Renewable and Sustainable Energy Reviews 193 (2024) 114288

Contents lists available at ScienceDirect

Renewable and Sustainable Energy Reviews

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

Application of graphene and graphene derivatives in cooling of

photovoltaic (PV) solar panels: A review
Li Teng Siow a, Jun Rong Lee b, Ean Hin Ooi a, **, Ee Von Lau a, *
a
School of Engineering, Monash University Malaysia, Jalan Lagoon Selatan, Bandar Sunway, 47500, Subang Jaya, Selangor, Malaysia
b
Department of Mechanical Engineering, Faculty of Engineering and Technology, Tunku Abdul Rahman University of Management and Technology, Jalan Genting
Kelang, Setapak, 53300, Kuala Lumpur, Malaysia

A R T I C L E I N F O A B S T R A C T

Keywords: Solar photovoltaic (PV) panels are often subjected to high temperature rise, causing their performance to
Graphene deteriorate. Graphene and graphene derivatives with superior in-plane thermal conductivity ranging up to
Solar photovoltaic 3000–5000 W/(m⋅K) have recently presented new opportunities for improving heat dissipation rates in engi­
Heat dissipation
neering applications. Cooling methods with the incorporation of graphene and its derivatives in different ap­
Cooling
Neutral density filter
proaches such as graphene-coated neutral density (ND) filters, graphene-enhanced thermal interface materials
Thermal interface material (TIM), graphene-enhanced phase change materials (PCM) and graphene nanoplatelets (GnP) nanofluids are
Phase change material reviewed in terms of their significances in promoting heat dissipation in solar PV panels. With a graphene-coated
Nanofluid ND filter, the focal spot temperature was reduced by 20 % compared to the infrared filter, and a 12 %
enhancement in efficiency was observed. Graphene-enhanced TIM reduced the temperature rise by 34 %
compared to the conventional TIM. The employment of GnP-enhanced PCM improved the power output and
efficiency of the solar PV system with lower average cell temperature achieved compared to other nanoparticles-
enhanced PCM. On the other hand, GnP nanofluid reduced the panel temperature by ~17 ◦ C, corresponding to
an increase of ~3 W in the power output. The surface temperature at the peak point was 35.8 % lower than the
conventional panel when graphene nanofluid was circulated in the solar PV system. These findings have not only
shed light on the application of graphene in assisting heat transfer for solar PV cooling, but also provide valuable
insights into its applicability across other diverse fields such as heat pipes, heat exchangers, and solar collectors.

from 15 % to 20 % [3,4], and is dependent on various factors, such as

dust accumulation across the solar PV panels, shading, wind speed, solar
1. Introduction irradiance, ambient temperature, and surface temperature of the solar
PV panels [5–11]. Excessive solar irradiance can cause waste heat gen­
The energy sector, specifically the non-renewable kind segment, is eration, which heats the PV panel and raises its surface temperature [12,
one of the largest contributors to the total global greenhouse gases 13]. This can negatively impact the conversion efficiency of the solar PV
(GHG) [1]. The emission of GHG into the atmosphere is inevitably a panels, which typically operate at a standard temperature of 25 ◦ C [14].
major contributor to global warming and climate change. This has led In hot and arid regions, however, the PV panels can heat up to 75 ◦ C,
many industries to adopt cleaner and renewable energy sources as part thus reducing its efficiency by 25 % [15]. For a mono-crystalline silicon
of their commitment to achieve a more environmentally friendly and PV cell, the relative change in the maximum power output (quantified as
sustainable outcome. Due to its zero GHG emissions, solar energy has the change in power output over its maximum power output) has been
become a popular energy source [2], which has prompted numerous determined to be − 0.002/◦ C [16], while the energy conversion effi­
research and development into the various aspects of solar energy to ciency decreases by 0.5 % for every 1 ◦ C rise in temperature [17].
improve its efficiency and applicability. This can be demonstrated by the Electrical efficiencies for amorphous, poly-crystalline, and
increase in the number of publications per year in the area of “solar mono-crystalline PV cells are found to decrease by 0.33 %, 0.51 %, and
energy” over the past two decades, as shown in Fig. 1. 0.84 %, respectively, for every 100 W/m2 increase in solar irradiation
The efficiency of a solar photovoltaic (PV) system typically ranges

* Corresponding author.
** Corresponding author.
E-mail addresses: ooi.ean.hin@monash.edu (E.H. Ooi), lau.ee.von@monash.edu (E.V. Lau).

https://doi.org/10.1016/j.rser.2024.114288
Received 6 October 2023; Received in revised form 20 December 2023; Accepted 9 January 2024
Available online 16 January 2024
1364-0321/© 2024 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).
L.T. Siow et al. Renewable and Sustainable Energy Reviews 193 (2024) 114288

Nomenclature CuO Copper oxide

TiO2 Titanium oxide
PV Photovoltaic SiO2 Silicon dioxide
CPV Concentrated photovoltaic MgO Magnesium oxide
PV/T Photovoltaic thermal MWCNT Multi-walled carbon nanotubes
CPVT Concentrated photovoltaic thermal PEG1000 Polyethylene glycol 1000
TEG Thermoelectric generator EG Expanded graphite
ND Neutral density PDMS Polydimethylsiloxane
GCND Graphene-coated neutral density SDBS Sodium dodecyl-benzene sulfonate
TIM Thermal interface material XRD X-ray diffraction
PCM Phase change material FESEM Field emission scanning electron microscopy
n-PCM Nano-enhanced phase change material VM Maximum operating voltage
MJSC Multi-junction solar cells IM Maximum operating current
FF Fill factor VOC Open-circuit voltage
CR Concentration ratio ISC Short-circuit current
GHG Greenhouse gases
GnP Graphene nanoplatelets Greek symbols
HyNF Hybrid nanofluid λ Wavelengths
Al Aluminium η Efficiency
Al2O3 Aluminium oxide

nanoplatelets into the working fluid of a sintered wick heat pipe was
found to reduce its thermal resistance by 48.4 % when compared to
using distilled water alone [34]. Similarly, an enhancement of 35.6 % in
convective heat transfer coefficient was recorded with 0.1 wt% gra­
phene nanofluid in a shell and tube heat exchanger compared to pure
water [35].
Since its discovery in 2004 [36], graphene has gained interests
among researchers as a medium for enhancing heat transfer in cooling
systems [37–40]. The enhanced heat transfer performance reported by
these studies have motivated the use of graphene to improve the effi­
ciency of the cooling systems employed in solar PV panels. The various
uses of graphene in solar PV cooling systems include using graphene as
selective absorber coating [41,42], the addition of graphene nano­
particles into thermal interface material (TIM) [43,44] and phase
change material (PCM) [45–55], and its incorporation into the working
fluid as a nanofluid [56–61].
Fig. 1. Number of publications related to the topic “solar energy” from 2003 to The mechanisms by which graphene enhances the heat transfer ef­
2022. Results were obtained from Web of Science’s “Topic” search with the ficiency of solar PV cooling systems in each of the aforementioned ap­
keyword “solar energy”, and document types refined by “Article or Proceed­ proaches can vary. Given the continued interest in both graphene and
ing Paper”. solar energy, a proper understanding of the utilisation of graphene in
solar PV cooling systems is crucial to advance the development of
[18]. Therefore, thermal management of solar PV cells is crucial to graphene-based solar PV cooling systems. It is thus the objective of this
prevent overheating and performance deterioration. study to provide an in-depth review and valuable insights into the
Numerous cooling systems have been developed and applied to application of graphene and its derivative in enhancing solar PV cooling;
dissipate heat from solar PV panels. These include air-cooled systems, hence, further exploiting their benefits in dissipating heat from solar PV
water-cooled systems, heat sinks, heat pipes, phase change materials, panels.
and forced nanofluids circulation [19–23], among others. However, the Section 2.0 of this paper first outlines the methodology of how this
working fluids and materials used in these cooling approaches are often review was conducted, Section 3.0 briefly describes the various cooling
reported to have low thermal conductivity, which causes low efficiency techniques in solar PV systems, followed by Section 4.0 which reviews
in solar PV systems [24,25]. In recent studies, the incorporation of the application of graphene in different solar PV cooling approaches.
highly thermally conductive nanomaterials into conventional material Section 5.0 then summarises the advantages, limitations, and signifi­
was found to enhance the thermal conductivity of the material, thus cance of graphene in cooling solar PV panels, while Section 6.0 provides
increasing the heat dissipation rate [26,27]. an outlook and future perspectives of graphene in solar PV cooling
One of these high thermal conductivity materials is graphene, a two- systems, with Section 7.0 concludes this review.
dimensional nanomaterial [28,29] that is often incorporated to enhance
heat transfer [30–32]. Graphene has an outstandingly high in-plane 2. Review methodology
thermal conductivity of 3000–5000 W/(m⋅K), which outperforms
commonly used metals, such as copper (390 ± 34 W/(m⋅K)) and 2.1. Scope identification
aluminium (214 ± 17 W/(m⋅K)) [33]. The high thermal conductivity of
graphene implies that only a minimal amount is required when added to This paper aims to review the various applications of graphene and
the working fluid of a cooling system to enhance the overall heat its derivatives in aiding the cooling and thermal management of solar PV
dissipation rate. For example, the addition of 0.1 wt% graphene systems. Therefore, the scope of this paper was identified by the

