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The stability, optical properties and solar-thermal conversion performance of SiC-MWCNTs hybrid nanofluids for the direct absorption solar collector (DASC) application
Solar Energy Materials & Solar Cells xxx (xxxx) xxx
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The stability, optical properties and solar-thermal conversion performance of SiC-MWCNTs hybrid nanofluids for the direct absorption solar collector (DASC) application Xiaoke Li a, *, Guangyong Zeng a, Xinyu Lei b a b
College of Materials and Chemistry & Chemical Engineering, Chengdu University of Technology, Chengdu, 610059, China Organic Chemicals Co., Ltd. Chinese Academy of Sciences, Chengdu, 610041, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: Hybrid nanofluids Solar-thermal conversion DASC Solar energy
Hybrid nanofluids have an advanced application prospect in the community of heat transfer fluids and partic ularly for the direct absorption solar collectors (DASCs). Therefore, stable ethylene glycol-based SiC-MWCNTs nanofluids with mass fractions ranging from 0.01% to 1% were prepared in this study. Combined with the unique properties of these two nanomaterials, the studied hybrid nanofluids displayed excellent stability, optical properties and photothermal conversion properties. The purpose of this paper is to simultaneously achieve the enhanced stability and high solar-thermal conversion efficiency of hybrid nanofluids used for DASC applications. The stability of hybrid nanofluids was confirmed. In addition, the hybrid nanofluids displayed an excellent solar irradiation absorption capacity in both visible and near-infrared regions (200–1100 nm). The fact proved that the hybrid nanofluid was effective working fluid in DASCs, where 0.5 wt% SiC-MWCNTs nanofluids could absorb 99.9% solar energy at only 1 cm path length. In addition, the solar-thermal conversion efficiency of hybrid nanofluids increased with the mass concentration. The maximum value of solar-thermal conversion efficiency was found to be 97.3% on 1 wt% SiC-MWCNTs nanofluid at 10 min, which was 48.6% higher than that of pure EG. The application potentials of SiC-MWCNTs hybrid nanofluids in low-temperature DASCs system were presented.
1. Introduction In the modern society with rapid economic development, fossil fuels such as oil and gas occupy the dominant position of human energy consumption. But a large amount of fossil fuel consumption will inevi tably lead to various environmental pollution problems, such as air pollution, haze, acid rain and greenhouse effect [1,2]. The increase of energy consumption and cost related to fossil fuels, along with the in crease of environmental pollution and the aggravation of greenhouse effect, has changed people’s attitude towards how to produce and use the energy of this planet. Renewable energy refers to a series of inexhaustible and environment-friendly energy sources that can be automatically renew able without human participation. With the decrease of fossil fuel re serves, solar energy has become an important part of human energy use and has been developing continuously. Solar energy is also widely considered to be the most abundant source of permanent energy on
earth [3]. However, how to effectively transfer or store solar energy becomes another focus of the scientific community, both now and in the near future. In 1975, the term of direct absorption solar collector (DASC) was firstly designed and proposed by Minardi and Chuang [4], which was used to absorb the solar radiation or energy directly through heat transfer fluid (HTF). In order to improve the solar radiation absorption capacity of traditional HTFs, such as water, ethylene glycol (EG) or heat thermal oil, the India ink was added into the base fluid. Although solar energy absorption efficiency has been improved to some extent, the suspension had inherent light and temperature instability or degrada tion defects, which limited the efficiency and development of DASC. Therefore, it is known that DASC has two major deficiencies: one is the poor stability of working fluids, and the other is the weak light capture capability. Thanks to the rapid development of nanotechnology, the concept of nanofluids rose in response to the proper time and conditions [5]. Nanofluid is a new type of heat transfer medium which is uniform and
* Corresponding author. No.1, Dongsan Road, Erxianqiao, Chenghua District, Chengdu, 610059, PR China. E-mail address: [email protected] (X. Li). https://doi.org/10.1016/j.solmat.2019.110323 Received 15 August 2019; Received in revised form 3 November 2019; Accepted 27 November 2019 0927-0248/© 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Xiaoke Li, Solar Energy Materials & Solar Cells, https://doi.org/10.1016/j.solmat.2019.110323
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more nanoparticles, which were later developed and termed as “hybrid nanofluids” [31–33]. Most studies have shown that the hybrid nano fluids could offer the possibility of matching solar radiation spectra and subsequently achieving broadband volumetric solar thermal absorption, based on the complementary optical absorption properties of different nanoparticles [34,35]. Qu et al. [36] experimentally prepared the CuO-MWCNTs hybrid nanofluid to directly capture of solar thermal energy. The results revealed that the spectral absorption characteristics of the studied hybrid nanofluids were greatly reinforced. Akilu et al. [37] investigated the thermos-physical properties of glycerol and EG mixture based SiO2–CuO/C hybrid nanofluid and an enhanced thermal conductivity of 26.9% was found. The experimental results indicated the hybrid an attractive HTF for solar energy transportation. Menbari et al. [38] focused on the stability and optical property of CuO–Al2O3 hybrid nanofluids for the direct absorption solar parabolic trough collector (DASPTC). The thermal efficiency of DASPTC system could be effec tively enhanced by increasing nanoparticle loading and nanofluid flow rate. The photo-thermal conversion performance of nanofluids con taining TiO2–Ag with core/shell structures was studied by Xuan and his group [39]. They found that the hybrid nanofluids absorbed light in a wide range from UV to near-infrared. They also prepared hybrid CNT-SiO2/Ag nanofluids for solar-thermal conversion application [40]. At the penetration distance of 1 cm, the 0.005 vol% of hybrid nanofluid gave a solar weighted absorption fraction of 74.5%. Chen et al. [41] chose CuO and antimony doped tin oxide (ATO) nanoparticles to pre pare the water-based hybrid nanofluids. The results showed that CuO-ATO nanofluids had excellent light absorption in both the visible light and near-infrared regions and enhanced photo-thermal conversion property. Specifically, the solar-thermal conversion efficiency of the hybrid nanofluids were estimated to be 92.5%. Wei et al. [42] dispersed the SiC–TiO2 nanoparticles in diathermic oil to fabricate nanofluids and found greater thermal conductivity of hybrid nanofluid than that of single nanofluid. In addition, Valizade et al. [43] conducted the spec trophotometric experiments on the produced porous metal foams and water based nanofluids from two materials namely SiC and CuO. Meh rali et al. [44] focused on the optical properties of graphene/silver hybrid plasmonic nanofluids. A collector efficiency of 77% was found at low concentration of 40 ppm. However, most of these studies used two types of spherical structured nanoparticles, which suffered from the disadvantages of instability, complexity and difficulty in large-scale production. Therefore, the new concept and system of hybrid nano fluids for DASCs applications are highly desired. And responding to this need will be meaningful to drive nanofluids application of solar system into the next stage. In our previous study [10], we found that SiC nanoparticles with spherical structure have considerable absorption in the near-infrared region. On the other hand, the MWCNTs nanofluids were demon strated that they could absorb over the majority of solar spectrum because of the unique properties of carbon nanotubes [22,24,45]. Kar ami et al. focused on the applications of MWCNTs nanofluids in volu metric solar collectors [46]. They found that the extinction coefficient of water-based MWCNTs nanofluids showed considerable improvement compared to the pure water even at low particle concentration. Based on the above facts, it was highly expected that the combination of the two materials will exhibit unique characteristics. Therefore, the SiC nano particles and MWCNTs were chosen to prepare the SiC-MWCNTs hybrid nanofluids in this paper. Considering cost factors, optical properties and stability of nanofluids, the mass ratio of 8:2 (SiC:MWCNTs) was adopted. EG was used as the base fluid due to its relatively wide usage temper ature range and wide application in the field of heat transfer. To our knowledge, there have been few papers studied the stability of hybrid nanofluids for DASCs in depth. However, stability is a prerequisite for the application of all nanofluids in DASC. In addition, it seems that no one has focused on the properties of SiC-MWCNTs nanofluids before. Therefore, the colloidal stability of SiC-MWCNTs nanofluids as the
Nomenclature Symbols m
ρ ζ Am Cp T(λ) K(λ)
Mass [g] Density [g/cm2] Absolute Zeta potential Solar-thermal conversion efficiency [%] Specific heat capacity [KJ/(Kg⋅K)] Transmittance [%] Extinction coefficient [cm 1]
Abbreviations HTF Heat transfer fluid SiC Silicon carbide MWCNTs Multiwalled carbon nanotubes EG Ethylene glycol DASC Direct absorption solar collector Subscripts hybrid Hybrid nanofluids b Base fluid nf Nanofluids
stable by dispersing nanoparticles (<100 nm) into traditional HTFs [6]. Till date, a large number of experiments and theoretical researches have been investigated with nanofluids based DASC systems [7]. The results of these studies showed that the nanofluids could effectively improve the solar-thermal conversion efficiency and work efficiency of solar thermal collector systems. For example, Karami et al. [8] studied the thermos-physical and optical properties of copper oxide (CuO) nanofluid as working fluid in DASC. They found that the absorption of solar energy of CuO nanofluids was about 2 times more than that of the water-EG mixture. Milanese et al. [9] focused on the application of water/Al2O3 nanofluids in the flat-panel solar collectors and a 25% enhancement of the convective heat transfer coefficient was recorded. Chen et al. [10] studied the thermos-physical and optical properties of SiC/ionic liquid nanofluid (Ionanofluids) and the results indicated that the studied nanofluid was a superior solar absorption choice, where the extinction coefficient of 0.03 wt % SiC Ionanofluids could increase about 5.8 cm 1. Most recently, our group designed a new method for preparing multi-walled carbon nanotubes (MWCNTs) nanofluids with high mass fraction [11]. The prepared MWCNTs nanofluids were ideal candidate as working fluids in volumetric solar receiver because nearly 100% inci dent solar energy could be harvested with only 0.01 wt% MWCNTs nanofluids. But the stability was still a challenge for the MWCNTs nanofluids, and the cost of MWCNTs was relatively high for large-scale preparation. In addition, nanofluids containing different types of nanoparticles, such as metallic monomers [12–14], oxides [15–18], and carbon nanomaterials [19–24], have been extensively studied for using in solar energy systems. Therefore, nanofluid is a prominent choice for the working fluid of the direct absorption solar collectors. It is noteworthy that up to now, most of the nanofluids related to the application of DASCs were single-component nanoparticles. However, the single-component nanofluids still have certain disadvantages in some aspects. For instance, although carbon nanotubes have extremely high thermal conductivity and excellent optical properties [25,26], they are relatively expensive and difficult to disperse uniformly [27]. On the other hand, ceramic oxides such as Al2O3, CuO, TiO2 and SiC have good dispersion, but their thermal conductivities are often not high; And the solar absorption capacities are limited due to the absorption spectra of these nanoparticles are mainly in the visible region, while the absorption in the near-infrared region is weak [8,28–30]. To overcome these dif ficulties, the researchers proposed to prepare nanofluids using two or 2
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working fluid for DASC systems was comprehensively determined firstly. In addition, the transmittance, extinction coefficient, solar weighted absorption fraction and solar-thermal conversion efficiency of hybrid nanofluids with different mass concentrations were experimen tally measured, scientifically analyzed and objectively discussed. The purpose of this paper is to simultaneously achieve the enhanced stability and high solar-thermal conversion efficiency of hybrid nanofluids used for DASC applications. It should be pointed out that the preparation method of hybrid nanofluids was different from the well-known two-step method. The straightforward strategy used in the current work was firstly reported by our group [11], which was capable of preparing nanofluids easily in large-scale for solar energy applications. The application of SiC-MWCNTs hybrid nanofluids in DASC is proposed in this paper. Since no one has done any research on the SiC-MWCNTs hybrid nanofluids before, we believe the current study could provide an insight into the understanding of colloidal stability, optical properties and solar-thermal conversion performance of nonmetallic oxides/carbon nanotubes hybrid nanofluids used in DASCs.
