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Effect of sonication time on the evaporation rate of seawater containing a nanocomposite
Journal Pre-proofs Effect of sonication time on the evaporation rate of seawater containing a nanocomposite Mohammad Mustafa Ghafurian, Zohreh Akbari, Hamid Niazmand, Roya Mehrkhah, Somchai Wongwises, Omid Mahian PII: DOI: Reference:
S1350-4177(19)31167-8 https://doi.org/10.1016/j.ultsonch.2019.104817 ULTSON 104817
To appear in:
Ultrasonics Sonochemistry
Received Date: Revised Date: Accepted Date:
25 July 2019 15 September 2019 30 September 2019
Please cite this article as: M.M. Ghafurian, Z. Akbari, H. Niazmand, R. Mehrkhah, S. Wongwises, O. Mahian, Effect of sonication time on the evaporation rate of seawater containing a nanocomposite, Ultrasonics Sonochemistry (2019), doi: https://doi.org/10.1016/j.ultsonch.2019.104817
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© 2019 Published by Elsevier B.V.
Effect of sonication time on the evaporation rate of seawater containing a nanocomposite
Mohammad Mustafa Ghafuriana,b, Zohreh Akbarib, Hamid Niazmandb*, Roya Mehrkhahc, Somchai Wongwisesd, Omid Mahiana* School of Chemical Engineering and Technology, Xi'an Jiaotong University, Xi'an, Shaanxi 710049, China
a
b
Department of Mechanical Engineering, Ferdowsi University of Mashhad, Mashhad, Iran cDepartment
dFluid
of Chemistry, Ferdowsi University of Mashhad, Mashhad, Iran
Mechanics, Thermal Engineering and Multiphase Flow Research Lab (FUTURE), Faculty of
Engineering, Department of Mechanical Engineering, King Mongkuts University of Technology Thonburi, Bangmod, Bangkok 10140, Thailand
1
Abstract Sonication time has a significant contribution to the stability and properties of nanofluids (mixtures of nanoparticles and a base fluid). Finding the optimum sonication time can help to save energy and ensure optimal design. The present study deals with the sonication time effect on the evaporation rate of seawater containing a nanocomposite (i.e., a mixture of multi-walled carbon nanotubes and graphene nanoplates). For indoor experiments, a solar simulator was employed as the radiation source. At first, the nanofluid with a concentration of 0.01% wt. was sonicated in an ultrasonic bath (40 kHz frequency and 200 W power) for different times of 30, 60, 90, 120, 180, 240 min, and the associated zeta potential values were recorded to evaluate the stability. Next, the best time function was used to appraise the effect of concentration variations (0.001, 0.002, 0.004, 0.01, 0.02 and 0.04% wt.) and the light intensities (1.6, 2.6, and 3.6 suns) on the rate of solar steam generation. The results indicate that for a concentration of 0.01% wt. and under 3.6 suns, the highest evaporation efficiency of 61.3% would be achieved at 120 min sonication time. Keywords: Stability, Ultrasonic time, Composite nanofluid, Solar evaporation
2
1. Introduction Nanofluids are considered suspensions formed by the dispersion of solid nanoparticles (NPs) in a base fluid. From the practical viewpoint, the base fluid is usually a kind of conventional liquids i.e. water [1,2], oil [3], or ethylene glycol [4]. The literature shows that the base fluid properties, notably thermal [5], electrical [6], and optical ones [7] are affected when NPs are added to it. Therefore, nanofluids are a good candidate for application in various fields such as automobile industries, energy conversion and storage, solar systems, and biomedical applications [8-10]. One of the most important indices in the study of nanofluids is stability level. Because the NPs tend to adhere to each other and aggregate due to Van der Waals forces [11], the stability is highly dependent on methods of preparing the nanofluid, nanoparticle characteristics, base fluid type, surfactant, pH, and ultrasonic parameters [12]. Generally, to improve the nanofluid stability, there are two physical and chemical methods by which the surface properties of the NPs are modified. The chemical methods include surfactant addition [13], pH adjustment [14], and functionalization of NPs [12,13]. The physical methods include the use of a homogenizer [12] and force exerted on NPs by ultrasonic waves [15] to break the intermolecular bonds between NPs. On occasion, both methods (physical and chemical) are simultaneously utilized for uniform