i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 3
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Investigation of optical properties and radiative transfer of sea water-based nanofluids for photocatalysis with different salt concentrations Cheng Ziming a, Wang Fuqiang a,*, Xie Yinmo a, Ma Lanxin a, Xu Huijin b, Tan Jianyu a, Bai Fengwu c a
School of Automobile Engineering, Harbin Institute of Technology at Weihai, 2, West Wenhua Road, Weihai, 264209, PR China b College of Pipeline and Civil Engineering, China University of Petroleum (Huadong), 66, West Changjiang Street, Qingdao, 266580, PR China c Key Laboratory of Solar Thermal Energy and Photovoltaic System, Institute of Electrical Engineerin(CAS), 6, Beiertiao Road, Beijing, 100190, PR China
article info
abstract
Article history:
Photocatalytic decomposition of sea water is an effective way to solve the future energy
Received 6 July 2017
crisis. However, there is a great deal of controversy about seawater's effect on photo-
Received in revised form
catalytic hydrogen production efficiency. In this paper, the catalyst particles are divided
31 August 2017
into large particles (c > 1) and small particles (c < 1). The Mie theory combined with Monte
Accepted 4 September 2017
Carlo method are used to analyze the influence of sea water on radiative transfer in the sea
Available online xxx
water-based nanofluids. The effects of the optical constants of the base fluid (fresh water and sea water) on the spectral extinction and spectral transmittance of the TiO2 nanofluids
Keywords:
are calculated. Results indicate that, for sea water-based nanofluids with small particles
Sea water
(c < 1), the spectral extinction coefficient increases and spectral transmittance decreases
Complex refractive index
with salt concentration increasing. For sea water-based nanofluids with large particles
TiO2 nanofluids
(c > 1), the spectral extinction coefficient and transmittance oscillate because of diffraction
Extinction coefficient
peaks.
Transmittance
© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction In the past centuries, fossil fuels have sustained the energy demands of human beings [1,2]. However, the world has been confronting an energy crisis due to depletion of nonrenewable resources and increased environmental problems [3e6]. On account of this, it is necessary and important to develop renewable and clean energy sources [7,8]. Many
regions and counties have been focused on vigorously searching for renewable and clean energy sources such as wind, solar, hydrogen, and tidal energy [9e16]. Hydrogen is likely to become an increasingly used energy source because it has many advantages [17e20]; for example, it has a high heating value and low emissions, and is environment-friendly, cleaner, and more sustainable [3]. In addition, the need for sustainable energy draws attention to hydrogen fuel [3], which is produced almost exclusively from
* Corresponding author. E-mail address:
[email protected] (W. Fuqiang). https://doi.org/10.1016/j.ijhydene.2017.09.044 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Ziming C, et al., Investigation of optical properties and radiative transfer of sea water-based nanofluids for photocatalysis with different salt concentrations, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/ j.ijhydene.2017.09.044
2
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 3
Nomenclature D m n k Q Qext Qabs Qsca fv N I g p P an bn jn ; cn pn tn S1 ; S2 Pn
diameter of the particle, nm complex index of refraction refractive index absorptive index efficiency factor extinction factor absorption factor scattering factor particle volume fraction number density of particles, m3 radiative density asymmetry factor probability density function cumulative distribution function Mie scattering coefficient Mie scattering coefficient RiccatieBessel functions directional dependent function directional dependent function amplitude function Legendre polynomial
Greek symbols c size parameter l Wavelength, nm F scattering phase function Q scattering angle s scattering coefficient, m-1 k absorption coefficient, m-1 b extinction coefficient, m-1 x random number u scattering albedo solid angle, sr U0 f azimuth angle Subscripts ext extinction sca scattering abs absorption f fluid p particle rel relative T monodispersed particles
fossil fuels through the steam reformation of methane [21]. Since the report by Fujishima and Honda on water splitting [22], hydrogen generation from water via solar energy using photocatalysts has been extensively studied [23e26]. Among all materials developed for photocatalytic applications, TiO2 remains the benchmark photocatalyst for hydrogen generation [27,28] because it is environmentally friendly, corrosion resistant, cost-effective, and a suitable band gap semiconductor (3.2 eV) [29]. Now, more and more research [30,31] turns to photocatalytic hydrogen production from seawater because of the lack of fresh water resources and the fact that seawater accounts for 96.5% of all water.
