Complementary enhanced solar thermal conversion performance of core-shell nanoparticles

Applied Energy 211 (2018) 735–742

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Applied Energy journal homepage: www.elsevier.com/locate/apenergy

Complementary enhanced solar thermal conversion performance of coreshell nanoparticles

T

⁎

Meijie Chen, Yurong He , Xinzhi Wang, Yanwei Hu School of Energy Science & Engineering, Harbin Institute of Technology, Harbin 150001, China Heilongjiang Key Laboratory of New Energy Storage Materials and Processes, School of Energy Science & Engineering, Harbin Institute of Technology, Harbin 150001, China

H I G H L I G H T S properties of core-shell NPs were discussed systematically. • Optical efficiency can be adjusted by the core-shell or mixing ratios of NPs. • Absorption parameters of the core-shell NPs for solar absorption were obtained. • Optimized • Efficiency of Au-decorated SiO NPs was superior to Au NPs and SiO NPs. 2

2

A R T I C L E I N F O

A B S T R A C T

Keywords: Solar thermal conversion Core-shell nanoparticle Finite difference time domain Optical properties

In this study, the properties of various types of core-shell nanoparticles (NPs) were evaluated using the finite difference time domain (FDTD) method towards the enhancement of solar absorption performance. Results showed that the resonance wavelength of SiO2@Au NPs lay in the 540–900 nm range, covering the near-infrared and visible regions. The resonance wavelength of SiO2@Ag NPs lay in the 390–830 nm range, covering the entire visible region. SiO2@Au nanofluid with a core-shell ratio of φ = 0.2 exhibited the highest solar absorption efficiency with 64% less Au consumption compared to pure Au NPs. For mixed nanofluids, the mixtures featuring core-shell ratios of 0.1 and 0.6 with mixing ratios of 0.5 for SiO2@Au and 0.6 for SiO2@Ag gave the highest absorption efficiencies. In addition, the peak solar absorption efficiency of a mixed nanofluid of SiO2@Au (φ = 0.1) and SiO2@Ag (φ = 0.4) with a mixing ratio of 0.58 was as high as 94.4%. Solar thermal conversion experiments revealed that, under the same conditions, a Au-decorated SiO2 nanofluid showed a comparable efficiency to the calculated solar absorption efficiency of the SiO2@Au core-shell nanofluid (∼95.2%); it was as high as 95.9%, higher than those of Au NPs and SiO2 NPs. These results showed that adjusting the core-shell ratios and tuning the mixing ratios of different nanofluids are two efficient methods to enhance the solar absorption efficiencies of SiO2@Au and SiO2@Ag NPs under the optimal conditions.

1. Introduction The energy crisis is one of the key issues in the modern world that must be overcome, and because of it, there are urgent demands for developing clean and sustainable energy sources for our future development [1,2]. Solar energy is a widespread and clean energy source. Efficient methods to transform solar energy into other forms include photovoltaic conversion [3,4], photochemical conversion [5], and photothermal conversion [6]. One of the most common and convenient methods to utilize solar energy is solar thermal conversion for a thermal storage system [7–10], which has attracted more attention. However, the solar thermal conversion efficiency is still too low at present owing ⁎

to the application of a single transparent fluid. The development of nanoscale control of nanoparticles (NPs) has enabled researchers to investigate their intense interactions with light for a wide range of applications: photothermal therapy [11], sensing [12], and light harvesting & conversion [13,14]. Moreover, this has shown significant technological interest in the effective harvesting and conversion of solar energy [15]. Recently, the addition of NPs to working media was introduced as a means to enhance either solar absorption or scattering for different conversion methods [16,17]. Nanofluid-based direct absorption solar collectors (DASCs) are a potential alternative to traditional solar collectors owing to the enhanced solar thermal conversion performances

Corresponding author at: School of Energy Science & Engineering, Harbin Institute of Technology, Harbin 150001, China. E-mail address: [email protected] (Y. He).

https://doi.org/10.1016/j.apenergy.2017.11.087 Received 30 August 2017; Received in revised form 17 November 2017; Accepted 20 November 2017 Available online 24 November 2017 0306-2619/ © 2017 Elsevier Ltd. All rights reserved.

