A review on the applications of nanofluids in solar energy systems Alibakhsh Kasaeian n ,Amin Toghi Eshghi, Mohammad Sameti Faculty of New Science & Technologies, University of Tehran, Iran a r t i c l e i n f o Article history: Received 27 June 2013 Received in revised form 31 August 2014 Accepted 1 November 2014 Keywords: Solar energy Nanofluids Heat transfer enhancement Review a b s t r a c t The negative impact of human activities on the environment receives tremendous attention, especially on the increased global temperature. To combat climate change, clean and sustainable energy sources need to be rapidly developed. Solar energy technology is considered as one of the ideal candidates, which directly converts solar energy into electricity and heat without any greenhouse gas In both areas, high-performance cooling, heating and electricity generation is one of the vital needs. Modern nanotechnology can produce metallic or nonmetallic particles of nanometer dimension have unique mechanical, optical, electrical, magnetic,and thermalproperties. Studies in this field indicate that exploiting nanofluid in solar systems, offers unique advantages over conventional fluids. In this paper, the applications of nanofluids on different types of solar collectors, photovoltaic systems and solar thermoelectrics are reviewed. Beside the wide range of energy conversion, the ef the energy storage system (ESS) have been reviewed. In the field of economics,nanotech reduces manufacturing costs as a result of using a low temperature process. & 2014 Elsevier Ltd. All rights reserved. Contents 1. Introduction . ....................................................................................................... 584 2. Applications of nanofluids in solar energy . ............................................................................... 585 2.1. Solar collector . ............................................................................................... 585 2.2. Evacuated solar collectors . ...................................................................................... 589 2.3. Photovoltaic thermal systems . ................................................................................... 589 2.4. Thermal energy storage. ........................................................................................ 590 2.5. Solar thermoelectric devices . .................................................................................... 592 2.6. Solar cells . ................................................................................................... 592 3. Economical and environmental aspects . ................................................................................. 593 3.1. Concluding remarks and directions for future work . ................................................................. 596 References . ............................................................................................................ 596 1. Introduction Heat transfer has many applications in industries with the aim of both increasing and decreasing temperature. The imperfection of thermal engineering devices is the low thermal conductivity of conventional fluids such as water, ethylene glycol, or oil. Nano- fluids have solved this constraint because of their remarkable heat transfer abilities. A fluid which contains nanometer-sized particles (1–100 nm in one dimension) is called nanofluid. Comparing to base fluids, nanofluids enhance the rate of heat transfer. Hence, they have a wide range of utility in industry, thermal generation, transportation and microelectronics. Adding nanometer-sized ticles to a fluid was initially investigated by Choi in 1995 [1], in which the results revealed better thermal conductivity. In the pasttwo decades,researchers have theoretically and experimentally surveyed the thermophysical characteristics o fluids.In many research, the intensification of heat transfer for nanofluids compared to conventional fluids has been proven [2– The study of Lee et al. [9] showed that Cu-water, Al 2 O 3 -water and CuO-ethyleneglycolnanofluidscause augmentation of thermal Contents lists available at ScienceDirect journalhomepage: www.elsevier.com/locate/rser Renewable and Sustainable Energy Reviews http://dx.doi.org/10.1016/j.rser.2014.11.020 1364-0321/& 2014 Elsevier Ltd. All rights reserved. n Corresponding author. Tel.: þ98 9121947510; fax: þ98 21 88617087. E-mail address: [email protected](A. Kasaeian). Renewable and Sustainable Energy Reviews 43 (2015) 584–598
Transcript
A review on the applications of nanofluids in solar energy systemsAlibakhsh Kasaeiann,Amin Toghi Eshghi,Mohammad SametiFaculty of New Science & Technologies,University of Tehran,Iran
a r t i c l ei n f o
Article history:Received 27 June 2013Received in revised form31 August 2014Accepted 1 November 2014
Keywords:Solar energyNanofluidsHeat transfer enhancementReview
a b s t r a c t
The negative impact of human activities on the environment receives tremendous attention,especiallyon the increased global temperature.To combat climate change,clean and sustainable energy sourcesneed to be rapidly developed.Solar energy technology is considered as one ofthe idealcandidates,which directly converts solar energy into electricity and heat without any greenhouse gas emissions.In both areas,high-performance cooling,heating and electricity generation is one ofthe vitalneeds.Modern nanotechnology can produce metallic or nonmetallic particles of nanometer dimensions whichhave unique mechanical,optical,electrical,magnetic,and thermalproperties.Studies in this fieldindicate that exploiting nanofluid in solar systems,offers unique advantages over conventionalfluids.In this paper,the applications of nanofluids on different types of solar collectors,photovoltaic systemsand solar thermoelectrics are reviewed. Beside the wide range of energy conversion, the efforts done onthe energy storage system (ESS)have been reviewed.In the field ofeconomics,nanotech reducesmanufacturing costs as a result of using a low temperature process.
& 2014 Elsevier Ltd.All rights reserved.
Contents
1. Introduction ........................................................................................................5842. Applications of nanofluids in solar energy ................................................................................585
2.1. Solar collector ................................................................................................ 5852.2. Evacuated solar collectors ....................................................................................... 5892.3. Photovoltaic thermal systems .................................................................................... 5892.4. Thermal energy storage......................................................................................... 5902.5. Solar thermoelectric devices ..................................................................................... 5922.6. Solar cells .................................................................................................... 592
3. Economical and environmental aspects ..................................................................................5933.1. Concluding remarks and directions for future work .................................................................. 596
Heat transfer has many applications in industries with the aimof both increasing and decreasing temperature.The imperfectionof thermal engineering devices is the low thermal conductivity ofconventionalfluids such as water,ethylene glycol,or oil.Nano-fluids have solved this constraint because of their remarkable heattransfer abilities. A fluid which contains nanometer-sized particles
(1–100 nm in one dimension) is called nanofluid.Comparing tobase fluids,nanofluids enhance the rate ofheat transfer.Hence,they have a wide range of utility in industry,thermal generation,transportation and microelectronics. Adding nanometer-sized par-ticles to a fluid was initially investigated by Choiin 1995 [1],inwhich the results revealed better thermal conductivity.
