10% mostly.

Fig. 10 shows the comparison of calculated and experi-mental results for seven days on hourly outlet fluid temper-

ature of pipes. From the figure it can be seen that the modelpredicts the test results very well.

In view of the complicated nature of this problem, it isconcluded that the model can be used to analyze and pre-dict the cyclic charging and discharging performance ofthe integrated system with cooling storage in soil and

ground-coupled heat pump.

5. Parametric studies

The charging and discharging performance of the inte-grated system involves many factors. A parametric studyprovides the information to determine the relative impor-tance of each factor and thus to improve the performanceof the integrated system. The parameters selected for inves-tigation were: (1) tube diameter, (2) charging inlet temper-ature, (3) moisture content of soil, (4) pipe distance, and (5)time of cooling pre-storage.

As a basis for comparison, a base run was completedwith the following parameters and operation conditions:(1) 15 mm nominal diameter high-density polyethylene(HDPE) pipe; (2) 5 C/12 C charging and discharginginlet temperature and 12 C far-field and initial soil temper-atures; (3) 0.5 m pipe (borehole) distance; (4) 50 m pipelength; (5) 1.5 m far-field radius; (6) 0.52/0.38 m3/h charg-ing and discharging volumetric flow rate (Reynolds num-ber is 2400); (7) 18% moisture content by dry soil weightwith kfr = 3.05 W/(m C) in the frozen soil andkufr = 2.18 W/(m C) in the unfrozen soil region; (8) threedays pre-storage and 30 days cyclic operation with 8 h dis-charging, 10 h charging and 6 h stopping.

In order to analyze the influence of each factor pre-sented above on the performance of the integrated system,the simulation of a single inner and outer pipe in the cyclicsteady operation was performed, and the results are asfollows.

5.1. Tube diameter

The tube diameter is an important parameter of the inte-grated system design. In order to analyze the effect of tubediameter on charging and discharging performance, anoperation condition with 20 mm diameter tube and theReynolds number 2400 in charging and discharging is per-formed. Fig. 11 shows the effect of tube diameter on dis-

0.5m

U-tube

Adiabatic

layer

sand

(a) Photos of the buried pipes in sand box

(b) Layout of the buried pipes in sand box

Fig. 7. Schematic diagram of the buried U-tube.

Table 1Properties of the heat exchanger pipe and the sand used in the experiment

Name Unit Value Name Unit Value

Dry density of sand kg m3 1535 Thermal conductivity of sand W m1 C1 1.19Moisture content by weight % 8.61 Specific heat of sand J kg1 C1 900Pipe material HDPE Thermal conductivity of pipe material W m1 C1 0.517Density of pipe material kg m3 953 Specific heat of pipe material J kg1 C1 2302.7Inner/outer diameter mm 12/16 Pipe legs distance mm 32Pipe distance M 0.5 Pipe length m 32.9



8/13

7:00 17:00 7:00 17:00 7:00 17:00 7:00 17:00 7:00 17:00 7:00 17:00 7:00 17:00-6.5-6.0-5.5-5.0-4.5-4.0-3.5-3.0-2.5-2.0-1.5-1.0-0.5

9.0

9.5

10.010.5

11.0

11.5

12.0

12.5

13.0

Temperature(C)

The time of day (h)

Experimental inlet temperature

Experimental outlet temperature

Simulated outlet temperature

Fig. 10. Comparison of fluid outlet temperature between simulation and test during the cyclic operation.

1:00 5:00 9:00 13:00 17:00 21:00 1:00 5:00 9:00 13:00 17:00 21:00

-7.0-6.5-6.0-5.5-5.0-4.5-4.0-3.5-3.0-2.5-2.0

Temperature(C)

Experimental inlet temperature

Experimental outlet temperature

Simulated outlet temperature

1:00 5:00 9:00 13:00 17:00 21:00 1:00 5:00 9:00 13:00 17:00 21:000.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

The time of day (h)

Temperaturedifference(C)

Experimental Results

Simulated Results

Fig. 8. Comparison of the fluid outlet temperature between simulation and test during the pre-storage period.

