1. Thermophysical properties of Single Wall Carbon Nanotubes
and its effect on exergy efficiency of a flat plate solar collector
Z. Said a,b
, R. Saidur d,⇑
, M.A. Sabiha b
, N.A. Rahim c
, M.R. Anisur e
a
Department of Engineering Systems and Management (ESM), Masdar Institute of Science and Technology, Abu Dhabi, United Arab Emirates
b
Department of Mechanical Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia
c
UM Power Energy Dedicated Advanced Centre (UMPEDAC), Level 4, Wisma R&D, University of Malaya, 50603 Kuala Lumpur, Malaysia
d
Centre of Research Excellence in Renewable Energy (CoRE-RE), King Fahd University of Petroleum and Minerals (KFUPM), Dhahran, 31261,
Saudi Arabia
e
Department of Mechanical & Aerospace Engineering, Monash University, Clayton, Victoria, 3168, Australia
Received 31 March 2014; received in revised form 18 February 2015; accepted 24 February 2015
Communicated by: Associate Editor Brian Norton
Abstract
In order to enhance thermal efficiency of a flat plate solar collector, the effects of thermo-physical properties of short Single Wall
Carbon Nanotubes (SWCNTs) suspended in water was investigated in this study. Sodium dodecyl sulphate was used as a dispersant
for preparing a stable nanofluid. Subsequently, the nanofluid was comprehensively characterized by particle size measurement and spec-
troscopic technique. Specific heat with the increase of particle loading and temperature was investigated. Thermal conductivity increment
also showed a linear dependence on particle concentration and temperature. Viscosity of the nanofluids and water reduced with the
increase of temperature and increased with the particle loading. Using improved thermo-physical properties of the nanofluid, the maxi-
mum energy and exergy efficiency of flat plate collector was enhanced up to 95.12% and 26.25% compared to water which was 42.07%
and 8.77%, respectively. This low exergy efficiency shows that flat plate collectors still require substantial enhancement. To the authors’
knowledge, SWCNTs–H2O was used as the functioning fluid for the first time to investigate both the thermos-physical properties as well
as the increase in thermal efficiency of a flat plate solar collector.
Ó 2015 Elsevier Ltd. All rights reserved.
Keywords: Specific heat; Thermal conductivity; Viscosity; Nanofluid; Exergy; SWCNTs
1. Introduction
Nanofluids are new addition to the family of fluids pre-
pared by immersing nanoparticles in conventional fluids
such as water, oils, ethylene glycol or coolants. In general,
these nanoparticles used in nanofluids are metals, metal
oxides or carbon nanotubes (CNTs), in diverse allotropic
forms. Choi et al. (2001) first reported studies on nanoflu-
ids and also explored the potentials of these nanofluids,
precisely in heat conduction applications. With regards to
thermal engineering applications, enhancement of upto
60% in thermal conductivity for water based nanofluids
was reported in literature (Keblinski et al., 2008; Yu
et al., 2008).
One of the utmost extraordinary findings of the last dec-
ade are carbon nanotubes (CNTs) (Iijima and Ichihashi,
http://dx.doi.org/10.1016/j.solener.2015.02.037
0038-092X/Ó 2015 Elsevier Ltd. All rights reserved.
⇑ Corresponding author at: Centre of Research Excellence in Renewable
Energy (CoRE-RE), King Fahd University of Petroleum and Minerals
(KFUPM), Dhahran, 31261, Saudi Arabia. Tel.: +966 13 860 4628; fax:
+966 13 860 7312.
E-mail addresses: saidur@kfupm.edu.sa, saidur912@yahoo.com
(R. Saidur).
www.elsevier.com/locate/solener
Available online at www.sciencedirect.com
ScienceDirect
Solar Energy 115 (2015) 757–769

2. 1993; Choi et al., 2001). Depending on their structure, they
have several unusual properties, such as high electrical and
thermal conductivities. In particular, thermal properties of
CNTs have attained a great deal of dedication (Tans et al.,
1997; Saito et al., 1998; Mizel et al., 1999; Hone et al., 2000;
Zhang et al., 2003; Wen and Ding, 2004; Duong et al.,
2008; Sun et al., 2008; Harish et al., 2012). Both experimen-
tally and numerically high thermal conductivities of CNTs
have been reported in literature (Berber et al., 2000; Kim
et al., 2001; Maruyama, 2003; Yu et al., 2005; Pop et al.,
2006). Therefore, CNTs are naturally expected to have
higher thermal conductivity enhancements in nanofluids
as compared to other nanoparticles. However, this unusual
increase could not be supported by consequent studies
reported (Xie et al., 2003; Wen and Ding, 2004; Assael
et al., 2005; Liu et al., 2005; Ding et al., 2006; Garg
et al., 2009). Since unique mechanical, electrical and struc-
tural properties are possessed by Single Wall Carbon
Nanotubes (SWCNT), they have attracted the attention
of the researchers (Dresselhaus and Avouris, 2001;
Baughman et al., 2002). SWCNT possesses outstanding
thermal and chemical stabilities with high-tensile strength
and extremely light weight (Jha and Ramaprabhu, 2012).
The specific heat of SWCNT has also been investigated
by several researchers (Mizel et al., 1999; Hone et al.,
2000; Zhang et al., 2003; Pradhan et al., 2009). Most of
the presented reports in literature are focused on multi
walled carbon nanotubes (MWCNTs), whereas limited
studies are found to be conducted on thermo-physical
properties of SWCNTs based nanofluid.
Several researchers have proposed different techniques
and models for obtaining stable nanofluid suspensions
(Li et al., 2007; Jiang et al., 2010; Ghadimi et al., 2011;
Said et al., 2013; Sajid et al., 2014). Some important
parameters such as the length of the CNTs, the purity level,
preparation method and pH of the solution and
thermophysical properties should be known in order to
make a direct comparison between experimental and theo-
ritical results. In this context, stable suspension of
SWCNTs based nanofluid using SDS surfactant was pre-
pared to get more accurate results.
Heat transfer enhancement using nanofluids in solar
thermal collectors is one of the main issues in saving
energy. Several studies related to nanofluids and its uses
in solar collectors are reported (Link and El-Sayed, 2000;
Kameya and Hanamura, 2011; Mercatelli et al., 2011a,b,
2012; Sani et al., 2011; Saidur et al., 2012). Tyagi et al.