In the pasttwo decades,researchershave theoretically andexperimentally surveyed the thermophysical characteristics of nano-fluids.In many research,the intensification ofheattransferfornanofluids compared to conventional fluids has been proven [2–8].The study of Lee et al.[9] showed that Cu-water,Al2O3-water andCuO-ethyleneglycolnanofluidscauseaugmentation ofthermal
Contents lists available at ScienceDirect
journalhomepage: www.elsevier.com/locate/rser
Renewable and Sustainable Energy Reviews
http://dx.doi.org/10.1016/j.rser.2014.11.0201364-0321/& 2014 Elsevier Ltd.All rights reserved.
Renewable and Sustainable Energy Reviews 43 (2015) 584–598
conductivity.In another study on engine oil containing 1.0% volumecarbon nanotube,160% enhancement in thermalconductivity wasobserved by Choi et al. [10]. Nanofluid minimum quantity lubrication(MQL), which has been recently mentioned, was investigated by Namet al.[11].They found that using nanofluid MQL in micro drillingprocess decreases the drilling torques and thrust forces. Many studieshave been carried out about the effect of nanofluids on convectiveheat transfer coefficient and friction factor [12–15]. Sundar et al. [16]reported the enhancement of convective heat transfer coefficient andfriction factorby adding Fe3O4 nanoparticles to water.Duangth-ongsuk and Wongwises [17] stated that water nanofluid consisting of0.2% volume TiO2 nanoparticles caused 6–11% enhancement in theheat transfer coefficient.
A group ofliteratures investigated the effects of nanoparticlesize and volume fraction on the heat transfer [18–25]. Wongchareeand Eiamsa-ard [26] studied CuO-water nanofluid in three differ-ent volume fractions of 0.3%,0.5%,0.7% for a laminar regime.Thereults exhibited an improvement of Nusselt number as nanofluidconcentration rose. Santra et al. [27] in their assessment of copper-water nanofluid for a range of Reynolds numbers (Re¼5 to 1500)and solid volume fraction between 0.00 and 0.05 assuming thefluid in two phases (Newtonian and non-Newtonian), observed theenhancement ofheat transfer with enrichment in solid volumefraction.Fotukian and Nasr-Esfahany [28]investigated the heattransfer features ofγ-Al2O3/water nanofluid in a circular tube witha solid volume fraction less than 0.2%.By adding nanoparticles towater,thermalconductivity augmented.Meanwhile,increasingsolid volume fraction beyond 0.2% caused no change in the heattransfer rate.Araniand Amani[29] in an experimentalresearchexamined TiO2-water nanofluid with Reynolds numbers between8000 and 51000 and volume fraction in the range of 0.002–0.02.Heat transferwas improved with increasing ofnanoparticlesvolume fraction.They also observed that at high Reynolds num-bers,more power is needed to overcome the pressure drop ofnanofluid,so it is not beneficial to use nanofluid at high Reynoldsnumbers compared to low Reynolds numbers.Sebdani et al.[30]investigated Al2O3-water in mixed convection in a square cavity atconstantRayleigh numbers,the results demonstrated the heattransfer reduction for low Reynolds number (Re¼1) while volumefraction was more than 0.05,but in high Reynolds number (10–100),increasingof nanoparticlespercentage,enhanced heattransfer. Also, for a constant Reynolds number, the effect of addingnanoparticles on heat transfer was correlated to Rayleigh number,so thataugmentation ofheattransfer continued untilRa¼10 3
while forRa¼10 4 and Ra¼10 5 heat transferdecreased withadding more nanoparticles [30].
The reports on the effect of nanoparticles size on the thermalconductivity are antagonist.A numericalmodeling research byLelea[31]showed that at constantReynoldsnumbers in amicrochannel heat sink, the enhancement of heat transfer reducesas Al2O3 nanoparticle diameter increased in base fluid.Teng et al.[32] surveyed the changes in heat transfer of Al2O3-water nano-fluid at different diameter size ofnanoparticles and a variety oftemperatures; they declared better thermal conductivity in smal-ler nanoparticle diameter.The interesting aspect of this study wasthat the heat transfer enhanced considerably at higher tempera-tures.In contrary,Beck et al.[33] observed reduction of thermalconductivity for water-based and ethylene glycol-based aluminawith decreasing in particle size.The same results were obtainedfor water-gold nanofluid by Shalkevich et al.[34].
Nanofluid may be utilized as a coolant for electronic devices.Recently they are used in heatsinks to improve thermalcon-ductivity [35–41].Ijam and Saidur [42] investigated the influenceof SiC-waterand TiO2-waternanofluidsas the coolantin aminichannelheat sink,the results exhibited an improvement inthermal conductivity compared to base fluid.In another study by
Selvakumarand Suresh [43]on CuO-water nanofluidsin anelectronic heat sink,the same results were obtained.Hung andYan [44] researched on a double-layered microchannelheat sinkand demonstrated that adding Al2O3 nanoparticles to water raisesthe thermal performance.Nanofluid is also capable to improve oilrecovery,Suleimanov etal.[45]demonstrated thatan aqueoussolution ofanionic surface-active agents with addition oflightnon-ferrous metal nanoparticles permitted a 70–90% reduction ofsurface tension on an oilboundary in comparison with surface-active agent aqueous solution and is characterized by a shift indilution.
By the rapid expansion in global population,demand for moreenergy sources is notrefutable.Since fossilenergy sources arebeing restricted, solar energy is acquiring worldwide attention as aproper alternative which is completely environmentally benign.Solar energy converting systems suffer from low efficiency; henceharvesting solar radiation with a high efficiency technology is thekey issue.Nanotechnology has opened a new field to solve thisdeficiency.Nanofluid plays a key role to enhance the efficiency insolar systems.In this paper,the previous studies on nanofluidapplications in solar systems has been reviewed and an analysisare carried out on the achievements.
2. Applications of nanofluids in solar energy
2.1.Solar collector
In solarcollectors,the absorbed incidentsolarradiation isconverted to heat.The working fluid conveys the generated heatfor different applications.Solar collectors are categorized in towtypes,non-concentrating and concentrating collectors [46].Non-concentrating solar collectors are usually used for low and med-ium temperature applications such as space heating and cooling,water heating,and desalination.While concentrating solar collec-torsare exploited in high temperatureapplicationssuch aselectricity generation.However these systems are acquiring moreand more attention,prevailing to low efficiency is still a big deal.Nanofluid has shown a good ability in enhancing the efficiency ofsolar systems.In this part,the research over employing nanofluidin solar collectors are reviewed.
