This document reviews research on nanomaterials for thermal energy storage technologies. It discusses various types of phase change materials and nanomaterials that have been investigated as possible materials for efficient thermal energy storage. Nanoadditives have been used to enhance the thermal properties of phase change materials by increasing their thermal conductivity. The document outlines different classification systems for phase change materials, nanomaterials, and nanofluids/nanocomposites developed for thermal energy storage. It also reviews various synthesis methods that have been used to prepare nanofluids and nanocomposites, including one-step direct evaporation and two-step methods.

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Nanoscience & Nanotechnology-Asia, 2013, 3, 000-000 1
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Review on Nanomaterials for Thermal Energy Storage Technologies
Hussain H. Al-Kayiema,*, Saw Chun Linb,* and Afolabi Lukmonc,*
a,b,c
Department of Mechanical Engineering, Universiti Teknologi PETRONAS, 31750 Tronoh, Perak, Malaysia
Abstract: To optimize the utilization of thermal conversion systems, it is essential to integrate them with thermal energy
storage. Among many types of base materials, the phase change materials are the most satisfactory mediums to store and
release the thermal energy due to their high latent heat of fusion. In general, the phase change materials have low thermal
conductivity. Nanoadditives have been investigated to further enhance the thermal properties of the phase change
materials. This paper reviews the research development of the various types of phase change materials, nanoadditives,
nanofluids, and nanocomposites as possible materials for efficient thermal energy storage. Some deficit in the literature
has been noticed on the dispersion of various types of nanoparticles in the various types of base materials. It is also
recommended that further studies are required to understand the stability of the nanofluids and nanocomposites due to a
large number of thermal cycles.
Keywords: Nanoadditives, Nanomaterials, Nanocomposites, Nanofluids, Nanoparticles, TES, PCM.
1. INTRODUCTION
Energy storage plays important roles in conserving the
over demand of energy for utilization during the demand. On
the other hand, many types of energy sources are intermittent
in nature like solar energy which is affected by cloudy and
dusty weather, as well as the non availability during the
night. In this sense, the thermal energy storages (TESs) play
essential roles in heat recovery and contribute considerably
in improving the performance of the thermal systems. TESs
are simply contained mediums which are able to store the
thermal energy (charging mode) and release the stored
thermal energy (discharging mode) to compensate for the
shortage in the main thermal source. Commonly, the storing
medium is a fluid or phase change material (PCM). In
general, the thermal conductivity of these TES-based
materials is poor leading to a slow charging and discharging
rate. The charging and discharging rate can be enhanced by
applying the heat transfer enhancement methods. The
literature shows numerous methods to enhance the thermal
conductivity of the TES materials varying from extended
surfaces and fins, bubble agitation, metal ring and metal
matrix insertion, encapsulation, and many others; among
them are the nanoadditives.
Nanomaterials are used as additives to enhance the
properties of base materials. When they are added to the
fluid, the produced mixture is denoted by nanofluid; while
by adding the nanomaterials to PCM, the production is
denoted by nanocomposites. Subsequently, they will be
denoted by nanofluids/composites. Some review articles
were published reporting progress on the enhancement in
*Address correspondence to these authors at the Department of Mechanical
Engineering, Universiti Teknologi PETRONAS, 31750 Tronoh, Perak,
Malaysia; Tel: ??????????????; Fax: ??????????????;
E-mails: hussain_kayiem@petronas.com.my, clsaw78@gmail.com,
afoolabs@yahoo.com
thermophysical properties investigated with different
nanomaterials for different applications [1 to 4]. Many other
reviews have emphasized the technological applications,
synthesis and preparation methods in the production of
nanomaterials [5 to 8]. The investigations on the characteristic
mechanism enhancement (conductive and convective heat
transfer) by these advanced materials have been carried out
experimentally, numerically and theoretically. They have
been aimed to determine the major phenomenon (Inter-layer
surface and Brownian motion, particle aggregate etc.) which
is responsible for the enhancement. The different synthesis
methods of preparation and the characterization of
nanofluids, suspension, and the convection and conduction
heat transfer in nanofluids have been reported by [9 to 11].
Within the realm of nanomaterials for thermal energy
applications, many researchers have focused on the thermal
conductivity effectiveness of nanofluids/composites which
can improve their thermophysical properties [12 to 17].
This paper focuses on the enhancement of TES by using
nanomaterials as additives to the base storage materials.
TESs as a whole and heat exchanger development in
particular are mostly concerned with heating and cooling
applications. The utilization of nanofluids/composites in
heating and cooling processes is viewed in relation to
different applications as grouped in Fig. (1).
The paper critically reviews the existing studies dealing
with the use of nanofluids and nanocomposites in TES
applications. The types and classifications of the nanomaterials
have been presented and discussed. The synthesis methods
used in the preparation of the nanofluids and nanocomposites
are also presented and discussed.
2. CLASSIFICATION OF WORKING MATERIALS
This section presents and discusses the classification of
the materials included in the process of TES enhancement.

2. 2 Nanoscience & Nanotechnology-Asia, 2013, Vol. 3, No. 1 Al-Kayiem et al.