However, the effect of seawater on photocatalysis is controversial, and a consensus on whether it has a positive or negative effect on the hydrogen production rate has not yet been reached. Several studies have been conducted to investigate the effect of seawater on photocatalysis. Peng et al. [32] studied the water-splitting property of the composite photocatalyst CdSePt/TiO2 in visible light. The experimental results showed that the catalytic activity of the photocatalytic hydrolysis of seawater was 33% higher than that of photolysis with pure water under optimal conditions. Sang et al. [33] studied the effects of two kinds of catalysts, La2Ti2O7 and CdS/TiO2, on natural seawater and fresh water. The results showed that the hydrogen generation rate from seawater was about half as much as that from pure water. The main research objective of Simamora et al. [34] was to prepare effective photocatalysts for the splitting of seawater for hydrogen. They used CuO/nano TiO2 to perform photocatalytic hydrogen production experiments on sea water and fresh water. The results were that the 2.5% CuO/nano TiO2 has 9.9 and 7.8 times more activity than nano TiO2 in the photocatalytic splitting of water and seawater, respectively, so the efficiency of seawater-based hydrogen production is still lower than that of fresh water. In addition, nanofluids are the absorbing and scattering medium due to the presence of catalyst particles in the fluid phase. And just because of this, it is valuable for researchers to study the radiation field of the nanofluids [35,36]. The problem of liquid-solid two-phase radiation transmission in photocatalytic reaction vessels was studied by Pasquali et al. [37] using the Monte Carlo method. They analyzed the catalyst particles present in the liquid reaction solution according to the absorption and scattering of the participant. The numerical results indicated that the optical thickness of the reaction solution was thin, and catalyst particles with low absorption albedo could improve the efficiency of the photocatalytic process. They mentioned the effect of optical properties of the base fluids, but they did not study it further. With the same purpose, Said et al. [38] had studied the effects of the size and concentration of the TiO2 nanoparticles on the extinction coefficient using the Rayleigh approach. Their results showed that a smaller particle size (<20 nm) has a nominal effect on the optical properties of nanofluids and the volume fraction is linearly proportional to the extinction coefficient. However, Said et al. [38] ignored the fact that the small particles would coalesce, and they did not research the influence of the largescale particles on the optical properties of nanofluids. Additionally, Rayleigh scattering theory is not suitable for large particles. In order to improve the utilization of light in the biofuel photo bioreactor, Li et al. [39] used a combination of theory and experiment to measure the spectral transmittance of microalgae nanofluids with different species and concentrations. Their experimental results showed that there were significant differences in the UVevisible band, and the differences at long wavelengths were very small. However, they had not taken into account the effects of different media on the light utilization of microalgae nanofluids. The radiation characteristics of six different nanoTiO2 particles were measured by Cabrera et al. [40]. Their results showed that the scattering extinction of nanoparticles plays a major role in radiative transfer, and the effects of particle scattering on
Please cite this article in press as: Ziming C, et al., Investigation of optical properties and radiative transfer of sea water-based nanofluids for photocatalysis with different salt concentrations, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/ j.ijhydene.2017.09.044
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 3
photocatalysis cannot be neglected. It is worth mentioning that Tan et al. [41] studied the light transmission process with scattering effect of different sizes of particles on the spectral characteristics of TiO2 nanofluids. And that study concluded that the scattering effects need to be considered during investigation of optical properties and radiative transfer of nanofluids. The literature survey indicated that there was the controversy regarding photocatalytic hydrogen production from seawater: some researches showed that seawater benefits photocatalytic hydrogen production [32], meanwhile some researches showed that seawater is negative effect on photocatalytic hydrogen production [33,34]. Many researchers tried to study it from the aspects of the optical properties of the photocatalyst particles [42], the reaction mechanism [32], presence of other components [43] and photocatalyst particle dynamics [44]. But few researches have studied the effects of optical constant variations of base fluid on the optical characteristics of reaction solution and radiative transfer in process of photocatalysis. And the particle size parameters of the catalyst were also not studied separately. In addition, the optical properties of nanofluids were mainly investigated using Rayleigh scattering theory, which is an approximation of Mie scattering theory and unsuitable for large particles. Therefore, this study investigated the optical properties and radiative transfer of sea water-based nanofluids for photocatalysis with different salt concentrations. The catalyst particles were divided into large particles (c > 1) and small particles (c < 1) by the size parameters as the standard. Combined with Mie theory, Monte Carlo method was developed to calculate the optical properties and radiative transfer of the nanofluids with TiO2. The effects of different refractive indexes n and different absorption coefficients k of the base solution on the optical properties of the reaction solution were investigated. The optical parameters of the catalyst particles TiO2, base solution, and cuvette were taken into account in the calculation. The effects of the optical constants of the base fluid (fresh water and sea water) on the spectral extinction and spectral transmittance of the TiO2 nanofluids are calculated.
3
absorptive indexes, respectively [46,47]. The spectral distribution of the complex refractive index of the anatase TiO2 photocatalyst particle for wavelengths between 300 and 1000 nm is presented logarithmically in Fig. 1 [48]. When a single spherical, isotropic, homogeneous particle with a constant diameter D interacts with an electromagnetic wave with wavelength l, the extinction efficiency factor Qext , scattering efficiency factor Qsca , and the scattering phase function FðQÞ are governed by Maxwell's equations and calculated using Mie theory [46,47]. Qext ðm; cÞ ¼
∞ 2 X ð2n þ 1ÞRefan þ bn g 2 c n¼1
(1)
Qsca ðm; cÞ ¼
∞ 2 X 2 ð2n þ 1Þ jan j2 þ jbn j c2 n¼1
(2)
Qabs ðm; cÞ ¼ Qext ðm; cÞ Qsca ðm; cÞ FðQÞ ¼ 2
i1 þ i2 c2 Qsca
(3)
(4)
where the symbol Re represents the real components, an and bn are the Mie scattering coefficients, and both of them can be calculated by the following formulas an ¼
j0n ðmcÞjn ðcÞ mjn ðmcÞj0n ðcÞ j0n ðmcÞxn ðcÞ mjn ðmcÞx0n ðcÞ
(5)
bn ¼
mj0n ðmcÞjn ðcÞ jn ðmcÞj0n ðcÞ mj0n ðmcÞxn ðcÞ jn ðmcÞx0n ðcÞ
(6)
where xn ¼ jn icn , jn and cn are RiccatieBessel functions, and both of them conform to the following recursive relationship: jnþ1 ðcÞ ¼
2n þ 1 jn ðcÞ jn1 ðcÞ c
(7)
cnþ1 ðcÞ ¼
2n þ 1 cn ðcÞ cn1 ðcÞ c
(8)
where
Methodology
j1 ðcÞ ¼ cosc j0 ¼ sinc
First, the optical properties of a single nanoparticle, including the absorption efficiency factor, scattering efficiency factor, and extinction efficiency factor, are calculated using Mie theory. When these parameters of a single nanoparticle are obtained, the independent regime is used to calculate the optical properties of the uniform photocatalyst particles in the reaction solution. Finally, the radiative transfer equation (RTE) is numerically solved by using the Monte Carlo method to calculate the spectral transmissivity of the photocatalytic reaction solution [45,46].