Tyagi et al.[47] theoretically investigated the performance of adirect absorption solar collector (DAC) exploiting aluminum-waternanofluid as the absorbing medium.Fig. 1 shows the schematic ofa nanofluid-based DAC of their study with glass surface on the topand completely isolated atthe bottom side.They supposed asteady-state two-dimensional model for heat transfer. By using thefollowing equation,the collector efficiency is obtained:
η¼ useful gainavailable energy¼
_mC pðToutT inÞAGt
ð1Þ
where _m is the mass flow rate flowing through the collector,cp isthe specific heat,Tin and Tout are the mean fluid inlet and outlettemperatures respectively,A is the area of the collector and Gtisthe solar flux incident on the solar collector.Fig.2 depicts thecollector efficiency versus the variation ofparticles size in therange of1–20 nm.The collectorefficiency increased graduallywith ascendance of nanoparticle size.They attributed this to theenhancement of absorption coefficient which is directly affectedby the term D2. From Fig.3, the augmentation ofcollectorefficiency is obvious as the volume fraction increases.This is dueto the enhanced attenuation ofsunlightpassesthrough thecollector.Since the attenuation varies exponentially with volumefraction,the efficiency initially increases rapidly at low concen-trations and then reaches an asymptotic value in higher concen-trations more than 1%.The resultrevealed that,under similar
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operating conditions,the efficiency fornanofluid-based directabsorption solar collectors is 10% higher compared to the conven-tional models using pure water.
Otanicar et al.[48] examined the effect of different nanofluids(carbon nanotubes,graphite and silver) on the performance of adirectabsorption collectorexperimentally and compared theresults with numerical models.Fig.4 demonstrates the schematicof the setup which isa micro solar-thermalcollectorwith a3 5 cm2 surface area and 150μm channeldepth;they used aSuper PAR64 lamp to simulate the solarspectrum.The sameequation ofTyagi's study (Eq.(1))was applied to evaluate theexperimentalefficiency ofthe collector.In the numericalmodel,they modified the work ofTyagiet al.[47]using radiativetransport equations (RTE) coupled to the energy equations whichinvolved emission term compared to the previous work.Fig.5exhibits the efficiency ofthe modeland experimentfor 30 nmgraphite spheres with a 5% discrepancy in comparison.Fig.6demonstratesthe experimentalresultsof collectorefficiencyversus volume fraction variations for different nanoparticles.Asit is shown, the efficiency ascended with enhancement of particlesconcentration but,after a volume fraction of5%,the efficiencydiminished slightly.The reason is that the transmittance of wateris approached atlow particle concentrations and little heatingoccurs,while athigh particle concentrationswe expect highabsorption ofsolarincidentforthe nanofluid.Allin alltheenhanced efficiency is due to three reasons,modification oftheopticalproperties ofthe fluid,heatloss reduction as the peaktemperature places away from surface,and thermalconductivityenhancement.According to thenew model expectation,theinfluence ofparticle size on the efficiency was in contrast with
Fig.3.Collector efficiency (Eq.1) as a function of the particle volume fraction (ƒv)(D¼5 nm) [47].
Fig.4.The micro-solarthermalcollector experimentalschematic in Otanicar'sstudy [48].
Fig.5.Comparison ofmodeling and experimentalresultsfor30 nm graphitespheres [48].
Fig.2.Collector efficiency (Eq.1) as a function of the particle size (D) (ƒv¼0.8%)[47].
Fig. 1.Schematic of the nanofluid-based direct absorption solar collector in Tyagi'sstudy [47].
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Tyagi's results.Otanicar modified Tyagi's model by accounting theheat loss in the collector and impact of particle size appears in theabsorption and scattering efficiency.As it is shown in Fig.7,theefficiency decreases by particle size enhancement.Tiwariet al.[49]studied a nanofluid-based flat-plate collector theoretically.They demonstrated enhancement ofcollector efficiency and thepotential of reducing 31% in CO2 emission in comparison with theconventionalmodels.Yousefi et al.[50] experimentally exploredthe effect of Al2O3-H2O nanofluid on the efficiency of a flat-platesolar collector,using ASHRAE standard to evaluate the efficiency.Results displayed 28.3% enhancementin efficiency for0.2 wt%nanoparticles concentration compared to pure water.In anotherstudy by Yousefi etal.[51]utilizing MWCNT-H2O nanofluidenhanced the efficiency of a flat-plate solar collector.
Taylor et al. [52] investigated utilizing nanofluid receiver in powertower solar collectors theoretically,they also applied nanofluid in alaboratory-scale dish receiver;in both cases the enhancementofefficiency was observed comparing to base fluid.Sani et al.[53] andMercatelli et al.[54] investigated the potentiality of utilizing single-wall carbon nanohorns (SWCNHs) in ethylene glycol suspension andnominated that as a good choice for exploiting in solar collectors.Khullar et al.[55] investigated the enhancement of solar irradianceabsorption capacity for nanofluid-based concentrating parabolic solarcollectors(NCPSC)theoretically and compared the resultswithexperimentaldata ofconventionalconcentrating parabolicsolarcollectors which demonstrated 5–10% higher efficiency as comparedto the conventional models.Kasaeian et al.[56,57] have studied the
heat transfer modeling for different nanofluids, they also investigatedthe heat transfer enhancement for Al2O3/synthetic oil nanofluid in aparabolic trough collector tube numerically [58].
Solar energy conversion to heat or electricity mainly utilizessurface absorbers.Temperature difference between absorber andheattransfer fluid is a common imperfection in these systemswhich happen due to thermal resistance at interfaces.One of thesolutions to reduce heatloss is volumetric absorption.In volu-metric absorption,solar radiation is absorbed by a volume of heattransfer fluid directly.Attaining better properties through addingsmall solid particles to base fluid was firstly suggested by Abdel-rahman et al. [59]. Some researchers declared utilizing the conceptof volumetric absorption in solar power collectors [60,61].Veer-aragavan et al.[62] made an analytical model for volumetric solarflow receivers,which employed nanoparticles suspended in thebase fluid and displayed an improvementin solarconversionefficiency by decreasing the temperature differencesbetweenthe absorber and fluid. Lenert and Wang [63] studied the influenceof different variations in nanofluid volumetric receivers theoreti-cally and experimentally.In their theoreticalpart,a one dimen-sionaltransientheat transfermodel was supposed and theenhancement ofreceiver efficiency with augmentation ofnano-fluid height (H) and incident solar flux was proved. The schematicof this model is shown in Fig.8.In the experimental part,carboncoated cobalt nanoparticles were added to Therminols VP-1in aliquid-based volumetricreceiver.For the temperaturesbelow700 K,enhancementof nanofluid heightlowered the receiver'sefficiency.For the temperatures between 800 and 1200 K,theefficiency enhanced while no effectwas observed for the tem-peratures above 1300 K.
