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Graphene nanofluids containing core-shell nanoparticles with plasmon resonance effect enhanced solar energy absorption
Solar Energy 158 (2017) 1–8
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Graphene nanofluids containing core-shell nanoparticles with plasmon resonance effect enhanced solar energy absorption
MARK
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Desong Fan, Qiang Li , Weibing Chen, Jia Zeng MIIT Key Laboratory of Thermal Control of Electronic Equipment, School of Energy and Power Engineering, Nanjing University of Science and Technology, Nanjing, Jiangsu 210094, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Solar absorption Graphene nanofluids Plasmon resonance Core-shell nanoparticles
Nanofluids are a kind of important working fluid in volumetric solar collector. Here, we presented a novel strategy to enhance the solar absorption properties of graphene nanofluids utilizing the plasmon resonance of core-shell nanoparticles. The preparation, micrograph, optical properties and thermal conductivity of nanofluid have been investigated by considering the effect of volume fractions, nanoparticles selection and temperature. Results show that the graphene-embedded Sn@SiO2@Ag nanofluid exhibits a strong absorption band in the range of 250–300 nm and 380–600 nm. The solar absorption performance of graphene nanofluids is enhanced significantly by the plasmon resonance absorption and thermal conduction bridge of graphene-embedded Sn@ SiO2@Ag core-shell nanoparticles. The solar absorptance performance of graphene nanofluids was enhanced 2.9 times by adding 0.4 g/L Sn@SiO2@Ag solutions. An enhancement in thermal conductivity of 11.3% was obtained at 20 °C and 16% enhancement at 50 °C for 0.3 g/L graphene-embedded Sn@SiO2@Ag nanofluids. It is concluded that the synergic effect of Sn@SiO2@Ag core-shell nanoparticles and graphene nanosheets increases both the solar absorption coefficient and thermal conductivity of the nanofluids.
1. Introduction Nowadays, climate abnormality, environmental deterioration, and air contamination have become the most challenges in our society, especially in the developing countries. It is thought that the combustion of fossil fuel during the conventional heat supply is the main cause of aggravating global environment and climate issue (Baharoon et al., 2015; Crabtree and Lewis, 2007). In China, the achievements of local government are closely related to the reduction of smog. It is reported that the Premier Keqing Li will launch a special fund to explore more effective way of fighting against the smog pollution. Therefore, there is an urgent need to decrease the dependence on fossil fuel by developing renewable energy technology. As a clean energy source, solar energy has been developed vigorously in the past decades (Zheng and Kammen, 2014). Among various technologies of solar energy utilization, solar-thermal conversion is seen as the most direct and promising approach (Weinstein et al., 2015). It employs solar thermal collector to transform sunlight from condensing mirror into heat energy, replacing heat from combustion in power production systems (Weinstein et al., 2015; Xu et al., 2016). However, the conventional solar thermal collector usually suffers a high temperature because of its high solar absorbing surface, leading to a significant radiative heat loss (∝T4), and
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consequently lowers the solar-thermal conversion efficiency, especially for applications involving concentrated solar power (CSP) (Lee et al., 2012a). In order to decrease the heat loss at high temperature, Abdelrahman et al. (1979) proposed a black-liquid collector in the 1970s, which it is also called volumetric solar thermal collector. In contrast to the conventional solar thermal collector, solar energy is directly absorbed by the working fluid in the black-liquid collector, decreasing in turn the surface temperature of collector, and then the radiative heat loss can be also reduced. Moreover, the overall thermal resistance is also lowered since the thermal resistance from hot absorbing surface to working fluid is eliminated (Taylor et al., 2011a). Recently, nanofluids (nano-sized particles suspended in base fluid) have been introduced to solar thermal collectors as the working fluid that directly absorbs the solar radiation. A series of explorations have been performed in order to further enhance the solar absorption of nanofluids. These explorations involve the nanoparticles material, size, shape, volume fraction, and so on (Karami et al., 2016; Saidur et al., 2012; Karami et al., 2014; Mercatelli et al., 2011; He et al., 2013; Luo et al., 2014; Han et al., 2011; Jin et al., 2016a). For example, Taylor et al. (2011a) reported that a 0.125% volume fraction of graphite resulting in approximately an 11% improvement in steady-state efficiency
Corresponding author. E-mail addresses: [email protected] (D. Fan), [email protected] (Q. Li).
http://dx.doi.org/10.1016/j.solener.2017.09.031 Received 16 May 2017; Received in revised form 13 September 2017; Accepted 14 September 2017 0038-092X/ © 2017 Published by Elsevier Ltd.
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over the base fluid. Saidur et al. (2012) suggested that only 1% volume fraction, the transmittance of aluminum nanofluid is significantly reduced by 60% in average comparing with the water fluids throughout the visible light region. Karami et al. (2014) researched the transmittance spectra of functionalized carbon nanotubes (f-CNTs) nanofluid. They found that f-CNTs considerably reduce the base fluid transmittance and enhance the amount of light-capture. Luo et al. (2014) analyzed the transmittance of C, Ag, TiO2, SiO2, Cu, Al2O3 and carbontubes nanofluid. They found that the absorption of SiO2 nanofluid is the worst, while the absorption of Cu and Al2O3 nanofluids are the highest. In these previous works, a series of investigation efforts showed that nanoparticles offer a potential of improving the solar absorption of fluids. For a volumetric absorbers based on nanofluids, however, its overall conversion efficiency is still low. Further investigation on the improvement of intrinsic optical properties of nanoparticles in base fluids has to be carried out. It is reported that light absorption can be enhanced at a certain frequency by a so-called localized surface plasmon resonance effect excited in metal nanoparticles (Lee et al., 2012b). If we can design the nanoparticles in based fluid to form plasmonic nanofluid, an improvement of light absorption is foreseeable. Although numerous studies have been reported on the surface plasmon in the field of photovoltaics, the plasmonic nanofluid investigation in solar collectors has only received very limited attention (Lee et al., 2012b; Filho et al., 2014). At present, several studies on the plasmonic nanofluid have been undertaken. Lee et al. (2012b) performed the radiative heat transfer analysis in plasmonic nanofluids. Their results be greatly enhanced based on the surface plasmons effect. The photothermal conversion characteristics of silver nanofluids were reported that silver particles have excellent photothermal conversion capability even under very low concentrations (Filho et al., 2014). They found that the best photothermal conversion performance was observed at the initial radiation period mainly due to the low heat loss and strong surface plasmon resonance effect of silver nanoparticles. Jeon et al. (2014) experimentally demonstrate the spectral tunability of plasmonic nanofluids based on Au nanorod with different aspect ratios, exhibiting nearly uniform absorption characteristic from the visible to near infrared region. Jin et al. (2016b) investigated the steam generation mechanism of gold nanoparticles-based solar volumetric receivers. Their results suggested that for future solar receiver design, more solar energy should be focused and trapped in the superheated region while minimizing the temperature rise of the bulk fluid. In this work, we prepared graphene oxide, graphene, Sn@SiO2@Ag core-shell nanoparticles, and their hybrid nanoparticles. Likewise, the corresponding nanofluids were also prepared by dispersing these nanoparticles into distilled water. The solar absorption properties and thermal conductivity of nanofluids were investigated to evaluate the potential of using the nanofluids for absorbing solar radiation in volumetric solar absorbers.