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L.T. Siow et al. Renewable and Sustainable Energy Reviews 193 (2024) 114288

Fig. 2. PRISMA framework for literature search of a new systematic review [62].

following keywords: “graphene”, “cooling”, “thermal management”, so allows only light at wavelengths (λ) that contribute to the electricity
and “solar PV”. conversion of the PV cells (ultraviolet and visible light) to pass through
while filtering out light at unwanted wavelengths (infrared light) that
contribute only to the heating of PV cells [63]. Since ultraviolet (290 ≤ λ
2.2. Search strategy ≤ 380 nm) and visible (380 ≤ λ ≤ 780 nm) lights make up about 49 % of
solar irradiation, the pre-illumination cooling technique is capable of
The abstract and citation database, Scopus, was used to search for halting slightly more than half of the solar irradiation that does not
literature based on the scope of this paper. The literature search was contribute to the generation of electricity in the PV cells [63].
performed according to the procedures outlined in the Preferred Pre-illumination cooling method is mostly employed in concentrated
Reporting Items for Systematic reviews and Meta-Analyses (PRISMA) photovoltaic (CPV) systems [41] and operated without the use of
(Fig. 2) [62]. The literature search was conducted by using the following external energy.
keywords string: “graphene AND (cooling OR thermal OR performance) Cooling via pre-illumination techniques can be achieved through a
AND (pv OR (solar AND pv) OR ((concentrated OR concentrator) AND variety of different approaches. These include the use of spectral beam
(photovoltaic OR solar)))”. Up to the date when this review was con­ splitter, holographic filter, luminescent filtering, and selective absorp­
ducted, 311 documents were found from the search. Following this, the tion filter [41]. Spectral beam splitter helps to redirect the shorter
documents were screened using the PRISMA framework as outlined in wavelengths of photons with energy higher than the bandgap energy to
Fig. 2, with the numbers in parentheses indicating the quantity of the solar cell surface for electrical energy conversion, while the longer
publications involved in the literature selection and screening process. wavelengths with lower energy photons are directed towards the heat
sink for thermal energy conversion [64]. A holographic filter utilises the
3. Cooling techniques in solar PV systems principles of refraction to divert the spectrum band of different wave­
lengths to a series of solar cells with different bandgaps to maximise the
3.1. Pre-illumination cooling techniques operation efficiency [65]. Transparent plastic dispersed with lumines­
cent species (usually organic dye molecules) is employed in the lumi­
Pre-illumination cooling method addresses overheating by decom­ nescent filtering method to absorb the incident light and emit it through
posing the sun spectrum before it reaches the solar PV cells [63]. Doing

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L.T. Siow et al. Renewable and Sustainable Energy Reviews 193 (2024) 114288

Fig. 3. Classification of solar PV cooling techniques.

a red-shifted wavelength of higher quantum efficiency, which in turn solar PV system.

could yield a near-unity conversion efficiency in the solar cell [66]. Among the different types of coatings available, carbon-based coat­
Selective absorber coating has high sunlight absorbance and low ings are of specific interest due to their reusability, low cost, good light-
infrared thermal emittance, which can maximise solar energy absorp­ to-heat conversion properties, maximum light absorption, enhanced
tion with minimal heat radiative loss, hence enhancing the photo­ optical absorption, great thermal stability, and high specific surface area
thermal conversion efficiency [67]. [41]. Graphene outperforms all the other carbon allotropes due to its
two-dimensional features that lead to high mechanical flexibility, high
3.2. Post-illumination cooling techniques degree of transparency, high charge mobilities, and superior thermal
conductivity [75,76]. Graphene can conduct heat horizontally with high
Post-illumination cooling techniques, on the other hand, utilise heat thermal stability due to its flat 2D band structure, thus making it more
receiver devices to dissipate heat generated from solar PV cells [63]. superior than other conventional solar absorber coatings [41]. Due to its
Available post-illumination cooling techniques employed in solar PV strong in-plane thermal conductivity, heat generated from the incident
systems can be classified into active cooling and passive cooling [9,19, light ray is dissipated horizontally by graphene at the upper layer before
21,68–70]. Active cooling employs mechanical equipment such as fans the incident light is transmitted further down into the lower layer [41].
or pumps to circulate the working fluid around the system to extract heat This is illustrated in Fig. 4.
from the solar PV through forced convection, whereas passive cooling A concentrated photovoltaic (CPV) system converts incoming solar
utilises devices such as heat sink, heat pipe, thermal interface material, light rays and concentrates them onto the smaller PV cells using lenses
and phase change material to perform heat transfer without the aid of and mirrors [77,78]. This technique increases the concentration ratio
external forces [71]. Fig. 3 summarises some of the post-illumination and forms a higher heat flux density on the solar cell [79], which pro­
cooling techniques available for solar PV cooling, their advantages vides higher power output and a reduction in the cost for multi-junction
and limitations, and examples of the techniques available. solar cells (MJSC) [41]. MJSC are solar cells that consist of multiple p-n
junctions made of different semiconductive materials, where the p-n
4. Application of graphene in solar PV cooling systems junction of each material responds to different wavelengths to produce
electric current [80]. With the use of MJSC, the absorption of a wider
4.1. Pre-illumination cooling range of wavelengths is possible, thus enhancing the efficiency of solar
cells [80]. However, due to the low internal resistance of the MJSC, it is
4.1.1. Graphene-coated neutral density (GCND) filter as a selective unable to withstand temperatures beyond 110 ◦ C. To overcome this,
absorber coating some researchers have resorted to graphene-coated neutral density (ND)
Photon energy absorbed by the solar cell plays a significant role in filters, which attenuate the transmittance of solar irradiance over a wide
determining the performance of solar PV panels. Photons that possess spectral band [41]. These graphene absorber coatings attenuate solar
energy lower than the energy bandgap of the PV cell material will not irradiance and allow photons with an energy that is more than and close
excite the electrons in the cell for the conversion into electrical energy to the bandgap energy to pass through efficiently, thus improving the
and subsequently converted into thermal energy [72]. On the other solar cell efficiency [41]. Graphene coatings benefit not only in CPV but
hand, photon energy that is greater than the bandgap energy passes on also in other solar PV systems such as ultrahigh CPV and multi-junction
extra energy to the electrons. This extra energy is also converted into PV [81].
thermal energy, which is not beneficial for improving the efficiency of Graphene-coated neutral density (GCND) filters with different
solar PV systems [72]. Thermal energy stored in solar PV cells can lead thicknesses, namely thin (2.2 μm), medium (6.3 μm), and thick (9.1 μm)
to detrimental effects on the circuit performance and consequently, the were employed by Alzahrani et al. [82] to investigate the thermal
overall conversion efficiency. A drastic drop in the open-circuit voltage characteristics of the GCND filters during the operation of CPV system.
(VOC) is observed when the solar PV cells experience ambient temper­ The authors found that the focal spot temperature was considerably
atures higher than 25 ◦ C [73]. By coating the solar PV cells with selective lower than the incident temperature when GCND filters were used, but
absorbers, the performance of the solar PV cells can be improved by the temperature on the graphene coating increased while the focal spot
allowing high solar energy absorption while minimising thermal emit­ temperature was maintained [82]. In the experiment where the GCND
tance to reduce thermal losses [74]. As such selective absorber coating is filter was utilised, the focal spot temperature of the CPV system was
a top choice when choosing a pre-illumination cooling technique for a reduced by 20 % and 12 % for medium and thin graphene coatings,

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L.T. Siow et al. Renewable and Sustainable Energy Reviews 193 (2024) 114288

