energy and exergy efficiency of a flat plate solar collector using ph treated al2o3 nanofluid

lable at ScienceDirect

Journal of Cleaner Production xxx (2015) 1e12

Contents lists avai

Journal of Cleaner Production

journal homepage: www.elsevier .com/locate/ jc lepro

Energy and exergy efficiency of a flat plate solar collector using pHtreated Al2O3 nanofluid

Z. Said a, b, *, R. Saidur c, M.A. Sabiha b, A. Hepbasli d, N.A. Rahim e

a Department of Engineering Systems and Management (ESM), Masdar Institute of Science and Technology, Abu Dhabi, United Arab Emiratesb Department of Mechanical Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysiac Centre of Research Excellence in Renewable Energy (CoRE-RE), King Fahd University of Petroleum and Minerals (KFUPM), Dhahran 31261, Saudi Arabiad Department of Energy Systems Engineering, Faculty of Engineering, Yasar University, 35100 Bornova, Izmir, Turkeye UM Power Energy Dedicated Advanced Centre (UMPEDAC), Level 4, Wisma R&D, University of Malaya, 50603 Kuala Lumpur, Malaysia

a r t i c l e i n f o

Article history:Received 29 April 2015Received in revised form21 July 2015Accepted 22 July 2015Available online xxx

Keywords:NanofluidFlat plate solar collectorEnergyExergyEfficiency improvementAl2O3

* Corresponding author. Department of Engineerin(ESM), Masdar Institute of Science and TechnologEmirates.

E-mail addresses: [email protected], zsaid@mas

http://dx.doi.org/10.1016/j.jclepro.2015.07.1150959-6526/© 2015 Elsevier Ltd. All rights reserved.

Please cite this article in press as: Said, Z., eJournal of Cleaner Production (2015), http:/

a b s t r a c t

Application of nanofluid to increase the thermal efficiency of a traditional solar collector is gettingtremendous attention among the scientific community. Al2O3ewater nanofluid, as a working fluid and itseffect on the energy and exergy efficiencies of a flat plate solar collector was examined experimentally.Volume fraction used for this study was 0.1% and 0.3%, while the size of the nanoparticles was ~13 nm.Experiments were carried out using a stable nanofluid which was obtained by controlling the pH of thesolution over a period of 30 days. The mass flow rates of the nanofluid varied from 0.5 to 1.5 kg/min.Energy and exergy efficiencies of a flat plate solar collector using water and nanofluids as working fluidswere matched. The results revealed that nanofluids increased the energy efficiency by 83.5% for 0.3% v/vand 1.5 kg/min, whereas the exergy efficiency was enhanced by up to 20.3% for 0.1% v/v and 1 kg/min.Thermal efficiency of the system was found to be more than 50% compared to the existing systemavailable in the literature. New findings on the stability and exergy analysis of the solar collector systemoperated with a pH controlled nanofluid are reported.

© 2015 Elsevier Ltd. All rights reserved.

1. Introduction

Solar energy research field is gaining increasing responsivenessin thermal applications, due to their extraordinary performance inenergy storage density and energy conversion efficiency. Solarthermal processing is being used as the cleaner pathways for theproduction of hydrogen, carbon nano particles, industrial carbonblack, and metals with substantially reduced CO2 (Ozalp et al.,2010). For many years solar collectors have been existed. In thecurrent years their practices are undergoing resurgence due to thefocus in renewable energy sources (Joshi et al., 2005; Lee andSharma, 2007; Sutthivirode et al., 2009; Fong et al., 2012; Tianand Zhao, 2013). Flat plate solar collectors have been broadlyused to enhance the working fluid temperature within the range of30 �Ce100 �C. The performance of a flat plate solar collector

g Systems and Managementy, Abu Dhabi, United Arab

dar.ac.ae (Z. Said).

t al., Energy and exergy effic/dx.doi.org/10.1016/j.jclepro.2

depends on the absorption of solar radiation, which then in theform of absorbed energy is transferred to the working fluid insidethe pipes of the solar collector (Kalogirou, 2004). Water, Ethyleneglycol, acetone or a combination of water and ethylene glycol canbe used as theworking fluid (Choi and Eastman,1995; Prasher et al.,2005), but the thermal conductivity of these fluids is low. Flat platesolar collectors are mainly used in domestic hot water system(Zambrana-Vasquez et al., 2015). Therefore, improving the perfor-mance of this type of solar collector is extremely crucial.

The first law of thermodynamics cannot identify the inner lossesfor calculating the flat plate solar collector's efficiency. However,second law of thermodynamics (i.e. exergy analysis) can determineand evaluate the causes of thermodynamic imperfection and ableto indicate the possibilities of thermodynamic improvement of asystem (Amini et al., 2007; Rosen, 2008). Though the first law ofthermodynamics has been used by engineers and scientists but inrecent years exergy concept has gained considerable interests inthe thermodynamic analysis of thermal processes (Koroneos andTsarouhis, 2012). Thus, the aim of this study will be on thecomprehensive energy and exergy investigation of a flat plate solarcollector operated with nanofluids.

iency of a flat plate solar collector using pH treated Al2O3 nanofluid,015.07.115

Delta:1_given name

Delta:1_surname

Delta:1_given name

Delta:1_surname

Delta:1_given name

mailto:[email protected]

mailto:[email protected]

www.sciencedirect.com/science/journal/09596526

http://www.elsevier.com/locate/jclepro

http://dx.doi.org/10.1016/j.jclepro.2015.07.115



Nomenclature

Ac collector area, m2

Cp specific heat, J/kg Kd diameter of pipe, mDH hydraulic diameter, 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_EX;sun exergy rate, Wf friction factorh specific enthalpy, J/kghin specific enthalpy at inlet, J/kghout specific enthalpy at outlet, J/kghnf heat transfer coefficient, W/m2

IT incident solar energy per unit area, W/m2

kp thermal conductivity of nanoparticle, W/m KK loss coefficient (dimensionless)ṁ mass flow rate, kg/sV velocity of fluid, m/sP pressure, PaJ specific exergy, J/kgq convective heat transfer rate, W_Qo heat loss rate to the ambient, W

_Qs energy rate engrossed, W_Qsun;in energy gain rate, WR ideal gas constant, J K�1 mol�1

Re Reynolds number (dimensionless)sa entropy generation to surrounding, J/kg Ksin entropy generation at inlet, J/kg Ksout entropy generation at outlet, J/kg KS absorbed irradiation, W/m2

