variation in cooling load of a moving air-conditioned train compartment under the effects of ambient...

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Applied Thermal Engineering 31 (2011) 1150e1162

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Applied Thermal Engineering

journal homepage: www.elsevier .com/locate/apthermeng

Variation in cooling load of a moving air-conditioned train compartment underthe effects of ambient conditions and body thermal storage

Weiwei Liu a,*, Qihong Deng a, Wenjie Huang b, Rui Liu b

a School of Energy Science & Engineering, Central South University, 932 South Road Yueshang, Changsha, Hunan 410083, Chinab Institute of City Railway Vehicle Research, Nanjing Puzhen Vehicle Factory, 5 Street Longhu, District Pukou, Nanjing 210031, China

a r t i c l e i n f o

Article history:Received 11 August 2010Accepted 2 December 2010Available online 15 December 2010

Keywords:Air-conditioningDynamic cooling loadTrain compartmentEnergy-saving

* Corresponding author. Tel.: þ86 731 88877175.E-mail address: [email protected] (W. Liu).

1359-4311/$ e see front matter � 2010 Elsevier Ltd.doi:10.1016/j.applthermaleng.2010.12.010

a b s t r a c t

A mathematic model was built to simulate dynamic cooling load of an air-conditioned train compart-ment. Using the model, the dynamic cooling loads of YZ25G train compartment were investigated underthe average ambient conditions during the hottest month July, when it travels in three main railway linesof China. The effects of ambient conditions and body thermal storage on the variation in the cooling loadwere discussed. The results indicated that the maximum total cooling loads were between 40.4 and43.8 kW, and the minimum between 4.5 and 33.7 kW. And significant differences in dynamic coolingload of train compartment were found between different regions (north/south and west/east) andbetween different periods of time (morning/night/afternoon). Compared with the dynamic cooling load,the steady cooling load calculated according to the national standard (TB1951-87) had a larger value. Thismeans TB1951-87 is likely to overestimate the actual cooling load of a train, which can lead to a waste ofenergy. The calculation of dynamic cooling load can provide an adequate basis to determine the coolingload for trains traveling in different regions and periods of time.

� 2010 Elsevier Ltd. All rights reserved.

1. Introduction

According to China Ministry of Railways (MOR), the railwaypassenger capacity reached 1525 million person-times in 2009 [1].Without doubt, trains become the most important means of longdistance transportation in China and are attached more and moreattention.

In most train compartments, air-conditioning systems areinstalled to provide a thermally-comfortable environment. However,energy consumption of the air-conditioned train compartments islarge [2], which is proving an increasing problem. Almost 70% of thetotal energy is consumedbyair-conditioningunits (ACU) ina train [3].At the same time, rising comfort expectations show the traincompartments in aworsening light [4], in view of large fluctuation inindoor air temperature experienced, due to the change of ambientconditions during a long distance travel. Therefore, serious energyconsumption and thermal discomfort in the air-conditioned traincompartment are becoming main challenges to the future develop-ment of railway transport in China.

The cooling load has a significant effect on the energy consump-tionofACUandthefluctuationof indoorair temperaturewhena train

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is moving. Up to now, the cooling load of a train compartment iscalculated as a steady value under the specific ambient conditionaccording to ChinaMOR standard (TB1951-87) [5], which is the basefor thedesignof theACU in train. However, the fact is that the coolingload of an air-conditioned train compartment is unsteady in theprocess moving from place to place because the ambient tempera-ture andhumiditychange evidentlywith spaceand time.Apparently,the result of TB1951-87 cannot reflect the variation in the coolingload fora train ina travel. It isnecessary toobtain thedynamic coolingload of a train compartment, which can provide an important basisfor better control of indoor air temperature and exploitation of thepotential for saving energy.

However, it is almost impossible to directly measure thedynamic cooling load when a train is running. An effective methodis to simulate the dynamic cooling load of a train by developing anappropriate model. Regretfully, in present almost no relativestudies were found in international journals, except the steadycooling load model of an underground railway carriage at constantambient condition [6,7]. Usually body heat storage is also a reasonto the variation in cooling load of a train. Therefore, this paperproposed a mathematical model to simulate the unsteady heattransfer process in a typical air-conditioned train compartmentunder variable ambient conditions, considering the thermal storageof train body. Using the model, the variation in cooling load of the

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Nomenclature

a absorptance of body surface for solar radiationAb internal surface area of train body (m2)Aw windows area (m2)c specific heat (kJ/kg K)ce transfer coefficient of the equipment cooling loadco transfer coefficient for occupancy cooling loadcr transfer coefficient for radiation cooling loadhai indoor air enthalpy (kJ/kg)hao outdoor air enthalpy (kJ/kg)J solar radiation (W/m2)k thermal conductivity (W/m K)mv mass flow rate of ventilation air (kg/s)MQ mean cooling load (kW)n total number of passengers/railway stationsn0 aggregation coefficientq1 latent heat emission per person (kW)qs sensible heat emission per person (kW)Qi cooling load at a railway station i (kW)QCðsÞ conduction dynamic cooling load (kW)QCbðsÞ cooling load due to heat conduction through train

body (kW)QCwðsÞ cooling load due to window conduction (kW)Qe total heat emission of equipments (kW)QEðsÞ equipment dynamic cooling load (kW)QOðsÞ occupancy dynamic cooling load (kW)QRðsÞ radiation dynamic cooling load (kW)QV ðsÞ ventilation dynamic cooling load (kW)QTotalðsÞ total dynamic cooling load (kW)tai indoor air temperature (�C)tao outdoor air temperature (�C)te solareair temperature (�C)twi internal surface temperature (�C)T temperature in train compartment body (K)U total heat transfer coefficient for windows (W/m2 K)V train velocity (km/h)VQ variable part of dynamic cooling load (kW)

DRH outdooreindoor relative humidity differenceDt outdooreindoor temperature difference (�C)x, y distances along coordinate axes (m)

Greek lettersai internal surface heat transfer coefficient (W/m2 K)ao external surface heat transfer coefficient (W/m2 K)b transmittance of the window glassr density of train body (kg/m3)s time (s)Dsi time distance between station i and its next station

i þ 1 (s)s shading coefficient of window curtain

Subscriptsai indoor airao outdoor airb train bodyC heat conductionCb heat conduction through train bodyCw heat conduction through train windowE equipmente equipment/combination of solareairi internal surface/the ith railway stationl latent heato external surfaceO occupancyR radiations sensible heatTotal total numberV ventilationw windowwi internal surface of wall

AbbreviationsACU air-conditioning unitCFD computational fluid dynamicsMOR Ministry of Railways

W. Liu et al. / Applied Thermal Engineering 31 (2011) 1150e1162 1151

train compartment is investigated during its long distance travel inChina. Results of the paper can be useful in designing andcontrolling ACU in the train compartment for energy conservationand better thermal comfort.

