A review of phase change materials (pcms) for cold energy storage applications

1. A review of Phase Change Materials (PCMs) for cold
energy storage applications
Rajeh,Taha Hussein
Nanjing University of Science and Technology, Department
of Energy and Power Engineering
Nanjing, China
eng.tahaalrajeh@hotmail.com
Prof. houlei Zhang
Nanjing University of Science and Technology, Department
of Energy and Power Engineering
Nanjing, China
zhangl@njust.edu.cn
Abstract— Thermal energy storage or TES is referred as
heat or cold storage. This energy can be utilized using various
practical applications in the field of Electrical and Mechanical
engineering. Cold storage operations can be performed using
Latent Heat, Sensible Heat and various chemical processes.
Solid- Liquid Phase Change Materials (PCMs) are discussed
because of latent heat, high energy storage and high charging
and discharging capabilities. The scope of this work is to
choose a suitable solid liquid material, identify its properties
and applications. Properties like phase change temperature,
enthalpy and cycling stability with reference to practical
applications of solid liquid materials are discussed.
Keywords—TES (Thermal Energy Storage), Latent Heat,
Sensible Heat, PCMs (Phase Change Materials), Cold Storage,
Enthalpy, Cycling stability.
I. INTRODUCTION
Thermal energy storage (TES) is currently the hot topic
of research to its applications in the field of energy, it
provides wide range of solutions in the field of renewable
energy applications. [1] TES comprise of two categories,
Sensible TES and Latent TES. [2] In sensible TES energy
storage is attained when temperature of storage medium is
changed and in latent TES energy storage is attained by
changing the material phase [2]. Latent TES is used more
efficiently because of its energy storage ability during phase
change, as it is greater than the energy storage in sensible
TES. Thermal cold storage (TCS) are the special type of
systems developed to keep the temperature of particular
environment to a very low level with the help of different
processes like using condensing machines etc [3].
Depending on the practical application TES may use various
steps but the process of TES in general is performed using
three steps i.e. Storage, Charging and Discharging. Use of
phase change materials (PCMs) with thermal storage mainly
cold storage avoids the additional equipments for better
efficiency. Typically, various solid-liquid materials are used
for this purpose due to their suitable phase change
temperature and large melting enthalpy [3].
A. Classification of Cold Strage
Classification of cold storage is based on the following
properties.
a) Temperature.
Classification of cold storage based on
temperature comprise of two sublets.
• Cold storage maintained above the zero
degree celcius temperature.
• Cold storage maintained below the zero
degree celcius temperature [4].
b) Construction.
Classification of cold storage based on
construction depends on which type of
material/product we want to preserve/store [5].
c) Load.
Classification on the basis of specific loads in cold
storage is also of importance, load is classified as:
• Sensible heat load.
• Latent heat load [2].
B. Classification of PCMs
Classification of PCMs mostly depend on the
application, the temperature change in that application,
limitations of heat storage and the storage system itself.
PCMs materials mostly depend on sensible heat storage and
latent heat storage. The expressions to calculate the sensible
heat storage and latent heat storage are mentioned
respectively [2,5,6].
∫=
2
1
.
T
T
Psensible dTCQ …………… Equation 1
∫ +=
2
1
.
T
T
SLatent dTCQ ∫+∆
2
1.
T
TPC
ls dTCH ….
Equation 2
The selection of material to be used for phase change TES is
very important and one should have the knowledge of
melting and freezing temperature according to practical
range of applications with chemical stability, non-corrosive,
non-toxic and minimum sub-cooling properties [5,6].
II. FACTORS NECESSARY FOR THE SELECTION OF PCM
To get efficient TES the selection of PCM plays an
important role, the properties of PCM like chemical and
kinetic properties play a major role in the development and
1

2. utilization of TES [5,7]. The following are the required
characteristics/properties required for a suitable PCM [8].
I. THERMOPHYSICAL
PROPERTIES OF PCM.
The Thermophysical properties for the selection of PCM
comprise the following important characteristics:
• Melting Temperature.
• Latent heat of fusion.
• Sensible heat storage.
• Thermal conductivity.
• Phase transformation.
• Nucleation and crystal growth.
ii. Chemical Properties of PCM.
The chemical properties for the selection of PCM required
for efficient TES are the following:
• Complete and reversible freeze and melt cycle.
• Non-corrosive.
• Non- Toxic.
• Recyclable.
Based on the chemical properties mentioned above the
phase change heat storage materials are further classified as
shown in figure 1. [ 3,5,7,8]
Figure 1: Classification of PCMs according to chemical
properties.
There are variety of non-commercial and commercial PCMs
based on low melting point temperature mentioned in [6,9].
Once the PCM material is selected for a particular
application based on the above factors the next important
step is the stability and design of cold storage system [9].
Few of the commercial PCMs are mentioned in the
following table along with their characteristics [2, 10].
Table 1: List of commericial PCMs
PCM
Name
Type of
Product
Melting
Temperatur
e Co
Heat of
fusion
(Kj/kg)
Density
Kg/
3
m
Source
2