_Sgen entropy generation rate, W/KTa ambient temperature, KTs sun temperature, KTsur surrounding/ambient temperature, KTW wall temperature, K_W work rate or power, WDl Length of pipe, mDh specific enthalpy change, J/kgDp pressure drop, PaDs change in entropy generation, J/kg Kɳo optical efficiency (dimensionless)hII exergetic efficiency (dimensionless)m viscosity, N s/m2

t transmittancea absorptanceF nanoparticles volume fraction, %r density, kg/m3

s overall entropy production, J/kg Kk thermal conductivity, W/m K

Z. Said et al. / Journal of Cleaner Production xxx (2015) 1e122

Challenged with insufficient energy and material resources andundesirable man made climate changes, science is searching fornew and innovative strategies to save, transfer, and store thermalenergy. Presently, one of the utmost intensively debated alterna-tives is the so-called nanofluids. The suspension of metal or metaloxide nanoparticles and CNTs in a base fluid is known as a nanofluid(Choi and Eastman, 1995). Nanofluids are a new and promisingoption as working fluids in thermosyphons, heat pipes, and solarcollectors (Buschmann, 2013; Said et al., 2015). Alumina is the mostcost effective and widely used material in the family of engineeringceramics (Haddad et al., 2014). A large number of Al2O3-basednanofluids are prepared by the two-step method using an ultra-sonic vibrator, which results in non-stable nanofluids for a longperiod of time. One of the utmost tasks to be accomplished is thestability of nanofluids (Wei et al., 2009; Yu and Xie, 2012) for thebetter thermal performance. Different approaches have beenselected by various authors for preparing stable suspensions usingdifferent surfactant, optimizing the pH, temperature for numerousnanoparticle based fluids, and by surface modification of the par-ticles. Studies on using high pressure homogenizer for preparationof Al2O3-based nanofluids are limited (Sridhara and Satapathy,2011; Bobbo et al., 2012).

Significant enhancements in the thermal conductivity, and heattransfer coefficient of working fluid, are known as the exceptionalphysical effects of nanofluids. Solid phase metals have higherthermal conductivity than the conventional fluids (Bejan and Kraus,2003). Therefore, metal nanoparticles suspended in fluids areanticipated to improve thermal conductivity compared to purefluids. Li and Peterson (Li and Peterson, 2006) dispersed oxidenanoparticles (CuO and Al2O3 with 6% and 10% volume fractions) ina liquid and reported enhancement in thermal conductivity at 34 �Cby a factor of 1.52 and 1.3, respectively. Grimm (Grimm, 1993)

Please cite this article in press as: Said, Z., et al., Energy and exergy efficJournal of Cleaner Production (2015), http://dx.doi.org/10.1016/j.jclepro.2

reported 100% improvement in the thermal conductivity of thenanofluid for 0.5e10 wt. % of alumina nanoparticles suspended inbase fluid.

Recently, several studies have used nanofluids in solar collectorsto improve the thermal performance of the system. The effect ofusing Al2O3 nanofluids in a flat plate solar collector as an absorbingmediumwas studied by Tiwari et al. (2013). The effect of particle %v/v and mass flow rate on the efficiency of the collector was alsoconsidered in their study. The authors found a 31.64% improvementin thermal efficiency for the 1.5% of Al2O3 nanofluid (Li andPeterson, 2006). A similar experiment was done by Yousefi et al.(2012a,b) to investigate the effect of Al2O3eH2O based workingfluid on the efficiency of a flat plate solar collector. Their resultshowed that the efficiency of solar collector was increased by 28.3%,while using 0.2% Al2O3 nanofluid instead of water as a workingfluid. Experimental investigations on the effect of Multi WalledCarbon Nanotubes (MWCNTs) water nanofluids on the energy ef-ficiency of flat plate solar collector by Yousefi et al. showed that theimprovement in the energy of the collector increased by 35%, using(MWCNTs) water nanofluid as the working fluid (Yousefi et al.,2012a; Yousefi et al., 2012b). Otanicar et al. (2010) experimentallyinvestigated different nanofluids, and the effect of these nanofluidson the efficiency of a micro-solar thermal collector. An efficiencyenhancement of up to 5% was reported by them using nanofluid asan absorption medium. Natarajan and Sathish used carbon nano-tubes as a medium of heat transport to enhance the thermal con-ductivity of base fluids, and reported improved efficiency of theconventional solar water heater (Natarajan and Sathish, 2009).Thus, it is important to improve the efficiency and performance ofthe solar thermal systems. To the best of our knowledge, we foundthat almost all of the previous works were directed on the appli-cations of nanofluids in collectors and solar water heaters (Otanicar


Z. Said et al. / Journal of Cleaner Production xxx (2015) 1e12 3

et al., 2010; Yousefi et al., 2012a; Yousefi et al., 2012b; Mahian et al.,2013; Tiwari et al., 2013). None of the above mentioned researchersused a pH control for longer stability of Al2O3eH2O nanofluid.

Based on the above literature study, it has been found that au-thors (Otanicar et al., 2010; Yousefi et al., 2012a; Yousefi et al.,2012b; Said et al., 2013b; Tiwari et al., 2013) focused on the firstlaw efficiency of the solar collector operated with nanofluids orMWCNTs. However, we have investigated exergy efficiency of thesolar collector operated with nanofluids. Moreover, first law effi-ciency of the present investigation found to be higher compared tothe existing systems. Lastly, stability of the nanofluid obtained byusing a pH control approach was found to be better than theexisting literature.

2. Methodology

2.1. Solar water heater

Solar water heaters are the natural and carbon free process toget hot water for many useful applications such as domestic, in-dustrial and commercial applications. A solar water heater basicallyconsists of a collector and insulated storage; collector is used forcollecting solar radiation from sun and storage tank for storing thehot water. Basic functioning of solar water heater is that solar en-ergy from the sun incident on the absorber panel coated withselected coating transfers the heat to the water flowing through thetubes and the water passing through the tube gets heated which isfinally delivered to the storage tank. In general, the temperature ofwater goes up to 60e70 �C on a good sunny day and is useful formany real life applications (Park et al., 2014).

Energy is based on the first law of thermodynamics and givesthe quantity of energy only. While exergy is based on the secondlaw of thermodynamics and represents the quality of energy andinvolves the irreversibility while analysing system efficiency.Exergy analysis identifies the causes, locations and magnitude ofthe system inefficiencies and provides the true measure how asystem approaches to the ideal (Dincer and Rosen).

2.1.1. Energy analysisAmount of energy conserved is overall the same but in different

forms of energy i.e. thermal, mechanical, internal, potential, kineticexperience measurable changes. The general energy balance equa-tion of the solar water heater (for a stationary process observedthrough a control volume) may be given as below (Ceylan, 2012):

Qc ¼ Qw þ Qb þ QL (1)

where, Qc presents the absorbed energy by the collector, Qw pre-sents the stored energy in the storage tank, Qb presents the storedenergy in the body and QL presents the lost energy (Chen et al.,2009). The gained useful energy in the tank by water is:

Qw ¼ Qc � Qb � QL (2)

The energy storage in the tank is related to the mass and thedifference in temperature between the initial and final temperatureof water in the storage tank (Chen et al., 2009; Esen et al., 2009).