23:00 2:00 5:00 8:00 11:00 14:00 17:00 20:00 23:00 2:00 5:00 8:00 11:00 14:00 17:00 20:000.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

2.6

2.8

3.0

H

ourlycoolstorage(MJ)

The time of day (h)

Experimental Data

Simulated Data

With 10% Error

Fig. 9. Comparison of the hourly charged cooling energy between simulation and test during the pre-storage period.



9/13

charging outlet temperature with the cyclic steady state. Itcan be seen that the daily average discharging outlet tem-perature of the inner and outer pipe increases 0.5 and

0.25 C with the increasing of the tube diameter, respec-tively. However as can be seen in Fig. 12 the total chargingand discharging energy have increased by 24.2% for theinner pipe, while 17.8% and 25.1% for the outer pipe ascompared with the base run operation.

5.2. Charging inlet temperature

In the charging process, the fluid inlet temperature influ-ences the soil frozen condition around the pipe, the amountof the stored cooling energy in the soil and the dischargingoutlet temperature. Fig. 13 shows that the daily averagepipe discharging outlet temperature has increased by

Fig. 11. Effect of pipe diameter on discharging outlet temperature.

(a) Inner Pipe

(b) Outer Pipe

Fig. 12. Effect of pipe diameter on total charging and discharging energy.

Fig. 13. Effect of charging inlet temperature on discharging outlettemperature.

(a) Inner Pipe

(b) Outer Pipe

Fig. 14. Effect of charging inlet temperature on total charging and

discharging energy.



10/13

1.8 C for the inner pipe and by 0.8 C for the outer pipe,respectively, while the pipe charging inlet temperatureincreased from 5 to 2 C. Moreover, the daily totalcharging and discharging energy has dropped about30.0% and 19.6% for the inner and outer pipe, respectively(Fig. 14). This indicates that the lower charging inlet tem-

perature helps in improving the operation performance ofthe integrated system, while it is not beneficial for the effi-ciency of the chiller. Therefore, under the condition withthe discharging temperature and energy satisfying the airconditioning system, the higher charging inlet temperatureis more economical.

5.3. Pipe (borehole) distance

The buried pipes in the integrated system serve as thethermal storage devices and geothermal heat exchangerssimultaneously, and for different operational modes, therequirements of the pipe distance are different. For example,

in the heating and cooling mode of GCHP system, a largerpipe distance is required, while it is reverse in the case ofcharging and discharging modes. Hence, the pipe distanceis an important parameter for the integrated system.Fig. 15 shows the relationship between the hourly discharg-ing outlet temperature and the pipe distance under the cyclicsteady condition. It can be seen that the hourly dischargingtemperature of the outer pipes has enhanced with increasingthe pipe distance, while for the inner pipe it has not beeninfluenced. Moreover, in the cyclic steady condition, thedaily total charging and discharging energy of the innerpipes keep constant in different pipe space. But for the outer

pipes, the daily total charging energy has increased nearly6.0% and the discharging energy has dropped by 23.3% withthe pipe distance increasing from 0.5 to 1.0 m (Fig. 16).

5.4. Cooling pre-storage time

Before the charging and discharging operation, the ini-tial soil temperature is higher and should be decreased toensure that the pipe discharging outlet temperature satisfiesthe air conditioning system. Therefore, the cooling pre-storage to the soil is necessary.

Fig. 17 shows the influence of cooling pre-storage timeon discharging outlet temperature of the inner and outerpipe in cyclic steady state. It can be seen that the hourlydischarging outlet temperature of the inner and outer pipehave the same results in the condition with one day andthree days pre-storage operation. Fig. 18 shows the effectof cooling pre-storage on the daily total charging and dis-charging cooling energy of the inner and outer pipe, and

Fig. 15. Effect of pipe distance on discharging outlet temperature.

(a) Inner pipe

(b) Outer pipe

Fig. 16. Effect of pipe distance on total charging and discharging energy.

Fig. 17. Effect of cooling pre-storage time on discharging outlet

temperature.



11/13

they have the same conclusions. In other words, when thesystem reaches steady state, the cooling pre-storage timedoes not influence the charging and discharging perfor-mance, but it affects the performance before the system

reaching steady (Fig. 19).

