2. (European Solar Thermal technology Platform), they will design
and establish “Active Solar Building” as a standard of new buildings
where solar energy will meet 100% of their heating and cooling
demand and “Active Solar Renovation” as a standard for the
refurbishment of existing buildings where solar thermal energy
will meet at least 50% of heating and cooling demand of the
renovated buildings (ESTIF, 2011). Solar radiation can also be con-
verted to electricity by photovoltaic technology which aims to
reduce 30% overall emission in Netherland by 2020 (Vasseur et al.,
2013). Energy performance can be enhanced by using a hybrid
photovoltaic and thermal (PVT) collector tool, which uses water as a
coolant. The electrical and thermal efficiencies of both can be
increased by improving the thermal contact between the thermal
absorber and the PV module (He et al., 2006). Solar collectors and
thermal energy storage systems are two important applications of
this technology (Tian and Zhao, 2013).
Several articles examine the prospects of improving collector
efficiency by using nanofluids on FPSC. According to
Duangthongsuk and Wongwises (2009), most of the solids have
higher thermal conductivity than that of conventional cooling
fluids (e.g. water, oil and ethylene). Otanicar and Golden (2009)
compared the environmental and economic influences of conven-
tional and nanofluid solar collectors. Otanicar et al. (2010) con-
ducted experiment on direct absorption solar collector using
different nanofluids. They used nanofluids as absorption medium
and found that the efficiency improved by 5% in a solar thermal
collector. The experimental result was also compared with a nu-
merical model. A novel approach for utilizing nanofluids to improve
the energy density and to enhance heat transfer from solar collector
to storage tanks was proposed by Das et al. (2008). Natarjan and
Sathish (2009) also used nanofluids in place of conventional heat
transfer fluid to enhance the efficiency of solar water heater. Tiwari
et al. (2013) used a Al2O3 nanofluid to improve the thermal effi-
ciency of a solar collector by 31.64%, when using an optimum Al2O3
particle volume fraction of 1.5%. A similar experiment was done by
Yousefi et al. (2012b) to investigate the effect of Al2O3eH2O nano-
fluid on the efficiency of FPSC. They improved the efficiency of a
solar collector by 28.3%, while using a volume fraction of 0.2% Al2O3
in their nanofluid. Yousefi et al. (2012b) increased the efficiency of
their solar collector by 28.3% with a Al2O3 nanofluid; and 35% using
a nanofluid based on multi wall carbon nanotubes based.
Gangadevi et al. (2013) conducted experiments that showed
that the thermal efficiency of a FPSC could be improved by
increasing the nanoparticle volume fraction and therefore the
nanofluid's thermal conductivity. Efficiency increased to about 30%
by using Al2O3 water nanofluid as a working medium. Masuda et al.
(1993) studied the thermo physical properties of (Al2O3 and TiO2)
immersed in water. They reported that the thermal conductivity of
Al2O3 water nanofluids and TiO2 water nanofluids at a volume
fraction of 4.3% are about 32% and 11% higher than pure water,
respectively.
Numerous additional authors (Reddy et al., 2012) (Turgut et al.,
2009; Wamkam et al., 2011; Xie et al., 2010) have studied the effect
of increasing the thermal conductivity by increasing the concen-
tration of nanoparticles in fluids. Wang et al. (2007) showed that
the thermal conductivity of nanofluids can be improved by
reducing the nanoparticle's size. Viscosity is directly related to
pumping power and pressure drop for a solar collector. Demir et al.
(2011) obtained numerical results showing that a high concentra-
tion of nanoparticles (Al2O3 or TiO2) in water yielded higher heat
transfer rates and pressure drops. Fotukian and Nasr Esfahany
(2010) experimentally examined the influence of dilute Al2O3/wa-
ter nanofluid on the heat transfer and the pressure drop inside a
circular tube under turbulent flow conditions. His work showed
that increasing the volume of nanoparticles to a base fluid,
improved the heat transfer coefficient. However, addition of
nanoparticles also increased the viscosity of fluid, requiring greater
energy for pumping the fluid in the circuits. This consequently
decreased the improvement in heat transfer efficiency.
Research has been conducted using Al2O3 based nanofluids, but
limited studies have been carried out on TiO2 nanofluids. Consid-
ering the increasing global demand for solar energy, it is very
important to find new, effective, and convenient approaches to
improve the efficiency rate of a FPSC. Thus, the aim of this experi-
ment is to study the effect of TiO2 nanofluid as a working fluid and
its effect on performance.
Abbreviations and nomenclature
Ac collector area, m2
Cp specific heat, J/kg K
d diameter of pipe, m
_Exin exergy rate at inlet, W
_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
t shear stress;
kp thermal conductivity of nanoparticle, W/m K
K loss coefficient (dimensionless)
_m mass flow rate, kg/s
_W work rate or power, W
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, JKÀ1
molÀ1
hin specific enthalpy at inlet, J/kg
hout specific enthalpy at outlet, J/kg
m coefficient of viscosity
Ts sun temperature, K
Tsur surrounding/ambient temperature, K
m viscosity, N s/m2
t transmittance
a absorptance
4 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
r density, kg/m3
s overall entropy production, J/kg K
f friction factor
h specific enthalpy, J/kg
_g shear strain rate.
Z. Said et al. / Journal of Cleaner Production 92 (2015) 343e353344
3. 2. Hypothetical background
All the analysis has been carried under steady-state and steady-
flow conditions. This investigation begins by analyzing solar water
heating (SWH) using the first and second laws of thermodynamics
applied to varying absorbing surfaces; in other words, an assess-
ment from the energetic and exergetic points of view.
