Performance evaluation of nanofluids in solar energy: a review of the recent literature
© Bozorgan and Shafahi; licensee Springer. 2015
Received: 4 September 2014
Accepted: 26 January 2015
Published: 20 May 2015
Utilizing nanofluid as an absorber fluid is an effective approach to enhance heat transfer in solar devices. The purpose of this review is to summarize the research done on the nanofluids’ applications in solar thermal engineering systems in recent years. This review article provides comprehensive information for the design of a solar thermal system working at the optimum conditions. This paper identifies the opportunities for future research as well.
Energy is an important entity for the economic development of any country. On the other hand, fossil fuels meeting a great portion of the energy demand are scarce and their availability is decreasing continously. Nowadays, solar systems play an important role in the production of energy from renewable sources by converting solar radiation into useful heat or electricity. Considering the environmental protection and great uncertainty over future energy supplies, solar energy is a better alternative energy form in spite of its its slightly higher operation costs. Heat transfer enhancement in solar devices is one of the significant issues in energy saving and compact designs. One of the effictive methods is to replace the working fluid with nanofluids as a novel strategy to improve heat transfer characteristics of the fluid. More recently reserachers have become interested in the use of nanofluids in collectors, water heaters, solar cooling systems, solar cells, solar stills, solar absorption refrigeration systems, and a combination of different solar devices due to higher thermal conductivity of nanofluids and the radiative properties of nanoparticle. How to select suitable nanofluids in solar applications is a key issue. The effectiveness of nanofluids as absorber fluids in a solar device strongly depends on the type of nanoparticles and base fluid, volume fraction of nanoparticles, radiative properties of nanofluids, temperature of the liquid, size and shape of the nanoparticles, pH values, and stability of the nanofluids . It was found that only a few review papers have discussed the capability of nanofluids to enhance the performance of solar systems [2-5].This paper compiles recent research in this field and identifies many issues that are open or even not commenced to investigate. It is authors’ hope that this review will be useful to determine the effectiveness of nanofluids in solar applications.
Literature review of recent years
Using nanofluids in solar collectors
Role of nanoparticles
Gan et al.  experimently showed that the radiation absorption of Al2O3 nanofluids is less than Aluminuim nanofluids. For nanofluids with Al2O3 particles, the situation is different because of the different optical properties of Al2O3. The weak radiation absorption of Al2O3 nanoparticles will not result in significant localized convective heat transfer from the particles to the base fluids. The use of Al2O3/water nanofluid as coolant was simulated for a silicon solar cell using the finite element method by Elmir et al. . They considered the solar panel as an inclined cavity with a slope of 30°. Application of nanofluids increased the average Nusselt number and rate of cooling. They reported 27% enhancement in the heat 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 by solving the radiative transport equations of particulate media and combining conduction and convection heat transfer equations. The nanofluid flows horizontally from right to left in a steady-state solar collector covered with a glass plate. A solar radiation simulator is used to validate their model. They prepared nanofluids by dispersing and oscillating TiO2, Al2O3, Ag, Cu, SiO2, graphite nanoparticles, and carbon nanotubes into Texatherm oil. Their results show that the use of nanofluid in solar collector can improve the outlet temperature and efficiency. They also found that the efficiency of most nanofluids are similar and larger than that of oil, except for TiO2.
Tang et al.  prepared the carbon nanotube/PEG/SiO2 composites with high thermal conductivity from multiwall carbon nanotubes (MWCNTs), poly (ethylene glycol) (PEG) and inorganic SiO2. These composites had higher thermal conductivity than traditional phase-change materials (PCMs) because of the high thermal conductivity of MWCNTs. Their results clearly showed that PEG/ SiO2/MWCNT composites can effectively improve the efficiency of solar energy applications.