Graphene oxide can be chemically reduced to graphene with various reducing agents, such as hydrazine monohydrate (Tung et al., 2009; Compton et al., 2010; Li et al., 2008). Here, 100 mg graphene oxide was added into 100 mL water, the mixture presents a color of reddish brown. After adding 50 mL 80% hydrazine monohydrate into the mixture, the mixture quickly becomes black. The solution was heated to 100 °C for refluxing 24 h. After refluxing, the suspension was cooled to room temperature, and was filtered through filter paper. The resultant was washed successively in deionized water and ethanol for 5 times. After each washing, the mixture was filtered through filter paper. The final graphene powders were obtained by vacuum-drying the resultant deposited on the filter paper as shown in Fig. 1(b). Tin nanoparticles were synthesized using a modified polyole wetchemical reduction process (Cingarapu et al., 2014). 3 g polyvinylpyrrolidone (PVP) and 50 mL tetraethylene glycol (TEG) was mixed at 140 °C by a magnetic stirrer. Then, SnCl2 solution (1 g in 10 mL of TEG) was added slowly into the reaction mixture until appearing yellow-brown solutions. When the temperature was decreased to 70 °C, a freshly prepared NaBH4 solution (3 g of NaBH4 in 40 mL of TEG) and the NaBH2 solution was added into the reaction solution to react about 90 min. After cooling to room temperature, the tin nanoparticles were obtained by in turn washing with ethanol, separating centrifugally, and ultrasonic dispersion with alcohol. Then the alcohol solution containing Sn nanoparticles was magnetic stirred at 30 °C, and 1 mL tetraethyl-ortho-silicate (TEOS) was added. After 45 min, 25 mL stronger ammonia water was added drop-wise to the reaction solution and stirred for 12 h. The schematic diagram was illustrated in Fig. 1(c). After centrifuging, the remained sediment was washed in alcohol and filtered in oily paper. Finally, the Sn@SiO2 nanoparticle was obtained by drying 5 h at room temperature under vacuum condition. The Sn@SiO2 nanoparticles were dispersed to 40 mL solution consisted of 0.5 g SnCl2 and 30% HCl to induce the absorption of Sn2+ ions on the silica sphere surface, during which HCl prohibited the hydroxylation of SnCl2. After 20 min, the spheres were rinsed for 5 times with deionized water, and moved into 40 mL silver ammonia solution (1.5 g AgNO3). After 10 min, the nanospheres were rinsed for three times with deionized water. Then, nanoparticles were dissolved in 70 mL ethanol, adding 0.05 mL formaldehyde and 6 mL silver ammonia solution to stirring 24 h at 70 °C. Finally, the Sn@SiO2@Ag solution was obtained. The corresponding nanofluids were also prepared by dispersing these nanoparticles into distilled water. 2.2. Characterization Transmission electron microscopy (TEM, Tecnai 12, Philips, Netherlands) was used to conducted the observation of the micrograph of graphene oxide. The TEM images of the graphene oxide are displayed in Fig. 2(a). Its solution exhibits a yellowish-brown color giving in Fig. 2(b). It can be seen that the graphene oxide sheets are almost smooth except a tiny fold. The few-layer graphene oxide sheets are less than 10 nm by the estimation of fold. After the reduction, the resultant graphene sheets become more corrugated than the graphene oxide sheets as shown in Fig. 2(c). Fig. 3 shows the Raman spectra of graphene and graphene oxide in the range of 200–1700 cm−1 collected at room temperature via Raman spectrometer made by Renishaw. It shows two characteristic peaks with the D band (1355 cm−1) and the G band (1600 cm−1). D band means the structure defects of graphene and G band means the in-plane vibration of sp2 C atoms. How to judge the quality of graphene is a key problem, D band involving a process of the double resonance Raman scattering defects, so the defects of graphene will be reflected in the D band. Usually, ID/IG is used as important parameters of defect density in the characterization of graphene (Li et al., 2011). After chemical reduction of GO, the ratio of the intensities of the D and G bands (ID/IG) reduces from 1.1 to 0.8, which is caused by most of the reduction of oxygen containing functional groups.
2. Experimental section 2.1. Nanofluid preparation Graphite flakes were oxidized using the improved Hummers’ method (Marcano et al., 2010). A mixture of H2SO4 and H3PO4 (270:30 mL) was added to another mixture of 3.0 g graphite flakes and 15.0 g KMnO4, and then the solution was stirred for 1 h at 35 °C through a thermostat or water bath. After stirring constantly for 12 h at 50 °C, the solution was cooled to room temperature. Then the cooled solution was poured into 350 mL ice bath. When the solution becomes yellow color after adding the H2O2 with concentration of 30% as shown in Fig. 2(b), the filtrate was centrifuged with 6000 rpm for 2 h. The remaining solid material was then washed continuously by water, 30% HCl, and ethanol. Finally, the graphene oxide nanoparticles were obtained after drying at room temperature in vacuum ambient. The preparation process was illustrated in Fig. 1(a). 2
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Fig. 1. A preparation schematic of (a) graphene oxide, (b) graphene, and (c) Sn@SiO2 nanoparticle.
Fig. 4 gives the TEM images of Sn@SiO2@Ag nanoparticles. It can be found that the spherical nanoparticles Sn@SiO2@Ag with different size about 200–300 nm exhibited a core-shell structure with Sn core and SiO2 shell. Moreover, Ag nanoparticles are attached densely to the surface of SiO2 shell. The core-shell structure drawing was given in Fig. 4(e). In order to understand the internal situation, the TEM image of nanofluids containing graphene sheets and Sn@SiO2@Ag nanoparticles was detected as shown in Fig. 5. The TEM image shows that the Sn@SiO2@Ag nanoparticles were encapsulated by graphene sheets. The composite textures in base fluid are crinkled and rough because the presence of flexible and ultrathin graphene sheets. The overlapping textures of graphene layers were observed at the edges and surface of individual Sn@SiO2@Ag nanoparticle. The similar structure was also observed in graphene-encapsulated silica spheres by Yang et al. (2010). Although the size of nanoparticles seems too large in our present work, we observed experimentally that the dispersant-free nanofluids can stabilize a week in water due to the large specific surface area of graphene sheets and the good dispersivity of core-shell nanoparticles. In order to obtain long-term stable nanofluids, the fine nanoparticles with
Fig. 3. Raman spectra of graphene oxide and graphene.
Fig. 2. (a) TEM images of graphene oxide, (b) graphene oxide solution photo, and (c) TEM images of graphene.
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Fig. 4. (a) Photos of Sn@SiO2@Ag-water nanofluids with increasing concentration, (b)–(d) TEM images of Sn@SiO2@Ag nanoparticles with different magnifications, and (d) Sn@SiO2@Ag core-shell structure drawing.
Fig. 5. (a)–(c) TEM images of graphene-embedded Sn@SiO2@Ag nanofluids, and (d) interaction schematic of sunlight and nanofluid.
size below 100 nm still need to be explored further in the future research.
3. Results and discussions 3.1. Solar absorption characteristics of graphene oxide nanofluids Nanofluids absorptivity was measured by Cary5000 spectrophotometer. The measured wavelength range is from 200 to 1300 nm including 83% of the total incident solar energy if the average surface temperature of sun is 5800 K. Nanofluid sample was enclosed in a quartz cuvette with a 10 mm path length. For each measurement, the transmittance spectrum will be corrected to represent only the nanofluid transmittance eliminating the effect of cuvette wall. The details method can refer to reference Taylor et al. (2011b). Fig. 6 shows that the transmittance spectra of graphene oxide nanofluids are dramatic increase in the wavelength range 200–700 nm. Then the transmittance spectra of graphene oxide nanofluids increase slowly in the wavelength range 700–1000 nm. The results show that the average transmittance of base fluid with water is about 97% in the wavelength range of 200–900 nm and the strong absorption bands exist
Fig. 6. Transmittance spectra of graphene oxide nanofluids with different nanoparticle volume fractions: (a) 0.05 g/L, (b) 0.10 g/L, (c) 0.15 g/L, (d) 0.20 g/L, (e) 0.25 g/L, (f) 0.30 g/L.
