...Organic pcm (review, 2016)

Developments in organic solid–liquid phase change materials
and their applications in thermal energy storage
R.K. Sharma a
, P. Ganesan a,⇑
, V.V. Tyagi b
, H.S.C. Metselaar a
, S.C. Sandaran c
a
Department of Mechanical Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia
b
DST Centre for Policy Research, B.B.A. University (Central University), Rai Bareilly Road, Lucknow, UP, India
c
University Technology Malaysia, Johor Bahru, Malaysia
a r t i c l e i n f o
Article history:
Received 4 December 2014
Accepted 29 January 2015
Keywords:
Thermal energy storage
Latent heat
Organic phase change materials
Encapsulation
Thermal conductivity enhancement
a b s t r a c t
Thermal energy storage as sensible or latent heat is an efficient way to conserve the waste heat and
excess energy available such as solar radiation. Storage of latent heat using organic phase change
materials (PCMs) offers greater energy storage density over a marginal melting and freezing temperature
difference in comparison to inorganic materials. These favorable characteristics of organic PCMs make
them suitable in a wide range of applications. These materials and their eutectic mixtures have been
successfully tested and implemented in many domestic and commercial applications such as, building,
electronic devices, refrigeration and air-conditioning, solar air/water heating, textiles, automobiles, food,
and space industries.
This review focuses on three aspects: the materials, encapsulation and applications of organic PCMs,
and provides an insight on the recent developments in applications of these materials. Organic PCMs have
inherent characteristic of low thermal conductivity (0.15–0.35 W/m K), hence, a larger surface area is
required to enhance the heat transfer rate. Therefore, attention is also given to the thermal conductivity
enhancement of the materials, which helps to keep the area of the system to a minimum. Besides, various
available techniques for material characterization have also been discussed. It has been found that a wide
range of the applications of organic PCMs in buildings and other low and medium temperature solar
energy applications are in abundant use but these materials are not yet popular among space applications
and virtual data storage media. In addition, it has also been observed that because of the low melting
point of organic PCMs, they have not yet been explored for high temperature applications such as in
power plants.
Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction
Thermal energy storage (TES) using phase change materials
(PCM) have been a key area of research in the last three decades
and more, and became an important aspect after the 1973–74
energy crisis. Depletion of the fossil fuels and increase in the ener-
gy demand has increased the gap between energy demand and its
supply. Excess energy stored in a suitable form has been able to
bridge this energy demand/supply gap significantly. TES can be
used for either short term or long-term storage. If the energy is
stored for a few hours, it is termed as short term storage and is
essential in many industrial and domestic applications; while if
energy is stored for a month or more, it is generally considered
as a long term storage device which may also be required in some
applications. Thermal energy storage plays a very important role
when energy demand and supply are not equal. Excess energy
http://dx.doi.org/10.1016/j.enconman.2015.01.084
0196-8904/Ó 2015 Elsevier Ltd. All rights reserved.
Abbreviations: ASHRAE, American Society of Heating, Refrigeration and Air-
Conditioning Engineers; CA, capric acid; CNF, carbon nanofiber; CNT, carbon
nanotube; CSP, concentrated solar power; DSC, differential scanning calorimeter;
DTA, differential thermal analysis; EG, expanded graphite; ETC, evacuated tube
collector; FESEM, field emission scanning electron microscope; FT-IR, Fourier
transformed infrared; GA, genetic algorithm; GNF, graphite nanofiber; GNP,
graphite nano particles; HDPE, high density polyethylene; HRHE, heat recovery
heat exchanger; HP, horse power; HSU, heat storage unit; IR, infrared; LA, lauric
acid; LDPE, low density polyethylene; LHTES, latent heat thermal energy storage;
MA, myristic acid; MEPCM, microencapsulated phase change material; MMA,
methyl methacrylate; MP, methyl palmitate; M.P., melting point; MS, methyl
stearate; NG, nano graphite; PA, palmitic acid; PCL, poly(e-caprolactone); PCM,
phase change materials; PEG, poly(ethylene glycol); PEO, poly(ethylene oxide);
PMMA, poly methyl methacrylate; PNHMPA, poly(N-hydroxymethyl acrylamide);
PSD, particle size distribution; PU, polyurethane; PVC, polyvinyl chloride; PVP,
polyvinyl pyrrolidone; PW, paraffin wax; RT, rubitherm; SA, stearic acid; SEM,
scanning electron microscope; St, styrene; TGA, thermogravimetry analysis; TEM,
transmission electron microscope; TES, thermal energy storage; TRNSYS, transient
system simulation tool; USD, United States Dollar.
⇑ Corresponding author. Tel.: +60 3 79675204/7670 (O); fax: +60 3 79674579 (O).
E-mail address: poo_ganesan@um.edu.my (P. Ganesan).
Energy Conversion and Management 95 (2015) 193–228
Contents lists available at ScienceDirect
Energy Conversion and Management
journal homepage: www.elsevier.com/locate/enconman

available in the off peak time can be stored in TES devices for later
use e.g. solar energy is available only in sunshine hours, thus, the
excess heat may be stored in the day time and used later in the
night hours. Energy storage helps in the saving of expensive fuels
and reduces the wastage of energy and capital cost which leads
to a cost effective system [1]. TES devices are majorly categorized
as sensible heat storage and latent heat storage (LHS) devices.
Although the most commonly used device in industrial applica-
tions for thermal energy storage, is the sensible heat storage but
the latent heat thermal energy storage (LHTES) devices have
attracted a wide range of industrial and domestic applications
and will be discussed in the later sections of this paper. LHTES pro-
vides large energy storage density with a smaller temperature
change when compared to sensible heat storage devices [2,3]. Pre-
vious studies have shown that PCM has the capability to store
about 3–4 times more heat per volume than is stored as sensible
heat in the temperature increment of 20 °C [4]. However, LHTES
devices confront the difficulties that arise when the latent heat
method is applied. This is due to the low thermal conductivity,
change in density, stability of thermal properties and subcooling
of PCMs.
There are a large number of review articles on the phase change
materials such as Zalba et al. [5], Farid et al. [6], Tyagi and Buddhi
[7], Sharma et al. [1], Cabeza et al. [10], Liu et al. [11], Tatsid-
jodoung et al. [12], Nkwetta and Haghighat [13], Pielichowska
and Pielichowski [14] and ample information on organic PCMs
are available in the literature, however, in a scattered manner. So
far, most of the review articles have focused on general classifica-
tions of the PCMs and have presented their applications irrespec-
tive of their organic/inorganic nature. Organic PCMs are a very
important class of materials because of their unique thermal prop-
erties such as congruent melting and narrow melting/freezing tem-
perature range. These properties make them suitable for many
applications in solar energy storage, textiles, and cooling of elec-
tronic devices. Organic PCMs are the most suitable materials for
cooling/heating of building. Sarier and Onder [15] presented a
review of organic PCMs suitability for textile industries. To the best
of our knowledge, no review article has been made available which
summarizes the classifications, thermal properties and applica-
tions of organic PCMs. Therefore, a review is required, which gives
a deeper insight to organic PCMs and their applications. This paper
reviews the present state of the art of the organic PCMs for thermal
energy storage and provides insights into the efforts that have been
made to develop new organic PCMs, showing enhanced thermal
performance. Attention is also given to the encapsulation methods
and thermal conductivity enhancements. Use of organic PCMs in
domestic and industrial applications such as in buildings, electron-
ic devices, refrigeration, solar energy, textile, automobiles and food
industry are broadly discussed.
2. Phase change materials
The PCMs are latent heat storage materials that have high heat
of fusion, high thermal energy storage densities compared to sen-
sible heat storage materials and absorb and release heat at a con-
stant temperature when undergoing a phase change process (e.g.
solid–liquid). The storage capacity of LHTES devices is given by
[16]:
Q ¼
Z Tm
Ti
mCpdT þ mamDhm þ
Z Tf
Tm
mCpdT ð1Þ
Q ¼ m½CspðTm TiÞ þ amDhm þ ClpðTf TmÞ ð2Þ
where Q is the storage capacity, Cp specific heat, Ti, Tm, and Tf are ini-
tial, melting and freezing temperature, and h is the enthalpy.
2.1. Classification of PCMs
Phase change materials are majorly classified as organic, inor-
ganic, and eutectic and a comprehensive classification was given
by Abhat [17] and shown in Fig. 1. Based on the melting/freezing
temperature and latent heat of fusion, a large number of organic
and inorganic materials can be treated as PCM. Even though, their
melting/freezing temperature lies in the operating range, many of
the PCMs do not satisfy the criteria required for an adequate ther-
mal energy storage device because no single material can have all
the properties required for TES. Therefore, the available materials
are to be used and their thermo physical properties are to be
improvised by making suitable changes in systems design or by
using external agents. For example, the thermal conductivity of
PCM can be increased by dispersion of metallic nanoparticle in
the PCM or by inserting metallic fins in the systems design and
supercooling can be suppressed by using a nucleating agent in
the PCM.
Organic PCMs such as paraffin wax consist of straight n-alkanes
chain (CH3–(CH2)–CH3) and fatty acids that are made up of straight
chain hydrocarbons and are relatively expensive and possess com-
bustible nature. Organic materials possess the capability of congru-
ent melting without phase separation. These compounds are
available in a wide range of melting points [19]. Paraffin is safe,
reliable, predictable, inexpensive, non-corrosive and chemically
inert and stable below 500 °C but possesses extremely low thermal
conductivity (0.1–0.3 W/m K) and is not suitable for encapsulation
in plastic containers. Organic PCMs will be discussed in detail in
the later sections of this paper.
Inorganic materials are generally hydrated salts and metallic
and have a large number of applications in solar energy [20,21].
As PCM, these materials are capable of maintaining the heat of
fusion (350 MJ/m3
) even after a large number of cycles and
relatively higher thermal conductivity (0.5 W/m °C), but they
melt incongruently. One of the cheapest inorganic materials which
is suitable to be used as thermal energy storage is Glauber salt
(Na2SO4H2O), which contains 44% Na2SO4 and 56% H2O in weight
and was studied by Telkes [22]. This salt has high latent heat
(254 kJ/kg) and melting point of about 32.4 °C but it is highly prone
to phase segregation and subcooling. The corrosion of salt on metal
Fig. 1. Classifications of phase change materials [1,5,17,18].
194 R.K. Sharma et al. / Energy Conversion and Management 95 (2015) 193–228