Taylor et al. [64] in their experiment investigated the possibilityof volumetric absorption fordirectsteam collection mediums.They used laser light as the radiation with wavelength of 532 nm.Three absorbing mediumsincluding black dyes,black paintedsurfaces and nanofluids were employed in the study.For purewater with black backing without nanofluid, the temperature rosebeyond 300 1C;but lower temperature was obtained for nano-fluids although vaporgeneration enhanced up to 50%.Theyconcludedthat applyingnanofluid enhances the volumetricabsorption;hence directsteam nanofluid collectors can signifi-cantly improve the efficiency ofthe lightto steam conversion.Kandasamy etal.[65,66]investigated the Hiemenz flow ofCu-nanofluid over a porous wedge plate which plays an importantrole in volumetric absorption of the incident solar radiation andtransferring the thermalenergy to the fluid.It should be notedthat if the nanofluid is too dense,light will be absorbed by a thinlayer and thermalenergy willeasily be lost.In the other side,ifthe concentration ofnanoparticles is too low,most oflight istransmitted;so obtaining a properportion ofnanoparticle forvolumetric absorption is essential. Fig. 9 demonstrates the thermal
Fig.7.Collector efficiency as a function of silver nanoparticle diameter (squares:bulk properties; circles: size-dependent properties) and volume fraction [48].
Fig.8.Schematic for the model formulation in Lenert and Wang's study of a 1-Dvolumetric solar receiver with a transparent top (y¼ 0) where τr¼1 and a specularreflective adiabatic bottom (y ¼ H) where ρr¼ 1 [63].
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resistances for a surface-based and for a volumetric-based collec-tor in a solar plant,so its obvious that the thermal resistances areclearly lower for a volumetric-based collector.Otanicar et al.[48]in theirstudy on a DAC,observed thatvolumetric absorptioncauses the maximum temperature to take place in the vicinity ofthe center rather than the collector surface; hence heat loss wouldbe minimum. Thisisan importantmechanism in volumetricreceivers that causes efficiency enhancement.
Nanoparticles would change the optical properties of base fluidthat are suspended in which candidate them to be exploited asopticalfiltersfor a variety ofapplications[67].Kameya andHanamura [68] studied the radiation absorption characteristics ofNi nanoparticles suspension, the absorption increased drastically forwavelengthsin visibleand near-infrared;also theabsorptioncoefficient remained constant for infrared region.They proposedit as a helpfulmechanism to be utilized in DAC.Han et al.[69]investigated the opticalproperties,the rheologicalbehaviors andthe thermal conductivity of carbon black-water nanofluid for solarabsorption purposes; the results exhibited the eminent potentialityof carbon black nanofluidfor using in solar systems. Saidur et al. [70]studied the effect of exploiting aluminum-water nanofluid in directsolarcollectors.Aluminum nanoparticlesenhanced the lightabsorption in visible light and shorter wavelengths despite of lowerextinction coefficient.Extinction coefficientand volume fractionwere linearly proportionate.The authors mention thatalthoughparticle size has the minimal influence on the optical properties,inorder to benefit Rayleigh distribution,the particle size should beunder 20 nm.Also to prevent agglomeration and stability ofthesuspension,after obtaining the optimized opticalproperties thevolume fraction should notascend any more.Taylor etal.[71]investigated the optical properties of various nanofluids those canbe efficiently applied in direct absorption solar collectors.Table 1shows the results of this study which are the nanoparticles with theneeded volume fraction and thicknessto absorb over95% ofincoming sunlight.
Stability of nanofluid is an important factor in its performancewhich is supplied usually via adding pH buffers,surfactants orchemical treatment.Many researchers have surveyed the effect ofpH fluctuations on the thermal conductivity of nanofluids [72–77].According to the DLOV theory [78] when the pH value of nanofluid
is equalor close to the pH ofisoelectric point(the pointthatmolecules carry no electrical charge), colloidal particles are instable.As the pH value of nanofluid diverges positively or negatively fromthe isoelectric pointthe particlescharge isenhanced and therepulsion forces between particles increase thus less agglomerationappears.This mechanism causes more stability of the suspensionwhich consequently leads to better thermal conductivity [79–81].
Yousefi etal.[82]in an experimentalstudy investigated theeffects of pH variation for MWCNT-H2O nanofluid on the efficiencyof a flat plate solar collector.A water-based MWCNT with 0.2 wt%and Triton X-100 as surfactant for dispersion of nanoparticles wereused.The schematic ofthe experimentis shown in Fig.10;theefficiency of the collector is described by Eq.(2).The parametersdescription and their numericalquantities are shown in Tables 2and 3 respectively.According to Eq.(2)and Table 3,Fig.11 isdepicted for the efficiency of a flat-plate solar collector importingMWCNT nanofluid at three different pH values where the mass flowrate is 0.0333 kg/s.With respectto water as the base fluid,theenhancement of efficiency is obvious for nanofluids. Also, in greaterdifferences between the pH of nanofluid and pH of isoelectric point,more enhancements in the efficiency are observed.
ηi¼ FRðταÞF RULTiT aGT
ð2Þ
However,the system efficiency can be improved by the nanofluids,sedimentation has been observed for the solid phase.Therefore,ifnanofluids have to be used,it is essential to prevent any potentialsedimentation of the solid phase.To achieve this aim,G.Colangeloet al. [83] analyzed the flat plate solar collectors' sedimentation andtested a suitable solution to avoid it.They investigated the stabilityof different water-Al2O3 nanofluids with 1 vol%, 2 vol% and 3 vol% inorder to select the most stable suspension.They used an opticalinvestigation to study the thermal conductivity,k,the heat transfercoefficient,h,and the sedimentation in a flat plate solar thermalcollector.The hot wire technique measurements showed that theenhancement for thermalconductivity is directly proportionaltothe volume fraction which reaches up to 6.7% for 3 vol% of Al2O3.The convective heat transfer coefficient was also enhanced for both
Fig. 10.The schematic of the experimental model used by Yousefi et al.[82].
Fig.9.Thermalresistance network - comparison between a conventionalsolarthermalplant and a nanofluid solar thermalplant.Rabs,Rcd,Rcv,RH. Ex,and Rabs'refer to the thermal resistance of solid surface absorption,conduction,convection,fluid-to-fluid heat exchange,and volumetric solar absorption heat transfer steps,respectively [52].