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Fig. 8. Average transmittance reduction ratio of different nanofluids at different concentration, Y-axis stands for (τ bf −τnf )/ τ bf , where τ bf and τnf is the average transmittance
Fig. 7. Transmittance of graphene nanofluids with different volume fractions.
of water base fluid and nanofluid, respectively.
at 900–1000 nm and again at 1200 nm. The nanofluids are essentially opaque in the wavelength range of 200–300 nm beyond the 0.1 g/L concentration. Although the transmittance is reduced significantly by adding graphene oxide blow 500 nm wavelength, the graphene oxide nanofluids still remain a high transmittance in the wavelength of 500–1000 nm even volume fraction near 0.30 g/L. It means that the ability to absorb sun light is still not enough in solar collector application because the wavelength range still contains 47% of total solar radiation, near a half of solar radiation energy. From the inset of Fig. 6, it clearly shows that the solution color deepened significantly with the increase of concentration. When the concentration increases to 0.30 g/ L, the solution keeps a yellowish-brown color, which does not show the expected black fluid with high solar absorption. The fact reveals that the solar absorption properties of graphene oxide nanofluids are not enough to apply in solar collector. 3.2. Solar absorption characteristics of graphene nanofluids Fig. 7 shows the transmittance spectra of graphene nanofluids at different nanoparticle concentration. It can be seen that the transmittance of nanofluids decreases significantly with the increase of graphene nanosheets. When 0.4 g/L graphene nanosheets are added into the base fluid, the average transmittance of flow fluid decreased from 88% with base fluid to 48%, the reduction ratio about 45%. It is suggested that a further performance improvement is possible for the use of graphene nanofluids in solar absorbers. With the increase of wavelength, the transmittance of nanofluid at different concentration retains a similar trend to that of base fluid except the absorption valley around wavelength 270 nm due to the graphene strong absorption. For graphene nanofluids, one can conclude that the average transmittance decreases at least 10% per 0.1 g/L increment of nanoparticles in the nanofluids as shown in Fig. 8. The fact reveals that the solar absorption of fluid can be enhanced greatly by increasing graphene nanosheets. Compared with the graphene oxide, the addition of graphene nanosheets is more attractive to enhance solar absorption of nanofluid because the transmittance spectra show an overall downturn in the whole spectra regions.
Fig. 9. Transmittance spectra of Sn@SiO2@Ag nanofluids at different volume fractions.
SiO2@Ag nanofluids show a similar trend comparing with that of base fluid in the wavelength range of 500–1300 nm. When Sn@SiO2@Ag nanoparticles volume fraction increases, the transmittance of nanofluid decreases drastically, thus exhibits an expected black color fluid as shown in Fig. 4(a). Compared with water base fluid, the average transmittance of Sn@SiO2@Ag nanofluid can be reduced to 17% with 0.4 g/L concentration, and the reduction ratio is up to 81%, which is much higher than that of graphene nanofluid. When the concentration increased to 0.8 g/L, the transmittance of nanofluid reduces to zero, which is an ideal result. However, it is not an ideal selection to realize a zero transmittance or perfect absorption by increasing limitlessly nanoparticle concentration because the heavy concentration can lead to some other problems such as particle agglomeration, dispersion, sedimentation, and heat loss (Taylor et al., 2011b), which have not yet been considered in present work. If the particle concentration is too low, the solar absorption is also insufficient. Therefore, it is suggested that the concentration of 0.4 g/L Sn@SiO2@Ag nanoparticles is sufficient to enhance the solar absorption in solar collector.
3.3. Solar absorption characteristics of Sn@SiO2@Ag nanofluids
3.4. Solar absorption characteristics of nanofluids containing graphene and Sn@SiO2@Ag nanoparticles
In order to further enhance the solar absorption performance of graphene nanofluids, we explore another strategy by employing the plasmon resonance of nanoparticles. As shown in Fig. 9, a solar absorption band was found in the wavelength range of 350–500 nm in Sn@SiO2@Ag nanofluids owing to the plasmon resonance of core-shell structure (Ding et al., 2006). The transmittance spectra of Sn@
Based on the above-mentioned strategy, we prepared the graphene nanofluids containing Sn@SiO2@Ag core-shell nanoparticles (0.2G & Sn@SiO2@Ag). The volume fraction of graphene nanosheets is 5
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Fig. 11. Extinction coefficient of graphene-embedded Sn@SiO2@Ag nanofluids. The solar irradiance data is obtained from reference Mecherikunnel and Duncan (1982). Fig. 10. Transmittance spectra of graphene-embedded Sn@SiO2@Ag nanofluids. λ
fixed to 0.2 g/L and that of Sn@SiO2@Ag core-shell nanoparticles changes from 0.1 g/L to 0.5 g/L. Their transmittance spectra were given in Fig. 10. A sharp peak is also observed around 320 nm due to the excited surface plasmon in the core-shell structure, resulting in a notable absorption in the vicinity of 440 nm wavelength. In addition, a weak transmittance peak appears at 230 nm due to the intrinsic absorption of graphene. One can found that a broadband absorption is formed in view of the synergistic enhancement of nanoparticle’s intrinsic absorption and surface plasmon, which is differ from the single peak absorption in Sn@SiO2@Ag nanofluids. The results show that the average transmittance of graphene fluid with 0.2 g/L concentration can be reduced to 49% only adding 0.1 g/L Sn@SiO2@Ag solution, which is apparent to enhance solar absorption. The average transmittance of Sn@SiO2@Ag nanofluids with concentration 0.4 g/L is further reduced to 11%, which is far below 69% of the graphene nanofluid with concentration 0.2 g/L, improving remarkably the solar absorption properties. If we ignore the reflection and scattering, the solar absorptance (αs ) of nanofluids can be evaluated simply as αs = 1−τnf from the transmittance (τnf ). This means that the solar absorptance of nanofluids consisting of 0.2 g/L graphene and 0.4 g/L Sn@SiO2@Ag solutions is 2.9 times larger than that of 0.2 g/L graphene nanofluid. It is suggested that the Sn@SiO2@Ag core-shell nanoparticles augments the solar absorption of graphene fluids due to the core-shell plasmon resonance effect (Wu et al., 2015). As shown in the above-mentioned analysis, the nanofluids containing graphene and Sn@SiO2@Ag core-shell nanoparticles are useful in volumetric solar collectors. In order to evaluate the radiative properties of the nanofluids, extinction coefficient k (λ ) is calculated from the transmittance data according to Bear-Lambert law T (λ ) = exp(−k (λ ) L) . The term T (λ ) and L is transmittance and the length of light crossing, and L = 10 mm. The extinction coefficient of nanofluids with different volume fraction is illustrated in Fig. 11. One can found that a broad peak exists around the wavelength of 440 nm due to the plasmon resonance absorption beside the intrinsic absorption peak (270 nm) of graphene. This means that the addition of Sn@ SiO2@Ag core-shell nanoparticles can significantly enhance the solar absorption ability of graphene nanofluids because the broad peak location is very close to the wavelength (480 nm) of maximum intensity of solar irradiance. The solar absorption ability of nanofluids is important in volumetric solar collectors. One always expects a larger absorption fraction of solar energy (AF) can be achieved through a minimum thickness and volume fraction of nanofluids. In this work, the absorption fraction AF is calculated from Eq. (1) (Zhang et al., 2014)
AF = 1−
max Is (λ )exp(−k (λ ) x ) dλ ∫λmin
λ
max Is (λ ) dλ ∫λmin
. (1)
In this case, Is (λ ) denotes the solar radiation intensity (Mecherikunnel and Duncan, 1982), k (λ ) is the extinction coefficient of nanofluids, and x is equal to the thickness of the nanofluids layer. λmin = 250 nm and λmax = 1300 nm are considered as integration bounds. The calculated absorption fraction is shown in Fig. 12. It can be found that the minimal thickness of nanofluids layer with AF = 100% decreases with increasing volume fraction of nanofluids. For example, the thickness is 6 cm for the nanofluids with containing 0.2 g/L graphene and 0.1 g/L Sn@SiO2@Ag nanoparticles, while the thickness decreases to 2 cm when the Sn@SiO2@Ag nanoparticles volume fraction increases to 0.5 g/L. According to the Eq. (1), the absorption fraction depends mainly on the extinction coefficient k (λ ) and the thickness (x) of nanofluids layer. Therefore, the enhancement of AF can be achieved by increasing k (λ ) or x. However, it is inadvisable to increase the thickness of nanofluids layer because the size of solar collector becomes bulky. In fact, it is only suitable selection to increase the k (λ ) by the addition of nanoparticles in base fluids when the minimum thickness layer of nanofluids be determined by considering the AF = 100%. 3.5. Thermal conductivity of nanofluids Nanofluids have been reported to exhibit superior heat transfer
Fig. 12. Absorbed energy fractions (AF) versus the penetration distance (x) of grapheneembedded Sn@SiO2@Ag nanofluids.