Qw ¼ mwCpw�Tf � Ti

�(3)

Using the above equation, Qb and QL can be given by:

Qb ¼ mbCb�Tf � Ti

�(4)

QL ¼ Ut�Tm;st � Ta

�(5)


where Ut is the coefficient of the total heat loss rate. For simplifi-cation, the mean system temperature can be taken as the arith-metic mean of the initial and final water temperature in the storagetank as given below:

Tm;st ¼Ti þ Tf

2(6)

The thermal efficiency of the flat plate solar collector (h), is theratio of energy storage in the storage tank to the total solar radia-tion on the collector, which can be expressed as (Al-Madani, 2006;Roonprasang et al., 2008; Ceylan, 2012):

h ¼mnf Cnf

�Tf � Ti

�ITAp

(7)

2.1.2. Exergy analysisExergy is the maximum output that can be achieved relative to

the environment temperature. The general equation of the exergybalance is (Suzuki, 1988; Farahat et al., 2009):

_Ein þ _Es þ _Eout þ _El þ _Ed ¼ 0 (8)

where _Ein is inlet exergy rate, _Es is stored exergy rate, _Eout is outletexergy rate, _El is leakage exergy rate, _Ed is destroyed exergy rate.

The inlet exergy rate measures the fluid flow and the absorbedsolar radiation rate. The inlet exergy rate with fluid flow can becalculated by Farahat et al. (2009) and Bejan (1988):

_Ein;f ¼ _mCp

�Tin � Ta � Ta ln

�TinTa

��þ _mDPin

r(9)

where DPin is the pressure difference of the fluid with the sur-roundings at entrance, r is fluid density.

The absorbed solar radiation exergy rate can be calculated as:

_Ein;Q ¼ hITAP

�1� Ta

Ts

�(10)

where Ts is apparent sun temperature and equals to 75% of black-body temperature of the sun (Bejan et al., 1981).

Total inlet exergy rate of the solar collector can be calculated as:

_Ein ¼ _Ein;f þ _Ein;Q (11)

At steady state conditions, where the fluid is flowing, the storedexergy rate is zero.

_Es ¼ 0 (12)

When only the exergy rate of outlet fluid flow is considered, theoutlet exergy rate can be defined as (Kotas, 1995):

_Eout;f ¼ � _mCp

�Tout � Ta � Ta ln

�ToutTa

��þ _mDPout

r(13)

The heat leakage from the absorber plate to the environmentcan be defined as the leakage exergy rate and calculated as (Guptaand Saha, 1990):

_El ¼ �UAPðTP � TaÞ�1� Ta

Tp

�(14)

where the overall heat loss coefficient U is optimized at 4:6797 Wm2K

(Farahat et al., 2009).



The destroyed exergy rate caused by the temperature differencebetween the absorber plate surface and the sun can be expressed as(Gupta and Saha, 1990):

_Ed;DTS ¼ �hITAPTa

�1Tp

� 1Ts

�(15)

The destroyed exergy rate by pressure drop is expressed bySuzuki (1988):

_Ed;DP ¼ � _mDPr

Ta ln�ToutTa

�ðTout � TinÞ

(16)

The destroyed exergy rate caused by the temperature differencebetween the absorber plate surface and the agent fluid can becalculated from Suzuki (1988):

_Ed;DTf ¼ � _mCpTa

�ln�ToutTin

�� ðTout � TinÞ

Tp

�(17)

So, the total destroyed exergy rate can be calculated from:

_Ed ¼ _Ed;DTS þ _Ed;DP þ _Ed;DTf (18)

The exergy destruction rate can also be expressed from:

_Ed ¼ Ta _Sgen (19)

where _Sgen is the overall rate of entropy generation and can becalculated from Bejan (1996a):

_Sgen ¼ _mCp lnToutTin

�_QS

Tsþ

_QO

Ta(20)

Where _QS is solar energy absorbed (W) by the collector surface asexpressed by Esen (2008):

_QS ¼ ITðtaÞAP (21)

And _QO is the heat loss to the environment (W),

_QO ¼ _QS � _mCPðTout � TinÞ (22)

Ultimately, combining all the expression above, the exergy ef-ficiency equation of the solar collector can be analyzed (Farahatet al., 2009):

hex ¼_m�Cp

�Tout � Tin � Ta ln

�ToutTin

�� DP

r

ITAp

�1� Ta

Ts

� (23)

3. Experimental descriptions

3.1. Material

Commercial spherical shape Al2O3 nanopowder (Product ID:718475) from Sigma Aldrich, Malaysia with 99.8% trace metal basisand an average diameter of ~13 nm was used for the experimentalinvestigation. Reagent grade chemicals were used in the experi-mental investigation. Distilled water was used as a base fluid whilehydrochloric acid (HCl-37%) was also used tomaintain the pH of thebase fluid.

3.2. Preparation method and characterization

The previous decade has seen the speedy progress of nanofluidscience in diverse aspects, where the researchers concentratedmostly on the improvement of heat transfer. Nevertheless


nanofluids preparation also deserves the similar devotion since thefinal properties of nanofluids are reliant on the stability of thedispersion (Haddad et al., 2014). The analytical analysis shows thatfor a given particle size and zeta-potential, pH value of 7e9 gives ahigher stability ratio (W) and the same range of pH value is alsoreported by Huang et al. (2009) for stability. Very few literature isavailable on the stability of Al2O3 nanofluids using different pHvalues (acidic or basic media) (Min et al., 2008; Sajid et al., 2014).Moreover, Al2O3 is totally insoluble in water and it is amphoteric innature. To functionalize nanoparticles HCl solution was used. Ionicstrength of base-fluid can be adjusted tuning pH value (Min et al.,2008). The above information suggests for a weak acidic base-fluid to get aforementioned pH value. For reducing the aggrega-tion and enhancing the dispersion behavior of Al2O3 nanoparticlessuspended in the base fluid, two notable methods were applied inthis study: (i) using pH 4 solution as the base fluid and (ii) usinghigh pressure Homogenizer (capacity up to 2000 bar) was used tooptimize the nanoparticles suspension (0.1% and 0.3%v/v) with pH4 solution. A high pressure Homogenizer is an acceptedmachine fordissolving the accumulated nanoparticle (Wei et al., 2009). Al2O3nanoparticles with 0.1% v/v and 0.3% v/v were added to pH 4 so-lution (base fluid) to obtain a homogeneously dispersed solution,after passing the solution through several cycles for 30 min in ahigh pressure Homogenizer. Addition of Al2O3 nanoparticles to thepH 4 solution tends to increase the pH value ranging from 6 to 9based on the % v/v (Haddad et al., 2014).

Field Emission Scanning Electron Microscopy (FESEM) andTransmission Electron Microscope (TEM) were employed toinvestigate the morphological characteristics of the nanoparticles.A Zeta-seizer Nano ZS (Malvern) was used to obtain the averagediameter of the nanoparticles immersed in the base fluids. DLSapproach is used to give the hydrodynamic radius of particles in thesolution. The stability time of Al2O3eH2O is further supported byvisual images shown in Fig. 5. A KD2 Pro thermal property analyzer(Decagon, USA)was employed to obtain thermal conductivity of thenanofluids.