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laminar and turbulent flow regimes where it increased up to 25%for water-Al2O3 nanofluids with 3 vol%.The authors found that theamount of the deposited solid phase is directly proportional to thevolume fraction of the solid phase.However,it is inversely propor-tionalto the average fluid velocity inside the top and bottomheaders.To reducesedimentation in flatplatesolarthermalcollectors, the velocity along the bottom header and the top headerwas kept constant by variation of the cross section areas.
2.2.Evacuated solar collectors
In the thermodynamic viewpoint,solar collectors with evacu-ated tubes have many advantages compared to flat-plate collec-tors.However,evacuated solar collectors are not yet competitivewith the conventionaltypes economically [84,85].Zambolin andDel Col [86]in an experimentalstudy examined the thermalperformance of a flat-plate and an evacuated tube collector oversimilar operating conditions.The results demonstrated a higherefficiency forevacuated collector.Lu etal.[87]evaluated the
influence ofwater-based CuO nanofluids upon an open thermo-syphon utilizing in a high-temperature evacuated tabular solarcollector(HTC).Compared to water,nanofluid improved thethermal performance of the evaporator; also a 30% enhancementwas observed for the evaporating heat transfer coefficient.Fig. 12shows the relation between evaporating heat transfer coefficient(he) and heat flux via different CuO nanoparticle concentrations(0.8–1.5 wt%).Shahi et al.[88]simulated thesteady naturalconvective flow and heattransfer for a single-ended evacuatedsolar tube over utilizing copper-water nanofluid.
2.3.Photovoltaic thermal systems
Hybrid photovoltaic thermalsystems consist of two parts,PVmodules and heat extraction part which cools PV module.Thesesystems are capable ofproducing electricaland thermalenergysimultaneously;hence the overallefficiency ofPV/T systems isgreater than PV systems [89–92].Consequently the effective costsfor PV/T systems are lower.Usually,the heat is rejected by air orwater in PV/T systems [93]. PVT/water systems take the advantageofa higher efficiency in comparison with PVT/air systems [94].Optimizing optical properties of the working fluid in PV/T systemscan improve the efficiency, it means that the more transmission ofthe visible lightand the more absorption ofthe solar infraredradiation,improve the performance of PV/T systems.
Zhao et al.[95] employed a damped oscillator Lorenz–Drudemodel to investigate the opticalproperties of working fluid in aPV/T system which satisfied the Kramers–Kronig relations.Theinverse method based on genetic algorithm was applied to obtainthe refraction from the transmittance on the absorptance.Theoptimization includesmaximizing both transmittance ofsolarincidents owning wavelength between 200 nm and 800 nm andabsorption ofinfrared partof solarradiation forwavelengthbetween 800 nm and 2000 nm which leaded to 92% absorption
Fig. 12.Effect of concentrations of nanofluids on the evaporating HTC [84].
Table 2Description of the parameters in Eq.(2).
Parameter FR τα UL Ti Ta Gt
Description heatremovalfactor
Absorptance-transmittance product
overall loss coefficient of solarcollector (W/m2K)
inlet fluid temperature ofsolar collector (K)
outlet fluid temperature ofsolar collector (K)
global solarradiation (W/m2)
Table 3FR(τα) and FRUL values of the solar collector for each test [82].
Basic fluid typeFRUL Uncertainty (%)(n¼5)
FR(τα) Uncertainty (%)(n¼5)
R2
Water 36.952 3.4 0.5005 3.8 0.975Nanofluid at
pH¼3.524.284 2.8 0.736 3.4 0.975
Nanofluid atpH¼6.5
38.841 4.5 0.742 5.5 0.986
Nanofluid atpH¼9.5
30.2 5.1 0.809 6.3 0.978
Fig. 11.The efficiency of flat-plate solar collector with MWCNT nanofluid as basefluid at three pH values as compared with water in 0.0333 kg/s mass flow rate [82].
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of the solar radiation and 89% transmittance of the visible light forthe optimized working fluid.
Proper spectral tuning for optical properties of working fluid inPV/T systemscan be done by nanofluids.Tayloret al.[67]investigated the optimization ofnanofluid-based opticalfilterstheoretically for PV/T systems.Five kinds of PV cells were chosenin their study (InGaP, CdTe, InGaAs, Si, Ge) to inquire the versatilityof nanofluid filters over the solar spectrum.The purpose was toattain an optimized modelfor working fluid to have maximumtransmittance between absorption spectrum ofeach cellandmaximize the absorption out of this range.Table 4 demonstratesthe optimum absorption spectrum for each PV cell.They focusedon core/shell nanoparticles. In these materials, the optical featuresare controllable by changing the shellto core radius ratio.Theyused the following equation to obtain the nanoparticles volume
fraction (fv) for optimizing.
σithparticle¼32fvQ ithext
D ð3Þ
where σ is the particle extinction coefficient,iis the ith particle,Q extrepresents the extinction efficiency of the particle and D is theparticle diameter.The efficiency ofthe nanofluid filterswereattained by Eq.(4).
η¼RlongλshortλEλTλdλRlongλshortλEλdλ
Rshortλ0 EλTλdλRshortλ0 Eλdλ
R4μmlongλEλTλdλR4μmlongλEλdλ
ð4Þ
Three types of particle nanofluid liquid filters were modeled indifferentparticle sizes and volume fractions.Fig.13 depicts theabsorption of the optimized nanofluid-based filter for Indium Gal-lium Phosphate cells in comparison with differentfluids.Table 5shows the results of this study.It was inferred that nanofluid-basefilters almost have the same performance with conventionalfiltersand as the small bulk of metal is needed for core/shell nanoparticles,these introduce low-cost filters to the industry.
Cuiand Zhu [96] studied the influence of nanofluid for a PV/Tsystem.In their research,MgO-water nanofluid was used as thecoolant which flowed on the top of silicon solar cells.The experi-ment setup is shown in Fig. 14.The observations revealed that theenhancement of both particles volume fraction and nanofluid filmthickness cause reduction in the output power of solar cells in PV/Tsystem which is attributed to the degradation of nanofluid trans-mittances.Fig.15 shows the transmittances of nanofluid in threedifferentvolume fractions.Figs.16 and 17 show the effects ofnanofluid volume fraction and film thickness on the output of solarcells in PV/T systems respectively. The output power for a silicon PVis also demonstrated in Fig. 17; it infers that the electrical output fora bare PV is higher in comparison with PV/T systems,while theoverall efficiency of PV/T systems is greater than PV systems.