Firstly, 1 wt% SiC-MWCNTs powder, 0.5 wt% PVP-K30 and corre sponding EG were mixed in the stirred vessel with 500 rpm. It should be noted that PVP-K30 was selected as dispersant due to its high temper ature resistance, low foamability and low viscosity [47]. Next, the pre-mixed suspension was pumped into the mill chamber and then the nanofluids would flow back into the stirrer vessel after sand milling. The above circulation process was set to run 120 min to disperse the hybrid nanofluids thoroughly. After successive reflux, about 0.8 L SiC-MWCNTs hybrid nanofluids (1 wt%) could be processed at once. Finally, the hybrid nanofluids were diluted to different required concentration by simple ultrasonic. 2.3. Stability tests Firstly, the Turbiscan LabExpert was used to investigate the stability of hybrid nanofluids (1 wt%, 24 h after preparation). The probe head of the instrument has a near infrared light source (λ ¼ 800 nm) and two synchronous detectors that receive the light transmitted through the sample and the backscattered light respectively [48]. The variation of backscattering value was proportional to the variation of particle con centration at each position of the test bottle (55 mm). Then the homo geneity and stability of samples could be characterized. In addition, a Zeta potential measurer (Brookhaven Instruments Corporation, USA) was used to predict long-term suspension stability of hybrid nanofluids with different concentrations (1, 7, 15, 30 days after preparation). The uncertainties of measurements were controlled within �5%. The absolute values of Zeta potential were recorded as the mean value of the five repeated measurements for each sample. The size dis tribution of SiC-MWCNTs in nanofluid was determined by a dynamic light scattering device (DLS, VASCO, France) and the measurement range was from 10 to 4000 nm. To further verify the application potential of SiC-MWCNTs hybrid nanofluids in DASC system, the thermal cycling tests were carried out. The nanofluids samples were first added into several borosilicate glass tubes. Then these tubes were heated from room temperature to 60 � C by a thermostatic water bath, and keeping the temperature for another 30 min. The sample was then cooled to room temperature by natural con vection, and the absorbance of sample was measured by UV–vis spec trophotometer (UV3802,UNICO, China) at 980 nm subsequently. The above procedure was defined as one cycle, and each sample was repeatedly performed for thirty cycles in three days. After every 10 cy cles, the samples were placed at room temperature overnight. The following day, all samples will be subjected to 30 min of ultrasonic treatment to break up the soft aggregates. The samples without ultra sonic treatment were analyzed according to the same procedure as control experiments.
2. Material and methods 2.1. Materials and preparation of binary nanoparticles Multi-walled carbon nanotubes (diameter of 20 nm, purity >95%) were purchased from Chengdu Organic Chemistry Co., Ltd., Chinese Academy of Sciences (Chengdu, China). The SiC nanoparticles (diameter of 40 nm) were provided by C.W. Nanotechnology Company (Shanghai, China). Theirs basic properties provided by the sellers are listed in Table 1. The base fluid EG, the surfactants polyvinyl pyrrolidone (PVP–K30) and hexane were purchased from KeLong Chemical Reagent Factory (Chengdu, China). The preparation process of binary nanoparticles was as follows. Firstly, the SiC and MWCNTs powders were weighted on an electronic balance (mass ratio ¼ 8:2) and then moved into a beaker containing hexane (200 mL). Subsequently, a magnetic stirrer was employed to mix the suspension for 20 min. Then the solution was subjected to ultrasonication homogenizer Sonifier 250 (Branson Ultrasonics, Danbury, USA) for 1 h. Finally, the powders collected by centrifugation were dried for 10 h in an oven at 65 � C. The dried binary nanoparticles were then ground to get the fine target product –– SiC-MWCNTs binary nano particles. The morphologies of SiC nanoparticles, MWCNTs and SiCMWCNTs binary nanoparticles were characterised by scanning elec tron microscopy (SEM, FEI, USA) (See Fig. 1). It could be observed that the morphology of SiC nanoparticles was nearly spherical, while that of MWCNTs was tubular. On the other hand, the dry SiC-MWCNTs binary nanoparticles were in a state of massive aggregation. In addition, nanoSiC and MWCNTs had formed “point-line” network structure.
2.4. Optical absorption properties of hybrid nanofluids
2.2. Preparation of hybrid nanofluids
The UV–vis spectrophotometer was used to measure the optical transmittance spectra of SiC-MWCNTs nanofluids in a wavelength range from 200 to 1100 nm with accuracy of �1 nm. The 10 mm quartz cu vettes were used to hold the nanofluid samples at room temperature (25 � 0.2 � C). The spectral transmittance of base fluid (EG) or nanofluids was measured against the standard reference (air) and during the measurements one cuvette was filled with base fluid (EG) or nanofluids and other cuvette was left empty (filled with air). Three times of mea surements were repeated for each sample to avoid the systematic errors of multiple reflection at the interfaces between liquid, glass and air. In addition, according to the Beer-Lambert law, the extinction coefficient (K(λ)) of hybrid nanofluids was calculated by Eq. (1) [23]:
In this part, a stirred media mill (PHN Labstar 0.3CA, PUHLER Smart Nano Tech Co., Ltd, China) was employed to prepare the EG-based SiCMWCNTs hybrid nanofluids. The schematic diagram of this equipment is shown in Fig. 2 and the details of it could be found in our former study [11]. Table 1 The basic properties of SiC nanoparticles and MWCNTs. Sample
Diameter
Length
Purity
Specific surface area
Density
MWCNTs
20 nm
~30 μm
254.87 m2/g
SiC
40 nm
/
>95 wt % 99.9 wt %
2.1 g/ cm3 3.2 g/ cm3
39.8 m2/g
TðλÞ ¼ expð KðλÞ ⋅ HÞ
(1)
where T(λ) means the transmittance, H presents the light path (10 mm). 3
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Fig. 1. Scanning electron microscopy image of (a) SiC nanoparticles; (b)MWCNTs; (C) SiC-MWCNTs binary nanoparticles.
Fig. 2. Schematic diagram of the stirred media mill in the preparation of hybrid nanofluids.