3.3. Experimental procedure

Fig. 1, shows the photographic image of the solar collector. Fig. 2,shows the schematic presentation of the solar collector. Theexperiment was performed at the University Malaya, Malaysia.Table 2, shows the environmental and analytical conditions for theflat plate solar collector. The properties Al2O3 used for thermalperformance calculations are presented in Table 1. For non-trackingsolar collection systems, the tilt angle has a predominant effect onthe quantity of energy that the system can intercept. An optimumtilt angle of this flat plate solar collector is taken as 22� for themaximum average daily radiation. An electric pump is used in thissolar collector system for the force convection heat transfer. Theheat generated from the collector cycle is absorbed by the tank,with a capacity of 50 L, as shown in Fig. 2. A heat exchanger is usedoutside the tank that transfers the heat load of the solar cycle to thewater. A flow meter is connected to the water pipe before theelectric pump (Fig. 2) tomeasure the flowof fluids. A simple valve isused to control the mass flow rate of the working fluid in the solarsystem. Five K type thermocouples were used to measure the fluidtemperatures at the entering and exit point of the solar collector asshown in Fig. 2. Then these were connected to a 10 channel datalogger for data storage and analysis. A Li-COR Pyranometer (PY82188) is used to measure the total solar radiation. The pressuredifference between the entry and exit point, was measured using apressure sensor. A PROVA (AV M-07) Anemometer is used tomeasure the wind speed. All the data were then transferred fromdata logger into the computer by an interface cable. Calibration of


Fig. 1. Photograph of the experimental setup: (a) Front view (b) Back view (c) Right side view & (d) Left side view.


the entire system was carried out several times to obtain accuratedata. All the instruments used for this experiment were calibratedaccording to the standards provided.

4. Testing method

An ASHARE Standard 93e2003 (Rojas et al., 2008) was used toassess the thermal performance of the collector. The incident ra-diation, ambient temperature inlet and outlet fluid temperatureswere measured and used for the thermal performance of the col-lector. Thermo-physical properties of Al2O3 and base fluid arepresented in Table 1. Specifications and input parameters werepresented in Table 2. These values are needed for the calculation ofthe first and second law efficiency analysis of the solar collector.

5. Uncertainty analysis

Uncertainty is needed to prove the accuracy of the experiments.There are two kinds of error which could take place for the presentstudy. One group could come from the direct measurement pa-rameters such as solar radiation flux (DGc),DT ,DP and the secondgroup of errors could come from the indirect measurements, suchas energy and exergy efficiencies. The following relations can beused based on the Luminosu and Fara (2005) method:

Dhex ¼DI:

_Exheatþ I

:_Exheat_Ex2heat

(24)

and


Dhen ¼ Dqa:

Gcþ qa

:DGc

G2c

(25)

where each error component can be evaluated through thefollowing relations:

DExheat ¼ DTTs

þ TaDTT2s

!AcðtaÞGc þ

�1� Ta

Ts

�AcðtaÞDGc (26)

DI:

¼ TaDS:

gen þ S:

genDT (27)

DS:

gen ¼�R ln

PoutPin

þ Cp lnTinTout

þ CpTout þ Tin

Ta

�Dm

:

þ GcAcðtaÞDTT2a

þm:Cp

1

Toutþ 1Tin

þ 2Ta

þ ðTout þ TinÞT2a

!DT þm

:R�

1Pout

þ 1Pin

�DP

þ AcðtaÞ�1Ts

þ 1Ta

�DGc

(28)

Dq:

a ¼ Cp

�Dm

: ðTout þ TinÞ þ 2m:DT

Ac

(29)

For this experiment, K-type thermocouples with an accuracy of±2.2 �C or ±0.75%, a PROVA (AV M�07) anemometer with accuracy


Fig. 2. Schematic diagram of the experimental setup.


±3%, pressure transducer with ±0.3% (at ±25 �C) accuracy (AZ82100, digital manometer) and Li-COR Pyranometer (PY 82188)with 1% accuracy were employed. Therefore, the maximum errorsfor the indirect measuring of energy and exergy efficiencies wereestimated to be ±4.52% and ±3.75% using Eqs. (24) and (25).

6. Results and discussion

6.1. Stability and characterization of nanofluids

Fig. 3 displays the particle size distributions with respect to theintensity acquired from the Zeta-seizer at dissimilar days. Fig. 4presents SEM and TEM images. The high-pressure Homogenizerwas used to homogeneously disperse the well-isolated primaryparticles. Therefore, better stability and size reduction of the

0

2

4

6

8

10

12

14

16

0.1 1 10 100 1000 10000

Inte

nsity

%

Size (nm)

1st Day

After 7 days

After 30 days

Fig. 3. Size presentation of watereAl2O3 0.1% nanofluids with pH 9.


nanoparticles was obtained since the high pressure Homogenizerwas found to provide long-term stable and well-dispersed nano-fluids and better particle breakdown.

Fig. 5 shows the visual appearances of the nanofluids with nosign of aggregation for a period of a 30 days.

According to earlier studies (Vatanpour et al., 2011; Said et al.,2013a; Said et al., 2013b), with higher concentration of nano-particles in a solution, nanoparticles tend to agglomerate, therefore,resulting in reduced stability of the nanofluids. It is witnessed thatthe stability of the prepared nanofluid with a lower % v/v ofnanoparticles immersed into the base fluid is stable for a longerperiod of time compared to the nanofluids with higher volumefraction. Nanofluids with an average particle size of 106 nm andwith a high zeta potential value of 58.4 mV is presented in Table 3.These values were obtained for more than a month period of time.It was witnessed that with the growing volume fraction, the pHshifts close to a basic value of (7e14). It was noted from theexperimental findings that pH 9 is the best value for a stablesolution.

6.2. Thermal performance

6.2.1. Solar radiationFig. 6 presents the recorded data for solar radiation on a clear

and cloudy day. This data is used for calculating the overall ener-getic and exergetic efficiency of a flat plate solar collector, pre-sented in Fig. 15.

6.2.2. Thermal conductivitySince energy and exergy efficiencies are dependent on the

thermal conductivity of a nanofluid, details of the thermal


Fig. 4. (a) SEM of Al2O3 nanoparticles. (b) TEM images of Al2O3/water using control pH ¼ 9.

Fig. 5. Prepared Al2O3 nanofluid solutions (a) Samples on the first day of preparations (b) Samples after 30 days of preparations.

Table 1Physical characteristics of Al2O3 and base fluid (Said et al., 2013a; Said et al., 2013b).

Particle & base fluid Average particle size (nm) Actual density (kg/m3) Cp(J/kg K) K (W/mK) Viscosity (m.Pa.s)

Al2O3 (gamma) 13 3960 773 40Al2O3eH2O (0.1%v/v) 13 1020.5 3841.1Al2O3eH2O (0.3%v/v) 13 1080.5 3159.3Water 997.1 4179 0.605 0.89

Table 2Specifications for the flat plate solar collector studied.