(2009) reported the efficiency enhancement for low values
of the volume fraction of nanoparticles. However, for a
volume fraction higher than 2%, the efficiency stayed close-
lypersistent. Otanicar et al. (2010) found that the additio-
nof a little amount of nanoparticles enhanced the
Nomenclature
Ac collector area (m2
)
Cp specific heat (J/kg K)
d diameter of pipe (m)
_Exin exergy rate at inlet (W)
Gc global solar irradiation
D difference
_Exout exergy rate at outlet (W)
_Exdest rate of irreversibility (W)
_Exheat exergy rate received from solar radiation (W)
_Exwork exergy output rate from the system (W)
_Exmass;in exergy rate associated with mass at inlet (W)
_Exmass;out
exergy rate associated with mass at outlet (W)
_Sgen entropy generation rate (W/K)
_Qsun;in energy gain rate (W)
s shear stress
I intensity of solar radiation (W/m2
)
Pnf nanofluid
kp thermal conductivity of nanoparticle (W/m K)
K loss coefficient (dimensionless)
_m mass flow rate (kg/s)
_W work rate or power (W)
g collector efficiency
P fluid pressure (Pa)
q convective heat transfer rate (W)
k thermal conductivity (W/m K)
_Qo heat loss rate to the ambient (W)
_Qs energy rate engrossed (W)
Ta ambient temperature (K)
R ideal gas constant (J/K mol)
hin specific enthalpy at inlet (J/kg)
hout specific enthalpy at outlet (J/kg)
l coefficient of viscosity
Tout output temperature (K)
bf basefluid
Ts sun temperature (K)
Tsur surrounding/ambient temperature (K)
M viscosity (N s/m2
)
s transmittance coefficient of glazing
F absorptance coefficient of plate
sa effective product transmittance–absorptance
U nanoparticles volume fraction (%)
sa entropy generation to surrounding (J/kg K)
sin entropy generation at inlet (J/kg K)
sout entropy generation at outlet (J/kg K)
Pm density (kg/m3
)
R overall entropy production (J/kg K)
F friction factor
H specific enthalpy (J/kg)
_c shear strain rate
Tin input temperature (K)
P nano particle
758 Z. Said et al. / Solar Energy 115 (2015) 757–769

3. efficiency until a volume fraction of approximately 0.5%.
However, further addition of volume fraction levels off or
even slightly reduced the efficiency. Taylor et al. (2011)
showed an efficiency enhancement of 10% using nanofluids.
He et al. (2011) reported that the CNT–H2O nanofluidare
more suitableas compared to TiO2–H2O in a vacuum tube
solar collector. Yousefi et al. (2012c) reported an energy
efficiency of 28.3% with 0.2 wt.% as compared to water in
a flat plate solar collector. Yousefi et al. (2012b) with the
similarsetup reported an efficiency enhancement of 35%
with 0.4 wt.% of MWCNT-H2O nanofluid. Again, with
same setup in Yousefi et al. (2012a, 2012c) studied the
effects of pH variation of the MWCNT–water nanofluid
on the efficiency of the flat plate collector. Tiwari et al.
(2013) showed an efficiency improvement of 31.64% using
Al2O3 nanofluid in flat plate solar collector.
Studies using SWCNTs based nanofluid as a working
medium was not reported anywhere in literature. In this
study, SWCNTs was characterized using TEM, Zeta
Siezer, UV–Vis spectroscopy as well as visual recordings.
Thermal conductivity with respect to different volume frac-
tion using a KD2 pro was measured. Specific heat and vis-
cosity were measured as well. The energy and exergy
efficiencies of a flat plate solar collector using SWCNTs
based nanofluid are examined experimentally to evaluate
the performance enhancement.
2. Theoretical background
Theoretical studies on energy and exergy analyses are
reported below in the sub sections.
2.1. Energy analysis
The thermal efficiency of the flat plate solar collector (g)
is defined in Eq. (1) (Sukhatme, 2008).
g ¼ _mCpðTout À TinÞ=IAc ð1Þ
2.2. Entropy analysis
In this analysis, the system is assumed to be steady flow
and steady state operation. Work transfer from the system
and heat transfer to the system are also considered positive.
Loss coefficient is only considered for the entrance effect. If
the influences of potential and kinetic energy deviations are
ignored, the typical exergy stabilities can be expressed in
the rate form as in Eq. (2) (Ucar and Inallı, 2006).
_Exheat À _Exwork À _Exmass;in À _Exmass;out ¼ _Exdest ð2Þ
The rate of the general exergy balance can also be com-
posed as in Eq. (3).
X
1À
T a
Tsur
_Qs À _m½ðhout ÀhinÞÀT aðsout ÀsinÞŠ ¼ _Exdest
ð3Þ
Solar energy _Qs is the energy absorbed by the collector
absorber surface (Esen, 2008). The enthalpy and entropy
deviations of the nanofluid in the collector are expressed
in Eq. (4) (Ucar and Inallı, 2006).
1 À
Ta
Tsur
IT ðsaÞAc À _mCp;nf ðT f ;out À Tf ;inÞ
þ _mCp;nf Ta ln
Tf ;out
T f ;in
À _mRTa ln
Pout
Pin
¼ _Exdest ð4Þ
The exergy destruction (or irreversibility) rate, _Exdest can be
sincerely appraised from the subsequent Eq. (5).
_Exdest ¼ Ta
_Sgen ð5Þ
As reported by Bejan (1996), in a non-isothermal solar flat
plate collector, the overall rate of entropy generation can
be written as in Eq. (6).
_Sgen ¼ _mCp ln
Tout
Tin
À
_Qs
T s
þ
_Qo
Ta
ð6Þ
In order to measure the total heat loss to the ambient, Eq.
(7) can be used.
_Qo ¼ _Qs À _mCpðTout À T inÞ ð7Þ
Finally, the exergy efficiency is calculated from Eq. (8).
gex ¼ 1 À
Ta
_Sgen
½1 À ðTa=TsÞŠ _Qs
ð8Þ
3. Materials and methods
3.1. Materials and data collection
Short SWCNTs (90% CNTs, 60% SWCNTs) of length
1–3 lm and diameter 1–2 nm were purchased from
Nanostructured Amorphous Materials, Inc, USA.
Sodium dodecyl sulphate (SDS, 92.5–100.5%, Sigma–
Aldrich) as surfactants, was used. Distilled water was used
as a base fluid.
TEM was used to characterize SWCNTs nanoparticles.
A Zeta-seizer Nano ZS (Malvern) was used to obtain the
average diameter of the nanoparticles immersed in the base
fluids. DLS (dynamic light scattering) approach is used to
give the hydrodynamic radius of the particles in solution.
Mettler toledo pH meter was used to measure the pH of
the solution. The Density Meter DA-130 N from Kyoto
Electronics is used to measure the density of the nanofluids.
Viscosity of nanofluid was measured using Brookfield vis-
cometer (DV-II + Pro Programmable Viscometer) which
was connected with a temperature-controlled bath.