2.5. Solar-thermal conversion performance of hybrid nanofluids
self-assembled by our group and the schematic diagram of the experi mental setup is shown as Fig. 3. Firstly, the aluminized paper was used to paste around and at the bottom of the flat bottom beakers (50 mL) to effectively reduce the absorption of light. Then, same amounts of SiCMWCNTs hybrid nanofluids with different mass concentration were added into the beakers. Shortly afterwards, the beaker was enclosed in an adiabatic foam block (thermal conductivity�0.036 (W/(m⋅K)) to minimize the heat loss during the experiment. A solar simulator (SOLAREGDE 700, Perfectlight Technology Co., Ltd., Beijing) was used as the radiative source, which matched with the AM1.5 spectrum well. The flux of incident radiation to the surface of the beaker was measured accurately by a solar radiation meter (HD2302, Delta-ohm, Italy) with accuracy of 0.9%. Three thermocouples (WRP-S, Feilong Instrument Co., Ltd., Shanghai) were fixed to the inner wall of the experimental device to measure the temperature of each point inside the nanofluid during heating. Another thermocouple was placed outside the device for ambient temperature measurement. The maximum measuring temper ature of thermocouple was 1800 � C, and the measuring error was 1.5 � C. All temperature measurements were performed by a data acquisition (34970 A, Agilent) with a time interval of 5 s. The ambient temperature was 20 � 1 � C. In the experiment, when the sample temperature did not change within 10 min, the light source was turned off. The uncertainties of the experiment resulted from the measurement
The solar-thermal conversion performance experimental setup was
Fig. 3. Schematic diagram of solar-thermal conversion performance experi mental setup. 4
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error of thermocouples (�1.5 � C) and solar radiation meter (�0.9%). Therefore, the uncertainty of the experiments could be expressed as Eq. (2) [49]: sffi� ffiffiffiffiffiffiffiffiffiffi� ffiffiffiffiffiffiffiffiffiffiffi� ffiffiffiffiffiffiffiffiffiffiffi� ffiffiffiffiffiffiffiffiffiffiffi� ffiffiffiffiffiffiffiffiffiffiffi� ffiffiffiffiffiffiffiffiffiffiffi� ffiffiffiffiffiffiffiffiffiffi� ffiffiffiffiffi 2 2 2 2 δT1 δT2 δT3 δqi ER ¼ (2) þ þ þ jT1 j jT2 j jT3 j jqi j
nanofluids was measured at different mass concentrations and the cor responding results could be found in Fig. 5. The Fig. 5a clearly showed that the ζ value decreased slightly with the mass concentration at room temperature, while that of all the nanofluids decreased with the increase of standing time. However, even after 30 days of storage, the nanofluids remained in a relatively stable state, because in general, the Zeta po tential greater than 45 mV means that the system was in an excellent stable state [51]. Considering the applications in DASC system, nano fluids are always affected by different temperature conditions. Hence, the ζ values of the fresh hybrid nanofluids at different temperature were also determined. According to Fig. 5b, temperature had obvious side effects on the stability of hybrid nanofluids. Specifically, the absolute ζ value of 1 wt% SiC-MWCNTs nanofluids decreased from 52.7 mV at 20 � C to 40.2 mV at 60 � C. The main reason of this phenomenon was that the high temperature will increase the probability and strength of the collision of nanoparticles and reduce the effect of dispersant, thus affecting the electrostatic balance of the suspension system. In addition, it was clear that the mean nanoparticle size in hybrid nanofluids (1 wt%) was around 217 nm (See Fig. 5c) after preparation, which was larger than that of primary particles, evincing the formation of clusters in the nanofluid. The average cluster size was found to be almost constant (around 221 nm) after 30 days of re-evaluation. The experimental re sults were in good agreement with Zeta potential experiment. In order to further verify the stability of SiC-MWCNTs hybrid nanofluids in DASC applications in this paper, the thermal cycling ex periments were conducted and Fig. 6 shows the experimental result. In this part, the relative absorbance (RA) was introduced as an evaluation index, which was defined as the ratio of the absorbance of heated SiCMWCNTs nanofluids to the initial absorbance of fresh hybrid nano fluids. Taken altogether, the RA values decreased with the number of heating circles. Only a 7.07% of decrement was found on the RA value of 1 wt% hybrid nanofluids without ultrasonic treatment after 30 cycles of thermal cycling. This indicated that the SiC-MWCNTs hybrid nanofluids owned application potential due to the long-term stability, which was in agreement with the experimental results of Zeta potential. On the other hand, significant fluctuations could be observed for each sample after every 10 heating cycles, but this trend could be easily reversed by ul trasonic treatment. Through the repeated heating would put a side effect
where the T1, T2 and T3 mean the measured temperature by thermo couples; the qi is the flux of incident radiation. According to the calibration analysis of the experimental measuring property for the measuring devices used, the resulting uncertainty value was �2.75%, which indicates that an acceptable and reliable engi neering border was achieved in the conducted tests. 3. Results and discussion 3.1. Stability It is well known that colloidal stability is a necessary and sufficient condition for the applications of nanofluids in solar energy systems [50]. The fresh samples with different mass concentrations are exhibited in Fig. 4a. The static settlement experiment, which is the most intuitive way to observe the stability of nanofluids, indicated that no visible stratification was found even a month later (See Fig. 4b). There were only some black flocs at the bottom of the bottle with high concentration of nanofluids (1 wt%). On the other hand, the stability of SiC-MWCNTs hybrid nanofluids (1 wt%, maximum concentration in this study) was measured for 30 min at 25 � C by the Turbiscan LabExpert. Fig. 4c shows the delta backscat tering (ΔBS) during experiment. As shown, only 0.2% of ΔBS was found at the bottom of test bottle (0–3 mm), while in other positions this fluctuation was practically negligible. The experimental data suggested that only a few nanoparticles were deposited at the bottom, while the remaining solid phase was quite stable in the hybrid nanofluids. Since the stability of colloidal systems is mainly the result of the interaction between particles and particle-fluid interfaces over time, the stability of EG-based SiC-MWCNTs nanofluids is closely related with their surface charge density. Therefore, the Zeta potential (ζ) of studied
Fig. 4. Sedimentation experiments at different standing time: (a) One day after preparation; (b) One month after preparation. (c) Delta backscattering of SiCMWCNTs nanofluid at 1 wt%. 5
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Fig. 5. (a) Zeta potential of SiC-MWCNTs nanofluids as a function of standing time; (b) as a function of temperature; (c) Size distribution of 1 wt% hybrid nanofluids at different standing time.
SiC-MWCNTs hybrid nanofluids are shown in Fig. 7. The transmittance spectra of hybrid nanofluids is exhibited in Fig. 7a. The spectral peaks found at about 920 nm and 980 nm were associated with the absorption from base fluid EG Ref. [52]. In general, the transmittance of hybrid nanofluids decreased significantly with the increase of powder loading. Even if the content of nanoparticles was only 0.01 wt%, the transmittance of ethylene glycol could be reduced by about 45%. In addition, the transmittance of 0.5 wt% object almost fell to zero on the tested wave length range and the sample with higher concentration (1 wt%) was completely black, which was below the detection limit of the UV–vis spectrophotometer. In conclusion, in order to maintain a transmittance of less than 5% at all wavelengths, the concentration of the bicomponent nanofluid should only be higher than 0.25 wt%. In addition, the K(λ) of SiC-MWCNTs hybrid nanofluids is shown in Fig. 7b, and the gray shadow represents the total incident solar irradiance at air mass 1.5 according to the ASTM G173-03. The exper imental results showed that the extinction coefficient increased with the mass concentration of hybrid nanofluids. For 0.5 wt% SiC-MWCNTs nanofluid, the average extinction coefficient was close to 9 cm 1 in the tested spectral range. However, the average extinction coefficient of pure EG was below 1.0 cm 1. Furthermore, the studied hybrid nano fluids seemed to be a more effective sunlight absorbing fluid than the CuO-ATO nanofluids [41], whose extinction coefficient was about 7 cm 1 on average for 0.12 vol%. In addition, to clarify the application prospect of SiC-MWCNTs hybrid nanofluids, the absorbed solar irradiance of hybrid nanofluids was calculated based on Eq. (3) [29] and the results are listed in Fig. 7c.
Fig. 6. The relative absorbance of SiC-MWCNTs nanofluids in different mass concentration as a function of thermal cycles.
on the stability of nanofluids inevitably, easy and necessary adjustments could be made according to the actual situation in the DASC applications. In conclusion, the SiC-MWCNTs hybrid nanofluids have satisfactory colloidal stability and potential applications in DASC systems.