3. SN33
Salt
solution
-33 245 1.24
Cristopia[1
1]
TH-31 n.a. -31 131 n.a. TEAP[12]
SN29
Salt
solution
-29 233 1.15
Cristopia[1
1]
SN26
Salt
soulution
-26 268 1.21
Cristopia[1
1]
TH-21 n.a. -21 222 n.a. TEAP[12]
SN21
Salt
solution
-21 240 1.12
Cristopia[1
1]
STL-
21
Salt
solution
-21 240 1.12
Mitsubishi
Chemical[
13]
SN18
Salt
solution
-18 268 1.21
Cristopia[1
1]
TH-16 n.a. -16 289 n.a. TEAP[12]
STL-
16
n.a. -16 n.a. n.a.
Mitsubshi
Chemical[
13]
SN15
Salt
solution
-15 311 1.02
Cristopia[1
1]
SN12
Salt
solution
-12 306 1.06
Cristopia[1
1]
STLN
10
Salt
solution
-11 271 1.05
Mistubishi
Chemical[
13]
SN10
Salt
solution
-11 310 1.11
Cristopia[1
1]
TH-10 n.a. -10 283 n.a. TEAP[12]
STL-6
Salt
solution
-6 284 1.07
Mitsubishi
Chemical[
13]
SN06
Salt
solution
-6 284 1.07
Cristopia[1
1]
TH-4 n.a. -4 286 n.a. TEAP[12]
STL-3
Salt
solution
-3 328 1.01
Mitsubishi
Chemical[
13]
SN03
Salt
solution
-3 328 1.01
Cristopia[1
1]
ClimS
el C7
n.a. 7 130 n.a.
Climator[1
4]
RT5 Paraffin 9 205 n.a.
Rubitherm
GmbH[15]
ClimS
el C15
n.a. 15 130 n.a.
Climator[1
4]
ClimS
el C23
Sal
hydrate
23 148 1.48
Climator[1
4]
RT25 Paraffin 26 232
Rubitherm
GmbH[15]
STL27
Salt
hydrate
27 213 1.09
Mitsubishi
Chemical[
13]
S27
Salt
hydrate
27 207 1.47
Cristopia[1
1]
RT30 Paraffin 28 206 n.a.
Rubitherm
GmbH[15]
TH29
Salt
hydrate
29 188 n.a. TEAP[12]
ClimS
el C32
Salt
hydrate
32 212 1.45
Climator[1
1]
RT40 Paraffin 43 181 n.a.
Rubitherm
GmbH[15]
STL47
Salt
hydrate
47 221 1.34
Mitsubishi
Chimecal[
13]
ClimS
EL
C48
n.a. 48 227 1.36
Climator[1
1]
STL52
Salt
hydrate
52 201 1.3
Mitsubishi
Chemical[
13]
RT50 Paraffin 54 195 n.a.
Rubitherm
GmbH[15]
STL55
Salt
hydrate
55 242 1.29
Mitsubishi
Chemical[
13]
TH58 n.a. 58 226 n.a. TEAP[12]
ClimS
EL
C58
n.a. 58 259 1.46
Climator[1
1]
RT65 Paraffin 64 207
Rubither
GmbH[15]
ClimS
el C70
n.a. 70 194 1.7
Climator[1
1]
RT80 Paraffin 79 209 n.a.
Rubitherm
GmbH[15]
TH89 n.a. 89 149 n.a. TEAP[11]
RT90 Paraffin 90 197 n.a.
Rubitherm
GmbH[15]
RT110 Paraffin 112 213 n.a.
Rubitherm
GmbH[15]
A. Steps for the Selection of PCMs for cold storage
The selection of perfect PCM is an important criterion
for a particular application in the field of cold storage. The
design of cold storage system mainly depends on exact
phase transition temperature [16]. Different analysis like
scanning calorimetric analysis at varying heating and
cooling temperatures provide details about the important
properties like exact phase transition temperature, latent
heat of fusion, thermal stability and reactivity [5,17].
3