Parameters of collector Value

Dimension L2000 mm�W1000 mm� T80 mmAperture area 1.84 m2

Weight 36 kgFrame Aluminum Alloy, AnodizedWorking fluids in flow ducts Water and Al2O3 based nanofluidAbsorption area, Ap 1.84 m2

Wind speed 5 m/sCollector tilt, bo 22�

Absorption rate 0.94Emittance 0.12Heat transfer coefficient 4.398Header material Copper TP2Header tube size F22 mm� t0.6 mm(2pcs)Riser tube material Copper TP2Riser tube size F10 mm� t0.45 mm (8pcs)



conductivity analysis with the variation of temperature, concen-trations are presented in this section.

Fig. 7 presents thermal conductivity of the experimental datawith the variation of % v/v of nanoparticles. Results of experimentalinvestigation were compared with the findings available in theliterature (Das et al., 2003; Yu and Choi, 2003; Xie et al., 2005).

Fig. 7 shows that with the growing volume fraction, the thermalconductivity of the nanofluids increases. Since higher concentra-tion of the nano-particles leads to higher thermal conductivity ofthe working fluid, thus, resulting in a better heat transfer rate forthe solar collector.

Fig. 8 presents thermal conductivity variation with temperatureand comparison with the existing literature.

The experimental results presented in Fig. 8 agree well with theliterature (Chon and Kihm, 2005). Improved thermal conductivityresults, due to larger temperature differences, which result inhigher speed of molecules, and greater impacts between nano-particles and the molecules of bulk liquid (Das et al., 2003; Chon


Table 3Zeta potential, particle size and pH values of Al2O3/water 13 nm particles suspendedin water.

Nominal particlesize

Zeta potential(mV)

Particle size (nm)from DLS usinghigh pressureHomogenizer

pH

13 nm 58.4 106 9.013 nm 54.3 109 8.113 nm 49.5 123.9 7.013 nm 35.9 126.4 6.1

0.98

1.03

1.08

1.13

1.18

1.23

1.28

0.000 0.010 0.020 0.030 0.040 0.050 0.060

Kef

f/Kb

% v/v(φ)

Das et al. 2003Xie et al. model 2005Yu and Choi model 2003Exp. Al2O3/water

Fig. 7. Models and investigational data on the thermal conductivity of Al2O3/waternanofluids at changing volume fractions.


and Kihm, 2005; Li and Peterson, 2006; Mintsa et al., 2009). Themain mechanism behind the thermal conductivity improvement innanofluids is said to be as the stochastic motion of the nano-particles. It was noted that with the increasing volume fraction, thetemperature difference between inlet and outlet was higher fornanofluids operated collector compared to water as shown inFig. 14. This indicates that nanofluids can be used for higher heattransfer rate. Higher temperatures of nanofluids results in moreactive Brownian motion of nanoparticles. The Brownian motion isdependent on the fluid temperature.

Fig. 9 illustrates the thermal conductivity improvement ofAl2O3ewater nanofluids at a volume fraction of 0.1%e0.3%,respectively. The thermal conductivity improvement is directlyproportional to the % v/v and surges up to 6.8% with 0.3% v/v ofAl2O3, it improved from 2.46% to 6.80%. The experimental resultspresented in Fig. 9 confirm the values achieved in other studies(Vatanpour et al., 2011).

As it is presented in Figs. 7e9 thermal conductivity improveswith the growing % v/v as well as with increasing temperatures. Anincrease in the heat loss may be faced in conventional collectorswith the increasing fluid temperature. However, the heat losses for0.3% v/v nanofluid are lower in comparison with 0.1% v/v.

6.2.3. Efficiencies with respect to mass flow rateThe flat plate solar collector was tested at different mass flow

rates of 0.5, 1.0 and 1.5 kg/min. Each investigation was repeated forseveral days, in order to achieve the best results with the least error.A flow meter was used for controlling the mass flow rate. In thispart, both 0.1% and 0.3% v/v of nanoparticles with a controlled pHwere used as a working fluid. The collector efficiency at mass flowrates of 0.5 kg/min, 1.0 kg/min and 1.5 kg/min were experimentallyinvestigated. The effect of mass flow rate on energy efficiency, en-tropy generation, exergy destruction and exergy efficiency arepresented in Figs. 10e13, respectively. It is noticed from the energy

0

200

400

600

800

1000

1200

1400

9.00 10.00 11.00 12.00 13.00 14.00 15.00 16.00 17.00 18.00

Sola

r ra

diat

ion

(W/m

²)

Time of day (hour)

Solar insolation on a clear day Solar insolation on a cloudy day

Fig. 6. Solar radiation on a clear day and cloudy day.


gain equation that the useful solar energy is directly proportional tomass flow rate for a certain temperature increase. It was clear thatwhen the mass flow rate, and the inlet temperature increased; thetemperature difference reduced. According to the Figs., the effi-ciency of the solar collector at low temperature differences de-creases as the mass flow rate increases.

6.2.4. First law based efficiencyTable below shows the experimental data obtained from the

setup with water and with Al2O3eH2O (Table 4).Energy efficiency has been estimated using Eq. (7) and input

data from Tables 1 and 2 presented in Fig. 10.Fig. 10 demonstrates that the collector efficiency enhances with

the rising volume fraction. It has been observed that solar collectoroperated with various concentrations of nanofluids has a higherthermal efficiency than the solar collected operated with water as aworking fluid. The increase in the efficiency can be a result of theincreased thermal conductivity, which gives an improved convec-tive heat transfer coefficient. An increase of 73.7% energy efficiencywas observed for 0.1% vol., whereas an increase of 83.51% energyefficiency was observed for 0.3% vol. at the same flow rate (1.5 kg/min). In case of water, a maximum energy efficiency of 42.07% isobserved for 0.5 kg/min, whereas an energy efficiency of 20.91% isobserved for 1.5 kg/min.

6.2.5. Entropy generation and exergy destruction of Al2O3 nanofluidHeat transfer is an irreversible, non-equilibrium process from

the thermodynamic viewpoint. Entropy generation was consideredto be a measure of irreversibility (Onsager, 1931a, 1931b; Kreuzer,1981). The irreversible losses affect the performance of the ther-mal devices that results in an increased entropy and decreasedthermal efficiency. It is essential to calculate the entropy generation

1.07

1.09

1.11

1.13

1.15

1.17

1.19

1.21

1.23

1.25

15 25 35 45 55 65 75

The

rmal

con

duct

ivity

enh

ancm

ent

(Knf

/Kb)

Temperature (°C)

Al2O3, ~13nm (Experimental) C.H. Chon et al. 2005

Fig. 8. Thermal conductivity of nanofluids with respect to increasing temperature.


R² = 0.9965

0.99

1.00

1.01

1.02

1.03

1.04

1.05

1.06

1.07

1.08

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35

Knf

/Kb

volume fraction, φ

Experimental data Das et al. (2003) Linear (Experimental data)

Fig. 9. Thermal conductivity of Al2O3eH2O with respect to volume fraction.

20

30

40

50

60

70

80

90

0.5 0.7 0.9 1.1 1.3 1.5

Ener

gy e

ffici

ency

, %

Flow rate, kg/min

0.30% 0.10% Water

Fig. 10. The energy efficiency at different mass flow rates and different volumefractions.