container is also a concern [23]. The use of thickening agents e.g.
Bentonite clay and nucleating agent e.g. Borax help to overcome
the subcooling but they reduce the heat transfer rate by lowering
the thermal conductivity. Generally, most of the salt hydrates
encounter the same problem. An extensive review of hydrated salts
was presented in chapter 1 of Lane [16] and Sharma et al. [24].
The eutectic is a composition of two or more components such
as organic–organic, organic–inorganic, inorganic–inorganic and
each of them change their phase congruently and form a mixture
of component crystal during crystallization [25]. Eutectics general-
ly melt and freeze congruently and leave no chances of separation
of components [1].
2.2. Thermo-physical properties
Since thermo-physical properties of PCMs vary from one
manufacturer to another, any of the available PCM cannot be sim-
ply used for designing an effective thermal energy storage device
[26]. The PCM to be used as thermal storage system should possess
the following thermal, physical, chemical, and economic properties
[1,17,27–29]:
Thermal properties:
Suitable phase change temperature.
High specific heat.
High latent heat.
High thermal conductivity in both phase liquid and solid.
The operating temperature of heating or cooling should be
matched with the phase change temperature of the PCM to be
selected for energy storage. High specific heat provides the addi-
tional sensible heat storage. High latent heat is desirable to store
the large amount of energy in a small volume of PCM i.e. to mini-
mize the physical size of thermal energy storage. Thermal conduc-
tivity should be high in order to minimize the temperature
gradient required for the melting and freezing of PCM.
Physical properties:
High density.
No or little subcooling during freezing.
Low vapor pressure.
Small volume change.
High density materials require relatively small storage contain-
ers and little subcooling avoid the temperature range required for
freezing or melting of the PCM and give a single value of phase
change temperature i.e. high nucleation rate. Low vapor pressure
and small volume change in the PCM help to reduce the complexity
of geometry of the container.
Chemical properties:
Prolonged chemical stability.
Compatible with capsule material.
Non-toxic, non-flammable, and non-explosive.
Continuous freezing and melting cycles may hamper the chemi-
cal composition of the PCM so it is highly desirable that the mate-
rial maintains its chemical stability over a long period of time. The
PCM is to be encapsulated so it is not expected to have any kind of
undesirable reaction to construction materials and from at safety
point of view, it should be non-toxic, non-flammable, and non-
explosive.
Economic properties:
Abundantly available.
Inexpensive.
Inexpensive and easy availability of PCMs is a highly desirable
characteristic.
2.3. Encapsulation of PCMs
Encapsulation is the technique used to hold the material in a
sealed container of certain volume in order to achieve the follow-
ing goals:
To avoid direct contact between the PCM and environment
which may be harmful for the environment or change the com-
position of the PCM.
To prevent the leakage of the PCM when it is in a liquid state.
To increase the heat transfer area.
Encapsulation of PCMs has significantly received the attention
of researchers in the last 20 years or more and different capsule
materials and their compatibility with PCM along with the differ-
ent geometries of encapsulation was discussed by Lane [20].
Encapsulation can be done in two possible ways, micro and macro
encapsulations [19,30]. Microencapsulation is a technique in which
a large number of PCM particles of 1–1000 lm diameter are
enclosed in a solid shell and then arranged in a continuous matrix
[31]. Microencapsulation has widely found its application in tex-
tiles [32–35], cosmetics [36,37], pharmaceuticals [38–40], and
buildings [41–43]. This encapsulation system suffers from low heat
transfer rates due to the low thermal conductivity of the matrix
materials and the chances of subcooling are higher. Another reason
for this low heat transfer rate is the rigidity of the matrix that pre-
vents the convective currents and forces all heat transfer to occur
only by conduction [27]. Microencapsulation requires skills to be
done and it is a relatively expensive process. Microencapsulated
phase change materials (MEPCMs) are expected to possess some
certain characteristics such as required morphology, uniform dia-
meter, thermal stability, shell mechanical strength, and penetra-
tion abilities [44]. MEPCMs are in the form of pouches, tubes,
sphere, panels or other receptacles and can be used directly as heat
exchangers or can be incorporated into building products. Alkan
et al. [45] in their literature survey indicates that urea–formalde-
hyde (UF) resin, melamine–formaldehyde (MF) resin and polyur-
ethanes (PU) are the most appropriate microcapsule shell
material. Macro encapsulation is very commonly used because of
its availability in various shapes and sizes. This is mainly used to
hold the liquid PCM and to prevent changes in its composition
due to contact with the environment. It also adds the mechanical
stability to a system if the container is sufficiently rigid. There
are numerous techniques adopted for microencapsulation such as
coacervation [46–49], suspension [33,50,51], emulsion [52–54],
condensation [55–58], and polyaddition polymerization [50,59,60].
In the coacervation method, more than one colloid is involved
and it results from the neutralization of the oppositely charged col-
loids in an aqueous solution [61]. Suspension polymerization is a
technique for encapsulation in which the PCM as core is filled in
a polymer shell. A monomer is dispersed in the form of droplets
in an appropriate medium and polymerization is initiated.
Sánchez-Silva et al. [62] studied the microencapsulation of various
PCMs by suspension copolymerization of styrene (St) and methyl
methacrylate (MMA). They investigated the influence of the
monomers/paraffin on the encapsulation process and thermo-phy-
sical properties. The ratio of MMA and St is reported to influence
the polymerization rate and affect the time at which the identity
point is reached. In a recent study [63], the effect of polymeric shell
dry glass transition temperature and the reaction temperature on
the microencapsulated paraffin prepared by suspension-like
copolymerization technique was studied. It was reported that,
R.K. Sharma et al. / Energy Conversion and Management 95 (2015) 193–228 195

when the shell dry glass transition temperature was at least 10 °C
above the reaction temperature, the irregular shape particles were
observed, while, when the transition temperature was between
10–20 °C, uniform and spherical particles were obtained. PMMA
microcapsules containing docosane [45], n-octacosane [52], and
n-eicosane [64] were prepared by emulsion polymerization and
their thermal properties were evaluated and in all three studies,
the thermal cycle test showed a good chemical stability. The
results showed that the PMMA microcapsules consisted of 28%
docusate, 43% n-octacosane, and 35% n-eicosane respectively and
the best thermo-physical properties as a PCM was shown by the
43% n-octacosane. Rao et al. [65] encapsulated the n-docosane in
the melamine resin shell by polycondensation technique. Melted
paraffin is emulsified in water using water-soluble mixers to
achieve the desired size and stability and then melamine resin is
added to the solution. Addition of acid initiates the polycondensa-
tion. MEPCM with a core mass fraction of 60% showed a latent heat
of 150 kJ/kg in the DSC test and the thermal cycling test found this
encapsulation process thermally and chemically stable and reli-
able. Recently, Zhang et al. prepared the MEPCM by encapsulating
n-octadecane as a core in silica shells at various pH values using
the interfacial polycondensation technique [66]. SEM images
showed the strong dependence of morphology and microstructure
on the acidity of the reaction solution. An optimum sample was
achieved at pH value of 2.89 and particle size of 17 lm.
Technological advancements have made it possible to encapsu-
late PCMs at nanoscale as well. Sukhorukov et al. [67] have shown
that nanocapsules are more stable than microcapsules. They found
that deformation in the 10 nm capsule is significantly lesser than
that in the 10 lm for the same force exerted on the capsule. Unlike
the microcapsules, in the nanoencapsulation technique the core
material is kept in the shell during freeze/thaw cycle [68]. The n-
tetradecane was encapsulated at nano level of diameter as
100 nm and urea and formaldehyde were used as the shell poly-
merization materials. The experiments were then conducted to test
the thermal reliability of the nanoencapsulated PCM in Ref. [68]. In
this study, at the stirring rate of 200 rpm and a pH value of 3–4, the
prepolymer solution was added drop by drop into oil/water emul-
sion. The stirring rate was increased to 500 rpm and maintained for
4 h, then the urea–formaldehyde polymer network was formed
and the oil/water interface, the PCM was then encapsulated. At
the mass content of 60% PCM, differential scanning calorimeter
(DSC) results indicated the high latent heat 134.61 kJ/kg. Very
recently, Tumirah et al. [69] prepared the n-octadecane filled
nanocapsules and carried out the thermo-physical characterization
for TES. The melting and freezing point were reported as 29.5 °C
and 24.6 °C respectively when tested by DSC. The influence of shell
material St/MMA on the encapsulation efficiency was evaluated
and the nanocapsules showed good thermal properties for the test-
ed 360 thermal cycles.
The effects of capsule geometry have also been reported in pre-
vious studies and found that the geometry can be a significant
parameter to improve the thermal performance of PCMs. Regular
geometries like square [70–73], cylindrical [74–78], and spherical
[79–83] have been extensively tested but the studies using irregu-
lar geometries such as triangular and trapezoidal are scarce. Dug-
girala et al. [84] investigated the solidification of binary mixture
of various concentrations of ammonia-water filled in trapezoidal
cavity. However, this study does not explicitly investigate the
effect of the trapezoidal cavity on the solidification rate and is
not based on solidification/melting of NEPCM. Recently, Sharma
et al. [85] numerically investigated the effect of trapezoidal cavity
on solidification of copper–water nanofluid. They performed the
CFD simulation for various aspect ratios and calculated the solidifi-
cation time for various initial fluid temperatures, cold wall tem-
peratures, and Grashof number. This shape of cavity was found
to be a controlling parameter for the total solidification time of
NEPCM.
Organic phase change materials show negligible or no super-
cooling during the freezing process and provide congruent melting,
while supercooling is one of the major problems in the inorganic
materials. In addition, comparatively they are more chemically
stable, non-corrosive, possess high latent heat and low vapor pres-
sure. Unlike the inorganic materials, most of the organic PCMs pos-
sess a sharp or narrower range of phase change temperature.
Inorganic materials have high thermal conductivity and are com-
paratively less expensive but suffer of decomposition and require
nucleating agents for crystallization. Encapsulation is essential
for PCMs to avoid their interaction with the external environment,
which helps to curb the chemical reactions to occur and to prevent
the leakage when they are in the molten stage. The encapsulation
types such as the macro, micro and nano encapsulation and the
techniques to encapsulate them play a very important role in
enhancing the heat transfer rate and their durability. Organic PCMs
show a good mixing property with construction materials and pro-
vide high energy density compared to inorganic materials. Among
all the methods available for microencapsulation, In situ polymer-
ization has been found as the most suitable technique.
3. Organic PCMs
Organic PCMs provide congruent melting and are further classi-
fied as paraffins and non-paraffins. These materials provide the
congruent melting without phase segregation over the large num-
ber of melting/freezing cycles at the cost of degrading latent heat
of fusion and do not suffer from supercooling. Hale [86] in 1971,
provided the data related to the material properties of more than
500 PCMs required by thermal design engineers to build efficient
thermal energy storage devices. This was followed by numerous
studies that focused on organic PCMs. Paraffin waxes, poly(ethy-
lene glycol)s, fatty acids and their derivatives are the major classi-
fication of organic PCMs, which undergo a solid–liquid phase
transition during heating and subsequent cooling. Polyalcohols
and polyethylene are the other groups of organic PCMs, which
undergo a solid–solid phase transition. Such a kind of phase tran-
sition occurs at a fix temperature by absorbing/releasing large
amounts of the latent heat. A detailed list of organic PCM candi-
dates can be found in the literature [5,6,10,17,87,88]. Apart from
the many listed advantages of organic PCMS, their major draw-
backs are low thermal conductivity which curb the charging/dis-
charging rate, super cooling effect in cooling cycles, and leakage
of PCM in the containers [89–91].
3.1. Paraffins
Paraffin or paraffin wax is a mixture of straight chain n-alkanes
which is represented by the chemical formula CnH2n+2, where
20 6 n 6 40. Depending on the chain length of the alkane, paraffins
may be even-chained (n-paraffin) or odd-chained (iso-paraffin)
[17]. Studies conducted over recent years on paraffins are summa-
rized in Table 1. The molecular chain of paraffin wax involves large
amount of latent heat during the crystallization/fusion. The melt-
ing temperature of these compounds increases with increase of
number of alkane chains in the molecules [92] as seen in Table 1.
This increment in the melting temperature is because of the elevat-
ed induced dipole attraction between n-alkane chains [15] e.g. the
melting point of C14 is 4.5 °C and that of C18 is 28 °C. Many previ-
ous studies [93–100] have shown that paraffin waxes are capable
to absorb, store, and release a great amount of heat over a large
number of phase change cycles. They are excellent materials for
energy storage, particularly in the buildings with a heat capacity

