Experimental Research on Thermal Characteristics of PCM Thermal [600054]

Applied Thermal Engineering
Manuscript Draft

Manuscript Number: ATE -2015-10287R1

Title: Experimental Research on Thermal Characteristics of PCM Thermal
Energy Storage Units

Article Type: Research Paper

Keywords: thermal energy storage unit;
phase change material;
multiple melting points;
cascading combination;
thermal characteristics

Abstract: To explore heat energy recycling and thermal management for
integration in electric vehicles (EVs), a phase change materials (PCMs)
thermal energy storage unit using flat tubes and corrugated fins is
designed. The investigation focuses on the thermal characteristics of the
PCM units, such as the temperature variation, he at volume, and heat
transfer time, etc. Meanwhile, the heat storage and release process will
be influenced by different inlet temperature, liquid flow rate, melting
point of the PCM, and the combination order of the units. The
experimental results show tha t the thermal energy storage unit realizes
small cell contact heat transfer and exhibits rapid heat storage and
release rates. Meanwhile, the cascading combination with multiple melting
points PCM units can help extend the range of applicable temperatures and
satisfy multiple heat sources with different temperature levels. In
addition, the adoption of the descending melting point order can improve
phase change uniformity and heat storage rate in PCM units, and the units
with the ascending melting point orde r perform better during the heat
release process.

Dear Editor and Reviewer s:
Thank you for your letter and for the reviewers’ comments concerning our manuscript entitled
“Experimental Research on T hermal Characteristics of PCM Thermal Energy Storage Units ”
(ATE-2015 -10287 ).
Those comments are all valuable and very helpful for revising and improving our paper . We
have studied the comments carefully and have made correction which we hope meet with approval.
Revised portion s are marked in red in the revised manuscript .

Responds to the reviewers’ comments:
Reviewer #1:
1. The manuscript presents an experimental analysis of a latent heat storage system
integrated into a test platform. The work is not original and present results rather trivial.
Very important parameters such as PCM thermo -physical properties, thermal energy
storage unit geometric elements, and PCM mass are not presented. In the absence of PCM
mass parameters the author mention such as time and accumulated/released heat d o not
have any relevance. Not even the PCM was specified, which raises serious question marks on
how the authors understand to structure and write a scientific article.
Response:
Thanks for the reviewer's suggestion . We are very sorry for our negligence o f the important
parameters . Indeed, t he former paper content and write does have a lot of shortage s, and we have
made a very big adjustment during this period . The PCM thermo -physical properties, thermal
energy storage unit geometric elements, and PCM mass have been added to the revised manuscript
in page 2-3 line 61-70.
The structure and geometric elements of the thermal energy storage unit is shown in Fig. 1 .
The core of unit consist ed of flat tube s and corrugated fins with louvers. Liquid flow ed inside the
flat tube s. A PCM w as added between the flat tubes , and the thickness of flat tube was 2 mm . The
corrugated fins with louvers were used to increase surface area for heat transfer. The thickness of
corrugated fin was 0.1 mm, and the wide of it was 4 cm . The mass of PCM is 1.15 kg in the
thermal energy storage unit. Thermo -physical properties of PCM s are shown in Table 1.
Response to Reviewers

Fig. 1 The structure and geometric elements of the thermal energy storage unit
Table 1 Thermo -physical properties of PCM
Melting point of
PCM (℃) Density
(Kg m-3) Specific heat
capacity
(KJ Kg-1K-1) Heat of fusion
(KJ Kg-1)
30
40
50 840
852
845 2.5
2.5
2.5 207
220
210

2. An interesting section of the manuscript is 3.4. However, the authors fail to present clearly
what happens if units are cascaded and what are the advantages of the cascade scheme over
single unit scheme. It is worth to elaborate and to rewrite this section in such way that the
points raised above are treated.
Response:
Thanks for the reviewer's suggestion . This section has been modified in page 9 -10 line
205-243 for the revised manuscript.
The c ascaded combination of thermal energy storage units using multiple melting point
PCM units is employed to help extend the applicable temperature range compared with the single
unit, and it is also benefited waste heat recycl ing from multiple heat source s with different
temperature level s such as the battery pack, power control unit ( PCU ), and motor for electric
vehicle. In add ition, we hope that the heat of the c ascaded thermal energy storage combination can
release rapidly in practical application such as battery preheating in cold weather, and find out the
effects of the different cascade order through experimental investigat ion.
Experimental results show that adopting the melting point descending order can further
increase the phase change uniformity of the units and the heat storage rate. Meanwhile, the unit s
with the ascending melting point order perform better than the unit s with the descending melting