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graphene approaches 0.72 W m−1 K−1, which is the maximum at room temperature in present experiment. The enhanced thermal conductivity may be attributed to the increase of conductivity chain. As seen in TEM picture that the graphene is a nanosheets shape and the Sn@SiO2@Ag is spherical particles. The addition of core-shell particles can form an effective conductivity path at between the graphene nanosheets. Temperature is also an important parameter to impact the thermal conductivity of nanofluids. In the case of G & Sn@SiO2@Ag suspension, the thermal conductivity was measured at temperature of 20–50 °C. As shown in Fig. 14, the thermal conductivity increases with increasing temperature. When the concentration of G & Sn@SiO2@Ag increases to 0.3 g/L, the thermal conductivity is enhanced 11.3% at 20 °C comparing with water base fluid, while the value is 16% at 50 °C. The notable enhancement can probably be attributed to the extremely high surface area of graphene nanosheets and the increased conductivity path through the embedded Sn@SiO2@Ag core-shell nanoparticles. The present results are much superior to what Vakili et al. (2016) had reported for 0.05 wt.% of graphene nanoplatelets in water base fluids. It is thought that the Brownian motion of nanoparticles in base fluids is the main contribution to the enhancement of thermal conductivity in nanofluids as it is one of the most dominated functions of temperature (Xuan and Li, 2000). In addition, Baby and Ramaprabhu (2010) suggested that the interaction of graphene layer with each other results in a readily electron hopping from one graphene to other in graphene-water nanofluids. In our case, however, the hopping becomes more convenient from one graphene layer to another via the conductivity bridge of Sn@SiO2@Ag core-shell nanoparticles. Moreover, the synergic enhancement of Sn@SiO2@Ag core-shell nanoparticles and graphene nanosheets is also favor to enhance the thermal conductivity of nanofluids.
Fig. 13. Thermal conductivity of nanofluids at 20 °C. In the case of 0.2G & Sn@SiO2@Ag nanofluid, the X-axis stands for the concentration of adding Sn@SiO2@Ag in 0.2 g/L graphene nanofluids.
Thermal conductivity (W/m.K)
0.80
G&Sn@SiO2@Ag 0.30 g/L G&Sn@SiO2@Ag 0.25 g/L G&Sn@SiO2@Ag 0.20 g/L G&Sn@SiO2@Ag 0.15 g/L G&Sn@SiO2@Ag 0.10 g/L G&Sn@SiO2@Ag 0.05 g/L
0.75
0.70
4. Conclusions
0.65
0.60
20
25
30 35 40 Temperature ( C)
45
In summary, we investigated the solar absorption performance of several nanofluids in volumetric solar collector based on the nanoparticles/nanosheets of graphene oxide, graphene, Sn@SiO2@Ag, or graphene-embedded Sn@SiO2@Ag. It is confirmed that the performance of graphene nanofluids is enhanced significantly through the plasmon resonance absorption and thermal conduction bridge of graphene-embedded Sn@SiO2@Ag core-shell nanoparticles. A strong absorption band is observed in the wavelength range of 250–300 nm and 380–600 nm. The solar absorption coefficient of graphene nanofluid is enhanced 2.9 times by the core-shell nanoparticles. An enhancement in thermal conductivity of 11.3% has been obtained at 20 °C for 0.3 g/L graphene-coated Sn@SiO2@Ag nanofluids and 16% enhancement for the same volume fraction at 50 °C. It is concluded that the synergic effect of the core-shell nanoparticles and graphene nanosheets increases both the solar absorption coefficient and thermal conductivity of the nanofluids. This will enhance the performance of volumetric solar collector.
50
Fig. 14. Temperature-dependent thermal conductivity of graphene-embedded Sn@ SiO2@Ag nanofluids.
properties compared with base fluids over the past decades (Xuan and Li, 2000; Yu et al., 2009). In volumetric solar collectors, the superior heat transfer properties are also equally important. It is known that the thermal conductivity of nanofluid is strongly dependent on the volume fraction dimensions, nanofluids temperature, and nanoparticles itself. In this work, we employ the transient hot-wire apparatus to measure the thermal conductivity of nanofluids. As shown in Fig. 13, the thermal conductivity at room temperature of 20 °C increases significantly with increasing nanoparticles volume fraction. For example, the thermal conductivity of base fluids is 0.62 W m−1 K−1, and it is 0.68 W m−1 K−1 for the graphene oxide suspension at 0.3 g/L volume fraction. The percentage of enhancement [(Knf −Kbf ) × 100]/ Kbf in thermal conductivity approaches to 9.7%, where Knf and Kbf denotes the thermal conductivity of nanofluids and base fluids, respectively. In the case of G & Sn@SiO2@Ag suspension, the graphene and core-shell nanoparticles were blended by the mass ration of 1:1. The thermal conductivity approaches 0.69 W m−1 K−1 in 0.3 g/L G & Sn@SiO2@Ag suspension. The percentage of enhancement is 11.3%. However, the thermal conductivity can be further enhanced by increasing additive concentration. For instance, in the case of 0.2G & Sn@SiO2@Ag suspension, when the graphene concentration was fixed at 0.2 g/L, the thermal conductivity suspension was increased about 0.01 W m−1 K−1 per 0.1 g/L increment of Sn@SiO2@Ag. The thermal conductivity of suspension with consisting of 0.4 g/L G & Sn@SiO2@Ag and 0.2 g/L
Acknowledgments D. Fan and Q. Li contributed equally to this work. This work is sponsored by the National Natural Science Foundation of China (Grant Nos. 51590901 and 51590903), and the Six Talent Peaks Project in Jiangsu Province (No. XNY-031). References Abdelrahman, M., Fumeaux, P., Suter, P., 1979. Study of solid-gas-suspensions used for direct absorption of concentrated solar radiation. Sol. Energy 22 (1), 45–48. Baby, T.T., Ramaprabhu, S., 2010. Investigation of thermal and electrical conductivity of graphene based nanofluids. J. Appl. Phys. 108 (12), 124308. Baharoon, D.A., Rahman, H.A., Omar, W.Z.W., Fadhl, S.O., 2015. Historical development of concentrating solar power technologies to generate clean electricity efficiently—a review. Renew. Sustain. Energy Rev. 41, 996–1027.