of 2.14–2.4 kJ/kg K, and latent heat of 200–220 kJ/kg. Paraffins dis-
play no phase segregation even after many phase transition cycles
and exhibit many favorable characteristics as PCMs such as they
are chemically inert, non-corrosive, colorless, durable, inexpensive,
available abundantly, ecologically harmless and nontoxic
[16,17,26,101]. On the contrary, Lane [16] reported that paraffin
shows slow oxidation when exposed to oxygen, therefore, it
requires leak-proof containers. They are generally compatible with
all metallic containers but on the other hand they make some plas-
tic containers soften [16]. Paraffins possess low thermal conduc-
tivity in the liquid state; therefore, high latent heat is required
during the freezing cycle. This property of the paraffin can be
improved by using metal fillers and other techniques in the base
materials, and this has been discussed in detail in Section 4.
3.2. Fatty acids
Non-paraffins are generally found in the form of acid and repre-
sented by the formula CH3(CH2)2nCOOH [88]. The fatty acids are
basically derived from vegetable and animal sources which ensure
nonpolluting source of supply [106] and are divided into six
groups: caprylic, capric, lauric, myristic, palmitic and stearic [17].
The individual material in this category has its own properties
unlike paraffins which have very similar properties [1]. When com-
pared with paraffins, these materials show excellent phase change
(solid–liquid) properties but are about three times more expensive
than paraffins [26]. An extensive survey of organic materials was
done by Abhat et al. [107] and Buddhi and Sawhney [108] and
the number of esters, fatty acids, alcohol’s and glycol’s were iden-
tified to be suitable as latent heat storage. These materials are
highly flammable and should avoid exposure to high temperature,
flames and oxidizing agents. Fatty acids and palmitoleic acids,
which have a low melting point, are the most common among
others in the category. The melting temperatures of fatty acids vary
from 5 to 70 °C and the latent heat from 45 to 210 kJ/kg [8]. These
materials have the capability to be retained in the shape of host
material due to their high surface tension of 2–3 104
N/cm. A
great insight of fatty acids was recently presented by Yuan et al.
[109]. Some of the fatty acids investigated in the past are presented
in Table 2. PCMs based on fatty acids can be categorized as follows
[110]:
1. Naturally occurring triglycerides.
2. Hydrates of acids of triglycerides and their mixtures.
3. Esters of the fatty acids of naturally occurring triglycerides.
4. Refined/synthesized triglyceride products produced by a com-
bination of fractionation and transesterification processes.
5. Synthesized triglyceride products using hydrogenation (or
dehydrogenation) and fractionation.
6. Synthesized triglyceride products using cis–trans isomerization
and fractionation.
7. Synthesized fatty acid derivatives that have the desired freezing
point temperatures.
8. Refined fatty acid hydrates that have the desired freezing point
temperatures.
Table 1
Thermo-physical properties of some paraffins, paraffin waxes, and its blends used as latent heat storage.
Compound Tm (°C) Hf (kJ/kg) Cp (kJ/kg K) k (W/m K) q (kg/m3
) Ref.
Decane 29.65 202 – – 726 (l) [92]
Undecane 25.6 177 – – 737 (l) [92]
Dodecane 9.6 216 – – 745 (l) [92]
Tridecane 5.4 196 2.21 (l) – 753 (l) [92]
Paraffin C14 4.5 165 – – – [17]
Tetradecane 5.5 227 2.07 (s) 0.15 825 (s) [92]
Paraffin C15–C16 8 153 2.2 (s) – – [17]
Paraffin C16–C18 20–22 152 – – – [7]
Paraffin C13–C14 22–24 189 2.1 0.21 790 (l) [7]
900 (s) [4]
Paraffin C18 28 244 2.16 0.15 814 [17]
Nonadecane 32 222 – – 785 [3]
Eicosane 36.6 247 788 [3]
Heneicozane 40.2 213 – – 791 [3]
Paraffin C20–C33 48–50 189 2.1 0.21 769 (l) [17]
912 (s)
Paraffin C22–45 58–60 189 2.1 0.21 795 (lC) [17]
920 (s)
1-Tetradecanol 38 205 – – 825 [102]
Paraffin C23–C45 62–64 189 2.1 0.21 0.915 [17]
Paraffin wax 64 173.6 – 0.167 (l) 790 (l) [103]
266.0 0.346 (s) 916 (s) [21]
Paraffin C21–C50 66–68 189 – 0.21 830 (l) [17]
930 (s)
Biphenyl 71 119.2 – – 994 (l) [103]
1166 (s) [21]
Propionamide 79 168.2 – – – [103]
Napthelene 80 147.7 2.8 0.132 (l) 976 (l) [103]
0.341 (s) 1145 (s) [104]
0.310 (s) [21]
Tetradecane + octadecane 4.02 to 2.1 227.52 – – – [8]
91.67% Tetradecane + 8.33% hexadecane 1.70 156.20 – – – [10]
Tetradecane + docosane 1.5–5.6 234.33 – – – [10]
Paraffin blend (n = 14–16) 5–6 152 – – 783 (s) [105]
Paraffin blend (n = 15–16) 8 147–153 – – 751.6 [105]
Paraffin blend (n = 16–18) 20–22 152 – – – [105]
Octadecane + heneicosane 25.8–26 193.93 – – – [10]
Octadecane + docosane 25.5–27 203.80 – – – [10]

9. Prepared mixtures produced by essentially any of the previous
processing approaches with other chemicals (preferable cheap
and nontoxic) to produce eutectic compositions with the
desired freezing point temperature range.
Fatty acids possess the superior properties such as congruent
melting, good chemical and thermal stabilities, nontoxicity,
biodegradability, and melting temperature range suitable for many
latent heat storage applications. They are also capable of thousands
of thermal (melting/freezing) cycles without any notable degrada-
tion in thermal properties [106,111,112]. The fatty acids and their
eutectic mixtures have been recently investigated extensively as
possible phase change materials for low/medium energy storage
applications such as solar energy storage and residential applica-
tions. Thermal properties of fatty acids such as capric, lauric, pal-
mitic and stearic acids were evaluated by Feldman et al. [111]
and they found that these materials are very promising to be used
as PCMs in space heating applications. The melting point of these
acids was measured between 30 °C and 65 °C and latent heat
between 153 kJ/kg and 182 kJ/kg. Feldman et al. [106,112,113] fur-
ther investigated the behavior of fatty acids and their thermal sta-
bility as PCMs. The thermal performance of myristic acid was
investigated by Sari and Kaygusuz [114] and they found that this
acid shows better stability at low temperature. They also observed
that this PCM is more effective when heat exchanger is in horizon-
tal position. A. Sari et al. studied the thermal performance of stearic
acid [116] and palmitic acid [117] as thermal energy storage. In
their later study, A. Karaipekli et al. [118] tested the eutectic mix-
ture of capric and stearic acid and found it as a potential material
for low temperature solar energy in building applications. Detailed
review on organic PCMs can be found in Rozanna et al. [110] and
Sarier and Onder [15].
In order to improve the thermal performance and widen the
application scope of organic PCMs, many researches have been
preparing the eutectics of fatty acids and other PCMs. For a low
temperature thermal energy storage, a mixture of capric and lauric
acids was evaluated as possible phase change material by L. Shieli
et al. [123]. Later Dimaano and Watanabe [124] in their research,
mixed pentadecane in the capric–lauric mixture and found that
50% of pentadecane in the mixture provides the highest heat
charged. The solid–liquid phase transition in lauric, palmitic, stea-
ric acid and their binary systems was studied by Zhang et al. [125]
and they found that thermal properties of 23% lauric–palmitic acid
eutectic system remained stable after 100 heating–cooling cycles
at 32.8 °C. Sari et al. [8] evaluated the thermal properties of lau-
ric–stearic, myristic–palmitic, and palmitic–stearic acid and tested
the thermal stability for 360 melting–freezing cycle and concluded
that these materials can be effectively used for a one year period.
Later Sari [9] studied the thermal performance of eutectic mixtures
of lauric–myristic acid, lauric–palmitic acid, and myristic–stearic
Table 2
Thermo-physical properties of some fatty acids used as latent heat storage.
Acid Tm (°C) Hf (kJ/kg) Cp (kJ/kg K) k (W/m K) q (kg/m3
) Ref.
Enanthic 7.4 107 – – – [119]
Butyric 5.6 126 – – – [111]
Caproic 3 131 – – – [111]
Propyl palmiate 10 186 – – – [102]
Pelargonic 12.3 127 – – – [140]
Isopropyl stearate 14–18 140–142 – – – [120]
Caprylic 16 148.5 – 0.149 (l) 862 (l) [17]
16.5 149 0.148 (l) 1033 (s) [103]
981 (s) [21]
Butyl stearate 19 140 – – – [102]
123–200 [120]
Dimethyl sabacate 21 120–135 – – – [120]
Undecylenic 24.6 141 – – – [121]
Vinyl stearate 27–29 122 – – – [120]
Undecylic 28.4 139 – – – [140]
Capric 31.5 153 – 0.149 (l) 886 (l) [17]
32 152.7 0.153 (l) 878 (l) [103]
[21]
Tridecylic 41.8 157 – – – [140]
Methyl-12 hydroxy-stearate 42–43 120–126 – – – [120]
Lauric acid 42–44 178 1.6 0.147 (l) 870 (l) [17]
44 177.4 862 (l) [103]
1007 (s)
Elaidic 47 218 – – 851 (l) [86]
Myristic 54 187 1.6 (s) – 844 (l) [17]
58 186.6 2.7 (l) 990 (s) [103]
49–51 204.5 [114]
Pentadecanoic 52–53 178 – – – [1]
Margaric 60 172.2
Palmitic 63 187 – 0.165 (l) 874 (l) [17]
61 203.4 – 0.159 (l) 847 (l) [122]
64 185.4 – 0.162 (l) 850 (l) [117]
Stearic 70 203 2.35 (l) 0.172 (l) 941 (l) [17]
69 202.5 848 (l) [122]
60–61 186.5 [116]
69.4 199 [86]
Nonadecylic 67 192 [15]
Arachidic 74 227 – – – [121]
Heneicosylic 73–74 193 [15]
Phenylacetic 16.7 102 [1]
Acetamide 81 241 – – – [86]