c1 ðcÞ ¼ sinc
The non-dimensional polarized intensities i1 and i2 in Eq. (3) are calculated from i1 ðc; m; QÞ ¼ jS1 j2 ; i2 ðc; m; QÞ ¼ jS2 j2 S1 ðQÞ ¼
The size parameter c ¼ pD l and complex index of refraction m ¼ n ik are the most important parameters for the nanoparticles, which have a constant diameter D, where l represents the wavelength, n and k represent the refractive and
∞ X ð2n þ 1Þ n¼1
S2 ðQÞ ¼
Optical properties of a single nanoparticle
c0 ðcÞ ¼ cosc
½an pn ðcos QÞ þ bn tn ðcos QÞ nðn þ 1Þ
∞ X ð2n þ 1Þ n¼1
½bn pn ðcos QÞ þ an tn ðcos QÞ nðn þ 1Þ
(9)
(10)
(11)
In the above the equations, S1 ðQÞ and S2 ðQÞ represent complex amplitude functions and the direction-dependent function.pn and tn are related to the Legendre polynomials Pn , expressed as
Please cite this article in press as: Ziming C, et al., Investigation of optical properties and radiative transfer of sea water-based nanofluids for photocatalysis with different salt concentrations, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/ j.ijhydene.2017.09.044
4
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 3
2.0
4.0
1.8
n k
3.6
1.6
Refractive index (n)
1.4 3.4
1.2 1.0
3.2
0.8
3.0
0.6
Absorptive index (k)
3.8
2.8 0.4 2.6
0.2 0.0
2.4 300
400
500
600
700
800
900
1000
Wavelength, nm Fig. 1 e Spectral distributions of the complex refractive index of TiO2 in the wavelength range of 300e1000 nm [48].
pn ðcos QÞ ¼
d ½Pn ðcos QÞ d cos Q
(12)
2
tn ðcos QÞ ¼ cos Qpn ðcos QÞ sin Q
d ½pn ðcos QÞ d cos Q
(13)
When the nanoparticles are dispersed in the base fluid, the relative size parameter and the normalized refractive index of the particles in the fluid are calculated with [48,49]: pDnf ¼ l
crel
mrel
When the optical properties of a single particle are obtained, the scattering coefficient sp;l , absorption coefficient kp;l , and scattering phase function FT;l ðQÞ for a group of monodispersed particles with independent scattering regimes can be computed with [45]:
kp;l ¼ NT
(14)
(18)
pD2 Qabs;l ; 4
(19)
fv ¼ NT
np ikp ¼ nf
pD2 Qsca;l ; 4
sp;l ¼ NT
pD3 6
(20)
(15)
In Eqs. (14) and (15), the symbol nf represents the refractive index of base fluid, and the symbols np and kp represent the refractive and absorptive indexes of the particle, respectively. g ¼ cos Q ¼
1 4p
Z Fp ðQÞcos QdU
(16)
4p
In Eq. (16), the symbol g represents the asymmetry factor. It is used to describe the directional scattering behavior, and related to the phase function.
Spectral transmissivity of the photocatalytic reaction solution The photocatalytic reaction solution consists of nanoparticles and the base fluid. For the pure base fluid, the spectral absorption coefficient of the base fluid can be calculated with Eq. (17): kf;l ¼
4pkf l
(17)
The symbol kf is the absorptive index of the base fluid.
Fig. 2 e Schematic of radiative transfer for nanofluids in a cuvette.
Please cite this article in press as: Ziming C, et al., Investigation of optical properties and radiative transfer of sea water-based nanofluids for photocatalysis with different salt concentrations, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/ j.ijhydene.2017.09.044
5
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 3
bl ¼ sl þ kl
(24)
When the optical properties of the nanofluids are obtained, the calculation model of the radiative transfer of nanofluids in a cuvette can be used as depicted in Fig. 2. Essentially, sea water-based nanofluids is a kind of semitransparent medium, the radiative transfer in sea water-based nanofluids follows the classic radiative transfer equation. The RTE of nanofluids can be written as [46,51]. dIl ðsÞ sl ¼ ðsl þ kl ÞIl ðsÞ þ kl Ibl ðsÞ þ ds 4p
Z
! ! ! Il s; U 0 Fl U 0; U dU0
4p
(25)
Fig. 3 e Micrograph of polystyrene standard particles with a radius of 200 nm obtained by SEM.
FT;l ðQÞ ¼
1 pD2 NT Qsca;l ; Fl ðQÞ sl 4
(21)
The symbol Il represents the spectral radiative intensity in ! the direction of U along path s; Ibl represents the spectral blackbody intensity; and U0 is the solid angle. To solve the RTE of nanofluids, the nanofluids are considered to be homogeneous media with specified scattering coefficient sl , absorption coefficient kl , and scattering phase function FTl ðQÞ, and the base fluid is considered to be non-scattering material.