point order during the heat release process.
3. The authors should justify choice of 0 °C as it is well below the PCM melting point.
Response:
The reviewer's question is very important . In fact, we consider using stored heat in PCM to
improve cold start performances of battery pack for electric vehicle s in low temperature regions.
The environment temperature was choice of 0 °C in this investigation and the PCM initial
temperature should be consistent with it. In fact, the refrigeration system of the e nvironmental
chamber is upgrading , and we hope that the further investigation will be conducted in lower
ambient temperature .
4. Inclusion in the manuscript title of some constructive details of the heat storage unit
“corrugated fins” is not justified by the manuscript content. The title itself is rather vague
for the manuscript content and misleading.
Response:
Thanks for the reviewer's suggestion . It is really true that the title itself is rather vague for the
manuscript content and misleading. In fact, the investigat ion is mainly focus on the thermal
characteristics of the designed thermal energy storage units and the effects of the inlet temperature,
liquid flow rate, melting point of the PCM, and comb ination order of the units . The thermal energy
storage unit and corrugated fins geometric elements have been added to the revised manuscript in
page 2 line 60-68. Meanwhile, the paper title has been modif ied to "Experimental Research on
Thermal Characteris tics of PCM Thermal Energy Storage Units ".
5. In addition, please rephrase (I strongly recommend language editing for this manuscript):
“… appeared more moderate from when the phase change heat storage starting ”
“was minimal for the 50 ℃ condition ”
…and many others
Response:
Thanks for the reviewer's suggestion and this manuscript has been revised by Elsevier
Language Editing during this period . The English writing problem s which the reviewer mentioned
has been modif ied in page 6 line 12 9-131.

6. What is the meaning of the adjective “essential” in the context: “essential heat storage
time” and“essential heat storage volume ”
Response:
I am sorry about that my express is not clear and precise . The meaning of the adjective
"essential " in the context is similar to the "m ajor". In fact, the PCM heat storage includes the latent
heat and the sensible heat; the former is more dominant in the early and middle periods than in the
latter periods , and the latent heat volume apparently exceeded the sensible he at volume for the
PCM . Meanwhile, the storage or release rate of the sensible heat is very slow in the last period ,
Therefore, the essential heat transfer processes was defined as the storage or release periods of the
70% heat volume , and the heat storage or release volume in the corresponding period was defined
as the essential heat storage or release volume . Some of the investigations and analyses were
performed during the essential heat storage and release periods.
7. I suggest reconsidering Highlights:
(1) I recommend using impersonal expressions
(2) Not justified
(3) Trivial result
(4) Trivial result
(5) Reconsider / Rephrase
Response:
Thanks for the reviewer's suggestion . The Highlights have been modified as below :
(1) A PCM thermal energy storage unit is designed.
(2)Thermal characteristic s of the energy storage unit are investigated.
(3) The designed PCM unit exhibits rapid heat storage and release rates .
(4) The descending melting point order can increase heat storage rate of units.
(5) The ascending melting point order can increase heat release rate of units.

Reviewer #2:
1. This manuscript introduces the experimental results of PCM storage process. It could
provide a reference to researchers. The experimental procedures are correct and we ll
organized. I recommend the manuscript being published.
Response:

Thanks for the reviewer's recognition to our research work. Indeed, t he former paper content
and write does have many shortage s, and we have made a very big adjustment during this period .
2. Some minor English writing problem should be modified, such as indicated in the
manuscript.
Response:
Thanks for the reviewer's suggestion and this manuscript has been revised by Elsevier
Language Editing during this period . The English writing proble ms which the reviewer mentioned
has been modif ied in page 6 lin e 129 -130 and page 8 line 170.
3. A question: why the temperature curve did not show a flat region as it reach the phase
change point?
Response:
The reviewer's question is very important . I am sorry about that my express is not clear and
precise . In fact, the temperature curve represent s the outlet liquid temperature of the PCM thermal
energy storage unit. As show in Fig. 4 (a), the heat storage volume (Qhs) rose as the outlet liquid
temper ature (Tol) increased in the phase change process . Due to the phase change rate of the
thermal energy storage unit rose, the absorbed heat by PCM decreased gradually , and the release
heat by liquid was decreased simultaneously . Therefore, the outlet liquid temperature and the
slopes of the Tol were increased , and the flat region not presented during the period.

（a）Heat storage process
Fig. 4 The variations in the outlet temperature and heat volume for several inlet liquid
temperatures

0100200300400500600700800
010203040506070
0 100 200 300 400 500 600 700
Qhs/kJTol /℃
Time/s50℃ Tol
60℃ Tol
70℃ Tol
50℃ Qhs
60℃ Qhs
70℃ Qhs

(1) A PCM thermal energy storage unit is designed.
(2) Thermal characteristic s of the energy storage unit are investigated.
(3) The designed PCM unit exhibits rapid heat storage and release rates .
(4) The descending melting point order can increase heat st orage rate of units.
(5) The ascending melting point order can increase heat release rate of units. Highlights (for review)