2.1. First law of thermodynamics (energy) analysis
A theoretical model can be derived using a thermal energy
balance equation (shown in Eq. (1)).
MpCp
À
dTp;ave
dt
Á
þ _mCpðTout À TinÞ ¼ h0IAc À Uc
À
Tp;ave À Te
Á
Ac
(1)
The thermal efficiency of the FPSC (h) is therefore:
h ¼ _mCpðTout À TinÞ=IAc (2)
2.2. Second law of thermodynamics (exergy) analysis
The exergy efficiency (hex) can be derived from the first and
second laws of thermodynamics. This is a steady flow system that
operates under steady state conditions. Both the potential and ki-
netic energy are negligible. The thermo physical properties of
nanofluids flowing in and out of the collector are constant. The
work transfer from the system and the heat transfer to the system
are positive. The loss coefficient only accounts for the entry effect.
Therefore the general exergy balance for a steady state and
steady flow process is:
_Exheat À _Exwork À _Exmass;in À _Exmass;out ¼ _Exdest (3)
Substituting terms into this equation yields:
X
1À
Ta
Tsur
_Qs À _W þ
X
_minJin À
X
_moutJout ¼ _Exdest (4)
X
1 À
Ta
Tsur
_Qs À _m½ðhout À hinÞ À Taðsout À sinÞŠ ¼ _Exdest (5)
where _Qs is the total rate of the exergy received from the solar
radiation by the collector absorber area (Esen, 2008);
_Qs ¼ IT ðtaÞAc ¼ S$Ac (6)
The change in enthalpy and entropy of the nanofluid in the
collector is:
Dh ¼ hout À hin ¼ Cp;nf
Tf ;out À Tf ;in
(7)
Ds ¼ sout À sin ¼ Cp;nf ln
Tf ;out
Tf ;in
À R ln
Pout
Pin
(8)
Replacing Eqs. (6)e(8) in Eq. (5) yields:
1 À
Ta
Tsur
IT ðtaÞAc À _mCp;nf
Tf ;out À Tf ;in
þ _mCp;nf Ta ln
Tf ;out
Tf ;in
À _mRTa ln
Pout
Pin
¼ _Exdest
(9)
where _Exdest is the exergy loss (or irreversibility) rate defined as:
_Exdest ¼ Ta
_Sgen (10)
Therefore, the exergy efficiency is given by the following
equation:
hex ¼ 1 À
Ta
_Sgen
½1 À ðTa=TsÞŠ _Qs
(11)
2.3. Pumping power and pressure drop
A pump is needed to circulate nanofluids all the way through the
system. The pumping power and pressure drop were calculated
using Eq. (12).
Dp ¼ f
rV2
2
Dl
d
þ K
rV2
2
(12)
The loss coefficient K is calculated using formulas that consist of
the density and kinematic viscosity of the heat transfer fluid.
Pumping power is calculated by Eq. (13) (Said et al., 2013b):
Pumping power ¼
_m
rnf
!
 Dp (13)
3. Methodology
Commercial spherical shape TiO2 (99.5% trace metal, Sigma
Aldrich, Malaysia) having an average diameter of ~21 nm were
used. Polyethylene glycol 400 (PEG) (Sigma Aldrich) was used as a
surfactant. Distilled water was used as a base fluid.
3.1. Stability characterization and data collection
Nanoparticles were suspended in base fluid to reduce the ag-
gregation of TiO2. Two methods were executed in this study. The
first method used polyethylene glycol 400 (PEG, Sigma Aldrich) as a
dispersant. The second method used a highly pressurized homog-
enizer (capacity of up to 1700 bar) to optimize the dispersion of
nanoparticles (0.1 vol% and 0.3 vol%) into the distilled water base
fluid. The homogenizer dispersed the well-isolated primary parti-
cles (Bobbo et al., 2012). The TiO2 nanoparticles with 0.1 vol% and
0.3 vol% were added to the base fluid to obtain a homogenously
dispersed solution, after passing the solution through 30 cycles in a
high pressure homogenizer. For the watereTiO2 nanofluid, the ratio
between the nanoparticles and the dispersant mass was 1:2.
The morphological characterization of the nanoparticles was
accomplished by a field emission scanning electron microscopy.
The SEM pictures of TiO2 are shown in Fig. 4. The actual dimensions
of the TiO2 nanoparticles are between 20 and 40 nm. A Zeta-seizer
Nano ZS was employed to examine the average size of the nano-
particles in the base medium, along with the zeta potential value.
The stability time of TiO2eH2O is further supported by visual im-
ages presented in this paper. The density of the nanofluid was
determined using the DA-130N Density Meter (Kyoto Electronics).
The thermal conductivity of the nanofluid was measured using a
KD2 Pro thermal property analyzer (Decagon Devices).
3.2. Location of the study
The solar collector was investigated at the University Malaya,
Malaysia for the experiment. The tilt angle has major impact on the
amount of energy that the system can capture, for non-tracking
solar collectors. The tilt angle used for this experiment is 22,
which is the optimal tilt angle to obtain the highest average daily
radiation.
Z. Said et al. / Journal of Cleaner Production 92 (2015) 343e353 345
4. 3.3. Experimental approach
The schematics of the solar collector and the experiment are
presented in Figs. 1 and 2, respectively. The dimensions of the solar
collectors are listed in Table 1. Table 1 also includes the physical
properties of the TiO2 nanoparticles and water used in the calcu-
lations. Table 2 presents the specifications of the FPSC that are
studied in this experiment. For the force convection system, an
electric pump is used in the solar water heating system. Fig. 2
shows that the tank with a capacity of 50 L absorbs the heat load
form the collector cycle. The heat load of the solar cycle is trans-
ferred to the water by a heat exchanger, which is used outside the
tank. Fig. 2 illustrates that a flow meter is linked to the water pipe
before the electric pump.