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Contents lists available at ScienceDirect
Solar Energy journal homepage: www.elsevier.com/locate/solener
Graphene nanofluids containing core-shell nanoparticles with plasmon resonance effect enhanced solar energy absorption
MARK
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Desong Fan, Qiang Li , Weibing Chen, Jia Zeng MIIT Key Laboratory of Thermal Control of Electronic Equipment, School of Energy and Power Engineering, Nanjing University of Science and Technology, Nanjing, Jiangsu 210094, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Solar absorption Graphene nanofluids Plasmon resonance Core-shell nanoparticles
Nanofluids are a kind of important working fluid in volumetric solar collector. Here, we presented a novel strategy to enhance the solar absorption properties of graphene nanofluids utilizing the plasmon resonance of core-shell nanoparticles. The preparation, micrograph, optical properties and thermal conductivity of nanofluid have been investigated by considering the effect of volume fractions, nanoparticles selection and temperature. Results show that the graphene-embedded Sn@SiO2@Ag nanofluid exhibits a strong absorption band in the range of 250–300 nm and 380–600 nm. The solar absorption performance of graphene nanofluids is enhanced significantly by the plasmon resonance absorption and thermal conduction bridge of graphene-embedded Sn@ SiO2@Ag core-shell nanoparticles. The solar absorptance performance of graphene nanofluids was enhanced 2.9 times by adding 0.4 g/L Sn@SiO2@Ag solutions. An enhancement in thermal conductivity of 11.3% was obtained at 20 °C and 16% enhancement at 50 °C for 0.3 g/L graphene-embedded Sn@SiO2@Ag nanofluids. It is concluded that the synergic effect of Sn@SiO2@Ag core-shell nanoparticles and graphene nanosheets increases both the solar absorption coefficient and thermal conductivity of the nanofluids.
1. Introduction Nowadays, climate abnormality, environmental deterioration, and air contamination have become the most challenges in our society, especially in the developing countries. It is thought that the combustion of fossil fuel during the conventional heat supply is the main cause of aggravating global environment and climate issue (Baharoon et al., 2015; Crabtree and Lewis, 2007). In China, the achievements of local government are closely related to the reduction of smog. It is reported that the Premier Keqing Li will launch a special fund to explore more effective way of fighting against the smog pollution. Therefore, there is an urgent need to decrease the dependence on fossil fuel by developing renewable energy technology. As a clean energy source, solar energy has been developed vigorously in the past decades (Zheng and Kammen, 2014). Among various technologies of solar energy utilization, solar-thermal conversion is seen as the most direct and promising approach (Weinstein et al., 2015). It employs solar thermal collector to transform sunlight from condensing mirror into heat energy, replacing heat from combustion in power production systems (Weinstein et al., 2015; Xu et al., 2016). However, the conventional solar thermal collector usually suffers a high temperature because of its high solar absorbing surface, leading to a significant radiative heat loss (∝T4), and
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consequently lowers the solar-thermal conversion efficiency, especially for applications involving concentrated solar power (CSP) (Lee et al., 2012a). In order to decrease the heat loss at high temperature, Abdelrahman et al. (1979) proposed a black-liquid collector in the 1970s, which it is also called volumetric solar thermal collector. In contrast to the conventional solar thermal collector, solar energy is directly absorbed by the working fluid in the black-liquid collector, decreasing in turn the surface temperature of collector, and then the radiative heat loss can be also reduced. Moreover, the overall thermal resistance is also lowered since the thermal resistance from hot absorbing surface to working fluid is eliminated (Taylor et al., 2011a). Recently, nanofluids (nano-sized particles suspended in base fluid) have been introduced to solar thermal collectors as the working fluid that directly absorbs the solar radiation. A series of explorations have been performed in order to further enhance the solar absorption of nanofluids. These explorations involve the nanoparticles material, size, shape, volume fraction, and so on (Karami et al., 2016; Saidur et al., 2012; Karami et al., 2014; Mercatelli et al., 2011; He et al., 2013; Luo et al., 2014; Han et al., 2011; Jin et al., 2016a). For example, Taylor et al. (2011a) reported that a 0.125% volume fraction of graphite resulting in approximately an 11% improvement in steady-state efficiency
Corresponding author. E-mail addresses: [email protected] (D. Fan), [email protected] (Q. Li).
http://dx.doi.org/10.1016/j.solener.2017.09.031 Received 16 May 2017; Received in revised form 13 September 2017; Accepted 14 September 2017 0038-092X/ © 2017 Published by Elsevier Ltd.
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over the base fluid. Saidur et al. (2012) suggested that only 1% volume fraction, the transmittance of aluminum nanofluid is significantly reduced by 60% in average comparing with the water fluids throughout the visible light region. Karami et al. (2014) researched the transmittance spectra of functionalized carbon nanotubes (f-CNTs) nanofluid. They found that f-CNTs considerably reduce the base fluid transmittance and enhance the amount of light-capture. Luo et al. (2014) analyzed the transmittance of C, Ag, TiO2, SiO2, Cu, Al2O3 and carbontubes nanofluid. They found that the absorption of SiO2 nanofluid is the worst, while the absorption of Cu and Al2O3 nanofluids are the highest. In these previous works, a series of investigation efforts showed that nanoparticles offer a potential of improving the solar absorption of fluids. For a volumetric absorbers based on nanofluids, however, its overall conversion efficiency is still low. Further investigation on the improvement of intrinsic optical properties of nanoparticles in base fluids has to be carried out. It is reported that light absorption can be enhanced at a certain frequency by a so-called localized surface plasmon resonance effect excited in metal nanoparticles (Lee et al., 2012b). If we can design the nanoparticles in based fluid to form plasmonic nanofluid, an improvement of light absorption is foreseeable. Although numerous studies have been reported on the surface plasmon in the field of photovoltaics, the plasmonic nanofluid investigation in solar collectors has only received very limited attention (Lee et al., 2012b; Filho et al., 2014). At present, several studies on the plasmonic nanofluid have been undertaken. Lee et al. (2012b) performed the radiative heat transfer analysis in plasmonic nanofluids. Their results be greatly enhanced based on the surface plasmons effect. The photothermal conversion characteristics of silver nanofluids were reported that silver particles have excellent photothermal conversion capability even under very low concentrations (Filho et al., 2014). They found that the best photothermal conversion performance was observed at the initial radiation period mainly due to the low heat loss and strong surface plasmon resonance effect of silver nanoparticles. Jeon et al. (2014) experimentally demonstrate the spectral tunability of plasmonic nanofluids based on Au nanorod with different aspect ratios, exhibiting nearly uniform absorption characteristic from the visible to near infrared region. Jin et al. (2016b) investigated the steam generation mechanism of gold nanoparticles-based solar volumetric receivers. Their results suggested that for future solar receiver design, more solar energy should be focused and trapped in the superheated region while minimizing the temperature rise of the bulk fluid. In this work, we prepared graphene oxide, graphene, Sn@SiO2@Ag core-shell nanoparticles, and their hybrid nanoparticles. Likewise, the corresponding nanofluids were also prepared by dispersing these nanoparticles into distilled water. The solar absorption properties and thermal conductivity of nanofluids were investigated to evaluate the potential of using the nanofluids for absorbing solar radiation in volumetric solar absorbers.