IA ðλÞ ¼ IAM1:5 ðλÞ � ½1
3.2. Optical properties
TðλÞ�
(3)
where IA ðλÞ is the absorbed solar irradiance of hybrid nanofluids; IAM1:5 ðλÞ means the incident solar irradiance at air mass 1.5. Similarly, the gray shadow means the total incident solar irradiance
Absorption performance of working fluid is a key point in the application of solar collector system. Herein, the optical properties of 6
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where l is the penetration distance. In the DASCs systems, a thin layer of working fluid (thickness of nanofluid layer in this case) would act as a surface absorber, where high surface temperatures and radiation losses will result in heat loss to the environment. In addition, if the collector height is not high enough, the solar energy cannot be effectively absorbed by the nanofluids, which greatly reduces the solar energy conversion efficiency. Therefore, it is necessary to optimize Am in en gineering practice until the working fluid achieves efficiency optimiza tion in DASC system. As displayed in Fig. 8, the SiC-MWCNTs hybrid nanofluids could greatly enhance the Am of pure EG and all hybrid nanofluids could absorb the incoming solar energy completely at 10 cm path length. For the hybrid nanofluid with 0.5 wt%, the Am could reach 99.9% or more at only 1 cm path length. This means that a thin layer of hybrid nanofluids can achieve high efficiency, rather than volumetrically by the HTF medium. On the basis of good stability, the SiC-MWCNTs hybrid nano fluids could be an ideal working fluid in the application of DASC systems. 3.3. Solar-thermal conversion performance The solar-thermal properties of nanofluids depend to a great extent on the relationship between the temperature change of nanofluids and solar radiation time. The bulk temperature change curves of SiCMWCNTs hybrid nanofluids with different mass concentrations were plotted in Fig. 9a. Obviously, the solar-thermal conversion performance of pure EG was significantly improved after the addition of SiC-MWCNTs nanoparticles. The temperature of all fluids increased with the irradia tion time, but the temperature rise rate and the maximum equilibrium temperature of the nanofluids were higher than those of pure EG. For example, the highest temperature difference of hybrid nanofluids with 0.1 wt% was 107.1 � C (irradiation time was 50 min), whose temperature rise rate was 2.142 � C/min. This was about 31.5% faster than that of base fluid. In addition, the maximum equilibrium temperature differ ence of hybrid nanofluids (1 wt%) was about 32 � C higher than that of pure EG at the same solar radiation time. Similar trend was reported on water-based CuO/MWCNTs hybrid nanofluids [36] and water-based Al2O3–Co3O4 hybrid nanofluids [53]. Therefore, the solar-thermal con version performance of base fluid (EG in this paper) could be signifi cantly improved by adding a small amount of hybrid nanoparticles. However, the SiC-MWCNTs hybrid nanofluids exhibited even better solar-thermal properties, which may be attributed to the enhancement of stability as discussed in Section 3.1.
Fig. 7. The (a) transmittance, (b) extinction coefficient and (c) the absorbed solar irradiance of SiC-MWCNTs hybrid nanofluids at different mass concentrations.
in Fig. 7c. The data in the figure indicated that most of the solar irra diation had been absorbed by SiC-MWCNTs nanofluids. And almost a 100% absorption of the solar irradiation was found on the 0.5 wt% hybrid nanofluid. Specifically, the solar weighted absorption fraction (Am) could describe the percentage of solar energy absorbed by the nanofluids, which was expressed as Eq. (4) [11]. R λmax IAM1:5 ðλÞ½1 expð KðλÞ⋅lÞ�dλ Am ¼ λmin (4) R λmax I ðλÞdλ λmin AM1:5
Fig. 8. The solar weighted absorption fraction of SiC-MWCNTs hybrid nano fluids at different mass concentrations. 7
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Fig. 9. (a) Temperature difference of SiC-MWCNTs nanofluids as a function of irradiation time; (b) The solar-thermal conversion efficiency of SiC-MWCNTs nanofluids at different irradiation time.
To further qualitatively analyze the solar-thermal conversion per formance of the studied hybrid nanofluids, the solar-thermal conversion efficiency was calculated by using Eq. (5) [11].
η¼
mnf Cp;nf ΔT GAΔt
believe that 0.5–1 wt% nanofluids are most suitable for DASCs appli cations. The concentration could be adjusted according to the actual situation. However, the solar-thermal properties of any nanofluid sys tem is extremely complex. This section only provided a limited under standing of the underlying factors affecting solar-thermal properties of SiC-MWCNTs hybrid nanofluids. It should be noted that the thermal conductivity, viscosity, specific heat capacity and frictional pressure drop et al. are also important properties of hybrid nanofluids for DASCs applications, and further quantitative analysis will be the focus of our future work.
(5)
where ΔT is the temperature rise of nanofluids after the irradiation time Δt. G represents the intensity of incident light, and A is the heat col lecting area at the top of the experimental device; mnf and Cp,nf are mass and specific heat capacity of nanofluids, respectively. Considering the additive amount of MWCNTs was very small and many existed papers had proved the effect of MWCNTs could be ignored [54]. It could be further simplified as follows,
η¼
ρb Cp;b LΔT GΔt
4. Conclusion In the present study, the EG-based SiC-MWCNTs hybrid nanofluids were prepared by an improved method based on mechanical milling technique, aiming at enhanced stability and solar-thermal conversion properties. Therefore, the stability, optical properties and solar-thermal conversion performance of the prepared hybrid nanofluids were evalu ated. To our knowledge, this is the first time to focus on the solarthermal properties of SiC-MWCNTs hybrid nanofluids. Through sys tematic research, the following main conclusions could be drawn:
(6)
where L means the height of nanofluids layer and ρb represents the density of EG. The formula was based on two assumptions. First of all, the bottom and surrounding walls of the experimental device had good thermal insulation performance. And the heat loss was mainly caused by the combined action of convection and radiation of the top surface, where was exposed to the surrounding atmosphere. The calculation results of the solar-thermal conversion efficiency are shown in Fig. 9b. The results clearly showed that the η values reached to the maximum at 10 min of solar radiation for all fluids, but then it became less efficient. This was mainly because the increased heat loss during nanofluid temperature rise [14,22]. It should be noted that, although similar radiation intensity was used, the solar-thermal con version efficiency values between different researches were incompa rable, because it could be very different with different receiver height. In the present study, the highest efficiency was found to be 97.3% on 1 wt% hybrid nanofluid at 10 min. However, this data dropped to 72.6% after 60 min irradiation time. In addition, a decrease in the solar-thermal conversion efficiency would occur if the irradiation time increased (e.g. 70 min). This phe nomenon could be explained as follow: in a certain irradiation time, the incident solar radiation occurred complete absorption. And the nano fluids reached to its maximum equilibrium temperature. The increase of adsorption energy led to the increase of surface temperature. And then, the heat lost to the environment increased as well [14]. From what has been discussed above, the EG-based SiC-MWCNTs hybrid nanofluids prepared in this paper have great application value and prospect in DASC as the heat conducting and collecting medium. Considering the properties of SiC-MWCNTs hybrid nanofluids, we
(I) The excellent stability of studied nanofluids were found through static experiment, Zeta potential measurement and Turbiscan LabExpert. In addition, the application potential of studied nanofluids in DASC systems was confirmed by thermal cycling experiments. (II) The hybrid nanofluids displayed an excellent solar irradiation absorption capacity in both visible and near-infrared regions (200–1100 nm). The fact proved that the hybrid could be effec tive working fluids in DASCs, where 0.5 wt% SiC-MWCNTs nanofluids could absorb 99.9% solar energy at only 1 cm path length. (III) The solar-thermal conversion efficiency of hybrid nanofluids increased with the mass concentration. The maximum value of solar-thermal conversion efficiency was found to be 97.3% on 1 wt% SiC-MWCNTs nanofluid at 10 min, which was 48.6% higher than that of pure EG. In conclusion, the promising features of stability, optical properties and solar-thermal conversion efficiency could ensure the SiC-MWCNTs hybrid nanofluids application potential in practical DASC system for different targets. We think 0.5–1 wt% nanofluids are most suitable concentration range for DASCs applications according to this study. And 8
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the concentration could be adjusted according to the actual situation.