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caused by NPs [18]. For DASCs, one of the key issues is the enhancement of the solar absorption performances of the working fluids to improve collector efficiency at a high working temperature. Therefore, researchers have made great efforts to improve the solar absorption performances of the working fluids. The solar thermal conversion characteristics of Ag NPs under realistic conditions could be enhanced by up to 144% with a very low Ag NP concentration (∼6.5 ppm) [19]. An experimental model that accounted for heat loss has been introduced to calculate the solar thermal conversion efficiency of a Au nanofluid (2.5 ppm), which could be up to 200% higher than that of water [20]. These enhanced performances could be attributed to the strong interactions of noble metals Au and Ag between light and NPs, resulting in surface plasmon resonance (SPR) around the NP surface at the resonance wavelength [21,22]. When the oscillation frequency of electrons is equal to the incident light frequency, SPR is significantly strong. Both the absorption and scattering properties of plasmonic NPs are enhanced. As discussed above, the unique optical, magnetic, and electronic properties of metal NPs enable photothermal, therapeutic, and electronic device applications, respectively. However, the limited range of the properties of simple spherical metal NPs has restricted their ability to function in many of these applications. The use of a shell layer is an efficient way to tune the optical properties of NPs and reduce the consumption of noble metals. Nanoshells, consisting of a dielectric core covered by a noble metal shell layer, also show tunable SPR. The SPR wavelengths are affected by the parameters of the core and shell layers, because of the interaction between the inner and outer surfaces of the metallic shell. For metal semiconductor core-shell nanostructures, the effect of the semiconductor shell on the metal NP, or vice versa, results in reduced charge recombination at the interface, leading to an enhanced efficiency in energy conversion and storage applications such as solar cells, fuel cells, rechargeable batteries, and super capacitors [23]. Tang et al. [24] reported that tuning the aspect ratios of Au@SiO2 coreshell nanorods could lead to increased cross-sectional scattering and spectrally absorbing energy density for perovskite solar cells. Kim et al. [25] reviewed “smart” core-shell composite NPs with multi-response mechanisms, for example, to temperature, light, or an applied magnetic field. Additionally, hydrogel-coated metal@silica NPs have been demonstrated to store drugs in a mesoporous silica interlayer to carry cargo to targeted sites [26]. Core-shell nanostructures have become new potential systems for the enhancement of given properties of the native components; this could be because of the effects of the interface between the core and shell [27]. The key is to develop tailored materials based on core-shell NPs, which would open the way to bifunctional NPs. The SPR wavelength of nanoshells can be tuned from the visible to the infrared domains [28]; this can be utilized efficiently to tune the radiative properties of plasmonic nanofluids used in solar thermal applications [29]. Results have shown that the structures of composite NPs themselves can simultaneously influence their average and nearfield radiative properties and, thus, those of plasmonic nanofluids. In other experiments, the optical absorption of a TiO2@Ag plasmonic nanofluid within the solar radiation spectrum was enhanced remarkably owing to the SPR effect on the Ag surface [30]. The scattering, absorption, or extinction coefficient for NPs with Si or SiC cores and Au, Ag, Cu, or Al shells were calculated using the Mie scattering theory and the Rayleigh scattering approximation by Wu et al. [31]. Results showed that the coupling effects between light and the coreshell interface can be utilized efficiently to tune the radiative properties of plasmonic nanofluids used in photothermal applications. In addition, the use of carbon@Au core-shell NPs dispersed in liquid water has been proposed for the enhancement of solar thermal energy conversion efficiency; it was demonstrated that carbon@Au NPs can theoretically enhance the intensity of the absorption peak while broadening the absorption band [32]. Li and Wang et al. [33–35] conducted serial experimental studies on the photothermal performance of Ag@TiO2

and other NPs with great absorptivity in the visible region. In the study of Taylor et al. [36], core-shell Ag@SiO2 NPs were dispersed in water to filter out the ideal spectrum for Si photovoltaic cells in hybrid photovoltaic/thermal collectors. These simulation and experimental results from previous studies showed that core-shell NPs could improve solar absorption ability by tuning the absorption peak location or broadening the absorption band. As described above, the improvement of solar absorption efficiency is significantly influenced by the particle material, size, shape, volume fraction, etc. However, simultaneous investigations of the thermal and optical properties have been rare, especially for the spectral properties that are critical for efficiency in photothermal applications. Moreover, optimization of the material design, core-shell ratio, and collector parameters (such as the NP volume fraction and collector height) have not been investigated systematically. New strategies to improve solar absorption performance by matching the NP absorption properties with the solar spectrum intensity are required in addition to these methods. Hence, in this study, the optical properties of various types of coreshell NPs were evaluated. The optimal combinations of core-shell NPs whose absorption characteristics fit the solar spectrum at the Earth’s surface are discussed in this paper; these would result in less noble metal consumption. In addition, the collector height and NP volume fraction were optimized. The solar absorption efficiencies of SiO2@Au and SiO2@Ag NPs were enhanced by adjusting their core-shell ratios or tuning their mixing ratios. 2. Theoretical modeling The extinction coefficient of a nanofluid can be determined by adding the individual contributions of the NPs and base fluid as follows [37]:

keλ,nf = keλ,np + keλ,bf

(1)

where keλ,nf, keλ,np, and keλ,bf are the spectral extinction coefficients of the nanofluid, NPs, and base fluid, respectively. Therefore, the optical properties of the base fluid (water) and NPs are evaluated separately. 2.1. Optical absorption of the base fluid Water is used as the base fluid owing to its good absorption in the near-infrared range. In a pure fluid, the effect of scattering can be neglected, so only the absorption effect is considered. Therefore, the spectral extinction coefficient of the base fluid (i.e., water) can be calculated as follows [37]:

keλ,bf ≈ kaλ,bf =

4πκ λ

(2)

where kaλ,bf is the spectral absorption coefficient of the base fluid, κ is the absorption index of water, and λ is the wavelength. Fig. 1 shows the absorption coefficient of water, which indicates that it has poor absorption ability in the ultraviolet and visible light spectral ranges. However, it is a strong absorber in the near-infrared range. Adding NPs to a base fluid can greatly improve the absorption ability in visible light. Therefore, the present study focused on improving solar absorption by changing the NP elements or mixing various NPs to broaden the solar absorption spectra. 2.2. Optical properties of the nanoparticles In this work, the finite difference time domain (FDTD) method was used to evaluate the optical properties of core-shell NPs, a model of which is shown in Fig. 2(A). An explicit time-marching algorithm is involved in this method for solving Maxwell equations on discretized spatial grids. The electromagnetic propagation in a system can be described by Maxwell equations [38]: 736

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Absorption coefficient / m-1

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10000

scattering regime, the scattered radiation from one source is not affected by that from other sources. Hence, the extinction caused by the nanofluid can be determined by adding the individual contributions from the NPs and base fluid; this has been reported in previous literature [40]. The extinction coefficient of all suspended NPs can be calculated as follows [41]:

1000 100 10

keλ,nf = keλ,np + keλ,bf =

1

600

1200

1800

2400

Wavelength / nm Fig. 1. Absorption coefficient of water. ⎯⎯⎯⇀

ε

⎯⎯⎯⇀ ⎯⇀ ⎯ ∂E = ∇ × H −J ∂t

(3)

⎯⎯⎯⇀ ∂H = −∇ × E ∂t

⎯⇀ ⎯

⎯⎯⎯⇀

J = σE

i=1

3fv,i Qext ,i 2di

(6)

λ

ηabs =

⎯⎯⎯⇀

μ

n

∑

where fv is the volume fraction of the NPs, d is the equivalent diameter of the NPs, Qext is the extinction efficiency of a single NP, and n is the number of different mixed components. According to the Beer-Lambert law [42], the radiation intensity decreases exponentially along its transfer direction. The percentage of solar energy absorbed by a nanofluid can be determined by using the solar weighted absorption fraction to describe the maximum absorption of solar energy by a nanofluid. Therefore, the maximum solar absorption efficiency ηabs can be defined as follows:

0.1 0.01

4πκ + λ

(4)

max I0,λ (1−e−keλ,nf h) dλ ∫λmin

λ

max I0,λ dλ ∫λmin

(7)

where I0,λ is the solar intensity (ASTM G173-03 AM1.5 Global) and h is the height of the nanofluid. In our previous study [20], the maximum solar absorption efficiency ηabs was used with Eq. (7) to evaluate the theoretical maximum solar thermal conversion efficiency of the fluid and provided results that showed good agreement, indicating that the maximum solar absorption efficiency can be used as a parameter for NP optimization and collector design without the need to consider the temperature field.