1
1
Experimental Research on T hermal Characteristics of PCM Thermal 2
Energy Storage Units 3
4
Tianshi Zhang a, b, Qing Gao a, b,*, Guohua Wang a, b, Zhenmin Yan b, Y.Y.Yan c, Cong Xiao d 5
（a. State Key Laboratory of Automotive Simulation and Control, Jilin University, 130025, Changchun, China ; 6
b. College of Automotive Engineering, Jilin University, 130025, Changchun, China ; 7
c. Faculty of Engineering, University of Nottingha m, Nottingham, NG72RD, UK; 8
d. FAW R&D Centre, China First Automobile Works Group Corp, 130011, Changchun, China ) 9
Email: zhangtianshi@jlu.edu.cn (*Corresponding Email: gaoqing@jlu.edu.cn ) 10
Abstract: 11
To explore heat energy recycling and thermal management for integration in electric vehicle s (EVs), a phase change material s 12
(PCM s) thermal energy storage unit using flat tubes and corrugated fins is designed . The investigation focuses on the thermal 13
characteristic s of the PCM units, such as the temperature variation , heat volume, and heat transfer time , etc. Meanwhile, the heat storage 14
and release process will be influenced by different inlet temperature, liquid fl ow rate, melting point of the PCM, and the combination 15
order of the units. The experimental results show that the thermal energy storage unit realize s small cell contact heat transfer and 16
exhibits rapid heat storage and release rate s. Meanwhile, t he cascad ing combination with multiple melting points PCM units can help 17
extend the range of applicable temperature s and satisfy multiple heat source s with different temperature level s. In addition, the adoption 18
of the descending melting point order can improve phase change uniformity and heat storag e rate in PCM units, and the units with the 19
ascending melting point order perform better during the heat release process. 20
21
Key words: thermal energy storage unit, phase change material , multiple melting points , cascad ing combination, thermal characteristic s 22
1 Introduction 23
Phase-change heat storage technologies have received considerable attention in the field of vehicle -mounted 24
waste heat utilization. For the traditional internal combustion engine car, Oskar Schatz [1] proposed the concept of a 25
heat battery, which adopts a phase change material (PCM) to store waste heat from engine cooling water. The heat 26
could be used to preheat the cabin and improve engine cold start performance in winter. E. Korin [2] used latent heat 27
energy to preheat a catalytic converter until it reach ed the optimum working temperature. Taha Aldoss [3] built a 28
mathematical model to investigate a lighting thermal management system with a PCM. In additio n, Adamczyk [4] 29
proposed a vacuum insulation method, and expe rimental result s showed that this method could efficiently reduce the 30
conduction and radiation heat losses, realiz ing heat preservation for a long time. 31
Compared with tradition al vehicles, extending the operational range a nd reducing energy consumption are more 32 Manuscript
Click here to download Manuscript: revised manuscript.docx Click here to view linked References

2
critical problems for EV s. Because pure electric vehicle s (PEV s) do not have engine waste heat , winter heating and 33
battery cold start s have become difficult technical problems and large obstacles for all climate applications. Al-Hallaj 34
[5] considered using heat stored in PCM s to improve the cold start performance s of battery pack s in cold regions. 35
Park [6] proposed charging the heat and electricity at the same time for EV s using a heat storage device, and 36
discussed coordinated control and application methods. Gao [7] et al. indicated that PCM s can absorb considerable 37
heat from batteries, PCU s, and motor s to make their working temperature s stable. In addition, waste heat can be used 38
to defrost and preheat the cabin facilitating heat recovery and energy savings . 39
Therefore, a thermal energy storage setup is important for vehicle -mounted heat energy reutilization and 40
complementation, and it will promote electric vehicle thermal management (VTM) integration involving the battery 41
pack s, motor s, power control units (PCU s), heat pump s (HPs), etc. At the same time, realizing rapid heat storage and 42
release is a key problem in vehicle application s. Therefore, heat transfer enhancement technologies are necessary and 43
important. At present, technological development mainly focuses on two aspects : the improvement of comprehensive 44
thermal conductivity and the optimization of the structure for heat exchange. 45
Siddique A et al. [8] filled the PCM with aluminum foam to enhance heat transfer and temperature uniformity . 46
The result s showed that the comprehensive thermal conductivity of the PCM increased. Fukai [9-10] proposed the 47
use of carbon fibers as heat transfer duct s inside PCM s. Mills [11] filled a PCM with a graphite matrix and 48
experimentally investigated the effects of the filling proportion on the thermal conductivity and latent heat. Banaszek 49
[12] designed a spiral thermal energy storage unit and conducted theoretical and experimental research on this type 50
of unit . E. Ass is [13] adopted a small spherical shell to encapsulate PCM and investigated the effects of different 51
diameters on the heat storage rate. 52
To realize heat energy recycling for electric vehicle and improve cold start performances of battery pack , a PCM 53
therma l energy storage unit using flat tube s and corrugated fins is designed and the PCM unit s were cascaded to help 54
extend the range of applicable temperature and satisfy multiple heat source s with different temperature level s. In fact, 55
the heat storage and release rate s of PCM units will be influenced by the different inlet temperature, liquid flow rate, 56
melting point of the PCM and the combination order of the units , and the thermal characteristic s of the PCM units 57
were investigat ed, such as the temperature variation , heat volume, and heat transfer time , etc. 58
2 Design of PCM unit and experiment al system 59
2.1 Thermal energy storage unit 60
The structure and geometric elements of the thermal energy storage unit is shown in Figure 1. The core of unit 61
consist ed of flat tube s and corrugated fins with louvers. Liquid flow ed inside the flat tube s. A PCM w as added 62
between the flat tubes , and the thickness of flat tube was 2 mm . The corrugated fins with louvers were used to 63
increase surface area for heat transfer . The thickness of corrugated fin was 0.1 mm, and the wide of it was 4 cm . The 64
mass of PCM is 1.15 in the thermal energy storage unit. Thermo -physical properties of PCM s are shown in Table 65

