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Radiative properties of ionic liquid-based nanofluids for medium-to-high-temperature direct absorption solar collectors
Solar Energy Materials & Solar Cells 130 (2014) 521–528
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Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat
Radiative properties of ionic liquid-based nanofluids for medium-to-high-temperature direct absorption solar collectors Long Zhang, Jian Liu, Guodong He, Zhuocheng Ye, Xiaoming Fang, Zhengguo Zhang n Key Laboratory of Enhanced Heat Transfer and Energy Conservation, The Ministry of Education, School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, China
art ic l e i nf o
a b s t r a c t
Article history: Received 25 January 2014 Received in revised form 22 May 2014 Accepted 23 July 2014
Here the radiative properties of the ionic liquid [HMIM][NTf2] and its nanofluids are investigated for the first time by the experimental and theoretical methods. [HMIM][NTf2] is almost transparent in the visible light range, while its optical absorption property can be significantly enhanced by dispersing a very low volume fraction of nanoparticles in it. At the volume fraction of 10 ppm, the extinction coefficient of the nanofluid containing the Ni nanoparticles with an average size of 40 nm is higher than that of the one containing the Cu nanoparticles with the similar average size, owing to their different complex refractive indexes. The nanofluid containing the carbon-coated Ni (Ni/C) nanoparticles exhibits lower transmittance and higher extinction coefficient, compared with the one containing the Ni nanoparticles with the similar average size. The radiative properties of the Ni/C nanofluids increase with the volume fraction of the nanoparticles. As the volume fraction is increased to 40 ppm, the absorbed energy fraction by the Ni/C nanofluid reaches up to almost 100% after the incident light only penetrate 1 cm. The excellent radiative properties of the IL-based nanofluids make them show promising to be used as the absorbers for medium-to-high-temperature direct absorption solar collectors. & 2014 Elsevier B.V. All rights reserved.
Keywords: Solar thermal utilization Direct absorption solar collector Nanofluid Radiative properties Ionic liquid
1. Introduction Solar thermal utilization is one of the most popular and effective means to utilize solar energy. Solar thermal collectors that capture the incoming solar radiation, convert it into heat, and then transfer the heat to a heat transfer fluid (HTF) by a cycling system [1] are pivotal devices in solar thermal systems. The performance of a solar collector plays a significant role in the whole solar energy utilization efficiency. Traditional solar collectors such as flat-plate solar collectors, which absorb the solar radiation by black-surface absorbers, have been widely used to collect heat for space heating, domestic hot water, etc. [1] Nonetheless, one inherent drawback in the traditional solar collectors is that the maximum temperature spot appears on the black surface, leading to a large temperature difference between the surface and the HTF [2]. As a result, the absorbed heat ineluctably losses to the surroundings, decreasing the performance of the traditional solar collectors [3]. To overcome this drawback in the traditional solar collectors, direct absorption solar collectors (DASCs) were originally proposed in the 1970s [4]. In a DASC, solar energy is directly
n
Corresponding author. Tel.: þ 86 20 8711 2845; fax: þ 86 20 8711 3870. E-mail address: [email protected] (Z. Zhang).
http://dx.doi.org/10.1016/j.solmat.2014.07.040 0927-0248/& 2014 Elsevier B.V. All rights reserved.
absorbed by a black liquid that functions as both the absorber and the HTF. The complete elimination of the temperature difference between the absorber and the HTF makes DASCs a novel type of high-performance solar collectors [2]. However, the initially used HTFs, prepared by adding black inks or dyes into some base liquids such as water, ethylene glycol, etc., show serious shortcomings such as the light-induced degradation and thermal degradation at the operating temperatures as well as low thermal conductivity. Consequently, the DASCs based on these black liquids do not exhibit high performance, inhibiting the development and application of the DASCs. Since 1990s, nanotechnology has been providing new development space for HTFs. Nanofluids, first coined by Choi in 1995 [5], is a kind of uniform suspension prepared by dispersing nanosized particles, fibers, or tubes with a definite proportion into base fluids. Numerous studies have demonstrated that nanofluids exhibit enhanced thermo-physical properties as compared to the corresponding base fluids, especially their thermal conductivity and convective heat transfer coefficients [6–10]. On the other hand, nanoparticles dispersed into base liquids offered a sharp enhancement in optical properties through absorption and scattering [11]. Furthermore, theoretical predictions and experimental investigations have proven that the receiver efficiency of the DASCs equipped with nanofluids is superior to that of the flat-plate
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Nomenclature D F fv I(λ) k Keλ Kaλ m mparticles n Qabs Qext Qscat
diameter of the particle (nm) the absorbed energy fraction (dimensionless) volume fraction (%) the total incident solar irradiance (W/m2) absorption index of the particle (dimensionless) extinction coefficient (1/cm) absorption coefficient (1/cm) relative complex refractive index (particles to basefluid) (dimensionless) complex refractive index of the nanoparticle (dimensionless) refraction index of the particle (dimensionless) absorption efficiency (dimensionless) extinction efficiency (dimensionless) scattering efficiency (dimensionless)
collectors [2,3,12–14], making nanofluids show great potentials for use as the absorbers in DASCs. To further increase the performance of DASCs until they can be commercialized, it is necessary to optimize the thermo-physical and optical properties of nanofluids, which are influenced by the material, size, shape, and volume fraction of nanoparticles [15]. Compared with thermophysical properties of nanofluids, their optical properties have not been paid much attention yet [16], which are more important to the absorbers for DASCs. Taylor et al. [17] combined experimental and theoretical methods to investigate the optical properties of the water- and oil-based nanofluids, in which Al, TiO2, Cu, and graphite nanoparticles were used as the nanoadditives. They found that the extinction coefficients from the model predictions agree well with the spectroscopic measurements for the water-based nanofluids containing graphite nanoparticles, but less well for metallic nanoparticles and/or oil-based nanofluids. Sani et al. [18–20] used a novel carbon nanomaterial, carbon nanohorns, to prepare water- and glycol-based nanofluids, and investigated their thermal and optical properties. They demonstrated that carbon nanohorn-based nanofluids not only possessed enhanced thermal properties as compared with the base liquids, but also showed more promising for use as the direct sunlight absorbers than the nanofluids containing amorphous carbon black. More recently, Saidur et al. [21] calculated the extinction coefficient of the waterbased aluminum nanofluids with varying sizes and volume fractions of the Al nanoparticles, and presented that the particle size had minimal influence on the optical properties of the nanofluids. All above results uniformly indicate that the optical properties of nanofluids dramatically increase by adding a low volume fraction of nanoparticles. However, all the aforementioned researches involve the nanofuids based on the traditional base liquids such as water, ethylene glycol, and thermal oil. The nanofluids based on water and ethylene glycol can be only applied in low-temperature solar thermal collectors. Although the thermal oil can keep the liquid phase up to about 300 1C, their applications are limited by some intrinsic disadvantages such as low decomposition temperature, inflammability, high vapor pressure, and low chemical stability. Therefore, novel nanofluids with excellent thermophysical and optical properties are needed to be developed for medium-to-high-temperature direct absorption solar collectors. Ionic liquids (ILs), composed of organic cations and organic or inorganic anions, are the group of salts with a wide liquid temperature range from room temperature to a maximum temperature above 400 1C [22]. A certain of convincing researches indicate that ILs can be used as a promising replacement of the
T ( λ) X y
spectral transmittance (dimensionless) thickness of the fluid layer (cm) optical path (mm)
Greek symbols
α λ
particle size parameter (dimensionless) wavelength (nm)
Subscripts abs ext EXP MOD scat
absorption extinction experimental result model result scattering
current HTFs, especially in medium- and high-temperature heat transfer systems, due to their favorable physical properties such as wide liquid temperature range, good thermal and chemical stability, low vapor pressure, and non-harmfulness [23–27]. Furthermore, several kinds of IL-based nanofluids (Ionanofluids) have been prepared and shown enhanced thermo-physical properties as compared with the pure ILs [28–33]. Note that the optical properties of Ionanofluids are more crucial to the performance of medium-to-high-temperature DASCs. In the current work, the radiative properties of Ionanofluids have been systematically investigated for the first time by varying particle material, volume fraction, and optical path length. The extinction coefficients of the Ionanofluids have been obtained based on theoretical predictions and experimental investigations. Moreover, the light absorbing capability of the Ionanofluids has been evaluated based on their measured spectrally-resolved optical properties.