5.5. Moisture content of soil

In the charging and discharging operation mode, the soilis taken as the cooling storage media, so the moisture con-

tent of soil will affect its thermal properties and the system

(a) Inner pipe

(b) Outer pipe

Fig. 19. Relations between daily charging/discharging energy and pre-storage time.

5 10 15 20 25 30 350.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

fr

ufr

Soilconductivity(Wm-1oC-1)

Soil moisture content W(%)

5 10 15 20 25 30 35

1000

1200

1400

1600

1800

2000

2200 c fr

cufr

Soils

pecificheat(Jkg-1oC-1)

Soil moisture content W(%)

Fig. 20. Influence of soil moisture content on soil thermal properties.

(a) Inner pipe

(b) Outer pipe

Fig. 18. Effect of cooling pre-storage on total charging and dischargingenergy.



12/13

performance. The soil thermal properties with differentmoisture content are shown in Fig. 20.

Fig. 21 shows the effect of moisture content of clay onthe hourly discharging outlet temperature of the inner pipein the steady state. It can be seen that the daily average dis-charging temperature of inner pipe decreases from 7.7 to6.9 C and 6.1 C with the soil moisture content increasingfrom 15% to 25% and 35%, respectively. Moreover as canbe seen in Fig. 22, the heat flux per-unit length of pipeincreases linearly with the moisture content of soil, andthe charging and discharging heat flux increases from25.8 to 34.7 W/m and from 32.3 to 43.5 W/m, respectively.

Therefore, the moisture content of soil is very importantparameter and can be done artificially in order to improvethe performance of the integrated system.

6. Conclusion

For the purpose of shifting the on-peak electric powerand utilizing the renewable geothermal energy, a new inte-

grated system with cooling storage in soil and ground-cou-

pled heat pump system is presented. A mathematical modelbased on the theory of heat transfer coupled with phasechange which describes the charging and discharging pro-cess of the integrated system has been established. A com-puter code has been completed to simulate the heat transferprocess with the freezing and unfreezing of soil water.

Moreover, the mathematical model has been validatedby laboratory tests for the charging and discharging cyclicoperation, and the results show that the mathematicalmodel predicts the test results very well. thus the mathe-matical model can be used to analyze the steady state per-formance of the soil charging and discharging process.

This paper presents a new integrated system in which theground coils operate in charging and discharging modes isstudied in detail. While the ground coils operate in differentmodes in a year, therefore, the further study for the wholeyear operational performance, cooling loss, economic anal-ysis and the optimal design of the integrated system will bedone in the near future.

Acknowledgement

This study was supported by the National Natural Sci-ence Foundation of China (Grant No. 50378024).

References

[1] D.A. Ball, R.D. Fischer, D.L. Hodgett, Design methods for ground-source heat pumps, ASHRAE Transactions 89 (2) (1983) 416440.

[2] M. Ramamoorthy, H. Jin, Optimal sizing of hybrid ground sourceheat pump systems that use a cooling pond as a supplemental heat

rejecter a system simulation approach, ASHRAE Transactions 107(1) (2001) 2637.

[3] S.P. Kavanaugh, Field test of vertical ground-coupled heat pump inAlabama, ASHRAE Transactions 98 (2) (1992) 607616.

[4] P.S. Doherty, S.A. Huthaili, S.B. Riffat, N. Abodahab, Groundsource heat pump-description and preliminary results of the eco housesystem, Applied Thermal Engineering 24 (2004) 26272641.

[5] C. Yavuzturk, J.D. Spitler, A transient two-dimensional finite volumemodel for the simulation of vertical U-tube ground heat exchangers,ASHRAE Transactions 105 (2) (1999) 465474.

[6] X.G. Li, J. Zhao, Q. Zhou, Inner heat source model with heat andmoisture transfer in soil around the underground heat exchanger,Applied Thermal Engineering 25 (2005) 15651577.

[7] V.C. Mei, C.J. Emerson, New approach for analysis of ground coildesign for applied heat pump systems, ASHRAE Transactions 91 (2)

(1985) 12161224.[8] Y.H. Bi, L.G. Chen, C. Wu, Ground heat exchanger temperature

distribution analysis and experimental verification, Applied ThermalEngineering 22 (2002) 183189.