Meanwhile, increasing the graphene deposition to two monolayers did

not show any positive effect on the FF due to optical loss [42].
The application of the GCND filter as a pre-illumination passive
cooling technique is highly advantageous, particularly in cooling CPV
systems. This pre-illumination cooling technique attenuates the solar
radiation before it reaches the PV panel, thus reducing the amount of
heat generated at the PV panel. With minimal heat generated at the PV
panel, the panel can sustain for a longer operation period without
damaging the solar cells. Moreover, this passive cooling method does
not require the use of external devices to execute the task, hence lower
maintenance costs. However, there is a major drawback in employing
the GCND filter for thermal management in the CPV system. As gra­
phene has to be coated on a substrate (often made of glass) when in use
for GCND filter, the optothermal properties of both the substrate and
graphene have to be similar to reduce the thermal stress on the GCND
filter. Alzahrani et al. [82] employed low-iron glass as the substrate and
reported breakage due to the thermal stress experienced by the sub­
strate. Since graphene has better optothermal properties than low-iron
Fig. 4. Light transmission mechanism through graphene coating [41]. glass, it is capable of retaining more heat [82]; hence, causing
low-iron glass with a lower thermal capacity to fail drastically. There­
respectively, compared to the infrared filter [41]. Moreover, an effi­ fore, the opto-thermal properties of the substrate material are another
ciency enhancement of 12 % was achieved with the use of medium factor to consider when adopting a GCND filter. Here, the study of
graphene coating under eight suns condition1 compared to the base case graphene coating on various types of glass (e.g. borosilicate glass, fused
which was under seven suns condition of 400 W/m2 [41]. Higher cell silica) to pre-examine the performance of the GCND filter and their
efficiency is achieved by reducing the focal spot temperature due to the employment feasibility in CPV systems could be further explored.
enhanced optothermal properties of graphene despite optical loss across Additionally, the economic feasibility of coating graphene on the ND
the wavelength range [41]. Similarly, another study also showed that filter must be accounted for cost-to-effectiveness evaluation. While
GCND with thicker coating exhibited higher cell efficiency compared to studies have demonstrated positive results using graphene as a thermal
thinner coating, as thicker coating prohibited a larger portion of solar filtering material for selective absorber coating in GCND filters for
irradiance from passing through, thus reducing the heat generated [82]. cooling CPV systems, further optimisation studies should be considered.
Apart from that, the presence of graphene coating plays a unique role For instance, while a thicker graphene coating may lead to a lower focal
in controlling the incident solar irradiance onto the PV cell. The reduced spot temperature, the cost of graphene coatings increases with the
transparency of graphene from colourless to dark due to the increase in coating thickness. Therefore, achieving a balance between performance
optical density has an inverse correlation to the heat generated on the and cost is crucial when employing GCND filters in CPV systems. Rather
solar PV cell [41]. Additionally, the focal spot temperature also dem­ than coating graphene across the entire substrate, focusing the graphene
onstrates an inverse correlation with the optical density. A decrease of coating specifically at the focal spot area could contribute to reducing
89 %, 70 %, and 39 % in focal spot temperature was observed for the coating costs. In terms of cost-effectiveness, the application of GCND
thick, medium, and thin graphene coatings, respectively when filter as a cooling method for a CPV system appears to be more suitable
compared to no coating [41]. Thicker coating results in a higher optical for small-scale domestic use, despite the requirement for a smaller
density and therefore, a lower total transmittance, optical efficiency, coating area.
and focal spot temperature [41,82]. Fig. 5 shows the working mecha­
nism of a CPV system with the incorporation of GCND.
Fill factor (FF) is another indicator that is often used to quantify the 4.2. Post-illumination cooling
performance of a solar PV system. FF is defined as the ratio of the
maximum output power to the ideal output power of the solar PV system 4.2.1. Passive cooling
[83]:
4.2.1.1. Graphene as thermal conductive filler in thermal interface mate­
VM × IM
FF = 1 rials (TIM). Thermal interface material (TIM) refers to an adhesive,
VOC × ISC
paste, gel, putty, grease, solder, phase change material (PCM), or
where VM is the maximum operating voltage, IM is the maximum oper­ advanced material such as a carbon matrix that is sandwiched between
ating current, VOC is the open-circuit voltage and ISC is the short-circuit two solid interfaces [84,85]. As these two proximate surfaces may not be
current. perfectly smooth even after fine polishing; hence, the formation of
A solar PV system with a larger FF is preferred as this indicates a microscopic air voids at the interface is unavoidable [86,87]. These air
higher efficiency [83]. The incorporation of a single monolayer of gra­ voids negatively affect the heat conduction across the surfaces due to the
phene onto a triple-junction solar cell resulted in a 35 % decrease in the low thermal conductivity of air. One way to remove these air voids is to
series resistance and a 4 % increment in FF when the concentration ratio use TIM to fill the air voids, hence improving the thermal contact and
is at 1000 suns. The increase in FF is caused by the superior thermal enhancing the heat transfer rate between the two surfaces [86].
conductivity of graphene, thus enhancing the power output [82]. To enhance thermal conductivity, TIM is generally constructed from
a composite material that consists of fillers such as metal or ceramic
particles as they provide better thermal conductance [88]. Metals with
1 large thermal conductivity, such as aluminium (204 W/(m⋅K) [89]),
The sun condition refers to the concentration level of the system and is also
known as the concentration ratio. The concentration ratio is the ratio of the boron nitride (250–300 W/(m⋅K) [89]), and copper (483 W/(m⋅K) [89])
concentrated energy density per unit area to the incident energy density. For are often used as thermally conductive fillers in TIM [84]. Durability is
example, when the sun condition is eight (or the concentration ratio is eight), it another important factor when it comes to selecting TIM. It should
means that the PV surface is exposed to light that is eight times stronger than demonstrate high stability and reliability over its service, as well as
normal sunlight [114]. strong corrosion resistance [88].

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L.T. Siow et al. Renewable and Sustainable Energy Reviews 193 (2024) 114288

Fig. 5. The working mechanism of a CPV system with the incorporation of GCND [41].

Solder-type TIM has a low melting point, which allows it to spread due to low deformability [91]. Meanwhile, polydimethylsiloxane
evenly in its molten state. Nevertheless, solder is relatively thicker than (PDMS), which is a type of soft silicon, has a lower compressive modulus
other TIM materials and more reactive, thus it is not an ideal candidate of 2–10 MPa. However, PDMS has a significantly higher viscosity of
for solar PV cooling [84]. Epoxy is commonly used as an adhesive in 3900–4150 mPa s, which makes the infusion of PDMS into the graphene
TIM. Cured epoxy can spread evenly through the air gaps. However, framework more challenging [91]. Table 1 summarises the thermal
cured epoxy may lead to a mismatch in the thermal expansion coeffi­ conductivity of different graphene-based TIM in different matrix types.
cient and delamination [85]. Hence, adhesive-type TIM may not be In the thermal management of PV solar cells, Mahadevan et al. [43]
practical in a solar environment. Paste or grease-type TIM has low vis­ developed a non-cured TIM with mineral oil and graphene fillers,
cosity, thus allowing it to flow easily between the contacting interfaces varying their weights to investigate its effectiveness in enhancing the
[85]. Since thermal paste uses polymers as its matrix, no curing is performance of a triple-junction solar cell. Unlike conventional
required; hence, allowing for easy installation and replication [85]. For epoxy-based curing TIM, non-curing TIM does not solidify completely,
these reasons, paste-type TIM is often preferable in solar applications. which prevents the formation of cracks in the bond line as a result of
Due to the superior thermal conductivity of graphene (3000–5000 repeated thermal cycling of the two interface materials with different
W/(m⋅K)) [89], it has gained positive attention for use as a thermally thermal expansion coefficients [101]. Non-cured graphene-enhanced
conductive filler in TIM [90]. In the earlier stages, graphene-based TIM TIM of 40 wt% graphene was found to reduce the temperature rise in the
was prepared by dispersing graphene sheets into a polymer matrix solar cells by 34 %, and the corresponding VOC drop was reduced by 44
through a solution or melt-blending process [91]. The thermal conduc­ % when compared to those employing commercial TIM under 200 suns
tivity of this dispersed graphene-based TIM can surpass the thermal illumination. Non-cured graphene-enhanced TIM of 40 wt% graphene
conductivity of the conventional TIM of 5 W/(m⋅K) only when the showed the greatest performance with minimal drop in VOC and minimal
concentration of graphene present in the polymer is between 20 and 50 rise in temperature, followed by 20 wt% graphene TIM, 10 wt% gra­
wt% [91]. A high percentage of graphene was observed to result in high phene TIM, commercial TIM, and lastly mineral oil. This shows that the
viscosity in the polymer and undesirable mechanical properties, thus addition of graphene filler in TIM is significantly effective in promoting
rendering the material unfit for practical applications [91]. With proper better thermal performance in triple-junction solar cells.
graphene particle shape, such as graphene nanoflakes or nanoplatelets, a Similarly, Saadah et al. [44] conducted experiments to explore how
satisfactory thermal conductivity of 7.3–10 W/(m⋅K) can be achieved the presence of graphene as an additional filler in commercial TIM
[92,93]. affected the temperature, VOC, and efficiency (η) change specifically in
Other than dispersed graphene-based TIM, graphene framework- concentrated solar PV cell. The commercial TIM initially consisted of
based TIM has been developed to improve the thermal conductivity of silver, aluminium oxide, and aluminium nitrite particles. Commercial
graphene-based TIM by infusing polymers into the three-dimensional liquid phase exfoliated graphene and a few-layer graphene mixtures
graphene framework [91]. Unlike dispersed graphene-based TIM, the were then added to the commercial TIM. TIM without graphene filler
graphene framework-based TIM recorded a higher thermal conductivity and TIM with graphene filler were utilised together with the control case
compared to dispersed graphene of similar filler content, with the without TIM. MJSC was held together to an aluminium heat sink with
highest recorded thermal conductivity in an epoxy-based TIM reaching the TIM positioned in the middle. When the MJSC was subjected to
35.5 W/(m⋅K) [91]. While the graphene framework exhibits superior concentrated light of 1000 suns illumination, the use of TIM with 4 wt%
thermal conductivity due to its continuous thermal transport paths graphene filler caused a voltage drop of 12 %, while the case without
formed within the polymer matrices, this graphene framework structure TIM had a voltage drop of 29 %. Utilising TIM with graphene thus
does have certain drawbacks. The graphene framework is produced reducing the power loss by 60 %. Comparisons between graphene TIM
based on a frame, in which the compressive modulus is determined by with different loading fractions, namely 2 wt% and 4 wt%, indicated
the base frame used [91]. When the epoxy is used as the base matrix, the that the latter recorded a lower voltage drop, suggesting that the higher
graphene framework formed is solid and hard, with a compressive concentration of graphene in the TIM can dissipate more heat due to the
modulus ranging between 2 and 5 GPa, which thus limits its use as TIM higher thermal conductivity. Saadah et al. [44] concluded that the