(5)

where ε and μ are the permittivity and permeability, respectively, E and H are electric and magnetic fields of the medium, respectively, J is the current density, and σ is the conductivity. These equations can be solved on discrete grids by replacing all of the derivatives with finitedifference expressions. The discretization of the computational domain is usually based on the Yee grid. During a computational process, the new value of the electric-field (or magnetic-field) component is calculated from the previous value and the adjacent node values of the magnetic-field (or electric-field) component. The mesh size is 1 × 1 × 1 nm. Perfectly matched layers (PMLs) are applied at the surfaces along the light incident direction to absorb nearly all of the incident waves. Fig. 2(B) shows a comparison between the simulated results from the FDTD method and Mie theory for Au NPs (radius = 50 nm). The absorption efficiency calculated using the FDTD method was in good agreement with the Mie theory results. The average error between the simulation results obtained using the FDTD method and those obtained using Mie theory was 2.3%, which indicates that accurate results can be obtained using the FDTD method.

3. Results and discussion 3.1. Optical properties of the NPs A typical core-shell NP with a radius R and a shell thickness tshell is shown in Fig. 2. The core-shell ratio (φ) of NPs is defined as tshell/R. The optical properties of SiO2@Au, SiO2@Ag, and Au@SiO2 NPs with different core-shell ratios were studied quantitatively. The radius of the NPs, R, was controlled to be 50 nm. Figs. 3–5 present the optical properties of core-shell NPs with different core-shell ratios, which were achieved by adjusting the shell thickness. The spectra range from 300 to 1200 nm, covering at least 80% of solar radiation. Fig. 3 shows the absorption efficiency of Au@SiO2 NPs with different core-shell ratios. When the core-shell ratio φ is between 0 and 1, the main absorption range lies in the visible light region, as shown in Fig. 3(A). Fig. 3(B) shows that φ has little effect on the peak absorption wavelength in the solar radiation range. Therefore, using small particles does not change the peak absorption wavelength of solar energy but does increase stability. However, the peak absorption efficiency of Au@

2.3. Solar absorption efficiencies of nanofluids The particles can be assumed to be in the independent scattering regime owing to the low NP volume fraction and the small NP size [39], so relatively simple calculations are required. In the independent

Fig. 2. (A) Computational model of an NP with a radius of 50 nm; (B) comparison between simulated results from the FDTD method and Mie theory for Au NPs.

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2.700 2.400 2.000 1.700 1.300 1.000 0.600 0.300 0.000

600

400

0.2

0.4

0.6

0.8

Peak wavelength Peak absorbance

700 2 600 1 500

400 0.0

1.0

0.2

0.4

shell-SiO 2

Qabs 5.600 5.000 4.200 3.500 2.800 2.100 1.400 0.700 0.000

900

600

0.2

0.4

0.6

0.8

1.0

(B) Peak wavelength / nm

Wavelength / nm

1200

300 0.0

0.6

0.8

0 1.0

Fig. 3. Optical properties of Au@SiO2 NPs: (A) absorption efficiencies with different core-shell ratios; (B) peak wavelengths and absorption efficiencies with different coreshell ratios.

shell-SiO2

SiO2 NPs decreases with increasing φ or decreasing Au core size. In fact, it is known that pure Au nanospheres that are 10–100 nm in size all have the same resonance wavelengths, but their optical resonance does not have the desired tunability [43]. Since changes in the core-shell ratio of Au@SiO2 NPs did not lead to the desired tunability of the optical resonance, the optical properties of SiO2@Au NPs were investigated, and the results are shown in Fig. 4. SiO2@Au NPs showed tunable absorption efficiencies that are comparable to or even higher than those of pure Au NPs when φ is between 0 and 0.5. Additionally, the resonance wavelength of the SiO2@Au NPs lies in the 540–900 nm range, covering the near-infrared and visible regions. Moreover, in the near-infrared region, they show the highest absorption efficiency in the tunable range, where the most solar energy can be absorbed by NPs and biological tissue transmissivity is the highest and which is out of the visible absorption range of hemoglobin of around 500–600 nm. Thus, SiO2@Au NPs are much more suited for solar heating, in vivo imaging, and therapy applications than pure Au NPs are. As one of the most popular plasmon metals, Ag has also attracted the interest of many researchers. Hence, the optical properties of SiO2@Ag NPs were also evaluated and are shown in Fig. 5. These NPs showed the same tendencies as the SiO2@Au NPs. The resonance wavelength of the SiO2@Ag NPs lies in the 390–830 nm range, covering the entire visible region. It showed great tunability when φ < 0.6. It is better to choose SiO2@Ag NPs with φ < 0.6 in order to reduce Ag metal consumption, since Ag consumption has little effect on plasmon resonance when φ > 0.6, as shown in Fig. 5(B), which could also be seen for SiO2@Au NPs when φ > 0.7. The electric fields around these three core-shell NPs (φ = 0.2) and their spectral absorption curves (Fig. 6) were examined in order to compare their optical properties. The electric fields of the metal in different locations (core or shell) in the core-shell NPs were examined. It can be seen that the electric field is only enhanced around the surface of the metal, while the SiO2 shell is almost transparent without any obvious electric field enhancement around its surface, as shown in Fig. 6(A). However, the electric field in the Au shell is greatly