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Credit author statement Xiaoke Li: Conceptualization, Methodology, Writing-Original draft preparation, Funding acquisition. Guangyong Zeng: Investigation, Data curation, Writing-Reviewing and Editing. Xinyu Lei: Resources, Investigation, Visualization. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments The authors would like to acknowledge the financial support of Research Foundation of Chengdu University of Technology (109122019KYQD-07545). The authors would like to acknowledge the tech nical support of Ceshigo Research Service Agency (www.ceshigo.com). The first author Dr. Xiaoke Li would like to thank his wife Mrs. Minqi He for her love and support. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.solmat.2019.110323. References [1] D. Coumou, S. Rahmstorf, A decade of weather extremes, Nat. Clim. Chang. 2 (2012) 491–496. [2] W. Park, M. Latif, Ensemble global warming simulations with idealized Antarctic meltwater input, Clim. Dyn. (2018) 1–17. [3] A.N. Al-Shamani, M.H. Yazdi, M.A. Alghoul, A.M. Abed, M.H. Ruslan, S. Mat, K. Sopian, Nanofluids for improved efficiency in cooling solar collectors – a review, Renew. Sustain. Energy Rev. 38 (2014) 348–367. [4] J.E. Minardi, H.N. Chuang, Performance of a “black” liquid flat-plate solar collector, Sol. Energy 17 (1975) 179–183. [5] S.U.S. Choi, Developments and Applications of Non-newtonian Flows, vol. 231, ASME, New York, 1995, pp. 99–102. _ [6] G. Zyła, J.P. Vallejo, J. Fal, L. Lugo, Nanodiamonds – ethylene Glycol nanofluids: experimental investigation of fundamental physical properties, Int. J. Heat Mass Transf. 121 (2018) 1201–1213. [7] E. Bellos, Z. Said, C. Tzivanidis, The use of nanofluids in solar concentrating technologies: a comprehensive review, J. Clean. Prod. 196 (2018) 84–99. [8] M. Karami, M.A. Akhavan-Behabadi, M. Raisee Dehkordi, S. Delfani, Thermooptical properties of copper oxide nanofluids for direct absorption of solar radiation, Sol. Energy Mater. Sol. Cells 144 (2016) 136–142. [9] M. Milanese, G. Colangelo, A. Cretì, M. Lomascolo, F. Iacobazzi, A. de Risi, Optical absorption measurements of oxide nanoparticles for application as nanofluid in direct absorption solar power systems – Part I: water-based nanofluids behavior, Sol. Energy Mater. Sol. Cells 147 (2016) 315–320. [10] W. Chen, C. Zou, X. Li, An investigation into the thermophysical and optical properties of SiC/ionic liquid nanofluid for direct absorption solar collector, Sol. Energy Mater. Sol. Cells 163 (2017) 157–163. [11] W. Chen, C. Zou, X. Li, Application of large-scale prepared MWCNTs nanofluids in solar energy system as volumetric solar absorber, Sol. Energy Mater. Sol. Cells 200 (2019) 109931. [12] C.L.L. Beicker, M. Amjad, E.P. Bandarra Filho, D. Wen, Experimental study of photothermal conversion using gold/water and MWCNT/water nanofluids, Sol. Energy Mater. Sol. Cells 188 (2018) 51–65. [13] X.J. Wang, X.F. Li, Y.H. Xu, D.S. Zhu, Thermal energy storage characteristics of Cu–H2O nanofluids, Energy 78 (2014) 212–217. [14] M. Chen, Y. He, J. Zhu, D. Wen, Investigating the collector efficiency of silver nanofluids based direct absorption solar collectors, Appl. Energy 181 (2016) 65–74. [15] Y. Hu, Y. He, Z. Zhang, D. Wen, Effect of Al 2 O 3 nanoparticle dispersion on the specific heat capacity of a eutectic binary nitrate salt for solar power applications, Energy Convers. Manag. 142 (2017) 366–373. [16] M. Milanese, G. Colangelo, A. Cretì, M. Lomascolo, F. Iacobazzi, A. de Risi, Optical absorption measurements of oxide nanoparticles for application as nanofluid in direct absorption solar power systems – Part II: ZnO, CeO2, Fe2O3 nanoparticles behavior, Sol. Energy Mater. Sol. Cells 147 (2016) 321–326.
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Contents lists available at ScienceDirect
Solar Energy Materials and Solar Cells journal homepage: http://www.elsevier.com/locate/solmat
The stability, optical properties and solar-thermal conversion performance of SiC-MWCNTs hybrid nanofluids for the direct absorption solar collector (DASC) application Xiaoke Li a, *, Guangyong Zeng a, Xinyu Lei b a b
College of Materials and Chemistry & Chemical Engineering, Chengdu University of Technology, Chengdu, 610059, China Organic Chemicals Co., Ltd. Chinese Academy of Sciences, Chengdu, 610041, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: Hybrid nanofluids Solar-thermal conversion DASC Solar energy
Hybrid nanofluids have an advanced application prospect in the community of heat transfer fluids and partic ularly for the direct absorption solar collectors (DASCs). Therefore, stable ethylene glycol-based SiC-MWCNTs nanofluids with mass fractions ranging from 0.01% to 1% were prepared in this study. Combined with the unique properties of these two nanomaterials, the studied hybrid nanofluids displayed excellent stability, optical properties and photothermal conversion properties. The purpose of this paper is to simultaneously achieve the enhanced stability and high solar-thermal conversion efficiency of hybrid nanofluids used for DASC applications. The stability of hybrid nanofluids was confirmed. In addition, the hybrid nanofluids displayed an excellent solar irradiation absorption capacity in both visible and near-infrared regions (200–1100 nm). The fact proved that the hybrid nanofluid was effective working fluid in DASCs, where 0.5 wt% SiC-MWCNTs nanofluids could absorb 99.9% solar energy at only 1 cm path length. In addition, the solar-thermal conversion efficiency of hybrid nanofluids increased with the mass concentration. The maximum value of solar-thermal conversion efficiency was found to be 97.3% on 1 wt% SiC-MWCNTs nanofluid at 10 min, which was 48.6% higher than that of pure EG. The application potentials of SiC-MWCNTs hybrid nanofluids in low-temperature DASCs system were presented.
1. Introduction In the modern society with rapid economic development, fossil fuels such as oil and gas occupy the dominant position of human energy consumption. But a large amount of fossil fuel consumption will inevi tably lead to various environmental pollution problems, such as air pollution, haze, acid rain and greenhouse effect [1,2]. The increase of energy consumption and cost related to fossil fuels, along with the in crease of environmental pollution and the aggravation of greenhouse effect, has changed people’s attitude towards how to produce and use the energy of this planet. Renewable energy refers to a series of inexhaustible and environment-friendly energy sources that can be automatically renew able without human participation. With the decrease of fossil fuel re serves, solar energy has become an important part of human energy use and has been developing continuously. Solar energy is also widely considered to be the most abundant source of permanent energy on
earth [3]. However, how to effectively transfer or store solar energy becomes another focus of the scientific community, both now and in the near future. In 1975, the term of direct absorption solar collector (DASC) was firstly designed and proposed by Minardi and Chuang [4], which was used to absorb the solar radiation or energy directly through heat transfer fluid (HTF). In order to improve the solar radiation absorption capacity of traditional HTFs, such as water, ethylene glycol (EG) or heat thermal oil, the India ink was added into the base fluid. Although solar energy absorption efficiency has been improved to some extent, the suspension had inherent light and temperature instability or degrada tion defects, which limited the efficiency and development of DASC. Therefore, it is known that DASC has two major deficiencies: one is the poor stability of working fluids, and the other is the weak light capture capability. Thanks to the rapid development of nanotechnology, the concept of nanofluids rose in response to the proper time and conditions [5]. Nanofluid is a new type of heat transfer medium which is uniform and
* Corresponding author. No.1, Dongsan Road, Erxianqiao, Chenghua District, Chengdu, 610059, PR China. E-mail address: [email protected] (X. Li). https://doi.org/10.1016/j.solmat.2019.110323 Received 15 August 2019; Received in revised form 3 November 2019; Accepted 27 November 2019 0927-0248/© 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Xiaoke Li, Solar Energy Materials & Solar Cells, https://doi.org/10.1016/j.solmat.2019.110323
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more nanoparticles, which were later developed and termed as “hybrid nanofluids” [31–33]. Most studies have shown that the hybrid nano fluids could offer the possibility of matching solar radiation spectra and subsequently achieving broadband volumetric solar thermal absorption, based on the complementary optical absorption properties of different nanoparticles [34,35]. Qu et al. [36] experimentally prepared the CuO-MWCNTs hybrid nanofluid to directly capture of solar thermal energy. The results revealed that the spectral absorption characteristics of the studied hybrid nanofluids were greatly reinforced. Akilu et al. [37] investigated the thermos-physical properties of glycerol and EG mixture based SiO2–CuO/C hybrid nanofluid and an enhanced thermal conductivity of 26.9% was found. The experimental results indicated the hybrid an attractive HTF for solar energy transportation. Menbari et al. [38] focused on the stability and optical property of CuO–Al2O3 hybrid nanofluids for the direct absorption solar parabolic trough collector (DASPTC). The thermal efficiency of DASPTC system could be effec tively enhanced by increasing nanoparticle loading and nanofluid flow rate. The photo-thermal conversion performance of nanofluids con taining TiO2–Ag with core/shell structures was studied by Xuan and his group [39]. They found that the hybrid nanofluids absorbed light in a wide range from UV to near-infrared. They also prepared hybrid CNT-SiO2/Ag nanofluids for solar-thermal conversion application [40]. At the penetration distance of 1 cm, the 0.005 vol% of hybrid nanofluid gave a solar weighted absorption fraction of 74.5%. Chen et al. [41] chose CuO and antimony doped tin oxide (ATO) nanoparticles to pre pare the water-based hybrid nanofluids. The results showed that CuO-ATO nanofluids had excellent light absorption in both the visible light and near-infrared regions and enhanced photo-thermal conversion property. Specifically, the solar-thermal conversion efficiency of the hybrid nanofluids were estimated to be 92.5%. Wei et al. [42] dispersed the SiC–TiO2 nanoparticles in diathermic oil to fabricate nanofluids and found greater thermal conductivity of hybrid nanofluid than that of single nanofluid. In addition, Valizade et al. [43] conducted the spec trophotometric experiments on the produced porous metal foams and water based nanofluids from two materials namely SiC and CuO. Meh rali et al. [44] focused on the