Graphene oxide can be chemically reduced to graphene with various reducing agents, such as hydrazine monohydrate (Tung et al., 2009; Compton et al., 2010; Li et al., 2008). Here, 100 mg graphene oxide was added into 100 mL water, the mixture presents a color of reddish brown. After adding 50 mL 80% hydrazine monohydrate into the mixture, the mixture quickly becomes black. The solution was heated to 100 °C for refluxing 24 h. After refluxing, the suspension was cooled to room temperature, and was filtered through filter paper. The resultant was washed successively in deionized water and ethanol for 5 times. After each washing, the mixture was filtered through filter paper. The final graphene powders were obtained by vacuum-drying the resultant deposited on the filter paper as shown in Fig. 1(b). Tin nanoparticles were synthesized using a modified polyole wetchemical reduction process (Cingarapu et al., 2014). 3 g polyvinylpyrrolidone (PVP) and 50 mL tetraethylene glycol (TEG) was mixed at 140 °C by a magnetic stirrer. Then, SnCl2 solution (1 g in 10 mL of TEG) was added slowly into the reaction mixture until appearing yellow-brown solutions. When the temperature was decreased to 70 °C, a freshly prepared NaBH4 solution (3 g of NaBH4 in 40 mL of TEG) and the NaBH2 solution was added into the reaction solution to react about 90 min. After cooling to room temperature, the tin nanoparticles were obtained by in turn washing with ethanol, separating centrifugally, and ultrasonic dispersion with alcohol. Then the alcohol solution containing Sn nanoparticles was magnetic stirred at 30 °C, and 1 mL tetraethyl-ortho-silicate (TEOS) was added. After 45 min, 25 mL stronger ammonia water was added drop-wise to the reaction solution and stirred for 12 h. The schematic diagram was illustrated in Fig. 1(c). After centrifuging, the remained sediment was washed in alcohol and filtered in oily paper. Finally, the Sn@SiO2 nanoparticle was obtained by drying 5 h at room temperature under vacuum condition. The Sn@SiO2 nanoparticles were dispersed to 40 mL solution consisted of 0.5 g SnCl2 and 30% HCl to induce the absorption of Sn2+ ions on the silica sphere surface, during which HCl prohibited the hydroxylation of SnCl2. After 20 min, the spheres were rinsed for 5 times with deionized water, and moved into 40 mL silver ammonia solution (1.5 g AgNO3). After 10 min, the nanospheres were rinsed for three times with deionized water. Then, nanoparticles were dissolved in 70 mL ethanol, adding 0.05 mL formaldehyde and 6 mL silver ammonia solution to stirring 24 h at 70 °C. Finally, the Sn@SiO2@Ag solution was obtained. The corresponding nanofluids were also prepared by dispersing these nanoparticles into distilled water. 2.2. Characterization Transmission electron microscopy (TEM, Tecnai 12, Philips, Netherlands) was used to conducted the observation of the micrograph of graphene oxide. The TEM images of the graphene oxide are displayed in Fig. 2(a). Its solution exhibits a yellowish-brown color giving in Fig. 2(b). It can be seen that the graphene oxide sheets are almost smooth except a tiny fold. The few-layer graphene oxide sheets are less than 10 nm by the estimation of fold. After the reduction, the resultant graphene sheets become more corrugated than the graphene oxide sheets as shown in Fig. 2(c). Fig. 3 shows the Raman spectra of graphene and graphene oxide in the range of 200–1700 cm−1 collected at room temperature via Raman spectrometer made by Renishaw. It shows two characteristic peaks with the D band (1355 cm−1) and the G band (1600 cm−1). D band means the structure defects of graphene and G band means the in-plane vibration of sp2 C atoms. How to judge the quality of graphene is a key problem, D band involving a process of the double resonance Raman scattering defects, so the defects of graphene will be reflected in the D band. Usually, ID/IG is used as important parameters of defect density in the characterization of graphene (Li et al., 2011). After chemical reduction of GO, the ratio of the intensities of the D and G bands (ID/IG) reduces from 1.1 to 0.8, which is caused by most of the reduction of oxygen containing functional groups.
2. Experimental section 2.1. Nanofluid preparation Graphite flakes were oxidized using the improved Hummers’ method (Marcano et al., 2010). A mixture of H2SO4 and H3PO4 (270:30 mL) was added to another mixture of 3.0 g graphite flakes and 15.0 g KMnO4, and then the solution was stirred for 1 h at 35 °C through a thermostat or water bath. After stirring constantly for 12 h at 50 °C, the solution was cooled to room temperature. Then the cooled solution was poured into 350 mL ice bath. When the solution becomes yellow color after adding the H2O2 with concentration of 30% as shown in Fig. 2(b), the filtrate was centrifuged with 6000 rpm for 2 h. The remaining solid material was then washed continuously by water, 30% HCl, and ethanol. Finally, the graphene oxide nanoparticles were obtained after drying at room temperature in vacuum ambient. The preparation process was illustrated in Fig. 1(a). 2
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Fig. 1. A preparation schematic of (a) graphene oxide, (b) graphene, and (c) Sn@SiO2 nanoparticle.
Fig. 4 gives the TEM images of Sn@SiO2@Ag nanoparticles. It can be found that the spherical nanoparticles Sn@SiO2@Ag with different size about 200–300 nm exhibited a core-shell structure with Sn core and SiO2 shell. Moreover, Ag nanoparticles are attached densely to the surface of SiO2 shell. The core-shell structure drawing was given in Fig. 4(e). In order to understand the internal situation, the TEM image of nanofluids containing graphene sheets and Sn@SiO2@Ag nanoparticles was detected as shown in Fig. 5. The TEM image shows that the Sn@SiO2@Ag nanoparticles were encapsulated by graphene sheets. The composite textures in base fluid are crinkled and rough because the presence of flexible and ultrathin graphene sheets. The overlapping textures of graphene layers were observed at the edges and surface of individual Sn@SiO2@Ag nanoparticle. The similar structure was also observed in graphene-encapsulated silica spheres by Yang et al. (2010). Although the size of nanoparticles seems too large in our present work, we observed experimentally that the dispersant-free nanofluids can stabilize a week in water due to the large specific surface area of graphene sheets and the good dispersivity of core-shell nanoparticles. In order to obtain long-term stable nanofluids, the fine nanoparticles with
Fig. 3. Raman spectra of graphene oxide and graphene.
Fig. 2. (a) TEM images of graphene oxide, (b) graphene oxide solution photo, and (c) TEM images of graphene.
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Fig. 4. (a) Photos of Sn@SiO2@Ag-water nanofluids with increasing concentration, (b)–(d) TEM images of Sn@SiO2@Ag nanoparticles with different magnifications, and (d) Sn@SiO2@Ag core-shell structure drawing.
Fig. 5. (a)–(c) TEM images of graphene-embedded Sn@SiO2@Ag nanofluids, and (d) interaction schematic of sunlight and nanofluid.
size below 100 nm still need to be explored further in the future research.