(A)

3

strengthened at the resonance wavelength, as shown in Fig. 6(B), leading to a remarkable enhancement of optical absorption compared to that of the Au core, as shown in Fig. 6(D). Electric field enhancement could also be observed around the Ag shell. Moreover, the electric field enhancement of the Ag shell was much stronger than that of the Au shell at their resonance wavelengths, and this could be attributed to the higher resonance quality factor of Ag [44]. The surface plasmon strength is directly proportional to the resonance quality factor; high resonance quality factors indicate strong plasmons. However, the peak absorption efficiency of the SiO2@Ag NPs is lower than that of the SiO2@Au NPs owing to the appearance of two absorption peaks for the SiO2@Ag NPs, which has also been observed for TiO2@Ag NPs [30]. 3.2. Calculated solar absorption efficiencies of nanofluids For solar thermal conversion applications based on nanofluids, good agreement between the calculated solar absorption efficiency and the experimental solar thermal conversion efficiency has been reported previously [20]. In this section, the calculated solar absorption efficiencies of the nanofluids for solar thermal conversion applications will be discussed. In addition, two methods to enhance solar absorption efficiency were studied. One involved adjusting the core-shell ratios of the NPs and the other involved tuning the mixing ratios of the different nanofluids. This would provide the optimal parameters for the maximum solar absorption efficiency. As discussed above in Section 3.1, SiO2@Au NPs show great tunable optical properties in the near-infrared and visible regions. Hence, the solar absorption efficiencies of SiO2@Au nanofluids with different NP volume fractions, nanofluid heights, and NP core-shell ratios were calculated and are shown in Fig. 7. The SiO2 nanofluid showed the lowest solar absorption efficiency. Generally speaking, Fig. 7(A) and (B) show that the solar absorption efficiency of SiO2@Au nanofluids increases with increasing NP volume fraction and height. Moreover, this increase is rapid at low NP volume fractions and low heights, and then slows down. The solar absorption efficiencies of SiO2@Au nanofluids are greatly enhanced by Au shells. The SiO2@Au nanofluid with the

900

Peak wavelength Peak absorbance

800

6 5

700

4

600

3

500 0.0

0.2

0.4

0.6

shell-Au

shell-Au

738

0.8

2 1. 0

Peak absorption efficiency / a.u.

0.0

(B) 800

Peak absorption efficiency / a.u.

Qabs

Peak wavelength / nm

Wavelength / nm

(A) 800

Fig. 4. Optical properties of SiO2@Au NPs: (A) absorption efficiencies with different coreshell ratios; (B) peak wavelengths and absorption efficiencies with different core-shell ratios.

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(B)

Qabs 5.900 5.200 4.400 3.700 2.900 2.200 1.400 0.7000 0.000

900

600

300 0.0

0.2

0.4

0.6

ϕ shell-Ag

0.8

840

Peak wavelength Peak absorbance

770

Peak wavelength / nm

Wavelength / nm

(A) 1200

700 630

4

490

3

420 350 0.0

1.0

0.2

E (N/C)

z / nm

20 0

60

ratio of 0.6 gave the highest absorption efficiency at 93.9%, which is 21.7% and 18.9% higher than those of its individual components, respectively. Based on the above simulation results, the mixtures containing SiO2@Au with φ = 0.1 and 0.6 and SiO2@Ag with φ = 0.1 and 0.6 showed the highest solar absorption efficiencies, indicating that these are the optimal combinations of core-shell NPs whose absorption characteristics fit the solar spectrum at the Earth’s surface well, according to Eqs. (6) and (7). For SiO2@Ag and SiO2@Au mixed nanofluids, Fig. 8(C) shows that the peak solar absorption efficiency was achieved with a mixing ratio of 0.58. Moreover, the peak solar absorption efficiency of a mixed nanofluid containing SiO2@Au (φ = 0.1) and SiO2@Ag (φ = 0.4) was as high as 94.4%, which is higher than that of a mixed nanofluid containing SiO2@Au (φ = 0.4) and SiO2@Ag (φ = 0.1) with less Au consumption. In conclusion, adjusting their coreshell ratios and tuning the mixing ratios of different nanofluids are two efficient methods to enhance the solar absorption efficiencies of SiO2@Au and SiO2@Ag NPs.