4. B. Problems in the selection of PCM
Heat transfer is the common problem of using latent heat
cold storage leading to incomplete melting, freezing and
loss in extraction of stored cold energy. Techniques like
Nano structures and encapsulation are deployed for the
shape stabilization of PCMs [4,5,6].
C. Nano structured PCMs
To avoid the problem of heat transfer and improve the
efficiency of PCM, nano structures are embedded in the
PCMs. The geometry of nano structures is very important
because the design of nano structures may affect the
Thermophysical properties of PCM [6,18]. For most of the
applications metal oxide and silver titania nano structures
are used enabling increase in thermal conductivity. The heat
release and the thermal conductivity properties of pure
PCMs and hybrid nano PCMS (HyNPCMs) are compared
and there is significant improvement while using HyNPCMs
[5,19,20].
D. Encapsulation of PCMs
The practical method to efficiently perform TES for
cold storage system is encapsulation. There are
various techniques of encapsulation: macro, micro
and nano encapsulation. Based on the temperature
importance we can select any of the technique for
cold storage systems which may result in energy
saving and system efficiency. All the techniques
define different applications and usability [5,19].
E. Shape Stabilized PCM
There is special category of PCM materials referred as
shape specialized PCMs comprising of working material
and supporting material [21]. The supporting material
remains in the solid phase even if the working material
undergoes a phase transition. The methods to develop the
materials are based on physical methods like blending and
adsorbing as well as chemical methods such as graft co-
polymerization [5,22]. This special category is developed
for different set of applications [ 22].
F. PCM Selection for Food storage and medical
applications
Various below zero degree Celsius PCMs have been
developed for different range of applications recently such
as food storage compartment, medical applications, dual
refrigeration, ice cream vending and frozen food
transportation etc. They have the property of improving life
of the storage material as well as saving electricity [5,19,
21].
The applications of PCM in the field of food storage and
medical applications has significant benefits like:
• Use of PCM prolongs the life of food or saving the
medical applications in a particular temperature
during off time refrigeration along with keeping
them fresh.
• It helps in saving the energy with the high
refrigeration temperature, as the melting
temperature of PCM is greater than refrigeration
temperature it will provide cold energy to the
system and the products [5,6].
The PCM selected for the cold storage system in the
current scenario is Eutectic PCM E-26 with the
following properties in table 2 [10].
Table 2: Selected PCM properties
PCM
Name
Latent
heat
capacity
KJ/kg
Melting
Temperatur
e Co
Heat of
fusion
(KJ/kg)
Densit
y
Kg/
3
m
Volumetric
heat
capacity
(MJ/ 3
m )
E-26 260 -26 3.67 1250 325
The exact measurement of temperature range for many
PCM applications is difficult to measure, to measure the
efficient phase transition temperature range for the materials
used of cold storage PCMs should follow a standard
scientific procedure. Differential Scanning Calorimeter
(DSC) technique is used for such a purpose. The
information obtained from DSC is of significance as it helps
in choosing the right material for every application. If the
phase transition for a material is not chosen according to the
application, it may result in system's inaccuracy and
resource loss. The accurate temperature range also specifies
the charging and discharging process of cold storage
material, the governing mathematical model for charging
and discharging of PCM for cold storage is expressed in the
following section [2].
G. Mathematical model for Heat load calculation of PCM
It is necessary to develop a mathematical model for food
and medical applications storage system so as to make
proper heat load calculations. As it’s the energy storage
model, it is based on joules unit for the system. The
following mathematical parameters are necessary for
thermal load calculations [5,7].
• Energy content (Q).
• Time (T).
• Heat leakage (W).
• Heat capacity (M pC ).
To calculate the energy content for the on duration of the
refrigerator the following mathematical relation is used:
11
1
*WtQ
where
QQQ
off
fi
=
=+
…………… Equation 3
Here iQ is the heat content at the on time of refrigeration,
fQ is the heat content at the off time of the refrigeration
and 1W is the average heat leakage from the ambient
temperature to cooling system. While offt is the off time
of compressor for a particular duration [7].
4