2. Experimental section 2.1. Materials [HMIM][NTf2] (CAS number 916729-96-9), provided by Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, was selected as the base liquid due to its high initial decomposition temperature of more than 400 1C. Ni and Cu nanoparticles with average sizes of 40 nm were supplied by Xuzhou Jiechuang New Material Technology Co., Ltd., China. Carbon-coated Ni (Ni/C) nanoparticles with an average size of 40 nm were purchased from NanoAmor, USA. The morphology of the Cu, Ni, and Ni/C nanoparticles was observed using a transmission electron microscope (Hitachi H-7650, Japan). TEM images of the Cu, Ni, and Ni/C nanoparticles are displayed in Fig. 1. It can be seen that all the nanoparticles are almost spherical, except a few larger particles, which are likely aggregates of the smaller ones. The average diameters of all the nanoparticles are estimated to be ca. 40 nm. 2.2. Preparation of Ionanofluids Ionanofluids were prepared by the two-step method [32,33]. A certain amount of nanoparticles were added into [HMIM][NTf2], followed by magnetic stirring for 15 min. The obtained suspensions were thoroughly dispersed for 30 min by an ultrasonic apparatus at 90 W (KQ2200DE, Kunshan of Jiangsu Equipment Company, China). All the obtained Ionanofluids without any
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Fig. 1. TEM images of the nanoparticles at the same average sizes of 40 nm (a–c) and the photograph of the carbon-coated Ni Ionanofluids at varying volume fractions (d). (a) Carbon coated Ni; (b) Ni; and (c) Cu.
Table 1 Ionanofluids containing different nanoparticles at varying volume fractions. Sample
A0 A1 A2 A3 A4 A5 A6 A7
Nanoparticle Material
Average particle size (nm)
Ni/C Ni/C Ni/C Ni/C Ni/C Ni Cu
40 40 40 40 40 40 40
Volume fraction (ppm)
0 2 6 10 30 40 10 10
surfactants could maintain stable for about 3 days. The volume fractions of the nanoparticles in the Ionanaofluids ranged from 2.0 ppm (fv ¼2.0E-6) to 40 ppm (fv ¼4.0E-5). Table 1 lists the details of all the as-prepared Ionanofluids containing different nanoparticles at varying volume fractions.
2.3. Characterization and measurements The room temperature transmission spectrum of [HMIM][NTf2] was recorded in the wavelength range from 200 to 2500 nm with a double-beam UV–vis-NIR spectrophotometer (PerkinElmer Lambda950) using a differential measurement technique [34]. First, the IL was enclosed in two cuvettes with different optical path lengths (2 mm and 10 mm); then, the monochromatic light source was sent through a beam-splitter, which directed the two beams to the two cuvettes with different path lengths. As a result, the output was a relative transmission between them. In the differential measurement technique, the effects of scattering and multiple reflections through media with different optical thicknesses were assumed to be negligible. In addition, the room
temperature transmission spectrum of deionized water was obtained with the same technique for comparison purpose. Room temperature transmission spectra of all the [HMIM] [NTf2]-based nanofluids containing different nanoparticles at varying volume fractions were recorded with the same spectrophotometer using a cuvette with an optical path length of 10 mm. In order to improve the accuracy of the measurement, an integrating sphere was employed to collect all the deviated light including the scattered light, which is caused by the nanoparticles added into the base fluid. The experimental extinction coefficient of a Ionanofluid was calculated based on the spectral transmittance T(λ) recorded with the spectrophotometer according to the Beer Lambert law [35]: TðλÞ ¼ e K eλ; Ionanofluids y
ð1Þ
where y is the optical path, i.e.10 mm.
3. Theoretical model An Ionanofluid is a two-phase system containing an IL and the added nanoparticles. The radiation intensity of an Ionanofluid is dominated by the absorptions of the IL and the nanoparticles along with the scattering of the nanoparticles, which are simultaneously affected by the unpredictable and unmeasurable internal interference among the nanoparticles [36]. In order to elucidate the optical radiation mechanisms in an Ionanofluid, the optical properties of the nanoparticles and the IL are modeled separately as follows. First of all, the absorption and scattering of the nanaoparticles are evaluated. The modes for the absorption and scattering of the nanoparticles depend upon several factors, such as particle material, particle size, shape, and volume fraction. The effect of the material is reflected by the complex refractive index of the bulk material. Different materials exhibit diverse optical properties,
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where α is the size parameter, which depends on the particle diameter D, and the incident wavelength λ. m is the relative complex refractive index of an Ionanofluid, mparticles and nfluid are the complex refractive indices of the nanoparticle and the IL, respectively. The extinction coefficients of all the nanoparticles in the base fluid can be calculated using the following equation [17]: K eλ; nanoparticles ¼
3f v Q ext ðα; mÞ 2D
ð6Þ
Actually, the absorption is the only consideration due to the negligible scattering of an IL. Therefore, the extinction coefficient Keλ,IL of the IL is approximately equal to its absorption coefficient Kaλ,IL. K eλ;IL K aλ;IL ¼ Fig. 2. Dependent and independent scattering regime map for different particulate systems (modified on the basis of Ref. [36]). (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
owing to their different complex refractive index. The effect of the particle shape is quite complicated, and there has not been reliable theory to discuss it. So the effect of the particle shape is beyond the scope of the present work, and will be study separately. The nanoparticles used in the current work are spherical. The particle size is reflected by a size parameter α, which is defined as α ¼ πD/ λ, where D is the particle diameter and λ is the wavelength. The scattering pattern, whether it is independent, or dependent, is mainly determined by the particle volume fraction fv and the size parameter α. Fig. 2 shows a scattering regime map about the boundary between dependent and independent scattering by Tien [37], when the particle concentration is less than 0.6 vol% and the particle size is very small, the scattering effect of nanoparticles in nanofluid can be considered to be independent, that means one individual particle was scarcely affected by the scattered wave from other particles. Meanwhile, in this case, the radiative properties of the nanoparticle are well-described by Rayleigh scattering theory [35], which is applicable to small, spherical particles and the medium to be nonabsorbing. In the current work, the average sizes of all the different nanoparticles were around 40 nm, and the volume fractions of the nanoparticles in the Ionanaofluids were in the range from 2.0E-6 (2 ppm) to 4.0E-5 (40 ppm), the parameters about α and fv of Ionanofluids tested have been labeled using a red outline in the Fig. 2. Apparently, the scattering effect of the nanoparticles in the Ionanofluids can be considered to be independent, and the scattering is also applied to Rayleigh scattering. Because the average particle size of nanoparticles in present work are 40 nm, and the wavelength of the most of incident light (200–2500 nm) is at least 10 times larger, of course, it means the partical size parameter α«1. As a result, the solution for Rayleigh scattering can be simplified using the Rayleigh approximation equations [35,36]. Therefore, extinction (Qext), the scattering (Qscat) and absorption (Qabs), and efficiencies of an individual particle can be solved using the following equations: Q ext ¼ Q abs þ Q scat
ð2Þ
2 8 m2 1 Q scat ¼ α4 2 3 m þ2
ð3Þ
2 m 1 α2 m2 1 m4 þ 27m2 þ 38 Q abs ¼ 4α Im 1þ 2 2 2 15 m þ2 m þ2 2m þ 3
ð4Þ
m¼
mparticles nfluid
ð5Þ
4π kf
λ
ð7Þ
where kf is the complex component of the refractive index for the IL, which varies as a function of wavelength. Finally, the total extinction coefficient of an Ionanofluid is defined as below: K eλ; Ionanofluids ¼ K aλ;IL þ K eλ; nanoparticles
ð8Þ
In our calculation, all relevant optical characteristics of the nanoparticles, including the index of refraction (n) and absorption (k), can be obtained from an optical properties handbook [38]. The optical characteristics of the bulk material graphite are particularly used as those of the carbon-coated nickel nanoparticles, and the refractive index of the IL is assumed to be constant for all wavelengths and equals to 1.43, which is provided by the commercial manufacture. Unfortunately, there is very less information on the complex component of the refractive index (kf) of ILs. Thus, the experimentally determined room temperature transmission spectrum of the IL is used in the model. According to Eqs. (2)–(8), we found that the extinction coefficient of Ionanofluids is a function of the size parameter (α), the relative complex refractive index (m), volume fraction (fv), and the wavelength of the incident light (λ). In this paper, we mainly investigated the effects of the relative complex refractive index (m) and volume fraction (fv) varies with the wavelength from 250 to 2200 nm. The extinction coefficient of Ionanofluids containing different kinds of nanoparticles (Ni/C, Ni and Cu) and Ni/C Ionanofluids at different volume fractions was calculated; the model predications were compared to experimental results shown in Figs. 6 and 8, the details are discussed in Section 4.