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Table 1
Thermal conductivity of dispersed graphene and graphene framework-based TIM.
Filler type Matrix type Fraction Thermal conductivity (W/(m⋅K)) Reference

Dispersed graphene-based TIM

Graphite nanoplatelet Epoxy ≈ 34 wt% 6.44 Yu et al. [94]
Graphene nanoflake Polytetrafluoroethylene ≈ 24 wt% 10.0 Jung et al. [92]
Multilayer graphene Epoxy 5.7 wt% 1.5 Shen et al. [95]
Graphene Epoxy 10 vol% 5.0 Feng et al. [96]
Graphene framework-based TIM
Graphene aerogel PDMS 1 wt% 0.68 Zhang et al. [97]
Graphene aerogel Epoxy 1.4 wt% 0.63 (in-plane), 2.13 (through-plane) Lian et al. [98]
Graphene aerogel PDMS 2 wt% 0.82 Song and Zhang [99]
Vertically aligned graphene framework Epoxy ≈ 33 wt% 17.2 (in-plane), 35.5 (through-plane) An et al. [100]

incorporation of graphene filler into commercial TIM outperforms the thermal conductivity from 0.212 to 0.262 W/(m⋅K), with a maximum
other TIMs without graphene due to enhanced thermal conductivity that enhancement of 23.5 %. Additionally, materials with a large latent heat
enhances the dissipation of heat from solar PV panels. can prolong the phase transition process and stall the increment in
The graphene-enhanced TIMs presented in these studies were pri­ temperature. Khanna et al. [112] experimentally performed thermal
marily prepared by mixing graphene fillers into the base TIM material. regulation on PV solar panels with PCM. The authors reported that a
However, challenges in thermal coupling may arise from this mixture, PCM with low latent heat and low melting point required a larger vol­
given the distinct thermal properties of TIM and graphene nanoparticles. ume when executing the task. As a result, when selecting a PCM, its
Additionally, conventional TIM, which originally comprised other melting point must be higher than the ambient temperature but lower
nanoparticles at a specific loading fraction [44], may face rheological than the operating temperature of the PV panel [112]. Various forms of
changes. For instance, the effective viscosity may be affected despite graphene and its derivatives (graphene nanoplatelets and expanded
enhancing the thermal conductivity of the TIM, when introducing graphite) were incorporated into PCM to enhance the thermal perfor­
additional graphene fillers into the conventional TIM. Therefore, the mance of solar PV systems. The performance achieved by the
selection of the base material used for producing a graphene-enhanced graphene-enhanced PCMs were compared with other nanoparticles
TIM is critical to prevent performance deterioration. Based on litera­ (multi-walled carbon nanotubes and magnesium oxide) enhanced PCM.
ture aforementioned, it is evident that higher heat transfer rates were Abdelrazik et al. [46] incorporated two different carbon-based nano­
recorded with lower panel temperature and voltage drop during the materials, namely multi-walled carbon nanotubes (MWCNT) and gra­
employment of graphene-enhanced TIM in cooling the PV panels. This phene nanoplatelets (GnP) into paraffin wax to study the thermal
makes graphene-enhanced TIM a promising candidate for dissipating management of hybrid photovoltaic thermal (PV/T) system. The tem­
heat from the PV panels. However, further investigations on the service perature of the PV panel was observed to decrease even when GnP was
life of graphene-enhanced TIM is crucial in determining its long-term loaded at a very low concentration of 0.1 wt%. When GnP was loaded at
application in solar PV cooling. Additionally, the occurrence of nano­ a concentration of 5.0 wt%, the panel’s temperature dropped by 3.9 %,
particle mass loss due to the effects of the cyclic heat loading experi­ subsequently leading to a 2.65 % enhancement in the PV output effi­
enced by the TIM should also be taken into consideration. With ciency. However, when MWCNT was loaded at a lower concentration
nanoparticle mass loss occurring from time to time, the thermal con­ (0.1 wt% to 1.0 wt%), the panel temperature was higher than the panel
ductivity is reduced, thus affecting the cooling effectiveness on the PV with pure paraffin wax as the PCM. This might be caused by the lower
panels. Therefore, the operational life of the graphene-enhanced TIM specific heat capacity of the MWCNT-enhanced-n-PCM. Comparatively,
must be accounted for during the design and fabrication processes. the GnP-enhanced-n-PCM has a higher specific heat capacity, which
showed a better cooling effect in the PV panel. Therefore, a higher
4.2.1.2. Graphene in phase change materials (PCM). Besides TIM, there output efficiency was recorded for GnP-enhanced-n-PCM when
is also a growing research interest in employing PCM for thermal compared to the MWCNT counterpart.
regulation in PV solar panels [102–104]. This is because PCM is capable In another study, Jamil et al. [51] investigated the effects of incor­
of absorbing large amounts of thermal energy in the form of latent heat porating three different nanoparticles, namely MWCNT, GnP, and
during phase change from solid to liquid while maintaining a constant magnesium oxide (MgO) into organic paraffin wax PT58. The nano­
temperature [105]. This is dissimilar to sensible heat, which is associ­ particles were loaded at the same weight concentration of 0.5 wt%. The
ated with an increase in temperature [106]. PCM in PV solar cooling results obtained confirmed that GnP, which possesses thermal conduc­
applications often involves only the solid and liquid phases and is tivity of 3000–5000 W/(m⋅K) [89] achieved the lowest average cell
generally made of a low thermal conductivity material ranging from 0.2 temperature of 56.41 ◦ C, which corresponded to highest electrical
to 0.5 W/(m⋅K) which limits its heat transfer rate [52]. power of 6.88 kW and electrical efficiency of 12.10 %. Meanwhile,
There are five factors that guide the selection of a good PCM, namely MWCNT with thermal conductivity of 2000–6000 W/(m⋅K) [89] and
the chemical, physical, thermal, economic, and kinetic properties [107]. magnesium oxide (MgO) with thermal conductivity of 48.4 W/(m⋅K)
The general requirement for an ideal PCM is for it to be highly thermally [113] recorded the average cell temperature of 56.52 ◦ C and 59.62 ◦ C,
conductive, possess large latent heat, have no supercooling, non-toxic, respectively. The ability of the nanoparticles to dissipate heat may be
non-corrosive, chemically and thermally stable, and low cost [106]. associated with their structures. MWCNT appears in a long tube-like
Out of all these attributes, the most significant but most difficult crite­ structure, MgO has a spherical shape, while GnP possesses a flat 2D
rion to achieve is high thermal conductivity [108]. Nano-enhanced structure. Consequently, GnP exhibits a higher surface area, enhancing
phase change material (n-PCM) was introduced to enhance the ther­ its effectiveness in dissipating heat.
mal conductivity of PCM by incorporating highly thermal conductive While the former studies utilised graphene and its derivatives as
nanomaterials into the base PCM [109]. A 33.34 % enhancement in additives to the PCM, others have employed graphene mixtures as the
thermal conductivity was achieved by Kumar et al. [110] when base PCM. The incorporation of polyethylene glycol 1000 (PEG1000) as
dispersing 2.0 wt% of SiO2 into paraffin wax. In a study by Yin et al. base PCM and expanded graphite (EG) as an inclusive particle in the
[111], graphene nanosheets at concentrations ranging from 0 to 200 formation of a composite PCM mixture and its suitability in aiding the
ppm were dispersed into liquid paraffin, resulting in a linear increase in cooling of solar PV panel was investigated by Senthilkumar et al. [53].

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L.T. Siow et al. Renewable and Sustainable Energy Reviews 193 (2024) 114288