3
1. 66
67
Fig. 1 The structure and geometric elements of the thermal energy storage unit 68
Table 1 Thermo -physical properties of PCM 69
Melting point of
PCM (℃) Density
(Kg/m3) Specific heat capacity
(KJ/Kg K) Heat of fusion
(KJ/Kg)
30
40
50 840
852
845 2.5
2.5
2.5 207
220
210
70
To concurrently recover waste heat from heat sources at different temperature s, a cascad ing heat storage 71
combination was built, which adopted multiple melting point PCM units throughout the experimental process. The 72
melting point of the PCM s include d 30℃, 40℃, and 50℃, and the corresponding heat storage unit s were called 30#, 73
40#, and 50# , respectively. The cascad ing combination s have two arrangements that make use of a change in the 74
direction of the flow of the liquid. These orders include the melting point ascending and descending order s, as show 75
in Figure 2. The solid arrow represents the forward flow direction, and the hollow arrow represents the backward 76
flow direction. 77
78
Fig. 2 Cascad ing combination of thermal energy storage units 79
80

4
2.2 System and apparatus 81
82
The experimental system consisted of the thermal energy storage unit, a refrigerating system, a heating tank, a 83
cooling tank, an expansion tank, an air duct, an axial fan, a radiator, pumps, flow distributors, regulating valves, an 84
agitator, filters , etc. The experimental system loop and apparatus are presented in Figure 3 (a) and Figure 3 (b) , 85
respectively. 86
87
（a）System loop 88
89
（b）Experimental apparatus 90
Fig. 3 Experimental system composition 91
The l iquid temperature was regulated using the heating tank, cooling tank, refrigerating system , and radiator. 92
The e xpansion tank was used to maintain the system pressure and to add liquid. The liquid was a 50% aqueous 93

5
ethylene glycol solution by volume . During the experimental process, insulation measures were adopted for the PCM 94
unit, heating tank, cooling tank, and pipes to reduce the environment impact. In addition, the test system mainly 95
include d a NI (National Instrument) data acquisition system, a temperature control unit, a turbine flow meter, 96
thermocouple s, etc. 97
2.3 Working conditions 98
To systematic ally investigate the heat transfer characteristics and effects of the inlet temperature, liquid flow 99
rate, PCM melting point, and order of the PCM units , a series of experimental conditions were defined. The authors 100
defined the liquid flow rate as 1 L/min with an initial PCM temperature of 0℃ and an inlet liquid temperature of 60℃ 101
as the basic condition s during the heat storage process. Other conditions are shown in Table 2. 102
Table 2 The w orking conditions during the heat storage process 103
liquid flow rate
(L/min） initial PCM temperature
(℃) inlet liquid flow
temperature (℃) melting point of
PCM (℃)
0.5
1
2
0 50
60
70 30
40
50
104
Furthermore, the authors defined the liquid flow rate as 1 L/min, the initial PCM temperature as 60℃, and the 105
inlet liquid flow temperature as 0℃ as the basic condition s of the heat release process . The other conditions are 106
shown in Table 3. 107
Table 3 The w orking conditions during the heat release process 108
liquid flow rate
(L/min） initial PCM temperature
（℃） inlet liquid flow
temperature （℃） melting point of
PCM（℃）
0.5
1
2
60 -10
0
10 30
40
50
109
In fact, the PCM heat storage includes the latent heat and the sensible heat; the former is more dominant in the 110
early and middle periods than in the latter periods. Therefore, the essential heat transfer period was defined as the 111
storage or release periods of the 70% heat volume , and the heat storage or release volume in the corresponding 112
period was defined as the essential heat storage or release volume . Some of the investigations and analyses were 113
performed during the essential heat storage and release periods. 114
3 Results and analysis 115
The performance of the thermal energy storage unit was evaluated in the basic condition. In addition, the 116
authors investigated the effects of the inlet liquid temperature, the liquid flow rate , and the PCM melting point and 117

6
comparatively analyzed the performance s of the different cascad ing combination order s. 118
3.1 Inlet temperature 119
The e ffects of the inlet liquid temperature were investigated based on the 40# PCM unit. In the heat storage 120
process, the inlet temperature of the unit was 50℃, 60℃, or 70℃. In the heat release process, the inlet liquid 121
temperature of the unit was -10℃, 0℃, or 10℃. Other parameters were consistent with the basic heat storage and 122
release conditions. 123
124
（a）Heat storage process （b）Heat release process 125
Fig. 4 The variations in the outlet temperature and heat volume for several inlet liquid temperatures 126
Figure 4 (a) demonstrates the variations in the outlet liquid temperature (Tol) of the thermal energy storage unit 127
and the heat storage volume ( Qhs) under different inlet liquid temperature conditions ranging from 50℃to 60℃ 128
during the heat storage process. Because the latent heat exceeded the sensible heat for the PCM , the outlet 129
temperature curves had smaller slopes at the beginning of the phase change heat storage. Furthermore, the slope of 130
Qhs was low for the 50℃condition , and the T ol curve even presented a smooth section during the phase change heat 131
storage period. The phase change heat storage rate was lowest for the 50℃ condition . Meanwhile, the slopes of the 132
Tol and Qhs curves were greatest and the heat storage rate s were the highest for the 70℃ condition in the same 133
period. When the inlet temperature was 60 ℃ (the basic heat storage condition), the essential heat storage time was 134
165 s, and the essential heat storage volume was 309.1 kJ. 135
Figure 4（b）demonstrates the variations in the outlet temperature (Tol) and the heat release volume ( Qhr) under 136
different inlet temperature conditions ranging from -10℃ to 10℃ in the heat release process. The slope of the Tol 137
curve and the heat release rate were at their lowest values for the 10 ℃ condition during the phase change heat 138
release period. Meanwhile, the slope of the Tol curve and the heat release rate were at their highest values for the -10℃ 139
condition in the same period. The essential heat release time was 180 s, and the essential heat r elease volume was 140
317.9 kJ for the 0℃ condition (the basic heat release condition). 141
The a bove -mentioned results prove that the newly design ed thermal energy storage unit exhibited a rapid 142
thermal response and heat transfer rate (storage and release). Additionally, in the experimental range, with increased 143
inlet liquid temperature s, the heat storage rate and Qhs also increased . With decreased inlet liquid temperature s, the 144
heat release rate and Qhr rose. 145
0100200300400500600700800
010203040506070
0 100 200 300 400 500 600 700
Qhs/kJTol /℃
Time/s50℃ Tol
60℃ Tol
70℃ Tol
50℃ Qhs
60℃ Qhs
70℃ Qhs
0100200300400500600700800
-10010203040506070
0 100 200 300 400 500 600 700
Qhr/kJTol/℃
Time/ s-10℃ Tol
0℃ Tol
10℃ Tol
-10℃ Qhr
0℃ Qhr
10℃ Qhr