D is the nanoparticle diameter, fv is the particle volume fraction, and NT is the number density of the particles; Qsca;l , Qabs;l and Fl ðQÞ can be obtained by using Eqs. (1e4). The total scattering coefficient sl , absorption coefficientkl , and extinction coefficient bl of the nanofluids are given by Ref. [50]:
Monte Carlo method
sl ¼ sp;l
(22)
1 Ds ¼ lnx1 b
kl ¼ kf ;l þ kp;l
(23)
The symbol x1 represents a uniformly distributed random number in the range of 0 and 1. As long as the
The problem of radiative transfer for nanofluids a cuvette is solved by the Monto Carlo method. When a photon bundle is transported in the reaction solution, it will be absorbed and scattered, and the free path length is calculated with [52,53]. (26)
1.0 0.9
Tested Calculated
0.8
Transmittance
0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 300
350
400
450
500
550
600
650
700
750
800
Wavelength, nm Fig. 4 e Comparison of spectral transmittance of the nanofluids between results calculated by the Monte Carlo method and test results from the UVeVis spectrophotometer. Please cite this article in press as: Ziming C, et al., Investigation of optical properties and radiative transfer of sea water-based nanofluids for photocatalysis with different salt concentrations, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/ j.ijhydene.2017.09.044
6
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 3
Fig. 5 e Nanoparticle cluster distribution in the nanofluids under different operating temperatures after the 30-min photocatalytic water splitting test conducted by the authors [56]. photon has moved, we determine whether the photon is to be scattered or absorbed by generating a random number x2 and compare this number with the single scattering albedo u, which is defined as u ¼ s=b. If x2 u, the photon will be scattered, otherwise the photon will be absorbed. x2
is a uniformly distributed random number in the range of 0 and 1. The scattering phase function FTl ðQÞ of the nanofluids can be obtained from Eq. (21). The scattering phase function satisfies the normalization condition [53]. 4000 3500
n=1.33 n=1.43 n=1.53 n=1.63
1500
Extinction coefficient (m-1)
Extinction coefficient (m-1)
2000
1000
500
n=1.33 n=1.43 n=1.53 n=1.63
3000 2500 2000 1500 1000 500 0
0 300
400
500
600
700
(a)D=20nm( χ
< 1)
300
800
400
Wavelength (nm)
(b)D=50nm( χ
n=1.33 n=1.43 n=1.53 n=1.63
700 600 500 400 300
600
700
800
< 1)
n=1.33 n=1.43 n=1.53 n=1.63
350
Extinction coefficient (m-1)
Extinction coefficient (m -1)
800
500
Wavelength (nm)
300
250
200
150 300
200 300
400
500
600
700
800
400
500
600
700
800
Wavelength (nm)
Wavelength (nm)
(c)D=500nm ( χ
>1)
(d)D=800nm ( χ
>1)
Fig. 6 e Spectral extinction of TiO2 nanofluids with different base fluid refractive indexes.
Please cite this article in press as: Ziming C, et al., Investigation of optical properties and radiative transfer of sea water-based nanofluids for photocatalysis with different salt concentrations, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/ j.ijhydene.2017.09.044
7
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 3
Zp
f ¼ 2px4
1 FTl ðQÞsin QdQ ¼ 1 2
(27)
0
then probability density function pl ðQÞfor scattering angle Q can be obtained (28)
To determine the value of scattering angle Q for a particular Monte Carlo realization, we set a random number x3 equal to the cumulative distribution function of nanofluids Pl ðQÞ[53]. ZQ
pl ðQ0 ÞdQ0 ¼ x3 ;
where, x3 and x4 are uniformly distributed random numbers between 0 and 1.
Model validations
1 pl ðQÞ ¼ FTl ðQÞsin Q 0 Q p 2
Pl ðQÞ ¼
(30)
0 Pl ðQÞ 1
(29)
0
and solve for scattering angle Q. The azimuth angle f is sampled from a uniform distribution between 0 and 2p:
In this study, an experiment was conducted to validate the Monte Carlo method dealing with the radiative transfer problem of nanofluids in a cuvette. Polystyrene standard particles with a radius of 200 nm were mono-dispersed in water to form the nanofluids. The micrograph of polystyrene standard particles with a radius of 200 nm obtained by a scanning electron microscope (SEM, TESCAN-VEGA-II) is shown in Fig. 3. The spectral complex refractive indexes of polystyrene were the same as those obtained by Ma et al. [54]. The volume fraction (fv) of polystyrene standard particles in the nanofluids was 0.0538%. A cuvette was used to keep
2000 3500
1800
k=0.1 k=0.01 k=0.001 k=0.0001
1400 1200 1000 800 600 400
2500 2000 1500 1000
200 0 300
400
500
600
k=0.1 k=0.01 k=0.001 k=0.0001
3000
Extinction coefficient (m-1)
Extinction coefficient (m-1)
1600
700
500 0
800
300
Wavelength (nm)
(b)D=50nm ( χ
700
800
< 1) k=0.1 k=0.01 k=0.001 k=0.0001
400
Extinction coefficient (m-1)
Extinction coefficient (m-1)
600
600
450
k=0.1 k=0.01 k=0.001 k=0.0001
700
500
Wavelength (nm)
(a)D=20nm (χ<1 ) 800
400
500 400 300 200
350 300 250 200 150 100
100 300
400
500
600
700
Wavelength (nm)
(c)D=500nm( χ
800
50 300
400
500
600
700
800
Wavelength (nm)
>1)
(d)D=800nm ( χ
>1)
Fig. 7 e Spectral extinction of TiO2 nanofluids with different base fluid absorption indexes.
Please cite this article in press as: Ziming C, et al., Investigation of optical properties and radiative transfer of sea water-based nanofluids for photocatalysis with different salt concentrations, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/ j.ijhydene.2017.09.044
8
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 3
nanofluids, and the optical path length of the cuvette was 1.0 mm. A double-beam UVeVis spectrophotometer (TUe1901, Persee) was used to measure the spectral transmittance of the nanofluids within the cuvette. The spectral transmittance of the nanofluids calculated by the Monte Carlo method is validated with the experimental values and is shown in Fig. 4. As seen from this figure, the solid lines representing the numerical calculated results agree well with the scatter points representing experimental values at all wavelengths.
Results and discussion During the photocatalytic water splitting test, nano-sized photocatalyst particles readily aggregate into large
secondary agglomerates even in the presence of the antiagglomeration agent, especially at high temperatures [55e58]. Fig. 5 presents the nanoparticle cluster distribution and radius variations of the nanoparticles in the nanofluids under different operating temperatures after the 30-min photocatalytic water splitting test conducted by the authors [56]. The size parameter of the photocatalyst nanoparticles in the reaction solution has a large variation. However, the pre-study indicated that the optical properties of nanofluids show a smooth variation with wavelength for small particles (c < 1), while they present oscillating variations with wavelength for large particles (c > 1). Therefore, the optical property analyses of TiO2nanofluids were conducted in two categories: nanofluids with small particles (c < 1) and nanofluids with large particles (c > 1).