Saidur et al.  investigated the effects of different parameters on the efficiency of a low-temperature nanofluid-based direct absorption solar collector (DAC) with water and aluminum nanoparticles. One big advantage of using low-temperature systems is that solar collectors can be relatively simple and inexpensive. Additionally, there are a number of working fluids suitable to low-temperature operation. Commonly used base liquids are water, oil, and ethylene glycol. They accounted for the effects of absorption and scattering within the nanofluid to evaluate the intensity distribution within the nanofluid by the radiative transfer equation (RTE). In order to calculate the spectral extinction coefficient of the nanofluid that is sum of scattering coefficient and absorption coefficient, they investigated the optical properties of the based fluid and nanoparticles separately. Their results revealed that Aluminum/water nanofluid with 1% volume fraction improves the solar absorption considerably. They found that the effect of particle size on the optical properties of nanofluid is minimal, but in order to have Rayleigh scattering the size of nanoparticles should be less than 20 nm. They also found that the extinction coefficient is linearly proportionate to volume fraction.
Role of base fluid
Colangelo et al.  experimently showed that the thermal conductivity improvement of the nanofluids with diathermic oil is greater than that with water in high temperature applications such as solar collectores. They observed that the thermal conductivity reduced with increasing the size of nanoparticles.
Hordy et al.  made four different nanofluids by dispersing plasma functionalized multi-walled carbon nanotubes (MWCNTs) in water, ethylene glycol, propylene glycol and Therminol VP-1 heat transfer fluids with the aid of an ultrasonic bath. They examined both the long-term and high-temperature stability of CNT nanofluids for use in direct solar absorption. In this work plasma treatment applied to modify the surface of the MWCNTs to improve their dispersion property within the base fluid. This study reported a quantitative demonstration of the high temperature and long-term stability of ethylene glycol and propylene glycol-based MWCNT nanofluids for solar thermal collectors.
The authors attributed this reduction to the self-lubrication characteristic of GE. In addition, the results obtained from the thermogravimetric analysis showed the high thermal stability of GE/BF4 nanofluids. Their measurements showed that this novel class of nanofluids is very suitable for high temperature applications such as solar collectors.
Ho et al.  found the optimal concentration of alumina nanoparticles in doped molten Hitec (a nitrate salt) by maximizing its specific heat capacity. High-temperature molten salt typically has a high heat capacity and is effective as a working fluid for concentrating solar power (CSP) systems. Their findings are as follows: 1- The addition of less than 2% Al2O3 nanoparticles significantly increases the specific heat of Hitec metal at low temperatures as seen in Figure 15, 2- For the volume fractions less than or equal to 0.5%, adding Al2O3 nanoparticles has a negative effect on the specific heat in temperature of 335°C, 3- At all temperatures, a concentration of 0.063 wt.% provides the maximum enhancement of specific heat about 19.9%, 4- The scanning electron microscopic (SEM) images show that, even at a relatively low concentration, nanoparticles aggregate as clusters with the size of 0.2 to 0.6 μm in the grain boundaries of Hitec, 5- The findings of this study suggest that the concentration that yields favorable uniform dispersion and optimal pattern of particles or clusters may maximize the specific heat. The simplified model of the solid-fluid interfacial area demonstrates that interfacial area is maximal at a concentration of 0.023 wt.%. As the nanoparticle concentration increases above 0.023 wt. %, the formed clusters become larger and the interfacial area density between the solid clusters and the base fluid decreases which may reduce the increase in specific heat capacity. According to the results obtained from this study, the maximum enhancement of the specific heat capacity occurs at concentration of 0.063 wt.% instead of 0.023 wt.%. Indeed, some agglomeration of nanoparticles forming submicrometer clusters may be the best for the enhancement of specific heat capacity. However, the total interfacial area at concentration of 0.063 wt. % was slightly less than its value at concentration of 0.023 wt. %.
Role of surfactants
Singh et al.  added Cu to commercial solar heat transfer fluids (Therminol 59 (TH59) and Therminol 66 (TH66)) by the combination of temperature and ultrasonic ripening processes. They stated that surfactant selection has an important role in preparing stable nanofluids. Choosing the right surfactant is mainly dependent on the properties of the base fluids and particles. For example, silicon oxide nanoparticles were successfully dispersed in TH66 using benzalkonium chloride (BAC, Acros Organics) as a surfactant but the use of BAC surfactant with Cu nanoparticles did not provide sufficient stability of suspension due to the lack of specific interaction between the nanoparticles and the surfactant molecules. The bi-layer arrangement of surfactant molecules should provide good adhesion to the nanoparticle surface and miscibility with the aromatic solvent. In this work, authors used a combination of oleic acid and BAC and a mixture of octadecyl thiol (ODT) and BAC surfactants to disperse Cu nanoparticles in TH66 and TH59, respectively. They observed that 3D Cu nanoparticle agglomerates do not break by conventional sonication with ultrasound gun without temperature ripening. They showed that a sonication time of about 4 h leads to the effective breakup of Cu agglomerates into individual grains at a 120°C. They also concluded that Cu/TH66 nanofluids appear to be more stable than the Cu/TH59 nanofluids because of the higher dynamic viscosity.