2.4.Thermal energy storage
Obtaining electricity from solar energy is applicable by usingphotovoltaic or solar-thermal energy conversion systems which ismore reliable and costeffective in large scalescomparing tophotovoltaic systems.A storage medium plays the key role insolar-thermal power plants which should take advantage of highthermalconductivity,also capable of operating at high tempera-tures.Some of materialsused as heat transferfluid in highthermal-energy storage are Na-K eutectics and alkalimetalsaltseutectics [97].Usually these materials have poor thermo-physicalcharacteristics [98].Increasing the thermal conductivity,the spe-cificheat capacity ofthe storage medium also the operatingtemperature of these materials willimprove the thermodynamicefficiency ofsystem.Importing gaseous working fluid ofsmall
Fig. 13.Indium gallium phosphate cell filter comparison. Absorptance is shown for:an idealfilter (arbitrary thickness),a ‘good’pure fluid (192 mm H2O),a conven-tionalthin film filter (w/200 mm H2O) and a nanofluid filter (20 mm thickness)[67].
Table 5Comparison table of nanofluid optical filters [67].
A.Kasaeian et al./ Renewable and Sustainable Energy Reviews 43 (2015) 584–598590
absorbing particles for solar-thermal electric generation was firstlymentioned by Hunt in 1978 [99].
Nanofluid has been introduced asthe viablesolution toimprove heat transfer properties also enhancing the specific heatcapacity (SHC) in some research.In Long Jianyou's research [100],adding 9 wt% aluminum nanoparticlesto paraffin and 0.5 wt%SDBS as dispersant,improved the heattransferpropertiesofparaffin for thermal energy storage applications.
Colangelo et al. [101], in an experimental work, investigated theheat transfer properties of diathermic oil through adding differentnanoparticles.This kind ofoilis exploited in high temperatureapplicationssuch as solarthermodynamic plants.In anotherexperimentalwork,Shin and Banerjee [97] investigated the SHCvariations of 1 wt% SiO2 nanoparticles in alkali metal chloride salteutecticswhich can be used forsolarthermal-energy storageapplications.They observed 14.5% augmentation in specific heatcapacity overneat chloride salteutectic.They enounced thatEq.(5),cannot explicate the anomalous enhancement of SHC:
cp;nf¼φðρcpÞnþð1 φÞðρcp;fÞf
φρnþð1 φÞρf ð5Þ
In anotherstudy by Tiznobaikand Shin [102]on high-temperature molten salt-based nanofluidsat 1% concentrationweightand differentsize ofnanoparticles,the enhancementofthe SHC was observed.Consequently,solar electricity cost degra-dation is expected due to reduced amount ofrequiring storagemedium and reduced size of thermal transport system. Nelson andBanerjee [103] observed 50% enhancement in SHC of nanofluidsover neat polyalphaolefin in their experiment.
In contrast,some studiesreportthe reduction ofSHC fornanofluids.O’Hanley etal.[104]examined the accordance oftheoreticalmodels with experimentalresults for obtaining SHCof nanofluids.Two models are proposed for evaluating SHC ofnanofluids.Model Ι[105–107] based on the mixing theory andmodel II [105,108–111] based on thermal equilibrium mechanismare brought in Eqs.(6) and (5),respectively.In Eq.(6),Cp,n,fis thenanofluid specific heat, Cp,nis the nanoparticle specific heat, Cp,fisthe base fluid specific heat andφ declares the volumetric fractionof nanoparticle.For Eq.(6),m nfis an arbitrary mass of nanofluidand volume Vnf,the nanofluid density isρn,f¼V nf/mnf¼ φ ρnþ(1 φ)ρf where ρn and ρf are the particle and fluid densitiesrespectively.The amount of the needed energy to elevate thenanofluid massisφVnfCp,nþ(1 φ)Cp,fhence the specific heatwould be calculated by Eq.(6).
cp;nf¼ φcp;nþð1 φÞcp;f ð6ÞFig. 16.Output power of solar cells in PV/T systems with different mass fractions ofnanofluids (film thickness: 4 mm) [95].
Fig. 17.Output power of solar cells in PV/T system with different film thicknesses(mass fraction: 0.02 wt%) [95].
Fig. 14.The experimental setup for PV/T system in Cui et al.[95].
A.Kasaeian et al./ Renewable and Sustainable Energy Reviews 43 (2015) 584–598 591
O’Hanley et al.[104] analyzed three nanofluids in their study,alumina-water,silica-water and copper oxide-water.The resultswere in a good agreement with model II, also reduction of SHC wasobserved with ascendanceof nanoparticlesvolume fraction.Figs.18–20 demonstrate the results at35 1C for alumina-water,copper oxide-water and silica-water,respectively.
An examination by Zhou etal.[106]on CuO/EG nanofluidsrevealed reduction of SHC through increasing CuO nanoparticlesvolume fraction.Zhou and Ni [104] detected reduction of SHC forwater-based Al2O3 with enhancementof nanoparticles volumefraction.The same results were obtained by Namburu et al.[112]on examining three samples,CuO/EG,SiO2/EG and Al2O3 whichwere compared with the pure EG.
2.5.Solar thermoelectric devices
In thermoelectricdevices,heat isconverted into electricitydirectly or indirectly which happens through the Seedbeck,Petlier
and Thomson associated effects [113–115].Thermoelectric deviceshave many applications in solar energy conversion,electronic cool-ing,vehicle air conditioners and refrigerators [116].Thermoelectricpower harvesting via solar energy is one ofthe alternatives inrenewable energy resources.Chang et al.[117] fabricated CuO thinfilms via electrophoresis deposition process with CuO nanoparticlessuspension and isopropanolas the dielectric.This CuO film wasadhered between a thermoelectric generator and dye-sensitizedsolar cells which elevated conversion efficiency ofsolar energy.Fig.21 shows a schematic of a solar-thermoelectric module.
2.6.Solar cells
The cooling improvementof solarcells leadsto the betterperformance of solar panels. Elmir et al. [118] simulated cooling fora solar cell by forced convection in the presence of a nanofluid. Thephysicalproperties were chosen for Al2O3-water nanofluid.Theresults unveiled that changing the solid volume fraction from 0.0%to 10% causes 27% increase ofthe heat transfer at low Reynoldsnumbers (Re¼ 5) which leads to better performance of the cell.Incontrary,Cuiand Zhu [96] mentioned the reduction in electrical
Fig. 20.Variation of specific heat capacity versus volume fraction for silica-water at35 1C [103].