acid as PCM and found that these PCMs have good thermal stability
for at least for four years. Subsequently Sari and Kaygusuz investi-
gated the thermal behavior of the eutectic mixture of myristic acid
and stearic acid that can be effectively used as PCMs in low tem-
perature thermal energy storages. Some of the studies carried out
on eutectic mixtures are shown in Table 3.
3.3. Sugar alcohol
Sugar alcohol, also known as polyalcohols are considered as
medium temperature (90–200 °C) PCMs and have received less
attention by researchers. Previous studies revealed that alcohols
such as xylitol, erythritol, and mannitol possess a much higher val-
ue of latent heat than other materials in this family. Alcohols have
been tested as potential phase change materials in the last four
decades. Hormansdorfer [131] first proposed the use of polyalco-
hols as PCM but their phase change behavior was discussed by Tal-
ja and Roos [132], and Kaizawa et al. [133]. They observed that
some polyalochols possess latent heat almost double than that of
the other organic PCMs. Among sugar alcohols, erythritol has
shown excellent suitability as a thermal energy storage material
[134]. Its melting point is 20 °C and latent heat of fusion is
339.8 kJ/kg. Shukla et al. [135] performed a thermal cycling test
of erythritol and observed no degradation for the 75 thermal cycles
and experienced a supercooling of 15 °C. At a certain temperature,
they observed the phase separation in the liquid state. When com-
pared to lower temperature organic PCMs, sugar alcohol exhibited
much larger degree of super cooling which can hamper the effi-
ciency of thermal energy storage and has been addressed in num-
ber of research articles [136–139]. These materials also undergo
the 10–15% volume expansion during melting [133].
Solé et al. [140] tested sugar alcohols D-mannitol, myo-inositol,
and galactitol as potential PCMs by thermal cycle test. It was found
that for the chosen set of parameters, myo-inositol sustained well
during the cycling test, however, FT-IR images show changes in
chemical structure, which does not affect the thermal properties.
Some polymorphic changes were noticed when myo-inositol was
analyzed between 50 and 260 °C but they were found almost dis-
appeared in the temperature range 150 to 260 °C. Galactitol
showed poor cycling stability and at 18th cycle, its freezing tem-
perature was measured as 60 °C, which was 102 °C before starting
the cycle test. D-Mannitol showed the reaction with oxygen in the
atmosphere which leads to the non-stable materials with a lower
thermal energy storage capacity.
Ali Memon et al. [141] developed a novel form stable alcohol
based PCM by preparing a composite of lauryl alcohol and kaolin
using vacuum impregnation method, Fig. 2(a). Simultaneous heat-
ing checked the exudation of this composite during impregnation
process. Leakage testing was performed to check the maximum
absorption ratio. The composite was placed in the oven at a certain
temperature above the melting point for 30 min. This composite
was checked for thermal reliability by performing cycling test.
An experimental set up, Fig. 2(b), which consist a test room, a
150 W infrared lamp (as heating source), a hollow PVC envelope,
and the thermocouple, was designed and developed. Leakage test
showed that the maximum 24% lauryl acid can be retained by this
composite. DSC measured the melting point of this composite as
19.14 °C, which is less than of the pure lauryl acid melting point
of 25 °C and latent heat of fusion of 48.08 kJ/kg, which is higher
when compared to that of the lauryl acid 205.4 kJ/kg. Thermal
cycling test revealed that after one month of complete cycling test,
the melting point of composite dropped by 0.39 °C and latent heat
of fusion was dropped by 0.7 kJ/kg.
3.4. Esters
Esters are derived from acids in which one hydroxyl (–OH)
group is replaced by one alkyl (–O) group. Fatty acid esters show
the solid–liquid transition over a narrow temperature range and
they can form the eutectics without or little subcooling [14]. Many
fatty acid esters are commercially available in large quantities for
applications in polymer, cosmetics, smart clothing [106,142–
149]. Solid–liquid transition of five fatty acid esters: methyl
stearate, methyl palmitate, cetyl stearate, cetyl palmitate and their
eutectic mixtures (methyl stearate–methyl palmitate, methyl
stearate–cetyl palmitate and methyl stearate–cetyl stearate) was
studied by Nikolić et al. [143] using DSC. The prepared samples
were tested for 50 thermal cycles in the temperature range 10
to 60 °C. These cycles were repeated after having stored these sam-
ples for 18 months and no change in thermo-physical properties
were measured. DSC results showed that up to 30% of esters can
be absorbed in the building materials. Stearic acid esters were pre-
pared, synthesized, characterized and their thermal properties
were tested by Sari et al. [150]. These ester compounds were syn-
thesized by reacting stearic acid with n-butyl alcohol, isopropyl
alcohol and glycerol and characterized by Fourier transform infra-
red spectroscopy (FT-IR) and H Nuclear Magnetic Resonance (H
NMR) techniques. FTIR images shows that in the range of 3200–
3650 cm1
, the hydroxyl absorption peaks disappear which means
all hydroxyl groups of alcohols have been transformed into ester
bonds. DSC results showed that there was no residual stearic acid
in the synthesized PCM. Prepared esters were tested for 200 ther-
mal cycles and no degradation in thermal properties was observed
after 100 cycles. High chain fatty acid esters of myristyl alcohol
[146,151] and 1-hexadecanol [147] were prepared and it was
observed that these materials are suitable for low temperature
thermal energy storage and possess superior thermal properties
and reliability. An organic ester PCM was prepared by mixing silver
nitrate into an organic PCM polyvinyl pyrrolidone (PVP) 4000 [152]
and their thermal properties were evaluated experimentally. Addi-
tion of silver nanoparticle in the PCM showed only physical
changes and no chemical changes were noticed. The thermal
Table 3
Thermo-physical properties of compound of fatty acids used as latent heat storage.
Compound (wt%) Tm (°C) Hf (kJ/kg) Ref.
CA–LA (90–10) 13.3 142.2 [27]
CA–LA (64–36) 19.62 149.95 [126]
CA–LA (65.1–34.9) 19.67 126.56 [123]
CA–LA (45–55) 17–21 143 [9]
CA–LA (70–30) 21.09 123.98 [127]
CA–MA (70–30) 21.79 123.62 [127]
CA–LA (66.75–33.25) 22.76 127.2 [128]
CA–PA (76.5–23.5) 23.12 156.44 [126]
CA–SA (70–30) 23.40 104.90 [127]
C14H28O2–C10H20O2 (34–66) 24 147.7 [1]
CA–SA (83–17) 24.68 178.69 [126]
CA–SA (83–17) 25.39 188.15 [126]
CA–MA (78.39–21.61) 26.02 155.2 [128]
CA–PA (70–30) 27.07 142.61 [127]
CA–PA (89–11) 28.71 141.4 [128]
LA–MA (60–40) 28.8–40.8 172 [9]
LA–MA–SA (55.8–32.8–11.4) 29.29 140.9 [129]
LA–MA–PA (55.24–29.75–15.02) 31.14 142.6 [130]
CA–SA (94.47–5.53) 31.17 156.8 [128]
LA–PA (65–35) 32.8–37.1 170.2 [9]
LA–SA (60.3–39.7) 33.8–47.6 189.8 [115]
MA–SA (50–50) 35.2–51.8 189.2 [9]
LA–SA (75.1–24.9) 36.9–37.6 183.4 [8]
MA–PA (50–50) 39.1–45.4 173.7 [8]
MA–PA (50–50) 47.91 153.12 [125]
PA–SA (60–40) 51.2–54.2 183.7 [8]
CA–LA (65–35) 18 148 [124]
CA–LA (45–55) 21 143 [102]

conductivity of the ester was enhanced from 0.284 to 0.765 W/
m K. The freezing and melting cycle was improved by 41–45.6%
respectively. Aydin and Okutan [153] prepared a mixture of poly-
urethane (PU) rigid foam and myristyl myristate (PCM). The DSC
analysis for this PU–PCM composite was carried out at 5 °C/min
heating and cooling rate for the determination of phase change
temperature, enthalpy and total absorbed heat. The DSC results
indicate that the total heat absorption capacity can be improved
by up to 34%, which represents the enhanced thermal energy stor-
age capacity. Xu et al. [154] prepared the esters of fatty acids
methyl palmitate (MP) and methyl stearate (MS) to meet the ther-
mal requirement of a building (25–40 °C) and their thermal relia-
bility were tested. When the mass ratio of MP/MS was 70/30, 60/
40, 50/50, and 40/60, the DSC results showed two distinctive
endothermic peaks. One of the peaks shows the melting point
and the other one is at 17.33 °C, which indicates that these MP/
MS binary mixtures may from a compound with an incongruent
melting point. The thermal cycling test shows that even after for
360 thermal cycles, the DSC results of binary mixture is the same
as for the fresh mixture.
3.5. Poly(ethylene glycol)s
Polyethylene glycol (PEG) also known as polyoxyethylene (POE)
or polyethylene oxide (PEO) is composed of dimethyl ether chains,
HO–CH2–(CH2–O–CH2–)n–CH2–OH. Having the hydroxyl group at
the end, they are soluble in water as well as in organic compounds.
PEGs in various grades (400, 600, 1000, 3400, 10,000, 20,000,
35,000, 100,000, and 1,000,000) [14] have been extensively inves-
tigated numerically and experimentally in the past and they are
found to be chemically and thermally stable, nonflammable, non-
toxic, non-corrosive and inexpensive [163–167]. As reported by
Sarier and Onder [15], the melting point and latent heat of fusion
increase with increasing molecular weight (MW), e.g. melting
point of PEG (MW 400) is 3.2 °C and latent heat 91.4 kJ/kg, PEG
(MW 2000) is 51 °C and latent heat 181.4 kJ/kg, and PEG (MW
20,000) is 68.7 °C and latent heat 187.8 kJ/kg. Ahmad et al. [168]
experimentally and numerically investigated the thermal perfor-
mance of a wallboard filled with PEG 600. The apparent heat capa-
city method was adopted to numerically simulate the phase
change process in a vertical panel. A sinusoidal variation of the out-
door temperature (Eq. (3)) was considered in the numerical
simulation, which represents the daily variation in the outside
temperature. Simulations started with the investigation using
paraffin in gypsum wallboard, later in order to overcome the con-
ductivity issue due to the presence of air, the polycarbonate and
PVC panels was filled with PEG 600 and investigated experimental-
ly. Results showed that use of polycarbonate panel filled with PEG
is not suitable for light envelope for buildings but PVC panels filled
with PEG 600 serves this purpose. During thermal cycle test, no
degradation in the thermal properties of PVC panel with PEG 600
was noticed in the 400 cycles. A numerical simulation model also
validated these experimental results.
Te ð
CÞ ¼ 24 þ 8 sinðxtÞ ð3Þ
Like other organic PCMs, PEG is also subjected to low thermal
conductivity and a large number of experimental and numerical
studies has been carried out to enhance this property. Wang
et al. [169] prepared a form stable composite by SiO2 into PEG
and this composite was characterized by SEM, FTIR and DSC.
PEG of molecular weight 10,000 and SiO2 composite was prepared
by dissolving PEG and SiO2 in water and stirred for 12 h. Then the
mixture was heated in the oven at 100 °C for 24 h, followed by
heating under a reduced pressure at 70 °C for 24 h. SEM images
showed that PEG is dispersed into the network of solid SiO2,
which shows the mechanical strength of the composite. Polariz-
ing optical microscope (POM) micrographs show that SiO2 serves
as supporting material and help to prevent leakage of liquid PEG.
DSC graphs showed that the latent heat of PEG/SiO2 composite is
less and thermal conductivity is more than that of plane PEG. The
thermal conductivity of solid PEG is 0.2985 W/m K, while it is
0.3615 W/m K for PEG/20% SiO2 (w/w) which is 21% higher and
0.5124 for PEG/50%SiO2, which is 71.7% higher than normal
PEG. More studies based on PEG/SiO2 composite can be found
in Refs. [170–174].
The blends of PEG and fatty acids have also been studied in the
past to obtain the desired range of melting temperature and latent
heat. It has been possible to obtain a homogeneous PEG/fatty acid
blend by mixing both materials in liquid state follow by subse-
quent freezing. An experimental study carried out by Pielichowski
and Flejtuch [175] indicated that the melting range of such blend
lies between 30 and 72 °C. They investigated the series of blends
of PEG with capric, lauric, myristic, palmitic, and stearic acid of dif-
ferent molecular weights as thermal energy storage material. Their
latent heat of fusion was observed between 168 and 208 kJ/kg,
which is higher than that of pure fatty acid, and PEG. In their
Fig. 2. (a) Vacuum impregnation method, (b) schematic of thermal performance test. Reprinted from [141] with permission from Elsevier.