2. YZ25G type train compartment

Currently, YZ25G train compartment is the main type running inChina. The construction of YZ25G semi-cushioned seat traincompartment (3.12 � 3.29 � 25.5 m) is shown in Fig. 1. Two ACUs(29 kW per unit) were separately installed on each side of thecompartment top.

3. Mathematical model

Amodel was developed to calculate the dynamic cooling load ofan air-conditioned train compartment during its travel. Usually thefollowing cooling loads need to be considered [6e8]:

� Conduction ðQCðsÞÞ� Radiation ðQRðsÞÞ� Ventilation ðQV ðsÞÞ

� Occupancy ðQOðsÞÞ� Equipment ðQEðsÞÞ

The treatment of these loads is shown below.

3.1. Conduction through the train compartment

The train compartment includes two parts: body (roof/floor/walls) and windows. The heat conduction through body andwindows were calculated respectively.

3.1.1. Body conductionThe two-dimension unsteady heat conduction equation was

used to describe the heat transfer processes of the train compart-ment body, expressed as follows [9]:

rcvTvs

¼ v

vx

�kvTvx

�þ v

vy

�kvTvy

�(1)

where r is the density of bodymaterial, c is the specific heat, k is thethermal conductivity, s is time and T is the temperature in the traincompartment body.

Fig. 1. Construction of YZ25G train compartment in China. Numbers in parentheses arethe inner/exterior areas. (a) Photo of YZ25G train compartment, (b) Body of YZ25G traincompartment.

W. Liu et al. / Applied Thermal Engineering 31 (2011) 1150e11621152

Based on equation (1), the cooling load due to the heat con-duction through train body (QCb) was calculated as,

QCbðsÞ ¼ aiAbðtwi � taiÞ (2)

where ai is the internal surface heat transfer coefficient, Ab is theinternal surface area of train body, twi is the internal surfacetemperature and tai is the indoor air temperature.

3.1.2. Window conductionThe thermal storage of the windows was neglected and the

cooling load due to window conduction (QCw) can be simply esti-mated using equation (3).

QCwðsÞ ¼ UAwðtao � taiÞ (3)

where U is the total heat transfer coefficient for windows, Aw is thearea of windows and tao is the outdoor air temperature.

3.1.3. Total conduction cooling loadThe total cooling load due to the heat conductionwas the summa-

tion of body and window conduction, illustrated as equation (4).

QCðsÞ ¼ QCbðsÞ þ QCwðsÞ (4)

3.2. Radiation into the train compartment

The train compartment absorbs part of the sunlight enteringthrough the windows and then heats the inside air via convectionheat transfer. The cooling load from the radiation into the traincompartment was estimated using [10]:

QRðsÞ ¼ scrbJ (5)

where s is the shading coefficient for the window curtain, cr is thetransfer coefficient for the radiation cooling load, b is the trans-mittance of the window glass and J is the solar radiation.

3.3. Ventilation

Ventilation air (outdoor air) is cooled down by the ACUs beforesupplied into the train compartment. The total cooling load (sensibleand latent) produced by the ventilation air is determined as:

QV ðsÞ ¼ mvðhao � haiÞ (6)

wheremV is the mass flow rate of ventilation air, hao and hai are therespective outdoor and indoor air enthalpies.

3.4. Occupancy

The occupancy load includes the sensible and latent heatemitted from passengers, which was calculated according toequation (7).

QOðsÞ ¼ coðqs þ qlÞnn0 (7)

where co is the transfer coefficient for the occupancy cooling load,qs and ql are the respective sensible and latent heat emission perperson, n is the total number of passengers and n0 is the coefficientof aggregation. Usually the change of the occupancy load is verysmall during a travel, so it can be regarded as steady.

3.5. Equipment

The equipments in YZ25G train compartment mainly includefluorescent luminaries, electronic water heaters and other electricalappliances. The cooling load due to the heat emission of equip-ments was given as,

QEðsÞ ¼ ceQe (8)

where ce is the transfer coefficient of the equipment cooling loadand Qe is the total heat emission of the equipments.

Like the occupancy load, the equipment load almost keepsunchanged.

3.6. Total dynamic cooling load

Based on the calculation of each cooling load as depicted before,the total dynamic cooling load can be obtained using equation (9).

QTotalðsÞ ¼ QCðsÞ þ QRðsÞ þ QV ðsÞ þ QOðsÞ þ QEðsÞ (9)

4. Calculation conditions

The simulation of dynamic cooling load was carried on YZ25Gtrain compartment. The calculation conditionswere listed as below.

4.1. Main railway lines

Three main railway lines in south of Beijing were selected,which are shown in Fig. 2.

Fig. 2. Railways in China. *Wuchang is a part of Wuhan.


Line I is the most important railway in northesouth direction,which connects two metropolis Beijing and Guangzhou. II and IIIare main lines in eastewest direction, which are in north and southof the Changjiang River, respectively. All the three lines are longdistance (about 2000 km) railways through several provinces onthe way.

The information on the three lines is listed in Table 1.

4.2. Ambient conditions

The ambient conditions at each railway station in the selectedrailway lines are given in Table 2, including ambient temperature,humidity, enthalpy and solar radiation intensity. The meteorolog-ical stations are in the same area as the corresponding railwaystations. All the meteorological data were the average of measuredvalues in the hottest month July during 1973e2003 [11].

4.3. Indoor conditions

According to TB1951-87 [5], a ventilation (outdoor air) rate of25m3/hper person in summerwas assumed. The conditionof the airin the train compartment was given as 26 �C, 55% and 56 kJ/kg d.a.

4.4. Body conduction calculation

4.4.1. Solareair temperatureIn a travel, the compartment body is heated by outside air and

sun radiation together, resulting in the body conduction load. In

order to consider the combined effects of air temperature and sunradiation, solareair temperaturewas used as given in equation (10).

te ¼ tao þ aJ=ao (10)

where te is the solareair temperature, a is the absorptance of bodysurface for solar radiation and ao is the external surface heattransfer coefficient.

The external surface heat transfer coefficient was assumedmainly convective and determined using [5]:

ao ¼ 9þ 3:5V0:66 ðV > 0Þ (11)

where V is the train running velocity, which was gotten by dividingthe total line distance by the total running time as given in Table 1.While the train stops (V ¼ 0), the external surface heat transfercoefficient was recommended as 16 W/m2 K [5].

For fuscous train body surface covered with dust, the absorp-tance for sun radiation used was 0.7 [5].

Table 3 shows the solareair temperature at each station for thethree lines.

4.4.2. Body materials and boundary conditionsAs illustrated in Fig. 3, the body (roof/floor/walls) is composed of

three kinds of materials. The material properties [12] and itsthickness for each layer are listed in Table 4.

Convection thermal boundary conditionwas applied to the trainbody. The internal surface heat transfer coefficient was suggestedas 8 W/m2 K [5].