7
3.2 Melting point 146
The effects of the melting point were investigated in the basic heat storage and release conditions, and the PCM 147
melting point was set to 30℃, 40℃, or 50℃ in the experimenta l process. 148
149
（a）Heat storage process （b）Heat release process 150
Fig. 5 The variations in the outlet liquid temperature and heat volume for several PCM melting points 151
Figure 5 (a) demonstrates the vari ations of the Tol and Qhs under different PCM melting points ranging from 152
30# to 50# during the heat storage process. It could be seen that the T ol slope curve was at its minimum value, and 153
the rising speed of the Qhs was slowest for the 50# condition during the phase change heat storage period, indicating 154
that the phase change heat storage rate was lowest. At the same time, the slope of the Tol curve slope was at its 155
maximum value, and the heat storage rate was highest in the 30# condition in the same period. When the PCM 156
melting point was 40#, the T ol curve presented three apparent rising sections, and the phase change heat storage rate 157
was between 30# and 50#. 158
Figure 5 (b) demonstrates the variations of the Tol and Qhr under different PCM melting poin ts from 30# to 50# 159
during the heat release process. The slope of the Tol curve was at its minimum value, and the rising speed of Qhr was 160
slowest under the 30# condition during the phase change heat release period. The slope of the Tol curve was at its 161
maximum value and the rising speed of Qhr was fastest under the 50# condition in the same period. Therefore, the 162
phase change heat release rate was highest for 50# and lowest for 30#. 163
164
（a）Essential heat transfer time 165
0 100 200 300 400 500 600 700 800
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0 100 200 300 400 500 600
Qhs/kJTol /℃
Time/s30# Tol
40# Tol
50# Tol
30# Qhs
40# Qhs
50# Qhs
0 100 200 300 400 500 600 700 800
010203040506070
0 100 200 300 400 500 600
Qhr/kJTol/℃
Time/s30# Tol
40# Tol
50# Tol
30# Qhr
40# Qhr
50# Qhr
050100150200250300350
heat storage heat releaseteht /s30#
40#
50#

8
166
（b）Heat power （c）Average outlet temperature 167
Fig. 6 Comparisons and analyses of PCMs with different melting points 168
Figure 6 illustrates the comparisons and analyses of PCMs with different melting point s in the essential heat 169
transfer (storage or release) periods. In the figure , as the PCM melting point increased, the essential heat transfer 170
time (t eht) was e xtended, the heat power ( qhp) decreased , and the average outlet temperature (T ao) rose gradually in 171
the basic heat storage condition. In the basic heat release conditi on, as the PCM melting point increased , the qhp and 172
Tao also rose, but t eht gradually decreased, and the higher melting point PCMs maintained higher outlet liquid 173
temperature s. 174
3.3 Liquid flow rate 175
The e ffects of the liquid flow rate were investigated bas ed on the 40# PCM unit, and the liquid flow rate was 176
0.5 L/ min, 1 L/min, or 2 L/min in the experiment al process. Other parameters were consistent with the basic heat 177
storage and release conditions. 178
Figure 7（a）demonstrates the variations of the Tol and Qhs under different liquid flow rate s ranging from 0 .5 179
L/min to 2 L/min during the heat storage process. As demonstrated, the slope of the Tol curve was at a maximum and 180
the rising speed of Qhs was fastest under the 2 L/min condition during the phase change heat storage period, which 181
means that the phase change heat storage rate was the fastest. At the same time, the rising speed of Qhs was slowest 182
and the heat storage rate was lowest under the 0.5 L/ min condition in the same period. When the liquid flow rate was 183
1 L/min, the phase change heat storage rate and rising speed of Qhs were between 0 .5 L/ min and 2 L/min. 184
185
（a）Heat storage process （b）Heat release process 186
Fig. 7 Variations in the outlet temperature an d heat volume for several liquid flow rate s 187
Figure 7（b）illustrates variations of the Tol and Qhr under different liquid flow rate s ranging from 0 .5 L/min to 188
0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50 4.00
heat storage heat releaseqhp/kW30#
40#
50#
0.00 5.00 10.00 15.00 20.00 25.00 30.00 35.00 40.00 45.00 50.00
heat storage heat releaseTao/℃30#
40#
50#
0.00 100.00 200.00 300.00 400.00 500.00 600.00 700.00 800.00
010203040506070
0.00 100.00 200.00 300.00 400.00 500.00 600.00 700.00 800.00 900.00 1000.00
Qhs/kJTol /℃
Time/s0.5L/min Tol
1.0L/min Tol
2.0L/min Tol
0.5L/min Qhs
1.0L/min Qhs
2.0L/min Qhs
0100200300400500600700800
0 10 20 30 40 50 60 70
0100 200 300 400 500 600 700 800 900 1000
Qhr/kJTol /℃
Time/s0.5L/min Tol
1.0L/min Tol
2.0L/min Tol
0.5L/min Qhr
1.0L/min Qhr
2.0L/min Qhr