Fig.18.Variation ofspecific heatcapacity versus volume fraction for alumina-water at 35 1C [103].
Fig. 19.Variation of specific heat capacity versus volume fraction for copper oxide-water at 35 1C [103]. Fig.21.Schematic diagram of a solar-thermoelectric module [116].
A.Kasaeian et al./ Renewable and Sustainable Energy Reviews 43 (2015) 584–598592
efficiency ofPV through adding MgO nanoparticles to water incomparison with exploiting water as the coolant.
In a differentapplication,Chen etal.[119]used TiO2-waternanofluid for coating TiO2 nanoparticles on the photoelectrodes ofDSSC utilizing Electric-Discharge Nanofluid-Process.Chang et al.[120]employed TiO2 nanofluidto create the anode ofDSSC viaelectrophoresis deposition.To evaluate the effects of nanofluid onsolar cells more studies are needed regarding newer temperature-dependent or temperature-independent models to evaluate ther-mophysical properties of nanofluid. For example using the modelspresented by Maiga etal.[121],Nguyen etal.[122],Koo andKleinstreuer[123],and Duangthongsuk and Wongwises[124],Chon et al.[125] can be beneficialto estimate the viscosity andthermal conductivity of nanofluid.
3. Economical and environmental aspects
Both economical and environmental aspects are important criter-ionsthatdefine the acceptability ofutilizing nanofluid in solar
systems.Otanicar and Golden [126]compared the economic andenvironmental features of nanofluid-based solar collectors with theconventionaltypes.The study was based on life cycle assessmentwhich is a capable methodology to evaluate the economicalandenvironmentalimpacts ofproducts.As itshown in Table 6,thecapital and maintenance costs are $120 and $20 higher for nanofluid-based collectors respectively for year life time. The payback period isless for the conventional collectors,but with assuming 15 years lifetime according to the better performance of nanofluid-based collec-tors,the life cycle savings would be nearly the same.
Fig.22 demonstrates a conventional collector beside ananofluid-based one; as it is seen a large portion of copper is replaced by steeland glass in the nanofluid-based collector, hence it results in 200 MJreduction in energy consumption for manufacturing nanofluid-basedcollectors compared to the conventional models.Also,according toTable 7,the total embodied energy is 9% lower for nanofluid-basedcollectors. Table 8 presents a comparison in the environmental viewpoint,as it is shown,the totalembodied energy is 9% lower for
Fig.22.Conventionalsolar collector (top) and nanofluid-based direct absorptioncollector (bottom) [125].
Table 7Embodied energy comparisons for conventional and nanofluid-based solar collec-tors [125].
Table 6Economic comparisons for conventional and nanofluid-based solar collectors [125].
Conventional solarcollector ($)
Nanofluid solarcollector ($)
Capital costsIndependent costs 200 200Area based costs 397.8 327.8Nanoparticles 188.79Total capital (one time cost)597.8 716.59Total maintenance (for 15year life)
96.23 115.35
Total costs 694.03 831.94
Electricity cost savings peryear
270.13 278.95
Years until electricitysavings ¼costs
2.57 2.98
Natural gas cost savings peryear
80.37 83.02
Years until natural gassavings ¼costs
8.64 10.02
Electricity priceNovember–March (per
kWh)0.08 0.08
May–October (per kWh) 0.09 0.09Daily service charge 0.25 0.25Gas price rate (per term)0.74 0.74Monthly service charge 9.70 9.70
Table 8Embodied energy emissions from a solar collector and consumer phase operationalenergy [125].
Emission Pollution from solar collectorembodied energy
Saving of solar collector
Conventional(kg)
Nanofluid-based (kg)
Conventional(kg)
Nanofluid-based (kg)
Carbondioxide(CO2)
599.77 564.94 1500.89 1550.33
Sulfur oxides(SOx)
0.51 0.48 0.83 0.85
Nitrogenoxides(NOx)
0.84 0.79 1.53 1.58'
A.Kasaeian et al./ Renewable and Sustainable Energy Reviews 43 (2015) 584–598 593
nanofluid-based collectors.Table 8 presents a comparison in theenvironmentalview point,as it is shown, manufacturingofnanofluid-based collectors comprises 34 kg less CO2 emissions andoperationally offset50 kg yearly,and 740 kg lessCO2 emissionsduring its lifetime in comparison to conventional models.
They also declared that ifutilization of nanofluid-based solarcollectors enhances up to 50% in Phoenix and Arizona, emission ofover 1,000,000 t of CO2,500 t of SOx,and 1000 t of NOx would be
reduced.Table 9 demonstrates the avoided damage costs which is$3 higher for nanofluid-based collectors and encompasses $1300savings over 15 years lifetime.
Taylor et al.[52] evaluated utilizing nanofluid receivers for a solarthermalpower planttheoretically.They appraised the amountofnanoparticles that would be needed for a solar thermal power plant(3 kg per each 1MWe) and with assuming the price of nanoparticles$1000/kg, the total capital investment would be $5/W where the costrise is less than 0.1% of the total capital investments.They proposedtwo conceptualdesigns for nanofluid receivers thatsubstitute theconventionalmodels in the solar plant which are cheaper than theconventionalceramicreceivers,Fig.23 depictsthese conceptualmodels.They used graphite/therminolVP-1 nanofluid in the modelwith a volume fraction less than 0.001%. The conservative calculationsexhibited that for a 100 MWe power tower solar plant operating inTucson- Arizona,more than about$3.5 million is obtained in theyearly revenue and the payback time of the plant reduces about twoyears.Khullar and Tyagi [127] surveyed the environmental impact ofnanofluid-based concentrating solar water heating system (NCSWHS).The results showed that utilizing NCSWHS can debate annualelec-tricity 1716 kWh/household/year,saving 206 kg/household /yearofliquefied petroleum gas (LPG) and the potentiality of reducing about2.2 103 kg of CO2/household/year.
Fig. 23.(a) Conceptual design of a nanofluid concentrating collector with glazing. (b) Conceptual design of a nanofluidconcentrating collector without glazing. (c) Conceptualdrawing of a conventional power tower solid surface absorber [52].
Table 9Yearly avoided damage costs for conventional andnanofluid-based solar collectors[125].