When the train is moving, ambient temperature and sun radi-ation change with space and time that, in turn, induces the variety

Table

1Inform

ationon

threemainrailw

aylin

esin

China.

Line

Station&time

12

34

56

78

910

1112

1314

I(229

4km

)T1

5Station

Beijing

Zhen

gzhou

Wuch

ang

Chan

gsha

Guan

gzhou

N.eS.

A.T

.11

:00

16:34

21:10

0:29

7:35

R.T

.0

5.6

10.2

13.5

20.6

T16

Station

Guan

gzhou

Chan

gsha

Wuch

ang

Zhen

gzhou

Beijing

S.eN.

A.T

.16

:48

23:44

3:06

7:46

13:18

R.T

.0

6.9

10.3

15.0

20.5

II(175

9km

)K13

51Station

Lian

yunga

ng

Xuzh

ouSh

angq

iuLanka

oZh

engz

hou

Sanmen

xia

Xi’a

nBao

jiTian

shui

Longx

iLanzh

ouE.eW

.A.T

.10

:36

13:30

15:54

17:21

19:05

22:41

2:07

4:28

7:37

9:55

12:48

R.T

.0.0

2.9

5.3

6.8

8.5

12.1

15.5

17.9

21.0

23.3

26.2

K13

52Station

Lanzh

ouGan

guTian

shui

Bao

jiW

einan

Sanmen

xia

Luoy

ang

Zhen

gzhou

Shan

gqiu

Xuzh

ouDon

ghai

Lian

yunga

ng

W.eE.

A.T

.1:26

4:52

6:07

8:53

12:11

14:38

16:41

18:36

23:54

2:20

4:48

6:06

R.T

.0.0

3.5

4.7

7.5

10.8

13.2

15.3

17.2

22.4

24.9

27.4

28.7

III(266

0km

)K79

Station

Shan

ghai

Han

gzhou

Quzh

ouYingtan

Zhan

gshu

Yichun

Zhuzh

ouLo

udi

Huaihua

Yupin

Kaili

Ansh

un

Qujing

Kunming

E.eW

.A.T

.19

:10

21:05

0:17

3:15

5:13

6:54

8:39

10:44

14:52

16:39

18:38

22:35

6:13

8:17

R.T

.0.0

1.9

5.1

8.1

10.0

11.7

13.5

15.5

19.7

21.5

23.4

27.4

35.0

37.1

K80

Station

Kunming

Qujing

Xuan

wei

Ansh

un

Kaili

Huaihua

Loudi

Zhuzh

ouYichun

Zhan

gshu

Yingtan

Quzh

ouHan

gzhou

Shan

ghai

W.eE.

A.T

.16

:25

18:15

20:29

1:50

5:59

9:44

13:58

15:48

17:35

19:24

21:06

23:54

2:49

4:54

R.T

.0.0

1.9

4.1

9.4

13.6

17.3

21.6

23.4

25.2

27.0

28.7

31.5

34.4

36.5

N.m

eansNorth,S

.mea

nsSo

uth,E

.mea

nsEa

st,W

.mea

nsW

est,A.T

.mea

nsAbs

olute

arrivingtimean

dR.T

.mea

nsRelativearrivingtime(h).


of the solareair temperature as shown in Table 3. In the presentsimulation, it was supposed that the change of the solareairtemperature between two railway stations was linear.

4.4.3. Grid and solverAs depicted in Fig. 1b, a two-dimension physical model with

structured mesh was created for train compartment with the pre-processor Gambit, version 2.0.4 [13]. By the test on grid-indepen-dence, mesh sizes of 1 �1 mm for the steel layer and 2 � 2 mm forthe other layers were used.

Based on the physical model of the train compartment body, thetwo-dimension unsteady heat conduction equation was solvedusing the CFD software Fluent 6.0 [14]. By comparison (1/15/30/60 s), a time step of 30 s was chose considering both precision andtime consuming.

4.4.4. Initial conditionsIt was assumed that the heat conduction through the immobile

train compartment (V¼ 0) reached steady at the start station in therailway line. Therefore, the corresponding steady temperaturedistribution in the body was applied as the initial condition for theunsteady simulation when the train was running.

4.5. Other calculation parameters

When evaluating the occupancy load, it was assumed that 118people (all seats taken) could be present in the train compartment.The values of sensible and latent heat emission per person at theindoor air temperature 26 �C were suggested as 69.8 and 46.5 Wrespectively, with a total of 116.3 W [5].

For the calculation of the equipment load, the total heat emis-sion of 3 kW was supposed in the train compartment.

The other coefficients for the calculation of the radiation,occupancy and equipment loads used in this simulation are listed inTable 5 [1,10,12,15].

5. Results

5.1. Variation in each part cooling load

The dynamic cooling loads in the three lines are separatelyshown in Figs. 3e5. It can be seen that the conduction, radiationand ventilation cooling loads variedwith the stations, which lead tothe change of the total cooling load, though the occupancy andequipment cooling loads kept constant due to the steady indoor airtemperature.

For the conduction cooling load, its value was in the range from�4 to 4 kW. The minus values often appeared during night,meaning that the heat conducted from indoor to outdoor becauseof the lower ambient temperature. In afternoon, the higher ambienttemperature led to a peak value of the conduction cooling load.

The radiation cooling load changed between 0 and 2 kW. Themaximum of the radiation cooling load often occurred in afternoondue to the high solar radiation. However, at night there was almostno sunlight entering the train compartment to produce coolingload.

Compared with the conduction and radiation cooling loads, thevariation in the ventilation cooling load was larger. Especially inlines II and III (eastewest), the maximal change could reach 32 kW(from �7 to 25 kW). It can be found that the ventilation coolingload when the train moving in the west cities was much smallerthan that in the east cities, because of the lower ambient temper-ature and humidity in the west of China.

Table 2Ambient conditions of each station at arriving time in three main railway lines.