9
2 L/min during the heat release process. It could be seen that the slope of the Tol curve was at its maximum value, 189
and the rising speed o f Qhr was fastest under the 2 L/min condition during the phase change heat release period, 190
which means that the phase change heat release rate was fastest. At the same time, the rising speed of Qhr was 191
slowest and the heat release rate was lowest under the 0.5 L/ min condition in the same period. When the liquid flow 192
rate was 1 L/min, the heat release rate and rising speed of Qhr were between 0 .5 L/min and 2 L/min. 193
Figure 8 demonstrates the comparisons and analyses of different liquid flow rate s in the range of essential heat 194
transfer (storage and release) periods. With increasing liquid flow rate, teht gradually decreased , and qhp increased . 195
Meanwhile, Tao increased in th e heat storage process but decreased gradually in the heat release process. 196
197
（a）Essential heat transfer time 198
199
（b）Heat power （c）Average outlet temperature 200
Fig. 8 Comparisons and analyses of different liquid flow rates 201
Furthermore, in the middle and later periods, the role of the liquid flow rate continued to decrease compared 202
with the early period. Therefore, heat transfer enhancement could not be realized through a continuous increase in 203
the liquid flow rate. 204
3.4 Comb ination order of the units 205
The cascaded combination of thermal energy storage units using multiple melting point PCM units is employed 206
to help extend the applicable temperature range , and it is also benefited waste heat recycl ing from multiple heat 207
source s with different temperature level s such as the battery pack, power control unit ( PCU ), and motor for electric 208
vehicle. In addition, the authors hope that the heat of the c ascad ing thermal energy storage combination can release 209
rapidly in practical application such as battery preheating in cold weather, and find out the effects of the different 210
cascade order through experimental investigation. 211
The thermal energy storage units include 30# PCM, 40# PCM , and 50# PCM, and the cascad ing combination 212
0.00 50.00 100.00 150.00 200.00 250.00 300.00 350.00
heat storage heat releaseteht /s0.5L/min
1L/min
2L/min
0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50 4.00
heat storage heat releaseqhp/kW0.5L/min
1L/min
2L/min
0.00 5.00 10.00 15.00 20.00 25.00 30.00 35.00 40.00 45.00 50.00
heat storage heat releaseTao/℃0.5L/min
1L/min
2L/min

10
has two orders including ascending melting point and descending melting point , as show in Fig. 2. The inlet and 213
outlet liquid temperature s of each unit were measured . The inlet temperature of the first unit was also the inlet 214
temperature of the cascad ing combination, and the outlet temperature of the last unit was also the outlet temperature 215
of the cascad ing combination according to the two orders. During the heat storage process, the initial temperature of 216
the cascad ing combination was 0 ℃, the inlet liquid temperature was 60 ℃, and the liquid flow rate was 2 L/min. 217
Meanwhile, the initial temperature of the cascad ing combination was 60℃, the inlet liquid temperature was 0 ℃, and 218
the liquid flow rate was still 2 L/min during the heat release proc ess. 219
220
（a）Melting point ascending order （b）Melting point descending order 221
Fig. 9 The variations of the outlet liquid temperature during the heat storage process 222
Figure 9 (a) demonstrates the variations in the outlet liquid temperature under the melting point ascending 223
order condition during the heat storage process. With the exception of 40# PCM, it could be seen that the outlet 224
temperature curves of the 30# PCM and 50# PCM did not present an obvious smooth section, and this phenomenon 225
means that the phase changes of 30# PCM and 50# PCM did not occur rapidly and completely in the early and 226
middle p eriods. By analyzing the results, the authors found that the melting point ascending order was not good for 227
the phase change uniformity during heat storage process. Fig ure 9（b）illustrates the variations in the outlet liquid 228
temperature under the melting point descending order condition during the heat storage process. Notably, all outlet 229
temperature curves exhibited obvious smooth sections in the same period, and the phase change was very uniform. 230
231
（a）Melting point descending order （b）Melting point ascending order 232
Fig. 10 Variations in the outlet liquid temperatures during the heat release process 233
010203040506070
0 200 400 600 800 1000Tol /℃
Time/ s30# unit
40# unit
50# unit
010203040506070
0 200 400 600 800 1000Tol /℃
Time/ s30# unit
40# unit
50# unit
010203040506070
0 200 400 600 800 1000Tol /℃
Time/s30# unit
40# unit
50# unit
010203040506070
0 200 400 600 800 1000Tol /℃
Time/ s30# unit
40# unit
50# unit