The composite PCM with 15 % EG mass was found to be optimal for solar temperature on the PV panel. This can be attributed to the superior
PV cooling applications due to its negligible PCM mass loss of 2.51 %. thermal conductivity possessed by GnP. Owing to the 2D structure of
Composite PCM of 85 % PEG1000 and 15 % EG was utilised in the GnP, it can create a wide and strong conduction network in the PCM,
experimental domain with a reference panel to examine the perfor­ unlike the spherical CuO nanoparticles. Therefore, the heat conduction
mance of the composite PCM. The utilisation of this composite PCM rate is higher in GnP-enhanced PCM.
recorded a lower temperature compared to the reference case, where the Sivashankar et al. [52] analysed the performance improvement in
temperature of the PV panel with PCM was maintained between 32 and CPV using GnP-enhanced n-PCM. The base PCM used was an organic
36 ◦ C. This study concluded that EG was used with PEG1000, the pro­ non-paraffin wax, OM35, and GnP of thickness between 6 and 10 nm
portion of EG added to the PCM plays a significant role in determining were introduced as the nanoparticles for the PCM infusion. Concentra­
both the chemical and thermal stability of the PCM. In addition, this tion ratio (CR), which is defined as the ratio of the concentrated energy
parameter also affects the thermal conductivity, which is essential to density per unit area to the incident energy density, was a parameter of
avoid performance deterioration. In another study, hybrid PCM with interest in the study [114]. When subjected to the same CR, the imple­
graphene and silver nanocomposites loaded into paraffin wax (PW70) mentation of n-PCM significantly reduced the heat sink base tempera­
was employed by Aslfattahi et al. [47] in a hybrid concentrated photo­ ture by 12 ◦ C, 16 ◦ C and 18 ◦ C when CR = 3, CR = 4, and CR = 5,
voltaic thermal (CPVT) system. The nanoparticles consisted in the respectively, compared to the case without PCM. A significant reduction
graphene-silver nanocomposites were at a ratio of 5:5. By increasing the in the base temperature was reported at the latent heat region when GnP
nanocomposite loading to a maximum of 0.3 wt%, the average tem­ was used due to higher thermal conductivity induced by the GnP
perature of the PV panel was reduced by a maximum of 2.7 %, which loading. However, the temperature increased again after the latent heat
corresponded to 1.33 % enhancement in the electrical efficiency as region due to the reduction in heat-storing capacity of the PCM in liquid
compared to the pure PCM case. Since more than one type of nano­ state. The employment of n-PCM with 0.5 vol% GnP contributed to a
particles are present in the hybrid PCM, the effect of individual nano­ maximum enhancement of 7 % for power output and 6 % for electrical
particles on the thermal performance is not known. Therefore, the efficiency when CR = 5. This study suggested that high performance of
results obtained are less comparable to those with only graphene the CPV cell can be achieved within the latent heat region, thus choosing
nanoparticles. However, hybrid PCM is more cost-effective compared to a PCM with both high latent heat and high volume concentration is
pure graphene PCM. This is because less amount of graphene nano­ essential.
particles is required to achieve a similar enhancement in heat transfer. On the other hand, numerical studies were also conducted to
Hence, the cost and performance must be balanced when employing investigate the impact of graphene loading into n-octadecane PCM for
nanoparticles-enhanced PCM in solar PV systems. regulating the thermal performance of the CPV system. Here, Zarma
Zaimi et al. [48] examined the addition of GnP and sodium et al. [54] integrated graphene-enhanced n-octadecane PCM with
dodecyl-benzene sulfonate (SDBS) to paraffin wax (base PCM) on the multi-cavity heat sinks arranged in parallel and series configurations
cooling effects of a PV panel, where SDBS acted as a surfactant to reduce respectively. PCM with 5 wt% graphene loading recorded the lowest
agglomeration. When 5 wt% GnP and SDBS were added to the paraffin solar cell temperature when compared to 2 wt% graphene loading and
wax, the PV panel surface temperature was 39.9 ◦ C, which was 31.6 ◦ C the base PCM cases, disregarding the cavities number and heat sink
lower than the conventional PV panel without PCM, and 23.3 ◦ C lower arrangement. The electrical and thermal efficiencies of the solar system
than PV with only paraffin wax as the PCM. In the later work by Zaimi increased with increasing graphene loading. With a higher loading
et al. [49], similar results were obtained for 5 wt% GnP-enhanced PCM fraction, the thermal conductivity of the enhanced-PCM is higher, thus
with SDBS being the most performed configuration. The PV surface dissipating heat from the solar cell at a faster rate.
temperature recorded was 39.8 ◦ C, which was 30.5 ◦ C and 24.4 ◦ C lower Similarly, the effects of GnP loading in the RT-35 PCM on a hybrid
than the PV panel without PCM and the PV panel with conventional PV/T system were evaluated numerically by Abdelrazik et al. [55]. The
PCM, respectively. With GnP incorporated, the thermal conductivity of average PV temperature was reduced with GnP-enhanced PCM of higher
the PCM increased. Hence, heat was dissipated at a higher rate and the nanoparticle concentration. This is because the thermal conductivity of
temperature on the PV surface was reduced. The maximum power the PCM increases when more nanoparticles are loaded. However,
output of the PV panel achieved by this 5 wt% GnP-enhanced PCM was increasing the thickness of the PCM at a fixed concentration did not
9.05 W, which was 16.5 % and 12.7 % higher than the case without PCM show any enhancement in the heat transfer. As the thickness of the PCM
and the case with conventional PCM, respectively. Although increases, the thermal conductivity of the PCM reduces thus limiting the
GnP-enhanced PCM shows great capability in dissipating heat from the heat dissipation rate. Therefore, the thickness of the PCM is an important
PV panel, the rheological behaviour such as viscosity should not be factor to account for better thermal management in the solar PV system.
neglected. When nanoparticles are loaded at high fractions, the viscosity Moreover, ambient conditions such as atmospheric temperature, solar
might increase and deteriorate the heat transfer performance. While concentration and seasons also govern the performance of the solar PV
most of the aforementioned PCM studies revolved around paraffin wax system. In a higher ambient temperature environment, the
as the base material, several studies focused on using different types of graphene-enhanced PCM demonstrates the ability to maintain the PV
PCM, including PEG 1000 [45], organic non-paraffin wax [52], n-octa­ temperature lower. This outcome substantiates the enhanced thermal
decane [54], and RT-35 PCM [55]. The thermal performance of the conductivity of the PCM, enabling it to extract more heat from the PV
graphene-enhanced PCM was also analysed numerically by Refs. [54, panel.
55]. Jahromi et al. [45] experimented with graphene nanoplatelets The integration of heat sinks and graphene-enhanced PCM has
(GnP)-enhanced PEG1000 PCM of different concentrations in the ther­ opened up opportunities to synergize different strategies for optimising
mal regulation of the solar PV system. With the highest GnP concen­ thermal management in solar PV panels. Colarossi and Principi [50]
tration at 3 wt%, the PV panel recorded the lowest temperature, incorporated aluminium fins, paraffin wax RT35 HC PCM and graphene
followed by 2 wt% and 1 wt% GnP-enhanced PCMs. Despite the increase nanoparticles to investigate the cooling effects on the PV panels. The
in viscosity of the GnP-enhanced PCMs with increasing weight fraction, reference panel was a common mono-crystalline PV panel, while the
the heat transfer performance of the PCM was not affected although enhanced panel was a mono-crystalline PV panel with aluminium fins
higher viscosity led to slower convection rate. Therefore, the heat and paraffin wax (with 2 wt% graphene) attached. The experiment took
transfer performance was dominated by the high thermal conductivity place in an indoor and an outdoor setting. In the indoor condition with
possessed by the GnP-enhanced PCMs which improved the heat con­ simulated solar radiation of 1000 W/m2, the reference panel measured
duction rate. When compared to copper oxide (CuO)-enhanced PCMs at an average temperature of 60.01 ◦ C, while the enhanced panel recorded
the same concentration, GnP-enhanced PCMs displayed a lower an average temperature of 54.48 ◦ C, which corresponded to a 3 %

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L.T. Siow et al. Renewable and Sustainable Energy Reviews 193 (2024) 114288

experiment was conducted in outdoor conditions, the enhanced panel

recorded a lower temperature during daytime when the experiment was
ongoing and the maximum difference in temperature recorded was
approximately 6 ◦ C lower than the reference panel. By incorporating
heat sinks with graphene-enhanced PCM, a satisfactory cooling effect is
achieved. As heat sinks are commonly used for dissipating heat in heat
transfer devices, they are available in different configurations such as
circular fins and plate fins. Optimising the heat sink configuration makes
it possible to define the preferred configuration for better thermal per­
formance, thus reducing the graphene concentration in the PCM to
achieve a similar cooling effect.
In the same numerical study conducted by Zarma et al. [54], it is
deduced that heat sink arrangement played a significant role in deter­
mining the performance of the solar PV system. When heat sinks and the
PCM was arranged in a series pattern, the heat transfer performance
deteriorated. Meanwhile, heat transfer performance was enhanced in
the parallel configuration. This can be explained by the different thermal
paths exhibited by the different configurations. In Fig. 6(b and c), it
showed that the PCM in series arrangement started to melt at the first
layer and heat was dissipated from each layer to the next layer. How­
ever, all the PCMs in parallel arrangement started to melt instanta­
neously. Therefore, PCM and heat sinks arranged in parallel are more
efficient in dissipating heat. Hence, the configurations must be chosen
carefully during the incorporation of graphene-enhanced PCM and heat
sinks in the thermal management of a solar PV system.
Table 2 below summarises the available literature for the graphene-
enhanced PCM in solar PV cooling systems.
The easy of installing PCM broadens its potential applications in heat
transfer systems. In solar PV cooling applications, PCM is often incor­
porated with heat sinks for better heat dissipation. Not only that, PCM
can work as a TIM to enhance the thermal contact between the interfaces
in the solar PV panels. As PCM needs to be in direct contact with the
Fig. 6. (a) Configuration of the PCM and heat sink arranged in parallel and solar PV panels to dissipate heat, the application of PCM in solar PV
series, the yellow arrows indicate the direction of the solar radiation [54]. The cooling is therefore impractical on a commercial scale due to the large
evolution of the solid and liquid interfaces of the PCM (red: liquid, blue: solid) amount of PCM required. The heat transfer performance of the
at (b) t = 300s and (c) t = 1800s [54]. The figures are reproduced with graphene-enhanced PCM is improved with increasing nanoparticle
permission from John Wiley and Sons (licenses number 5681680394036 and 5, concentration, however, the increase in viscosity should not be neglec­
681,681,035,521). ted. Nanoparticle mass loss might happen due to the cyclic heat loading
that occurs during the operation of the solar PV panel, thus deteriorating
increase in the electric yield by the enhanced panel. It is noteworthy that the heat transfer performance of the graphene-enhanced PCM.
only 14 min were required for the enhanced panel to reach steady-state
condition, while the reference panel required an additional 6 min, thus
proving a higher thermal conductivity in the enhanced panel. When the

Table 2
Summary of the performance achieved by the graphene-enhanced PCM.
Published work Nanoparticles and concentration Base PCM Significances