2.1. Phase Change Materials, (PCM)
The trends of using PCMs as base materials for TES
have been increasing tremendously since the energy
crisis during the period of 1973 – 1974. There have been wide
research applications on PCMs such as space heating, space
cooling, water heating, food storage, electricity generation,
air conditioning etc. revealed [18]. The PCMs consist
of organic, inorganic and eutectic materials as shown in
Fig. (2).
Paraffins are mainly hydrocarbons. Non-paraffins are
fatty acids and form the largest group in the phase change
materials. Inorganic PCMs are divided into salt hydrate and
metallic. Hydrate salt undergoes hydration and dehydration
of the salt during the phase change but not all of the the
hydrate salts melt congruently. Metallics are low melting
metals and eutectics. These metallics are very light and have
a high thermal conductivity like metal but are low in terms
of the latent heat of fusion. Another alternative in PCMs is
eutectics. Eutectics are compositions of two or more
components such as a combination of organic and organic,
inorganic and inorganic, and inorganic and organic. Eutectics
have shown a positive sign to solve the incongruent melting
of hydrate salts. Mixing two types of hydrate salts,
(inorganic and inorganic) as an example, to form a eutectic,
helps to prevent incongruent melting and atthe same time
reduces the melting point and improves the thermal
conductivity. The advantages of organics compared to
inorganics are stated in Table 1.
Although organic PCMs possess lower latent heat
compared to inorganic PCMs, the thermal cycles are stable
and have a low super cooling effect. It is important to have
stable thermal cycles. In such, the storage and release of the
latent heat will be stable with no weight loss happening [20].
However, the thermal conductivity of PCMs is low. The
introduction of nanomaterials mixed with PCMs will
enhance the thermal conductivity.
2.2. Nanomaterials
Nanomaterials can be classified into organics and
inorganics as shown in Fig. (3), which has been concluded
from [21 and 22]. Organic nanomaterials consist of
fullerenes, carbon nanotubes (CNT), single-walled carbon
nanotubes (SWCNT), multi-walled carbon nanotubes
(MWCNT), graphite and nanofibers. Most of the organic
nanomaterials are carbon-based nanomaterials [23]. While,
metal and metal oxide-based nanomaterials such as
aluminium, zinc, copper, iron, aluminium oxide, iron oxide,
titanium oxide are categorized asinorganic nanomaterials
[24]. Quantum dots, such as CdSe, ZnS, ZnOetc are
metalloid nanomaterials and are also categorized as
inorganic nanomaterials [25 and 26]. Hybrid nanomaterials
are a new class included in the chart in Fig. (3). Hybrid
nanomaterials are the combination of organic – organic
nanomaterials, organic - inorganic nanomaterials and
inorganic – inorganic nanomaterials through synthesis such
as chemical vapor deposition (CVD), Electrospinning, atom
transfer radical polymerization (ARTP) etc. [27 and 28].
Fig. (1). Nanomaterials for TES Applications.

3. Review on Nanomaterials for Thermal Energy Storage Technologies Nanoscience & Nanotechnology-Asia, 2013, Vol. 3, No. 1 3
Although, nanomaterials possess many advantages, there
are risks and they having a toxic effect on humans and the
environment. Reference [29] studied 935 nanomaterials
including their particle size and surface area measurement.
The investigation also revealed 483 toxicity effects of
nanomaterials including CNT. CNT nanomaterials have
stable characteristics, are not reactive with air and have a
high thermal conductivity as compared to metal
nanomaterials. Carbon-based nanomaterials have been
prioritized and widely studied by many researchers as
summarized by, e.g., [29 and 30].
2.3. Classification of Nanocomposites and Nanofluids
Nanocomposites and nanofluids are the mixtures
resulting from the incorporation of nanoadditives with the
base materials of TES as classified in Fig. (4). Nanocomposites
can be organics and inorganics. Most of the nanofluids
reported for TES are water based mixtures. Some other base
fluids may be ethylene, glycol, and oil.
Many reported nanocomposites are mainly the organic-
based such as paraffin blended nanocomposites, fatty acid
blended nanocomposites, HDPE blended nanocomposites
etc. [31 and 32]. The organic-based nanocomposites are also
called polymer nanocomposites [33]. Nanofluids, as thermal
energy storage, have been investigated by many researchers
but most of the studies have been focused on indoor
experiments [34 and 35].
The term nanofluid was first introduced by the Argonne
National Laboratory (ANL) through the seminal work of Dr.
Choi and his team. They developed the noble concept of
nanofluids in 1995 [36]. Nanofluids are nanoscale colloidal
suspensions containing nanomaterial such as nanofibers,
nanorods, nanosheets, nanoparticles, nanodroplets, nanowires
or nanotubes in a conventional base fluid like ethylene or
tri-ethylene-glycols, water, bio-fluids, polymer solutions,
oil and other common coolants. These nanofluids possess
a higher specific surface area and dispersion stability,
adjustable properties including thermal conductivity and
surface wettability, reduced particle clogging and pumping
as compared to conventional solid-liquid suspension for heat
transfer intensification [37].