11
Figure 10 (a) demonstrates the variations in the outlet liquid temperature under the melting point descending 234
order condition during the heat release process. As shown, the phase change was not very uniform in the early and 235
middle periods. Figure 10（b）demonstrates the variations in the outlet liquid temperature under the melting point 236
ascending order condition during the heat release process. As shown, the phase change uniformity was better than the 237
descending order in the same period. 238
In addition, when the heat storage and release times reached 240 s, 70% of the heat volume was stored or 239
released. Analyses were also performed during the essential heat storage and release periods. At the same tim e, the 240
Qhs of the descending order condition was more than the Qhs of the ascending order condition by approximately 89 241
kJ in the same heat storage period. At the same time, the Qhr of the ascending order condition was more than the Qhr 242
of the descending order condition by approximately 88 kJ in the same heat release period . 243
4 Conclusions 244
A PCM thermal energy storage unit using flat tubes and corrugated fins was designed . The thermal 245
characteristics and the effects of the influence facto rs were investigated which include the inlet temperature, the 246
liquid flow rate , the melting point of the PCM , and the combination order of the units. Experimental results show 247
that the designed PCM thermal energy storage unit realizes small cell contact heat transfer and exhibits rapid heat 248
storage and release rates. The cascad ing combination of the PCM unit s can help extend the range of applicable 249
temperature and satisfy multiple heat source s with different temperature level s. During the heat storage process, 250
adopting the melting point descending order can further increase the phase change uniformity of the units and the 251
heat storage rate. Meanwhile, the unit s with the ascending melting point order perform better than the unit s with the 252
descending melting point order during the heat release process. 253
Furthermore, the phase change heat storage and release rates increase with increasing temperature differences 254
between the PCM melting point and the inlet liquid temperature of the un it, and this characteristic affects the melting 255
and solidification of the PCM. Meanwhile, the heat storage and release rates increase with increasing liquid flow 256
rates, but the effects are diminishing in the middle and later periods. The average outlet liquid temperature increase 257
in the heat storage process , but decreases in the heat release process. These findings are conducive to the selection of 258
the optimal condition , and establishment of the control strategy for the PCM utilization in further study. 259
Acknowledgements 260
The authors gratefully acknowledge the financial support from the NSFC (National Natural Science Foundation 261
of China) under grant No. 51376079, Department of Science & Technology of Jilin Province development plan item 262
(No. 20130204018GX), and Science & Technology Development Plan item (No. 14KG096) of Changchun, Jilin 263
province. 264

12
Nomenclature 265
Tol outlet liquid temperature ( ℃) 266
Qhs heat storage volume (kJ) 267
Qhr heat release volume (kJ) 268
teht essential heat transfer time (s) 269
qhp heat power (kW) 270
Tao average outlet temperature ( ℃) 271
272
Subscripts 273
274
ol outlet liquid 275
hs heat storage 276
hr heat release 277
eht essential heat transfer 278
hp heat power 279
ao average outlet 280
References 281
[1] Schatz O. Cold Started Improvement with a Heat Store. SAE Paper 910305, 1991. 282
[2] Korin E ，Reshef R，Tshernichovesky D and Sher E. Improving Cold -Start Functioning of Catalytic Converters by 283
Using Phase -Change Materials ，SAE Paper 980671, 1998. 284
[3 ]Aldoss T, Ewing D J, Zhao Y and Ma L. Numerical Investigation of Phase Change Materials for Thermal 285
Management Systems . SAE Paper 2009 -01-0171, 2009. 286
[4 ]Adamczyk A A, Hubbard C P, Ament F, Oh S H, M. J. Brady and M. C. Yee. Experimental and Modeling 287
Evaluations of a Vacuum -Insulated Catalytic Converter. SAE Paper1999 -01-3678, 1999. 288
[5] Al -Hallaj S, Selman JR. A novel thermal management system for EV batteries using phase change material 289
(PCM). Journal of the Electrochemical Society 2000,147:3231 – 6. 290
[6] June Kyu Park, Hee Sang Park, Apparatus and method for air conditioning ve hicle interior using battery charge 291
control of electric vehicle, US Patent20110118919A1;2011. 292
[7]Tianshi Zhang, Chun Gao, Qing Gao, Guohua Wang, MingHui Liu, Yuanke Guo, Cong Xiao and Y.Y. Yan. Status 293
and development of electric vehicle integrated thermal management from BTM to HV AC. Applied Thermal 294
Engineering. 75, 2015,1 -12. 295
[8] Siddique A. Khateeb, Shabab Amiruddin, Mohammed Farid, etal. Thermal management of Li -ion battery with 296
phase change material for electric scooters: experimental validation [J]. Jo urnal of Power Sources 142 (1 -2), 297
2005.3:345 -353. 298
[9] Andrew Mills a, b, Mohammed Farid a, J.R. Selman b, Said Al -Hallaj b, Thermal conductivity enhancement of 299

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phase change materials using a graphite matrix, Applied Thermal Engineering 26 (2006) 1652 –1661 . 300
[10] J. Fukai, M. Kanou, Y. Kodama, O. Miyatake, Thermal conductivity enhancement of energy storage media 301
using carbon fibers, Energy Convers. Manage. 41 (2000) 1543 –1556. 302
[11] J. Fukai, Y. Morozumi, Y. Hamada, O. Miyatake, Effect of carbonfiber brushes on conductive heat transfer in 303
phase change materials,Int. J. Heat Mass Transfer 45 (2002) 4781 –4792. 304
[12]BANASZEK J, DOMANSKIR, REBOWM. Experimental study of solid -liquid phase change in a spiral heat 305
energy storage unit[ J] . Applied Thermal Engineering, 1999, 19(1):1253 -1277. 306
[13] E. Assis, L. Katsman, G. Ziskind, R. Letan. Numerical and experimental study of melting in a spherical shell . 307
International Journal of Heat and M ass Transfer, 2007;50:1790 -1804. 308
309