5. From equation 3
11 *WtQQQ offfi =−= ……… Equation 4
For the off-duration time of the compressor for a particular
duration the heat content is calculated as:
1)( QQQQ ifev +−= …………. Equation 5
Equation 5 represents the steady state of the system for the
on time of the system, during that time the average heat
leakage remains constant.
And
oneev tWQ *= ………….............. Equation 6
From equations 4, 5, 6, we can derive that
onone tWtWW ** 11 −= …………. Equation 7
Which implies that:
eoffon
on
W
W
tt
t 1
=
+
…………………. Equation 8
To calculate the cooling compartment when,
=fT average high temperature
=iT average low temperature
ififp QQTTMC −=− )( …………. Equation 9
11 WtQQQ offif ==− ………….…. Equation 10
)(
1
if
offp
TT
W
tMC
−
= …………..… Equation 11
The MATLAB representation for the above equations is
shown in figure 2.
Figure 2: Equations Simulation
III. STRUCTURE OF COLD STORAGE SYSTEM
The structure of cold storage systems mainly depends on
the type of PCM used, so far the discussion is based on
different PCM types such as organic, inorganic, and liquid
metals etc with their applications varying from simple
systems such as thermal gloves to complex systems such as
spacecraft [23]. Each and every application offer certain
design challenges. The primary concern in developing such
systems is linked with the PCM transition from solid to
liquid or vice versa state. The main objective to select a
suitable PCM with suitable temperature range for melting
temperature of PCM and the cold storage system, keeping in
view that the melting temperature of PCM should be greater
than the temperature of the refrigeration system [24].
The design structure for cold storage should be based on
five key standards [25].
i. Design cost
The design cost of the system should be low
resulting in a system that would be easily
designed and tested at various levels.
ii. Design Standards
The design standard should be stringent to
ensure system's efficiency.
iii. Design size
The size of the system design should be such
that it could be used industrially as well as
domestically.
iv. Design usability
It deals with the usage of system in different
operating conditions at different temperature
ranges.
v. Safety
The designed system should be safe and secure
for the users as well as for different products.
The approach to select a material and design the application
system based on this selection is approached in two steps
[2, 5, 25].
1. Ideal system
To develop mathematical model for an ideal
system we have the following three cases:
• Case I (Ideal system in ambient air)
In this scenario, the system temperature is controlled
and it is considered that it only uses sensible heat. The
object loses thermal energy, cools down and vice versa.
To develop the mathematical model the following
assumptions are required to be fulfilled.
• The ambient air temperature should be constant.
• The object air heat transfer coefficient should be
constant.
• The object should be isothermal.
The mathematical equations governing such a scenario
are given below:
).(.
..
ambobjobj
objp
TTA
T
dt
d
cmQ
dt
d
−−
==
α
……….…. Equation 12
5

6. Using the above assumptions, we can conclude that the
energy lost by the object is equal to the energy transferred to
ambient air [5].
).(
.
.
ambobj
p
obj
obj TT
cm
A
T
dt
d
−
−
=
α
…… Equation 13
)
.
.
exp().)0((
)(
p
obj
ambobj
ambobj
cm
A
TtT
TtT
α−
−=
=−
……….
Equation 14
tTTA
tQhmH
ambpcobj
pcpc
∆−
=∆=∆=∆
)..(.
..
α

………….. Equation15
The following differential equation calculates the change in
temperature in the ideal scenario.
).(.
.
ambpcobj
pc
TTA
hm
t
−
∆
=∆
α
……….. Equation 16
The heat transfer area, heat transfer coefficient, mass of
object and heat capacity are the driving factors to transfer
energy as shown in equation below.
).
.
/1/1
1
.
exp(
).)((
).
.
.
exp().)0(()(
t
cm
k
A
TtT
t
cm
kA
TtTTtT
p
ins
obj
ambobj
p
effobj
ambobjambobj
+
−
−=
−−==−
α
Equation …… 17
At the phase transition the latent heat is released which
results in change in temperature and the heat loss at that
time is compensated as long as the object undergoes a phase
transition [2,5,25].
In such a case the temperature is controlled until the energy
loss take place, the change in temperature can now be
calculated as:








−
∆
=∆
).(.
..
ambpcobj
pc
TTA
hm
t
α
…………. Equation 18
• Case II (Ideal system with insulation)
To slow down the heat transfer and keep the temperature
consistent the thin insulation layer is introduced to reduce
cold loss. This technique is very commonly applied to
thermal energy cold storage. This insulation layer is used to
reduce the thermal resistance, the following assumption are
necessary for the modeling of this system [5,26].
• The area of the heat transfer should remain
constant because of the thin insulation.
• The insulation layer should not store any heat
energy.
The following mathematical equation represents the thermal
conductivity of insulation material.