-40

-40

-60

-60 0

20

40

3.3. Solar thermal conversion experiments of Au-decorated SiO2 nanofluids The solar thermal conversion performances of water, SiO2, Au, and Au-decorated SiO2 nanofluids were investigated. The sample

(B)

SiO2@Au NP

12.0 0 6.50 0 6.00 0 5.50 0 5.00 0 4.50 0 4.00 0 3.50 0 0.00 0

60

-60

-40

-20

x / nm E (N/C)

SiO2@Ag NP

(C)

32.00 8.000 6.000 5.500 5.000 4.500 3.500 3.000 0.000

40

z / nm

20 0 -20 -40 -60 -6 0

-4 0

-2 0

0

x / nm

20

40

60

4.5

Absorption efficiency

60

E (N/C)

0

-20

-2 0

2 1.0

0.8

20

-20

-4 0

0.6

40

z / nm

6.000 5.500 5.000 4.500 4.000 3.500 3.000 2.500 0.000

40

-6 0

0.4

Fig. 5. Optical properties of SiO2@Ag NPs: (A) absorption efficiencies with different core-shell ratios; (B) peak wavelengths and absorption efficiencies with different coreshell ratios.

shell-Ag

Au@SiO2 NP

(A)

5

560

core-shell ratio of 0.2 gave the highest solar absorption efficiency while consuming 64% less Au than pure Au NPs. This indicates that core-shell NPs with a plasmonic metal shell are a potential medium for enhancing solar absorption. For a solar absorption efficiency of 90%, the optimal core-shell ratio of SiO2@Au NPs is ∼0.2, the optimal NP volume fraction is ∼24 ppm, and the optimal collector height is ∼6 mm. In order to enhance solar absorption ability further, mixed nanofluids were studied. For the simulation procedure of the mixed nanofluids, the total NP volume fraction and collector height were 20 ppm and 5 mm, respectively. Different mixed nanofluids were obtained by changing the additive amount of SiO2@Au or SiO2@Ag NPs. For the mixed nanofluids, different core-shell ratios will be discussed. First, SiO2@Au and SiO2@Ag nanofluids with φ = 0.1 were chosen owing to their high absorption abilities with a peak at 800–900 nm, which is difficult to achieve with pure Au and Ag spheres. Fig. 8 shows that the solar absorption ability can be enhanced significantly by adjusting the core-shell ratios (from 0.2 to 1.0) of NPs mixed with the chosen nanofluids (φ = 0.1). For SiO2@Au mixed nanofluids, the mixture containing SiO2@Au with φ = 0.1 and 0.6 in a ratio of 0.5 gave the highest absorption efficiency at 94.3%, which is 17.5% and 13.3% higher than those of its individual components, respectively. For SiO2@Ag mixed nanofluids, the mixture containing SiO2@Ag with φ = 0.1 and 0.6 in a 60

6

Peak absorption efficiency / a.u .

M. Chen et al.

(D)

0

x / nm

20

40

60

Au@SiO 2 NP SiO2@Au NP

3.6

SiO2@Ag NP

2.7 1.8 0.9 0.0 300

600

900

Wavelength / nm

739

1200

Fig. 6. Electric fields around core-shell NPs (φ = 0.2) and their spectral absorption NPs curves: (A) Au@SiO2 (λpeak = 557 nm); (B) SiO2@Au NPs (λpeak = 697 nm); (C) SiO2@Ag NPs (λpeak = 631 nm); (D) absorption efficiency curves of the different NPs.

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Fig. 7. Solar absorption efficiencies of SiO2@Au nanofluids with different (A) NP volume fractions, (B) nanofluid heights, and (C) NP core-shell ratios.

with Au NPs, the peak absorbance wavelength shifted to 536 nm with a decrease in absorbance from 0.658 to 0.505. However, it can be seen that the absorption range of the Au-decorated SiO2 NPs is significantly wider. Then, the solar thermal conversion performances of water, SiO2, Au, and Au-decorated SiO2 nanofluids were evaluated by irradiating

preparation and solar thermal conversion experiments are described in detail in the supporting information. The absorbance spectra of the fluids are shown in Fig. 9(A). The SiO2 NPs showed low absorption ability in the 300–900 nm range. The peak absorbance wavelength for the Au NPs was 524 nm. After the surface of the SiO2 NPs was decorated

Fig. 8. Solar absorption efficiencies of mixed nanofluids: (A) SiO2@Au mixed nanofluids; (B) SiO2@Ag mixed nanofluids; (C) SiO2@Au and SiO2@Ag mixed nanofluids.