optical properties of graphene/silver hybrid plasmonic nanofluids. A collector efficiency of 77% was found at low concentration of 40 ppm. However, most of these studies used two types of spherical structured nanoparticles, which suffered from the disadvantages of instability, complexity and difficulty in large-scale production. Therefore, the new concept and system of hybrid nano fluids for DASCs applications are highly desired. And responding to this need will be meaningful to drive nanofluids application of solar system into the next stage. In our previous study [10], we found that SiC nanoparticles with spherical structure have considerable absorption in the near-infrared region. On the other hand, the MWCNTs nanofluids were demon strated that they could absorb over the majority of solar spectrum because of the unique properties of carbon nanotubes [22,24,45]. Kar ami et al. focused on the applications of MWCNTs nanofluids in volu metric solar collectors [46]. They found that the extinction coefficient of water-based MWCNTs nanofluids showed considerable improvement compared to the pure water even at low particle concentration. Based on the above facts, it was highly expected that the combination of the two materials will exhibit unique characteristics. Therefore, the SiC nano particles and MWCNTs were chosen to prepare the SiC-MWCNTs hybrid nanofluids in this paper. Considering cost factors, optical properties and stability of nanofluids, the mass ratio of 8:2 (SiC:MWCNTs) was adopted. EG was used as the base fluid due to its relatively wide usage temper ature range and wide application in the field of heat transfer. To our knowledge, there have been few papers studied the stability of hybrid nanofluids for DASCs in depth. However, stability is a prerequisite for the application of all nanofluids in DASC. In addition, it seems that no one has focused on the properties of SiC-MWCNTs nanofluids before. Therefore, the colloidal stability of SiC-MWCNTs nanofluids as the
Nomenclature Symbols m
ρ ζ Am Cp T(λ) K(λ)
Mass [g] Density [g/cm2] Absolute Zeta potential Solar-thermal conversion efficiency [%] Specific heat capacity [KJ/(Kg⋅K)] Transmittance [%] Extinction coefficient [cm 1]
Abbreviations HTF Heat transfer fluid SiC Silicon carbide MWCNTs Multiwalled carbon nanotubes EG Ethylene glycol DASC Direct absorption solar collector Subscripts hybrid Hybrid nanofluids b Base fluid nf Nanofluids
stable by dispersing nanoparticles (<100 nm) into traditional HTFs [6]. Till date, a large number of experiments and theoretical researches have been investigated with nanofluids based DASC systems [7]. The results of these studies showed that the nanofluids could effectively improve the solar-thermal conversion efficiency and work efficiency of solar thermal collector systems. For example, Karami et al. [8] studied the thermos-physical and optical properties of copper oxide (CuO) nanofluid as working fluid in DASC. They found that the absorption of solar energy of CuO nanofluids was about 2 times more than that of the water-EG mixture. Milanese et al. [9] focused on the application of water/Al2O3 nanofluids in the flat-panel solar collectors and a 25% enhancement of the convective heat transfer coefficient was recorded. Chen et al. [10] studied the thermos-physical and optical properties of SiC/ionic liquid nanofluid (Ionanofluids) and the results indicated that the studied nanofluid was a superior solar absorption choice, where the extinction coefficient of 0.03 wt % SiC Ionanofluids could increase about 5.8 cm 1. Most recently, our group designed a new method for preparing multi-walled carbon nanotubes (MWCNTs) nanofluids with high mass fraction [11]. The prepared MWCNTs nanofluids were ideal candidate as working fluids in volumetric solar receiver because nearly 100% inci dent solar energy could be harvested with only 0.01 wt% MWCNTs nanofluids. But the stability was still a challenge for the MWCNTs nanofluids, and the cost of MWCNTs was relatively high for large-scale preparation. In addition, nanofluids containing different types of nanoparticles, such as metallic monomers [12–14], oxides [15–18], and carbon nanomaterials [19–24], have been extensively studied for using in solar energy systems. Therefore, nanofluid is a prominent choice for the working fluid of the direct absorption solar collectors. It is noteworthy that up to now, most of the nanofluids related to the application of DASCs were single-component nanoparticles. However, the single-component nanofluids still have certain disadvantages in some aspects. For instance, although carbon nanotubes have extremely high thermal conductivity and excellent optical properties [25,26], they are relatively expensive and difficult to disperse uniformly [27]. On the other hand, ceramic oxides such as Al2O3, CuO, TiO2 and SiC have good dispersion, but their thermal conductivities are often not high; And the solar absorption capacities are limited due to the absorption spectra of these nanoparticles are mainly in the visible region, while the absorption in the near-infrared region is weak [8,28–30]. To overcome these dif ficulties, the researchers proposed to prepare nanofluids using two or 2
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working fluid for DASC systems was comprehensively determined firstly. In addition, the transmittance, extinction coefficient, solar weighted absorption fraction and solar-thermal conversion efficiency of hybrid nanofluids with different mass concentrations were experimen tally measured, scientifically analyzed and objectively discussed. The purpose of this paper is to simultaneously achieve the enhanced stability and high solar-thermal conversion efficiency of hybrid nanofluids used for DASC applications. It should be pointed out that the preparation method of hybrid nanofluids was different from the well-known two-step method. The straightforward strategy used in the current work was firstly reported by our group [11], which was capable of preparing nanofluids easily in large-scale for solar energy applications. The application of SiC-MWCNTs hybrid nanofluids in DASC is proposed in this paper. Since no one has done any research on the SiC-MWCNTs hybrid nanofluids before, we believe the current study could provide an insight into the understanding of colloidal stability, optical properties and solar-thermal conversion performance of nonmetallic oxides/carbon nanotubes hybrid nanofluids used in DASCs.
Firstly, 1 wt% SiC-MWCNTs powder, 0.5 wt% PVP-K30 and corre sponding EG were mixed in the stirred vessel with 500 rpm. It should be noted that PVP-K30 was selected as dispersant due to its high temper ature resistance, low foamability and low viscosity [47]. Next, the pre-mixed suspension was pumped into the mill chamber and then the nanofluids would flow back into the stirrer vessel after sand milling. The above circulation process was set to run 120 min to disperse the hybrid nanofluids thoroughly. After successive reflux, about 0.8 L SiC-MWCNTs hybrid nanofluids (1 wt%) could be processed at once. Finally, the hybrid nanofluids were diluted to different required concentration by simple ultrasonic. 2.3. Stability tests Firstly, the Turbiscan LabExpert was used to investigate the stability of hybrid nanofluids (1 wt%, 24 h after preparation). The probe head of the instrument has a near infrared light source (λ ¼ 800 nm) and two synchronous detectors that receive the light transmitted through the sample and the backscattered light respectively [48]. The variation of backscattering value was proportional to the variation of particle con centration at each position of the test bottle (55 mm). Then the homo geneity and stability of samples could be characterized. In addition, a Zeta potential measurer (Brookhaven Instruments Corporation, USA) was used to predict long-term suspension stability of hybrid nanofluids with different concentrations (1, 7, 15, 30 days after preparation). The uncertainties of measurements were controlled within �5%. The absolute values of Zeta potential were recorded as the mean value of the five repeated measurements for each sample. The size dis tribution of SiC-MWCNTs in nanofluid was determined by a dynamic light scattering device (DLS, VASCO, France) and the measurement range was from 10 to 4000 nm. To further verify the application potential of SiC-MWCNTs hybrid nanofluids in DASC system, the thermal cycling tests were carried out. The nanofluids samples were first added into several borosilicate glass tubes. Then these tubes were heated from room temperature to 60 � C by a thermostatic water bath, and keeping the temperature for another 30 min. The sample was then cooled to room temperature by natural con vection, and the absorbance of sample was measured by UV–vis spec trophotometer (UV3802,UNICO, China) at 980 nm subsequently. The above procedure was defined as one cycle, and each sample was repeatedly performed for thirty cycles in three days. After every 10 cy cles, the samples were placed at room temperature overnight. The following day, all samples will be subjected to 30 min of ultrasonic treatment to break up the soft aggregates. The samples without ultra sonic treatment were analyzed according to the same procedure as control experiments.
2. Material and methods 2.1. Materials and preparation of binary nanoparticles Multi-walled carbon nanotubes (diameter of 20 nm, purity >95%) were purchased from Chengdu Organic Chemistry Co., Ltd., Chinese Academy of Sciences (Chengdu, China). The SiC nanoparticles (diameter of 40 nm) were provided by C.W. Nanotechnology Company (Shanghai, China). Theirs basic properties provided by the sellers are listed in Table 1. The base fluid EG, the surfactants polyvinyl pyrrolidone (PVP–K30) and hexane were purchased from KeLong Chemical Reagent Factory (Chengdu, China). The preparation process of binary nanoparticles was as follows. Firstly, the SiC and MWCNTs powders were weighted on an electronic balance (mass ratio ¼ 8:2) and then moved into a beaker containing hexane (200 mL). Subsequently, a magnetic stirrer was employed to mix the suspension for 20 min. Then the solution was subjected to ultrasonication homogenizer Sonifier 250 (Branson Ultrasonics, Danbury, USA) for 1 h. Finally, the powders collected by centrifugation were dried for 10 h in an oven at 65 � C. The dried binary nanoparticles were then ground to get the fine target product –– SiC-MWCNTs binary nano particles. The morphologies of SiC nanoparticles, MWCNTs and SiCMWCNTs binary nanoparticles were characterised by scanning elec tron microscopy (SEM, FEI, USA) (See Fig. 1). It could be observed that the morphology of SiC nanoparticles was nearly spherical, while that of MWCNTs was tubular. On the other hand, the dry SiC-MWCNTs binary nanoparticles were in a state of massive aggregation. In addition, nanoSiC and MWCNTs had formed “point-line” network structure.