3.2. Specific heat
In the measurement of thermo-physical properties, the
term “specific” means the measure is an intensive property,
wherein the quantity of substance must be specified. For
specific heat capacity, mass is the specified quantity (unit
quantity). The specific heat capacity determines the convec-
tive flow nature of the nanofluid, and it necessarily depends
Z. Said et al. / Solar Energy 115 (2015) 757–769 759

4. on the volume fraction of the nanoparticles. Considering
the fact that very limited experimental data on specific heat
capacity values for various water-based nanofluids at dif-
ferent concentrations are available, the value of the specific
heat capacity is estimated using theoretical models. A heat-
flux-type Differential Scanning Calorimeter (PerkinElmer’s
DSC 4000) was used to measure the nanofluid specific heat
capacities. The Differential Scanning Calorimeter (DSC)
measures the heat flux into a sample as a function of tem-
perature for a user-prescribed heating regime. The classical
three-step DSC procedure was followed to measure specific
heat capacity (Ho¨hne et al., 2003). Therefore, this instru-
ment is used to measure the experimental values of water
based nanofluids. The specific heat capacity of nanofluids,
calculated at any particle concentration, which is valid for
homogeneous mixtures (Syam Sundar and Sharma, 2008),
is given by:
CPnf ¼
ð1 À /ÞðqCP Þbf þ /ðqCP ÞP
ð1 À /Þqbf þ /qP
ð9Þ
3.3. Thermal conductivity
The thermal conductivities of the tested nanofluids were
extracted by the ‘k’ module which contained a Decagon
Device KD2 Pro thermal property analyser. Equipped with
the optional KS-1 transient hotwire sensor, capable of
reading a fluid’s thermal conductivity from À323 K to
423 K with a maximum deviation of 5.0% reported and
was tested for accuracy under the experimentation parame-
ters. The sensor’s stem was vertically inserted in a jar
through the lid of a small container filled with USP glycer-
ine, which was in turn completely submerged in the
Polyscience Circulating Water Bath. Applying this method,
it was possible to accurately test the KD2 Pro at tempera-
tures ranging from 298 K to 323 K. Thermal conductivity
readings were found to be within 0.3% deviation from cali-
bration values until 323 K. Above this temperature, natural
heat flux in the glycerine caused micro-convection currents
to affect the hotwire stem surface and amplify the readings
by 4.2%, still within the acceptable ±5.0% tolerance.
3.4. Experimental procedure
The experimental set up of the solar collector and the
schematic diagram of the experiment are presented in
Figs. 1 and 2 respectively. The dimensions of the solar col-
lectors are listed in Table 1. The experiment was carried out
at University Malaya, Malaysia. The collector position was
fixed at 22° angle, for the maximum solar radiation absorp-
tion. Table 1 presents the specifications of the flat plate
solar collector that are considered in this experiment. For
the force convection system, an electric pump is used in
the solar water heating system. A radiator is used for cool-
ing the water inlet temperature. It is shown in Fig. 2 that
the tank which has a capacity of 50 L absorbs the heat load
from the collector cycle. All the data were later transferred
into the computer via interfaces. Calibration of the entire
system was taken several times.
ASHARE Standard 93-2003 (Standard, 1977) is used to
evaluate the thermal performance of the flat-plate solar col-
lector. The flow rates of 0.5, 1.0 and 1.5 kg/min are used to
test the flat plate solar collector.
3.5. Error analysis in measurements
Two groups of errors are reported in our measurements.
One group could come from the direct measurement
parameters such as solar radiation flux (DGc), DT, DP
and the second group of errors could come from the indi-
rect measurements, such as energy and exergy efficiencies.
The following relations can be used based on the
Luminosu and Fara (2005) method:
Dgex ¼
D_I
_Exheat
þ
_I _Exheat
_Ex2
heat
ð10Þ
Fig. 1. The experimental setup used for this study: (a) front view, (b) back view, (c) left side view and (d) right side view.
760 Z. Said et al. / Solar Energy 115 (2015) 757–769

5. and
Dgen ¼
D _qa
Gc
þ
_qaDGc
G2
c
ð11Þ
where each error component can be evaluated through the
following relations:
DExheat ¼
DT
T s
þ
TaDT
T2
s
AcðsaÞGc þ 1 À
Ta
T s
AcðsaÞDGc
ð12Þ
D_I ¼ TaD_Sgen þ _SgenDT ð13Þ
_DSgen ¼ R ln
Pout
Pin
þ Cp ln
Tin
Tout
þ Cp
T out þ T in
Ta
D _m
þ GcAcðsaÞ
DT
T 2
a
þ _mCp
1
Tout
þ
1
Tin
þ
2
Ta
þ
ðTout þ TinÞ
T2
a
DT
þ _mR
1
Pout
þ
1
Pin
DP
þ AcðsaÞ
1
Ts
þ
1
Ta
DGc ð14Þ
where Pin and Pout are the pressure difference of the agent
fluid with the surroundings at entrance and exit of the
solarcollector.
D_qa ¼ Cp
D _mðT out þ TinÞ þ 2 _mDT
Ac
ð15Þ
The total uncertainties of the measurements are estimated
to be ±3.0% for solar radiation, ±1.60% for the nanofluid
and water temperatures, ±3.32% for pressures and
±3.02% for massflow rate. Therefore, the maximum
errors for the indirect measuring of energy and exergy
efficiencies were estimated to be ±0.1 and ±0.14 using
Eqs. (10) and (11).
Fig. 2. Schematic presentation of the experimental set up.
Table 1
Specifications for the flat plate solar collector.
Parameters of collector Value
Frame Aluminum alloy
Glazing 4 mm tempered texture glass
Working fluids in flow ducts Water and SWCNTs based nanofluid
Absorption area, Ap 1.84 m2
Wind speed 2–4 m/s
Collector tilt, bo 22°
Absorption rate 0.94
Emittance 0.12
Heat transfer coefficient 4.398
Header material Copper TP2
Header tube size £22 mm  t0.6 mm (2 pcs)
Riser tube material Copper TP2
Riser tube size £10 mm  t0.45 mm (8 pcs)
Z. Said et al. / Solar Energy 115 (2015) 757–769 761

6. 4. Results and discussion
4.1. Nanofluid characterization and stability
The highly hydrophobic (tending to repel or fail to mix
with water) nature of SWCNTs makes very hard to dis-
perse them in water. Preparation of a stable and homoge-
nous dispersion is a vital prerequisite for a nanofluid. In
this present work, 0.1 and 0.3 vol.% dispersant was used
to prepare the nanofluid suspension. Fig. 3 shows the
visualization of SWCNTs using transmission electron
microscope (TEM), whereas Fig. 4 shows SWCNTs nano-
fluid with SDS after a period of 30 days. Chemical struc-
ture of Sodium dodecyl sulphate is presented in Fig. 5.
Stable nanofluid suspension was prepared by adding
necessary loading of SWCNTs. For this pupose, the
SWCNT and SDS density was considered to be 2.1 g/cm3
and 1.01 g/cm3
respectively. Ratio of 1:1 was employed
for the SWCNT nanoparticles and SDS. The dispersions
were subjected to a tip sonication using an ultrasonic pro-
cessor for 1:30 h. Same sonication conditions were used for
the samples of different volume concentrations. It was also
noticed that about 4–5% of the volume was lost during the
tip sonication, and the losses were taken into account dur-
ing the preparation of the nanofluid solutions.
SDS-dispersed SWCNTs were further characterized
using UV–Vis spectroscopyup to the range of 1100 nm,
presented in Fig. 6. The nanofluid solutions were diluted
to perform the measurements with the base fluid. Fig. 6
shows a typical absorption spectrum obtained from
SWCNTs dispersed in water using SDS. Sharp peaks are
witnessed in the absorption spectrum, which are mainly
due to the characteristic of isolated nanotubes.