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Fig. 9. Solar thermal conversion performances of nanofluids: (A) absorbance spectra of the nanofluids before and after experiments; (B) temperature increases of different fluids under solar illumination (10 suns); (C) solar thermal conversion efficiencies of different fluids.

4. Conclusions

them for 5 min with a solar simulator. The absorbance spectra of the fluids before and after the solar thermal conversion experiments in Fig. 9(A) show that the working fluids showed good thermal stability under strong solar irradiation and that the average absorbance changes were only 0.66%, 0.28%, and 0.35% for SiO2, Au, and Au-decorated SiO2 nanofluids, respectively. The temperature increases of the fluids are shown in Fig. 9(B). The Au-decorated SiO2 nanofluid showed the highest temperature increase. The temperature increase of the SiO2 nanofluid was nearly identical to that of water. During the short illumination period, the temperature increases were nearly linear with time. Therefore, the temperature increase rate ΔT/t could be obtained through linear fitting of the data in Fig. 9(B). The solar thermal conversion performances of the fluids were calculated using Eq. (S1) in the supporting information and are shown in Fig. 9(C). The Au-decorated SiO2 nanofluid showed the highest solar thermal conversion efficiency (95.9%), which was 3.37 times that of pure water. Since the absorption ability of the SiO2 NPs was low, their solar thermal conversion efficiency was only 3.9% higher than that of pure water. The Au nanofluid showed a solar thermal conversion efficiency of 90.4%, which was 217.2% higher than that of pure water. However, it was still lower than that of the Au-decorated SiO2 nanofluid (232.6%). This could be attributed to the fact that the SiO2 NPs can increase the transmission path of the photon via multi-scattering effect [45]. On the other hand, the Au NPs had great photon-absorption ability. Owing to the interactions of the Au and SiO2 NPs with photons, the absorption range of the Audecorated SiO2 NPs was significantly wider than that of the pure Au NPs, as shown in Fig. 9(A). Moreover, the Au-decorated SiO2 nanofluid showed comparable efficiency to the calculated solar absorption efficiency of the SiO2@Au core-shell nanofluid (∼95.2%) under the same conditions described in Section 3.2. As mentioned above, the Au-decorated SiO2 NPs and SiO2@Au core-shell NPs showed superior solar thermal conversion performances compared to those of Au NPs and SiO2 NPs in the solar thermal conversion experiments.

In this study, the properties of various core-shell NPs were evaluated using the FDTD method to determine the optimal combinations of coreshell NPs whose absorption or scattering characteristics fit the solar spectrum at the Earth’s surface and with low noble metal consumption. Results showed that the core-shell ratio φ of Au@SiO2 NPs had little effect on the peak absorption wavelength in the solar radiation range. However, the metal shell had a significant effect on the tunable absorption efficiency. SiO2@Au NPs showed a tunable SPR wavelength between 540 and 900 nm, covering the near-infrared and visible regions. The optical properties of the SiO2@Ag NPs showed the same tendencies as those of the SiO2@Au NPs. The resonance wavelength of the SiO2@Ag NPs lay in the 390–830 nm range, covering the entire visible region. Two methods, adjusting their core-shell ratios and tuning their mixing ratios, were applied to enhance the solar absorption efficiencies of SiO2@Au and SiO2@Ag NPs. The optimal core-shell ratio and mixing ratio were obtained based on different NP combinations. The SiO2@Au nanofluid with φ = 0.2 gave the highest solar absorption efficiency while consuming 64% less Au than pure Au NPs. For mixed nanofluids, the mixtures featuring core-shell ratios of 0.1 and 0.6 with mixing ratios of 0.5 for SiO2@Au and 0.6 for SiO2@Ag gave the highest absorption efficiencies. Moreover, the peak solar absorption efficiency of a mixed nanofluid of SiO2@Au (φ = 0.1) and SiO2@Ag (φ = 0.4) with a mixing ratio of 0.58 was as high as 94.4%. The solar thermal conversion experiments revealed that the Au-decorated SiO2 nanofluid showed comparable efficiency to the calculated solar absorption efficiency of the SiO2@Au core-shell nanofluid (∼95.2%) under the same conditions; it was as high as 95.9%, higher than those of Au NPs and SiO2 NPs. These results showed that adjusting their core-shell ratios and tuning the mixing ratios of different nanofluids are two efficient methods to enhance the solar absorption efficiencies of SiO2@Au and 741

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SiO2@Ag NPs under the optimal conditions.