Experimental studies
Jahromi et al. [45] GnP, 3 wt% PEG 1000 GnP-enhanced PCM recorded lower panel temperature compared to CuO-enhanced PCM
Abdelrazik et al. [46] GnP, 5 wt% Paraffin wax Panel temperature dropped by 3.9 %, leading to a 2.65 % enhancement in the PV output
efficiency
Aslfattahi et al. [47] Graphene-silver Paraffin wax Average temperature of the PV panel was reduced by 2.7 %, which corresponded to 1.33 %
nanocomposites, 0.3 wt% (PW70) enhancement in the electrical efficiency
Zaimi et al. [48] GnP and SDBS, 5 wt% Paraffin wax PV panel surface temperature was 31.6 ◦ C lower than the conventional PV panel without PCM
Zaimi et al. [49] GnP and SDBS, 5 wt% Paraffin wax Maximum power output of the PV panel was 16.5 % higher than the case without PCM
Colarossi and Principi Graphene, 2 wt% Paraffin wax (RT35 Maximum difference in temperature recorded was approximately 6 ◦ C lower than the
[50] HC) reference panel
Jamil et al. [51] GnP, 0.5 wt% Paraffin wax (PT58) GnP outperformed MWCNT and MgO with lowest average cell temperature recorded, leading
to highest electrical power and efficiency
Sivashankar et al. GnP, 0.5 vol% Non-paraffin wax Maximum enhancement of 7 % for power output and 6 % for electrical efficiency
[52] (OM35)
Senthilkumar et al. Expanded graphite, 15 wt% PEG 1000 Temperature of the PV panel with PCM was maintained between 32 and 36 ◦ C
[53]
Numerical studies
Zarma et al. [54] Graphene, 5 wt% n-octadecane PCM with 5 wt% graphene loading recorded the lowest solar cell temperature disregarding the
cavities number and heat sink arrangement
Abdelrazik et al. [55] GnP, 1 wt%, 5 wt%, 10 wt%, 20 Paraffin wax (RT35) Average PV temperature was reduced with GnP-enhanced PCM of higher nanoparticle
wt% concentration

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L.T. Siow et al. Renewable and Sustainable Energy Reviews 193 (2024) 114288

4.2.2. Active cooling mixing different nanoparticles (graphene nanoplatelets, aluminium and
aluminium oxide) in water and ethylene glycol. A list of recent research
4.2.2.1. Graphene nanofluids as working fluids. Various types of heat works on nanofluids used in solar PV cooling is provided in Table 3. It is
transfer fluids such as air, water, and ethylene glycol have been used to noteworthy that with the use of nanofluid as the coolant in the solar PV
dissipate heat from solar PV systems [115]. However, the low thermal cooling system, the overall performance (temperature, power output
conductivity of these conventional fluids has limited the heat transfer and efficiency) surpassed the case without nanofluid.
rate between the media, thus affecting their effectiveness [116]. Solid Several studies have been conducted to investigate the effects of
particles such as metallic and non-metallic powders were suspended in graphene nanofluid on the performance of solar PV panels [56–61]. In
conventional fluids to boost their thermal conductivity [117]. However, one of these studies, the effects of mass flow rate and concentration of
researchers found that this was ineffective for heat transfer enhance­ the graphene nanofluid (GnP in water) on the efficiency of a hybrid solar
ment because the large suspended particles (micro-to milli-sized parti­ photovoltaic thermal (PV/T) system was reported [56]. According to
cles) caused problems such as abrasion and clogging, which may affect Venkatesh et al. [56], at the time when solar irradiance peaked and the
their thermal conductivity [117]. The discovery of nanofluids by Choi mass flow rate was at 0.075 kg/s, increasing the GnP volume concen­
and Eastman [118] has since led to numerous studies on their use as tration from 0 to 0.3 vol% resulted in a reduced panel temperature from
working fluids for heat transfer enhancement. 60 ◦ C to approximately 43 ◦ C. The power output was then observed to
Nanofluid consists of nanoparticles with sizes ranging from 1 to 100 increase from around 7 W–10 W. The increase in mass flow rate from
nm that are dispersed within a base fluid. The nanoparticles are mainly 0.065 kg/s to 0.085 kg/s also showed a reduction in the maximum panel
classified into three categories, namely metal-based, carbon-based, and temperature from 60 ◦ C to 45 ◦ C when the nanofluid concentration was
nanocomposites [119]. The addition of solid nanoparticles of higher fixed at 0.3 vol%. Furthermore, when the GnP nanofluid was at the
thermal conductivity into the base fluid was observed to increase the highest concentration of 0.3 vol%, the power output, useful heat, elec­
overall thermal conductivity of the nanofluid; hence, improving the heat trical efficiency, and PV/T efficiency were found to increase with mass
transfer coefficient and making nanofluid a promising candidate for flow rate. Therefore, it is deduced that the concentration of the nano­
promoting heat transfer [120]. As such, studies have revolved around fluid affects its thermal conductivity, subsequently influencing the panel
photovoltaic thermal (PV/T) systems which use nanofluid as the cooling temperature, power output, and system efficiency of a solar PV/T sys­
media to reduce the surface temperature of PV cells by extracting heat tem. The increased mass flow rate also enhances the convective heat
from the panel to maximise its efficiency [115].
Graphene nanoparticles have gained significant attention as a
compelling component in the production of nanofluids for heat transfer
enhancement in solar PV cooling due to their excellent thermal, elec­
trical, and optical properties. Graphene nanoplatelets (GnP) is a type of
graphene nanoparticles comprising multiple graphitic layers held
together by van der Waals forces [121]. GnP has large specific surface
area and thermal conductivity [121], which makes it suitable for
nanofluids synthesis. The characterisations of GnP through X-ray
diffraction (XRD) and field emission scanning electron microscopy
(FESEM) are shown in Fig. 7. The highest diffraction peak, corre­
sponding to the miller indices of (0 0 2) at 2θ = 26.46◦ in the XRD
spectrum (Fig. 7 (a)), confirms the presence of multi-layered graphene
nanoplatelets [122]. Further analysis using FESEM (Fig. 7 (b)) confirms
the stacking of 2D graphene nanoplatelets in layers.
The thermal conductivity enhancement of a nanofluid can be ob­
tained by manipulating the ratio of the thermal conductivity of the
prepared nanofluid to the thermal conductivity of the base fluid, where a Fig. 8. Percentage enhancement of thermal conductivity for 0.5 % vol gra­
ratio greater than unity implies an improvement in the nanofluid ther­ phene nanoplatelets (GnP, 5–10 nm), 0.5 % vol aluminium (Al, 80 nm), and 0.5
mal conductivity over the base fluid [30]. Fig. 8 shows the percentage % vol aluminium oxide (Al2O3, 11 nm) in water and ethylene glycol, respec­
enhancement of thermal conductivity for nanofluids fabricated from tively at 30 ◦ C [116,123].

Fig. 7. The (a) XRD spectrum and (b) FESEM analysis of the GnP [122]. The figures are reproduced with permission from Elsevier (license number 5678120381064).

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L.T. Siow et al. Renewable and Sustainable Energy Reviews 193 (2024) 114288

Table 3
Recent research on nanofluids (water-based) used in solar PV cooling.
Published work Nanoparticles Significance

Murtadha et al. Titanium dioxide (TiO2), 28 nm PV cooled by 3 wt% TiO2 nanofluid achieved the highest efficiency of 19.23 % among the other cases
[124] with 2 wt% TiO2, 1 wt% TiO2, water cooled and uncooled
Janardhana et al. Silicon dioxide (SiO2), 30 nm The temperature of the PV panel cooled by 0.2 wt% SiO2 nanofluid was about 34 ◦ C after 420 min,
[125] while the temperature of the conventional PV was about 79 ◦ C
Sangeetha et al. Multi-wall carbon nanotubes (MWCNT, 45–50 nm), With 0.3 vol% MWCNT nanofluid, the electrical power and efficiency were enhanced by 45 % and 47
[126] Al2O3 (35–55 nm), TiO2 (35–50 nm) %, respectively, followed by 0.3 vol% Al2O3 nanofluid with a 36 % increase in electrical power and a
33 % increase in efficiency. Meanwhile, 0.3 vol% TiO2 nanofluid had an increment of 25 % and 27 % in
electrical power and efficiency, respectively.