Yousefi et al. [26,27] studied the effect of Al2O3 (15 nm) and MWCNT (10-30 nm) water nanofluid on the efficiency of a flat plate solar collector experimentally. The weight fractions of the nanoparticles were 0.2% and 0.4%, and the experiments were performed with and without Triton X-100 as surfactant. Their findings showed that the surfactant presence in the nanofluid extremely affects solar collector’s efficiency.
Lenert et al.  presented a combined modeling and experimental study to optimize the performance of a cylindrical nano-fluid volumetric receiver. They concluded that the efficiency is more than 35% when nanofluid volumetric receivers are coupled to a power cycle and optimized with respect to the optical thickness and solar exposure time. This study provides an important perspective in the use of nanofluids as volumetric receivers in concentrated solar applications. In this work, 28 nm carbon-coated cobalt (C-Co) nanoparticles dispersed and suspended in Therminol VP-1 after 30 min in a sonication bath without any surfactant.
Role of the pH
Using nanofluids in photovoltaic/thermal (PV/T) system
The influence of nanofluid on different solar thermal applications
Type of application
Luo et al. 
TiO2, Al2O3, Ag, Cu, SiO2, graphite, and carbon nanotubes in Texatherm oil
DAC solar collector
use of nanofluid in the solar collector can improve the outlet temperature and the efficiency
Rahman et al. 
Cu, Al2O3 and TiO2 in water
triangular shape solar collector
Results showed 24.28% improvement for Gr=106 at 10% volume fraction of copper particles. the convective heat transfer performance is better when the solid volume fraction is kept at 0.05 or 0.08.
Faizal et al. 
CuO, SiO2, TiO2 and Al2O3 in water
results confirmed that higher density and lower specific heat of nanofluids offers higher thermal efficiency than water and therefore can reduce the solar collector area about 25.6%, 21.6%, 22.1% and 21.5% for CuO, SiO2, TiO2 and Al2O3 nanofluids. Environmental damage cost is also lower with the nanofluid based solar collector
Parvin et al. 
Increasing the particles concentration raises the fluid viscosity and decreases the Reynolds number and consequently decreases heat transfer. There is a need to find the optimum volume fraction for each application
Ladjevardi et al. 
Their numerical results showed that nanofluid collector thermal efficiency increases about 88% compared with the pure water collector with the inlet temperature of 313 K. It also can be increased to 227% with the inlet temperature of 333 K.
Said et al. 
single wall carbon nanotubes, Al2O3, TiO2 and SiO2
flat plate solar collector
It was observed that SWCNTs nanofluids could reduce the entropy generation by 4.34% and enhance the heat transfer coefficient by 15.33%
Saidur et al. 
direct absorption solar collector
Their results revealed that Aluminum/water nanofluid with 1% volume fraction improves the solar absorption considerably. They found that the effect of particle size on the optical properties of nanofluid is minimal, but in order to have Rayleigh scattering the size of nanoparticles should be less than 20 nm. They also found that the extinction coefficient is linearly proportionate to volume fraction
Sokhansefat et al. 
parabolic trough collector tube
Nanofluid enhanced convective heat transfer coefficient.
Hordy et al. 
multi-walled carbon nanotubes/water ethylene glycol, propylene glycol
quantitative demonstration of the high temperature and long-term stability of ethylene glycol and propylene glycol-based MWCNT nanofluids for solar thermal collectors
Said et al. 
Al2O3, water, ethylene glycol
Their results showed that nanofluids pressure drop at a low concentration flowing in a solar collector is slightly higher than the base fluid.
