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A SIMULATION OF AN UNDERGROUND HEAT STORAGE SYSTEMUSING MIDNIGHT ELECTRIC POWER AT PARK DOME KUMAMOTO

Kouji Sakai*, Osamu Ishihara*, Kengo Sasaguchi*,Hiroyuki Baba** and Takamitsu Sato***

* Kumamoto University, 2-39-1, Kurokami, Kumamoto, 860-8555, Japan ** Kyushyu Electric Power Company Inc., 2-1 Watanabe-dori, Fukuoka, 810, Japan *** Fujita Corporation, 4-6-15 Sendagaya, Shibuya-ku, 151, Japan

ABSTRACTThe purpose of this study is to investigate howmuch the peak daytime demand for electricity isreduced by an underground heat storage systemthat uses surplus electricity during the nighttime.In this paper, we report on a numericalsimulation method and the results from anenergy performance simulation of this system atPark Dome Kumamoto, a Kumamoto Prefectureindoor athletic facility. The simulation resultswere in good agreement with the measurements,and that shows that this simulation method isapplicable for the prediction and optimization ofan underground heat storage system for otherconditions.

INTRODUCTIONPark Dome, a Kumamoto prefecture indoorathletic facility, was built in 1997(Figure 1).With a field of about 11,000 square meters, theentire Park Dome is located under a dome witha double pneumatic-film structure. The designutilizes and conserves natural energy by a swirlflow natural ventilation system, undergroundthermal tunnel and an underground heat storageair-conditioning system using reduced-ratenighttime electric power.

An underground heat storage system has beenadopted to reduce power requirements for air-conditioning in lounge area(Figure 2),particularly, during periods of peak daily load.The area of the lounge is 204 square meters.The lounge is located on the western side of theathletic field and the designed open space.

An air-source heat pump is used as the heatsource for this system. The thermal energy madeby heat pump in nighttime, is storedunderground in polyethylene pipes(Figure 3). Indaytime, the thermal energy is extracted fromunderground, and provided for lounge by an air-conditioning unit.

Figure 1. Outside view of Park Dome

Figure 2. Inside view of Lounge

Figure 3. Polyethylene pipes

Ring truss opening

Entrance

PV system

Pivoted doorsLounge

Top-light opening

2

Thermal Tunnel

Body of Heat Storage

Lounge

�������

���

���� ���

Air Conditioning Unit

Heat Pump Unit

Storage Pump

Backup Pump

ExtactPump

Figure 4. An underground heat storage system in Park Dome

150 150150 150

150150

150600

� ������

Lounge

Floor slab

Gravel

SoilPipe

Steel netLEVEL1

LEVEL2

LEVEL3

LEVEL4

Figure 5. Section of the body of heat storage

An Underground Heat Storage System

Figure 4, shows the underground heat storagesystem at Park Dome. The system consists offive units, a heat storage, heat source, air-conditioning, control and measurement unit.The thermal energy is stored in the earththrough polyethylene pipes using midnightelectric power, which is the extracted to be usedas a heat source for air-conditioning.

The earth beneath the athletic fields serves as aheat storage area, where the pipes(with 0.025diameter and a total length of 4,800 meter) areembedded. The pipe depth was determined from60 to 105cm under ground level afterconsidering the availability of execution due tothe presence of base beams. The cooling/heatingwater flows into the pipes diverged through aheader. The storage pipe arrangement consistsof 12 horizontal levels and 4 vertical levels withinterval of 150mm, the length of each pipebeing 100 meter. A section of the pipe systemlayout is shown in Figure 5. The pipe intervalshave been determined after a case study using asimulation code(Ishihara et al., 1997).

In order to store energy in the earth, thecirculating water first flows into the top level

and this flows out through the deepest levelpipes, and reversely if energy should beextracted from the storage system, because thereturn water temperature becomes lower and notvice versa. We also decided to lay sprinklerpipes at the top level in order to supplysufficient water to the soil, so as to preventpossible heat-characteristic modificationscaused by a change in the water content of thesoil during heat storage, and especially to avoidreduction of the thermal conductivity of the soilwhile the heating system is in operation.Furthermore, the fin ability of the pipesimproved by using a steel net.

Heat is generated by this system through a heatpump chilling unit, a circulating pump, and abuck-up pump, in case supplement heat isnecessary or heat is insufficiently stored. Airconditioning on the other hand consists of anAir-handling unit, a duct and a circulating pump.Heat storage, heat extraction and back up, areautomatically set into operation by a timer and abulb control function depending on the watertemperature.