The visual observations of sedimentation of SWCNTs
are illustrated in Fig. 7. The samples with different volume
fraction were poured into transparent cells right after pre-
paration. The visual images of the 1st day and the 30th
days are presented and show no visible sedimentation.
The pH of the (SWCNT + SDS)/water nanofluid for
0.1 vol.% was obtained to be 7.0. The nanofluids remained
highly stable showing no visible signs of sedimentation
even after 1 month of incubation.
Fig. 3. TEM visualization of SWCNTs (length 1–3 lm and diameter
1–2 nm) nanoparticleimage captured at an acceleration on 200 kV.
Fig. 4. TEM visualization of SWCNTs nanofluid with SDS image after
30 days.
Fig. 5. Chemical structure of Sodium dodecyl sulphate (NaC12H25SO4).
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0.45
300 400 500 600 700 800 900 1000 1100
Absorbance
Wavelength (nm)
Fig. 6. UV–Vis absorbance spectrum of SWCNTs dispersed in water
using SDS.
Fig. 7. Prepared SWCNTs nanofluid solutions (a) samples on the first day
of preparations and (b) samples after 30 days of preparations.
762 Z. Said et al. / Solar Energy 115 (2015) 757–769

7. 4.2. Particle size measurements
SWCNTs–water nanofluids with SDS as dispersant were
tested in several concentrations. An anionic dispersant was
chosen based on reports present in literature (Sun et al.,
2008). The investigated fluids were as follows.
Water +0.1 and 0.3 vol.% SDS at 0.1 and 0.3 vol.%
SWCNTs, respectively.
Fig 8 presents the mean particle diameters as a function
of time for the nanofluid in static mode.
In order to improve the stability of this suspension,
same SDS: SWCNTs ratio was tested. The result is shown
in Fig. 9. Here, the suspension contains 0.1 vol.% of
SWCNTs and shows a very stable behavior for 30 days,
keeping a mean diameter of about 130 nm.
The measured zeta potential was around À42 mV as
anticipated in the case of anionic dispersant (Li et al.,
2007; Sun et al., 2008) for all the studied SWCNTs-nano-
fluids, supporting their non-aggregating tendency. Owing
to the strong opacity of the solutions at 0.1 and
0.3 vol.%, they were diluted with distilled water to perform
the zeta potential measurements. Fig. 10 presents the effects
of zeta potential on the stability of suspension properties.
In conclusion, based on the higher zeta potential value
(À42 mV) and maintaining a mean diameter of about
130 nm for 30 days, proved the water-based nanofluids
containing SWCNTs and SDS as a very stable nanofluid.
Further investigation on their properties is underway.
Table 2 presents the experimentally measured thermo-
physical properties.
4.3. Specific heat
Fig. 11 shows the specific heat of SWCNTs nanoparticle
with respect to changing temperature.
The specific heat of SWCNTs compared to water is
much lower. Therefore, higher volume fraction of
SWCNTs in water could result in greater reduction in the
specific heat of SWCNTs based water nanofluid. This
experimental data agree well with data reported by other
researchers (Hone et al., 2000; Zhang et al., 2003;
Pradhan et al., 2009).
In Fig. 12, the result indicates that the specific heat
capacity of SWCNTs nanofluid decreases gradually with
increasing volume concentration of nanoparticles. It is
observed that the specific heat reduces gradually with the
increasing temperature. For 0.3 vol.% of SWCNTs nano-
fluid, it is observed that the specific heat reduces by a large
margin compared to 0.1 vol.% of SWCNTs particle load-
ing. From Fig. 12, it is noted that, the specific heat drops
almost linearly until 331 K of temperature and then a sud-
den rise is observed in the specific heat beyond this tem-
perature. After this point, no further drop is noticed,
suggesting the critical point at which the boiling starts to
take place and therefore, showing inconsistency in experi-
mental data. The reason behind the critical point is the
increased thermal conductivity of SWCNTs based nano-
fluidwhich is supported by the findings presented in ther-
mal conductivity section. Therefore, by the increase in
nanoparticle fraction, the portion of heat absorption with
these lower specific heat nanoparticles is increased and lead
to the decreasing in nanofluid specific heat. Eq. (9), cannot
be used to predict the tendency for fluid with nanoparticle
inclusions. The decline in specific heat for 0.3 vol.% of
SWCNTs nanofluid cannot be explained by the theoretical
model. This may be due to the reason that Eq. (9) does not
take temperature into account. Hence further studies need
to be carried out to provide models that can explain this
sudden decline in specific heat of nanofluids with respect
to temperature.
Qualitatively, the solid-liquid interface may change the
phonon vibration mode near the surface area of a nanopar-
ticle and thus change the specific heat capacity of nano-
fluid. The high specific interfacial area of nanoparticle
can adsorb liquid molecules to its surface and form liquid
layers, which will reversely constrain nanoparticle and
turns its free-boundary surface atoms to be non-free inter-
ior atoms (Wang et al., 2006). Specific heat obtained from
the experimental measurements is used for our study.
4.4. Thermal conductivity
Thermal conductivity of SWCNTs/water nanofluid with
volume fractions of 0.1 and 0.3 vol.% was experimentally
0
1
2
3
4
5
6
7
8
9
10
1 10 100 1,000 10,000
Intensity(%)
Size (nm)
0.1 vol. % SWCNTs 0.3 vol. % SWCNTs
Fig. 8. Particle size distribution (Z-average = 139.3 d nm) after 3 days
preparation of sample (0.1 vol.% SWCNTs + 0.1 vol.% SDS)/distilled
water and particle size distribution (Z-average = 135.5 d.nm) after
preparation of sample (0.3 vol.% SWCNTs + 0.3 vol.% SDS)/distilled
water.
80
100
120
140
160
180
200
0 5 10 15 20 25 30
Diameter(nm)
Number of Days
SWCNTs+SDS (0.1 vol. %)
Fig. 9. Diameter in relation to the time elapsed from the day of
preparation, for water containing 0.1 vol.% SWCNTs + 0.1 vol.% SDS.
Z. Said et al. / Solar Energy 115 (2015) 757–769 763

8. measured. Fig. 13 shows the thermal conductivity versus
different SWCNTs volume fractions measured at different
temperatures. Thermal conductivity enhanced with the
increasing volume fraction of SWCNTs in a linear fashion.
Fig. 13 is showing the difference between the thermal
conductivity of SWCNTs/water nanofluid and the experi-
mental data of Harish et al. (2012) with respect to water.
SWCNTs/water nanofluid showed higher thermal conduc-
tivity enhancement compared to both water and Harish
et al. (2012) data. A linear increase is found for
SWCNTs/water nanofluid. The SWCNTs form a saturat-
ing network which results in an improved energy transport
thereby increasing the effective conductivity of the fluid.
The thermal conductivity enhancement witnessed in the
current investigation supports the mechanism of particle
clustering in improving the thermal conductivity of the
fluid (Harish et al., 2012). It needs to be pointed out that
the thermal conductivity enhancement stayed nearly the
same for one month.