No. Station T (�C) RH (%) h (kJ/kg) SRI (W/m2) Station T (�C) RH (%) h (kJ/kg) SRI (W/m2)

Line I T15 East West Roof Floor T16 East West Roof Floor

1 Beijing 28.3 69.6 71.9 98.0 97.8 580.5 145.1 Guangzhou 30.2 75.2 83.6 30.3 157.3 91.4 22.92 Zhengzhou 30.5 64.7 76.6 99.5 285.2 312.9 78.2 Changsha 26.7 88.4 77.8 0 0 0 03 Wuchang 29.4 77.4 81.5 0 0 0 0 Wuchang 27.3 81.5 75.4 0 0 0 04 Changsha 26.6 88.9 77.7 0 0 0 0 Zhengzhou 25.1 85.1 70.3 158.6 70.3 197 49.35 Guangzhou 28.4 84.9 82.6 196.1 99.0 279.9 70.0 Beijing 29.9 65.8 74.8 84.5 280.2 478.3 119.6

Line II K1351 North South Roof Floor K1352 North South Roof Floor

1 Lianyungang 28.1 77.3 76.7 176.0 214.5 512.4 128.1 Lanzhou 18.8 72.1 48.6 0 0 0 02 Xuzhou 31.0 61.7 76.3 142.2 162.0 411.5 102.9 Gangu 19.0 81.2 52.1 0 0 0 03 Shangqiu 31.1 66.1 79.8 61.3 75.5 346.4 86.6 Tianshui 19.7 79.7 53.7 30.7 30.7 141.0 35.24 Lankao 30.4 67.4 78.0 33.0 97.2 173.7 43.4 Baoji 24.9 78.4 66.9 83.5 84.8 194.6 48.65 Zhengzhou 28.6 72.0 75.2 0 0 0 0 Weinan 28.3 66.9 72.0 153.5 211.5 541.8 135.56 Sanmenxia 27.0 82.6 75.6 0 0 0 0 Sanmenxia 32.5 62.2 82.0 80.1 101.6 356.3 89.17 Xi’an 24.6 80.7 66.7 0 0 0 0 Luoyang 30.5 64.7 76.5 96.8 117.7 302.0 75.58 Baoji 23.4 84.6 65.0 0 0 0 0 Zhengzhou 29.1 70.2 75.9 27.3 61.5 70.6 17.79 Tianshui 21.0 75.4 55.7 74.7 76.7 331.4 82.9 Shangqiu 25.5 87.1 72.3 0 0 0 010 Longxi 23.0 65.6 57.2 129.0 183.4 540.0 135.0 Xuzhou 24.6 88.1 69.7 0 0 0 011 Lanzhou 26.0 48.3 56.1 56.2 208.2 705.3 176.3 Donghai 24.1 89.8 68.2 27.6 27.6 97.3 24.312 Lianyungang 24.9 86.3 70.4 66.3 66.3 209.2 52.3

Line III K79 North South Roof Floor K80 North South Roof Floor

1 Shanghai 27.2 81.8 75.7 0 0 0 0 Kunming 22.6 71.2 61.5 116.6 135.5 418.1 104.52 Hangzhou 27.2 83.1 76.1 0 0 0 0 Qujing 19.2 76.7 54.1 30.0 61.1 94.8 23.73 Quzhou 26.3 85.4 74.3 0 0 0 0 Xuanwei 17.5 84.1 52.2 0 0 0 04 Yingtan 26.3 84.9 74.3 0 0 0 0 Anshun 22.3 84.9 64.4 0 0 0 05 Zhangshu 26.6 83.1 74.2 11.9 11.9 103.2 25.8 Kaili 21.4 87.5 62.5 1.9 1.9 3.7 0.96 Yichun 25.2 89.6 73.0 32.4 32.4 164.9 41.2 Huaihua 28.4 74.5 76.9 94.6 132.4 598.7 149.77 Zhuzhou 28.7 73.3 76.4 59.3 60.4 432.2 108.1 Loudi 33.2 57.1 80.6 87.3 129.8 632.0 158.08 Loudi 29.5 64.5 78.7 88.9 135.4 631.9 158.0 Zhuzhou 33.3 56.9 80.4 52.4 77.3 386.0 96.59 Huaihua 32.5 60.3 82.1 70.0 87.4 406.7 101.7 Yichun 30.9 67.1 79.7 25.6 79.9 119.3 29.810 Yupin 29.6 61.2 73.2 72.5 100.1 250.6 62.7 Zhangshu 29.7 74.9 81.1 0 0 0 011 Kaili 26.4 64.9 67.1 19.0 360.4 187.2 46.8 Yingtan 29.0 78.8 80.9 0 0 0 012 Anshun 23.4 79.1 65.2 0 0 0 0 Quzhou 26.5 84.9 74.8 0 0 0 013 Qujing 15.2 92.9 48.7 0 0 0 0 Hangzhou 25.2 90.1 72.7 0 0 0 014 Kunming 18.4 87.2 55.6 38.3 38.3 99.4 24.9 Shanghai 26.2 18.7 74.1 19.1 19.1 91.3 22.8

T means temperature, RH relative humidity, h enthalpy, SRI solar radiation intensity.


5.2. Proportion of each part cooling load to the total

Because of the distinct value and variation of each partcooling load, their contributions to the total cooling load shouldbe different. Here the proportion of one part cooling load to thetotal cooling load was calculated by the ratio of mean for thispart to mean for the total in a railway line. It was assumed thatthe trend of the cooling load between two stations is linear.Therefore, the mean of the cooling load for one line can becalculated as follows,

MQ ¼Xn�1

i¼1

�QiþQiþ1

2�Dsi

��Xn�1

i¼1

Dsi ði¼ 1;2;3;.;n�1Þ (12)

whereMQ is the mean cooling load, Qi is the cooling load at stationi, Δsi is the time distance between station i and its next station i þ 1and n is the total number of the stations.

According to Table 6, both the mean ventilation and occupancycooling loads took large proportions to the mean total cooling load.Except for the line II, the proportion of the former was even higherthan that of the latter. Considering its evident variation with theoutside conditions, the ventilation cooling load was the mostimportant factor to determine the trend of the total cooling load ina train, which was also reflected in Figs. 3e5.

On the other hand, the contributions of the mean conductionand radiation cooling loads to the mean total cooling load werevery limited due to their slight proportion (<5%). In lines II (K1352)and III, the proportion of the mean conduction cooling load was

even close to 0%, because in some areas the heat conducted frominside to outside (minus), while in the other areas the heat con-ducted from outside to inside (positive) as shown in Figs. 4 and 5,resulting in the mean just slightly higher than 0 kW.

5.3. Maximum, minimum and mean total cooling loads

The trend of the total cooling load in each line was also distinct.Even for the same line, the change of the total cooling loadmight betotally different if the train run in the opposite direction. Themaximum, minimum and mean were used to evaluate the changeof the total cooling load for each line, as listed in Table 7.

The difference in the maximum total cooling load for the threelines was small varying between 40.4 and 43.8 kW. Usually, thetotal cooling load reached its maximum during afternoon (exceptfor T15) in the east cities.

However the trend was totally different for the minimum totalcooling load. First, the difference between the three lines was big.Especially for line I, its minimumwas almost 2e8 times more thanthe other two lines. Second, the minimum often appeared duringmorningornight at the stations located in thenorthorwestof China.

For the mean total cooling load, it is clear from Table 7 that theline I had a larger value than lines II and III, while the differencebetween lines II and III was small.

The comparison on the maximum, minimum and mean reflec-ted that the variation in the total cooling load for the train runningin northesouth direction (line I) was much smaller than that forrunning in eastewest direction (lines II and III). It indicated that the

Table 3Solareair temperature of each station at arriving time in three main railway lines.