4. Results and discussion 4.1. Optical properties of IL Fig. 3 displays the room temperature transmission spectrum of [HMIM][NTf2], together with that of deionized water. It can be seen that the transmittance of the IL is close to zero in the UV region (o 300 nm), and dramatically increases to 100% as the wavelength increases from 300 nm to 400 nm. As wavelength continues to rise from 400 to 800 nm, the transmittance of the IL generally maintains a high value until it specifically decreases in the near to mid-infrared range (800–2500 nm), due to the resonance of the organic groups in the IL. It is indicated that [HMIM] [NTf2] is almost completely transparent in the visible-light range. Compared with the IL, water exhibits superior optical absorption property at the wavelengths of more than 900 nm. In addition, water, which can absorb 13% of the solar energy, has been demonstrated to be a better solar absorber than ethylene glycol, propylene glycol, or Therminol VP-1 [39]. However, water is not suitable for use in medium- and high-temperature solar thermal
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Fig. 3. Room temperature transmission spectrum of [HMIM][NTf2], together with that of deionized water.
525
Fig. 4. Room temperature transmission spectra of the Ionanafluid containing 10 ppm of the Ni/C nanoparticles passing through different optical path lengths.
systems due to its boiling point of 100 1C. Obviously, the IL with a melting point below100 1C as well as a wide liquid temperature range from room temperature to a maximum temperature of more than 400 1C, is a promising base liquid for the nanofluids to be used in medium- and high-temperature solar thermal systems. 4.2. Optical properties of Ionanofluids 4.2.1. Effect of optical path length The room temperature transmission spectrum of the Ionanofluid containing 10 ppm of the Ni/C nanoparticles has been measured under different optical path lengths (2, 5 and 10 mm). As shown in Fig. 4, the transmittance of the Ionanofluid decreases with the increase in the optical path length, while no red or blueshift in the transmittance peaks is observed. As the optical path length is increased, the light transmitting distance will lengthen accordingly; consequently, the probability of the collision between the light and the particles together with the opportunity of light scattering will increase, both of which would lead to an attenuation in the transmitting light intensity. The variation of the radiative property with the optical path length needs to be taken into consideration during the design of a DASC based on an Ionanofluid. 4.2.2. Effect of nanoparticle material Fig. 5 shows room temperature transmission spectra of the [HMIM][NTf2]-based Ionanafluids containing different nanoparticles with similar average sizes of ca. 40 nm at the same volume fraction of 10 ppm. For comparison purpose, the transmission spectrum of [HMIM][NTf2] is also displayed in Fig. 5. It is obvious that all the Ionanofluids show lower transmittances than the IL in the whole wavelength range, though the transmittance of the Ionanofluids changes with the wavelengths in the same trends as the pure IL does. It is suggested that the absorption property of the IL can be improved by dispersing the nanoparticles in it, even if the volume fraction of the nanoparticles is as low as 10 ppm. Specifically, in the visible range (400–800 nm), the transmittance decreases from 100% for the pure IL to 90% for the Cu Ionanofluid, to 50–60% for the Ni one, and to 30% for the Ni/C one. Although the morphology and average size of the Cu nanoparticles are similar to those of the Ni nanoparicles (Fig. 1b and c), the Ni Ionanofluid exhibits much better absorption property than the Cu one at the same volume fraction of 10 ppm. It is revealed that the material of the nanoparticles has an obvious effect on the optical absorption property of the Ionanofluids. As a result, it is important to select
Fig. 5. Room temperature transmission spectra of the Ionanafluids containing different kinds of nanoparticles with the same average sizes of ca. 40 nm at the same volume fraction of 10 ppm.
a suitable material of nanoparticles for the Ionanofluids to be used as the absorbers in DASCs. Furthermore, the Ni/C Ionanofluid shows superior absorption property to the Ni one, suggesting that coating the surfaces of Ni nanoparticles with carbon further improves the optical absorption of the Ionanofluids due to the predictable broadband absorption of carbon in the visible and near-IR spectrum. The Ionanofluid containing the Ni/C nanoparticles exhibits the lowest transmission as compared to the other two ones at the same volume fraction (fv ¼10 ppm). Fig. 6 shows the experimental extinction coefficients of the IL and its Ionanofluids obtained from their aforementioned transmittance according to the Beer Lambert law. In addition, their calculated extinction coefficients obtained from the theoretical model based on the Rayleigh approximation equations are also displayed in Fig. 6. It can be seen that the extinction coefficient of the IL is very low in the visible-light range. The extinction coefficients of all the Ionanofluids containing different nanoparticles at a low volume fraction of 10 ppm are higher than that of the IL in the whole wavelength range. Specifically, the extinction coefficient increases almost one magnitude for the Cu Ionanofluid and two magnitudes for the Ni and Ni/C ones. In the wavelength below 1200 nm, the experimental extinction coefficients do not match well with the model predictions, especially for the Cu and Ni Ionanofluids. One possible reason for the deviations can be attributed to the surface oxidation of the metal nanoparticles. As we know, when the size of a metal material reaches to nanoscale,
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Fig. 6. Comparisons on the extinction coefficients between the experimental results and the model predictions for the Ionanofluids containing different kinds of nanoparticles with the same average sizes of ca. 40 nm at the same volume fraction of 10 ppm, together with the experimental extinction coefficient of [HMIM] [NTf2].
Fig. 7. Room temperature transmission spectra of the Ni/C Ionanafluids at different volume fractions.
its specific surface area and surface energy increase sharply, resulting in the oxidization occurring on the surfaces of the metal nanoparticles. Consequently, the metal nanoparticles have different optical characteristics from its bulk metal. In addition, the agglomeration of the nanoparticles leads to an increase in the particle size, resulting in a deviation from the Rayleigh approximation. Since the difference between the experimental results and the model predictions limits within a one magnitude, the total trend of the extinction coefficients obtained from the Rayleigh approximation can still reasonably account for the effect of material on the optical properties of the Ionanofluids, which is reflected by their different complex refractive indexes.