Fig. 14 presents the effective thermal conductivity for
two volume fractions at different temperatures. Additional
Table 2
Experimental thermo-physical properties of SWCNTs/water and base fluid at room temperature.
Particle and base fluid Average particle
size (nm)
Actual density
(kg/m3
)
Cp (J/kg K) K (W/mK) Viscosity
(m Pa s)
pH
SWCNTs D = 1–2 nm 2100 600 $3500 (Pop et al., 2006)
L = 1–3 lm
Water 998.8 4179 0.605 0.89
SWCNTs/water (0.1%v/v SWCNTs + 0.1%v/v SDS) 1007 4104 0.651 7.0
SWCNTs/water (0.3%v/v SWCNTs + 0.3%v/v SDS) 1024 3845 0.691 7.0
3.100
3.300
3.500
3.700
3.900
4.100
4.300
4.500
280 290 300 310 320 330 340 350
SpecificHeat(J/g*°C)
Temperature (K)
Specific Heat (J/g*°C) of Water Specific Heat (J/g*°C) of 0.1 vol. % SWCNT
Specific Heat (J/g*°C) of 0.3 vol. % SWCNT
Fig. 12. Specific heat of SWCNTs based nanofluids with increasing
temperature and volume fraction.
450
500
550
600
650
700
750
273 283 293 303 313 323 333 343 353 363
SpecificHeat(J/g*°C)
Temperature (K)
Specific Heat (J/g*°C) of SWCNTs
Fig. 11. Specific heat of SWCNTs with respect to temperature.
Fig. 10. Effects of zeta potential on suspension properties.
764 Z. Said et al. / Solar Energy 115 (2015) 757–769

9. increase in effective thermal conductivity is observed with
the increasing temperature. A maximum conductivity
enhancement of 91% is obtained at a temperature of
323 K for 0.3 vol.% volume fraction, whereas, the minimum
enhancement in conductivity is 12% for 0.1 vol.% volume
fraction of SWCNTs at 298 K in comparison with water.
Nanoparticles tend to aggregate, with the period of
time, which aretemperature dependent and tend to increase
with the growing size of the aggregates as a substantial
amount of time is frequently consumed to heat the fluid
during measurements (Gharagozloo and Goodson, 2010).
Effective thermal conductivity with respect to heating and
cooling is measured in order to examine this mechanism
in Fig. 15.
From Fig. 15, it is evident that the fluid effective thermal
conductivity remains the same for both the heating and
cooling phase. Hysteresis effect was not observed for
SWCNTs/water, which therefore does not support “the
time dependent aggregation” (Gharagozloo and
Goodson, 2010; Harish et al., 2012), as a possible mecha-
nism for the temperature dependent thermal conductivity
enhancement.
The effective conductivity enhancement remains the
same with respect to temperature irrespective of whether
the fluid is heated or cooled with minor errors. The
agglomeration of nanoparticles and the formation of clus-
ters can increase the thermal conductivity. Increment in
thermal conductivity was reported to be 7% for a volume
fraction of 1% of MWCNT based nanofluid (Xie et al.,
2003). Another researcher reported an increment of about
40% at a volume fraction of 0.6% of MWCNTs based
nanofluid at room temperature (Assael et al., 2005). 80%
enhancement was reported by Ding et al. (2006) for
1 wt.% of MWCNTs at a temperature of 303 K. Nasiri
et al. (2012) reported an enhancement of 35% at a tempera-
ture of 323 K using 0.25 wt.% of MWCNTs. Our results
are supported by these findings.
4.5. Viscosity
The difference in temperature-dependent thermal con-
ductivity variation could be a possible indication of the
critical role of Brownian motion in the fluid. Gupta and
Kumar (2007) reported an enhancement of 6% in thermal
conductivity at higher temperatures. Due to improved diffu-
sion of heat walkers enhanced thermal conductivity was
reported (Duong et al., 2008). Translational diffusion coef-
ficient of SWCNTs/water was reported to be much lower
compared to water, ranging from 0.3 to 6 lm2
/s
(Tsyboulski et al., 2008). Broersma theory is used to esti-
mate the rotational diffusion (Dr) of SWCNTs (Broersma,
2004; Tsyboulski et al., 2008; Harish et al., 2012).
Dr ¼
3kB
pg
lnðL=dÞ À c
L3
ð16Þ
In Eq. (16), L and d denote the length and diameter of the
nanotube, respectively. kB is the Boltzmann constant, T is
the fluid temperature, g is the fluid viscosity and c is the
end correction coefficient (usually c is assumed to be
0.83). Eq. (16) shows that the rotational diffusion is inver-
sely proportional to the cube length of the SWCNTs.
Viscosity with respect to changing volume fraction and
temperature is presented in Fig. 16. It is observed from
Fig. 16, with the increasing temperature, the viscosity of
the fluid decreases. The decrease in the fluid viscosity
0.55
0.65
0.75
0.85
0.95
1.05
1.15
1.25
295 300 305 310 315 320 325
Thermalconductivity(W/m.k)
Temperature (K)
water Harish et al. (2012) SWCNTs+SDS (0.1 vol. %) SWCNTs+SDS (0.3 vol. %)
Fig. 13. Temperature-dependent thermal conductivity in (SWCNTs +
SDS)/water nanofluid.
1.05
1.15
1.25
1.35
1.45
1.55
1.65
1.75
1.85
1.95
295 300 305 310 315 320 325
ThermalConductivityRatio(Keff/Kf)
Temperature (K)
SWCNTs+SDS (0.1 vol. %) SWCNTs+SDS (0.3 vol. %)
Fig. 14. Thermal conductivity increase as a function of fluid temperature
in water.
1.00
1.10
1.20
1.30
1.40
1.50
1.60
295 300 305 310 315 320 325
ThermalConductivityRatio(Keff/Kf)
Temperature (K)
Heating Up Cooling Down
Fig. 15. Comparisons of thermal conductivity improvement during the
heating and cooling process in water (SWCNTs: 0.1 vol.%).
Z. Said et al. / Solar Energy 115 (2015) 757–769 765

10. improves the rotational diffusion of SWCNTs. As men-
tioned above, the given length of the SWCNTs used for
this study is from 1 to 3 lm, with a diameter of 1–2 nm.
Given Eq. (16) and these parameters, it is therefore, possi-
ble to conclude that the enhancement in the thermal con-
ductivity is due to the presence of shorter SWCNTs,
resulting in higher rotational diffusion.
With the increasing temperature, the viscosity of the
nanofluids and water, both reduced. An increase of 39%
in viscosity isobserved for SWCNTs and SDS suspension
at a volume fraction of 0.3 vol.%. An increase in viscosity
is observed with the increasing volume fraction of
SWCNTs and SDS. This strong increase in viscosity will
have adverse effects in practical applications of such
nanofluids.