No. Station V (km/h) ao (W/m2 K) Solareair temperature (�C) Station V (km/h) ao (W/m2 K) Solareair temperature (�C)

Line I T15 East West Roof Floor T16 East West Roof Floor

1 Beijing 0 16 32.6 32.6 53.7 34.6 Guangzhou 0 16 31.5 37.1 34.2 31.22 Zhengzhou 112 88 29.1 29.1 32.9 29.5 Changsha 112 88 30.4 31.5 30.9 30.43 Wuchang 31.3 32.8 33.0 31.1 Wuchang 26.7 26.7 26.7 26.74 Changsha 29.4 29.4 29.4 29.4 Zhengzhou 27.3 27.3 27.3 27.35 Guangzhou 26.6 26.6 26.6 26.6 Beijing 26.4 25.7 26.7 25.5

Line II K1351 North South Roof Floor K1352 North South Roof Floor

1 Lianyungang 0 16 35.8 37.5 50.5 33.7 Lanzhou 0 16 18.8 18.8 18.8 18.82 Xuzhou 73.6 68.7 29.9 30.3 33.3 29.4 Gangu 65.1 64.1 19.0 19.0 19.0 19.03 Shangqiu 32.4 32.7 35.2 32.0 Tianshui 20.0 20.0 21.2 20.14 Lankao 31.7 31.9 34.6 32.0 Baoji 25.8 25.8 27.0 25.45 Zhengzhou 30.7 31.4 32.2 30.8 Weinan 30.0 30.6 34.2 29.86 Sanmenxia 28.6 28.6 28.6 28.6 Sanmenxia 33.4 33.6 36.4 33.57 Xi’an 27.0 27.0 27.0 27.0 Luoyang 31.6 31.8 33.8 31.38 Baoji 24.6 24.6 24.6 24.6 Zhengzhou 29.4 29.8 29.9 29.39 Tianshui 23.4 23.4 23.4 23.4 Shangqiu 25.5 25.5 25.5 25.510 Longxi 21.8 21.8 24.4 21.8 Xuzhou 24.6 24.6 24.6 24.611 Lanzhou 24.3 24.9 28.5 24.4 Donghai 24.4 24.4 25.2 24.412 Lianyungang 25.6 25.6 27.2 25.5

Line III K79 North South Roof Floor K80 North South Roof Floor

1 Shanghai 0 16 27.2 27.2 27.2 27.2 Kunming 0 16 27.7 28.5 40.9 27.22 Hangzhou 77.6 70.9 27.2 27.2 27.2 27.2 Qujing 80.6 70.2 19.5 19.8 20.1 19.43 Quzhou 26.3 26.3 26.3 26.3 Xuanwei 17.5 17.5 17.5 17.54 Yingtan 26.3 26.3 26.3 26.3 Anshun 22.3 22.3 22.3 22.35 Zhangshu 26.7 26.7 27.6 26.9 Kaili 21.4 21.4 21.4 21.46 Yichun 25.5 25.5 26.8 25.6 Huaihua 29.3 29.7 34.4 29.97 Zhuzhou 29.3 29.3 33.0 29.8 Loudi 34.1 34.5 39.5 34.88 Loudi 30.4 30.8 35.7 31.1 Zhuzhou 33.8 34.1 37.1 34.39 Huaihua 33.2 33.4 36.5 33.5 Yichun 31.2 31.7 32.1 31.210 Yupin 30.3 30.6 32.1 30.2 Zhangshu 29.7 29.7 29.7 29.711 Kaili 26.6 30.0 28.2 26.9 Yingtan 29.0 29.0 29.0 29.012 Anshun 23.4 23.4 23.4 23.4 Quzhou 26.5 26.5 26.5 26.513 Qujing 15.2 15.2 15.2 15.2 Hangzhou 25.2 25.2 25.2 25.214 Kunming 18.8 18.8 19.4 18.6 Shanghai 26.4 26.4 27.1 26.4

V means velocity and ao external surface heat transfer coefficient.


effect of different regions on the train compartment cooling loadwas significant.

5.4. Difference in total cooling load between different regions

The differences in the total cooling load between north andsouth and between west and east were shown in Table 8, respec-tively. For line I, Wuchang is the division between north and south.Sanmenxia is the division betweenwest and east for line II and Kailifor line III.

According to Table 8, the mean total cooling load during thetravel in south was bigger than that in north. The maximumdifference was about 6 kW for T16 running in the direction fromsouth to north. When running in the contrary direction (T15), thedifference was very small (0.8 kW).

More significant difference in the mean total cooling load can beseen between west and east, which was more than 12 kW. Espe-cially for line III, the mean total cooling load in the east was about 2times larger than that in the west.

The main reason to the region difference in the mean totalcooling load was the distinct weather in different regions of China.As list in Table 2, the ambient humidity in the north cities was lowerthan that in the south cities, resulting in a smaller ventilationcooling load that, in turn, leads to a lower total cooling load. Thesimilar situation existed in the comparison on the humiditybetween the west and east cities. However, not only the ambienthumidity but also the temperature was much lower in the west,therefore the difference between the west and east was moresignificant as mentioned before.

5.5. Difference in total cooling load between different periods oftime

As illustrated before, the maximum of the total cooling loadoften occurred during afternoon, while the minimum appeared atmorning or night. It seems that the period of time (morning/afternoon/night) was also a factor resulting in the change of thecooling load for the train compartment.

The comparison on mean total cooling load between morning,afternoon and night was done in Table 8. Here nighttime wasa period from 19:00 to 6:00 (almost no sun radiation). Duringdaytime, morning and afternoon was between 6:00 and 12:00 andbetween 12:00 and 19:00, respectively.

Obviously, the mean total cooling load in afternoon was largerthan those in morning and at night (except for T15), because ofthe higher ambient temperature, humidity and solar radiation.For T15, the mean total cooling load in afternoon was almost thesame as that in morning or at night, with a very slight discrep-ancy of 0.8 kW.

As revealed by Table 8, the mean total cooling load at night wasbigger than that in morning for lines I and II, while for line III theresult reversed. Even in the same line (especially in I and II), thevalue of the difference can be very different when the train runningin the contrary direction. For example, the mean total cooling loadsin morning and at night were very close for T15 and K1352,however the difference between themwas more than 4 kW for T16and K1351.

This can be explained by the additional effect of differentregions. According to Figs. 3e5, if longer period during morning

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45

a

b

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21Time (h)

Wk(daol

gnilooC

)

Conduction Radiation VentilationOccupancy Equipment Total

Beijing

11:00

Zhengzhou

16:34

Wuchang

21:10 Changsha

0:29

Guangzhou

7:35

T15 Beijing-Guangzhou

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Guangzhou

16:48Changsha

23:44 Wuchang

3:06 Zhengzhou

7:46

Beijing

13:18

T16 Guangzhou-Beijing

Nighttime

Nighttime

Fig. 3. Dynamic cooling load of train compartment in railway line I. (a) T15 Beijing-Guangzhou (b) T16 Guangzhou-Beijing.


was spent in traveling in the west or north cities while most periodof night was spent in the east or south cities, the mean total coolingload inmorning would be lower than that at night. It is the same forthe comparison between the case that the train runs in the west ornorth cities during night and the case that the train runs in the eastor south cities during morning. Therefore, the effect of region led tothe distinct difference between morning and night for differentrailway lines.

