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Bozorgan and Shafahi Micro and Nano Systems Letters (2015) 3:5 DOI 10.1186/s40486-015-0014-2
REVIEW Open Access
Performance evaluation of nanofluids in solarenergy: a review of the recent literatureNavid Bozorgan1* and Maryam Shafahi2
Utilizing nanofluid as an absorber fluid is an effective approach to enhance heat transfer in solar devices. Thepurpose of this review is to summarize the research done on the nanofluids applications in solar thermalengineering systems in recent years. This review article provides comprehensive information for the design of asolar thermal system working at the optimum conditions. This paper identifies the opportunities for future researchas well.
Keywords: Nanofluids; Solar energy; Solar systems; Heat transfer enhancement
IntroductionEnergy is an important entity for the economic develop-ment of any country. On the other hand, fossil fuels meet-ing a great portion of the energy demand are scarce andtheir availability is decreasing continously. Nowadays,solar systems play an important role in the productionof energy from renewable sources by converting solarradiation into useful heat or electricity. Considering theenvironmental protection and great uncertainty overfuture energy supplies, solar energy is a better alternativeenergy form in spite of its its slightly higher operationcosts. Heat transfer enhancement in solar devices is oneof the significant issues in energy saving and compactdesigns. One of the effictive methods is to replace theworking fluid with nanofluids as a novel strategy to im-prove heat transfer characteristics of the fluid. Morerecently reserachers have become interested in the useof nanofluids in collectors, water heaters, solar coolingsystems, solar cells, solar stills, solar absorption refri-geration systems, and a combination of different solar de-vices due to higher thermal conductivity of nanofluids andthe radiative properties of nanoparticle. How to selectsuitable nanofluids in solar applications is a key issue. Theeffectiveness of nanofluids as absorber fluids in a solar de-vice strongly depends on the type of nanoparticles andbase fluid, volume fraction of nanoparticles, radiative
* Correspondence: N.Bozorgan@gmail.com1Mechanical Engineering Department, Abadan Branch, Islamic AzadUniversity, P.O.B. 666, Abadan, IranFull list of author information is available at the end of the article
2015 Bozorgan and Shafahi; licensee SpringeCommons Attribution License (http://creativecoreproduction in any medium, provided the originwaiver (http://creativecommons.org/publicdomastated.
properties of nanofluids, temperature of the liquid, sizeand shape of the nanoparticles, pH values, and stability ofthe nanofluids . It was found that only a few review pa-pers have discussed the capability of nanofluids to enhancethe performance of solar systems [2-5].This paper com-piles recent research in this field and identifies many is-sues that are open or even not commenced to investigate.It is authors hope that this review will be useful to deter-mine the effectiveness of nanofluids in solar applications.
Literature review of recent yearsUsing nanofluids in solar collectorsRole of nanoparticlesGan et al.  experimently showed that the radiation ab-sorption of Al2O3 nanofluids is less than Aluminuimnanofluids. For nanofluids with Al2O3 particles, the situ-ation is different because of the different optical proper-ties of Al2O3. The weak radiation absorption of Al2O3nanoparticles will not result in significant localized con-vective heat transfer from the particles to the base fluids.The use of Al2O3/water nanofluid as coolant was simu-lated for a silicon solar cell using the finite elementmethod by Elmir et al. . They considered the solarpanel as an inclined cavity with a slope of 30. Applicationof nanofluids increased the average Nusselt number andrate of cooling. They reported 27% enhancement in theheat transfer rate for 10% alumina nanofluid at Re = 5.Luo et al.  simulated the performance of a DAC
solar collector with nanofluids using a 2D model bysolving the radiative transport equations of particulate
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Bozorgan and Shafahi Micro and Nano Systems Letters (2015) 3:5 Page 2 of 15
media and combining conduction and convection heattransfer equations. The nanofluid flows horizontallyfrom right to left in a steady-state solar collector coveredwith a glass plate. A solar radiation simulator is used tovalidate their model. They prepared nanofluids by dis-persing and oscillating TiO2, Al2O3, Ag, Cu, SiO2,graphite nanoparticles, and carbon nanotubes into Tex-atherm oil. Their results show that the use of nanofluidin solar collector can improve the outlet temperatureand efficiency. They also found that the efficiency ofmost nanofluids are similar and larger than that of oil,except for TiO2.Rahman et al.  performed a numerical study for a
triangular shape solar collector with nanofluids by Galerkinweighted residual finite element method for a widerange of Grashof numbers (Gr). The corrugated bottomis kept at a constant high temperature and the side wallsof the triangular enclosure are kept at a low temperatureas seen in Figure 1. It is assumed that both fluid phase andnanoparticles are in thermal equilibrium and there is noslip between them. Nanofluid is Newtonian and incom-pressible, and flow is laminar and unsteady. Constantthermophysical properties are considered for the nano-fluid except for the density variation in the buoyancyforces determined by using the Boussinesq approximation.