[9] C. Yavuzturk, A.D. Chiasson, Performance analysis of U-tube,concentric tube, and standing column well ground heat exchangersusing a system simulation approach, ASHRAE Transactions 108 (1)(2002) 925938.

[10] F.J. Lenarduzzi, H.S. Radhakrishna, C.B.H. Cragg, The importanceof grouting to enhance the performance of earth energy system,ASHRAE Transactions 106 (1) (2000) 424434.

[11] Y. Gu, D.L. ONeal, Modeling the effect of backfills on U-tube groundcoil performance, ASHRAE Transactions 104 (2) (1998) 356362.

[12] M. Parent, A simplified tool for assessing the feasibility of groundsource heat pump projects, ASHRAE Transactions 107 (1) (2001)

120129.

Fig. 21. Effect of soil moisture content on discharging outlet temperature.

Fig. 22. Effect of moisture content on heat flux per-unit length pipe.



13/13

[13] M.A. Bernier, Uncertainty in the design length calculation or verticalground heat exchangers, ASHRAE Transactions 108 (1) (2002) 939943.

[14] H. Esen, M. Inalli, M. Esen, Numerical and experimental analysis ofa horizontal ground-coupled heat pump system, Building andEnvironment 42 (2007) 11261134.

[15] P. Cui, H.X. Yang, Z.H. Fang, Heat transfer analysis of ground heatexchangers with inclined boreholes, Applied Thermal Engineering 26(2006) 11691175.

[16] H. Esen, M. Inalli, M. Esen, Technoeconomic appraisal of a groundsource heat pump system for a heating season in eastern Turkey,Energy Conversion and Management 47 (2006) 12811297.

[17] H. Esen, M. Inalli, M. Esen, A techno-economic comparison ofground-coupled and air-coupled heat pump system for space cooling,Building and Environment 42 (2007) 19551965.

[18] J.W. Lund, D.H. Freeston, World-wide direct uses of geothermalenergy 2000, Geothermics 30 (2001) 2968.

[19] Y.Z. Yu, Heat Pump Theory and Application, Mechanical IndustryPress, Beijing, 1993.

[20] P. Ying, Ground source heat pump in China, Modern Air Condi-tioning (3) (2001) 18.

[21] H.T. Shin, Y.P. Lee, J. Jurng, Spherical-shaped ice particle produc-tion by spraying water in a vacuum chamber, Applied ThermalEngineering 20 (2000) 439454.

[22] I.W. Eames, K.T. Adref, Freezing and melting of water in sphericalenclosures of the type used in thermal (ice) storage systems, AppliedThermal Engineering 22 (2002) 733745.

[23] B.K. Soltan, M.M. Ardehali, Numerical simulation of water solid-ification phenomenon for ice-on-coil thermal energy storage applica-tion, Energy Conversion and Management 44 (2003) 8592.

[24] G.P. Henze, M. Krarti, M.J. Brandemuehl, Guidelines for improvedperformance of ice storage systems, Energy and Buildings 35 (2003)111127.

[25] G. Hellstrom, S.A. Larson, Seasonal thermal energy storage thehydrock concept, Bulletin of Engineering Geology and the Environ-ment 60 (2001) 145156.

[26] H. Ohga, K. Mikoda, Energy performance of borehole thermalenergy storage systems, in: Seventh International IBPSA Conference,Brazil, 2001, pp. 10091016.

[27] F. Kabus, P. Seibt, Aquifer thermal energy storage for the BerlinReichstag building-new seat of the german parliament, in: Proceed-ings World Geothermal Congress 2000, 2000, Japan, pp. 36113615.

[28] Y.S. Yu, Charging and discharging characteristics research on theintegrated system with soil cooling thermal storage and ground-coupled heat pump, Ph.D. Dissertation, Harbin, Harbin Institute ofTechnology, 2004.


Integrated Ground-Source Heat Pump With Cooling Storage

Documents

Transcript of Integrated Ground-Source Heat Pump With Cooling Storage