transfer, dissipating heat at a higher rate and thereby reducing the panel two techniques to further enhance heat dissipation in the PV/T system.
temperature. On a different note, numerical studies were also performed by Rejeb
Alshikhi and Kayfeci [57] investigated the daily variation of thermal et al. [58] who investigated the effects of using graphene/water nano­
efficiency of a solar system using GnP nanofluid, hybrid nanofluid fluid of 0.5 wt% concentration in cooling the hybrid concentrated
(HyNF) consisting of GnP and Al2O3, and water as coolant in PV/T photovoltaic thermal (CPVT) system with thermoelectric generator
systems. It is determined that the highest thermal efficiency was ach­ (TEG) modules. The results revealed that the maximum temperature of
ieved when using GnP nanofluid as the cooling media compared with the CPV with graphene nanofluid cooling was 0.3 ◦ C and 0.6 ◦ C lower
HyNF and water. However, the thermal efficiency achieved by GnP was than water cooling under cloudy and sunny conditions, respectively. The
less consistent. This was due to factors such as varying wind speed, great thermal properties of graphene enhance the convective heat
changes in solar irradiation, and humidity as the experiment was con­ transfer between the cooling fluid and enhance thermal conductivity
ducted in an uncontrolled environment [57]. Apart from that, GnP between the fluid and the tube wall. This led to higher fluid temperature
possesses a higher thermal conductivity than Al2O3, where higher at the outlet, which proves that the graphene nanofluid is capable of
thermal conductivity can enhance the thermal efficiency of the PV/T dissipating heat from the CPVT-TEG system. However, when comparing
system [127]. The study demonstrated the effectiveness of GnP nano­ the performance of the hybrid CPVT-TEG system to the conventional
fluid as the cooling media for PV/T systems. On the other hand, Alous CPVT system without TEG, the temperature of the CPV in the conven­
et al. [61] synthesised two types of water-based nanofluids using tional system recorded a lower temperature than in the hybrid case. This
different carbon-based nanoparticles, namely MWCNT and GnP. The is because the high thermal resistance of the TEG modules limits the
concentration of the nanofluids in this study was fixed at 0.5 wt%. The cooling performance of the heat transfer fluids, thus the nanofluid
nanofluids were used as the coolant in the PV/T system. In this study, cooling effect is negligible while TEG modules were incorporated in the
GnP nanofluid also recorded the lowest temperature of 48.3 ◦ C on the PV CPVT system. Therefore, the CPVT system configuration is an important
surface and the highest PV efficiency of 14.4 %, which outperformed factor to consider when employing nanofluid for thermal management
water (50.0 ◦ C and 13.5 %) and MWCNT nanofluid (49.6 ◦ C and 13.6 %), to avoid undesirable effects.
highlighting the efficacy of GnP nanofluid as a cooling medium. Table 4 below summarises the available literature for the application
The cooling effectiveness of GnP nanofluid was also studied in micro- of graphene nanofluids in solar PV cooling systems.
sized channels that were placed in contact with the backside of a solar Due to its superior thermal conductivity, graphene is considered a
PV panel [60]. In this study, the authors reported that the data recorded suitable nanoparticle for the fabrication of nanofluids. Studies have
for the measured PV efficiency was least scattered compared to proven the effectiveness of graphene nanofluid in enhancing heat
non-cooling and water-cooling conditions, regardless of fluid pumping transfer performance in solar PV systems, with lower PV panel tem­
flow rates. The efficiency of the non-cooled condition dropped drasti­ peratures recorded. Nanofluid cooling is a practical choice for com­
cally in the second hour, whereas for the nanofluid-cooled case, the mercial use, as the nanofluid can be circulated all over the solar PV
efficiency dropped for a longer duration. Despite the increase in surface panels in the solar farms. However, the nanofluid cooling approach is an
temperature, the drop in efficiency for GnP nanofluid-cooling condition active cooling technique, which requires external devices such as pumps
was still considered well maintained, which suggested that the and pipes to circulate the nanofluid around the system. This has
outstanding thermal properties of GnP nanofluid are capable of cooling increased the implementation cost of the cooling system, which requires
the panels for a longer duration, rather than the non-cooling and additional costs for maintaining the heat transfer devices over a long
water-cooling case. operational period.
In another study by Wahab et al. [59], the integrated effectiveness of While there is evidence supporting the use of graphene nanofluid as
graphene nanofluid and phase change material (PCM) within the hybrid an effective heat dissipation approach in solar panels, there are still
PV/T system was investigated. Graphene nanofluid of different con­ limitations and challenges that must be addressed. One of the main
centrations and RT-35HC (pure and organic paraffin PCM) were incor­ challenges of graphene nanofluid is its poor stability due to aggregation.
porated as the heat dissipation methods in the hybrid PV/T system. For nanofluid to exist in a stable state, the rate of aggregation must not
When graphene nanofluid with a concentration of 0.1 vol% was circu­ be significant. The aggregation rate can be determined from the fre­
lated at a flow rate of 40 LPM, the surface temperature peaked at quency and probability of collisions between the particles and the fluid
46.24 ◦ C. This temperature was 14.76 % lower than the panel that used [113]. However, due to its hydrophobic nature, graphene has a low
PCM and circulated distilled water, and 35.8 % lower than the con­ affinity towards the base fluid. This leads to a high Hamaker constant
ventional panel without the use of PCM and working fluid. However, which results in strong van der Waals attraction forces rather than
when the nanofluid’s concentration was increased to 0.15 vol%, the repulsive forces [128]. Hence, negative charges on the nanoparticles
surface temperature increased. This may be explained by the agglom­ tend to neutralize the positive charges around them [30], which causes
eration of nanoparticles that occurred in the base fluid, thus reducing aggregation and nanofluid instability. This hinders the large surface area
the Brownian motion of the nanoparticles, resulting in poor heat ab­ to volume ratio property of dispersed nanoparticles and eventually
sorption. This study provides a new opportunity to combine PCM and causes the nanofluid to lose its thermal properties. To overcome this,
graphene nanofluid to enhance the heat transfer rate in a hybrid PV/T surfactants have been added to the nanofluid to enhance its stability.
system. Another study to incorporate graphene-enhanced PCM with Surfactants consist of a hydrophilic head, a hydrophobic tail, and a long
graphene nanofluid can be done to study the combined effect of these chain of hydrocarbons [113]. With the addition of surfactants, the

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L.T. Siow et al. Renewable and Sustainable Energy Reviews 193 (2024) 114288

Table 4
Summary of the graphene nanofluids used for solar PV cooling.
Published work Nanofluid Nanoparticles concentration and Significances
flow rate

Venkatesh et al. GnP in 0.3 vol% at 0.075 kg/s (fixed mass Panel temperature reduced from 60 ◦ C to 43 ◦ C
[56] water flow rate)
0.3 vol% at 0.085 kg/s (fixed Maximum panel temperature reduced from 60 ◦ C to 45 ◦ C
concentration)
Alshiki and Kayfeci GnP 0.5 wt% and 0.5 LPM GnP nanofluid achieved the highest thermal efficiency compared to hybrid nanofluid (GnP and Al2O3)
[57] and water
Rejeb et al. [58] Graphene 0.5 wt% (unknown flow rate) Maximum temperature on CPV was 0.6 ◦ C lower than water cooling under sunny condition
Wahab et al. [59] Graphene 0.1 vol% at 40 LPM Panel surface temperature was 35.8 % lower than the conventional panel without any cooling
techniques
Moh et al. [60] GnP 0.44 LPM and 1.11 LPM The PV efficiency was maintained for longer hour, compared to water-cooled and non-cooled case
which dropped drastically in the second hour
Alous et al. [61] GnP 0.5 wt% at 0.5 LPM PV recorded lowest temperature of 48.3 ◦ C and highest efficiency at 14.4 %

hydrophobic nature of the graphene can be altered to be hydrophilic, viscosity, leading to increased friction factor and pressure drop [132].
hence easing the dispersion of graphene nanoparticles in the base fluid. However, an increment in thermal conductivity is possible by increasing
On the other hand, the increase in the concentration of graphene the nanoparticle concentration. Therefore, the concentration of nano­
nanoparticles can enhance the thermal conductivity of nanofluid. particles must be carefully optimised to prevent performance deterio­
However, when the concentration of graphene nanoparticles increases, ration. From an economic standpoint, the fabrication of graphene
the viscosity of the fluid also increases. The increase in viscosity is due to nanoparticles is costly. This is because the synthesis process is exhaus­
the higher frequency of collisions between the particles, hence causing tive due to the chemicals used, energy consumed and a rigorous process
higher internal friction [30] and a higher shear rate in the fluid. As a to be adhered in order to achieve the correct 2D structure, high purity
consequence of the increased viscosity, a higher pumping power is and defect-free graphene nanoparticles [30]. Hence, the
required to overcome the pressure drop throughout the fluid flow [30]. cost-to-effective ratio of using graphene in solar PV cooling must be
Increasing the nanoparticles concentration also leads to an increase in evaluated. From an environmental perspective, the impact induced by
the nanoparticles aggregation rate due to the decrease in nanoparticles graphene on the environment should be given utmost priority so that its
spacing and increase in gravitational force, resulting in a less stable handling and disposal can be carried out without risking the environ­
nanofluid [129]. Hence, an optimum graphene concentration must be ment [30].
chosen to minimise cost due to the trade-off between thermal conduc­ Moreover, ambient condition (e.g., wind speed, ambient tempera­
tivity, fluid viscosity, and nanofluid stability. ture, and humidity) varies across the globe, and the effectiveness of the
While graphene is composed of graphite, which is a relatively inex­ graphene-enhanced solar PV cooling system is highly dependent on the
pensive material, the fabrication process of graphene nanofluid is ambient condition. For example, the results obtained in regions with hot
complicated. Advanced equipment is needed to prepare a stable and climate might not be the same in regions with cold climate. Therefore,
well-dispersed graphene nanofluid [130]. Therefore, the challenge of more comprehensive studies are needed to further understand the in­
using graphene nanofluid is the costly and complex fabrication process, fluence of ambient conditions of different regions on the system per­
which prevents graphene nanofluid from being adopted in common formance before graphene-incorporated solar PV cooling systems are
industry practice. Although a few aspects require careful consideration, ready to be employed on the commercial scale. As significant results
graphene nanofluid is still one of the promising working fluids to be used were obtained by researchers when incorporating graphene in solar PV
for heat transfer enhancement due to its enhanced thermal conductivity cooling, interested parties in this field should invest in developing the
compared to conventional heat transfer fluids. use of graphene solar PV cooling at the commercial scale, further vali­
dating the ability of graphene to dissipate heat from solar PV panels.
5. Advantages, limitations, and significance of graphene in
cooling solar PV panels 7. Conclusions