Figure 1 (a) Schematic of the triangular shape collector (b) 3D view of a s
Nevertheless, they have not mentioned the particles dia-meters. The authors concluded that high value of both Grand solid volume fraction confirms better heat transferthrough convection and conduction. Results showed24.28% improvement for Gr = 106 at 10% volume fractionof copper particles. For lower values of Gr number, con-duction is the primary mode of heat transfer for any valueof solid volume fractions. The results showed that theconvective heat transfer performance is better when thesolid volume fraction is kept at 0.05 or 0.08. This studyalso showed that cu-water nanofluid is the best nanofluidfor the augmentation of heat transfer.Faizal et al.  investigated the thermal performance
of nanofluid solar collector and its contribution sizereduction to estimate the cost saving. Their findings in-dicated that efficiency of solar collector with nanofluidsis calculated by the function of working fluid density,specific heat and mass flow rates. The results confirmedthat higher density and lower specific heat of nanofluidsoffers higher thermal efficiency than water and can re-duce the solar collector area about 25.6%, 21.6%, 22.1%and 21.5% for CuO, SiO2, TiO2 and Al2O3 nanofluids asseen in Figure 2. Hence, it will reduce the weight, energyand cost to manufacture the collector. The average valueof 220 MJ embodied energy can be saved for each
olar thermal collector filled with nanofluid .
Figure 2 Percentage of size reduction for solar collector by applying different nanofluids.
Bozorgan and Shafahi Micro and Nano Systems Letters (2015) 3:5 Page 3 of 15
collector, 2.4 years payback period can be achieved andaround 170 kg less CO2 emissions will be the result ofusing nanofluid based solar collector compared to a con-ventional one. Environmental damage cost is also lowerwith the nanofluid based solar collector.Parvin et al.  numerically investigated the effects of
the nanoparticle volume fraction ( = 0%, 1%, 3%, 5%and 7%) and the Reynolds number (Re = 200, 400, 600,800 and 1000) on the temperature distribution, rate ofentropy generation, and collector efficiency. The work-ing fluid was incompressible Cu-water nanofluid under alaminar regime. Their findings were as follows: a) Increas-ing the particles concentration raises the fluid viscosityand decreases the Reynolds number and consequentlydecreases heat transfer. b) It is important to find theoptimum volume fraction of nanoparticle for each ap-plication. c) The collector efficiency can be enhancednearly 2 times by using Ag-water and Cu-water nano-fluids with concentration of 3% as seen in Figure 3 d)The entropy generation is enhanced up to = 3% asseen in Figure 3. After this level, adding more nanopar-ticles makes no changes in mean entropy generation.
Figure 3 Collector efficiency (), mean entropy generation (S) and Bejan n
Ladjevardi et al.  numerically studied the effectsof using nanofluid on the performance of a solar col-lector as seen in Figure 4 considering the differentdiameter and volume fractions of graphite nanopar-ticles. They observed that in the infrared domain,the water optical characteristics are dominant while inthe UV and visible ranges extinction coefficients aredependent on nanoparticle volume fractions. The ex-tinction coefficient is calculated from the absorptionand scattering efficiencies in this research. Their nu-merical results showed that nanofluid collector ther-mal efficiency increases about 88% compared with theone in pure water collector with the inlet temperatureof 313 K. It also can be increased to 227% with the inlettemperature of 333 K.Filho et al.  studied silver nanoparticles as direct
sunlight absorbers for solar thermal applications. Theirresults showed that the maximum stored thermal en-erg