4.6. Energy and exergy efficiencies using SWCNTs based
nanofluid
4.6.1. Entropy generation and exergy destruction
Entropy is produced in irreversible processes. Therefore,
for the energy optimization analysis, it is essential to mea-
sure the entropy generation or exergy destruction due to
heat transfer and viscous friction as a function of the
design variables selected (Onsager, 1931b,a; Kreuzer,
1981). Fig. 17 provides the solar insolation data recorded
for a clear and cloudy day used for the performance mea-
surement of the solar collector.
Experimental data of solar water heating systems with
and without nanofluids are various days and different flow
rate is provided in Table 3.
Fig. 18 presents the entropy generation and exergy
destruction with respect to mass flow rate and different vol-
ume concentrations of water and nanofluid. Eqs. (5) and
(6) are used to obtain exergy destruction and entropy
generation, respectively.
As shown in Fig. 18, the entropy generation is reduced
up to 32.21 M/K for 0.1 vol.% SWCNTs, for a mass flow
rate of 0.5 kg/min. For 0.3 vol.% the entropy generation
is reduced to 37.51%, for a mass flow rate of 0.5 kg/min,
whereas, for water with similar mass flow rate, the reduc-
tion in entropy generation is 43.53 M/K. Therefore, from
the obtained results, it can be said that the entropy can
be reduced with the least volume fraction of SWCNTs
used, compared to higher volume fractions.
The other axis of Fig. 18 illustrates the exergy destruc-
tion with respect to mass flow rate and changing volume
fraction. Similar behavior as that of entropy generation is
observed for exergy destruction as well. With 0.1 vol.% of
SWCNTs and a mass flow rate of 0.5 kg/min, the exergy
destruction reduced to 1037.11 W. For 0.3 vol.% of
SWCNTs and the similar mass flow rate as of earlier case;
the exergy destruction reduced to 1200.39 W. In case of
water, the lowest exergy destruction was observed
1423.69 W for a mass flow rate of 0.5 kg/min. From these
observations, SWCNTs based water with as low as
0.1 vol.% is very useful in reducing the entropy generation
and exergy destruction.
4.6.2. Effect of SWCNTs on the output temperature
Fig. 19 demonstrates the effect of mass flow rate and
volume fraction on output temperature. As recognized,
the output temperature is one of the most effectiveparame-
ter that affects the energy efficiency of a flat plate solar col-
lector directly. It is increased intensely with the growing
output temperature.
As illustrated in Fig. 19, a greater difference between the
water inlet temperature and ambient temperature results in
an enhanced exergy efficiency of the flat plate solar collec-
tor. This enhancement is due to the increasing temperature
of the absorber’s plate along with rising inlet water tem-
perature. The main reason of exergy loss in a collectoris
the difference between the temperature of the solar radia-
tion and the absorber plate temperature, since the rising
temperature of the absorber flat, results in a higher differ-
ence and subsequently reduced collector exergy loss.
4.6.3. Energy and exergy efficiencies
To ensure the best results with least error, each
investigation was repeated for several days. Different mass
flow rates and changing volume fraction of nanoparticles
are used to present the energy efficiency and exergy
0.40
0.50
0.60
0.70
0.80
0.90
1.00
1.10
1.20
1.30
1.40
295 300 305 310 315 320 325 330
Viscosity
Temperature (K)
Water 0.1 vol. % SWCNTs+0.1 vol. % SDS 0.3 vol. % SWCNTs+0.3 vol. % SDS
Fig. 16. Viscosity of SWCNTs/water nanofluid with respect to volume
fraction and temperature.
0
100
200
300
400
500
600
700
800
900
1000
1100
1200
9.00 10.00 11.00 12.00 13.00 14.00 15.00 16.00 17.00 18.00
SolarRadiation(W/m²)
Time of Day (hour)
Solar insolation on a clear day
Solar insolation on a cloudy day
Fig. 17. Solar insolation recordings on a clear and cloudy day for
experimental study.
766 Z. Said et al. / Solar Energy 115 (2015) 757–769

11. efficiency of the solar collector in Fig 20. This efficiency was
evaluated using Eqs. (10)–(14) and input Tables 2 and 3.
As shown in Fig. 20 and 0.3 vol.% of SWCNTs and a
mass flow rate of 0.5 kg/min enhanced the energy efficiency
up to 95.12%, whereas for 0.1 vol.% of SWCNTs the
energy efficiency improved up to 89.26%, for the same mass
flow rate. The highest energy efficiency record for water
was 42.07% for a mass flow rate of 0.5 kg/min. The other
axis in Fig. 20 shows the exergy efficiency with respect to
changing mass flow rate and volume fraction. As it is
shown, the exergy efficiency enhanced up to 26.25%, using
0.3 vol.% of SWCNTs and a mass flow rate of 0.5 kg/min.
Under the similar conditions for 0.1 vol.% of SWCNTs the
exergy efficiency enhanced up to 22.35%, whereas the high-
est exergy efficiency measured for water under similar con-
ditions was 8.77%. This highest energy and exergy
efficiency is the result of higher thermal conductivity, which
is supported by the results presented in Figs. 13 and 14.
According to the reports, it is obvious that the energy
and exergy efficiencies have contradictory behaviors in
numerous cases. Rising fluid inlet temperature results in a
reduced energy efficiency of collector. However, it results
in an overall increased exergy efficiency even to its maxi-
mum. Correspondingly, rising mass flow rate results in
improved energy efficiency of the collector; however, this
has an inverse effect on exergy efficiency. Most of the
exergy destructions arise during the absorbing process in
the collector’s absorber plate. Rising inlet water tempera-
ture and reducing water mass flow rate can be effective
on reducing these destructions.
5. Conclusions
Water-based nanofluids, obtained by dispersing
SWCNTs nanoparticles are investigated in this study. By
Table 3
Experimental data of the solar water heating system with and without nanofluids at various days.
Local time (h) Volume
concentration (%v/v)
Solar
radiation (W/m2
)
Water temperature (°C) Mass flow
rate (kg/min)
Ambient
temperature (°C)
Wind
velocity (m/s)
Inlet Outlet
12:30 Water + 0.1% of SWCNTs 730.2 44.2 58.9 0.5 33.3 2.75
13:00 834.2 48.1 59.7 0.5 33.9 2.64
13:30 851.4 48.4 65.3 0.5 34.2 3.55
14:30 885.4 48.4 67.7 0.5 35.1 3.38
12:50 981.1 40.2 48.1 1.5 33.1 2.66
13:30 985.2 40.8 48.8 1.5 33.9 2.75
14:00 992.4 40.9 47.9 1.5 33.6 3.25
12:30 Water only 716.3 43.4 49.2 1.0 33.1 3.00
13:00 759.8 44.2 49.9 1.0 33.3 3.25
13:30 765.8 48.5 54.6 1.0 33.4 3.38
0
5
10
15
20
25
30
20
30
40
50
60
70
80
90
100
0.5 0.7 0.9 1.1 1.3 1.5
Exergyefficiency,%
Energyefficiency,%
Flow rate, kg/min
0.30% 0.10% Water 0.10% 0.30% Water
Fig. 20. The energy efficiency (solid lines) and exergy efficiency (dotted
lines) at different mass flow rates and different volume fractions for
SWCNTs based nanofluid.