Advanced fabrication technologies provide great
opportunities to actively process materials at the micro and
nanometer scales. Nanostructured or nanophase materials are
made of nanometer-sized substances engineered on the
atomic or molecular scale to produce either new or enhanced
physical properties not exhibited by conventional bulk
solids. Much emphasis has been given to this new material
technologicalinnovation in recent times by several
researchers particularly over the past ten years [38]. Yet
further investigations are needed to be carried out to
determine the major factors influencing the performance of
nanofluids. The major driving force for nanofluid researches
lay in its verserange of applications and inert potentials.
3. SYNTHESIS TECHNIQUES
One of the major difficulties in nano technologies lies in
the synthesis of the nanofluids/composites and their control
over the size and dimension. Since the early nineties,
Fig. (2). Basic categoriesof phase change materials [19].
Table 1. Comparison of Organic and Inorganic Materials for
Thermal Storage [20]
Organics Inorganics
1. Advantages
Non corrosives Greater phase change enthalpy
Low or non-undercooling -
Chemical and thermal stability -
2. Disadvantages
Lower phase change enthalpy Undercooling
Lower thermal conductivity Corrosive
Non-flammable Phase separation
Phase segregation, lack of
thermal stability

4. 4 Nanoscience & Nanotechnology-Asia, 2013, Vol. 3, No. 1 Al-Kayiem et al.
innumerable efforts have been given by the researchers in the
field of nanofluid/compositesynthesis. Upon that effort,
several techniques have been evolved and considered as
standard nanofluid/composite synthesis procedures. The
synthesis of nanofluids/composites with respect to heating
and cooling applications in thermal energy storage is the
primary focus in this review paper. The following procedures
have beenadopted for the preparation of nanofluids/
composites.
3.1. Synthesis and Preparation of Nanofluids
The adequate preparation of nanofluids is an essential
step towards experimental studies because nanofluids are not
just the dispersion of solid particles in a fluid. Special
features are expected such as a stable and uniform
suspension, durable suspension, and the low agglomeration
of particles and stable chemical change of the fluid.
Nanofluids are produced by dispersing nanometer-scale solid
particles into base liquids such as water, ethylene glycol, oil
etc. In the synthesis of nanofluids, agglomeration is a major
problem.
There are mainly two methods to produce nanofluids:
3.1.1. One-step Direct Evaporation Method
This method involves one single process of
simultaneously producingand dispersing nanoparticles
directly into the base fluid; this is best known for metallic
nanofluids. The Vacuum Evaporation Running Oil Substrate
(VEROS) technique was the first known single-step direct
evaporation method used. Although, it was deficient due to
Fig. (3). Classification of nanomaterials.
Fig. (4). Classification of enhanced TES nanofluids/composites.

5. Review on Nanomaterials for Thermal Energy Storage Technologies Nanoscience & Nanotechnology-Asia, 2013, Vol. 3, No. 1 5
its subsequent difficulty to separate the particles from the
fluids to produce dry nanoparticles, the VEROS method was
modified by employing high pressure magnetron sputtering
for the preparation of the suspensions with different metal
particles such as silver (Ag) and iron (Fe) [39]. Cu vapor
wasdirectly condensed into nanoparticles by contact with a
flowing low-vapor pressure liquid (EG) using the modified
VEROS technique. Cu-ethylene glycol nanofluids were
developed by the one-step physical vapor condensation
method [40]. Stable ethanol-based nanofluids containing
silver nanoparticles can be produced by the one-step
microwave-assisted reduction of silver nitrate (AgNO3) with
polyvinylpyrrolidone (PVP) as the stabilizing agent [4].
Various methods have been tried to produce different kinds
of nanoparticles and nanosuspensions. The initial materials
tried for nanofluids were oxide particles primarily because
they were easy to produce and chemically stable in solutions.
Various investigators have produced aluminum oxide
(Al2O3)and copper oxide (CuO) nanopowders by an inert gas
condensation process. They were found to be 2–200 nm-
sized particles. The major problem with this method is its
unsuitability to produce pure metallic nanoparticles. The
problem of agglomeration is reduced to a good extent by
using a direct evaporation condensation method.
3.1.2. Two-step Method
This method is the most widely used method for
preparing nanofluids. The process involves:
i Firstly, producing the nanomaterial (nanofibers, nanorods,
nanosheets, nanoparticles, nanodroplets, nanowires or
nanotubes) in dry powder using various methods like
(chemical: vapor condensation, micro-emulsions, spray
pyrolysis, thermal spraying; or physical: grinding, inert-
gas condensation).
ii Then, dispersingthe nano-sized powders into the
base fluid and mixing with the aid of mechanical
agitation: high-shear mixing, homogenizing, ball milling
or ultrasonication).
The two-step method has a high tendency of
agglomeration problems due to the high surface area
contaction of the nanoparticles. The agglomeration can be
reduced with the addition of surfactants; however, the
effectiveness of the surfactant under high temperature
applications is yet to be established fully. The method is the
most economic method to produce nanofluids on alarge
scale, because nanopowder synthesis techniques have been
scaled up to industrial production levels. Several other
methods (chemical vapor condensation, chemical vapor
deposition, LASER deposition, inert gas condensation etc.)
have been employed towards the production of nanoparticles
and suspensions. Some good overviews of the synthesis
methods have beenhighlighted by [5]. Zinc oxide (ZnO)
nanoparticles have been dispersed in deionized water to
enhance the critical heat flux (CHF); the enhancement
obtained was in the concentration range of 0.001 – 0.1%
[41].