1.0
1.0
0.8
n=1.33 n=1.43 n=1.63
0.6
Transmittance
Transmittance
0.8
0.4
n=1.33 n=1.43 n=1.63
0.6
0.4
0.2
0.2
300
400
500
600
700
0.0
800
300
400
Wavelength (nm)
500
600
700
800
Wavelength (nm)
(a)D=20nm ( χ
(b)D=50nm ( χ
< 1)
< 1)
0.88 0.84
n=1.33 n=1.43 n=1.63
0.82
0.86
Transmittance
0.80
Transmittance
n=1.33 n=1.43 n=1.63
0.78 0.76
0.84
0.82
0.74
0.80 0.72
0.78
0.70 300
400
500
600
700
300
400
500
600
700
800
Wavelength (nm)
Wavelength (nm)
(c)D=500nm( χ
800
> 1)
(d)D=800nm ( χ
>1)
Fig. 8 e Spectral transmittance of TiO2 nanofluids with different base fluid refractive index.
Please cite this article in press as: Ziming C, et al., Investigation of optical properties and radiative transfer of sea water-based nanofluids for photocatalysis with different salt concentrations, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/ j.ijhydene.2017.09.044
9
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 3
Effects of refractive index of base fluid on spectral extinction Fig. 6 presents the spectral extinction of TiO2 nanoscale and submicron photocatalyst particle reaction solution with change in the base fluid's refractive index and wavelength. As described in Fig. 6(a) and (b), the extinction coefficient of the nanoscale photocatalyst particles (c < 1) increases with the increase in the refractive index of the base fluid, especially in the range of 300 nme350 nm, and decreases smoothly with the increase in wavelength. In addition, comparing Fig. 6(a) and (b), it is apparent that the extinction coefficient of the nanoscale photocatalyst particles (c < 1) increases with the increase in the diameter of the particles. Fig. 6(c) and (d) show that the extinction coefficient of the submicron-scale photocatalyst particles (c > 1) changes with the increase in the wavelength and the different base fluid refractive indexes; it basically stays the same when the wavelength varies in the
range of 300e340 nm and refraction and long-term volatility appear after a brief calm. In addition, in this wavelength area several curves are almost coincident, so the effect of the base fluid refractive index on the spectral extinction of the reaction solution is weak. The base fluid refractive index affects the spectral extinction coefficient because the wavelength shortens when light transfers from the optical thinner medium to the optical denser medium and is easily attenuated. For the large particles, the diffraction effect need to be considered.
Effects of absorption index of base fluid on spectral extinction When the refractive index of base fluid is constant, four different absorption indexes, k ¼ 0.1, k ¼ 0.01, k ¼ 0.001, and k ¼ 0.0001, are investigated. Fig. 7 shows the absorption
1.0
1.0
0.8
Transmittance
Transmittance
0.8
k=0.1 k=0.01 k=0.001 k=0.0001
0.6
0.4
k=0.1 k=0.01 k=0.001 k=0.0001
0.6
0.4
0.2
0.2
0.0 300
300
400
500
600
700
400
500
600
700
800
Wavelength (nm)
800
Wavelength (nm)
(a)D=20nm ( χ
(b)D=50nm ( χ
< 1)
< 1)
0.90
0.86
k=0.01 k=0.001 k=0.0001
0.84
0.88
0.82
0.86
Transmittance
Transmittance
k=0.01 k=0.001 k=0.0001
0.80 0.78 0.76
0.84
0.82
0.74 0.80
0.72 0.70
0.78
300
400
500
600
700
300
400
500
600
700
800
Wavelength (nm)
Wavelength (nm)
(c)D=500nm( χ
800
>1)
(d)D=800nm ( χ
>1)
Fig. 9 e Spectral transmittance of TiO2 nanofluids with different base fluid absorption indexes.
Please cite this article in press as: Ziming C, et al., Investigation of optical properties and radiative transfer of sea water-based nanofluids for photocatalysis with different salt concentrations, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/ j.ijhydene.2017.09.044
10
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 3
Effects of refractive index of base fluid on spectral transmittance When the absorption index of the base fluid is maintained atk ¼ 0, base fluids with different refractive indexes,n ¼ 1.33, n ¼ 1.43, andn ¼ 1.63, are investigated (when n ¼ 1.53, the calculation encounters convergence difficulties because of the subtle difference between the refractive indexes of the base fluid and cuvette). Fig. 8 illustrates the spectral transmittance with different refractive indexes of the base fluid and different size parameters of the photocatalyst particles. Fig. 8(a) and (b) show that when the size parameter is less than 1, the spectral transmittance increases with the increase in wavelength; it has a rapid rise, especially in the range of 300e400 nm, but in contrast, it increases with the decrease in the refractive index of the base fluid. As seen in Fig. 8(c) and (d), the spectral transmittance for different refractive indexes of the base fluid does not increase monotonically with the increase in the wavelength when the size parameter is larger than 1. Unlike for a small size parameter, the spectral transmittance of large
Effects of absorption index of base fluid on spectral transmittance With the refractive index of the base fluid is maintained at n ¼ 1.33, base fluids with different absorption indexes are investigated. Fig. 9 illustrates the change in spectral transmittance of nanofluids containing different sizes ofTiO2 particles along with different absorption indexes of the base fluid. Fig. 9(a) and (b)show that when the size parameter is less than 1.0, the spectral transmittance increases with increasing wavelength and also increases with the decrease in the absorption index of the base of the base fluid. As seen in Fig. 9(c) and (d), the spectral transmittance with different absorption indexes of the base fluid does not increase monotonically with the increase in the wavelength when the size parameter is larger than 1. Unlike the nanofluids with small particles, the transmittance of the nanofluids with large particles does not increase with the increase in the absorption of the base fluid.
Effects of complex refractive index of base fluid on spectral extinction According to the above parts of the paper, a conclusion can be drawn that different n or different kaffect the spectral extinction coefficient and spectral transmittance. Thus, pure water and two different salt concentration solutions were studied. The spectral complex refractive index of concentration of 0 g/L, 60 g/L and 360 g/L salt solutions, complex refractive index of the cuvette [59], and photocatalyst particles were taken into consideration, and the complex refraction index of them is shown in Fig. 10.
n-360g/L k-360g/L n-60g/L k-60g/L n-0g/L k-0g/L
1.44 1.42
Refraction index (n)
size parameter photocatalyst particles increases slightly as the refractive index of the base fluid decreases.