1300

1400

1500

1600

1700

1800

1900

0.5 0.7 0.9 1.1 1.3 1.5

Exer

gy d

estru

ctio

n, W

Flow rate, kg/min

0.30% 0.10% Water

Fig. 12. Exergy destruction rate with respect to mass flow rate and volume fraction.

3

5

7

9

11

13

15

17

19

21

23

0.5 0.7 0.9 1.1 1.3 1.5

Exer

gy e

ffici

ency

, %

Flow rate, kg/min

0.30% 0.10%Water

Fig. 13. Exergy efficiency with respect to mass flow rate and volume fraction.


or exergy destruction resulting from the heat transfer and viscousfriction as a function of the design variables selected for the optimalanalysis. Fig. 11 presents the entropy generation ( _Sgen) with regardto mass flow rate and different volume concentrations of nano-fluids. Entropy generation ( _Sgen) is calculated using Eq. (20) anddata from Tables 1 and 2

Fig. 11 shows the exergy destruction with respect to different %v/v and mass flow rate. Reduced exergy destruction with theincreasing flow rate compared to water is observed for Al2O3eH2O,which results in lower exergy destruction. Entropy generationfound to be much lower compared to water. The exergy destruction(or irreversibility) rate, shown in Fig. 12, is calculated using Eq. (19)and data from Tables 1 and 2

4042444648505254565860

0.5 0.7 0.9 1.1 1.3 1.5

Entro

py g

ener

atio

n, M

/K

Flow rate, kg/min

0.30% 0.10% Water

Fig. 11. Entropy generation with respect to mass flow rate and volume fraction.


A decrease in the entropy generation is observed with theincreasing volume fraction. This happens because of the increasingheat flux along the absorber plate, thus governing the irreversibilityturn out. As a result, enhanced thermal conductivity is obtainedwith the increment in the volume fraction of the nanoparticles,further proceeding to increase thermal conductance. Therefore, adecline in the irreversibility is observed as a consequence of heattransfer, which has a far greater effect compared to that of theviscous effects of entropy generation. On the other hand, with thegrowing nanoparticles volume fraction, the useful viscosity ofnanofluids is generated. The useful viscosity of nanofluids gives riseto the fluid friction involvement in entropy generation, presentedin Fig.12. For amass flow rate of 1.5 kg/min, nanofluid with 0.3% v/v

Fig. 14. Output temperature with respect to mass flow rates and volume fraction.


9

11

13

15

17

19

21

23

10

20

30

40

50

60

70

80

8.45 9.45 10.45 11.45 12.45 13.45 14.45 15.45 Ove

rall

exer

getic

(2nd

Law

) E

ffic

ienc

y (%

)

Ove

rall

ener

getic

(1st

Law

) Eff

icie

ncy

(%)

Operating Time

Overall Energetic (1st Law) Efficiency (%)Overall exergetic (2nd Law ) Efficiency (%)

Fig. 15. Variation of overall energetic and exergetic efficiencies over time.


showed the least exergy destruction of 1432.37 W, followed by1709.4 W for 0.1% v/v of nanofluid and 1855.7 W for water. It hasalso been observed that with the rising mass flow rate of nano-fluids, the efficiency of the solar collector also improves. Whereas,with rising mass flow rate the temperature difference decreases.This can be due to the reason that with lower mass flow ratenanofluid has more time to absorb solar radiations and gain moreheat.

6.2.6. Second law/exergy based efficiency of Al2O3 nanofluid asworking fluid

Fig. 13 displays the behavior of the exergy efficiency as a func-tion of the nanoparticles volume fraction and mass flow rate of thefluid. Exergy efficiency was calculated using Eq. (23) and inputTables 1 and 2

Based on Bejan's work (Bejan et al., 1981; Bejan 1996b, 1996c),this analysis is carried out. The study is, however, used for flat platecollectors as entropy generation minimization is vital to hightemperature systems. Maximization of the power output is thesame as the minimization of the entropy generation rate. It iswitnessed that the exergy efficiency, reduces with the growing % v/v as well as with the growing mass flow rate. Al2O3eH2O nanofluidshows higher values of efficiency compared to water base fluid. Byusing Al2O3eH2O nanofluid in a solar collector as a working me-dium, exergy efficiency can be enhanced. Al2O3eH2O can be a goodoption as an absorbing medium because its exergy efficiency ishigher compared to water. Experimental results also reveal that bysuspending small amount of nanoparticles up to 0.1%, the exergyefficiency could be enhanced by 20.3% compared the conventionalfluid.

Table 4Experimental data of the solar water heating system with and without nanofluids.

Local time (h) Volume concentration(% v/v)

Solar radiation(W/m2)

Water temperat

Inlet

12:30 Water þ 0.1% of Al2O3 839.9 50.113:00 898.1 51.613:30 981.2 53.414:30 890.5 53.112:50 981.1 51.813:30 1066.0 52.514:00 1200.0 53.712:30 Water only 839.9 47.313:00 898.1 45.313:30 890.5 48.5


6.2.7. Effect of Al2O3 nanofluid on output temperatureThe effects of mass flow rate and volume fraction on the output

temperatures of nanofluids operated solar collector compared towater as a working fluid are shown in Fig. 14.

As known the output temperature is one of the most effectiveparameters that affects the energy efficiency of a flat plate solarcollector directly. It dramatically increased with rising outputtemperature. Solar collectors, operated with nanofluids, providehigher efficiency due to the higher output temperatures. The spe-cific reason for higher output temperature is, the more nano-particles in the base fluid. As we know, specific heat is defined as,“The heat required to raise the temperature of a unit mass of asubstance by one unit of temperature.” It is clear from the definitionthat any substance, which has a lower specific heat, should providehigher temperature for equal heat flow.

6.2.8. Effect of overall first and second law efficiencies of Al2O3

nanofluidThe overall First and Second Law efficiencies with respect to

time is presented in Fig. 15. Using Eqs. (7) and (23), input data fromTables 1 and 2 and solar radiation from Fig. 6, first law and secondlaw based thermal efficiencies has been estimated and presented inFig. 15.

Improved energetic and exergetic efficiencies are witnessed forthe studied nanofluids. From Figs. 6 and 15, it was noticed that themaximum irreversibility occurs at noon, when solar radiation wasmaximized; it decreased as solar radiation decreased. The tem-perature difference between the collector and the ambient has anideal point for exergy efficiency, and a larger difference could resultin lower exergy efficiency. However, an increase in the temperaturedifference decreases the energy efficiency due to the possibility ofmore heat losses to ambient. According to the results from theexperiments, Al2O3eH2O nanofluid is found to bemore appropriateas a working medium for flat plate solar water heater than water.