3.2. Synthesis and Preparation of Nanocomposites
Nanocomposites, because of their controlled and
advanced properties, are used in a wide range of applications
in various fields, such as medicine, textiles, cosmetics,
agriculture, optics, food packaging, optoelectronic devices,
semiconductor devices, aerospace, construction and
catalysis.
Nanoparticles are dispersed into polymeric materials to
form polymer nanocomposites. There are three major groups
of nanocomposite matrices depending on the temperature
applications: Polymer, Metal and Ceramic nanocomposites.
Polymer matrices are the most commonly based materials
used especially for low temperature applications of less than
250o
C. Polymer nanocomposites are mostly polymer blended
with inorganic and organic nanoparticles. These polymer
nanocomposites represent a new class of materials that
exhibit improved performance compared to the base
materials. Incorporation of inorganic nanoparticles into a
polymer matrix can significantly affect the properties of the
matrix. The resulting nanocomposites might exhibit improved
thermal, mechanical, rheological, electrical, catalytic, fire
retardant and optical properties. The properties of polymer
nanocomposites depend on the type of nanoparticles that are
dispersed, their size and shape, their concentration and their
interactions with the polymer matrix. Particle agglomeration
is a major problem in polymer nanocomposites, too. It is
extremely hard to produce evenly dispersed nanoparticles in
a polymer matrix due to the nanoparticle agglomeration as a
result of their specific surface area and volume effects.
Modification of the surface of the inorganic particles
improves the interfacial interactions between the inorganic
particles and the polymer matrix. The two methods bywhich
the surface modifications are accomplished are: first, through
surface absorption or reaction with small molecules, such as
silane coupling agents, and the second method is based on
grafting polymeric molecules through covalent bonding to
the hydroxyl groups existing on the particles [42]. The
advantage of the second procedure over the first lies in the
fact that the polymer grafted particles can be designed with
the desired properties through a proper selection of the
species of the grafting monomers and the choice of grafting
conditions. The investigations on the use of nanomaterials
for storing thermal energy have gained prominence with
many researchers exploring different techniques and the
applications of the utilization of this new technological
advancement in the field of nanotechnology.
Micron scale additives have experienced hindrances in
the development of thermal enhancing technologies,
especially with the problem of particle clogging and
sediments. On the other hand, the nanoscale additives have
drastically improved the thermal properties of the heat
transfer of fluids. Also, they are able to produce an ultrafine
performance for cooling in electronics, enhanced heat
transfer surface techniques and improving the specific heats.
4. APPLICATIONS OF NANOFLUIDS/COMPOSITES
IN TES
Nanoparticles have one dimension that measures in 100
nanometers or less and the properties of the bulk material
changes with changes in the nanoscale particles [43]. The
verse changes in the physio-chemical properties are partly
due to the kinetic movement of the nanoparticles at the
surface layer. They exhibit greater surface area per weight

6. 6 Nanoscience & Nanotechnology-Asia, 2013, Vol. 3, No. 1 Al-Kayiem et al.
than larger particles which causes them to be more reactive
and have a continuous random motion. The dispersion of the
nanoparticles in polymers, metals, ceramics and fluids has
further opened pathways for more engineering of flexible
composites that exhibit advantageous thermophysical
properties [44]. These new hybrid nanofluids/composites are
applied in many technological applications ranging from
biomedical and transportation to energy etc.
The application of nanoparticles dispersed into base
fluids for TES is still a new research area. The amount of
nanoparticles needing to be dispersed into base fluids to suit
certain applications for specific TES is still under study and
investigation. To date, carbon nanotubes are the most widely
used additives as their thermophysical properties have been
established and the advantages have been proven compared
to other nanoparticles.
TES is a combination of different technologies that store
thermal energy in a reservoir for later use. They can be used
to balance energy demand at peak and off peak periods.
Among the several diverse technologies in TES are the
heating and cooling applications especially with respect to
buildings. As an example to the statistical data, the total area
of China’s residential buildings is about 40 billion m2
. The
total national building energy consumption (TNBEC) is 16
billion tons of standard coal which accounts for 20.7% of the
total end energy consumption [45]. Likewise, in the United
States, buildings account for 40% of all energy used in
the United States. This sector consumes more energy
than either industrial or transportation, surpassing industry
as the number one consuming sector in 1998 [46]. The
transportation sector (Automobile), micro-electronics and
electrical smart devices, medical, spacecraft and agriculture
(livestock) requires heating and cooling systems either for
enhancing efficiency or for thermal comfort. Many works
have been carried outon efficient trends in the heating
and cooling of systems for general applications but with
fewer investigations being conducted on the nanomixture
application for heating and cooling using TES.