1.40
1E-7
1.38 1E-8
1.36 1.34
Absorption index (k)
index's effect on the spectral extinction coefficient of the reaction solution with different particle diameters. As described in Fig. 7(a) and (b), the extinction coefficient decreases sharply to zero at a wavelength of 320 nm when k ¼ 0.01, k ¼ 0.001, andk ¼ 0.0001, and the size parameter of the photocatalyst particle is small (c < 1). When k ¼ 0.1, the extinction coefficient decreases smoothly with the increase in wavelength. Fig. 7(c) and (d) show that when the size parameter is larger than 1 (c > 1), the extinction coefficient remains approximately constant in the range of 300e330 nm and fluctuates when l>330 nm. Overall, the absorption action is enhanced with the increase in the absorption index of the base fluid, so the spectral transmittance is impacted by it.
1.32 1.30 300
400
500
600
700
1E-9 800
Wavelength(nm) Fig. 10 e Spectral distributions of the complex refractive index of different NaCl concentration solutions in the wavelength range of 300e800 nm [59]. Please cite this article in press as: Ziming C, et al., Investigation of optical properties and radiative transfer of sea water-based nanofluids for photocatalysis with different salt concentrations, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/ j.ijhydene.2017.09.044
11
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 3
1400
800
Extinction coefficient (m-1)
1200 1000
Extinction coefficient (m-1)
c=360g/L c=60g/L c=0g/L
800 600 400 200 0
360g/L 60g/L 0g/L
700 600 500 400 300 200
300
400
500
600
700
800
300
400
Wavelength (nm)
500
600
(a) D=20nm( χ
(b) D=500nm( χ
< 1)
1.0
0.84
360g/L 60g/L 0g/L
0.82
0.92
Transmittance
Transmittance
0.7
Transmittance
c=360g/L c=60g/L c=0g/L
0.8
0.5 0.4
0.91
400
500
600
700
800
0.2 400
0.78 0.76
0.72
Wavelength (nm)
300
0.80
0.74
0.90
0.3
800
>1)
0.9
0.6
700
Wavelength (nm)
500
600
700
800
(c) D=20nm( χ
300
400
500
600
700
800
Wavelength (nm)
Wavelength (nm)
< 1)
(d) D=500nm ( χ
>1)
Fig. 11 e Spectral extinction coefficients and spectral transmittances of TiO2 nanofluids with different base fluids.
The effect of the different kinds of base fluid with different complex refractive indexes on the spectral extinction coefficient and spectral transmittance is presented in Fig. 11. Fig. 11(a) and (c) show that although there is a very slight difference in the complex refractive index between the two kinds of base fluid, the effect on the spectral extinction coefficient and spectral transmittance of the nanofluids with small particles is relatively obvious when the wavelength is below 330 nm. In addition, Fig. 11(b) and (d) show the results of the nanofluids with large particle: the spectral transmittance increases with the decrease in salt concentration in the range of 300e350 nm. However, there is no obvious regular pattern when l > 350 nm.
Conclusion In this study, the effects of optical constant variations of base fluid on the optical characteristics of reaction solution and
radiative transfer in process of photocatalysis were investigated. A Monte Carlo method combined with the Mie scattering phase function was developed to calculate the optical properties and radiative transfer of TiO2 nanofluids with the consideration of scattering effects. The particle size parameter was adopted as the criteria to divide the nanofluids into two categories: nanofluids with small particles (c < 1) and nanofluids with large particles (c > 1). The following conclusions have been drawn: (1) The spectral extinction coefficient of nanofluids with small particles decreases and the spectral transmittance of them increases with the increase in the refractive index of the base fluid. And the spectral extinction coefficient increases and spectral transmittance decreases with the increase in the absorption index. (2) The spectral extinction coefficient and spectral transmittance of nanofluids with large particles change not
Please cite this article in press as: Ziming C, et al., Investigation of optical properties and radiative transfer of sea water-based nanofluids for photocatalysis with different salt concentrations, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/ j.ijhydene.2017.09.044
12
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 3
noticeable with the change in the refractive index and absorption index of the base fluid. Besides, due to diffraction peaks, the spectral transmittance and spectral extinction coefficient oscillate with the wavelength. (3) The spectral extinction coefficient increases and the spectral transmittance decreases with increasing salt concentration, especially for nanofluids with small particles in the short wavelength region; thus, the utilization of light increases with increasing salt concentration, and higher salt concentration is beneficial for light absorption in photocatalysis. (4) The change in the spectral extinction coefficient and spectral transmittance of the nanofluids with large particles for increasing salt concentrations is not obvious, so higher salt concentrations do not improve the light absorption in photocatalysis.
Acknowledgements This work was supported by the National Natural Science Foundation of China (Nos. 51676061, 51406239), and the China Postdoctoral Science Foundation (No. 2016T90283), and the fundamental research funds for the central universities (HIT.NSRIF.201706).