4.2.3. Effect of volume fraction Fig. 1d shows the photograph of the Ionanofluids containing the Ni/C nanoparticles at varying volume fractions. Their colors gradually become dark as the volume fraction of the Ni/C nanoparticles is increased from 0 to 40 ppm, implying that the optical absorption property of the Ni/C Ionanofluids is enhanced with the increase in the volume fraction of the nanoparticles. The Ionanofluids containing 30 and 40 ppm of the Ni/C nanoparticles are black. Compared with the traditional black liquids, the nanofluids composed of [HMIM][NTf2] and the Ni/C nanoparticles do not possess those serious shortcomings such as light-induced degradation, thermal degradation at the operating temperatures, and low thermal conductivity. Fig. 7 shows room temperature transmission spectra of the Ionanofluids containing the Ni/C nanoparticles at different volume fractions. It is obvious that the transmittance of the Ionanofluids gradually decreases with the increase in the volume fraction of the Ni/C nanoparticles in the wavelength range from 300 nm to 2200 nm. Even if the volume fraction is as low as 2 ppm (fv ¼ 0.0002%), the transmittance of the Ionanofluid remarkably decreases to around 75% as compared with 100% of the IL. As the volume fraction of the Ni/C nanoparticles is increased to 40 ppm (fv ¼ 0.004%), only less than 3% of the light is transmitted in the whole wavelength range, indicating a significant decrement in the transmittance. The excellent optical absorption property of the Ni/C Ionanofluids makes them show promising for use as the absorbers in DASCs. Fig. 8 shows the comparisons on the extinction coefficients between the experimental results and the model predictions for the Ionanofluids containing the Ni/C nanoparticles at different
Fig. 8. Comparisons on the extinction coefficients between the experimental results and the model predictions for the Ni/C Ionanafluids at different volume fractions.
volume fractions. In the visible-light range (from 400 nm to 800 nm), the Ionanofluid containing 2 ppm of the Ni/C nanoparticles shows an increment of more than one magnitude as compared with the IL. The extinction coefficients of the Ni/C Ionanofluids increase with the volume fraction of the Ni/C nanapartilces, which can be well explained by the Rayleigh approximation equations. According to Eqs. (6) and (8), the extinction coefficient of an Ionanofluid is linearly proportional to the volume fraction of the nanoparticles. The extinction coefficient of the Ionanofluids with 40 ppm of the Ni/C nanoparticles reaches to as high as around 5 cm 1. Moreover, for the Ni/C Ionanofluids at the volume fractions between 2 ppm and 40 ppm, their extinction coefficients derived from the theoretical model based on the Rayleigh approximation equations agree well with the experimental results obtained from the transmittances measured by the spectrophotometer. 4.3. Absorbed energy fractions of the Ni/C Ionanofluids The investigations on the spectrally-resolved optical properties of nanofluids allow us to evaluate their sunlight extinction behavior as well as their energy storage capability, providing very useful information for the absorber selection and collector optimization. To quantitatively evaluate the solar absorption capability of the Ionanofluids, an absorbed energy fraction, F, is defined to describe the proportion of the solar energy absorbed across an Ionanofluid layer, which means that the light passing through the
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absorb the incoming solar radiation completely. For a given Ionanofluid, the collector height should be consistent with the optical path length. If the collector height is too low, the solar radiation cannot be thoroughly absorbed by the nanofluid, resulting in a low receiver efficiency; if the collector height is too high, the solar radiation will be mainly absorbed on the top layer of the nanofluid, leading to a temperature difference in the nanofluid along with an increase in heat loss [3,40]. Therefore, the volume fraction of nanoparticles and the collect height should be chosen very precisely in order to achieve a maximum efficiency of the DASC. A further study on the performance of the DASCs based on the Ionanofluids is in progress.
5. Conclusions Fig. 9. Absorbed energy fractions (F) versus the penetration distance (X) for the Ionanofluids containing the Ni/C nanoparticles at different volume fractions.
Ionanofluid for a distance of X is no more available to be transmitted and has been stored in the Ionanofluid (either for direct absorption or scattering followed by absorption process). F can be calculated using the following equation [18]: R λmax F ¼ 1
K eλ;Ionanofluids X dλ λmin IðλÞe R λmax λmin IðλÞdλ
ð9Þ
where I(λ) is the total incident solar irradiance in the wavelength range (200–2500 nm), which can be approximated using Planck's equation for the body radiation at a temperature of 5800 K [2]. X is the thickness of the fluid layer, which is equivalent to the height (H) of a DASC. Fig. 9 illustrates the absorbed energy fraction (F) at varying penetration distance (X) for the Ni/C Ionanofluids at different volume fractions. It is obvious that F of all the Ionanofluids containing the carbon-coated Ni nanoparticles are much higher than that of the IL even at the Ni/C volume fraction as low as 2 ppm. Specifically, it only needs 1 cm of the fluid layer for the Ionanofluid containing 40 ppm of the Ni/C nanoparticles to absorb almost 100% of the incident light while it needs at least 10 cm for the pure IL to reach the same fraction. The results indicate that the Ni/C Ionanofluids show great potential for use as the absorbers in the medium-to-high-temperature DASCs. The investigations on the effects of optical path length, nanoparticle material, and volume fraction of nanopartilces on the radiative properties of the Ionanofluids provide very useful information for the optimization of a DASC. According to Eq. (9), F of a fluid absorber in a DASC depends on two factors, extinction coefficient (Keλ) and path length (X). The increases in Keλ and X enhance F of the fluid, resulting in an improvement in the efficiency of the DASC. As described in Section 4.2, Keλ is influenced by several factors including nanoparticle material, volume fraction, etc. It has been shown that, at the same volume fraction, the Ionanofluid containing the carbon-coated Ni nanoparcles with an average size of 40 nm exhibits the lowest transmittance and the highest extinction coefficient as compared with the ones containing the Cu and Ni nanoparticles with the similar average sizes. Once the nanoparticle material is determined, F of the nanofluid can be optimized by adjusting the two parameters, volume fraction (fv) and collector height (H). For a given collector height, F of the nanofluid increases with its volume fraction until F reaches up to 100%. However, as the volume fraction continues to increase, all of the incoming solar radiation will be absorbed with a thin layer of the nanofluid. In that case, the absorbed thermal energy is apt to lose to the environment. On the other hand, if the volume fraction is too low, the nanofluid cannot
The radiative properties of [HMIM][NTf2] and its nanofluids have been investigated for the first time, aiming at developing a novel kind of HTFs for medium-to-high-temperature DASCs. The optical absorption property of the IL can be greatly enhanced by adding a low volume fraction of nanopartcles in it. The transmittance of the Ionanofluid decreases with the increase of optical path length. The Ionanofluid containing the Ni/C nanoparticles exhibits the lowest transmittance and highest extinction coefficient as compared with the ones containing the Ni and Cu nanoparticles with the similar average size at the same volume fraction of 10 ppm. The Ni/C Ionanofluids exhibit a decrease in transmittance with the increase in the volume fraction of the nanoparticles, and their extinction coefficient predicted by Rayleigh scattering theory are consistent with the experimental results, implying this theory is appropriate for predicting the extinction coefficients of these two phase systems. At the volume fraction of 40 ppm, only less than 3% of the light is transmitted in the whole wavelength range for the Ni/C Ionanofluid, and its absorbed energy fraction reaches up to almost 100% after the incident light only penetrate the Ionanofluid for a distance of 1 cm. The excellent radiative property of the Ni/C Ionanofluid makes it show promising for use as the absorber in solar thermal collectors, especially in the medium-to-high-temperature DASCs.