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Acknowledgment This work was financially supported by the National Natural Science Foundation of China (grant no. 51676060), the Natural Science Funds of Heilongjiang Province for Distinguished Young Scholars (grant no. JC2016009), and the Science Creative Foundation for Distinguished Young Scholars in Harbin (Grant no. 2014RFYXJ004). Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.apenergy.2017.11.087. References [1] Duić N, Guzović Z, Kafarov V, Klemeš JJ, vad Mathiessen B, Yan J. Sustainable development of energy, water and environment systems. Appl Energy 2013;101:3–5. [2] Lund H, Mathiesen BV. Energy system analysis of 100% renewable energy systemsthe case of Denmark in years 2030 and 2050. Energy 2009;34(5):524–31. [3] Gao X, Liu J, Zhang J, Yan J, Bao S, Xu H, Qin T. Feasibility evaluation of solar photovoltaic pumping irrigation system based on analysis of dynamic variation of groundwater table. Appl Energy 2013;105:182–93. [4] Fahrenbruch A, Bube R. Fundamentals of solar cells: photovoltaic solar energy conversion. Elsevier; 2012. [5] Kalyanasundaram K. Photophysics, photochemistry and solar energy conversion with tris (bipyridyl) ruthenium (II) and its analogues. Coord Chem Rev 1982;46:159–244. [6] Gómez-Villarejo R, Martín EI, Navas J, Sánchez-Coronilla A, Aguilar T, Gallardo JJ, et al. Ag-based nanofluidic system to enhance heat transfer fluids for concentrating solar power: nano-level insights. Appl Energy 2017;194:19–29. [7] Wang W, Yang X, Fang Y, Ding J, Yan J. Preparation and thermal properties of polyethylene glycol/expanded graphite blends for energy storage. Appl Energy 2009;86(9):1479–83. [8] Wang W, Yang X, Fang Y, Ding J, Yan J. Enhanced thermal conductivity and thermal performance of form-stable composite phase change materials by using βAluminum nitride. Appl Energy 2009;86(7):1196–200. [9] Wang W, Guo S, Li H, Yan J, Zhao J, Li X, Ding J. Experimental study on the direct/ indirect contact energy storage container in mobilized thermal energy system (MTES). Appl Energy 2014;119:181–9. [10] Wang W, Li H, Guo S, He S, Ding J, Yan J, Yang J. Numerical simulation study on discharging process of the direct-contact phase change energy storage system. Appl Energy 2015;150:61–8. [11] Moghimi SM, Hunter AC. Poloxamers and poloxamines in nanoparticle engineering and experimental medicine. Trends Biotechnol 2000;18(10):412–20. [12] Saha K, Agasti SS, Kim C, Li X, Rotello VM. Gold nanoparticles in chemical and biological sensing. Chem Rev 2012;112(5):2739–79. [13] Potenza M, Milanese M, Colangelo G, de Risi A. Experimental investigation of transparent parabolic trough collector based on gas-phase nanofluid. Appl Energy 2017;203:560–70. [14] Neumann O, Feronti C, Neumann AD, Dong A, Schell K, Lu B, Nordlander P. Compact solar autoclave based on steam generation using broadband light-harvesting nanoparticles. Proc Natl Acad Sci USA 2013;110(29):11677–81. [15] Colangelo G, Favale E, Miglietta P, de Risi A, Milanese M, Laforgia D. Experimental test of an innovative high concentration nanofluid solar collector. Appl Energy 2015;154:874–81. [16] Nakayama K, Tanabe K, Atwater HA. Plasmonic nanoparticle enhanced light absorption in GaAs solar cells. Appl Phys Lett 2008;93(12):121904. [17] Derkacs D, Lim SH, Matheu P, Mar W, Yu ET. Improved performance of amorphous silicon solar cells via scattering from surface plasmon polaritons in nearby metallic nanoparticles. Appl Phys Lett 2006;89(9):093103. [18] Otanicar TP, Phelan PE, Prasher RS, Rosengarten G, Taylor RA. Nanofluid-based direct absorption solar collector. J Renew Sustain Energy 2010;2(3):033102.

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