references
[1] Muradov N. Hydrogen via methane decomposition: an application for decarbonization of fossil fuels. Int J Hydrogen Energy 2001;26(11):1165e75. [2] Liu SH, Syu HR. One-step fabrication of N-doped mesoporous TiO2, nanoparticles by self-assembly for photocatalytic water splitting under visible light. Appl Energy 2012;100(8):148e54. [3] Balat M. Potential importance of hydrogen as a future solution to environmental and transportation problems. Int J Hydrogen Energy 2008;33(15):4013e29. [4] Muradov N. Low to near-zero CO2 production of hydrogen from fossil fuels: status and perspectives. Int J Hydrogen Energy 2017;42:4058e88. [5] Wang FQ, Lai QZ, Han HZ, Tan JY. Parabolic trough receiver with corrugated tube for improving heat transfer and thermal deformation characteristics. Appl Energy 2016;164:411e24. [6] Li D, Li ZW, Zheng YM, Liu CY, Ahmed KH, Liu XY. Thermal performance of a PCMefilled doubleeglazing unit with different thermophysical parameters of PCM. Sol Energy 2016;133:207e20. [7] Cheng ZD, He YL, Cui FQ. Numerical investigations on coupled heat transfer and synthetical performance of a pressurized volumetric receiver with MCRTeFVM method. Appl Therm Eng 2013;50(1):1044e54. [8] Chen X, Xia XL, Meng XL, Dong XH. Thermal performance analysis on a volumetric solar receiver with double-layer ceramic foam. Energ Convers Manage 2015;97:282e9. n MI, Valenzuela L, Zarza E. Thermal analysis of solar [9] Rolda receiver pipes with superheated steam. Appl Energy 2013;103(1):73e84. [10] Li YX, Lin SY, Peng SQ, Lu GX, Li SB. Modification of ZnS1-x0.5yOx(OH)y-ZnO photocatalyst with NiS for enhanced visiblelight-driven hydrogen generation from seawater. Int J Hydrogen Energy 2013;38(36):15976e84.
[11] Wang YJ, Xu JL, Liu QB, Chen YY, Liu H. Performance analysis of a parabolic trough solar collector using Al2O3/synthetic oil nanofluids. Appl Therm Eng 2016;107:469e78. [12] Wang FQ, Tang ZX, Gong XT, Tan JY, Han HZ, Li BX. Heat transfer performance enhancement and thermal strain restrain of tube receiver for parabolic trough solar collector by using asymmetric outward convex corrugated tube. Energy 2016;114:275e92. [13] Wang FQ, Ma LX, Cheng ZM, Tan JY, Huang X, Liu LH. Radiative heat transfer in solar thermochemical particle reactor: a comprehensive review. Renew Sust Energ Rev 2017;73:935e49. [14] Khanna S, Singh S, Kedare SB. Explicit expressions for temperature distribution and deflection in absorber tube of solar parabolic trough concentrator. Sol Energy 2015;114:289e302. [15] Aggrey M, Tunde BO, Josua PM. Multieobjective and thermodynamic optimistic on of a parabolic trough receiver with perforated plate inserts. Appl Therm Eng 2015;77:42e56. [16] Wang FQ, Cheng ZM, Tan JY, Yuan Y, Shuai Y, Liu LH. Progress in concentrated solar power technology with parabolic trough collector system: a comprehensive review. Renew Sust Energ Rev 2017;79:1314e28. [17] Kim J, Kang M. High photocatalytic hydrogen production over the band gap-tuned urchin-like Bi2S3-loaded TiO2, composites system. Int J Hydrogen Energy 2012;37(10):8249e56. [18] Li YX, Gao D, Peng SQ, Lu G, Li SB. Photocatalytic hydrogen evolution over Pt/Cd0.5S from saltwater using glucose as electron donor: an investigation of the influence of electrolyte NaCl. Int J Hydrogen Energy 2011;36(7):4291e7. [19] Li YX, He F, Peng SQ, Lu GX, Li SB. Photocatalytic h evolution from NaCl saltwater over ZnS1-x-0.5yOx(OH)y-ZnO under visible light irradiation. Int J Hydrogen Energy 2011;36(17):10565e73. [20] Jin H, Lu Y, Liao B, Guo LJ. Hydrogen production by coal gasification in supercritical water with a fluidized bed reactor. Int J Hydrogen Energy 2010;35(13):7151e60. [21] Levent M, Gunn DJ, El-Bousiffi MA. Production of hydrogenrich gases from steam reforming of methane in an automatic catalytic microreactor. Int J Hydrogen Energy 2003;28(9):945e59. [22] Fujishima A, Honda K. Electrochemical photolysis of water at a semiconductor electrode. Nature 1972;238:37e8. [23] Vagia EC, Muradov N, Kalyva A, T Raissi A, Qin N, Srinivasa AR. Solar hybrid photo-thermochemical sulfurammonia water-splitting cycle: photocatalytic hydrogen production stage. Int J Hydrogen Energy 2017;42(32):608e24. [24] Li YX, He F, Peng SQ, Gao D, Lu GX, Li SB. Effects of electrolyte NaCl on photocatalytic hydrogen evolution in the presence of electron donors over Pt/TiO2. J Mol Catal A-Chem 2011;341(1e2):71e6. [25] Lin Y, Yang SY, Liu YP, Zhang SS, Wang HJ, Yu H, et al. In-situ photo-deposition CuO1-x cluster on TiO2 for enhanced photocatalytic H2 production activity. Int J Hydrogen Energy 2017;42(31):942e50. [26] Wang XJ, Zhang SS, Xie YB, Wang HJ, Yu H, Shen YX, et al. Branched hydrogenated TiO2 nanorod arrays for improving photocatalytic hydrogen evolution performance under simulated solar light. Int J Hydrogen Energy 2016;41(31):20192e7. [27] Samokhvalov A. Hydrogen by photocatalysis with nitrogen codoped titanium dioxide. Renew Sust Energ Rev 2017;72:981e1000. [28] Li YX, Xiang Y, Peng SQ, Wang XW, Zhou L. Modification of Zr-doped titania nanotube arrays by urea pyrolysis for enhanced visible-light photoelectrochemical H2 generation. Electrochim Acta 2013;87(1):794e800.