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Contents lists available at ScienceDirect
Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat
Radiative properties of ionic liquid-based nanofluids for medium-to-high-temperature direct absorption solar collectors Long Zhang, Jian Liu, Guodong He, Zhuocheng Ye, Xiaoming Fang, Zhengguo Zhang n Key Laboratory of Enhanced Heat Transfer and Energy Conservation, The Ministry of Education, School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, China
art ic l e i nf o
a b s t r a c t
Article history: Received 25 January 2014 Received in revised form 22 May 2014 Accepted 23 July 2014
Here the radiative properties of the ionic liquid [HMIM][NTf2] and its nanofluids are investigated for the first time by the experimental and theoretical methods. [HMIM][NTf2] is almost transparent in the visible light range, while its optical absorption property can be significantly enhanced by dispersing a very low volume fraction of nanoparticles in it. At the volume fraction of 10 ppm, the extinction coefficient of the nanofluid containing the Ni nanoparticles with an average size of 40 nm is higher than that of the one containing the Cu nanoparticles with the similar average size, owing to their different complex refractive indexes. The nanofluid containing the carbon-coated Ni (Ni/C) nanoparticles exhibits lower transmittance and higher extinction coefficient, compared with the one containing the Ni nanoparticles with the similar average size. The radiative properties of the Ni/C nanofluids increase with the volume fraction of the nanoparticles. As the volume fraction is increased to 40 ppm, the absorbed energy fraction by the Ni/C nanofluid reaches up to almost 100% after the incident light only penetrate 1 cm. The excellent radiative properties of the IL-based nanofluids make them show promising to be used as the absorbers for medium-to-high-temperature direct absorption solar collectors. & 2014 Elsevier B.V. All rights reserved.
Keywords: Solar thermal utilization Direct absorption solar collector Nanofluid Radiative properties Ionic liquid
1. Introduction Solar thermal utilization is one of the most popular and effective means to utilize solar energy. Solar thermal collectors that capture the incoming solar radiation, convert it into heat, and then transfer the heat to a heat transfer fluid (HTF) by a cycling system [1] are pivotal devices in solar thermal systems. The performance of a solar collector plays a significant role in the whole solar energy utilization efficiency. Traditional solar collectors such as flat-plate solar collectors, which absorb the solar radiation by black-surface absorbers, have been widely used to collect heat for space heating, domestic hot water, etc. [1] Nonetheless, one inherent drawback in the traditional solar collectors is that the maximum temperature spot appears on the black surface, leading to a large temperature difference between the surface and the HTF [2]. As a result, the absorbed heat ineluctably losses to the surroundings, decreasing the performance of the traditional solar collectors [3]. To overcome this drawback in the traditional solar collectors, direct absorption solar collectors (DASCs) were originally proposed in the 1970s [4]. In a DASC, solar energy is directly
n
Corresponding author. Tel.: þ 86 20 8711 2845; fax: þ 86 20 8711 3870. E-mail address: [email protected] (Z. Zhang).
http://dx.doi.org/10.1016/j.solmat.2014.07.040 0927-0248/& 2014 Elsevier B.V. All rights reserved.
absorbed by a black liquid that functions as both the absorber and the HTF. The complete elimination of the temperature difference between the absorber and the HTF makes DASCs a novel type of high-performance solar collectors [2]. However, the initially used HTFs, prepared by adding black inks or dyes into some base liquids such as water, ethylene glycol, etc., show serious shortcomings such as the light-induced degradation and thermal degradation at the operating temperatures as well as low thermal conductivity. Consequently, the DASCs based on these black liquids do not exhibit high performance, inhibiting the development and application of the DASCs. Since 1990s, nanotechnology has been providing new development space for HTFs. Nanofluids, first coined by Choi in 1995 [5], is a kind of uniform suspension prepared by dispersing nanosized particles, fibers, or tubes with a definite proportion into base fluids. Numerous studies have demonstrated that nanofluids exhibit enhanced thermo-physical properties as compared to the corresponding base fluids, especially their thermal conductivity and convective heat transfer coefficients [6–10]. On the other hand, nanoparticles dispersed into base liquids offered a sharp enhancement in optical properties through absorption and scattering [11]. Furthermore, theoretical predictions and experimental investigations have proven that the receiver efficiency of the DASCs equipped with nanofluids is superior to that of the flat-plate
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Nomenclature D F fv I(λ) k Keλ Kaλ m mparticles n Qabs Qext Qscat
diameter of the particle (nm) the absorbed energy fraction (dimensionless) volume fraction (%) the total incident solar irradiance (W/m2) absorption index of the particle (dimensionless) extinction coefficient (1/cm) absorption coefficient (1/cm) relative complex refractive index (particles to basefluid) (dimensionless) complex refractive index of the nanoparticle (dimensionless) refraction index of the particle (dimensionless) absorption efficiency (dimensionless) extinction efficiency (dimensionless) scattering efficiency (dimensionless)
collectors [2,3,12–14], making nanofluids show great potentials for use as the absorbers in DASCs. To further increase the performance of DASCs until they can be commercialized, it is necessary to optimize the thermo-physical and optical properties of nanofluids, which are influenced by the material, size, shape, and volume fraction of nanoparticles [15]. Compared with thermophysical properties of nanofluids, their optical properties have not been paid much attention yet [16], which are more important to the absorbers for DASCs. Taylor et al. [17] combined experimental and theoretical methods to investigate the optical properties of the water- and oil-based nanofluids, in which Al, TiO2, Cu, and graphite nanoparticles were used as the nanoadditives. They found that the extinction coefficients from the model predictions agree well with the spectroscopic measurements for the water-based nanofluids containing graphite nanoparticles, but less well for metallic nanoparticles and/or oil-based nanofluids. Sani et al. [18–20] used a novel carbon nanomaterial, carbon nanohorns, to prepare water- and glycol-based nanofluids, and investigated their thermal and optical properties. They demonstrated that carbon nanohorn-based nanofluids not only possessed enhanced thermal properties as compared with the base liquids, but also showed more promising for use as the direct sunlight absorbers than the nanofluids containing amorphous carbon black. More recently, Saidur et al. [21] calculated the extinction coefficient of the waterbased aluminum nanofluids with varying sizes and volume fractions of the Al nanoparticles, and presented that the particle size had minimal influence on the optical properties of the nanofluids. All above results uniformly indicate that the optical properties of nanofluids dramatically increase by adding a low volume fraction of nanoparticles. However, all the aforementioned researches involve the nanofuids based on the traditional base liquids such as water, ethylene glycol, and thermal oil. The nanofluids based on water and ethylene glycol can be only applied in low-temperature solar thermal collectors. Although the thermal oil can keep the liquid phase up to about 300 1C, their applications are limited by some intrinsic disadvantages such as low decomposition temperature, inflammability, high vapor pressure, and low chemical stability. Therefore, novel nanofluids with excellent thermophysical and optical properties are needed to be developed for medium-to-high-temperature direct absorption solar collectors. Ionic liquids (ILs), composed of organic cations and organic or inorganic anions, are the group of salts with a wide liquid temperature range from room temperature to a maximum temperature above 400 1C [22]. A certain of convincing researches indicate that ILs can be used as a promising replacement of the
T ( λ) X y
spectral transmittance (dimensionless) thickness of the fluid layer (cm) optical path (mm)
Greek symbols
α λ
particle size parameter (dimensionless) wavelength (nm)
Subscripts abs ext EXP MOD scat
absorption extinction experimental result model result scattering
current HTFs, especially in medium- and high-temperature heat transfer systems, due to their favorable physical properties such as wide liquid temperature range, good thermal and chemical stability, low vapor pressure, and non-harmfulness [23–27]. Furthermore, several kinds of IL-based nanofluids (Ionanofluids) have been prepared and shown enhanced thermo-physical properties as compared with the pure ILs [28–33]. Note that the optical properties of Ionanofluids are more crucial to the performance of medium-to-high-temperature DASCs. In the current work, the radiative properties of Ionanofluids have been systematically investigated for the first time by varying particle material, volume fraction, and optical path length. The extinction coefficients of the Ionanofluids have been obtained based on theoretical predictions and experimental investigations. Moreover, the light absorbing capability of the Ionanofluids has been evaluated based on their measured spectrally-resolved optical properties.