A simple valve is connected after the electric pump, which
regulates the mass flow rate of the working fluid. Five K thermo-
couples are used to assess the fluid temperatures at: the inlet and
outlet of the solar collector, the left and right panels of the solar
collector, and the outside environment. A 10 channel data logger is
connected to the sensors. The Li-COR Pyranometer (TES 1333R Solar
Power Meter) was used to record the total solar radiation whereas
PROVA (AV M-07) Anemometer was used to record the speed of the
wind. A pressure sensor was used to measure the pressure differ-
ence between the inlet and outlet of the solar collector. All of the
data were later transferred into the computer via USB interfaces.
Measurements were taken multiple times and averaged to reduce
experimental error.
3.4. Testing process
The ASHARE Standard 93-2003 (Standard, 1977) was used to
evaluate the thermal performance of FPSC. The incident radiation,
ambient temperature, outlet fluid temperature and inlet fluid
temperature are used to obtain the thermal performance of the
solar collector.
3.5. Inaccuracy analysis
Two potential sources of error were considered. The first error
arises from the direct measurement parameters such as:DGc, DT,
DP, and m. The second error arises from indirect measurements
such as: energy and exergy efficiencies. The subsequent relations
are derived from the Luminosu et al. (Luminosu and Fara, 2005)
method:
Dhex ¼
DI
:
E
:
xheat
þ
I
:
E
:
xheat
E
:
x2
heat
(14)
and
Dhen ¼
Dqa
:
Gc
þ
qa
:
DGc
G2
c
(15)
where each error component can be evaluated through the
following relations:
DExheat ¼
DT
Ts
þ
TaDT
T2
s
!
AcðtaÞGc þ
1 À
Ta
Ts
AcðtaÞDGc (16)
DI
:
¼ TaDS
:
gen þ S
:
genDT (17)
DS
:
gen ¼
R ln
Pout
Pin
þCp ln
Tin
Tout
þCp
Tout þTin
Ta
Dm
:
þGcAcðtaÞ
DT
T2
a
þm
:
Cp
1
Tout
þ
1
Tin
þ
2
Ta
þ
ðTout þTinÞ
T2
a
!
DT
þm
:
R
1
Pout
þ
1
Pin
DP þAcðtaÞ
1
Ts
þ
1
Ta
DGc
(18)
Dq
:
a ¼ Cp
Dm
:
ðTout þ TinÞ þ 2m
:
DT
Ac
!
(19)
4. Results and discussion
The errors are ±2.25 W/m2
, ±1 C, ±0.00144 kg/min and ±1.1%,
in measuring the solar radiation, temperature, pressure and mass
flow rate, respectively. Hence, the maximum errors (uncertainties)
associated with energy and exergy efficiencies are estimated to be
±0.04 and ±0.14 using Eqs. (14) and (15).
Fig. 1. Experimental setup for the study conducted: (a) Front part (b) Back part; photographs.
Z. Said et al. / Journal of Cleaner Production 92 (2015) 343e353346
5. 4.1. Nanofluids stability characterization
Fig. 3 displays the particle size distribution according to the
intensity obtained from the Zeta-seizer for 30 days. Fig. 4 presents
SEM and TEM images for characterization of the nanoparticles. The
best appropriate preparation technique and surfactant type and
concentration have been used in order to optimise the stable
nanofluids by examining the average size distribution of the par-
ticles with respect to time. In fact, after dispersing in the base fluids,
these nanoparticles can aggregate and settle down, resulting in
lower stability of the nanofluid and therefore affecting their
application. On the other hand, the surrounding face of the TiO2
nanoparticles is adsorbed by the PEG molecules, creating a com-
pressed layer around the particles. Steric effects stabilize the fluid
and result in the formation of more dense aggregates (Alphonse
et al., 2009).
Fig. 5 shows the visual appearances of the nanofluids with no
sign of aggregation for a period of a month.
Since these fluids will likely be used in a FPSC, where forced
circulation takes place resulting in a continuous mixing condition,
the settling effects of the nanoparticles are negligible. Measure-
ments revealed that the nanoparticles were larger than the size
listed by the supplier. The measured average diameter is 127 nm
and 136 nm for of 0.1vol% and 0.3vol%, respectively. This shows a
tendency of Titania particles in liquid media to aggregate, however
they are still distinctly nanometric.
Nanofluids with an average particle size of 225.9 nm were ob-
tained for the samples after 30 days, with high zeta potential value
of 41.8 mV (presented in Table 3). Zeta potential higher than 30 mV
was obtained for all the samples prepared.
4.2. Density of TiO2 nanofluid
The density of the nanofluids is proportional to the volume ratio
of nanoparticles (solid) and base fluid (liquid) in a system. The base
fluid also plays a significant role in the density of the nanofluids,
whereas the other parameters, such as nanoparticles shape, size,
zeta potential and additives, do not affect the density of the
nanofluids. Pak and Cho (1998) conducted an experiment (at a
single temperature of 25 C) for Al2O3 and TiO2 nanofluids up to 4
vol% to prove the mixing theory. Not enough density measurement
data was available for various nanofluids at varying temperatures in
the literature. Therefore, comprehensive measurements were car-
ried out to obtain this data, as well as verify the applicability of Eq.
Fig. 2. The presentation of the experimental setup in schematic diagram.