Liu et al. 
Grapheme/ ionic liquid 1-hexyl-3-methylimidazolium tetrafluoroborate
They observed 15.2%-22.9% enhancement in thermal conductivity using 0.06% volume graphene in the temperature range from 25 to 200°C. Their results showed that GE is a better nanoadditive for nanofluids than other carbon materials and metal nanoparticles
Ho et al. 
Alumina/ doped molten Hitec
concentrating solar power systems
The addition of less than 2% Al2O3 nanoparticles significantly increases the specific heat of Hitec metal at low temperatures
Singh et al. 
Cu/Therminol 59 (TH59) and Therminol 66 (TH66)
They stated that surfactant selection has an important role in preparing stable nanofluids. Choosing the right surfactant is mainly dependent on the properties of the base fluids and particles
Yousefi et al. 
flat plate solar collector
They found that increasing or decreasing the pH with respect to the pH of the isoelectric point (IEP) would enhance the positive effect of nanofluids on the efficiency of the solar collector
Sardarabadi et al. 
Thermal efficiency of the PV/T collector for the two cases of 1 and 3 wt% of silica/water nanofluid increased 7.6% and 12.8%, respectively.
Kabeel et al. 
water desalination unit
the water cost can be decreased from 16.43 to 11.68 $/m3 at ϕ=5%
Kabeel et al. 
using nanofluids improves the solar still water productivity by about 116% and 76% with and without operating the vacuum fan
Al-Nimr et al. 
shallow solar pond
energy stored in the nanofluid pond is about 216% more than the energy stored in the brine pond
Liu et al. 
maximum and mean values of the collecting efficiency of the collector with open thermosyphon using nanofluids increased 6.6% and 12.4%, respectively.
Using nanofluids in solar stills
Kabeel et al.  used Al2O3 nanoparticles with water inside a single basin solar still. Their results showed that using nanofluids improves the solar still water productivity by about 116% and 76% with and without operating the vacuum fan. The authors attributed this increment to the increase of evaporation rate inside the still. Utilizing nanofluid increases the rate of evaporation. In addition, due to this vacuum inside the still the evaporation rate increases further and the productivity increases compared with the still working at atmospheric conditions.
Using nanofluids in solar pond
Al-Nimr et al.  presented a mathematical model to describe the effects of using silver-water nanofluid on the thermal performance of a shallow solar pond (SSP) and showed that the energy stored in the nanofluid pond is about 216% more than the energy stored in the brine pond. The upper layer of the pond is made of mineral oil and the lower layer is made of silver (Ag) water-based nanofluid. Their results showed that for solar radiation of 1000 W/m2, the nanofluid pond required a depth less than 25 cm in order to absorb the light, while the brine pond depth must be more than 25 m to absorb the same amount of light. They attributed the increase of stored energy to the increase in thermal conductivity of the base fluid due to the nanoparticles addition that leads to uniform temperature distribution within the layer with reduction in heat losses.
Using nanofluids in the solar collector integrated with open thermosyphon
Liu et al.  experimentally showed that the solar collector integrated with open thermosyphon has a much better collecting performance compared to the collector with concentric tube and its efficiency could be improved by using CuO/water nanofluid as the working fluid as well. Their results showed that the maximum and mean values of the collecting efficiency of the collector with open thermosyphon using nanofluids increased 6.6% and 12.4%, respectively.
Nanofluids have been utilized to improve the efficiency of several solar thermal applications. Theoretical and experimental studies on solar systems proved that the system performance enhances noticeably by using nanofluids. A number of investigations presented the existence of an optimum concentration for nanoparticles in the base fluid. Adding nanoparticles beyond the optimum level no longer enhances the efficiency of the solar system.
Optimal conditions are a function of nanoparticles size and concentration, base fluid, surfactant and pH as discussed throughout this article. Nanofluid utilization in the solar thermal systems is accompanied by important challenges including high cost of production, instability, agglomeration and erosion. This review article is an attempt to elucidate the advantages and disadvantages of nanofluids application in the solar system.
The authors would like to express their appreciation to the Islamic Azad University of Abadan Branch for providing financial support.
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