+
−
−=












+
−
−=−
=
=
t
cm
k
A
TTt
cm
k
A
TTTtT
p
ins
obj
ambtobj
p
ins
obj
ambtobjambobj
.
.
/1/1
1
.
).(.
.
/1/1
1
.
exp
).()(
)0(
)0(
α
α
Equation 19
• Case III (Ideal system, Insulation, with PCM)
This approach uses insulation layer along with PCM
material to stabilize temperature in the system and
reduce the heat loss thus improving the system
efficiency and reducing heat loss [5, 26].
The following simplifications are necessary for the
system's mathematical modeling.
• The temperature of PCM and the object should be
the same.
• There should be a constant temperature in a system
so that a small temperature difference could be
recognized between PCM and the object.
The change in temperature while perfecting the above
simplification can be calculated as:
).(.
.
ambpceffobj
pc
TTkA
hm
t
−
∆
=∆ …….. Equation 20
The above equation also shows for how long the
temperature of insulated object with PCM will be stable,
thus preventing the energy loss [5,27].
6

7. 2. Practical systems
The above cases discussed are based on ideal systems and
can't be used in practical scenarios, for a practical scenario it
is very difficult to keep the temperature of insulation layer,
PCM and the system to remain stable. While looking into
the heat transfer coefficients the temperature of PCM,
insulation layer and system itself are at different states.
To keep the temperature constant for all the three sublets,
the PCM could be placed around the internal surface of
insulation so that the temperature remains close to the phase
change temperature [28].
The PCM will create the isothermal enclosure for the
system during the phase change [5].
IV. APPLICATIONS OF PCM (E-26) FOR MULTIPURPOSE
TRANSPORT BOXES AND CONTAINERS
There are wide range of applications for multipurpose
transport boxes not limited to food only but could be used
for medical applications, in different sizes based on
requirements of the system[10,27]. Special type of
insulation material along with PCM are used to ensure the
storage of materials inside the system so that they are
effective in saving energy, an example of such system is
developed by va-Q-tec AG in which we can obtain thermal
conductivity at least five folds better than by using only
conventional insulators. The product's thermal conductivity
is about 4-5 * 10 3− W/mK [5,28]. Which saves energy
equivalent to 10W and without reducing the storage space.
It has been tested that the PCM can help maintain a
temperature of -20 C0
and below can be maintained for
more than four days. The company has also produced
transport containers deployed with super insulations and
PCM and does not required any external or internal
electricity source used to maintain a temperature of -18
C0
or below for four days [5,28].Figure(3) shows some
pictures of transport boxes with PCM for different
purposes.
Figure 3: transport boxes with PCM for different purposes (pictures: down
va-Q-tec,above left delta T Gesellschaft für Medizintechnik mbH, above
right transport box[5].
Conclusion
PCMs has a strong potential in the field of industrial as well
as domestic applications due to its extraordinary stability to
store energy and release it at the time of necessity. There are
various research and development challenges for PCM
materials, the major challenge among those is keeping the
stable temperature for the system. Few of the problems like
corrosion, low heat transfer and super cooling could be
handled by the use of nano-structured encapsulation, which
opens a new technological path in the field of thermal
energy system. By observing the graph from appendix and
figure(4), it is clear that when the system is without energy
source/power source its temperature changes, in our case the
temperature of our system will tend to increase. To prevent
the system to observe the phenomena and keep our products
as trade mark. The melting temperature at any case remains
higher than the refrigeration.
Phase transition takes place at that instance and the system
with PCMs installed will tend to stabilize the temperature.
So, that the product inside could be used/Preserved for a
long period at a particular temperature.
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Appendex
Figure 4
Code
clear, close all
clc
Tobj = [0 -2 -4 -6 -8 -10 -12 -14];
Tamb = [ 4 2 0 -2 -4 -6 -8 -10];
t= 0:-1:-25
Tr = Tobj-Tamb;
A = 0.25:0.1:0.28;
keff = 0.65:0.1:0.68;
X = A*keff;
t= 3600:1000:6600;
Y = X*t;
%alpha = 0.58:0.01:0.61;
Cp = 3.67:0.1:3.70;
m = 0.3;
Q = m*Cp;
xxx = exp (-Y/Q);
aaa= exp (Y/Q);
figure
subplot (2,1,1)
plot(t,xxx)
title('cold')
subplot (2,1,2)
plot(t,aaa)
hold off
title('hot')
8