3. Results and discussions 3.1. Solar absorption characteristics of graphene oxide nanofluids Nanofluids absorptivity was measured by Cary5000 spectrophotometer. The measured wavelength range is from 200 to 1300 nm including 83% of the total incident solar energy if the average surface temperature of sun is 5800 K. Nanofluid sample was enclosed in a quartz cuvette with a 10 mm path length. For each measurement, the transmittance spectrum will be corrected to represent only the nanofluid transmittance eliminating the effect of cuvette wall. The details method can refer to reference Taylor et al. (2011b). Fig. 6 shows that the transmittance spectra of graphene oxide nanofluids are dramatic increase in the wavelength range 200–700 nm. Then the transmittance spectra of graphene oxide nanofluids increase slowly in the wavelength range 700–1000 nm. The results show that the average transmittance of base fluid with water is about 97% in the wavelength range of 200–900 nm and the strong absorption bands exist
Fig. 6. Transmittance spectra of graphene oxide nanofluids with different nanoparticle volume fractions: (a) 0.05 g/L, (b) 0.10 g/L, (c) 0.15 g/L, (d) 0.20 g/L, (e) 0.25 g/L, (f) 0.30 g/L.
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Fig. 8. Average transmittance reduction ratio of different nanofluids at different concentration, Y-axis stands for (τ bf −τnf )/ τ bf , where τ bf and τnf is the average transmittance
Fig. 7. Transmittance of graphene nanofluids with different volume fractions.
of water base fluid and nanofluid, respectively.
at 900–1000 nm and again at 1200 nm. The nanofluids are essentially opaque in the wavelength range of 200–300 nm beyond the 0.1 g/L concentration. Although the transmittance is reduced significantly by adding graphene oxide blow 500 nm wavelength, the graphene oxide nanofluids still remain a high transmittance in the wavelength of 500–1000 nm even volume fraction near 0.30 g/L. It means that the ability to absorb sun light is still not enough in solar collector application because the wavelength range still contains 47% of total solar radiation, near a half of solar radiation energy. From the inset of Fig. 6, it clearly shows that the solution color deepened significantly with the increase of concentration. When the concentration increases to 0.30 g/ L, the solution keeps a yellowish-brown color, which does not show the expected black fluid with high solar absorption. The fact reveals that the solar absorption properties of graphene oxide nanofluids are not enough to apply in solar collector. 3.2. Solar absorption characteristics of graphene nanofluids Fig. 7 shows the transmittance spectra of graphene nanofluids at different nanoparticle concentration. It can be seen that the transmittance of nanofluids decreases significantly with the increase of graphene nanosheets. When 0.4 g/L graphene nanosheets are added into the base fluid, the average transmittance of flow fluid decreased from 88% with base fluid to 48%, the reduction ratio about 45%. It is suggested that a further performance improvement is possible for the use of graphene nanofluids in solar absorbers. With the increase of wavelength, the transmittance of nanofluid at different concentration retains a similar trend to that of base fluid except the absorption valley around wavelength 270 nm due to the graphene strong absorption. For graphene nanofluids, one can conclude that the average transmittance decreases at least 10% per 0.1 g/L increment of nanoparticles in the nanofluids as shown in Fig. 8. The fact reveals that the solar absorption of fluid can be enhanced greatly by increasing graphene nanosheets. Compared with the graphene oxide, the addition of graphene nanosheets is more attractive to enhance solar absorption of nanofluid because the transmittance spectra show an overall downturn in the whole spectra regions.
Fig. 9. Transmittance spectra of Sn@SiO2@Ag nanofluids at different volume fractions.
SiO2@Ag nanofluids show a similar trend comparing with that of base fluid in the wavelength range of 500–1300 nm. When Sn@SiO2@Ag nanoparticles volume fraction increases, the transmittance of nanofluid decreases drastically, thus exhibits an expected black color fluid as shown in Fig. 4(a). Compared with water base fluid, the average transmittance of Sn@SiO2@Ag nanofluid can be reduced to 17% with 0.4 g/L concentration, and the reduction ratio is up to 81%, which is much higher than that of graphene nanofluid. When the concentration increased to 0.8 g/L, the transmittance of nanofluid reduces to zero, which is an ideal result. However, it is not an ideal selection to realize a zero transmittance or perfect absorption by increasing limitlessly nanoparticle concentration because the heavy concentration can lead to some other problems such as particle agglomeration, dispersion, sedimentation, and heat loss (Taylor et al., 2011b), which have not yet been considered in present work. If the particle concentration is too low, the solar absorption is also insufficient. Therefore, it is suggested that the concentration of 0.4 g/L Sn@SiO2@Ag nanoparticles is sufficient to enhance the solar absorption in solar collector.
3.3. Solar absorption characteristics of Sn@SiO2@Ag nanofluids
3.4. Solar absorption characteristics of nanofluids containing graphene and Sn@SiO2@Ag nanoparticles
In order to further enhance the solar absorption performance of graphene nanofluids, we explore another strategy by employing the plasmon resonance of nanoparticles. As shown in Fig. 9, a solar absorption band was found in the wavelength range of 350–500 nm in Sn@SiO2@Ag nanofluids owing to the plasmon resonance of core-shell structure (Ding et al., 2006). The transmittance spectra of Sn@
Based on the above-mentioned strategy, we prepared the graphene nanofluids containing Sn@SiO2@Ag core-shell nanoparticles (0.2G & Sn@SiO2@Ag). The volume fraction of graphene nanosheets is 5
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Fig. 11. Extinction coefficient of graphene-embedded Sn@SiO2@Ag nanofluids. The solar irradiance data is obtained from reference Mecherikunnel and Duncan (1982). Fig. 10. Transmittance spectra of graphene-embedded Sn@SiO2@Ag nanofluids. λ
fixed to 0.2 g/L and that of Sn@SiO2@Ag core-shell nanoparticles changes from 0.1 g/L to 0.5 g/L. Their transmittance spectra were given in Fig. 10. A sharp peak is also observed around 320 nm due to the excited surface plasmon in the core-shell structure, resulting in a notable absorption in the vicinity of 440 nm wavelength. In addition, a weak transmittance peak appears at 230 nm due to the intrinsic absorption of graphene. One can found that a broadband absorption is formed in view of the synergistic enhancement of nanoparticle’s intrinsic absorption and surface plasmon, which is differ from the single peak absorption in Sn@SiO2@Ag nanofluids. The results show that the average transmittance of graphene fluid with 0.2 g/L concentration can be reduced to 49% only adding 0.1 g/L Sn@SiO2@Ag solution, which is apparent to enhance solar absorption. The average transmittance of Sn@SiO2@Ag nanofluids with concentration 0.4 g/L is further reduced to 11%, which is far below 69% of the graphene nanofluid with concentration 0.2 g/L, improving remarkably the solar absorption properties. If we ignore the reflection and scattering, the solar absorptance (αs ) of nanofluids can be evaluated simply as αs = 1−τnf from the transmittance (τnf ). This means that the solar absorptance of nanofluids consisting of 0.2 g/L graphene and 0.4 g/L Sn@SiO2@Ag solutions is 2.9 times larger than that of 0.2 g/L graphene nanofluid. It is suggested that the Sn@SiO2@Ag core-shell nanoparticles augments the solar absorption of graphene fluids due to the core-shell plasmon resonance effect (Wu et al., 2015). As shown in the above-mentioned analysis, the nanofluids containing graphene and Sn@SiO2@Ag core-shell nanoparticles are useful in volumetric solar collectors. In order to evaluate the radiative properties of the nanofluids, extinction coefficient k (λ ) is calculated from the transmittance data according to Bear-Lambert law T (λ ) = exp(−k (λ ) L) . The term T (λ ) and L is transmittance and the length of light crossing, and L = 10 mm. The extinction coefficient of nanofluids with different volume fraction is illustrated in Fig. 11. One can found that a broad peak exists around the wavelength of 440 nm due to the plasmon resonance absorption beside the intrinsic absorption peak (270 nm) of graphene. This means that the addition of Sn@ SiO2@Ag core-shell nanoparticles can significantly enhance the solar absorption ability of graphene nanofluids because the broad peak location is very close to the wavelength (480 nm) of maximum intensity of solar irradiance. The solar absorption ability of nanofluids is important in volumetric solar collectors. One always expects a larger absorption fraction of solar energy (AF) can be achieved through a minimum thickness and volume fraction of nanofluids. In this work, the absorption fraction AF is calculated from Eq. (1) (Zhang et al., 2014)
AF = 1−
max Is (λ )exp(−k (λ ) x ) dλ ∫λmin
λ
max Is (λ ) dλ ∫λmin
. (1)
In this case, Is (λ ) denotes the solar radiation intensity (Mecherikunnel and Duncan, 1982), k (λ ) is the extinction coefficient of nanofluids, and x is equal to the thickness of the nanofluids layer. λmin = 250 nm and λmax = 1300 nm are considered as integration bounds. The calculated absorption fraction is shown in Fig. 12. It can be found that the minimal thickness of nanofluids layer with AF = 100% decreases with increasing volume fraction of nanofluids. For example, the thickness is 6 cm for the nanofluids with containing 0.2 g/L graphene and 0.1 g/L Sn@SiO2@Ag nanoparticles, while the thickness decreases to 2 cm when the Sn@SiO2@Ag nanoparticles volume fraction increases to 0.5 g/L. According to the Eq. (1), the absorption fraction depends mainly on the extinction coefficient k (λ ) and the thickness (x) of nanofluids layer. Therefore, the enhancement of AF can be achieved by increasing k (λ ) or x. However, it is inadvisable to increase the thickness of nanofluids layer because the size of solar collector becomes bulky. In fact, it is only suitable selection to increase the k (λ ) by the addition of nanoparticles in base fluids when the minimum thickness layer of nanofluids be determined by considering the AF = 100%. 3.5. Thermal conductivity of nanofluids Nanofluids have been reported to exhibit superior heat transfer
Fig. 12. Absorbed energy fractions (AF) versus the penetration distance (x) of grapheneembedded Sn@SiO2@Ag nanofluids.