1000
1100
1200
1300
1400
1500
1600
1700
1800
1900
30
33
36
39
42
45
48
51
54
57
0.5 0.7 0.9 1.1 1.3 1.5
Exergydestruction,W
Entropygeneration,M/K
Flow rate, kg/min
0.30% 0.10% Water Water 0.10% 0.30%
Fig. 18. Entropy generation (dotted line) and exergy destruction (solid
line) for SWCNTs based nanofluid at different mass flow rate and volume
fraction.
45
49
53
57
61
65
69
73
77
0.5 0.7 0.9 1.1 1.3 1.5
Flow rate, kg/min
0.30%
0.10%
Water
Fig. 19. SWCNTs based nanofluid at different mass flow rate and volume
fraction and its effect on output temperature.
Z. Said et al. / Solar Energy 115 (2015) 757–769 767

12. using DLS equipment, different preparation techniques
were compared. The mean diameter distribution variation
with time is measured by the size measurement technique,
therefore, showing the tendency of nanoparticles to settle
down. Moreover, the tendency of the nanoparticles to
aggregate is shown by the zeta potential measurement.
Stability analysis of SWCNTs nanofluid is carried out
using all these measurements, coupled with the visual
observation of the suspension. Distilled water containing
0.1 and 0.3 vol.% SDS and 0.1 and 0.3 vol.% SWCNTs,
respectively, proved to be very stable for at least 30 days.
The increase in density for both the investigated volume
fractions was almost negligible. Specific heat increased with
the volume fraction and dropped with the increasing tem-
perature. A maximum conductivity enhancement of 91%
is obtained at a temperature of 323 K for 0.3 vol.% volume
fraction, whereas, the minimum enhancement in conductiv-
ity is 12% for 0.1 vol.% volume fraction of SWCNTs at
298 K. Further investigation is required to confirm whether
the enhancement of thermal conductivity is abnormal.
With the increasing temperature, a reduction in viscosity
of the nanofluids and wateroccurred. An increase of 39%
in viscosity was observed for SWCNTs and SDS suspen-
sion at 0.3 vol.%.
According to the mentioned results, keeping the inlet
water temperature higher than the ambient temperature
as well as a lower flow rate may result in improved overall
performance. Based on the experimental results, the maxi-
mum energy and exergy efficiency of the flat plate collector
is close to 95.12% and 26.25% compared to water which
was 42.07% and 8.77%, respectively. This low exergy effi-
ciency shows that the flat plate collectors still need signifi-
cant improvement.
For future progression, it is recommended to perform
an exergoeconomic analysis, which is a combination of
exergy and economics, and provides useful insights into
the relations between thermodynamics and economics.
Acknowledgement
This research is supported by UM High Impact
Research Grant UM-MOHE UM.C/HIR/MOHE/ENG/
40 from the Ministry of Higher Education, Malaysia.
References
Assael, M. et al., 2005. Thermal conductivity enhancement in aqueous
suspensions of carbon multi-walled and double-walled nanotubes in the
presence of two different dispersants. Int. J. Thermophys. 26 (3), 647–664.
Baughman, R.H. et al., 2002. Carbon nanotubes – the route toward
applications. Science 297 (5582), 787–792.
Bejan, A., 1996. Entropy Generation Minimization: The Method of
Thermodynamic Optimization of Finite-size Systems and Finite-time
Processes. CRC Pressi llc.
Berber, S. et al., 2000. Unusually high thermal conductivity of carbon
nanotubes. Phys. Rev. Lett. 84 (20), 4613.
Broersma, S., 2004. Rotational diffusion constant of a cylindrical particle.
J. Chem. Phys. 32 (6), 1626–1631.
Choi, S. et al., 2001. Anomalous thermal conductivity enhancement in
nanotube suspensions. Appl. Phys. Lett. 79 (14), 2252–2254.
Ding, Y. et al., 2006. Heat transfer of aqueous suspensions of carbon
nanotubes (CNT nanofluids). Int. J. Heat Mass Transfer 49 (1), 240–
250.
Dresselhaus, M.S., Avouris, P., 2001. Introduction to Carbon Materials
Research. Carbon Nanotubes. Springer, pp. 1–9.
Duong, H.M. et al., 2008. Calculated thermal properties of single-walled
carbon nanotube suspensions. J. Phys. Chem. C 112 (50), 19860–
19865.
Esen, H., 2008. Experimental energy and exergy analysis of a double-flow
solar air heater having different obstacles on absorber plates. Build.
Environ. 43 (6), 1046–1054.
Garg, P. et al., 2009. An experimental study on the effect of ultrason-
ication on viscosity and heat transfer performance of multi-wall
carbon nanotube-based aqueous nanofluids. Int. J. Heat Mass
Transfer 52 (21), 5090–5101.
Ghadimi, A. et al., 2011. A review of nanofluid stability properties and
characterization in stationary conditions. Int. J. Heat Mass Transfer
54 (17), 4051–4068.
Gharagozloo, P.E., Goodson, K.E., 2010. Aggregate fractal dimensions
and thermal conduction in nanofluids. J. Appl. Phys. 108 (7), 074309.
Gupta, A., Kumar, R., 2007. Role of Brownian motion on the thermal
conductivity enhancement of nanofluids. Appl. Phys. Lett. 91 (22),
223102.
Harish, S. et al., 2012. Temperature dependent thermal conductivity
increase of aqueous nanofluid with single walled carbon nanotube
inclusion. Mater. Exp. 2 (3), 213–223.
He, Y. et al., 2011. Experimental study on the light-heat conversion
characteristics of nanofluids. Nanosci. Nanotechnol. Lett. 3 (4), 494–496.
Ho¨hne, G. et al., 2003. Differential scanning calorimetry. Springer.
Hone, J. et al., 2000. Quantized phonon spectrum of single-wall carbon
nanotubes. Science 289 (5485), 1730–1733.
Iijima, S., Ichihashi, T., 1993. Single-shell carbon nanotubes of 1-nm
diameter. Nature 363, 603–605.
Jha, N., Ramaprabhu, S., 2012. Thermal and electrical conductivity
studies of metal oxide-multiwalled carbon nanotubes nanocomposite
based nanofluids. J. Nanofluid 1 (1), 63–70.
Jiang, W. et al., 2010. Modeling of nanoparticles’ aggregation and
sedimentation in nanofluid. Curr. Appl. Phys. 10 (3), 934–941.
Kameya, Y., Hanamura, K., 2011. Enhancement of solar radiation
absorption using nanoparticle suspension. Sol. Energy 85 (2), 299–307.
Keblinski, P. et al., 2008. Thermal conductance of nanofluids: is the
controversy over? J. Nanopart. Res. 10 (7), 1089–1097.
Kim, P. et al., 2001. Thermal transport measurements of individual
multiwalled nanotubes. Phys. Rev. Lett. 87 (21), 215502.
Kreuzer, H.J., 1981. Nonequilibrium Thermodynamics and its Statistical
Foundations. Clarendon Press, Oxford and New York, 455 p., 1.