4.1. Heating and cooling of Buildings
Space cooling in industrial areas is an enormous scientific
challenge, which also applies to many other diverse
production areas, including transportation, manufacturing,
microelectronics, sporting arenas etc. Traditionally, the
heating and cooling of a building or space has been in
practice for many decades. The construction of buildings
amidst trees to aid for cooling in hot weather, or the use of
bricks in the building of walls to aids cooling in hot weather.
The need for the effective utilization of TES has led to the
use of PCM in different building technologies to aid cooling
and heating depending on the desired comfort. PCMs have a
high latent heat storage capacity and as such havebeen
considered for thermal storage in building applications [47].
With the advent of PCM impregnated in trombe walls,
wallboards, shutters, under-floor heating systems and ceiling
boards can be used as a part of the building for heating and
cooling applications [48]. The relatively low thermal
conductivity of most PCMs brings a furtherset back in
adequate utilization, hence the need to integrate the PCM
with nanomaterial to enhance the thermophysical properties
has been a recent development. Many literatures have shown
the possibilities of the development and use of PCM
nanocomposites to enhance the TES capacity in building
applications.
The application of PCM enhanced nanocomposites in
buildings or spaces mainly focuses on using natural heat that
is solar radiation for heating purposesincold weather and for
cooling during hot weather conditions using man-made heat
or cold sources. In any case, storage of thermal energy is
important to match availability and demand with respect to
time and also with respect to power.
Table 2 summarizes the reported studies dealing with the
use of nanomaterials in TES for the heating and cooling of
buildings.
4.2. Heating and Cooling of Electrical and Electronic
Components
The technological advancement in the development of
smart materials and components to further reduce the weight
and size of electromechanical devices has made greater the
need for an efficient heat dissipation mechanism especially
on the microelectronic devices which operate at high speeds,
on higher power engines, and brighter optical devices. This
has driven increased thermal loads that require advances in
Table 2. Nanomaterial usage in TES for Building Heating and Cooling
Authors / Year Nanomaterials & Composites Mechanism/Applications Remarks
[49] Granqvist
et al., 2007
Electro- chromic material with
nanostructure photocatalyst
Smart material for a
benign indoor
environment
The solar photocatalysts are used in air purification and the
nanostructured control properties of smart windows to reduce heat
absorptance thereby reducing the demand for ventilation.
[50] Colella
et al., 2012
Paraffin Graphite composite
Piping network for district
heating
The thermal conductivity of the paraffin-based (HTF) was enhanced
with a 15% graphite mixing fraction. The charging behavior was
improved and the discharging extendibility was observed without
physical damage to the (LHTES) units’ medium.
[51] Sciacovelli
et al., 2012
Graphite matrix composite
Cylindrical shell thermal
storage unit for space
heating
The PCM was enhanced by a 10% increase in the thermal
conductance and the efficiency of the system was determined as
a28% increment.

7. Review on Nanomaterials for Thermal Energy Storage Technologies Nanoscience & Nanotechnology-Asia, 2013, Vol. 3, No. 1 7
cooling. Conventionally, heat dissipation is a factor of
increasing the area available for exchanging heat and the use
of a better conductive fluid [52]. However, this approach
undesirably increases the size of the thermal network system;
hence, the need arises for novel coolants with improved
performance. The use of oscillatory heating pipes, heat sinks
and micro channels to reduce the heat dissipated in a
compact microelectronic device integrated with micro-
electromechanical systems (MEMs) all have limitations. The
extremely high rates of heat transfer obtained by employing
micro-channels make them an attractive alternative to
conventional methods of heat dissipation. This is especially
so in applications related to the cooling of micro-electronics.
The researcher [53] carried out a compilation and analysis of
the results from investigations on fluid flow and heat transfer
in micro- and mini-channels and micro-tubes. The advent of
nanofluids has opened new possibilities of integrating the
MEMs with nanocomposites/fluids for the purpose of heat
removal and cooling in micro-electronic systems. Fig. (5)
describesthe CNT’s assembled structure for use in micro-
channel cooling.
Thermal management issues are limiting barriers to the
high density electronic packaging and miniaturization. In the
electronic industry, improvement of the thermal performance
of cooling systems together with the reduction of their
required surface area has always been a great technical
challenge. The continually increasing power of micro-
processors and other electronic components requires a search
for a more efficient heat dissipating system. Different
approaches have been investigated in times past and more
recently in the use of nanoadditives incorporated with the
conventional heat transfer fluids. The reported works on the
use of nanofluids/composites in electronics and computers
for heating and cooling applications of micro-electronic
devices integrated with micro-electromechanical systems
(MEMS) for heat dissipated and heat transfer system are
summarized in Table 3. Nanoparticles of metals and metal
oxides are mostly used as nanoadditives to the base materials
[55, 56, 57, 58 and 59].
4.3. Automobile Applications
Radiant energy that falls on the vehicle through the
windows of the car is another storage source for automobiles
during daytime especially at times of high solar intensity.
Various methods to store this kind of energy and use it when
needed have been tried and used so far. Effective TES of this
Fig. (5). (a) Positioning and (b) soldering of the flip chip on the Cu landing pads of the substrate (this structure also served as a reference).