Table 1
Physical characteristic of TiO2 and base fluid (Bayat and Nikseresht, 2012; Kamyar
et al., 2012; Said et al., 2013a; Sridhara and Satapathy, 2011).
Particle
base fluid
Average particle
size (nm)
Actual density
(kg/m3
)
Cp(J/kg K) K (W/mK) Viscosity
(mPa s)
TiO2 21 4230 692 8.4
Water 997.1 4179 0.605 0.89
Table 2
Specification and environmental conditions for the FPSC.
Parameters of collector Value
Frame Aluminum alloy
Glazing 4 mm tempered texture glass
Working fluids in flow ducts Water and TiO2 based nanofluid
Absorption area, Ap 1.84 m2
Wind speed 5 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 F22 mm  t0.6 mm (2 pcs)
Riser tube material Copper TP2
Riser tube size F10 mm  t0.45 mm (8 pcs)
Z. Said et al. / Journal of Cleaner Production 92 (2015) 343e353 347
6. (20). Density with respect to concentration and temperature are
presented in Fig. 6 and Fig. 7.
rnf ¼
m
V
nf
¼
mf þ mp
Vf þ Vp
¼
rf Vf þ rpVp
Vf þ Vp
¼
À
1 À fp
Á
rbf þ fprp
(20)
where, 4p ¼ Vp=Vf þ Vp is the volume fraction of the nanoparticles.
The density of the nanofluids of TiO2 using distilled water as the
base fluid was measured at different temperature and different
concentrations (vol%). It was found that the density decreases as
either temperature or volume concentration increases. The density
values at 25 C of TiO2 nanofluid showed an outstanding agreement
with Pak and Cho equation (1998). The highest deviance between
the experimental values and the model is 0.22% for TiO2, which is
very small and below the minimum acceptable limit (1%).
4.3. Thermal conductivity
Previous research shows that the stability of nanoparticles re-
duces when the concentration of nanoparticles increases, because
the nanoparticles tend to agglomerate (Said et al., 2013b) (Said
et al., 2013a) (Vatanpour et al., 2011) (Said et al., 2013c). In order
to get stability for a longer period of time, nanoparticles with a
lower volume fraction can be immersed into a base fluid, which is
comparatively more stable than the nanofluids with higher volume
fraction.
Fig. 8 shows the improvement of thermal conductivity at a
volume fraction of 0.1%e0.3%, respectively using TiO2eH2O nano-
fluids. The figure shows that thermal conductivity rises with
increased volume concentration and also temperature. These re-
sults closely matched with the results obtained by Fedele et al.
(2012). The experimental outcomes demonstrated a peak in the
improvement factor in this range of volume fractions in the tem-
perature range calculated, which suggests that an optimum size
exists for dissimilar nanoparticle and base fluid mixtures. This
occurrence can be neither projected nor clarified using the hypo-
thetical models presently existing in the literature.
The thermal conductivity of 0.3 vol % of TiO2 enhances up to 6%
and from the measured data, the thermal conductivity is found to
be directly proportional to the volume fraction. The thermal con-
ductivity surges to 6% from 2.4%. Therefore, the correlation between
the volume fraction and the thermal conductivity is directly pro-
portional. The values used in the experimental results presented
are confirmed with other researchers in Fig. 8 (Chon and Kihm,
2005; Das et al., 2003; Fedele et al., 2012; Mintsa et al., 2009) (Li
and Peterson, 2006). To get more heat rate from the solar collec-
tor, it is necessary to have higher thermal conductivity, which is
possible when the concentration of nano particles is higher. It was
also observed that the difference between the inlet and the outlet is
higher comparing to water which is the ultimate result of the
increasing volume fraction. The heat loss is less from the collector
using nanofluid than water and thus the nanofluid can be used to
obtain as much higher heat transfer rate. From Fig. 8, it is obvious
that the increased volume fraction and the rising temperatures
increase the thermal conductivity as well. In conventional collec-
tors, the increasing fluid temperature may cause an increase in the
heat loss.
4.4. Viscosity of TiO2 nanofluid
In this content, an important matter is to achieve nanofluid
physical information, and one of the possible techniques is through
a comprehensive rheological investigation (Chen and Ding, 2009).
In this experiment, three kinds of studies, namely viscosity as a
function of shear rate, temperature as well as at different mass
concentrations, have been conducted.
Uncertainty in experimental data is illustrated in Fig. 9. The
experimental values of the viscosity of the nanofluid and the
Fig. 3. Size of watereTiO2 0.1% nanofluids with Polyethylene glycol 400, for a period of
30 days.
Fig. 4. (a) SEM image of TiO2 nanoparticles. (b) TEM image of TiO2/water using control pH ¼ 9.
Z. Said et al. / Journal of Cleaner Production 92 (2015) 343e353348
7. ASHARE data match fairly with a highest deviation of ±2.0% with
temperature ranging from 0 C to 80 C. The machine error results
in the deviation at low shear rates.
The governing equation for the Newtonian behavior of a fluid is
represented by
t ¼ m _g (21)
The relation between the shear stress and the shear rate is linear
in a Newtonian fluid, passing through the origin; the constant of
proportionality is called the coefficient of viscosity. In a non-
Newtonian fluid, the line never passes through the origin. From
55 C, the TiO2 nanofluid with 0.1 vol% was Newtonian, whereas
below this temperature, TiO2 nanofluid with 0.1 vol% was non-
Newtonian. For 0.3 vol% of TiO2, a Newtonian behavior was
observed in the entire temperature range in Fig. 10. It is noticed
from the results that the viscosity for the fresh samples and that of
the samples after running in FPSC are not the same anymore. The
deviation among the values for the similar volume fractions is
obtained. This finding is not explained anywhere else in the liter-
ature, therefore opening scope for new research field of nanofluids
and their effects on applications in the long run.