Li, X. et al., 2007. Evaluation on dispersion behavior of the aqueous
copper nano-suspensions. J. Colloid Interface Sci. 310 (2), 456–463.
Link, S., El-Sayed, M.A., 2000. Shape and size dependence of radiative,
non-radiative and photothermal properties of gold nanocrystals. Int.
Rev. Phys. Chem. 19 (3), 409–453.
Liu, M.-S. et al., 2005. Enhancement of thermal conductivity with
carbon nanotube for nanofluids. Int. Commun. Heat Mass 32 (9),
1202–1210.
Luminosu, I., Fara, L., 2005. Determination of the optimal operation
mode of a flat solar collector by exergetic analysis and numerical
simulation. Energy 30 (5), 731–747.
Maruyama, S., 2003. A molecular dynamics simulation of heat conduction
of a finite length single-walled carbon nanotube. Microscale
Thermophys. Eng. 7 (1), 41–50.
Mercatelli, L. et al., 2011a. Scattering and absorption properties of carbon
nanohorn-based nanofluids for sol. energy applications. J. Eur. Opt.
Soc. – Rapid, 6.
Mercatelli, L. et al., 2011b. Absorption and scattering properties of
carbon nanohorn-based nanofluids for direct sunlight absorbers.
Nanoscale Res. Lett. 6 (1), 1–9.
768 Z. Said et al. / Solar Energy 115 (2015) 757–769

13. Mercatelli, L. et al., 2012. Carbon nanohorn-based nanofluids: character-
ization of the spectral scattering albedo. Nanoscale Res. Lett. 7 (1),
1–5.
Mizel, A. et al., 1999. Analysis of the low-temperature specific heat of
multiwalled carbon nanotubes and carbon nanotube ropes. Phys. Rev.
B 60 (5), 3264.
Nasiri, A. et al., 2012. Effect of CNT structures on thermal conductivity
and stability of nanofluid. Int. J. Heat Mass Transfer 55 (5), 1529–
1535.
Onsager, L., 1931a. Reciprocal relations in irreversible processes. I. Phys.
Rev. 37 (4), 405.
Onsager, L., 1931b. Reciprocal relations in irreversible processes. II. Phys.
Rev. 38 (12), 2265.
Otanicar, T.P. et al., 2010. Nanofluid-based direct absorption solar
collector. J. Renew. Sust. Energy 2, 033102.
Pop, E. et al., 2006. Thermal conductance of an individual single-wall
carbon nanotube above room temperature. Nano Lett. 6 (1), 96–100.
Pradhan, N. et al., 2009. The specific heat and effective thermal
conductivity of composites containing single-wall and multi-wall
carbon nanotubes. Nanotechnology 20 (24), 245705.
Said, Z. et al., 2013. Experimental investigation of the thermophysical
properties of Al2O3-nanofluid and its effect on a flat plate solar
collector. Int. Commun. Heat Mass 48 (0), 99–107.
Saidur, R. et al., 2012. Evaluation of the effect of nanofluid-based
absorbers on direct solar collector. Int. J. Heat Mass Transfer.
Saito, R. et al., 1998. Physical Properties of Carbon Nanotubes. World
Scientific.
Sajid, M.H. et al., 2014. A time variant investigation on optical properties
of water based Al2O3 nanofluid. Int. Commun. Heat Mass 50, 108–
116.
Sani, E. et al., 2011. Potential of carbon nanohorn-based suspensions for
solar thermal collectors. Sol. Energy Mater. Sol. Cell 95 (11), 2994–
3000.
Standard, A., 1977. 93–77, Methods of Testing to Determine the Thermal
Performance of Solar Collectors. American Society of Heating,
Refrigeration and Air Conditioning Engineers, New York (USA).
Sukhatme, S.P., 2008. Sol. Energy: Principles of Thermal Collection and
Storage. Tata McGraw-Hill Education.
Sun, Z. et al., 2008. Quantitative evaluation of surfactant-stabilized single-
walled carbon nanotubes: dispersion quality and its correlation with
zeta potential. J. Phys. Chem. C 112 (29), 10692–10699.
Syam Sundar, L., Sharma, K., 2008. Thermal conductivity enhancement
of nanoparticles in distilled water. Int. J. Nat. 1 (1), 66–77.
Tans, S.J. et al., 1997. Individual single-wall carbon nanotubes as
quantum wires. Nature 386 (6624), 474–477.
Taylor, R.A. et al., 2011. Applicability of nanofluids in high flux solar
collectors. J. Renew. Sust. Energy 3, 023104.
Tiwari, A.K. et al., 2013. Solar water heating using nanofluids – a
comprehensive overview and environmental impact analysis. Int. J.
Emerg. Technol. Adv. Eng. 3 (3), 221–224.
Tsyboulski, D.A. et al., 2008. Translational and rotational dynamics of
individual single-walled carbon nanotubes in aqueous suspension.
ACS Nano 2 (9), 1770–1776.
Tyagi, H. et al., 2009. Predicted efficiency of a low-temperature nanofluid-
based direct absorption solar collector. J. Sol. Energy Eng. 131 (4).
Ucar, A., Inallı, M., 2006. Thermal and exergy analysis of solar air
collectors with passive augmentation techniques. Int. Commun. Heat
Mass 33 (10), 1281–1290.
Wang, B.-X. et al., 2006. Surface and size effects on the specific heat
capacity of nanoparticles. Int. J. Thermophys. 27 (1), 139–151.
Wen, D., Ding, Y., 2004. Effective thermal conductivity of aqueous
suspensions of carbon nanotubes (carbon nanotube nanofluids). Int. J.
Thermophys. Heat Transfer 18 (4), 481–485.
Xie, H. et al., 2003. Nanofluids containing multiwalled carbon nanotubes
and their enhanced thermal conductivities. J. Appl. Phys. 94 (8), 4967–
4971.
Yousefi, T. et al., 2012a. An experimental investigation on the effect of pH
variation of MWCNT–H2O nanofluid on the efficiency of a flat-plate
solar collector. Sol. Energy 86 (2), 771–779.
Yousefi, T. et al., 2012b. An experimental investigation on the effect of
MWCNT–H2O nanofluid on the efficiency of flat-plate solar collectors.
Exp. Therm. Fluid Sci. 39, 207–212.
Yousefi, T. et al., 2012c. An experimental investigation on the effect of
Al2O3–H2O nanofluid on the efficiency of flat-plate solar collectors.
Renew. Energy 39 (1), 293–298.
Yu, C. et al., 2005. Thermal conductance and thermopower of an
individual single-wall carbon nanotube. Nano Lett. 5 (9), 1842–1846.
Yu, W. et al., 2008. Review and comparison of nanofluid thermal
conductivity and heat transfer enhancements. Heat Transfer Eng. 29
(5), 432–460.
Zhang, S. et al., 2003. Specific heat of single-walled carbon nanotubes.
Phys. Rev. B 68 (7), 075415.
Z. Said et al. / Solar Energy 115 (2015) 757–769 769