MEASUREMENT

In order to improve and optimize theeffectiveness of the earth heat storage system,measurements were conducted at 150 differentpoints of the system and the data obtained werethen stored in a MO-disk of a personal computerin a 5 minute interval. The measurementsconcerned soil temperature, soil moisturecontent, water flow, water temperature, airflowvolume in the duct and air temperature. Fromthat stage on, heat storage/extraction and thesystem’s overall efficiency were estimatedaccording to the measurement results.

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The measurements were made during thesummer season in 1998. During themeasurement, air-conditioning unit is operatedby a timer from 10:00 to 18:00, the heat pumpunit is operated by a timer from 22:00 to 8:00.

SIMULATIONFor the purpose of general improvement of anunderground heat storage system, we developedthe simulation code based on finite differentialmethod. In the thermal conductivity analysis inburying the heat transfer pipe in the soil, thereare many cases in which solid part and flow inthe pipe are solved in the separate equation. Inthis method, the contact between pipe and soilmust be beforehand defined.

In the meantime, the heat balance equation ofthe flow in the pipe is able to express theconvection-diffusion equation. Then, weadopted the convection-diffusion equation forsolving underground heat storage as shown inTable 1.

Control volume method (Patanker, 1980) is usedin the discretion, and it defines temperaturemesh center. The difference scheme on the timeterm adopted on perfect implicit method,convective term on up-wind scheme, diffusionterm on the central scheme. Figure 6 shows themodel of underground heat storage part. Theheat storage part. The heat storage part isparallelepiped of 2.665m width, 2.25m depth,100m length. The heat storage part is beingdivided into 93�44�100 in unequal intervalorthogonal mesh. The physical property for thesimulation is shown at Table 2. The flowchart isshown in Figure 7.

The heat pump chilling unit is modified, fromthe measurement results, by adding temperaturegradient of 5.6 deg.C in compressor of 2operations and 2.6 deg.C in the 1 operationnight. The change of the water temperature inheat storage in the chiller unit is sending it outto the soil. The air conditioning load in thedaytime is being calculated according to theequation in Table 1 which made ambienttemperature and time (24 hours are controlled)to be a variable in order to reproduce the airconditioner conversive heat quantity by theobservation.

RESULTSThe measurement result of the temperature inthe heat storage part is shown in Figure 8, andthe calculation result is shown in Figure 9.

100,560mm

2,250mm

2,329mm

�

��

Storage pipe �������� ����

������� ���

Figure 6. Simulation model

START

body of heat storage configuration

pipe configuration

time

initial condition

Set on velocity vector

22 - 8:00(storage mode)

8 - 10:00/18 –22:00 10 - 18:00(extract mode)

Pipe Water velocity1,500[m/h]

Pipe water velocity0[m/h]

Pipe Water velocity1,375[m/h]

Water, soil Temperature[SOR]

Calculation period?

END

Figure 7. Flow chart

Table 1. Basic equation

∂θ∂

∂∂=

∂θ∂

+∂θ∂

jjj

j

xk

xx

u

t(1)

where k is; solid part : λ/cρ, water part : 4.08.0 PrRe023.0=Nu Heat pump: 1 compressor drive, temp. gradient of 2.6 deg.C 2 compressor drive, temp. gradient of 5.6 deg.C Air conditionning load:[W] (by measurment) =-2343*t+1938*θo+9665

Table 2. Thermal constant of materialsHeat conductivity

λ[W/m � C]Thermal capacity

Cρ[kJ/m3� C]

Soil 0.64 2302.3Gravel 0.62 2344.2Concrete 1.35 1929.8

Wire netting 45.00 3759.0Heat storage pipe 0.37 ��

Water �� 4173.4Insulation 0.06

4

8

10

12

14

16

18

20

22

24

9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 1 2 3

So

il te

mp

erat

ure

[deg

.C]

GL -35cm GL -60cm GL -75cmGL -90cm GL -105cm GL -135cm

Figure 8. Fluctuation of soil temperature (Measurement, 1998/7/9 – 8/4)

8

10

12

14

16

18

20

22

24

9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 1 2 3

So

il te

mp

erat

ure

[deg

.C]

GL -35cm GL -60cm GL -75cmGL -90cm GL -105cm GL -135cm

Figure 9. Fluctuation of soil temperature (Simulation, 1998/7/9 – 8/4)

8

10

12

14

16

18

20

22

24

9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 1 2 3

Wat

er te

mp

erat

ure

[deg

.C]

measurement

calculation

Figure 10. Fluctuation of outlet water temperature in heat storage/extract pipe

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� � � � �� �� �� �� � � �� �� �� �� �� �� �� [deg.C]

Figure 11. Soil temperature distribution (heat storage and extract process)

From 1998/7/9 to 7/17 we carried out thestorage operation only, and after 7/18 we carriedout both the storage and extract operation. Onthe closure day of 7/21 and 28 there is storageoperation only.