2.4. Optical absorption properties of hybrid nanofluids
2.2. Preparation of hybrid nanofluids
The UV–vis spectrophotometer was used to measure the optical transmittance spectra of SiC-MWCNTs nanofluids in a wavelength range from 200 to 1100 nm with accuracy of �1 nm. The 10 mm quartz cu vettes were used to hold the nanofluid samples at room temperature (25 � 0.2 � C). The spectral transmittance of base fluid (EG) or nanofluids was measured against the standard reference (air) and during the measurements one cuvette was filled with base fluid (EG) or nanofluids and other cuvette was left empty (filled with air). Three times of mea surements were repeated for each sample to avoid the systematic errors of multiple reflection at the interfaces between liquid, glass and air. In addition, according to the Beer-Lambert law, the extinction coefficient (K(λ)) of hybrid nanofluids was calculated by Eq. (1) [23]:
In this part, a stirred media mill (PHN Labstar 0.3CA, PUHLER Smart Nano Tech Co., Ltd, China) was employed to prepare the EG-based SiCMWCNTs hybrid nanofluids. The schematic diagram of this equipment is shown in Fig. 2 and the details of it could be found in our former study [11]. Table 1 The basic properties of SiC nanoparticles and MWCNTs. Sample
Diameter
Length
Purity
Specific surface area
Density
MWCNTs
20 nm
~30 μm
254.87 m2/g
SiC
40 nm
/
>95 wt % 99.9 wt %
2.1 g/ cm3 3.2 g/ cm3
39.8 m2/g
TðλÞ ¼ expð KðλÞ ⋅ HÞ
(1)
where T(λ) means the transmittance, H presents the light path (10 mm). 3
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Fig. 1. Scanning electron microscopy image of (a) SiC nanoparticles; (b)MWCNTs; (C) SiC-MWCNTs binary nanoparticles.
Fig. 2. Schematic diagram of the stirred media mill in the preparation of hybrid nanofluids.
2.5. Solar-thermal conversion performance of hybrid nanofluids
self-assembled by our group and the schematic diagram of the experi mental setup is shown as Fig. 3. Firstly, the aluminized paper was used to paste around and at the bottom of the flat bottom beakers (50 mL) to effectively reduce the absorption of light. Then, same amounts of SiCMWCNTs hybrid nanofluids with different mass concentration were added into the beakers. Shortly afterwards, the beaker was enclosed in an adiabatic foam block (thermal conductivity�0.036 (W/(m⋅K)) to minimize the heat loss during the experiment. A solar simulator (SOLAREGDE 700, Perfectlight Technology Co., Ltd., Beijing) was used as the radiative source, which matched with the AM1.5 spectrum well. The flux of incident radiation to the surface of the beaker was measured accurately by a solar radiation meter (HD2302, Delta-ohm, Italy) with accuracy of 0.9%. Three thermocouples (WRP-S, Feilong Instrument Co., Ltd., Shanghai) were fixed to the inner wall of the experimental device to measure the temperature of each point inside the nanofluid during heating. Another thermocouple was placed outside the device for ambient temperature measurement. The maximum measuring temper ature of thermocouple was 1800 � C, and the measuring error was 1.5 � C. All temperature measurements were performed by a data acquisition (34970 A, Agilent) with a time interval of 5 s. The ambient temperature was 20 � 1 � C. In the experiment, when the sample temperature did not change within 10 min, the light source was turned off. The uncertainties of the experiment resulted from the measurement
The solar-thermal conversion performance experimental setup was
Fig. 3. Schematic diagram of solar-thermal conversion performance experi mental setup. 4
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error of thermocouples (�1.5 � C) and solar radiation meter (�0.9%). Therefore, the uncertainty of the experiments could be expressed as Eq. (2) [49]: sffi� ffiffiffiffiffiffiffiffiffiffi� ffiffiffiffiffiffiffiffiffiffiffi� ffiffiffiffiffiffiffiffiffiffiffi� ffiffiffiffiffiffiffiffiffiffiffi� ffiffiffiffiffiffiffiffiffiffiffi� ffiffiffiffiffiffiffiffiffiffiffi� ffiffiffiffiffiffiffiffiffiffi� ffiffiffiffiffi 2 2 2 2 δT1 δT2 δT3 δqi ER ¼ (2) þ þ þ jT1 j jT2 j jT3 j jqi j
nanofluids was measured at different mass concentrations and the cor responding results could be found in Fig. 5. The Fig. 5a clearly showed that the ζ value decreased slightly with the mass concentration at room temperature, while that of all the nanofluids decreased with the increase of standing time. However, even after 30 days of storage, the nanofluids remained in a relatively stable state, because in general, the Zeta po tential greater than 45 mV means that the system was in an excellent stable state [51]. Considering the applications in DASC system, nano fluids are always affected by different temperature conditions. Hence, the ζ values of the fresh hybrid nanofluids at different temperature were also determined. According to Fig. 5b, temperature had obvious side effects on the stability of hybrid nanofluids. Specifically, the absolute ζ value of 1 wt% SiC-MWCNTs nanofluids decreased from 52.7 mV at 20 � C to 40.2 mV at 60 � C. The main reason of this phenomenon was that the high temperature will increase the probability and strength of the collision of nanoparticles and reduce the effect of dispersant, thus affecting the electrostatic balance of the suspension system. In addition, it was clear that the mean nanoparticle size in hybrid nanofluids (1 wt%) was around 217 nm (See Fig. 5c) after preparation, which was larger than that of primary particles, evincing the formation of clusters in the nanofluid. The average cluster size was found to be almost constant (around 221 nm) after 30 days of re-evaluation. The experimental re sults were in good agreement with Zeta potential experiment. In order to further verify the stability of SiC-MWCNTs hybrid nanofluids in DASC applications in this paper, the thermal cycling ex periments were conducted and Fig. 6 shows the experimental result. In this part, the relative absorbance (RA) was introduced as an evaluation index, which was defined as the ratio of the absorbance of heated SiCMWCNTs nanofluids to the initial absorbance of fresh hybrid nano fluids. Taken altogether, the RA values decreased with the number of heating circles. Only a 7.07% of decrement was found on the RA value of 1 wt% hybrid nanofluids without ultrasonic treatment after 30 cycles of thermal cycling. This indicated that the SiC-MWCNTs hybrid nanofluids owned application potential due to the long-term stability, which was in agreement with the experimental results of Zeta potential. On the other hand, significant fluctuations could be observed for each sample after every 10 heating cycles, but this trend could be easily reversed by ul trasonic treatment. Through the repeated heating would put a side effect
where the T1, T2 and T3 mean the measured temperature by thermo couples; the qi is the flux of incident radiation. According to the calibration analysis of the experimental measuring property for the measuring devices used, the resulting uncertainty value was �2.75%, which indicates that an acceptable and reliable engi neering border was achieved in the conducted tests. 3. Results and discussion 3.1. Stability It is well known that colloidal stability is a necessary and sufficient condition for the applications of nanofluids in solar energy systems [50]. The fresh samples with different mass concentrations are exhibited in Fig. 4a. The static settlement experiment, which is the most intuitive way to observe the stability of nanofluids, indicated that no visible stratification was found even a month later (See Fig. 4b). There were only some black flocs at the bottom of the bottle with high concentration of nanofluids (1 wt%). On the other hand, the stability of SiC-MWCNTs hybrid nanofluids (1 wt%, maximum concentration in this study) was measured for 30 min at 25 � C by the Turbiscan LabExpert. Fig. 4c shows the delta backscat tering (ΔBS) during experiment. As shown, only 0.2% of ΔBS was found at the bottom of test bottle (0–3 mm), while in other positions this fluctuation was practically negligible. The experimental data suggested that only a few nanoparticles were deposited at the bottom, while the remaining solid phase was quite stable in the hybrid nanofluids. Since the stability of colloidal systems is mainly the result of the interaction between particles and particle-fluid interfaces over time, the stability of EG-based SiC-MWCNTs nanofluids is closely related with their surface charge density. Therefore, the Zeta potential (ζ) of studied
Fig. 4. Sedimentation experiments at different standing time: (a) One day after preparation; (b) One month after preparation. (c) Delta backscattering of SiCMWCNTs nanofluid at 1 wt%. 5
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Fig. 5. (a) Zeta potential of SiC-MWCNTs nanofluids as a function of standing time; (b) as a function of temperature; (c) Size distribution of 1 wt% hybrid nanofluids at different standing time.
SiC-MWCNTs hybrid nanofluids are shown in Fig. 7. The transmittance spectra of hybrid nanofluids is exhibited in Fig. 7a. The spectral peaks found at about 920 nm and 980 nm were associated with the absorption from base fluid EG Ref. [52]. In general, the transmittance of hybrid nanofluids decreased significantly with the increase of powder loading. Even if the content of nanoparticles was only 0.01 wt%, the transmittance of ethylene glycol could be reduced by about 45%. In addition, the transmittance of 0.5 wt% object almost fell to zero on the tested wave length range and the sample with higher concentration (1 wt%) was completely black, which was below the detection limit of the UV–vis spectrophotometer. In conclusion, in order to maintain a transmittance of less than 5% at all wavelengths, the concentration of the bicomponent nanofluid should only be higher than 0.25 wt%. In addition, the K(λ) of SiC-MWCNTs hybrid nanofluids is shown in Fig. 7b, and the gray shadow represents the total incident solar irradiance at air mass 1.5 according to the ASTM G173-03. The exper imental results showed that the extinction coefficient increased with the mass concentration of hybrid nanofluids. For 0.5 wt% SiC-MWCNTs nanofluid, the average extinction coefficient was close to 9 cm 1 in the tested spectral range. However, the average extinction coefficient of pure EG was below 1.0 cm 1. Furthermore, the studied hybrid nano fluids seemed to be a more effective sunlight absorbing fluid than the CuO-ATO nanofluids [41], whose extinction coefficient was about 7 cm 1 on average for 0.12 vol%. In addition, to clarify the application prospect of SiC-MWCNTs hybrid nanofluids, the absorbed solar irradiance of hybrid nanofluids was calculated based on Eq. (3) [29] and the results are listed in Fig. 7c.