further research Pielichowska et al. [176] investigated the thermal
properties of blend of PEG 10,000 with lauric acid and stearic acid.
The results revealed that the crystallinity of PEG/lauric acid was
52% and PEG/stearic acid 43%. FT-IR images confirm the presence
of hydrogen bonds in blend, both in liquid and solid state that
defines their synergies. During phase change process, a synergistic
effect was observed due to formation and decay of hydrogen bond-
ing. In the similar studies done by Pielichowski and Flejtuch [177],
Flejtuch [178], and Pielichowski and Flejtuch [179], the blend of
PEG of different grades and fatty acids were prepared and tested
for their thermal reliability as energy storage materials and was
found that these materials are potential materials to be used as
TES.
3.6. Bio-based PCMs
Bio-based PCM is a new category of organic PCMs and are
obtained from underused raw materials, such as soybean oils,
coconut oils, palm oils and beef tallow. Compared to organic
PCMs, they possess higher latent heat, better chemical stability,
are less flammable and remain stable for thousands of melting/
freezing cycels without oxidation. They have the capability to
be manufactured such that their melting point can be varied in
the temperature range of 22.77 and 77.83 °C so that can be
used in a variety of applications, also, they offer excellent capa-
bility for microencapsulation [180]. However, like other organic
PCMs, bio-based PCMs also suffer from low thermal conductivity
and leakage problem [181]. Bio-based PCMs are required to be
incorporated into porous materials, such as gypsum wallboard,
concrete, and others [182]. Jeong et al. [183] used xGnP for
improving the thermal conductivity of bio-based PCM which
contained soybean oils, coconut oils, and beef tallow and pre-
pared a form stable material using vacuum impregnation
method. They found a hike of 375% in thermal conductivity of
bio-based PCM compared to bio-PCM at the cost of 25% reduc-
tion in latent heat. Subsequently, Yu et al. [184] prepared a
bio-based PCM composite with xGnP and CNT and performed
the thermal evaluation test and observed 336% hike in thermal
conductivity of bio-based PCM with xGnP 5.0 wt%. TGA curves
showed that both composite are thermally stable and a slight
decrease in latent heat was seen. Very recently, Jeong et al.
[185] adopted the vacuum impregnation method to prepare
the bio-based PCM with boron nitride. A hike of 477% in thermal
conductivity was measured and TGA showed the good thermal
stability in composite.
Organic PCMs are generally considered as a low (30 to 80 °C)
and medium (90–227 °C) melting point materials and possess the
characteristics of congruent melting. In comparison to inorganic
materials, they exhibit better thermal and chemical stability, even
after high number of thermal cycles. They possess high latent heat
per unit weight (120–270 kJ/kg), low vapor pressure and exhibit
little or no subcooling. While, inorganic PCMs require nucleating
and thickening agents to minimize the subcooling and they are
highly reactive to metal materials. These properties of organic
PCMs make them appropriate for buildings and low and moderate
temperature solar energy collectors. Apart from numerous
advantages, organic PCMs suffer from low thermal conductivity
(0.15–0.35 W/m K), high volume change (up to 20%) during phase
transition and they are flammable. Significant efforts have been
made in the past to mitigate these shortcomings. For example,
the dispersion of nano metal particles and insertion of metal
matrix have been found a successful way to enhance the thermal
conductivity of materials, which is discussed in detail in Section 4.
Being a new category of materials, bio-based PCM have not been
explored much. It has been seen that organic PCMs have their
largest market in Europe while America is the largest market for
inorganic and Bio-pased PCMs.
4. Techniques to improve thermal conductivity of organic PCMs
An organic phase change materials has a well-known drawback
of having low thermal conductivity, Tables 1–4, which substantial-
ly limits the heat transfer rate during phase transition. Despite of
the high energy density of organic PCMs, the slow heat transfer
rate during phase change process, limits their applicability in many
domestic and commercial applications. A large number of research
articles have been published in the last two decades, reporting the
enhanced thermal conductivity of these materials by various
means, such as dispersion of high conductivity solid particles
(micro/nano size) in the PCM [5], insertion of metal matrices
[5,186], chunks of metal (stainless steel and copper) pieces [187],
carbon fibers [188,189], and impregnation of porous graphite
matrix in the PCM [187,190–192]. Jegadheeswaran and Pohekar
[193] presented a detailed review on performance enhancement
in latent heat thermal storage system. They discussed the various
techniques such as impregnation of porous materials, dispersion
of high conductivity particles, placement of metal structures, and
use of high conductivity and low-density materials to enhance
the thermal conductivity of PCM. A nano composite of graphene
and 1-octadecanol (stearyl alcohol) was prepared by Yavari et al.
[194] and thermal conductivity was investigated as a function of
grapheme content. They observed the 2.5 times high conductivity
of composite by the addition of 4% (by weight) graphene at the loss
of 15.4% heat of fusion. Cui et al. [195] added the carbon nanofiber
(CNF) and carbon nanotube (CNT) in the PCM (soya wax and paraf-
fin wax) and observed that addition of both CNF and CNT increases
the thermal conductivity of base PCM. A detailed review of studies
regarding the thermal conductivity enhancement can be found in
[11,196]. In the past graphite has been extensively used as heat
transfer enhancer due to its high thermal conductivity.
[4,130,197–201]. Fan et al. [202] investigated the effect of carbon
nanofillers (CNT, CNF, and GNP) on thermal conductivity of paraffin
based PCM by hot wire method. The concentrations were varied
from 1% to 5% at an increment of 1% and found that the GNP
enhanced the thermal conductivity most, approx. 164% at the load-
ing of 5% w/w. Li [203] prepared a composite of paraffin and nano
graphite (NG) and reported the high thermal conductivity of this
composite. Nano graphite of particle diameter 35 nm was added
to paraffin in 1%, 4%, 7%, and 10% w/w at 60 °C. The thermal con-
ductivity of paraffin was measured 0.1264 W/m K and the conduc-
tivity of the composite PCM were measured to be 2.89 times and
7.41 times higher for 1% and 10% NG respectively. Nano graphite
in different forms has been added to the numerous organic PCMs
to enhance their thermal conductivity [202,204–208,181,209–
211].
Review of the experimental and numerical work reveals that
the placement of fixed structure (metal matrix and fins) and dis-
persion of micro/nano solid particles of metals such as copper, alu-
minum, nickel, stainless steel and carbon nano fiber and their
oxides have been primarily in use as thermal conductivity enhan-
cer with negligible change in melting point of PCM. Nano particles
are preferred over the micro sized particles because of their prop-
erties to behave like fluid and avoid the clogging during flow in
pipes. Although high-thermal conductivity fillers can enhance the
thermal conductivity of organic PCMs, the amount of fillers are
required to be minimum in order to preserve a high energy density.
Foams of copper, nickel, aluminum and other metals have reported
the enhanced thermal conductivity of organic PCMs but the solid
thermal conductivity of foam struts limits this enhancement. Fil-
lers such as CNFs, CNTs, graphene, and graphene flakes possess

the high thermal conductivity, but when mixed with PCMs,
composite exhibits the moderate increment in the thermal conduc-
tivity unless the volume fraction of these fillers is very high, which
significantly reduces the energy density of PCM.
5. Characterization techniques
There are numerous techniques available to characterize the
thermal and chemical properties of organic PCMs. The morphology
of the reference material is investigated by spectrum electron
microscope (SEM) [45,212–215] and particle size distribution
(PSD) [45,216,217]. SEM produces images of a sample by focusing
an electron beam on it and PSD list the values that define the rela-
tive amount of a substance into mixture. PSD sometime is also
knows as grain size distribution. Chemical compatibility of the
mixture is tested using Fourier transform infrared spec-
troscopy (FT-IR) technique in which infrared (IR) radiation is
passed through sample. Some of the IR radiation is transmitted
through the sample and some absorbed into it. There have been
numerous studies which have identified the properties of materials
using FT-IR technique [43,52,111,218–222]. Differential scanning
calorimeter (DSC) [43,45,117,199,212,223,224] and differential
thermal analysis (DTA) [225] are the measurement techniques
used to determine the latent heat of fusion, heat capacity and melt-
ing temperature of a material. In DSC, the sample and the reference
material (with known thermal properties) are maintained at the
same temperature and the thermal properties of sample materials
are calculated by measuring the difference of heat absorbed
between the sample and the reference. While in DTA, the heat is
applied to the sample only and the properties are evaluated by
measuring the temperature difference between sample and refer-
ence material. Yinping and Yi [226] proposed a method, called T-
history method for measurement of melting temperature, degree
of supercooling, thermal conductivity, specific heat, and heat of
fusion of PCMs. Later this method was modified by Refs.
[227,228] to make it more suitable for appropriate measurements.
SEM, FT-IR, TEM, DSC, TGA and hot wire methods have been
widely used to measure the thermal properties of pure and com-
posite PCMs. Therefore, to ensure the accurate measurement and
Table 4
Fatty acid derivatives, esters, and fatty alcohol.
Compound Tm (°C) Hf (kJ/kg) Cp (kJ/kg K) k (W/m K) q (kg/m3
) Ref.
Erythritol tetralaurate 9.03 161.39 – – – [155]
1-Decanol 6 206 – – 830.1 (s) [21]
Propyl palmitate 10 186 – – – [110]
Erythritol tetramyristate 10.82 190.90 – – – [155]
Isopropyl palmitate 11 95–100 – – – [5]
Ethyl myristate 11 184 – – – [86]
Isopropyl stearate 14–18 140–142 – – – [5]
Butyl stearate 18–23 123–200 – 0.21 – [10]
Erythritol tetrapalmitate 21.9–25.6 199–203 – – – [156]
1-Dodecanol 26 200 – – – [10]
Vinyl stearate 27–29 122 – – – [5]
Methyl palmitate 29 205 – – – [86]
Methyl stearate 29 169 – – – [10]
Erythritol tetrastearate 30.1–35.6 205.7–211.9 – – – [156]
Glycerol trimyristate 31.96 154.3 – – – [144]
1-Tetradecanol 38 205 – – – [5]
Cetyl laurate 38.24 192.2–198.9 1.65 (s) – – [147]
2.17 (l)
Galactitol hexa laurate 40.21 157.60 – – – [157]
Stearyl laurate 42.21 201.03–201.53 1.97 (s) – – [158]
2.31 (l)
Methyl eicosanoate 45 230 – – – [86]
Galactitol hexa myristate 45.98 172.80 – – – [157]
Stearyl myristate 48.86 203.39–203.53 2.07 (s) – – [158]
2.33 (l)
Cetyl myristate 49.44 222.0–228.4 1.97 (s) – – [147]
2.44 (l)
Cetyl palmitate 51.21 214.6–220.3 2.51 (s) – – [147]
2.93 (l)
Methyl behenate 52 234 – – – [86]
Ethyl tetracosanoate 54 218 – – – [86]
Methyl oxalate 54.3 178 – – – [1]
Cetyl stearate 54.63 212.1–216.3 1.99 (s) – – [147]
2.6 (l)
Sorbitol 55 166 – – – [159]
Stearyl palmitate 57.34 219.74–219.88 1.55 (s) – – [158]
1.89 (l)
1,4 Butanediol stearic acid 58 186 – – – [160]
Glycerol tripalmitate 58.50 185.9 – – – [144]
1,4 Butanediol palmitic acid 61 188 – – – [160]
Glycerol tristearate 63.45 149.4 – – – [144]
1,4 Butanediol behenic acid 74 209 – – – [160]
Xylitol 94 246 – – – [159]
92.9 260
Lactitol 146 135 – – – [161]
Maltitol 150 159.7 – – – [161]
D-Mannitol 166 279 [162]
Galactitol 179.8 246.4 – – – [140]
Myo-inositol 224–227 266 – – – [159]