From Figure 8 and 9, it appears that there isgood correlation between the measured andcalculated values of inside soil temperatures.The results of the simulation of the soiltemperature at GL-105cm are actually lowerthan the measurement; however, there is closecorrelation of the trend of the temperaturereduction in the precooling stage, the fluctuation

on the closure day, and lower limit of soiltemperature.

The fluctuation of outlet temperature of the heatstorage pipe is shown in Figure 10. At the airconditioning period, the return watertemperature (the inlet temp. of air conditioner)from the soil, it is about 2 deg.C larger thanmeasurement. The other period, the differenceof temperature has positioned within 1 deg.C.

Figure 11 shows, simulation result of the soiltemperature at the heat storage part centralevery 2 hours in July 24th 22:00 � 25th 20:00.In the night, soil temperature around the pipes

22:00 1998/7/24 0:00 1998/7/25

4:00 1998/7/25 6:00 1998/7/25

10:00 1998/7/25 12:00 1998/7/25

16:00 1998/7/25 18:00 1998/7/25 20:00 1998/7/25

2:00 1998/7/25

14:00 1998/7/25

8:00 1998/7/25

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becomes gradually lower. Soil temperaturedistribution in which the heat storage ends at8:00, minimum becomes 10 deg.C.

The soil temperature is kept low until 10:00when the extraction of heat begins. In theextraction of heat from 10:00 to 18:00, the pipecirculating water flows from the fourth level,and it escapes from the first level. Therefore, thetemperature gradually rises between first leveland second level. It is considered from thefluctuation of first temperature distribution, thatonly the pipe on the first level seems to workeffectively for extraction and storage.

OPTIMIZATIONIn this paper, it was proven that simulationresult corresponded to the observation resultwell. Then, using developed program, 16 caseswere calculated in order to clarify therelationship between volume of heat storagebody and air conditioning load.

The capacity of the heat storage body iscorrespondent estimated by calculating ydirection (Figure 6) length of the heat storagebody by changing. And, it also changes thecirculating water flow volume. The pipeposition lying in the ground and spacing of thepipes were identical to Figure 5. The analysiscondition were made to be the pre-coolingperiod until the heat pump was stopped, and itdoes 1 week storage and extract afterwards.

The period of pre-cooling is shown in Figure 12.The X-axis shows the length of pipe and area ofheat storage part and the Y-axis shows air-conditioning load and flow rare. It can beobserved from the figure that the optimum rangeof this system is the case in which theprecooling period is the 4 - 7 day.

CONCLUSIONSIn this paper, we reported on the outline of theheat storage system and simulated thermalperformance of heat storage part. The resultsobtained to this stage are as follows:

The measured value and calculated valuecorrespond well to the variation of thetemperature of the soil inside.

The simulation results prove that this systemcan be very effectively applied in realarchitectural projects and contribute inalleviating cooling loads in Kumamoto.

0

20

40

60

80

100

120

140

160

180

0 25 50 75 100 125

1day

2days

4days

7days

over 8days

Ishihara et al.(1993):7days

Park dome: 7days

Length[m]

Area [m2]

Air-

con.

load

[kW

h/d

ay]

Wat

er f

low

vo

l.[l/

min

]

0 50 100 150 200 250

100

0

200

300

Figure 12. Optimum range of this system

ACKNOWLEDGEMENTSWe express sincere gratitude to Mr. Kanazawaand Mr. Nakahara of the students in IshiharaLaboratory, Kumamoto University for theirextensive assistance offered for themeasurements in this study.

REFERENCESIshihara, O., Q. Zhang et al., “Development of theEarth Storage Heat-Pomp System”, The 5thInternational Energy Conference, 18-22 October,Seoul Korea, pp.360-370, 1993.

Ishihara, O., K. Sakai, et al., “Development of anUnderground Heat Storage System Using MidnightElectric Power, Part.1”, The 7th InternationalConference on Thermal Energy Storage, 1997.

Patanker, S.V. “Numerical Heat Transfer and FluidFlows”, McGraw-Hill Book Company, 1980.

NOMENCLATURE

u: water velocity of pipe[m/h]

θ: solid/water temperature[deg.C]

λ: Heat conductivity[W/mdeg.C]

Cρ:Thermal capacity[kJ/m3deg.C]

αc: coefficient of heat transfer rate by

convection[W/m2]

Nu: Nusselt number

Re: Reynolds number

Pr:Prandtl number