(c) Solder paste dispensing, CNT array positioning, and (d) soldering on the Cu coated backside of the chip. (e) Field emission scanning
electron microscopy image of an assembled structure (scale bar: 500 μm) [54].

8. 8 Nanoscience & Nanotechnology-Asia, 2013, Vol. 3, No. 1 Al-Kayiem et al.
waste heat can be used in the preheating of the engine,
preheating the catalytic converter, heating and cooling the
passenger compartment and defrosting the car windows. A
huge amount of energy is lost due to incomplete combustion
in automobile engines, as such considerable amounts of
gasoline are consumed thereby adding to environmental
pollution. Also, a significant amount of thermal energy
generated by automobile engines is wasted to the
environment especially in the case of diesel engines. Hence,
various heat recovery and reuse systems are under
investigation for probable use of nanomaterials as thermal
energy storage. Engine cylinders (liners) are being envisaged
to be coated with nanocrystalline ceramics, such as zirconia
and alumina, for purposes of heat retention and a more
efficient combustion system. The conventional heat transfer
fluid like ethylene glycol and a water mixture in the
automotive coolant have a relatively poor heat transfer due to
low thermal conductivity [60]. The addition of nanoparticles
to this engine coolant has the potential to improve the
cooling rate of automotive and heavy-duty engines. Such
improvements will reduce the size of the radiator which will
result in smaller and lighter coolant systems and reduce heat
waste. The studies on the performance enhancement by
integration with nano-TES are summarized in Table 4.
4.4. Solar Thermal Heating
Water heating is a major source of energy consumption
in domestic and commercial buildings where low temperatures
are mostly required. The primary energy sources used to
generate hot water are non-renewable. Regrettably, this
condition remains despite efforts to develop and promote the
use of renewable, solar thermal water heaters for over a
century [66]. Solar thermal collector systems are by far the
most reversed means of harvesting solar radiation to be used
for the heating of water. There are many types of solar
Table 3. Nanomaterials as TES in Electrical, Electronic and Computer Components
Authors /
Year
Nanomaterials &
Composites
Technology
/Applications
Remarks
[55]
Nguyen
et al.,
2007
Silver oxide
nanoparticle–
water mixture
Liquid Cooling
/Fluidic circuit of micro-
electronic devices
The inclusion of nanoparticles into distilled water has produced a considerable
enhancement of the cooling block convective heat transfer coefficient. For a particular
nanofluid with a 6.8% particle volume concentration, heat transfer coefficient has been
found to increase as much as 40% compared to the base fluid.
Experimental results have also shown that the nanofluids with a 36 nm particle size
provided higher convective heat transfer coefficients than the ones given by nanofluids
with 47 nm particles.
[56]
Putra et al.,
2011
Alumina–water
and Titania–water
nanofluids
Thermo-electric / Heat
pipe liquid block
The critical heat flux wasreduced due to a reduced temperature difference between the
heated wall and the coolant. The CPU temperatures as well as the thermal resistance
werereduced by 4-6o
C below the ambient temperature. The Al2O3 showed the best
result with the reduced thermal temperature of 23.9o
C, while TiO2 and distilled water
showed 24.2o
C and 26.5o
C, respectively, at a 1% mixture concentration.
[57]
Selvakumar
et al.,
2012
CuO / water
nanofluids
Heat sink / Compact
Electronic cooling
applications
The interface temperature of the water block wasmeasured and a maximum reduction
of 1.15°C was observed when nanofluids of 0.2% volume fraction were used as the
working fluid compared to deionized water. The convective heat transfer coefficient of
the water block was found to increase with the volume flow rate and the nanoparticle
volume fraction and the maximum rise in the convective heat transfer coefficient
wasobserved as 29.63% for the 0.2% volume fraction compared to deionized water.
The average increase in the pumping power was 15.11% for the nanofluid volume
fraction of 0.2% compared to deionized water.
[58]
Rafati et al.,
2012
Silica, Alumina
and Titania
nanoparticles
dispersed in base
fluid
Heat pipe /
Thermal performance in
computer cooling and
micro-electromechanical
systems (MEMS)
The use of nanofluids resulted in the considerable reduction of the processor operating
temperature as compared to the pure base fluid. The surface heat transfer wasalso
reduced. The largest decrease observed was for alumina nanofluids, which decreased
the processor temperature from 49.4o
C to 43.9o
C for 1.0% of the volumetric
concentration. The flow rate was 1.0 L per minute when compared with the pure base
fluid with the same flow rate.
[59]
Jeng et al.,
2013
Al2O3/
Water nanofluid
Hybrid cooling system /
vapor compression
refrigeration system
(VCRS) for micro-
electronic devices and
components
The hybrid cooling system for CPUs combinedthe advantages of a liquid cooling system
with an Al2O3/water nanofluid and a VCRS with a hydrocarbon refrigerant. The experimental
results demonstrated that the maximum cooling capacities of the liquid cooling system,
VCRS, and hybrid cooling system were 450 W, 270W and 540 W, respectively. The
Al2O3/water nanofluid increasedthe heat dissipation performance and the power
consumption of the water pump, yet decreasedthe surface temperature of the heater.