Table 1 Thermo -physical properties of PCM
Melting point of
PCM (℃) Density
(Kg/m3) Specific heat capacity
(KJ/Kg K) Heat of fusion
(KJ/Kg)
30
40
50 840
852
845 2.5
2.5
2.5 207
220
210

Table 2 The working conditions during the heat storage process
liquid flow rate
(L/min） initial PCM temperature
(℃) inlet liquid flow
temperature (℃) melting point of
PCM (℃)
0.5
1
2
0 50
60
70 30
40
50

Table 3 The working conditions during the heat release process
liquid flow rate
(L/min） initial PCM temperature
（℃） inlet liquid flow
temperature （℃） melting point of
PCM（℃）
0.5
1
2
60 -10
0
10 30
40
50
Table(s)

Fig. 1 The structure and geometric elements of the thermal energy storage unit

Fig. 2 Cascad ing combination of thermal energy storage units

（a）System loop
Figure(s)

（b）Experimental apparatus
Fig. 3 Experimental system composition

（a）Heat storage process （b）Heat release process
Fig. 4 The variations in the outlet temperature and heat volume for several inlet liquid temperatures

（a）Heat storage process （b）Heat release process
Fig. 5 The variations in the outlet liquid temperature and heat volume for several PCM melting points
0100200300400500600700800
010203040506070
0 100 200 300 400 500 600 700
Qhs/kJTol /℃
Time/s50℃ Tol
60℃ Tol
70℃ Tol
50℃ Qhs
60℃ Qhs
70℃ Qhs
0100200300400500600700800
-10010203040506070
0 100 200 300 400 500 600 700
Qhr/kJTol/℃
Time/ s-10℃ Tol
0℃ Tol
10℃ Tol
-10℃ Qhr
0℃ Qhr
10℃ Qhr
0 100 200 300 400 500 600 700 800
010203040506070
0 100 200 300 400 500 600
Qhs/kJTol /℃
Time/s30# Tol
40# Tol
50# Tol
30# Qhs
40# Qhs
50# Qhs
0 100 200 300 400 500 600 700 800
010203040506070
0 100 200 300 400 500 600
Qhr/kJTol/℃
Time/s30# Tol
40# Tol
50# Tol
30# Qhr
40# Qhr
50# Qhr

（a）Essential heat transfer time

（b）Heat power （c）Average outlet temperature
Fig. 6 Comparisons and analyses of PCMs with different melting points

（a）Heat storage process （b）Heat release process
Fig. 7 Variations in the outlet temperature an d heat volume for several liquid flow rates

050100150200250300350
heat storage heat releaseteht /s30#
40#
50#
0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50 4.00
heat storage heat releaseqhp/kW30#
40#
50#
0.00 5.00 10.00 15.00 20.00 25.00 30.00 35.00 40.00 45.00 50.00
heat storage heat releaseTao/℃30#
40#
50#
0.00 100.00 200.00 300.00 400.00 500.00 600.00 700.00 800.00
010203040506070
0.00 100.00 200.00 300.00 400.00 500.00 600.00 700.00 800.00 900.00 1000.00
Qhs/kJTol /℃
Time/s0.5L/min Tol
1.0L/min Tol
2.0L/min Tol
0.5L/min Qhs
1.0L/min Qhs
2.0L/min Qhs
0100200300400500600700800
0 10 20 30 40 50 60 70
0100 200 300 400 500 600 700 800 900 1000
Qhr/kJTol /℃
Time/s0.5L/min Tol
1.0L/min Tol
2.0L/min Tol
0.5L/min Qhr
1.0L/min Qhr
2.0L/min Qhr

（a）Essential heat transfer time

（b）Heat power （c）Average outlet temperature
Fig. 8 Comparisons and analyses of different liquid flow rates

（a）Melting point ascending order （b）Melting point descending order
Fig. 9 The variations of the outlet liquid temperature during the heat storage process

0.00 50.00 100.00 150.00 200.00 250.00 300.00 350.00
heat storage heat releaseteht /s0.5L/min
1L/min
2L/min
0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50 4.00
heat storage heat releaseqhp/kW0.5L/min
1L/min
2L/min
0.00 5.00 10.00 15.00 20.00 25.00 30.00 35.00 40.00 45.00 50.00
heat storage heat releaseTao/℃0.5L/min
1L/min
2L/min
010203040506070
0 200 400 600 800 1000Tol /℃
Time/ s30# unit
40# unit
50# unit
010203040506070
0 200 400 600 800 1000Tol /℃
Time/ s30# unit
40# unit
50# unit

（a）Melting point descending order （b）Melting point ascending order
Fig. 10 Variations in the outlet liquid temperatures during the heat release process

010203040506070
0 200 400 600 800 1000Tol /℃
Time/s30# unit
40# unit
50# unit
010203040506070
0 200 400 600 800 1000Tol /℃
Time/ s30# unit
40# unit
50# unit