2. Experimental section 2.1. Materials [HMIM][NTf2] (CAS number 916729-96-9), provided by Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, was selected as the base liquid due to its high initial decomposition temperature of more than 400 1C. Ni and Cu nanoparticles with average sizes of 40 nm were supplied by Xuzhou Jiechuang New Material Technology Co., Ltd., China. Carbon-coated Ni (Ni/C) nanoparticles with an average size of 40 nm were purchased from NanoAmor, USA. The morphology of the Cu, Ni, and Ni/C nanoparticles was observed using a transmission electron microscope (Hitachi H-7650, Japan). TEM images of the Cu, Ni, and Ni/C nanoparticles are displayed in Fig. 1. It can be seen that all the nanoparticles are almost spherical, except a few larger particles, which are likely aggregates of the smaller ones. The average diameters of all the nanoparticles are estimated to be ca. 40 nm. 2.2. Preparation of Ionanofluids Ionanofluids were prepared by the two-step method [32,33]. A certain amount of nanoparticles were added into [HMIM][NTf2], followed by magnetic stirring for 15 min. The obtained suspensions were thoroughly dispersed for 30 min by an ultrasonic apparatus at 90 W (KQ2200DE, Kunshan of Jiangsu Equipment Company, China). All the obtained Ionanofluids without any
L. Zhang et al. / Solar Energy Materials & Solar Cells 130 (2014) 521–528
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Fig. 1. TEM images of the nanoparticles at the same average sizes of 40 nm (a–c) and the photograph of the carbon-coated Ni Ionanofluids at varying volume fractions (d). (a) Carbon coated Ni; (b) Ni; and (c) Cu.
Table 1 Ionanofluids containing different nanoparticles at varying volume fractions. Sample
A0 A1 A2 A3 A4 A5 A6 A7
Nanoparticle Material
Average particle size (nm)
Ni/C Ni/C Ni/C Ni/C Ni/C Ni Cu
40 40 40 40 40 40 40
Volume fraction (ppm)
0 2 6 10 30 40 10 10
surfactants could maintain stable for about 3 days. The volume fractions of the nanoparticles in the Ionanaofluids ranged from 2.0 ppm (fv ¼2.0E-6) to 40 ppm (fv ¼4.0E-5). Table 1 lists the details of all the as-prepared Ionanofluids containing different nanoparticles at varying volume fractions.
2.3. Characterization and measurements The room temperature transmission spectrum of [HMIM][NTf2] was recorded in the wavelength range from 200 to 2500 nm with a double-beam UV–vis-NIR spectrophotometer (PerkinElmer Lambda950) using a differential measurement technique [34]. First, the IL was enclosed in two cuvettes with different optical path lengths (2 mm and 10 mm); then, the monochromatic light source was sent through a beam-splitter, which directed the two beams to the two cuvettes with different path lengths. As a result, the output was a relative transmission between them. In the differential measurement technique, the effects of scattering and multiple reflections through media with different optical thicknesses were assumed to be negligible. In addition, the room
temperature transmission spectrum of deionized water was obtained with the same technique for comparison purpose. Room temperature transmission spectra of all the [HMIM] [NTf2]-based nanofluids containing different nanoparticles at varying volume fractions were recorded with the same spectrophotometer using a cuvette with an optical path length of 10 mm. In order to improve the accuracy of the measurement, an integrating sphere was employed to collect all the deviated light including the scattered light, which is caused by the nanoparticles added into the base fluid. The experimental extinction coefficient of a Ionanofluid was calculated based on the spectral transmittance T(λ) recorded with the spectrophotometer according to the Beer Lambert law [35]: TðλÞ ¼ e K eλ; Ionanofluids y
ð1Þ
where y is the optical path, i.e.10 mm.
3. Theoretical model An Ionanofluid is a two-phase system containing an IL and the added nanoparticles. The radiation intensity of an Ionanofluid is dominated by the absorptions of the IL and the nanoparticles along with the scattering of the nanoparticles, which are simultaneously affected by the unpredictable and unmeasurable internal interference among the nanoparticles [36]. In order to elucidate the optical radiation mechanisms in an Ionanofluid, the optical properties of the nanoparticles and the IL are modeled separately as follows. First of all, the absorption and scattering of the nanaoparticles are evaluated. The modes for the absorption and scattering of the nanoparticles depend upon several factors, such as particle material, particle size, shape, and volume fraction. The effect of the material is reflected by the complex refractive index of the bulk material. Different materials exhibit diverse optical properties,
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where α is the size parameter, which depends on the particle diameter D, and the incident wavelength λ. m is the relative complex refractive index of an Ionanofluid, mparticles and nfluid are the complex refractive indices of the nanoparticle and the IL, respectively. The extinction coefficients of all the nanoparticles in the base fluid can be calculated using the following equation [17]: K eλ; nanoparticles ¼
3f v Q ext ðα; mÞ 2D
ð6Þ
Actually, the absorption is the only consideration due to the negligible scattering of an IL. Therefore, the extinction coefficient Keλ,IL of the IL is approximately equal to its absorption coefficient Kaλ,IL. K eλ;IL K aλ;IL ¼ Fig. 2. Dependent and independent scattering regime map for different particulate systems (modified on the basis of Ref. [36]). (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
owing to their different complex refractive index. The effect of the particle shape is quite complicated, and there has not been reliable theory to discuss it. So the effect of the particle shape is beyond the scope of the present work, and will be study separately. The nanoparticles used in the current work are spherical. The particle size is reflected by a size parameter α, which is defined as α ¼ πD/ λ, where D is the particle diameter and λ is the wavelength. The scattering pattern, whether it is independent, or dependent, is mainly determined by the particle volume fraction fv and the size parameter α. Fig. 2 shows a scattering regime map about the boundary between dependent and independent scattering by Tien [37], when the particle concentration is less than 0.6 vol% and the particle size is very small, the scattering effect of nanoparticles in nanofluid can be considered to be independent, that means one individual particle was scarcely affected by the scattered wave from other particles. Meanwhile, in this case, the radiative properties of the nanoparticle are well-described by Rayleigh scattering theory [35], which is applicable to small, spherical particles and the medium to be nonabsorbing. In the current work, the average sizes of all the different nanoparticles were around 40 nm, and the volume fractions of the nanoparticles in the Ionanaofluids were in the range from 2.0E-6 (2 ppm) to 4.0E-5 (40 ppm), the parameters about α and fv of Ionanofluids tested have been labeled using a red outline in the Fig. 2. Apparently, the scattering effect of the nanoparticles in the Ionanofluids can be considered to be independent, and the scattering is also applied to Rayleigh scattering. Because the average particle size of nanoparticles in present work are 40 nm, and the wavelength of the most of incident light (200–2500 nm) is at least 10 times larger, of course, it means the partical size parameter α«1. As a result, the solution for Rayleigh scattering can be simplified using the Rayleigh approximation equations [35,36]. Therefore, extinction (Qext), the scattering (Qscat) and absorption (Qabs), and efficiencies of an individual particle can be solved using the following equations: Q ext ¼ Q abs þ Q scat
ð2Þ
2 8 m2 1 Q scat ¼ α4 2 3 m þ2
ð3Þ
2 m 1 α2 m2 1 m4 þ 27m2 þ 38 Q abs ¼ 4α Im 1þ 2 2 2 15 m þ2 m þ2 2m þ 3
ð4Þ
m¼
mparticles nfluid
ð5Þ
4π kf
λ
ð7Þ
where kf is the complex component of the refractive index for the IL, which varies as a function of wavelength. Finally, the total extinction coefficient of an Ionanofluid is defined as below: K eλ; Ionanofluids ¼ K aλ;IL þ K eλ; nanoparticles
ð8Þ
In our calculation, all relevant optical characteristics of the nanoparticles, including the index of refraction (n) and absorption (k), can be obtained from an optical properties handbook [38]. The optical characteristics of the bulk material graphite are particularly used as those of the carbon-coated nickel nanoparticles, and the refractive index of the IL is assumed to be constant for all wavelengths and equals to 1.43, which is provided by the commercial manufacture. Unfortunately, there is very less information on the complex component of the refractive index (kf) of ILs. Thus, the experimentally determined room temperature transmission spectrum of the IL is used in the model. According to Eqs. (2)–(8), we found that the extinction coefficient of Ionanofluids is a function of the size parameter (α), the relative complex refractive index (m), volume fraction (fv), and the wavelength of the incident light (λ). In this paper, we mainly investigated the effects of the relative complex refractive index (m) and volume fraction (fv) varies with the wavelength from 250 to 2200 nm. The extinction coefficient of Ionanofluids containing different kinds of nanoparticles (Ni/C, Ni and Cu) and Ni/C Ionanofluids at different volume fractions was calculated; the model predications were compared to experimental results shown in Figs. 6 and 8, the details are discussed in Section 4.