Please cite this article in press as: Ziming C, et al., Investigation of optical properties and radiative transfer of sea water-based nanofluids for photocatalysis with different salt concentrations, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/ j.ijhydene.2017.09.044
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 3
[29] Ramalingam RJ, Kannan AM. Hydrogen production by an anaerobic photocatalytic reforming using palladium nanoparticle on boron and nitrogen doped TiO2 catalysts. Int J Hydrogen Energy 2013;135:72e8. [30] Kumar SG, Devi LG. Review on modified TiO2 photocatalysis under UV/Visible light: selected results and related mechanisms on interfacial charge carrier transfer dynamics. J Phys Chem A 2011;115(46):13211e41. [31] Fujishima A, Zhang X, Tryk AD. TiO2 photocatalysis and related surface phenomena. Surf Sci Rep 2008;63:515e82. [32] Peng SQ, Liu XY, Ding M. Preparation of CdS-Pt/ TiO2Composite and the properties for splitting sea water into hydrogen under visible light irradiation. J Mol Catal 2013;27(5):459e66. [33] Ji SM, Jun H, Jang JS. Photocatalytic hydrogen production from natural seawater. J Photoch Photobio A 2007;189(1):141e4. [34] Simamora AJ, Hsiung TL, Chang FCl. Photocatalytic splitting of seawater and degradation of methylene blue on CuO/nano TiO2. Int J Hydrogen Energy 2012;37(18):13855e8. [35] Wang WZ, Wang YM, Shi GQ. Experimental investigation on the infrared refraction and extinction properties of rock dust in tunneling face of coal mine. Appl Opt 2015;54:10532e40. [36] Ren YT, Qi H, He MJ, Ruan ST, Ruan LM, Tan HP. Application of an improved firework algorithm for simultaneous estimation of temperatureedependent thermal and optical properties of molten salt. Int Commun Heat Mass 2016;77:33e42. [37] Pasquali M, Santarelli F, Porter JF. Radiative transfer in photocatalytic systems. Aiche J 2010;42(2):532e7. [38] SaidZ,SajidMH,SaidurR,MahdirajiGA,RahimNA.Evaluatingtheoptical propertiesofTiO2nanofluidforadirectabsorptionsolarcollector.Numer HeatTrAAppl2015;67(9):1010e27. [39] Li XC, Zhao JM, Liu LH, Zhang L. Optical extinction characteristics of three biofuel producing microalgae determined by an improved transmission method. Particuology 2017;33:1e10. [40] Marı´a IC, Orlando MA, Alberto EC. Absorption and scattering coefficients of titanium dioxide particulate nanofluids in water. J Phys Chem 1996;27(1):11e4. [41] Tan JY, Xie YM, Wang FQ, Jing L, Ma LX. Investigation of optical properties and radiative transfer of TiO2 nanofluids with the consideration of scattering effects. Int J Heat Mass Tran 2017;115:1103e12. [42] Ahmad SHA, Saidur R, Mahbubul IM, Al-Sulaiman FA. Optical properties of various nanofluids used in solar collector: a review. Renew Sust Energ Rev 2017;73:1014e30. [43] Deguchi S, Takeichi T, Shimasaki S, Ogawa M, Isu N. Photocatalytic hydrogen production from water with nonfood hydrocarbons as oxidizing sacrifice agents. Aiche J 2011;57(8):2237e43.
13
[44] Tang YC, Hu C, Wang YZ. Recent advances in mechanisms and kinetics of TiO2. Prog Chem 2002;14(3):192e9. [45] Ma LX, Tan JY, Zhao JM, Wang FQ, Wang CA. Dependent scattering and absorption by densely packed discrete spherical particles: effects of complex refractive index. J Quant Spectrosc Ra 2017;196:94e102. [46] Modest MF. Radiative heat transfer. 2nd ed. San Diego: Academic Press; 2003. [47] Yu QZ. Principle of radiant heat exchange. Harbin: HIT Press; 2000. [48] Weber MJ. Handbook of optical materials. New York: CRC press; 2003. [49] Tan H, Qi H, Ruan LM, Chen Q, Ren YT. Simultaneous retrieval of the complex refractive index and particle size distribution. Opt Express 2015;23(15):19328e37. [50] Segelstein DJ. The complex refractive index of water. University of Missouri; 1981. PhD thesis. [51] Sakthivel S, Kisch H. Daylight photocatalysis by carbonmodified titanium dioxide. Angew Chem Int Ed 2003;42(40):4908e11. [52] Ma CY, Zhao JM, Liu LH, Tan JY. GPU-accelerated inverse identification of radiative properties of particle suspensions in liquid by the Monte Carlo method. J Quant Spectrosc Ra 2016;172:146e59. [53] Ma LX, Wang FQ, Wang CA, Wang CC, Tan JY. Monte Carlo simulation of spectral reflectance and BRDF of the bubble layer in the upper ocean. Opt Express 2015;23(9):24274e89. [54] Ma XY, Lu JQ, Brock RS, Jacobs KM, Yang P, Hu XH. Determination of complex refractive index of polystyrene microspheres from 370 to 1610 nm. Phys Med Biol 2003;48:4165e72. [55] Wang XZ, He YR, Liu X, Zhu JQ. Enhanced direct steam generation via a bio-inspired solar heating method using carbon nanotube films. Powder Technol 2017;321:276e85. [56] Liang HX, Wang FQ, Cheng ZM, Hu SHP, Xiao B, Gong XT, et al. Analyzing the effects of reaction temperature on photothermo chemical synergetic catalytic water splitting under full-spectrum solar irradiation: an experimental and thermodynamic investigation. Int J Hydrogen Energy 2017;42:12133e42. [57] Song D, Wang Y, Jing D, Geng J. Investigation and prediction of optical properties of alumina nanofluids with different aggregation properties. Int J Heat Mass Tran 2016;96:430e7. [58] Song D, Hatami M, Wang Y, Jing DW, Yang Y. Prediction of hydrodynamic and optical properties of TiO2/water nanofluids considering particle size distribution. Int J Heat Mass Tran 2016;92(1):864e76. [59] Li X, Liu L, Zhao J, Tan JY. Optical properties of sodium chloride solution within the spectral range from 300 to 2500 nm at room temperature. Appl Spectrosc 2015;69(5):635e40.
Please cite this article in press as: Ziming C, et al., Investigation of optical properties and radiative transfer of sea water-based nanofluids for photocatalysis with different salt concentrations, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/ j.ijhydene.2017.09.044