7. Conclusions

An experimental study was carried out to assess the energeticand exergetic efficiencies and the effect of pH control onAl2O3eH2O nanofluid as a working medium in a flat plate solarwater heater. The effect of mass flow rate, nanoparticles volumefraction, and the effect of pH on the energy and exergy efficiency ofthe collector is examined. The obtained stability of nanofluid wasmore than a month. The thermal conductivity improvement, ob-tained by KD2 Pro, is directly proportional to the % v/v and surgesup to 6.8%with 0.3% v/v of Al2O3. The results obtained, showed that,in contrast with water as the working medium, nanofluids increasethe first law efficiency by 83.5% for 0.3% v/v and 1.5 kg/min,whereas the second law efficiency was enhanced by up to 20.3% for

ure (�C) Mass flowrate (kg/min)

Ambient temperature(�C)

Wind velocity(m/s)

Outlet

67.1 0.5 32.9 2.9572.5 0.5 34.7 2.4574.1 0.5 33.8 3.2269.6 0.5 35.5 3.1865.9 1.5 36.2 2.6666.7 1.5 36.4 2.7570.2 1.5 38.0 3.2552.3 1.0 34.5 3.0054.6 1.0 35.9 3.2554.6 1.0 36.8 3.38



0.1% v/v and 1 kg/min. Increasing volume flow rate can increase theefficiency of the system but the exergy efficiency will decrease. Thecollector efficiency is directly proportional to the nanoparticleconcentration. It has been also observed that as the mass flow rateof the nanofluids is increased, the efficiency of the solar collectoralso improved. Whereas, with an increase in the mass flow rate thetemperature difference decreases. According to the results from theexperiments, Al2O3eH2O nanofluid is found to bemore suitable as aworking medium for flat plate solar water heater than water.

To our knowledge, this is the first study, which is executed onthe energetic and exergetic efficiency analysis of a flat plate solarcollector using controlled pH for Al2O3eH2O, both theoretically andexperimentally. Accordingly, given the changing functioning andatmospheric conditions that can occur in practice, the probability ofobtaining different results must be taken into account, in order toascertain the effect of the altered variables on the collector effi-ciency and the characteristic parameters of the solar collector.

8. Future recommendations

For both scientific investigation and systems, stability of nano-fluids suspension is an important challenge. Supplementaryemphasis should be given to the long term stability of nanofluids.Additionally, the stability of nanofluids in the practical conditions,should be given much more attention. Further research is neededfor thermal operations at higher temperatures, which wouldtremendously beneficial for high-temperature solar energy ab-sorption and high-temperature energy storage.

Therefore, there is a possibility for development in harvestingthe solar energy. It is recommended that the higher efficiency of thecollector can be achieved by reducing the losses and preventing thesettling of the nanoparticles. The size, form and % v/v of thenanoparticles in the nanofluids directly influence the collector ef-ficiency. More progress in the nanofluids properties and theirappropriate use in the solar energy harvesting will result in furtherresearch in this field.

Acknowledgments

This research is supported by UM High Impact Research GrantUM-MoE UM.C/HIR/MoE/ENG/40 from the Ministry of Education,Malaysia.

References

Al-Madani, H., 2006. The performance of a cylindrical solar water heater. Renew.Energy 31 (11), 1751e1763.

Amini, S., Remmerswaal, J., Castro, M., Reuter, M., 2007. Quantifying the quality lossand resource efficiency of recycling by means of exergy analysis. J. Clean. Prod.15 (10), 907e913.

Bejan, A., 1988. Advanced Engineering Thermodynamics. Wiley Inter-sciences, NewYork.

Bejan, A., 1996a. Entropy Generation Minimization: the Method of ThermodynamicOptimization of Finite-size Systems and Finite-time Processes. CRC Press.

Bejan, A., 1996b. Entropy generation minimization: the new thermodynamics offinite-size devices and finite-time processes. J. Appl. Phys. 79 (3), 1191e1218.

Bejan, A., 1996c. Method of entropy generation minimization, or modeling andoptimization based on combined heat transfer and thermodynamics. Rev.g�en�erale Therm. 35 (418), 637e646.

Bejan, A., Kearney, D., Kreith, F., 1981. Second law analysis and synthesis of solarcollector systems. J. Sol. Energy Eng. 103 (1), 23e28.

Bejan, A., Kraus, A.D., 2003. Heat Transfer Handbook. John Wiley & Sons.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.

Buschmann, M.H., 2013. Nanofluids in thermosyphons and heat pipes: overview ofrecent experiments and modelling approaches. Int. J. Therm. Sci. 72, 1e17.

Ceylan, I., 2012. Energy and exergy analyses of a temperature controlled solar waterheater. Energy Build. 47 (0), 630e635.


Chen, B.-R., Chang, Y.-W., Lee, W.-S., Chen, S.-L., 2009. Long-term thermal perfor-mance of a two-phase thermosyphon solar water heater. Sol. Energy 83 (7),1048e1055.

Choi, S.U., Eastman, J., 1995. Enhancing Thermal Conductivity of Fluids withNanoparticles. Argonne National Lab., IL (United States).

Chon, C., Kihm, K., 2005. Thermal conductivity enhancement of nanofluids byBrownian motion. J. Heat. Transf. 127 (8), 810e810.

Das, S.K., Putra, N., Thiesen, P., Roetzel, W., 2003. Temperature dependence ofthermal conductivity enhancement for nanofluids. J. Heat. Transf. 125 (4),567e574.

Esen, H., 2008. Experimental energy and exergy analysis of double-flow solar airheater having different obstacles on absorber plates. Build. Environ. 43 (6),1046e1054.

Esen, H., Ozgen, F., Esen, M., Sengur, A., 2009. Modelling of a new solar air heaterthrough least-squares support vector machines. Expert Syst. Appl. 36 (7),10673e10682.

Farahat, S., Sarhaddi, F., Ajam, H., 2009. Exergetic optimization of flat plate solarcollectors. Renew. Energy 34 (4), 1169e1174.

Fong, K., Lee, C.K., Chow, T.T., 2012. Comparative study of solar cooling systems withbuilding-integrated solar collectors for use in sub-tropical regions like HongKong. Appl. Energy 90 (1), 189e195.

Grimm, A., 1993. Powdered aluminum-containing heat transfer fluids. Germanpatent DE 4131516: A1.

Gupta, K.K.D., Saha, S.K., 1990. Energy analysis of solar thermal collectors. Renew.Energy Environ. 283e287.

Haddad, Z., Abid, C., Oztop, H.F., Mataoui, A., 2014. A review on how the researchersprepare their nanofluids. Int. J. Therm. Sci. 76, 168e189.

Huang, J., Wang, X., Long, Q., Wen, X., Zhou, Y., Li, L., 2009. Influence of pH on thestability characteristics of nanofluids. In: Photonics and Optoelectronics, 2009.SOPO 2009. Symposium on, IEEE.

Joshi, S., Bokil, R., Nayak, J., 2005. Test standards for thermosyphon-type solar do-mestic hot water system: review and experimental evaluation. Sol. Energy 78(6), 781e798.

Kalogirou, S.A., 2004. Solar thermal collectors and applications. Prog. EnergyCombust. Sci. 30 (3), 231e295.