4.5. Time range for experiment
The hourly time range of the experiment was 9:00 am to
5:00 pm (MST) in order to utilize the availability of the sun in
Malaysia. To minimize the variance of solar radiation, the experi-
ment was done in consecutive days. Solar radiation data (on both a
clear day and a cloudy day) are included in Fig. 11.
Numerous test runs were performed by distributing all the
presented data. Quasi steady conditions were used in each test run
(divided into several test periods) (Ucar and Inallı, 2006).
4.6. Mass flow rate
The mass flow rate of a nanofluid was varied using a flow meter.
Nanofluids with 0.1 vol% and 0.3 vol% nanoparticle were used as
working fluids. Experiments were performed at mass flow rates of
0.5, 1.0 and 1.5 kg/min to investigate the efficiency of the collector.
Fig. 5. Prepared TiO2 nanofluid solutions (a) Samples on the first day of preparations (b) Samples after 30 days of preparations.
Table 3
Zeta potential, particle size and pH values of TiO2/water 21 nm particles suspended
in water.
Nominal
particle size
Zeta
potential (mV)
Particle size (nm)
from DLS using
high pressure
homogenizer
No. of days
21 nm 48.6 126.9 1st
21 nm 46.2 160.7 7th
21 nm 45.8 202.0 21st
21 nm 41.8 225.9 30th
Fig. 6. Density vs. concentration graph of TiO2ewater nanofluid at different
concentrations.
Fig. 7. Density vs. temperature graph of TiO2ewater nanofluid at different
concentrations.
Z. Said et al. / Journal of Cleaner Production 92 (2015) 343e353 349
8. The results, presented in Figs. 12e16, show that the mass flow rate
has an impact on the efficiency of the collector. The relationship
between solar energy and the mass flow rate is shown in Eq. (22).
_Qu ¼ _mCpðTout À TinÞ (22)
Eq. (22) shows that the solar energy is directly proportional to
the mass flow rate.
4.7. Energy and exergy efficiencies
Fig. 12 shows the energy and exergy efficiency of the FPSC. This
was determined by varying the mass flow rate of the nanofluid (for
various volume fractions) by 0.5, 1.0 and 1.5 kg/min. Each investi-
gation was repeated for several days to determine measurement
error. This efficiency was evaluated using Eqs. (2) and (11),
respectively and input from Tables 2 and 3.
From Fig. 12 it can be seen that the efficiency of the solar col-
lector decreases as mass flow rate reduces. However, the efficiency
of the solar collector increases as the volume fraction of the
nanoparticles increases. A similar result has been reported in a past
journal article (Otanicar and Golden, 2009). This increase in effi-
ciency is due to the increase in the nanofluid's thermal conduc-
tivity, which elevates the convective heat transfer coefficient. The
energy efficiency for the TiO2eH2O nanofluid increased by 76.6% for
0.1 vol% and 0.5 kg/min, whereas energy efficiency increased by
67.9% for 0.3 vol% and 0.5 kg/min. For water the highest energy
efficiency that was obtained was 42.1% for a flow rate of 0.5 kg/min.
As it seen in Fig. 12 reducing the flow rate to approximately 0.5 kg/
min causes a considerable reduction in the absorber plate's tem-
perature. The decrease in temperature gradient between the
absorber plate and the environment reduces the overall heat loss
coefficient, improving the thermal efficiency of the collector.
In a flat plate solar collector, entropy generation reduction is
more significant for higher temperature systems (Bejan, 1996)
(Bejan et al., 1981). The reduction of the entropy generation rate is
equivalent to an increase in power output. Exergy efficiency re-
duces as the volume fraction reduces or mass flow rate increases.
The TiO2eH2O nanofluid shows improved values of efficiency
matched to water as a base fluid. By using the TiO2eH2O nanofluid
in a solar collector as an operational medium, the exergy efficiency
can be improved. This makes it a better alternative than water as an
absorbing medium. The experimental results show that by adding
in a small amount of nanoparticles (up to 0.1 vol%); the exergy ef-
ficiency is improved by 16.9%, compared to a pure water coolant. As
Fig. 8. Thermal conductivity of TiO2eH2O nanofluid with different volume fraction
and changing temperature.
Fig. 9. Uncertainty in viscosity of TiO2ewater nanofluid with 0.1% volume concen-
tration and at 30 C.
Fig. 10. Viscosity of TiO2ewater nanofluid with different volume concentration and
different temperature.
Fig. 11. Solar radiation readings for a clear day and on a cloudy day.
Fig. 12. The energy efficiency (No Fill) and exergy efficiency (solid Fill) of a FPSC at
different mass flow rates and different volume fractions for TiO2eH2O nanofluid.
Z. Said et al. / Journal of Cleaner Production 92 (2015) 343e353350
9. it is shown in Fig. 12, reducing the flow rate to approximately
0.5 kg/min increases the collector exergy. The exergy loss at high
mass flow rates is primarily due to heat transfer from the absorber
plate to the working fluid. The main cause of exergy loss in collector
is heat transfer due to the difference between the temperature of
the absorber plate and the solar radiation. The increase in the
absorber plate temperature leads to an increase in the difference
which consequently decreases collector exergy loss. It can be seen
that the minimum exergy efficiency belongs to the higher mass
flow rate.
4.8. Entropy generation and exergy destruction
Entropy is produced in irreversible processes. Therefore, for the
energy optimization analysis, it is necessary to assess entropy
generation or exergy destruction due to heat transfer and viscous
friction as a function of the design variables selected (Kreuzer,
1981) (Onsager, 1931a, b). Fig. 13 presents the entropy generation
and exergy destruction with regard to mass flow rate and different
volume concentrations.