Table 5Other calculation coefficients used in the simulation.

No. Item Value Reference

1 Shading coefficientfor the window curtain

0.64 TB 1951e1987 [5]

2 Transmittance for thewindow glass

0.6 TB 1951e1987 [5]

3 Absorptance for train 0.7 TB 1951e1987 [5]

6. Discussion

6.1. The effect of body thermal storage on train cooling load

It is well known that body thermal storage results in attenuationand delay of the transfer from heat gain to cooling load. In a train,the body thermal storage has effects on the conduction, radiation,occupancy and equipment cooling loads (the ventilation load is

Table 4Material properties and thickness for each layer in train body.

No. Layer Material Thickness(mm)

Density(kg/m3)

Specificheat (kJ/kg K)

Thermalconductivity(W/m K)

1 Outboard Steel 2 7850 0.48 502 Middle Glass

wool90/87/74 200 1.22 0.08

3 Inboard Wood 20 300 1.89 0.093

handled in the ACU). These effects were considered in the presentmodel using the methods as follows.

(1) For the calculation of the conduction load, the heat conductionthrough train compartment was simulated based on a two-dimension unsteady conduction equation;

(2) The transfer from the heat gain to the other cooling loads wasestimated by introducing the corresponding transfer coefficients.

body surface4 Aggregation coefficient 0.955 TB 1951e1987 [5]5 Transfer coefficient for

radiation cooling load0.7 Xu and Hu 1997 [10]

6 Transfer coefficient foroccupancy cooling load

0.95 Zhang and Teng 1993 [15]

7 Transfer coefficient forequipment cooling load

0.75 Zhang and Teng 1993 [15]

8 Thermal conductivityfor window glass(thickness: 12 mm)

0.76 W/m K GB 50176-93 [12]

Table 6Proportion of each part cooling load to the total (%).

Line Conduction Radiation Ventilation Occupancy Equipment

T15 5 2 55 32 6T16 2 2 55 35 6K1351 2 2 44 44 8K1352 0 2 44 46 8K79 0 1 47 44 8K80 0 1 47 44 8

The value was the ratio of the mean for one part cooling load to the mean for thetotal in a railway line.


The effect of body thermal storage on the conduction load can befound by the comparison between the steady and unsteady values.Here, the steady values were calculated using the one-dimensionsteady state equation with overall heat transfer coefficients. Fig. 6shows the trends of the conduction cooling load for K1352 whenusing the steady and unsteady methods respectively. According toFig. 6, the peak load with the unsteady method lags behind thatwith the steady method (about 2 h) and its value was lower(0.2 kW), which indicate the attenuation and delay in the transferfrom conduction heat gain to cooling load due to the body thermalstorage. Similar trends were found in the other lines.

More detailed results are listed in Table 9. Though the thermalstorage effect in the conduction cooling load was clear, the differ-ence (<0.5 kW at most cases) between the steady and unsteady

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Conduction ROccupancy E

Lianyungang10:36

Xuzhou13:30

Shangqiu15:54

Lankao17:21

Zhengzhou19:05

Sanmenxi22:41

K1351 Liangy

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Lanzho u1:26

Gangu4:52

Tianshui6:07

Baoji8:53

Weinan12:11

Sanmenxia14:38

K1352 Lanzhou

Ni

Nighttime

Fig. 4. Dynamic cooling load of train compartment in railway line II. (a

methods can be ignored compared with the total cooling load,because of the very low proportion (<5%) of the conduction coolingload (see Table 6). Therefore, it seems that the one-dimensionsteady state equation is appropriate to calculate the conductioncooling load for air-conditioning train compartments, consideringthe balance between precision and time consuming.

On the other hand, there are radiation heat exchanges betweendifferent surfaces (i.e. the internal surface, human body andequipment surface) in the rain compartment. Under the effect ofthermal storage, the internal radiation heat was stored in the trainbody, and then transferred to cooling load via convection heatexchange. However, the effect of body thermal storage was limited,as revealed in the heat conduction. And, it was supposed that theheat gains from the occupancy (fixed number) and equipment wereconstant in the present model. Therefore, instead of the compli-cated calculation of the internal radiation heat exchange, the cor-responding transfer coefficients were used to estimate the changeof the radiation, occupancy and equipment cooling loads, based onthe successive steady state calculations in the present model.

6.2. The effect of ambient conditions on train cooling load

The significant effect of ambient conditions can be found withthe relationship between the ambient conditions and dynamiccooling load. Fig. 7 shows the quantitative relationship between the

e (h)

adiation Ventilationquipment Total

a

Xi'an2:07

Baoji4:28

Tianshui7:37

Longxi9:55

Lanzhou12:48

ungang-Lanzhou

14 15 16 17 18 19 20 21 22 23 24 25 26 27

15 16 17 18 19 20 21 22 23 24 25 26 27 28 29

e (h)

adiation Ventilationquipment Total

Luoyang16:41

Zhengzhou18:36 Shangqiu

23:45

Xuzhou2:20

Donghai4:48

Lianyungang6:06

-Liangyungang

ghttime

Nighttime

) K1351 Liangyungang-Lanzhou, (b) K1352 Lanzhou-Liangyungang.

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Shanghai

19:10

Hangzhou

21:05

Quzhou

0:17

Yingtan

3:15

Zhangshu

5:13

Yichun

6:54

Zhuzhou

8:39Loudi

10:44

Huaihua

14:52

Yupin

16:39

Kaili

18:38

Anshun

22:35

Qujing

6:13

Kunming

8:17

K79 Shanghai-Kunming

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0 1 23 4 5 6 7 8 9 1011 12 1314 15 1617 18 1920 2122 23 2425 26 2728 29 3031 32 3334 35 3637 38

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Kunming16:25

Qujing18:15

Xuanwei20:29

Anshun1:50

Kaili5:59

Huaihua9:44

Loudi13:58

Zhuzhou15:48

Yichun17:35

Zhangshu19:24

Yingtan21:06

Quzhou23:54

Hangzhou2:49

Shanghai4:54

K80 Kunming-Shanghai

Nighttime

Nighttime

Nighttime

Nighttime

Fig. 5. Dynamic cooling load of train compartment in railway line III. (a) K79 Shanghai-Kunming, (b) K80 Kunming-Shanghai.


outdooreindoor temperature difference and the variable part ofthe dynamic cooling load. Here the variable part of the dynamiccooling load includes the conduction, radiation and ventilationcooling loads. As shown in Fig. 7, the trend of the variable part withthe outdooreindoor temperature difference was almost linear witha positive correlation. The regression equation and its determina-tion coefficient R2 were also given in equation (13). The high valueof R2 means that the change of ambient temperature (the indoorcondition was steady) can explain about 89% of the variation in thevariable part of the dynamic cooling load.