Koroneos, C., Tsarouhis, M., 2012. Exergy analysis and life cycle assessment of solarheating and cooling systems in the building environment. J. Clean. Prod. 32,52e60.

Kotas, T.J., 1995. The Exergy Method of Thermal Plant Analysis. Krieger PublishCompany, Malabar, FL.

Kreuzer, H.J., 1981. Nonequilibrium Thermodynamics and its Statistical Foundations.Clarendon Press, Oxford and New York, 455 p. 1.

Lee, D.W., Sharma, A., 2007. Thermal performances of the active and passive waterheating systems based on annual operation. Sol. Energy 81 (2), 207e215.

Li, C.H., Peterson, G., 2006. Experimental investigation of temperature and volumefraction variations on the effective thermal conductivity of nanoparticle sus-pensions (nanofluids). J. Appl. Phys. 99 (8), 084314.

Luminosu, I., Fara, L., 2005. Determination of the optimal operation mode of a flatsolar collector by exergetic analysis and numerical simulation. Energy 30 (5),731e747.

Mahian, O., Kianifar, A., Kalogirou, S.A., Pop, I., Wongwises, S., 2013. A review of theapplications of nanofluids in solar energy. Int. J. Heat Mass Transf. 57 (2),582e594.

Min, Y., Akbulut, M., Kristiansen, K., Golan, Y., Israelachvili, J., 2008. The role ofinterparticle and external forces in nanoparticle assembly. Nat. Mater. 7 (7),527e538.

Mintsa, H.A., Roy, G., Nguyen, C.T., Doucet, D., 2009. New temperature dependentthermal conductivity data for water-based nanofluids. Int. J. Therm. Sci. 48 (2),363e371.

Natarajan, E., Sathish, R., 2009. Role of nanofluids in solar water heater. Int. J. Adv.Manuf. Tech. 1e5.

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., Phelan, P.E., Prasher, R.S., Rosengarten, G., Taylor, R.A., 2010. Nano-fluid-based direct absorption solar collector. J. Renew. Sust. Energy 2 (3).

Ozalp, N., Epstein, M., Kogan, A., 2010. Cleaner pathways of hydrogen, carbon nano-materials and metals production via solar thermal processing. J. Clean. Prod. 18(9), 900e907.

Park, S., Pandey, A., Tyagi, V., Tyagi, S., 2014. Energy and exergy analysis of typicalrenewable energy systems. Renew. Sust. Energy Rev. 30, 105e123.

Prasher, R., Bhattacharya, P., Phelan, P.E., 2005. Thermal conductivity of nanoscalecolloidal solutions (nanofluids). Phy. Rev. Lett. 94 (2), 025901.

Rojas, D., Beermann, J., Klein, S., Reindl, D., 2008. Thermal performance testing offlat-plate collectors. Sol. Energy 82 (8), 746e757.

Roonprasang, N., Namprakai, P., Pratinthong, N., 2008. Experimental studies of anew solar water heater system using a solar water pump. Energy 33 (4),639e646.

Rosen, M.A., 2008. Allocating carbon dioxide emissions from cogeneration systems:descriptions of selected output-based methods. J. Clean. Prod. 16 (2), 171e177.

Said, Z., Kamyar, A., Saidur, R., 2013a. Experimental investigation on the stabilityand density of TiO2, Al2O3, SiO2 and TiSiO4. In: IOP Conference Series: Earth andEnvironmental Science. IOP Publishing.


http://refhub.elsevier.com/S0959-6526(15)01046-X/sref1





















































































































































Said, Z., Sajid, M., Alim, M., Saidur, R., Rahim, N., 2013b. Experimental investigationof the thermophysical properties of Al2O3-nanofluid and its effect on a flat platesolar collector. Int. Commun. Heat. Mass Transf. 48, 99e107.

Said, Z., Sabiha, M., Saidur, R., Hepbasli, A., Rahim, N., Mekhilef, S., Ward, T., 2015.Performance enhancement of a flat plate solar collector using TiO2 nanofluidand polyethylene glycol dispersant. J. Clean. Prod.

Sajid, M., Said, Z., Saidur, R., Adikan, F., Sabri, M., Rahim, N., 2014. A time variantinvestigation on optical properties of water based Al2O3 nanofluid. Int. Com-mun. Heat. Mass Transf. 50, 108e116.

Sridhara, V., Satapathy, L.N., 2011. Al2O3 based nanofluids: a review. Nanoscale Res.Lett. 6 (1), 1e16.

Sutthivirode, K., Namprakai, P., Roonprasang, N., 2009. A new version of a solarwater heating system coupled with a solar water pump. Appl. Energy 86 (9),1423e1430.

Suzuki, A., 1988. General theory of exergy balance analysis and application to solarcollectors. Energy 13 (2), 153e160.

Tian, Y., Zhao, C.-Y., 2013. A review of solar collectors and thermal energy storage insolar thermal applications. Appl. Energy 104, 538e553.

Tiwari, A.K., Ghosh, P., Sarkar, J., 2013. Solar water heating using nanofluids-acomprehensive overview and environmental impact analysis. Int. J. Emerg.Technol. Adv. Eng. 3 (3), 221e224.

Vatanpour, V., Madaeni, S.S., Moradian, R., Zinadini, S., Astinchap, B., 2011. Fabri-cation and characterization of novel antifouling nanofiltration membrane


prepared from oxidized multiwalled carbon nanotube/polyethersulfone nano-composite. J. Membr. Sci. 375 (1), 284e294.

Wei, X., Zhu, H., Kong, T., Wang, L., 2009. Synthesis and thermal conductivity ofCu2O nanofluids. Int. J. Heat Mass Transf. 52 (19), 4371e4374.

Xie, H., Fujii, M., Zhang, X., 2005. Effect of interfacial nanolayer on the effectivethermal conductivity of nanoparticle-fluid mixture. Int. J. Heat Mass Transf. 48(14), 2926e2932.

Yousefi, T., Shojaeizadeh, E., Veysi, F., Zinadini, S., 2012a. An experimental investi-gation on the effect of pH variation of MWCNTe H2O nanofluid on the efficiencyof a flat-plate solar collector. Sol. Energy 86 (2), 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 solarcollectors. Renew. Energy 39 (1), 293e298.

Yu, W., Choi, S., 2003. The role of interfacial layers in the enhanced thermal con-ductivity of nanofluids: a renovated Maxwell model.” J. Nanopart. Res. 5 (1e2),167e171.

Yu, W., Xie, H., 2012. A review on nanofluids: preparation, stability mechanisms,and applications. J. Nanomater. 2012, 1.

Zambrana-Vasquez, D., Aranda-Us�on, A., Zabalza-Bribi�an, I., Ja~nez, A., Llera-Sastresa, E., Hernandez, P., Arrizabalaga, E., 2015. Environmental assessment ofdomestic solar hot water systems: a case study in residential and hotel build-ings. J. Clean. Prod. 88, 29e42.














































































energy and exergy efficiency of a flat plate solar collector using ph treated al2o3 nanofluid

Documents