Fig. 13 shows that increasing the volume fraction of the nano-
fluid reduces entropy generation. The thermal conductivity im-
proves as the volume concentration of the nanoparticles increases.
The subsequent higher heat transfer reduces the irreversibility
generated in the system. This has a much greater effect than the
losses associated with viscous flow. The lowest entropy generation
is observed for 0.1 vol%, and 0.3 vol% of the TiO2eH2O nanofluid,
whereas the highest was observed for water, the values are 33.43 J/
K, 40.95 J/K and 43.54 J/K, respectively for a flow rate of 0.5 kg/min.
The TiO2eH2O nanofluid shows reduced exergy destruction
with the increasing flow rate (compared to water), resulting in
lower exergy destruction. Entropy generation is also less than water
as a working fluid. Similar patterns were observed for the exergy
destruction. Therefore, the TiO2eH2O nanofluid performs better
than using water as a working fluid.
4.9. Output temperature
Fig. 14 shows the effect of mass flow rate on the output tem-
perature. The output temperature has a strong and direct propor-
tional effect on the energy efficiency of a FPSC.
Solar collectors that use nanofluids are more efficient than
conventional collectors, due to their higher output temperature.
Specific heat is defined as, “The heat required to raise the tem-
perature of a unit mass of a substance by one unit of temperature.”
It is clear from the definition that any substance, which has a lower
specific heat, should provide greater temperature for equal heat
flow.
4.10. Overall energetic and exergetic efficiencies using TiO2eH2O
nanofluid
The overall energetic and exergetic efficiencies over the testing
period for our experiment is presented in Fig. 15. The improved
energetic and exergetic efficiencies are witnessed for the studied
nanofluid. It can also be observed from Figs. 11 and 15 that the
maximum irreversibility occurs at noon, when the solar radiation is
maximum and decreases as the solar energy decreases.
The temperature difference between the collector and the
environment has an ideal point for the exergy efficiency, and a
larger difference can result in lower exergy efficiency. However, an
increase in temperature difference reduces the energy efficiency,
because more heat is lost to the environment. According to the
results from the experiments, the TiO2eH2O nanofluid is found to
be more appropriate as a working medium for the flat plate solar
water heater than water. The energy efficiency increased by 76.6%
for 0.1 vol% and 0.5 kg/min, whereas the highest second law effi-
ciency achieved is 16.9% for 0.1 vol% and 0.5 kg/min.
4.11. Pumping power and pressure drop for TiO2 nanofluid
For the nanofluid to be used in the solar collector appropriately,
it is important to investigate the flow resistance of nanofluids in
order to increase the heat transfer. The pressure drop of the
TiO2eH2O nanofluids in a FPSC is examined, while considering
laminar flow.
Fig. 13. Effect of mass flow rate and volume fraction on entropy generation (No fill)
exergy destruction (solid fill) for TiO2eH2O nanofluid.
Fig. 14. Output temperature (Solid Fill) and input temperature (No Fill) with respect to
changing mass flow rate for TiO2eH2O nanofluid.
Fig. 15. Overall energetic and exergetic efficiencies over the testing period for the
experimental work.
Z. Said et al. / Journal of Cleaner Production 92 (2015) 343e353 351
10. Fig. 16 shows the effect of the nanofluid on the pumping power
and the pressure drop as a function of volume concentration and
volume flow rate (for laminar flow), respectively. These two pa-
rameters are calculated using Eqs. (12)e(13) and Tables 1 and 2.
The results show that the friction factors of the nanofluids are
very similar to water. This implies that the pumping power
required for nanofluids (with low volume fractions) are the same as
water (Gherasim et al., 2011).
This analysis shows that the addition of nanoparticles in a base
fluid (water) enhances its thermal conductivity and viscosity,
which increases its friction factor. When the nanofluid properties
are properly characterized, the friction factors of the TiO2 nanofluid
are largely in agreement with the classical friction factor theory for
single-phase flow (Tang et al., 2013). Therefore, the friction factor
relationship for the single phase flow can be applied for nanofluids.
However, there was no evidence that use of nanofluids would cost
any significant addition of pumping power.
5. Conclusion
TiO2eH2O nanofluid sustained stable for a period of more than
one month. The thermal conductivity improvement is directly
related to the volume fraction and enhances up to 6% with 0.3 vol%
of TiO2. Viscosity increases with particle loading and reduces with
rising temperature. The energy efficiency increased by 76.6% for 0.1
vol% and 0.5 kg/min, whereas the highest exergy efficiency ach-
ieved is 16.9% for 0.1 vol% and 0.5 kg/min, using the nanofluids in
comparison to the water. The solar collector efficiency using the
TiO2eH2O nanofluid has higher energy and exergy efficiencies than
water.
It can be concluded that the investigation reported here will
deliver the solar investigators with knowledge about enhancing the
solar collector systems using nanofluids. This knowledge is also
required for recognizing energy and exergy conservation oppor-
tunities of these systems.
Acknowledgement
This research is supported by UM High Impact Research Grant
UMRG Project: RP015B-13AET from the Ministry of Higher Educa-
tion Malaysia.
References
Alphonse, P., Bleta, R., Soules, R., 2009. Effect of PEG on rheology and stability of
nanocrystalline titania hydrosols. J. Colloid Interf. Sci. 337, 81e87.
Bayat, J., Nikseresht, A.H., 2012. Thermal performance and pressure drop analysis of
nanofluids in turbulent forced convective flows. Int. J. Therm. Sci. 60, 236e243.