9. Review on Nanomaterials for Thermal Energy Storage Technologies Nanoscience & Nanotechnology-Asia, 2013, Vol. 3, No. 1 9
collectors such as the flat plate collector, evacuated tube
collector, concentrated collector and integrated collector
[67]. The ideal thermal collector should efficiently absorb
solar radiation and convert it to a thermal energy, and
minimize heat losses in the system. The major setbacks of
the thermal collectors are the intermittent nature of solar
energy thereby requiring a storing medium. PCMs have been
used as storage mediums to store and release latent heat for
integrated solar collectors; while some integrated collectors
use water as the storage medium. However, PCMs are
proven to have higher heat storage capacity because of the
latent heat. During the daytime, PCMs will absorb heat from
solar radiation to store thermal energy while in the night the
PCMs will release the thermal energy to heat the water [68
and 69]. The low thermal conductivity of the PCM requires
enhancement, mainly on the thermal conductivity. Nano-
materials have been incorporated with PCMs to enhance the
heat transfer thermal conductivity and heat transfer rate.
Advanced heat transfer fluid and nanocomposites cascaded
with well transitioning temperatures in various designs has
improved the thermal storage capacity of solar thermal
collector systems. Experimental results of [68] demonstrate
thermal conductivity enhancement by 12.2% by dispersion
of 1% of the nano Cu powder in paraffin wax as a base
material. By increasing the dispersed nano Cu to 2%, by
weight, the thermal conductivity of the nanocomposite is
enhanced by 24%.
There have been many attempts to enhancethe
performance of solar thermal systems by integration with
nano-TES. Table 5 displays the achievements gained by
nanoadditives to the TES base materials.
Researchers [70, 71, 72, and 75] mainly conducted the
experimental characterization in the laboratory environment
to improve the PCMs and heat transfer fluids by dispersion
of nanoadditives. Only researchers [73 and 74] tested the
blended nanofluid for the solar water heating application on
the real operational condition. The solar collector
performance was enhanced by 25% and 30% by using the
nanofluids. Researcher [76] experimentally investigated the
direct absorption of solar radiation on nanofluids but no solar
collector model was revealed However, the researcher
proposed a 1% nanofluid concentration and a lower than 20
nm aluminium nanoparticle diameter for the maximum
absorption of the daylight scattering of solar radiation. There
is a lack of solar thermal heating and cooling experiments
incorporated with nanomaterials being applied at in-situ
projects. This will provide wider opportunitiesto test
nanocomposites and nanofluids withsolar collector underreal
operational conditions.
Table 4. Enhancing the IC Engine Performance by Nanomaterials as TES
Authors / Year
Nanomaterials &
Composites
Technology/Application Comments
[61]
Peyghambarzadeh
et al., 2011
Al2O3-nanoparticle
and conventional
heat transfer fluid
(water)
Heat transfer
/Automobile radiator
coolant
The Al2O3 – water nanofluid enhanced the heat transfer rate of the automobile
radiator by 45% compared with pure water. Increasing the fluid flow rate
increased the heat transfer performance and gave a slight variation between the
inlet water temperature to the radiator and the ambient. The cooling effectiveness
yielded a better performance and reduced the radiator space and weight.
[62] Gumus et al.,
2009
Salt Hydrate
(Na2SO4. 10H2O)
as the PCM
Thermal energy storage
system for pre-heating of
internal combustion
engine
The engine temperature was increased to 17.4o
C by pre-heating and the pre-
heating took500s. The maximum thermal efficiency recorded was57.5% and
2277kJ of heat absorbed during charging. Emissionswithpre-heating of the
enginewas less than normal engines where CO and HC emissions decreased by
about 64% and 15%, respectively, with the effect of pre-heating the engine.
[63] Bewilogua et
al., 2009
TiAIN
base
Nanocomposites
Coating and self-
cleansing surface
Higher surface hardness wasattained compared to the conventional coating and
enormous improvement in heat resistance. Addition of nanomaterial to improve
the coating material property gave improved properties and enhanced the system
performance.
[64] Pandiyarajan
et al., 2011
PCM
Heat exchanger in diesel
engine exhaust
The effectiveness of the heat recovery exchanger (HRHE) reached 99%
efficiency. 10-13% wasted heat wasrecovered using the (HRHE). The cascading
of the exhaust increased the temperature to 14.8o
C. The disadvantage of the
(HRHE) is the external space and added weight. The integration of nanomaterial
to the PCM couldreduce the space and use of an external storage heat medium,
and increase the temperature as well as efficiency.
[65] Bokde et al.,
2013
PCM
Catalytic converter for
pre-heating
An average of 23% and 21% reduction in %CO and HC emission occurred during
the cold start emission. The temperature of the engine increased by 12% and the
charging period was extended thereby allowing for an increased temperature and
reducing the cold start effect. Further modification of the catalytic converter
integrated with nanomaterial to improve the thermal property of the PCM further
increased the efficiency of the system.

10. 10 Nanoscience & Nanotechnology-Asia, 2013, Vol. 3, No. 1 Al-Kayiem et al.