Fig. 6. The relative absorbance of SiC-MWCNTs nanofluids in different mass concentration as a function of thermal cycles.
on the stability of nanofluids inevitably, easy and necessary adjustments could be made according to the actual situation in the DASC applications. In conclusion, the SiC-MWCNTs hybrid nanofluids have satisfactory colloidal stability and potential applications in DASC systems.
IA ðλÞ ¼ IAM1:5 ðλÞ � ½1
3.2. Optical properties
TðλÞ�
(3)
where IA ðλÞ is the absorbed solar irradiance of hybrid nanofluids; IAM1:5 ðλÞ means the incident solar irradiance at air mass 1.5. Similarly, the gray shadow means the total incident solar irradiance
Absorption performance of working fluid is a key point in the application of solar collector system. Herein, the optical properties of 6
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where l is the penetration distance. In the DASCs systems, a thin layer of working fluid (thickness of nanofluid layer in this case) would act as a surface absorber, where high surface temperatures and radiation losses will result in heat loss to the environment. In addition, if the collector height is not high enough, the solar energy cannot be effectively absorbed by the nanofluids, which greatly reduces the solar energy conversion efficiency. Therefore, it is necessary to optimize Am in en gineering practice until the working fluid achieves efficiency optimiza tion in DASC system. As displayed in Fig. 8, the SiC-MWCNTs hybrid nanofluids could greatly enhance the Am of pure EG and all hybrid nanofluids could absorb the incoming solar energy completely at 10 cm path length. For the hybrid nanofluid with 0.5 wt%, the Am could reach 99.9% or more at only 1 cm path length. This means that a thin layer of hybrid nanofluids can achieve high efficiency, rather than volumetrically by the HTF medium. On the basis of good stability, the SiC-MWCNTs hybrid nano fluids could be an ideal working fluid in the application of DASC systems. 3.3. Solar-thermal conversion performance The solar-thermal properties of nanofluids depend to a great extent on the relationship between the temperature change of nanofluids and solar radiation time. The bulk temperature change curves of SiCMWCNTs hybrid nanofluids with different mass concentrations were plotted in Fig. 9a. Obviously, the solar-thermal conversion performance of pure EG was significantly improved after the addition of SiC-MWCNTs nanoparticles. The temperature of all fluids increased with the irradia tion time, but the temperature rise rate and the maximum equilibrium temperature of the nanofluids were higher than those of pure EG. For example, the highest temperature difference of hybrid nanofluids with 0.1 wt% was 107.1 � C (irradiation time was 50 min), whose temperature rise rate was 2.142 � C/min. This was about 31.5% faster than that of base fluid. In addition, the maximum equilibrium temperature differ ence of hybrid nanofluids (1 wt%) was about 32 � C higher than that of pure EG at the same solar radiation time. Similar trend was reported on water-based CuO/MWCNTs hybrid nanofluids [36] and water-based Al2O3–Co3O4 hybrid nanofluids [53]. Therefore, the solar-thermal con version performance of base fluid (EG in this paper) could be signifi cantly improved by adding a small amount of hybrid nanoparticles. However, the SiC-MWCNTs hybrid nanofluids exhibited even better solar-thermal properties, which may be attributed to the enhancement of stability as discussed in Section 3.1.
Fig. 7. The (a) transmittance, (b) extinction coefficient and (c) the absorbed solar irradiance of SiC-MWCNTs hybrid nanofluids at different mass concentrations.
in Fig. 7c. The data in the figure indicated that most of the solar irra diation had been absorbed by SiC-MWCNTs nanofluids. And almost a 100% absorption of the solar irradiation was found on the 0.5 wt% hybrid nanofluid. Specifically, the solar weighted absorption fraction (Am) could describe the percentage of solar energy absorbed by the nanofluids, which was expressed as Eq. (4) [11]. R λmax IAM1:5 ðλÞ½1 expð KðλÞ⋅lÞ�dλ Am ¼ λmin (4) R λmax I ðλÞdλ λmin AM1:5
Fig. 8. The solar weighted absorption fraction of SiC-MWCNTs hybrid nano fluids at different mass concentrations. 7
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Fig. 9. (a) Temperature difference of SiC-MWCNTs nanofluids as a function of irradiation time; (b) The solar-thermal conversion efficiency of SiC-MWCNTs nanofluids at different irradiation time.
To further qualitatively analyze the solar-thermal conversion per formance of the studied hybrid nanofluids, the solar-thermal conversion efficiency was calculated by using Eq. (5) [11].
η¼
mnf Cp;nf ΔT GAΔt
believe that 0.5–1 wt% nanofluids are most suitable for DASCs appli cations. The concentration could be adjusted according to the actual situation. However, the solar-thermal properties of any nanofluid sys tem is extremely complex. This section only provided a limited under standing of the underlying factors affecting solar-thermal properties of SiC-MWCNTs hybrid nanofluids. It should be noted that the thermal conductivity, viscosity, specific heat capacity and frictional pressure drop et al. are also important properties of hybrid nanofluids for DASCs applications, and further quantitative analysis will be the focus of our future work.
(5)
where ΔT is the temperature rise of nanofluids after the irradiation time Δt. G represents the intensity of incident light, and A is the heat col lecting area at the top of the experimental device; mnf and Cp,nf are mass and specific heat capacity of nanofluids, respectively. Considering the additive amount of MWCNTs was very small and many existed papers had proved the effect of MWCNTs could be ignored [54]. It could be further simplified as follows,
η¼
ρb Cp;b LΔT GΔt
4. Conclusion In the present study, the EG-based SiC-MWCNTs hybrid nanofluids were prepared by an improved method based on mechanical milling technique, aiming at enhanced stability and solar-thermal conversion properties. Therefore, the stability, optical properties and solar-thermal conversion performance of the prepared hybrid nanofluids were evalu ated. To our knowledge, this is the first time to focus on the solarthermal properties of SiC-MWCNTs hybrid nanofluids. Through sys tematic research, the following main conclusions could be drawn:
(6)
where L means the height of nanofluids layer and ρb represents the density of EG. The formula was based on two assumptions. First of all, the bottom and surrounding walls of the experimental device had good thermal insulation performance. And the heat loss was mainly caused by the combined action of convection and radiation of the top surface, where was exposed to the surrounding atmosphere. The calculation results of the solar-thermal conversion efficiency are shown in Fig. 9b. The results clearly showed that the η values reached to the maximum at 10 min of solar radiation for all fluids, but then it became less efficient. This was mainly because the increased heat loss during nanofluid temperature rise [14,22]. It should be noted that, although similar radiation intensity was used, the solar-thermal con version efficiency values between different researches were incompa rable, because it could be very different with different receiver height. In the present study, the highest efficiency was found to be 97.3% on 1 wt% hybrid nanofluid at 10 min. However, this data dropped to 72.6% after 60 min irradiation time. In addition, a decrease in the solar-thermal conversion efficiency would occur if the irradiation time increased (e.g. 70 min). This phe nomenon could be explained as follow: in a certain irradiation time, the incident solar radiation occurred complete absorption. And the nano fluids reached to its maximum equilibrium temperature. The increase of adsorption energy led to the increase of surface temperature. And then, the heat lost to the environment increased as well [14]. From what has been discussed above, the EG-based SiC-MWCNTs hybrid nanofluids prepared in this paper have great application value and prospect in DASC as the heat conducting and collecting medium. Considering the properties of SiC-MWCNTs hybrid nanofluids, we
(I) The excellent stability of studied nanofluids were found through static experiment, Zeta potential measurement and Turbiscan LabExpert. In addition, the application potential of studied nanofluids in DASC systems was confirmed by thermal cycling experiments. (II) The hybrid nanofluids displayed an excellent solar irradiation absorption capacity in both visible and near-infrared regions (200–1100 nm). The fact proved that the hybrid could be effec tive working fluids in DASCs, where 0.5 wt% SiC-MWCNTs nanofluids could absorb 99.9% solar energy at only 1 cm path length. (III) The solar-thermal conversion efficiency of hybrid nanofluids increased with the mass concentration. The maximum value of solar-thermal conversion efficiency was found to be 97.3% on 1 wt% SiC-MWCNTs nanofluid at 10 min, which was 48.6% higher than that of pure EG. In conclusion, the promising features of stability, optical properties and solar-thermal conversion efficiency could ensure the SiC-MWCNTs hybrid nanofluids application potential in practical DASC system for different targets. We think 0.5–1 wt% nanofluids are most suitable concentration range for DASCs applications according to this study. And 8
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the concentration could be adjusted according to the actual situation.
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