minimize the uncertainties in the measured values it is necessary
that these equipment be calibrated before they are used. It will also
help to avoid the repetitive measurement, which is done to ensure
to get the correct data. Most of the previous studies have concen-
trated on the measurement of the melting temperature and latent
heat of prospective materials and very few studies have considered
the variation of thermo-physical properties like conductivity, den-
sity and viscosity with temperature. Almost all previous studies
reported the use of DSC for thermal property evaluation. This tech-
nique use a very small and require almost uniform sample of PCM
which is unrealisteic in the case of PCM based composite materials
because they are not homogeneous.
6. Applications of organic PCMs
Because of the inherent characteristics of little subcooling and
phase transition over a narrow temperature range, organic PCMs
have been found to be suitable for many domestic and industrial
applications.
6.1. Cooling/heating of buildings
Consumption of electricity is significantly varied during the day
and night, summer and winter seasons, according to the demand
by industrial, commercial and residential activities. Because of this
variation, the pricing of energy use is also varied during peak and
off peak season. A suitable thermal energy management system
can help to keep the energy stored in off peak season, which can
be used during peak season when the demand is more. For this
purpose, an encapsulated phase change material can be used to
enhance the thermal energy storage in building walls, floor and
ceiling. The capsule surface absorbs the solar energy and maintains
the internal temperature of the building for a longer time. The
American Society of Heating, Refrigerating and Air-conditioning
Engineers (ASHRAE) has suggested the room temperature as
23.5–25.5 °C in the summer and 21.0–23.0 °C in the winter [229]
so in the building application the PCM of temperature range of
20–30 °C is preferred. Hariri and Ward [230] were the first to
review the work done on applications of thermal energy storage
in buildings which mainly concentrated on theoretical aspects of
sensible and latent heat energy storage. PCMs in buildings can be
used in three different ways [7]: (i) in building walls, (ii) in ceilings
and floors, and (iii) in heat and cold storage units. In the first two
cases, the heat is automatically stored or released based on the
ambient temperature so they are categorized as passive systems.
Whereas the third type is an active system, in which heat is made
available on demand. Some of the major applications of organic
PCMs in the building are discussed below.
6.1.1. PCM walls and wallboard
Wallboards are easily available, effective and comparatively less
expensive to use in the buildings and these characteristic make
them highly suitable for PCM encapsulation. In wallboards, the
PCM is imbedded into a gypsum board, plaster or other building
structures. Stovall and Tomlinson [231] reported that a normal
wallboard can contain up to 30% of PCM. In their study, they found
that it is a good energy saver for a passive solar system with a pay-
back period of five years. Neeper [232] in his report mentioned that
PCM wallboards in HVAC field save the electricity. In an early work
[233], an experimental and numerical study was carried out over a
gypsum board impregnated with PCM (butyl stearate) in a direct
gain outdoor test room and it was observed that the room tem-
perature can be reduced by a maximum 4 °C during the day time.
Thermal dynamics of the PCM (fatty acid and paraffin wax)
impregnated wallboards which is subjected to diurnal variation
of room temperature was carried out by Neeper [234]. The results
of this study showed that when the PCM melting temperature is
close to the room temperature, the wallboard stores maximum
diurnal energy and this energy decreases if the phase change
occurs over a range of temperatures, Fig. 3.
Feldman et al. [113] carried out a thermal analysis of PCM
impregnated wallboards. They used a binary mixture of methyl
palmitate (93–95 wt%) and methyl stearate (7–5 wt%) as PCM.
They observed that the total energy storage capacity of such an
impregnated wallboard in the temperature range of 23–26.5 °C is
at least twelve times higher than that of the wallboards without
PCM in this temperature range. Shilei [123,235] used the mixture
of capric acid and lauric acid as PCM for wallboard and carried
out the thermal stability using DSC. They found that even after a
large number of cycles, this mixture sustained its thermophysical
properties and the PCM impregnated wallboards could substantial-
ly reduce the energy cost of HVAC systems. Kuznik and Virgone
[236] experimentally investigated the thermal performance of
PCM copolymer composite wallboard in a full scale test room for
the summer, mid-season and winter and found that the PCM wall-
boards can reduce the overheating effect for all cases. Later Kuznik
et al. [237] monitored a building for almost a year for heat varia-
tion in two identical rooms. One room was equipped with DuPont
de NemoursÒ
PCM wallboard and another was without any wall-
board. They found that the PCM wallboard work very well when
the outside temperature lies in the range of melting temperature
of PCM. Effect of natural convection on PCM board (Fig. 4) was
experimentally investigated by Liu and Awbi [238]. They found
that the flux density of the PCM wall is almost twice than that of
a normal wall. Also, the heat-insulation performance of a PCM wall
is better than that of an ordinary wall during the charging process,
while during the discharging process; the PCM wall releases more
heat energy. The calculated convective heat transfer coefficient
was high for PCM wall due to the increased energy exchange
between the wall and the indoor air.
A numerical investigation of the transient heat transfer through
a typical building exterior wall (Fig. 5) with a PCM layer for two
different periods of time: 6 days in the winter and 6 days in the
summer was carried out by Izquierdo-Barrientos et al. [239]. In
both cases, the orientation of the wall, the position of the PCM in
the wall, and the phase change temperature has been varied to find
the optimal parameters to minimize the energy fluctuations. They
found that the power needed for HVAC system to overcome the
thermal load is reduced. They also observed that in the winter
Fig. 3. Diurnal energy storage versus melt temperature for an interior wall.
Reprinted from [234] with permission from Elsevier.

season, the total heat lost during the day was reduced and during
the night, it increased whereas this is the reverse in the summer.
Oliver [240] prepared a new construction material: gypsum board
containing PCM 45 wt% and carried out its thermal characteriza-
tion. They found that a 1.5 cm-thick board of gypsum with PCMs
stores 5 times more thermal energy of a laminated gypsum board,
and the same energy as a 12 cm-thick brick wall within the com-
fort temperature range (20–30 °C). An experimental study of the
thermal characterization of a Mediterranean residential building
with PCM (BASF–MicronalÒ
PCM melting point 23 °C) impregnated
walls was carried out by Mandilaras et al. [241]. This house was
kept unoccupied and no energy system was installed in it for one
year of the monitoring period. Results of this study showed that
the thermal mass of this building is enhanced during late spring,
early summer, and autumn. They also observed the depreciation
in the decrement factor by 30–40% and increase in time lag of
approximately 100 min. A nano-PCM (n-heptadecane + graphite
nanosheets) enhanced wallboard was prepared by Biswas et al.
[242] and they simulated the thermal performance of this system,
Fig. 6, numerically and validated experimentally. Three tem-
perature range were selected for this study: 23.3 °C that was
well-above the phase change temperature range, 22 °C that was
near the higher end of the melting temperature range and 21 °C
that was at about the center of the melting range. No change in
heat gain was observed at 22 and 23.3 °C but at 21 °C cooling set
point, during peak summer, the nano-PCM wallboard reduced the
peak heat gains and also delayed the heat ﬂowing into the interior
space.
Lai and Hokoi [243] experimentally investigated the thermal
behavior of wallboards containing microencapsulated phase
change materials (MEPCM) embedded with a honeycomb structure
(Fig. 7). Results indicated that MEPCM + honeycomb exhibits the
better control of surface temperature and it is suitable for use in
places where the exterior surface temperature must be controlled.
Recently a new kind of composite PCM was developed by Sari [223]
which consists of polyethylene glycol (PEG 600) as the base PCM
with gypsum and natural clay and their thermal analysis was car-
ried out. The maximum absorption ratio of PEG 600 in gypsum-
based and natural clay-based composites was found to be 18 wt%
and 22 wt%, respectively. Thermal cycling test shows that this
material has good thermal and chemical stability along with good
thermal reliability. This material also showed excellent cooling and
heating performance.
Fig. 4. Sensor arrangement on PCM surface. Reprinted from [238] with permission
from Elsevier.
Fig. 5. wall layers of the typical external wall (base composite wall) used in the
simulations. Reprinted from [239] with permission from Elsevier.
Fig. 6. (a) Numerical model of nanoenhanced wallboard, (b) wallboard embedded with nanoPCM for experimental study. Reprinted from [242] with permission from Elsevier.

6.1.2. PCM Floors and ceilings for passive solar heating
Being important parts of a room, the ceiling and floor can be uti-
lized for heating and cooling in buildings effectively. Organic PCM
is effectively encapsulated in the ceiling in many ways to store the
solar radiation. Fig. 8 shows an insulation system proposed by
Turnpenny et al. [244]. They developed a latent heat storage unit
by incorporating the embedded heat pipes in phase change mate-
rial. This system stored coolness during the night and released it in
the daytime. Stalin et al. [245] designed a ceiling fan with PCM as
shown in Fig. 9. They placed a circular disc of PCM (paraffin wax)
30 cm aloft the fan. Inside the circular disc of PCM the small alu-
minum tubes are fitted which has the inlet from water tank of
the residence and has the outlet to the environment. They
observed that this modification in the ceiling fan is quite effective
Fig. 7. Installation locations of the mPCM honeycomb modules. Reprinted from [243] with permission from Elsevier.
Fig. 8. Outline of heat pipe/PCM insulation system. Reprinted from [244] with permission from Elsevier.