Cost($/kg)
Damage costs avoided ($)
Conventional solarcollector
Nanofluid based solarcollector
Carbon dioxide(CO2)
0.03 48.72 50.45
Sulfur oxides(SOx)
12.13 9.60 9.95
Nitrogen oxides(NOx)
18.40 27.13 28.12
Total 85.45 88.52
A.Kasaeian et al./ Renewable and Sustainable Energy Reviews 43 (2015) 584–598594
Table 10Summary of the previous research works on the application of nanofluids in solar systems.
Researcher and type ofstudy
Field of study Nanofluid type Particlesize (nm)
Findings
Tyagi et al.[47](Theoretically)
Direct absorption solar collectorAluminum/water 0–20 Efficiency increases with enhancement of volume fraction up to 2% and beyond thatremains nearly constant.Enhancement of collector efficiency with ascendance of nanoparticle size.
Otanicar et al.[48](Theoretically andexperimentally)
Direct absorption solar collectorGraphite/water Silver/water Carbon nanotube/water
6–40 nmdiameter,
Efficiency decreases through particle size enhancement.
1000–5000 nmlength
Efficiency increases with enhancement of volume fraction up to 5% and beyond that it mayeven diminish.
Volumetric absorption causes the maximum temperature to take place in the vicinity of thecenter rather than the collector surface; hence heat loss would be minimum.
Yousefi et al.[50](Experimentally)
Flat plate solar collector Al2O3-H2O and Triton X-100 is used as thesurfactant
15 For 0.2 wt% nanoparticles concentration,the efficiency enhanced 28.3%.
Surfactant caused 15.63% enhancement in efficiency.Yousefi et al.[51]
(Experimentally)Flat plate solar collector MWCNT-H 2O and Triton X-100 is used as the
surfactant10–30 For 0.2 wt% MWCNT without surfactant,the efficiency decreases while using surfactant
enhances the efficiency.For 0.4 wt% MWCNT without surfactant,the efficiency enhances.
Taylor et al.[52](Theoretically andexperimentally)
Nanofluid-based concentratingsolar thermal system
Therminol VP-1 as the base fluid and aluminum,copper,graphite,and silver as nanoparticles
20 Efficiency improvement in solar thermal systems
Khullar et al.[55](Theoretically)
Concentrating parabolic solarcollector
5–10% higher efficiency for nanofluid based concentrating parabolic solar collectors ascompared to the conventional model.
Kasaeian et al.[58](Theoretically)
Parabolic trough collector tubeAl2O3/synthetic oil nanofluid Enhancement of the nanoparticles concentration increases the convective heat transfercoefficient.In a constant mass flow rate,by increasing temperature,concentration on heat transfer coefficient diminishes.
Lenert and Wang [63](Theoretically andexperimentally)
28 The receiver efficiency increases with increasing nanofluid height and incident solar flux.
A 35% improvement in the receiver efficiency is expected when nanofluid volumetricreceivers are coupled to a power cycle.
Taylor et al.[67](Theoretically)
Optimization of nanofluid-basedoptical filters for PV/T systems
Al,Ag,SiO2,Au nanoparticles suspended in wateror VP-1
2–40 The optical properties of base fluid are tunable through utilizing low-cost nanofluid-basedfilters in PV/T systems.
Saidur et al.[70](Theoretically)
Direct absorption solar collectorAluminum-water o20 The light absorption in visible light and shorter wavelengths enhances despite of lowerextinction coefficient.Extinction coefficient and volume fraction were linearly proportionate.
Taylor et al.[71](Theoretically andexperimentally)
Direct absorption solar collectorWater as the base fluid and TiO2,Al,Au,Ag,Cu,graphite as nanoparticles
30 For nanofluid layer thickness Z10 and nanoparticle volume fractions less than 1 10over 95% of incident sunlight can be absorbed.
Therminol VP-1 as the base fluid and Al,Ag,Cu,graphiteas nanoparticles
40
Yousefi et al.[82](Experimentally)
Flat plate solar collector (MWCNT)/water in different pH values and TritonX-100 as the surfactant
10–30 Greater differences between the pH of nanofluid and pH of isoelectric point cause moreenhancements in the efficiency.
Lu et al. [86] (Experimentally)Evacuated tabular solar collectorCuO/water 50 Evaporating heat transfer coefficient increases about 30% and the optimal massconcentration is 1.2%.
Cui and Zhu [95](Experimentally)
PV/T system MgO 10 Enhancement of both particles volume fraction and nanofluid film thickness decreases theoutput power of solar cells in PV/T system.
Elmir et al.[118](Theoretically)
Solar cell cooling Al2O3 water Changing the solid volume fraction from 0.0% to 10% causes 27% enhancement of the heattransfer at low Reynolds numbers.
Table 10 presents a summary of the previous research work onthe application of nanofluids in solar systems.
3.1.Concluding remarks and directions for future work
Based on the literatures,the improved thermal conductivity ofnanofluid isthe most importantfactorforenhancing theefficiency in solar systems but a higher solid volume fractiondoes not always enhance the efficiency.The results on the effect of nanoparticle size for solar collectorefficiency are antagonist (see Refs.[47,48]),which needs moreexperimental research to do on the particle size effect.Volumetric absorption of nanofluid in solar collectors reducesthe thermal resistance at interfaces and minimizes temperaturedifference between absorber and heat transfer fluid; hence ahigher efficiency is expected.As the pH value of nanofluid diverges positively or negativelyfrom the isoelectricpoint,lessagglomeration appearsandconsequently leads to better thermal conductivity.The experi-mental study [82] exhibited the influence of this mechanism inimprovement of efficiency for a flat plate collector.Utilizing nanofluid in solar systems comprises many environ-mentaland economicalbeneficialaspects such as reducing ofCO2 emission through enhancing the efficiency,also less emis-sion in manufacturing process of nanofluid-based collectors. Asmentioned in the literature,Taylor [52] declared $3.5 millionmore in the yearly revenue for a 100 MW power tower solarplant exploiting nanofluid as the heat transfer medium.
This work focused on the characterization of various nanofluidsin solar systems,however further research is needed for betterunderstanding the effects of utilizing nanofluids in solar systemsas itdepends on many parameters such as particle size,poly-dispersity ofparticles,agglomeration,etc.The challenge is thelimitation of nanoparticles; also their specifications are not accu-rate. Hence development of the particle production and decreasingin costs is essentialfor the nanofluid research.Regarding to thescarce experimental work on some solar systems such as thermo-electric cells, parabolic trough systems, solar ponds or photovoltaicthermal systems,more data are needed to verify the performancevia imparting nanofluids.
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