4. Results and discussion 4.1. Optical properties of IL Fig. 3 displays the room temperature transmission spectrum of [HMIM][NTf2], together with that of deionized water. It can be seen that the transmittance of the IL is close to zero in the UV region (o 300 nm), and dramatically increases to 100% as the wavelength increases from 300 nm to 400 nm. As wavelength continues to rise from 400 to 800 nm, the transmittance of the IL generally maintains a high value until it specifically decreases in the near to mid-infrared range (800–2500 nm), due to the resonance of the organic groups in the IL. It is indicated that [HMIM] [NTf2] is almost completely transparent in the visible-light range. Compared with the IL, water exhibits superior optical absorption property at the wavelengths of more than 900 nm. In addition, water, which can absorb 13% of the solar energy, has been demonstrated to be a better solar absorber than ethylene glycol, propylene glycol, or Therminol VP-1 [39]. However, water is not suitable for use in medium- and high-temperature solar thermal
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Fig. 3. Room temperature transmission spectrum of [HMIM][NTf2], together with that of deionized water.
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Fig. 4. Room temperature transmission spectra of the Ionanafluid containing 10 ppm of the Ni/C nanoparticles passing through different optical path lengths.
systems due to its boiling point of 100 1C. Obviously, the IL with a melting point below100 1C as well as a wide liquid temperature range from room temperature to a maximum temperature of more than 400 1C, is a promising base liquid for the nanofluids to be used in medium- and high-temperature solar thermal systems. 4.2. Optical properties of Ionanofluids 4.2.1. Effect of optical path length The room temperature transmission spectrum of the Ionanofluid containing 10 ppm of the Ni/C nanoparticles has been measured under different optical path lengths (2, 5 and 10 mm). As shown in Fig. 4, the transmittance of the Ionanofluid decreases with the increase in the optical path length, while no red or blueshift in the transmittance peaks is observed. As the optical path length is increased, the light transmitting distance will lengthen accordingly; consequently, the probability of the collision between the light and the particles together with the opportunity of light scattering will increase, both of which would lead to an attenuation in the transmitting light intensity. The variation of the radiative property with the optical path length needs to be taken into consideration during the design of a DASC based on an Ionanofluid. 4.2.2. Effect of nanoparticle material Fig. 5 shows room temperature transmission spectra of the [HMIM][NTf2]-based Ionanafluids containing different nanoparticles with similar average sizes of ca. 40 nm at the same volume fraction of 10 ppm. For comparison purpose, the transmission spectrum of [HMIM][NTf2] is also displayed in Fig. 5. It is obvious that all the Ionanofluids show lower transmittances than the IL in the whole wavelength range, though the transmittance of the Ionanofluids changes with the wavelengths in the same trends as the pure IL does. It is suggested that the absorption property of the IL can be improved by dispersing the nanoparticles in it, even if the volume fraction of the nanoparticles is as low as 10 ppm. Specifically, in the visible range (400–800 nm), the transmittance decreases from 100% for the pure IL to 90% for the Cu Ionanofluid, to 50–60% for the Ni one, and to 30% for the Ni/C one. Although the morphology and average size of the Cu nanoparticles are similar to those of the Ni nanoparicles (Fig. 1b and c), the Ni Ionanofluid exhibits much better absorption property than the Cu one at the same volume fraction of 10 ppm. It is revealed that the material of the nanoparticles has an obvious effect on the optical absorption property of the Ionanofluids. As a result, it is important to select
Fig. 5. Room temperature transmission spectra of the Ionanafluids containing different kinds of nanoparticles with the same average sizes of ca. 40 nm at the same volume fraction of 10 ppm.
a suitable material of nanoparticles for the Ionanofluids to be used as the absorbers in DASCs. Furthermore, the Ni/C Ionanofluid shows superior absorption property to the Ni one, suggesting that coating the surfaces of Ni nanoparticles with carbon further improves the optical absorption of the Ionanofluids due to the predictable broadband absorption of carbon in the visible and near-IR spectrum. The Ionanofluid containing the Ni/C nanoparticles exhibits the lowest transmission as compared to the other two ones at the same volume fraction (fv ¼10 ppm). Fig. 6 shows the experimental extinction coefficients of the IL and its Ionanofluids obtained from their aforementioned transmittance according to the Beer Lambert law. In addition, their calculated extinction coefficients obtained from the theoretical model based on the Rayleigh approximation equations are also displayed in Fig. 6. It can be seen that the extinction coefficient of the IL is very low in the visible-light range. The extinction coefficients of all the Ionanofluids containing different nanoparticles at a low volume fraction of 10 ppm are higher than that of the IL in the whole wavelength range. Specifically, the extinction coefficient increases almost one magnitude for the Cu Ionanofluid and two magnitudes for the Ni and Ni/C ones. In the wavelength below 1200 nm, the experimental extinction coefficients do not match well with the model predictions, especially for the Cu and Ni Ionanofluids. One possible reason for the deviations can be attributed to the surface oxidation of the metal nanoparticles. As we know, when the size of a metal material reaches to nanoscale,
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Fig. 6. Comparisons on the extinction coefficients between the experimental results and the model predictions for the Ionanofluids containing different kinds of nanoparticles with the same average sizes of ca. 40 nm at the same volume fraction of 10 ppm, together with the experimental extinction coefficient of [HMIM] [NTf2].
Fig. 7. Room temperature transmission spectra of the Ni/C Ionanafluids at different volume fractions.
its specific surface area and surface energy increase sharply, resulting in the oxidization occurring on the surfaces of the metal nanoparticles. Consequently, the metal nanoparticles have different optical characteristics from its bulk metal. In addition, the agglomeration of the nanoparticles leads to an increase in the particle size, resulting in a deviation from the Rayleigh approximation. Since the difference between the experimental results and the model predictions limits within a one magnitude, the total trend of the extinction coefficients obtained from the Rayleigh approximation can still reasonably account for the effect of material on the optical properties of the Ionanofluids, which is reflected by their different complex refractive indexes.