for cooling purposes. In an early work done by Bénard et al. [246], a
PCM based solar roof of a chicken brooder was designed and tested
experimentally. Two semi-circular tanks, each containing 42 kg of
paraffin wax were laced below a glass roof. This design was found
to be excellent to maintain the temperature in the range of 22–
30 °C. Stritih and Butala [247] experimentally analyzed the cooling
building using paraffin (melting point 22 °C) impregnated ceiling
board as shown in Fig. 10. They monitored the cooling of PCM at
night for seven consecutive days and it was observed that during
this period the outside temperature remained stable. The amount
of cold released from the PCM was calculated from zero to
300 min. They found that this system was very helpful for the cool-
ing of buildings.
Floor heating is also very important as they may provide a com-
fortable indoor environment than convective heating systems
[248]. Organic PCMs have a great potential to be used as thermal
energy storage in floors. Some of the organic PCMs used in floor
heating are given in Table 5. Lin et al. [249] experimentally inves-
tigated the thermal performance of under-floor electric heating
system (Fig. 11) with a shape-stabilized phase change material
(75% paraffin wax + 25% polyethylene). This system can charge
heat by using cheap nighttime electricity and discharge the heat
stored at daytime. During the duration of the experiment the aver-
age indoor temperature was 31 °C and the temperature difference
between day and night was 12 °C. This resulted in higher indoor
temperature with no change in the temperature swing.
6.1.3. Other PCM applications in buildings
Organic PCMs have a promising applications in different sec-
tions of building also, such as, trombe wall, shutter, tiles, building
blocks and air based heating system. The trombe wall as shown in
Fig. 12, a thick and south facing wall, painted black and made of
heat storage material, i.e. PCM and normally used in the winter
season to keep the room warm. A thick layer of glass or plastic
glazing is installed a few inches from the solid wall. During day-
time, the glazed wall (PCM) absorbs the incoming solar radiation
and gets melted. During night, this absorbed heat is released inside
to warm the space. Trombe walls have the capability to provide up
to 42% of total heating load required for a large room [251]. The
early work of 70s, 80s, and 90s on buildings integrated with
trombe wall is very well reviewed by Tyagi and Buddhi [7]. Cas-
tellón et al. [252] experimentally investigated the effect of trombe
wall in the building envelope. To do this experiment, nine cubicles
of the same size were prepared. Two with concrete, five with con-
ventional bricks, and two with alveolar bricks and one cubicle of
each typology was integrated with a PCM. This experiment was
performed in real conditions in Puigverd de Lleida (Lleida, Spain).
A trombe wall was added to both concrete walls and all brick cubi-
cle were equipped with domestic heat pumps. It was observed that
in the concrete cubicles, the temperature oscillation was reduced
up to 4 °C. During the winter, the trombe wall cubicle was able
to keep the concrete cubicle warm. Khalifa and Abbas [253] carried
a comparative numerical study using three different materials
namely, concrete, the hydrated salt (CaCl26H2O), and paraffin
wax (n-eicosane). They carried out this investigation using differ-
ent wall thickness and found that 8-cm thick wall made up of
hydrated salt performed better than the 5-cm wall made up of
paraffin. Trigui et al. [254,255] prepared a composite (paraffin/
resin based) for trombe wall and they observed that the paraffin
based composite materials have high heat storage capability and
an enhanced heat transfer rate.
Shutters are the exterior shading devices, which are installed on
the outer side of windows to reduce the heat gain in the room. Dur-
ing daytime, they are kept open to absorb the solar radiation and
melt the PCM and at night, they are closed to minimize the heat
losses through the window and release the absorbed heat inside
Fig. 9. Design of ceiling fan with PCM [245].
Fig. 10. Principal function of PCM ‘‘free-cooling system’’: (left) cooling of PCM at night, (right) cooling of building during the day. Reprinted from [247] with permission from
Elsevier.
Table 5
Organic PCM used for floor heating [250].
Compound Melting point (°C) Heat of fusion (kJ/kg)
Paraffin C16–C18 20–22 152
Polyglycol E600 22 127.2
Paraffin C13–C24 22–24 189
1-Dodecanol 26 200
Paraffin C18 27.5 243.5
Vinyl Stearate 27–29 122
1-Tetradecanol 38 205
Paraffin C16–C28 42–44 189
Paraffin C20–C33 48–50 189
Paraffin wax 63 173.6

the room and keep it warm. The organic PCM has been greatly used
in the shutter and presented in previous studies. A numerical
investigation of the performance of PCM shutter was carried out
by Alawadhi [256] using different PCMs and different quantity.
He used n-octadecane, n-eicosane, and P116 paraffin as PCM for
this study. The results indicate that P116 is found to be capable
of reducing heat gains by approximately 16%. He also reported that
if the thickness of P116 PCM shutter is kept at 3 cm, approximately
a 23% reduction in the heat gain can be achieved. Soares et al. [257]
numerically investigated the effects of shutters in a building envel-
ope located in Coimbran, Portugal and optimized it using a 2
dimensional simulation model. Results revealed that the optimal
melting temperature of the PCM in this location is 20 °C and the
total energy stored was 20501.3 kJ during the cycle of a complete
day.
Use of organic PCMs for heat storage in building blocks and
other building construction materials is increasing. Hawes and
Feldman [30] examined the heat absorption in the PCM concrete.
The effect of temperature, PCM viscosity, concrete density, and
hydrogen bonding on PCM penetration was also reported. Cabeza
et al. [258] experimentally investigated the thermal performance
of Micronal (M.P. = 26 °C) impregnated concrete blocks and they
found that energy storage in the PCM impregnated wall has
improved thermal inertia compared to the conventional concrete
without PCM. Recently, organic PCMs have been used in many
experimental and numerical studies [259–262] and found that
organic PCM impregnated concrete walls show greater ability to
store thermal energy.
The suitability of organic PCMs in building structures is
immense. They have been able to balance out the discrepancies
between energy demand and energy supply. For further studies
on the applications of organic PCMs in the building readers are
advised to go through the review articles by Zalba et al. [5], Baetens
et al. [263], Farid et al. [6], Demirbas [88], Tyagi and Buddhi [7],
Jeon et al. [250], Shi et al. [262], Zalba et al. [264], Osterman
et al. [265], Bastani et al. [266], and Memon [267].
6.2. Organic PCMs used for cooling of electronic devices and
domestic/commercial refrigeration
Temperature of the heat sinks that are attached to electronic
devices increases when they are operated under transient condi-
tions. U.S. force once indicated that more than 50% of the failures
in electronic devices are due to overheating in them [268]. One
effective way to limit this temperature rise is by increasing the
thermal capacitance of these devices by absorbing the excess heat.
Inclusion of a material, which can absorb this heat and undergo the
phase change, has been considered as an effective way to enhance
the heat capacitance. Organic PCMs based thermal management
system for electronic devices has shown a great potential in the
past. However, the low thermal conductivity of these materials
limits their application in electronic devices because it reduces
the heat dissipation rate from cell to PCMs. Insertion of metallic
fins and absorbing PCMs into graphite matrix or in metal foam
have shown the significant enhancement in the thermal conduc-
tivity of PCMs. Paraffin and other hydrocarbon based organic PCMs
are primarily considered in the cooling of electronic devices [269].
Thermal conductivity of organic PCMs can be effectively
enhanced by inserting thermal enhancer made of metal, generally
Fig. 11. Under-floor electric heating system (a) schematic of electric floor heating system with shape-stabilized PCM plates, (b) electric heaters, (c) shape-stabilized PCM
plates, (d) wood floor. Reprinted from [249] with permission from Elsevier.
Fig. 12. Sketch of PCM Trombe wall.

in the form of fins. Tan and Tso [270] carried out an experimental
study on the cooling of hand-held electronic devices using an
organic PCM, n-eicosane (m.p. = 36 °C). The temperature distribu-
tion in this device was compared between with and without a
PCM as shown in Fig. 13 and was found that using PCM was able
to maintain the temperature below the allowable limit of 50 °C
for almost 2 h of the transient operation. This study was extended
by Fok et al. [271] by using fins. The PCM was filled between fins.
Three types of orientation of the fins, shown in Fig. 14, were used
to see their effect on temperature distribution. It was found that fin
with PCM is very effective for thermal management; however, the
orientation of fins does not contribute significantly. Later this work
was validated numerically by Wang and Yang [272]. They devel-
oped a three dimensional computational model and the simula-
tions were conducted for different amount of fins (0, 3 and 6
fins), various heating power levels (2 W, 3 W, and 4 W), different
orientation test (vertical, horizontal, and slant), and charge and
discharge modes. They found that a setup with six fins was more
thermally stable than other setups. Shaikh and Lafdi [273] pre-
pared a composite of organic PCM (paraffin) and CNT and tested
its thermal performance experimentally and numerically. The ther-
mal control system was tested for three cases, (i) system without
CNT and PCM, (ii) system with CNT but without PCM, and (iii) sys-
tem with CNT and PCM. Results of this study revealed that use of
CNTs as additive in the PCM significantly enhances the thermal
performance of the system. Weng et al. [274] investigated the ther-
mal performance of a heat pipe using organic PCM as thermal ener-
gy storage. Experiments were carried out for different PCM (lauric
acid, palmitic acid, and tricosane) different fan voltage (3.5 V and 5
V), heating power (20, 30, and 40 W) and the volume of PCM (85 cc
and 100 cc). Results revealed that lauric acid takes 1041 s to attain
a temperature 60 °C while palmitic acid and tricosane take 581 and
973 s respectively to reach at 60 °C at a heating power of 20 W. For
other heating power, also, lauric acid took least time but tricosane
was chosen for the rest of this study because of its highest latent
heat. This study finally reported that the use of tricosane as a
PCM can reduce the fan power consumption up to 46%.
Baby and Balaji [275] experimentally investigated the perfor-
mance of a finned heat sink filled with organic PCM n-eicosane.
Plate and pin fin arrangement with PCM were adopted in this
study. They found that this setup was useful for stretching the
duration of operation of electronic devices. Later [276], the authors
carried out a similar experimental study using paraffin wax and n-
eicosane as PCMs. In this study, the authors used different volume
fractions of pin fin arrangement (0, 33, 72, and 120 pin fins). An
enhancement factor of 24 for a power level of 7 W and volume
fraction of PCM 1, the operation time for the heat sink with 72 fins
with n-eicosane was observed when compared to those in the
absence of PCM. Finally, the authors used actual experimental data
for genetic algorithm (GA) coupled with feed forward back
propagation artificial neural network technique to obtain the opti-
mized configuration of heat sink. Further, the trained network was
used as surrogate to experiments, which results in maximization of
operating time of device using GA optimization. The predicted val-
ues were validated with respect to independent measurements
and optimal solution was found to hold for paraffin wax based heat
sinks also. Jaworski [277] carried out a thermal performance test of
a heat spreader equipped with organic PCM lauric acid for cooling
of electronic devices. The thin pipes of this spreader were filled
with lauric acid, Fig. 15. This kind of design offered two advan-
tages; one is high heat transfer surface (due to a large number of
pipes) and high thermal capacity (due to the presence of PCM).
Tubes were able to accommodate 12–17 g of lauric acid, which
were able to increase the thermal capacity to three times more.
Mahmoud et al. [278] experimentally investigated the effect of
PCM and design of heat sink on the thermal management of the
electronic device. Six PCMs (two inorganic hydrated salt mixtures,
two organic substances mixture and one paraffin wax) were used
in this study. In addition, six heat sink designs were tested: one
with single cavity, two with the parallel fin arrangement, two with
the cross fin arrangement, and one with a honeycomb insert inside
the single cavity. They observed that an inorganic mixture of calci-
um chloride and salt performed best among all selected PCMs
because it has the lowest melting temperature. Ling et al. [279]
extensively reviewed the thermal management systems developed
using PCM for electronic devices. This review is concerned with the
three main applications of thermal management system in
electronics; electronic components, Li-ion batteries, and photo-
voltaic modules. The authors have mentioned that the thermal
Fig. 13. Distribution of temperature with time. Reproduced from. Reprinted from
[270] with permission from Elsevier.
Fig. 14. Three different orientation of heat sink [271].