The unexplained part by the single parameter regressionequation is associated with the effects of ambient humidity, sunradiation and thermal storage of the compartment. As shown inequation (14), it can be known that the value of R2 increased to 98%,

Table 7Maximum, minimum and mean total cooling loads (kW).

Line I

T15 T16

Max. Value 42.4 43.8Station Guangzhou GuangzhouTime 7:35 16:48

Min. Value 33.7 29.3Station Beijing ZhengzhouTime 11:00 7:46

Mean 38.5 35.2

when the outdooreindoor temperature and relative humiditydifferences were included together. This means the humidity isanother important factor to the variable cooling load. The ambienthumidity also has a positive effect on the cooling load, as indicatedby the regression equation (14).

VQ ¼ 2:414Dt þ 14:517�R2 ¼ 0:892; Sig: < 0:001

�(13)

VQ ¼ 2:913Dt þ 35:657DRH þ 6:687�R2 ¼ 0:976; Sig: < 0:001

�(14)

where Dt means outdooreindoor temperature difference, DRHoutdooreindoor relative humidity difference, VQ variable part of

II III

K1351 K1352 K79 K80

40.4 42.7 42.7 41.6Shangqiu Sanmenxia Huaihua Loudi15:54 14:38 14:52 13:5813.5 5.2 4.5 8.8Tianshui Lanzhou Qujing Xuanwei7:37 1:26 6:13 20:2928.4 26.8 28.1 28.5

Table 8Comparison on mean total cooling loads between different regions and differentperiods of time (kW).

Line Region Period of time

North South West East Morning Afternoon Night

T15 38.1 39.0 e e 38.8 38.0 38.8T16 32.0 38.3 e e 31.4 39.4 35.9K1351 e e 20.8 37.2 20.3 35.3 28.5K1352 e e 20.3 32.3 23.2 37.7 23.4K79 e e 16.3 34.9 27.7 37.0 25.2K80 e e 16.2 35.8 31.4 34.2 25.1

Nighttime: 19:00e6:00 (almost no sun radiation); Morning: 6:00e12:00 andafternoon 12:00e19:00.


dynamic cooling load, R2 determination coefficient, and Sig.< 0.001means the regression equation was significant.

The regression equations indicate that the ambient temperatureand humidity are the most important factors inducing the variationof the cooling load during a train’s travel (Figs. 3e5) and thedifference in cooling load between different regions and periods oftime (see Table 8).

Both equations (13) and (14) provide simple and quickmodels topredict the variable part of dynamic cooling load with a smalldiscrepancy (high value of R2), considering the effect of the ambientconditions. At constant indoor air temperature, the occupancy andequipment cooling loads are invariable, if the occupant number andequipments do not change. Therefore, the total dynamic coolingload is the sum of the variable and invariable parts.

As revealed in this study, the variation in the cooling load ofa train compartment during its travel was big, especially for thetrain in westeeast railway lines. This can cause a fluctuation ofmore than 4 �C in air temperature in a train compartment [16].Therefore, it was suggested that a frequency conversion refrigera-tion system should be developed for the ACUs in trains of China todecrease its indoor air temperature fluctuation within 1 �C, which,at the same time, could reduce the energy consumption of thecompressors [16]. However, one of the key problems is how toobtain a reliable value of the real-time cooling load quickly whenthe train is moving. It seems that the regression equations (13) and(14) may provide an effective method to solve the key problem, byinstalling air temperature and humidity sensors in and out the traincompartment, respectively.

6.3. Comparison between the present model and TB1951-87

Up to now, the cooling load of a train compartment is deter-mined based on China MOR standard (TB1951-87). The standard

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Steady Unsteady

Fig. 6. Trends of the conduction cooling load for K1352 using steady and unsteadymethods.

TB1951-87 suggested the steady cooling load under the specificambient condition as the basis to design the ACU for all trainsmoving in south of Beijing. The main ambient calculation param-eters in summer provided in TB1951-87 are listed as follows:

� Ambient temperature ¼ 35 �C� Relative humidity ¼ 60%� Solar radiation intensity: the corresponding values in Wuhanwere recommended

� Ventilation rate: 20e25 m3/h per person

Table 10 gives the cooling load of YZ25G train compartmentunder a ventilation rate of 25 m3/h per person based on the stan-dard TB1951-87. Compared with the maximum of the corre-sponding part of the dynamic cooling load, the result of TB1951-87was bigger, with a difference of about 14 kW in values of the totalcooling load. Two main reasons were applied to these differences:

(1) The present model employed the actual real-time ambientparameters (the average during the hottest month July) ofdifferent railway lines as the calculation conditions. However,TB1951-87 used the assumed worst ambient conditions insummer and did not considered the significant differencebetween different railway lines;

(2) The present model considered the transfer from heat gain tocooling load, while TB1951-87 treated the heat gain as thevalue of the cooling load.

Apparently, the effects of body thermal storage and variableambient conditions were neglect in TB1951-87, which lead toa much larger calculation value.

The refrigerating capacity of the ACU in train compartments isdetermined based on the result of TB1951-87. For example, inpresent YZ25G train compartment is equipped with two ACUs witha total refrigerating capacity of 58 kW, which is a little more thanthe total value (TB1951-87) listed in Table 10. However, thecomparison reflected that the standard TB1951-87 might over-estimate the actual cooling load of YZ25G train compartment much,which might be a main reason to large energy consumption of theACUs in train compartments [2] and the uncomfortable thermalenvironment causing passengers’ feelings of cool or cold [4].

6.4. Urgent need of calculating dynamic cooling load for trains inChina

In the past, the railways in China were limited and most of themlocated in the east region. However, with the rapid development ofrailway transport in China, more and more trains run in the westregion. At the same time, trains running in short-time period (i.e.the high-speed trains) are increasing after the 6th speed-up ofChinese railways since 2007. As found in this study, the cooling loadfor trains traveling in the west region was significantly lower thanthat in the east region due to different weather conditions (seeTable 8). And the obvious difference in the cooling load of traincompartment was also found between afternoon and morning/night, as illustrated in Table 8. These significant differences in thecooling load should be seriously considered when designingthe refrigerating capacity of ACUs for a moving train. Therefore, thesimplex ambient condition and method for the calculation ofcooling load recommended by the standard TB1951-87 releasedtwenty years ago are likely to induce large discrepancy under thenew situation of railway transport in China, which can lead toa waste of energy and discomfort of indoor thermal environment.The calculation of dynamic cooling load provides an adequate basis

Table 9Comparison between the conduction cooling loads based on unsteady and steady calculation methods (kW).