Bejan, A., 1996. Entropy generation minimization: the new thermodynamics of
finiteesize devices and finiteetime processes. J. Appl. Phys. 79, 1191e1218.
Bejan, A., Kearney, D., Kreith, F., 1981. Second law analysis and synthesis of solar
collector systems. J. Sol. Energy Eng 103, 23e28.
Bobbo, S., Fedele, L., Benetti, A., Colla, L., Fabrizio, M., Pagura, C., Barison, S., 2012.
Viscosity of water based SWCNH and TiO2 nanofluids. Exp. Therm. Fluid Sci. 36,
65e71.
Chen, H., Ding, Y., 2009. Heat transfer and rheological behaviour of nanofluidsea
review. In: Advances in Transport Phenomena. Springer, pp. 135e177.
Chon, C., Kihm, K., 2005. Thermal conductivity enhancement of nanofluids by
Brownian motion. J. Heat. Transf. 127, 810.
Das, S.K., Putra, N., Thiesen, P., Roetzel, W., 2003. Temperature dependence of
thermal conductivity enhancement for nanofluids. J. Heat. Transf. 125, 567e574.
Das, S.K., Choi, S.U., Yu, W., Pradeep, T., 2008. Nanofluids: Science and Technology.
Wiley-Interscience, Hoboken, NJ.
Davis, W., Martín, M., 2014. Optimal year-round operation for methane production
from CO2 and water using wind and/or solar energy. J. Clean. Prod. 80, 252e261.
Demir, H., Dalkilic, A., Kürekci, N., Duangthongsuk, W., Wongwises, S., 2011. Nu-
merical investigation on the single phase forced convection heat transfer
characteristics of TiO2 nanofluids in a double-tube counter flow heat exchanger.
Int. Commun. Heat. Mass Transf. 38, 218e228.
Duangthongsuk, W., Wongwises, S., 2009. Measurement of temperature-dependent
thermal conductivity and viscosity of TiO2-water nanofluids. Exp. Therm. Fluid
Sci. 33, 706e714.
Esen, H., 2008. Experimental energy and exergy analysis of a double-flow solar air
heater having different obstacles on absorber plates. Build. Environ. 43,
1046e1054.
ESTIF, 2011. Solar Heating and Cooling for a Sustainable Energy Future in Europe.
Report was prepared by the European Solar Thermal Technology Platform
(ESTTP). http://www.estif.org/fileadmin/estif/content/projects/downloads/
ESTTP_SRA_RevisedVersion.pdf.
Fedele, L., Colla, L., Bobbo, S., 2012. Viscosity and thermal conductivity measure-
ments of water-based nanofluids containing titanium oxide nanoparticles. Int. J.
Refrig. 35, 1359e1366.
Fotukian, S., Nasr Esfahany, M., 2010. Experimental investigation of turbulent
convective heat transfer of dilute g-Al2O3/water nanofluid inside a circular tube.
Int. J. Heat. Fluid Fl 31, 606e612.
Fu, Y., Liu, X., Yuan, Z., 2015. Life-cycle assessment of multi-crystalline photovoltaic
(PV) systems in China. J. Clean. Prod. 86, 180e190.
Gangadevi, R., Senthilraja, S., Imam, S.A., 2013. Efficiency analysis of flat plate solar
collector using Al2O3/water nanofluid. Image 2, 3.
Gherasim, I., Roy, G., Nguyen, C.T., Vo-Ngoc, D., 2011. Heat transfer enhancement
and pumping power in confined radial flows using nanoparticle suspensions
(nanofluids). Int. J. Therm. Sci. 50, 369e377.
Gibbs, J.W., 1873. A Method of Geometrical Representation of the Thermodynamic
Properties of Substances by Means of Surfaces. Connecticut Academy of Arts
and Sciences.
He, W., Chow, T.-T., Ji, J., Lu, J., Pei, G., Chan, L.-s., 2006. Hybrid photovoltaic and
thermal solar-collector designed for natural circulation of water. Appl. Energy
83, 199e210.
Fig. 16. Pumping power (dotted line) and pressure drop (solid line) for TiO2/water nanofluid versus volume fraction and mass flow rate.
Z. Said et al. / Journal of Cleaner Production 92 (2015) 343e353352
11. Kamyar, A., Saidur, R., Hasanuzzaman, M., 2012. Application of computational fluid
dynamics (CFD) for nanofluids. Int. J. Heat Mass Transf. 55, 4104e4115.
Koroneos, C.J., Nanaki, E.A., 2012. Life cycle environmental impact assessment of a
solar water heater. J. Clean. Prod. 37, 154e161.
Kreuzer, H.J., 1981. Nonequilibrium Thermodynamics and its Statistical Foundations.
Clarendon Press, Oxford and New York, 455 p. 1.
Li, C.H., Peterson, G., 2006. Experimental investigation of temperature and volume
fraction variations on the effective thermal conductivity of nanoparticle sus-
pensions (nanofluids). J. Appl. Phys. 99, 084314.
Luminosu, I., Fara, L., 2005. Determination of the optimal operation mode of a flat
solar collector by exergetic analysis and numerical simulation. Energy 30,
731e747.
Masuda, H., Ebata, A., Teramae, K., Hishinuma, N., 1993. Alteration of thermal
conductivity and viscosity of liquid by dispersing ultra-fine particles. Netsu
Bussei 7, 227e233.
Mintsa, H.A., Roy, G., Nguyen, C.T., Doucet, D., 2009. New temperature dependent
thermal conductivity data for water-based nanofluids. Int. J. Therm. Sci. 48,
363e371.