management system for electronic devices mainly uses organic
PCMs. Although inorganic PCMs have excellent thermal conduc-
tivity, they have high chances of supercooling and corrosion. Ollier
et al. [280] proposed a novel design of thermal management sys-
tem for electronic cooling by using PCM (paraffin) and CNT, as
shown in Fig. 16(a). They designed a silicon composite with CNTs
and PCM. A cavity etched in the silicon is filled by the composite
of CNT and PCM, it is then covered by a second silicone part. When
the heat flux increases, the PCM absorbs a part of it and limits the
temperature. The CNT structure drives the flux into the PCM and
reduces the thermal resistance of the composite structure. The
authors suggested that this type of a system is very much useful
when the environment temperature is below the phase change
temperature. As shown in Fig. 16(b), the composite structure of
PCM and CNT is able to reduce the temperature of the device
significantly.
Domestic refrigerator is one of the most energy consuming
appliances in the house. However, the use of PCM as thermal ener-
gy storage in the refrigeration system is a new technique to
improve the performance of these devices by reducing the
electricity uses. The use of phase change material improves the
performance of the device by increasing the heat transfer rate
and enhancing the coefficient of performance (COP). In general,
inorganic PCMs are very popular for domestic refrigeration system
and the use of organic PCMs are yet to be explored in this area.
Recently few studies have been carried out using organic PCM in
refrigeration and air-conditioning, these are discussed below.
Ahmed et al. [281] modified the conventional technique of insu-
lation in refrigerated truck trailer by using paraffin based PCMs.
They proposed a design as shown in Fig. 17, which has a closed
loop system consisting of a chiller and two heat exchangers to pro-
vide the chilling effect. The results revealed that the peak heat
transfer rate were reduced significantly, resulting in potential
energy saving and reduced pollution from the diesel engine of
the truck. The indoor of the trailer would also experience lower
temperature oscillations, which helps to provide more stable
operation and control, longer operating life of the equipment and
Fig. 15. Heat spreader for PCB cooling with PCM filled pipe-fins. Reprinted from [277] with permission from Elsevier.
Fig. 16. (a) Composite structure, (b) transient temperature evolution at the interposer surface. Reprinted from [293] with permission from Elsevier.
Fig. 17. Schematic diagram of the cooling of the refrigerated truck. Reprinted from
[281] with permission from Elsevier.

energy conservation. An average daily heat flow reductions of
16.3% in the refrigerated compartment was also observed.
Gil et al. [282] performed an experimental investigation of high
temperature thermal energy storage using organic PCM hydro-
quinone (m.p. = 172.4 °C, enthalpy = 225 kJ/kg) for solar cooling
refrigeration system. This solar cooling system can be installed in
the real application using an absorption chiller and Fresnel collec-
tor. Two similar storage tanks (one with another without fins) were
constructed to carry out this study. An addition of fins enhanced
the thermal conductivity of the PCM. From the findings of this
study, the research had successfully built storage a tank using
5 Ton of PCM to work in a real solar cooling installation in Seville,
Spain. PCMs in the refrigeration system for packaging of perishable
goods are also a very interesting application. Recently Hoang et al.
[283] carried out a numerical study for two different mass fraction
of Rubitherm RT5 as PCM having phase transition temperature
around 5 °C encapsulated in polycaprolactone. The results showed
that the encapsulated PCM bear the better thermal buffering capa-
city compared to the cardboard.
6.3. Solar energy storage
Direct solar radiation is considered as one of the prospective
sources of energy and this can be effectively utilized in the large-
scale by proper storage of it. PCMs are one of the most effective
techniques to store the solar energy during daytime, which can
be utilized at nighttime or in cloudy days. General principle of
the solar energy storage and technologies to store it was summa-
rized in Ref. [284] and latent heat storage for solar energy was dis-
cussed in detail by Lane [16,20] and Dincer and Rosen [21]. PCMs
have a huge scope of applications in solar energy storage, such as
water and air heating, drying, solar cooking systems and solar ther-
mal power plants which can be divided in two major groups, low
temperature and high temperature solar energy storage. For solar
energy storage, the natural substances, such as salt hydrates, paraf-
fins and fatty acids in the melting point range 0–150 °C are consid-
ered suitable for use. Sharma et al. [1] presented a detailed review
on tested PCMs for solar energy storage to date. In this current
review paper, we have divided this section into two major sub-
sections, viz.: (i) low temperature energy storage, which includes
water, air, and dryer heating using solar energy, and (ii) high tem-
perature energy storage e.g. solar cooker.
6.3.1. Low temperature solar energy storage using organic PCMs
Use of solar radiation for water and air heating and in dryer are
considered as the low temperature applications of solar energy.
These applications are normally operated in the temperature range
of 0–80 °C.
6.3.1.1. Solar water heating. Solar water heating is relatively inex-
pensive and simple to fabricate and maintain. Barry [285] designed
one of the first kinds of solar water heating system as shown in
Fig. 18. A copper made upwardly tapered coil is fitted inside the
dome shape shell. The lower end of this coil is connected to inflow,
which is connected to the bottom of the water container. The
upper end of the coil is connected to the outlet of hot water, which
is eventually connected to the top of storage tank. The solar water
heater integrated with PCM is the upgraded version of the conven-
tional solar water heaters which takes the advantage of ability of
the PCM to store excess energy and which can be utilized in off-
peak hours. As an example, the solar energy available in daytime
can be used to charge the PCM and in the night hours when solar
radiation is not available, this stored energy can be used to heat
up the water. Prakash et al. [286] developed a built-in thermal
energy storage type water heater which contains a layer of PCM
at bottom which helped to provide hot water during off sunshine
hours and substitute hot water by cold water. They analyzed the
performance of such a system for two depths of PCMs and flow
rates and found that this system is a potential solar water heater
with improved heating characteristics.
Organic PCMs as thermal energy storage media have been
extensively used in the past and a detailed review was presented
by Shukla et al. [287]. Bansal and Buddhi [288] theoretically stud-
ied a cylindrical storage as a part of domestic hot water system
with a flat plate collector for it charging and discharging. During
the charging of PCM (paraffin wax P-16 and stearic acid), the cylin-
drical capsule is in the close loop with a solar water heater, and
while discharging, the liquid flowing in the storage unit absorbs
the stored energy in PCM. The performance of a PCM based solar
water heating system with a heat pump was investigated by Kay-
gusuz [289] for the data collected during November to April. The
solar collector used in this system was constructed by modifying
the flat-plate water-cooled collectors and the absorber unit con-
sists of nine copper tubes of 1.8 m length and 0.022 m external dia-
meter. Sheet iron was used to construct the storage tank with
diameters of 1.30 m and 3.20 long, which contain the PCM, filled
PVC containers. This design was helpful to save the energy
substantially.
Al-Hinti et al. [290] experimentally investigated the effect of the
paraffin filled capsules contained in a container used for water
heating by solar radiation. This system consisted of four south fac-
ing flat plate 1.94 m 0.76 m 0.15 m collectors with a tilt angle
30°. This system works on the principle of open and closed loop
system, which is enabled by the set of three valves, connected to
the hot water storage tank. 1 kg of paraffin is filled in a thin walled
cylindrical aluminum container of 1.3 l each. Thirty-eight such
containers are fixed into a storage tank made up of steel having a
length of 675 mm, inner diameter 450 mm and a volume of
107.4 l; see Fig. 19(a). The total volume of PCM containers are
49.4 l and remaining 58 l volume is occupied with water. Results
revealed that over the test period of 24 h, the water temperature
was measured to be 30 °C higher than the ambient temperature
as shown in Fig. 19(b).
Paraffin was encapsulated in spherical capsules as PCM in a
jacket shell type solar tank and the effect of PCM encapsulation
on water heater was investigated by Fazilati and Alemrajabi
[291]. One hundred and eighty spherical capsules made up of HDPE
having 38 mm diameter each, which occupy the 55% of the total
volume of the tank was embedded inside the tank. Copper wire
380 mm long and 0.3 mm in diameter was inserted into the
Fig. 18. Design of solar water heater [285].

paraffin to enhance its thermal conductivity. It was observed that
the use of PCM in the tank increased the storage density by 39%
and the exergy efficiency by 16%. It was also observed that such
PCM inbuilt solar water heater can supply hot water for up to
25% longer time. Khalifa et al. [292] designed a storage tank con-
sists of six 80 mm diameter copper pipes connected in series and
paraffin wax filled in between the pipes as shown in Fig. 20. Var-
ious performance factors such as top loss coefficient, water useful
heat gain and heat transferred between water and the PCM were
considered to evaluate the thermal performance of such system.
Experiments were conducted on clear and semi-cloudy days of
January, February and March. Results indicate that the plate tem-
perature increases up to a distance of 2.5 m from entrance, after
which a nearly steady temperature is noticed for the remaining
7.6 m of the total length (see Fig. 20(b)) which is in the contrast
to previously published results by many researchers.
Very recently, Mahfuz et al. [293] experimentally investigated
the thermal behavior of paraffin integrated solar water heating
system. Their proposed system, shown in Fig. 21 is made of three
major components, a solar collector unit, a shell ad tube thermal
energy storage, and insulated water storage tank. During sunny
hours, the valve 1 is open and valve 2 remained closed. The cold
water from the water storage tanks passes through the solar collec-
tor and gains the heat and flows back to the storage tank. A part of
this hot water goes through the thermal energy storage tank for
charging of PCM. The excess water will automatically flow out of
this tank and move towards the main water storage tank. In the
night when there is no sun light, valve 2 is opened to allow the
water to pass through the PCM tank so that it will extract the heat
from PCM, get heat up, and flow back to the main storage tank.
Results show that when the water flow rate is 0.033 kg/min the
energy efficiency of such system is 63.88% while it is 77.41% when
the flow rate is 0.167 kg/min. For the first flow rate the total life
cycle cost was calculated as $ 654.61 while for the later one the
total cost was predicted to be USD 609.22 that can be interpreted
as the flow rate increases, the life cycle cost decreases. Chaabane
et al. [294] carried out a numerical study on PCM integrated solar
water heater system. They used one organic PCM, myristic acid,
and one organic–inorganic mixture of Rubitherm 42-graphite for
this investigation. Results show that myristic acid integrated water
heating system performs better than others under same environ-
mental conditions do.
Huang et al. [295] proposed a new design of solar water heating
system which has a PCM floor (capric acid) in it. This system con-
sists of two heating layers of capillary plaits above and below the
PCM layer as shown in Fig. 22(a) which is to expedite the heat
storage process in the PCM layer. The two heating layers consist
of buried capillary plaits (thermal conductivity = 0.22 W/m K,
internal diameter = 2.5 mm, and external diameter = 4.3 mm) and
concrete. During heating period, the upper heating layer provides
Fig. 19. (a) cross sectional view of the storage tank, (b) temperature variations with time. Reprinted from [290] with permission from Elsevier.
Fig. 20. (a) Schematic diagram of storage tank, (b) variation of the pipe surface temperature with distance from entrance. Reprinted from [292] with permission from Elsevier.

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