The advantages, limitations, and experimental significances of all the In this review, it is determined that graphene is an excellent material
cooling methods presented in this review paper are summarised in to be used in solar PV panels for heat transfer enhancement due to its
Table 5. superior optical, mechanical and thermal properties, in both pre-
illumination cooling and post-illumination cooling. Four solar PV cool­
6. Outlooks and future perpectives ing methods enhanced by graphene were discussed, namely neutral
density filter, thermal interface material, phase change material, and
Despite the discovery of graphene two decades ago, the application nanofluid. By comparing the experimental data, it was found that the
of graphene-based cooling systems for solar PV cooling remains at the optimal setting of the experimental setup for each method is unique and
experimental stages. The use of graphene-incorporated cooling systems has its advantages and disadvantages depending on the various factors
at the commercial scale is still limited. of implementation.
The incorporation of graphene nanoparticles in solar PV cooling Pre-illumination cooling techniques attenuate the solar radiation
requires careful consideration, specifically of the material properties, its before it reaches the PV panel. This can reduce the heat generated on the
cost-effectiveness and the environmental impact. For example, the PV surface, thus minimising the damage to the solar PV panel due to
addition of nanoparticles into conventional TIM and PCM alters the high temperatures. However, coating graphene on a glass substrate is
shape stability (e.g. volume change, leakage) and phase change prop­ costly, and the substrate might experience thermal stress that eventually
erties (e.g. melting point, latent heat), respectively [131]. Thus, a full causes drastic failure in cooling the solar cells. Therefore, pre-
understanding of these properties is important when developing illumination cooling with a GCND filter is less practical for long-term
graphene-enhanced material. The stability of the nanofluid is also a and large-scale usage.
major concern. The addition of nanoparticles into the conventional heat On the other hand, post-illumination cooling deals with thermal
transfer fluid (e.g. water) often caused agglomeration and increased management on the solar PV panel due to the heat generated. The PV

12
L.T. Siow et al. Renewable and Sustainable Energy Reviews 193 (2024) 114288

Table 5
Summary of advantages, limitations and experimental significances of graphene in cooling solar PV panels.
Cooling Advantages Limitations Significances
Method

Neutral Density • High transparency • Substrates have low thermal • Focal spot temperature was reduced by 20 % and 12 % when medium and
Filter • High mechanical flexibility conductivity resulting in difficulty in thin graphene coating were used respectively in the GCND compared to
• Increased light absorption heat dissipation the infrared filter [41]
• Passive cooling • Substrates have lower optothermal • 12 % enhancement in efficiency was achieved by medium graphene
• Simple fabrication properties, which induce thermal stress coating under eight suns condition compared to seven suns condition
• Low maintenance and cause failure [41]
• Low weight addition • Lower optical efficiency • A decrement of 35 % on the series resistance with an increment of 4 % on
the FF when CR was at 1000 suns when one monolayer of graphene was
incorporated into a triple-junction solar cell [42]
Thermal • Passive cooling • Compatibility issues potentially • Non-cured graphene-enhanced TIM of 40 wt% graphene reduced the
Interface • Reduced thermal contact changing mechanical properties and temperature rise by 34 % and VOC drop by 44 % as compared to the
Material resistance viscosity commercial TIM under 200 suns illumination [43]
• Minimise air gaps • Scattering of the matrix which affects the • Graphene-enhanced TIM with 4 wt% graphene loading reported a
• Avoid reaction on the contacting properties of graphene voltage drop of 12 %, whereas commercial TIM had a voltage drop of 29
surfaces % when the MJSC was subjected to concentrated light of 1000 suns [44]
• Easy installation
• Non-permanent
• Lightweight
Phase Change • Passive cooling • Limited period of thermal regulation • With the highest GnP concentration at 3 wt%, the PV panel recorded the
Material • Low maintenance (phase transition region) lowest temperature, followed by 2 wt% and 1 wt% GnP-enhanced PCMs
• Low cost • Low heat transfer in the liquid phase [45]
• Allows thermal recovery • Volume variation during phase transition • When GnP was loaded at 5.0 wt% in paraffin wax, the panel’s
• High latent heat capacity temperature dropped by 3.9 % and corresponded to a 2.65 %
• Nanoparticle additives preserve enhancement in the PV’s output efficiency [46]
the latent heat capacity • Average temperature of the PV panel was reduced by 2.7 % and electrical
• Thermally stable for cyclic efficiency increased by 1.33 % when graphene-silver nanocomposites of
process 0.3 wt% were loaded into paraffin wax [47]
• Non-corrosive • With 5 wt% GnP and SDBS added to the paraffin wax, the PV panel’s
• Non-toxic surface temperature was 39.9 ◦ C, which was 31.6 ◦ C lower than the
conventional PV panel without PCM, and 23.3 ◦ C lower than the PV with
paraffin wax as the PCM [48]
• 5 wt% GnP-enhanced PCM with SDBS recorded lowest PV surface tem­
perature of 39.8 ◦ C and power output of 9.05 W, which was higher than
the case with conventional PCM [49]
• The incorporation of aluminium fins and graphene-enhanced PCM re­
ported an average panel temperature of 54.48 ◦ C, while the base panel
with no heat sink and PCM had an average panel temperature of
60.01 ◦ C, which the enhanced panel showed a 3 % increase in the electric
yield [50]
• The enhanced panel with aluminium fins and graphene-enhanced PCM
had an approximately 6 ◦ C lower on the maximum difference in tem­
perature as compared to the reference panel during the daytime [50]
• PV with n-PCM of 0.5 wt% GnP showed the lowest average cell
temperature of 56.41 ◦ C, highest maximum electrical power of 6.88 kW,
and highest maximum electrical efficiency of 12.10 % compared to n-
PCM of MWCNT and MgO of same concentration [51]
• Employment of n-PCM with 0.5 vol% GnP enhanced 7 % in power output
and 6 % in electric efficiency when CR = 5 [52]
• PCM made of 85 % PEG1000 and 15 % expanded graphite was capable of
maintaining the PV panel’s temperature at 32–36 ◦ C, which was lower
than the reference case [53]
• PCM with 5 wt% graphene loading recorded the lowest solar cell
temperature disregarding the cavities number and heat sink arrangement
[54]
• Average PV temperature was reduced with GnP-enhanced PCM of higher
nanoparticle concentration, but the PV temperature increased with
increasing PCM thickness [55]
Nanofluid • High heat transfer area to • Aggregation of nanoparticles affecting • With 0.3 vol% of GnP in the nanofluid, panel temperature was reduced
volume ratio heat transfer strength by ~17 ◦ C from the maximum temperature of ~60 ◦ C, while power
• Adjustable properties through • High viscosity output increased from ~7 W to 10 W [56]
varying the concentration of • Expensive fabrication process • When the GnP concentration fixed at 0.3 vol% and mass flow rate at
particles 0.085 kg/s, reduction in panel temperature was observed from 60 ◦ C to
• Low erosion and corrosion 45 ◦ C [56]
• GnP nanofluid recorded the highest thermal efficiency in a daily
variation, followed by HyNF of Al2O3 and GnP, and lastly water [57]
• Maximum temperature of the CPV with graphene nanofluid cooling was
0.3 ◦ C and 0.6 ◦ C lower than water cooling under cloudy and sunny
conditions [58]
• When 0.10 vol% graphene nanofluid was circulated at a flow rate of 40
LPM, the surface temperature at the peak point of 46.24 ◦ C was achieved,
which was 14.76 % lower than the panel with PCM and distilled water
circulated, and 35.8 % lower than the conventional panel without PCM
and working fluid [59]
(continued on next page)

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L.T. Siow et al. Renewable and Sustainable Energy Reviews 193 (2024) 114288

Table 5 (continued )
Cooling Advantages Limitations Significances
Method

• With the use of GnP nanofluid, no drastic drop in efficiency was observed
compared to the no cooling and water cooling case [60]
• GnP nanofluid recorded the lowest temperature of 48.3 ◦ C on the PV
surface and the highest PV efficiency of 14.4 %, which outperformed
water (50.0 ◦ C and 13.5 %) and MWCNT nanofluid (49.6 ◦ C and 13.6 %)
[61]

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Declaration of competing interest and arid regions. Energy Convers Manag 2021;243:114328. https://doi.org/
10.1016/j.enconman.2021.114328.
[16] Chander S, Purohit A, Sharma A, Arvind, Nehra SP, Dhaka MS. A study on
The authors declare the following financial interests/personal re­
photovoltaic parameters of mono-crystalline silicon solar cell with cell
lationships which may be considered as potential competing interests: temperature. Energy Rep 2015;1:104–9. https://doi.org/10.1016/j.
Lau Ee Von reports financial support was provided by Ministry of Higher egyr.2015.03.004.
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tion (MOHE) Malaysia for the Fundamental Research Grant Scheme
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