Station 1 2 3 4 5 6 7 8 9 10 11 12 13 14

T15 Steady 2.47 1.11 0.62 0.11 0.68Unsteady 2.57 2.53 0.86 0.44 0.53Difference �0.10 �1.43 �0.24 �0.33 0.15D/T (%) �0.30 �3.62 �0.60 �0.91 0.36

T16 Steady 1.29 0.13 0.24 0.01 1.09Unsteady 1.34 0.37 0.20 0.12 0.71Difference �0.04 �0.24 0.04 �0.10 0.38D/T (%) �0.10 �0.66 0.11 �0.35 1.08

K1351 Steady 2.52 1.32 1.23 0.98 0.47 0.18 �0.25 �0.47 �0.61 �0.03 0.62 2.52Unsteady 2.61 1.47 1.34 1.27 1.02 0.39 0.03 �0.27 �0.52 �0.45 0.14 2.61Difference �0.10 �0.14 �0.11 �0.29 �0.55 �0.21 �0.28 �0.20 �0.08 0.42 0.48 �0.10D/T (%) �0.25 �0.38 �0.26 �0.76 �1.58 �0.62 �1.15 �0.89 �0.63 2.61 3.00 �0.25

K1352 Steady �1.26 �1.27 �1.01 0.02 0.99 1.54 1.14 0.65 �0.09 �0.25 �0.25 0.02 �1.26Unsteady �1.30 �1.32 �1.30 �0.72 0.36 1.05 1.37 1.15 0.23 �0.09 �0.14 �0.22 �1.30Difference 0.04 0.05 0.29 0.74 0.63 0.49 �0.22 �0.49 �0.32 �0.17 �0.10 0.24 0.04D/T (%) 0.76 0.57 2.81 2.96 1.96 1.14 �0.60 �1.38 �1.04 �0.61 �0.40 0.84 0.76

K79 Steady 0.21 0.22 0.05 0.05 0.19 0.00 0.84 1.17 1.53 0.90 0.34 �0.47 �1.96 �1.28Unsteady 0.23 0.23 0.16 0.07 0.08 0.12 0.18 0.74 1.38 1.41 0.97 �0.03 �1.59 �1.76Difference �0.02 �0.01 �0.10 �0.01 0.11 �0.13 0.66 0.43 0.15 �0.51 �0.63 �0.44 �0.38 0.47D/T (%) �0.05 �0.02 �0.31 �0.04 0.33 �0.40 1.87 1.12 0.36 �1.51 �2.26 �1.91 �8.33 4.01

K80 Steady 1.07 �1.14 �1.55 �0.67 �0.83 0.95 1.85 1.65 1.01 0.67 0.55 0.09 �0.15 0.12Unsteady 1.11 0.25 �0.96 �1.03 �0.79 �0.01 1.42 1.73 1.62 1.17 0.83 0.43 0.05 0.04Difference �0.04 �1.38 �0.58 0.36 �0.04 0.96 0.43 �0.07 �0.60 �0.50 �0.28 �0.34 �0.19 0.07D/T (%) �0.18 �11.07 �6.61 1.69 �0.20 2.68 1.03 �0.17 �1.50 �1.24 �0.70 �1.03 �0.64 0.23

Difference ¼ SteadyeUnsteady. D/T means the proportion of the difference between steady and unsteady to the total cooling load.

y = 2.414x + 14.517

R2 = 0.892

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Outdoor-indoor temperature difference (°C)

Var

iabl

e pa

rt o

f dy

nam

ic c

oolin

g lo

ad (

kW)

Fig. 7. Relationship between outdooreindoor temperature difference (x) and variablepart of dynamic cooling load (y).


to determine the cooling load for trains traveling in differentregions and periods of time.

In this study, a preliminary model was built to simulate thedynamic cooling load of a moving air-conditioned train. Followingpoints need be considered in order to improve the present model infuture.

Table 10Comparison between the results of the present model and China MOR standardTB1951-87 (kW).

Method Conduction Radiation Ventilation Occupation &equipment

Total

TB1951-87 5.5 5.1 31.1 16.1 57.8The present model 3.4 1.6 26.5 14.7 43.9Difference 2.1 3.5 4.6 1.4 13.9

The result of TB1951-87 was done under the conditions: a ventilation rate of 25 m3/h per person, indoor temperature of 26 �C, relative humidity of 55% and train speedof 120 km/h. The result of the present model was the maximum of the corre-sponding part of the dynamic cooling load under the average ambient conditionsduring the hottest month July.

(1) The occupancy load is not always constant during a longdistance travel, because of the variation in number of passen-gers. Therefore, a model to predict the change of passengersshould be included;

(2) The effects of infiltration on the dynamic cooling load need beevaluated considering the velocity pressure associatedwith thetrain movement;

(3) A model will be built to adequately depict the change ofambient conditions for different railway lines in China.

7. Conclusions

This study presented the dynamic cooling load for YZ25G trainsrunning in three main railways of China. Based on the results,following main conclusions can be achieved:

(1) The effect of ambient conditions (air temperature andhumidity) on the dynamic cooling load of a train compartmentwas significant, while the effect of body thermal storage waslimited. Under the average ambient conditions during thehottest month July, the maximum total cooling loads for thethree railway lines were between 40.4 and 43.8 kW, andthe minimum between 4.5 and 33.7 kW;

(2) The ventilation cooling load was the most important factor todetermine the trend of the total cooling load, while thecontributions of the conduction and radiation cooling loads tothe total cooling load were very limited due to their slightproportion (<5%);

(3) The region difference in the total cooling load was found. Themean total cooling load for trains running in west region wassignificantly lower than that in east region;

(4) There was evident difference in the total cooling load whentrains running during different periods of time. The mean totalcooling load for trains traveling in afternoon was larger thanthose in morning and at night;

(5) Compared with the maximum of the dynamic cooling load, thecalculation result of steady cooling load based on China MORstandard (TB1951-87) had a larger value, which means thestandard TB1951-87might overestimate the actual cooling loadof a train compartment much during its traveling;


(6) The calculation of dynamic cooling load can provide anadequate basis to determine the cooling load for trains movingin different regions and periods of time. Due to the limitedeffect of body thermal storage, the successive steady stateapproach is appropriate to simulate the change of train coolingload considering the balance between precision and timeconsuming.

Acknowledgements

The project is financially supported by National Key Project ofScientific and Technical Supporting Programs of China(2008BAJ12B03) andNanjing Puzhen Vehicle Factory in China. ThanksforMissDiyuYang’s careful edit on tables andfigures in this paper. Andalso, the authors thank the reviewers for their valuable comments.

References

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variation in cooling load of a moving air-conditioned train compartment under the effects of ambient...

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