4.2.3. Effect of volume fraction Fig. 1d shows the photograph of the Ionanofluids containing the Ni/C nanoparticles at varying volume fractions. Their colors gradually become dark as the volume fraction of the Ni/C nanoparticles is increased from 0 to 40 ppm, implying that the optical absorption property of the Ni/C Ionanofluids is enhanced with the increase in the volume fraction of the nanoparticles. The Ionanofluids containing 30 and 40 ppm of the Ni/C nanoparticles are black. Compared with the traditional black liquids, the nanofluids composed of [HMIM][NTf2] and the Ni/C nanoparticles do not possess those serious shortcomings such as light-induced degradation, thermal degradation at the operating temperatures, and low thermal conductivity. Fig. 7 shows room temperature transmission spectra of the Ionanofluids containing the Ni/C nanoparticles at different volume fractions. It is obvious that the transmittance of the Ionanofluids gradually decreases with the increase in the volume fraction of the Ni/C nanoparticles in the wavelength range from 300 nm to 2200 nm. Even if the volume fraction is as low as 2 ppm (fv ¼ 0.0002%), the transmittance of the Ionanofluid remarkably decreases to around 75% as compared with 100% of the IL. As the volume fraction of the Ni/C nanoparticles is increased to 40 ppm (fv ¼ 0.004%), only less than 3% of the light is transmitted in the whole wavelength range, indicating a significant decrement in the transmittance. The excellent optical absorption property of the Ni/C Ionanofluids makes them show promising for use as the absorbers in DASCs. Fig. 8 shows the comparisons on the extinction coefficients between the experimental results and the model predictions for the Ionanofluids containing the Ni/C nanoparticles at different
Fig. 8. Comparisons on the extinction coefficients between the experimental results and the model predictions for the Ni/C Ionanafluids at different volume fractions.
volume fractions. In the visible-light range (from 400 nm to 800 nm), the Ionanofluid containing 2 ppm of the Ni/C nanoparticles shows an increment of more than one magnitude as compared with the IL. The extinction coefficients of the Ni/C Ionanofluids increase with the volume fraction of the Ni/C nanapartilces, which can be well explained by the Rayleigh approximation equations. According to Eqs. (6) and (8), the extinction coefficient of an Ionanofluid is linearly proportional to the volume fraction of the nanoparticles. The extinction coefficient of the Ionanofluids with 40 ppm of the Ni/C nanoparticles reaches to as high as around 5 cm 1. Moreover, for the Ni/C Ionanofluids at the volume fractions between 2 ppm and 40 ppm, their extinction coefficients derived from the theoretical model based on the Rayleigh approximation equations agree well with the experimental results obtained from the transmittances measured by the spectrophotometer. 4.3. Absorbed energy fractions of the Ni/C Ionanofluids The investigations on the spectrally-resolved optical properties of nanofluids allow us to evaluate their sunlight extinction behavior as well as their energy storage capability, providing very useful information for the absorber selection and collector optimization. To quantitatively evaluate the solar absorption capability of the Ionanofluids, an absorbed energy fraction, F, is defined to describe the proportion of the solar energy absorbed across an Ionanofluid layer, which means that the light passing through the
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absorb the incoming solar radiation completely. For a given Ionanofluid, the collector height should be consistent with the optical path length. If the collector height is too low, the solar radiation cannot be thoroughly absorbed by the nanofluid, resulting in a low receiver efficiency; if the collector height is too high, the solar radiation will be mainly absorbed on the top layer of the nanofluid, leading to a temperature difference in the nanofluid along with an increase in heat loss [3,40]. Therefore, the volume fraction of nanoparticles and the collect height should be chosen very precisely in order to achieve a maximum efficiency of the DASC. A further study on the performance of the DASCs based on the Ionanofluids is in progress.
5. Conclusions Fig. 9. Absorbed energy fractions (F) versus the penetration distance (X) for the Ionanofluids containing the Ni/C nanoparticles at different volume fractions.
Ionanofluid for a distance of X is no more available to be transmitted and has been stored in the Ionanofluid (either for direct absorption or scattering followed by absorption process). F can be calculated using the following equation [18]: R λmax F ¼ 1
K eλ;Ionanofluids X dλ λmin IðλÞe R λmax λmin IðλÞdλ
ð9Þ
where I(λ) is the total incident solar irradiance in the wavelength range (200–2500 nm), which can be approximated using Planck's equation for the body radiation at a temperature of 5800 K [2]. X is the thickness of the fluid layer, which is equivalent to the height (H) of a DASC. Fig. 9 illustrates the absorbed energy fraction (F) at varying penetration distance (X) for the Ni/C Ionanofluids at different volume fractions. It is obvious that F of all the Ionanofluids containing the carbon-coated Ni nanoparticles are much higher than that of the IL even at the Ni/C volume fraction as low as 2 ppm. Specifically, it only needs 1 cm of the fluid layer for the Ionanofluid containing 40 ppm of the Ni/C nanoparticles to absorb almost 100% of the incident light while it needs at least 10 cm for the pure IL to reach the same fraction. The results indicate that the Ni/C Ionanofluids show great potential for use as the absorbers in the medium-to-high-temperature DASCs. The investigations on the effects of optical path length, nanoparticle material, and volume fraction of nanopartilces on the radiative properties of the Ionanofluids provide very useful information for the optimization of a DASC. According to Eq. (9), F of a fluid absorber in a DASC depends on two factors, extinction coefficient (Keλ) and path length (X). The increases in Keλ and X enhance F of the fluid, resulting in an improvement in the efficiency of the DASC. As described in Section 4.2, Keλ is influenced by several factors including nanoparticle material, volume fraction, etc. It has been shown that, at the same volume fraction, the Ionanofluid containing the carbon-coated Ni nanoparcles with an average size of 40 nm exhibits the lowest transmittance and the highest extinction coefficient as compared with the ones containing the Cu and Ni nanoparticles with the similar average sizes. Once the nanoparticle material is determined, F of the nanofluid can be optimized by adjusting the two parameters, volume fraction (fv) and collector height (H). For a given collector height, F of the nanofluid increases with its volume fraction until F reaches up to 100%. However, as the volume fraction continues to increase, all of the incoming solar radiation will be absorbed with a thin layer of the nanofluid. In that case, the absorbed thermal energy is apt to lose to the environment. On the other hand, if the volume fraction is too low, the nanofluid cannot
The radiative properties of [HMIM][NTf2] and its nanofluids have been investigated for the first time, aiming at developing a novel kind of HTFs for medium-to-high-temperature DASCs. The optical absorption property of the IL can be greatly enhanced by adding a low volume fraction of nanopartcles in it. The transmittance of the Ionanofluid decreases with the increase of optical path length. The Ionanofluid containing the Ni/C nanoparticles exhibits the lowest transmittance and highest extinction coefficient as compared with the ones containing the Ni and Cu nanoparticles with the similar average size at the same volume fraction of 10 ppm. The Ni/C Ionanofluids exhibit a decrease in transmittance with the increase in the volume fraction of the nanoparticles, and their extinction coefficient predicted by Rayleigh scattering theory are consistent with the experimental results, implying this theory is appropriate for predicting the extinction coefficients of these two phase systems. At the volume fraction of 40 ppm, only less than 3% of the light is transmitted in the whole wavelength range for the Ni/C Ionanofluid, and its absorbed energy fraction reaches up to almost 100% after the incident light only penetrate the Ionanofluid for a distance of 1 cm. The excellent radiative property of the Ni/C Ionanofluid makes it show promising for use as the absorber in solar thermal collectors, especially in the medium-to-high-temperature DASCs.
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