Natarajan, E., Sathish, R., 2009. Role of nanofluids in solar water heater. Int. J. Adv.
Manuf. Tech. 1e5.
Nguyen, M., Ziemski, M., Vink, S., 2014. Application of an exergy approach to un-
derstand energy demand of mine water management options. J. Clean. Prod. 84,
639e648.
Onsager, L., 1931a. Reciprocal relations in irreversible processes. I. Phys. Rev. 37, 405.
Onsager, L., 1931b. Reciprocal relations in irreversible processes. II. Phys. Rev. 38,
2265.
Otanicar, T.P., Golden, J.S., 2009. Comparative environmental and economic analysis
of conventional and nanofluid solar hot water technologies. Environ. Sci.
Technol. 43, 6082e6087.
Otanicar, T.P., Phelan, P.E., Prasher, R.S., Rosengarten, G., Taylor, R.A., 2010. Nano-
fluid-based direct absorption solar collector. J. Renew. Sust. Energ. 2, 033102.
Pak, B.C., Cho, Y.I., 1998. Hydrodynamic and heat transfer study of dispersed fluids
with submicron metallic oxide particles. Exp. Heat Transf. Int. J. 11, 151e170.
Reddy, M., Rao, V.V., Reddy, B., Sarada, S.N., Ramesh, L., 2012. Thermal conductivity
measurements of ethylene glycol water based TiO2 nanofluids. Nanosci.
Nanotechnol. Lett. 4, 105e109.
Said, Z., Kamyar, A., Saidur, R., 2013a. Experimental investigation on the stability
and density of TiO2, Al2O3. In: SiO2 and TiSiO4, IOP Conference Series: Earth and
Environmental Science. IOP Publishing, p. 012002.
Said, Z., Sajid, M., Alim, M., Saidur, R., Rahim, N., 2013b. Experimental investigation
of the thermophysical properties of Al2O3-nanofluid and its effect on a flat plate
solar collector. Int. Commun. Heat. Mass Transf. 48, 99e107.
Said, Z., Sajid, M., Saidur, R., Kamalisarvestani, M., Rahim, N., 2013c. Radiative
properties of nanofluids. Int. Commun. Heat. Mass Transf. 46, 74e84.
Sridhara, V., Satapathy, L.N., 2011. Al2O3 based nanofluids: a review. Nanoscale Res.
Lett. 6, 1e16.
Standard, A., 1977. Methods of Testing to Determine the Thermal Performance of
Solar Collectors, 93e77. American Society of Heating, Refrigeration and Air
Conditioning Engineers, New York (USA).
Tang, C.C., Tiwari, S., Cox, M.W., 2013. Viscosity and friction factor of aluminum
oxideewater nanofluid flow in circular tubes. J. Nanotech. Eng. Med. 4, 021004.
Tian, Y., Zhao, C.-Y., 2013. A review of solar collectors and thermal energy storage in
solar thermal applications. Appl. Energy 104, 538e553.
Tiwari, A.K., Ghosh, P., Sarkar, J., 2013. Solar water heating using nanofluids-a
comprehensive overview and environmental impact analysis. Int. J. Emerg.
Tech. Adv. Eng. 3, 221e224.
Turgut, A., Tavman, I., Chirtoc, M., Schuchmann, H., Sauter, C., Tavman, S., 2009.
Thermal conductivity and viscosity measurements of water-based TiO2 nano-
fluids. Int. J. Thermophys. 30, 1213e1226.
Ucar, A., Inallı, M., 2006. Thermal and exergy analysis of solar air collectors with
passive augmentation techniques. Int. Commun. Heat. Mass Transf. 33,
1281e1290.
Vasseur, V., Kamp, L.M., Negro, S.O., 2013. A comparative analysis of photovoltaic
technological innovation systems including international dimensions: the cases
of Japan and The Netherlands. J. Clean. Prod. 48, 200e210.
Vatanpour, V., Madaeni, S.S., Moradian, R., Zinadini, S., Astinchap, B., 2011. Fabri-
cation and characterization of novel antifouling nanofiltration membrane pre-
pared from oxidized multiwalled carbon nanotube/polyethersulfone
nanocomposite. J. Membr. Sci. 375, 284e294.
Wamkam, C.T., Opoku, M.K., Hong, H., Smith, P., 2011. Effects of pH on heat transfer
nanofluids containing ZrO2 and TiO2 nanoparticles. J. Appl. Phys. 109, 024305.
Wang, X.-Q., Mujumdar, A.S., 2007. Heat transfer characteristics of nanofluids: a
review. Int. J. Therm. Sci. 46, 1e19.
Xie, H., Yu, W., Chen, W., 2010. MgO nanofluids: higher thermal conductivity and
lower viscosity among ethylene glycol-based nanofluids containing oxide
nanoparticles. J. Exp. Nanosci. 5, 463e472.
Xydis, G., 2013. On the exergetic capacity factor of a windesolar power generation
system. J. Clean. Prod. 47, 437e445.
Yousefi, T., Shojaeizadeh, E., Veysi, F., Zinadini, S., 2012a. An experimental investi-
gation on the effect of pH variation of MWCNTeH2O nanofluid on the efficiency
of a flat-plate solar collector. Sol. Energy 86, 771e779.
Yousefi, T., Veysi, F., Shojaeizadeh, E., Zinadini, S., 2012b. An experimental investi-
gation on the effect of Al2O3eH2O nanofluid on the efficiency of flat-plate solar
collectors. Renew. Energ. 39, 293e298.
Z. Said et al. / Journal of Cleaner Production 92 (2015) 343e353 353