5. CONCLUSION
The literature survey has demonstrated that the TESs are
effective when integrated with thermal conversion systems
like heat exchangers. However, the performance of the TESs
could be considerably improved by improving the thermal
conductivity of the TES base materials via nanoadditives.
Among the major base materials that have been used and
investigated are the PCMs. The paper has reviewed the
available PCMs, nanocomposites and nanofluids for TES
applications. Almost all of the researches agreed on the
enhancement of the thermal properties of the base materials
but with some inconsistency in the results. As the case is,
further investigations are required to reach standardized
status on the effect of the nanoadditives to the base
materials. In spite of the numerous investigations on the
nanoadditives and theirenhancement of the thermophysical
properties of the TES base materials, more investigations are
needed to understand the dispersion and settlement of the
nanoparticles in the base materials. Also, it is recommended
that more work is required to study the thermal cycle’s effect
on the stability of the nanofluids/composites.
CONFLICT OF INTEREST
• The main author would like to acknowledge Universiti
Teknologi PETRONAS for sponsoring the work
under the Universiti Internal Research Fund (URIF no.
19/2012).
• The second author would like to acknowledge the
Ministry of Higher Education, Malaysia, (MOHE) for
sponsoring his PhD scholarship.
• The third author would like to acknowledge Universiti
Teknologi PETRONAS for granting his PhD under the
GA scheme.
ACKNOWLEDGEMENTS
The main author would like to acknowledge Universiti
Teknologi PETRONAS for sponsoring the work under the
University Internal Research Fund (URIF no. 19/2012). The
second author would like to acknowledge the Ministry of
Higher Education, Malaysia, (MOHE) for sponsoring his
PhD scholarship. The third author would like to acknowledge
Table 5. Summary of Nano-TES Integration to Enhance the Solar Collectors
Authors / Year
Nanomaterials
Composites Nanofluids
Technology/
Application
Remarks
[70] Sani et al.,
2010
Single wall carbon
nanohorns (SWCNHs) in
an aqueous suspension
Solar Collector
Heat Exchanger
Thermal conductivity was increased to 10% with the investigated concentration.
The spectral transmission showed that single wall carbon nanohorns (SWCNHs)
improved the platonic properties of the fluid, leading to a significant increase of
the light extinction level even at a very low concentration
[71] Jung et al.,
2011
Mica nanoparticle and pure
nitrate salt eutectic
Concentrated
Solar Power
(CSP)
There was a 13-15% enhancement on the specific heat at the solid phase and 13-
19% at the liquid phase compared to a pure nitrate salt eutectic in 60:40 molar
ratio. Enhancement in the specific heat value in the liquid phase increasedwith an
increase in the mass concentration
[72] Shin et al.,
2011
Silica nanoparticle
dispersed to carbonate salt
eutectic
Concentrated
Solar Power
(CSP)
A 24% enhancement at the solid phase and 75% at the liquid phasewerereported
with an error analysis of 3.3% and 1.5% for the solid and liquid phases. The
enhancement increased with an increase in the volume ratio of the mixture.
[73] Lu et al.,
2011
CuO nanofluid
Evacuated Solar
collector
The CuO nanofluid concentration range of 0.8 % - 1.5% was tested. 1.2% was
found to provide the highest thermal conductivity. The performance of the
evaporator improved by 30% as compared to ionized water.
[74] Yousefi et
al., 2012
Multi-walled carbon
nanotube (MWCNT)
nanofluid
Flat Plate Solar
Collector
0.2% of the MWCNTs were investigated with water as the base fluid. Different pH
rangeswere selected. The nanofluid withthe pH of 6.5 achieved the highest heat
absorption coefficient, FRUL (38.84), while the nanofluid with the pH of 9.5
(FRUL=30.2). The enhancement increasedthe collector efficiency by 25%.
[75] Poinern et
al., 2012
Carbon nanowalls (CNWs)
in an aqueous solution
Solar Thermal
Collector
The largest CNS mass content (0.04 g) nanofluid had the largest temperature
enhancement of 8.1°C, which clearly demonstrates the efficient absorption
capabilities of the CNS nanofluids towards solar irradiation The results indicate
that functionalized CNS nanofluids have the potential to effectively improve the
solar absorption capabilities of direct-absorption solar collectors
[76] Saidur et
al., 2012
Aluminium nanofluid
Direct Solar
Absorption
Researchers have investigated experimentally the effect of the absorption
coefficient and the scattering coefficient range of the 0.1 – 2.5% concentration of
aluminium nanofluid. The nanoparticle diameters chosen werefrom 1 – 20 nm. The
absorption and scattering coefficient was themaximum at the solar wavelength of
0.3 μm before starting to decrease for all the parameters. The scattering coefficient
showed a good value only at the solar wave length of 0.2μm.

11. Review on Nanomaterials for Thermal Energy Storage Technologies Nanoscience & Nanotechnology-Asia, 2013, Vol. 3, No. 1 11
Universiti Teknologi PETRONAS for granting his PhD
under the GA scheme.
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Received: May 19, 2013 Revised: June 06, 2013 Accepted: July 02, 2013