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graphene approaches 0.72 W m−1 K−1, which is the maximum at room temperature in present experiment. The enhanced thermal conductivity may be attributed to the increase of conductivity chain. As seen in TEM picture that the graphene is a nanosheets shape and the Sn@SiO2@Ag is spherical particles. The addition of core-shell particles can form an effective conductivity path at between the graphene nanosheets. Temperature is also an important parameter to impact the thermal conductivity of nanofluids. In the case of G & Sn@SiO2@Ag suspension, the thermal conductivity was measured at temperature of 20–50 °C. As shown in Fig. 14, the thermal conductivity increases with increasing temperature. When the concentration of G & Sn@SiO2@Ag increases to 0.3 g/L, the thermal conductivity is enhanced 11.3% at 20 °C comparing with water base fluid, while the value is 16% at 50 °C. The notable enhancement can probably be attributed to the extremely high surface area of graphene nanosheets and the increased conductivity path through the embedded Sn@SiO2@Ag core-shell nanoparticles. The present results are much superior to what Vakili et al. (2016) had reported for 0.05 wt.% of graphene nanoplatelets in water base fluids. It is thought that the Brownian motion of nanoparticles in base fluids is the main contribution to the enhancement of thermal conductivity in nanofluids as it is one of the most dominated functions of temperature (Xuan and Li, 2000). In addition, Baby and Ramaprabhu (2010) suggested that the interaction of graphene layer with each other results in a readily electron hopping from one graphene to other in graphene-water nanofluids. In our case, however, the hopping becomes more convenient from one graphene layer to another via the conductivity bridge of Sn@SiO2@Ag core-shell nanoparticles. Moreover, the synergic enhancement of Sn@SiO2@Ag core-shell nanoparticles and graphene nanosheets is also favor to enhance the thermal conductivity of nanofluids.
Fig. 13. Thermal conductivity of nanofluids at 20 °C. In the case of 0.2G & Sn@SiO2@Ag nanofluid, the X-axis stands for the concentration of adding Sn@SiO2@Ag in 0.2 g/L graphene nanofluids.
Thermal conductivity (W/m.K)
0.80
G&Sn@SiO2@Ag 0.30 g/L G&Sn@SiO2@Ag 0.25 g/L G&Sn@SiO2@Ag 0.20 g/L G&Sn@SiO2@Ag 0.15 g/L G&Sn@SiO2@Ag 0.10 g/L G&Sn@SiO2@Ag 0.05 g/L
0.75
0.70
4. Conclusions
0.65
0.60
20
25
30 35 40 Temperature ( C)
45
In summary, we investigated the solar absorption performance of several nanofluids in volumetric solar collector based on the nanoparticles/nanosheets of graphene oxide, graphene, Sn@SiO2@Ag, or graphene-embedded Sn@SiO2@Ag. It is confirmed that the performance of graphene nanofluids is enhanced significantly through the plasmon resonance absorption and thermal conduction bridge of graphene-embedded Sn@SiO2@Ag core-shell nanoparticles. A strong absorption band is observed in the wavelength range of 250–300 nm and 380–600 nm. The solar absorption coefficient of graphene nanofluid is enhanced 2.9 times by the core-shell nanoparticles. An enhancement in thermal conductivity of 11.3% has been obtained at 20 °C for 0.3 g/L graphene-coated Sn@SiO2@Ag nanofluids and 16% enhancement for the same volume fraction at 50 °C. It is concluded that the synergic effect of the core-shell nanoparticles and graphene nanosheets increases both the solar absorption coefficient and thermal conductivity of the nanofluids. This will enhance the performance of volumetric solar collector.
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Fig. 14. Temperature-dependent thermal conductivity of graphene-embedded Sn@ SiO2@Ag nanofluids.
properties compared with base fluids over the past decades (Xuan and Li, 2000; Yu et al., 2009). In volumetric solar collectors, the superior heat transfer properties are also equally important. It is known that the thermal conductivity of nanofluid is strongly dependent on the volume fraction dimensions, nanofluids temperature, and nanoparticles itself. In this work, we employ the transient hot-wire apparatus to measure the thermal conductivity of nanofluids. As shown in Fig. 13, the thermal conductivity at room temperature of 20 °C increases significantly with increasing nanoparticles volume fraction. For example, the thermal conductivity of base fluids is 0.62 W m−1 K−1, and it is 0.68 W m−1 K−1 for the graphene oxide suspension at 0.3 g/L volume fraction. The percentage of enhancement [(Knf −Kbf ) × 100]/ Kbf in thermal conductivity approaches to 9.7%, where Knf and Kbf denotes the thermal conductivity of nanofluids and base fluids, respectively. In the case of G & Sn@SiO2@Ag suspension, the graphene and core-shell nanoparticles were blended by the mass ration of 1:1. The thermal conductivity approaches 0.69 W m−1 K−1 in 0.3 g/L G & Sn@SiO2@Ag suspension. The percentage of enhancement is 11.3%. However, the thermal conductivity can be further enhanced by increasing additive concentration. For instance, in the case of 0.2G & Sn@SiO2@Ag suspension, when the graphene concentration was fixed at 0.2 g/L, the thermal conductivity suspension was increased about 0.01 W m−1 K−1 per 0.1 g/L increment of Sn@SiO2@Ag. The thermal conductivity of suspension with consisting of 0.4 g/L G & Sn@SiO2@Ag and 0.2 g/L
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