dispersion of NPs in the base fluid. The stability level of nanofluids is deeply affected by the nature of the NPs and base fluid. The NPs are divided into hydrophilic and hydrophobic groups. Hydrophilic NPs are usually stable in polar fluids (like water), whereas hydrophobic ones are stable in non-polar fluids (like oils). Given the conditions, there is no need to add an additive to stabilize the nanofluid. What is required to prepare a stable nanofluid composed of hydrophobic NPs in the polar base fluid and vice versa is the addition of a stabilizer, for example, a surfactant. In fact, as a linking agent, the surfactant establishes intermolecular bonds between the NPs and base fluid molecules [12,14]. Several researchers have investigated the surfactant effect on the stability and properties of the nanofluid. For example, Jong Choi et al. [16], Zareei et al. [11] and Soltanimehr and Afrand [16], experimentally investigated the effects of different surfactants on the stability nanofluids consisting of carbon nanotubes [16], aluminum oxide (Al2O3) [11] and MWCNT [16]. Their results indicated although the use of a surfactant is considered an economical and straightforward method to achieve desirable stability, it causes to make a resistance between NPs and base fluid, resulting in a change in NP properties, particularly thermal and optical ones. To tackle this problem, NPs are modified, thereby functionalizing their 3
surface. By this process, covalent bonds of the particles are destroyed and an acidic or alkali agent is attached to the NP surface, causing NPs to separate from each other, followed by improvement of the nanofluid stability. This method is widely employed in stabilizing carbon nanotubes (CNTs) [17,18]. One of the methods applied to assess the nanofluid stability is pH adjustment [19], which is deemed another chemical method for stabilizing the nanofluid in some references [12,14]. Firstly, it is necessary to define the isoelectric point to explain the pH effect. The isoelectric point is the point at which the electrical surface charge of a particle is zero. The pure charge of the nanofluid is strongly influenced by the nanofluid pH so that under various pH values, it might become negative or positive. At the isoelectric point, the suspension is completely unstable and the value of the zeta potential parameter is zero [12]. As the suspension pH gets farther away from the isoelectric point pH, the stability of the nanofluid will increase. Researchers such as Zareei [11] and Khairul [20] have studied this parameter in detail for aluminum oxide [10,19] and copper oxide NPs. In addition to chemical approaches, there are physical methods to stabilize nanofluids. One of these methods is the use of ultrasonic waves in which a high frequency is produced in the nanofluid by an ultrasonic probe (direct method) [15], ultrasonic bath (indirect method) [21], or high-pressure homogenizer [12]. There are some studies in the literature on the effect of ultrasonic waves on the stability and characteristics of nanofluids. The literature shows that the stability of some nanofluids such as water-based Mg(OH)2 in the absence of a surfactant [22] and water-based TiO2 nanofluids in the presence of anionic surfactant SDS [23] is enhanced with an increase in ultrasonic time. Other researchers investigated the effect of sonication time on nanofluid stability and found that there is an optimal time for sonication. Before the optimal time, the stability has an ascending trend with sonication time, and after that point, it follows the opposite trend. This was true for titanium oxide/water nanofluids in the absence of a surfactant [15], Cu/EG nanofluid [4], aluminum oxide/water nanofluid [24], and water-based nanofluid of magnesium hydroxide[22] in the presence of a surfactant. It has also been mentioned that increasing the ultrasonic time provides the force required to separate the NPs from each other, causing the NPs to disperse within the base fluid and their stability to increase. When the ultrasonic time is longer than the optimal time, the probability of NPs colliding with each other is 4
very high, triggering the Van der Waals attraction force between the NPs, and as a result, the particles are absorbed into each other once again. Under the mentioned circumstances, aggregation occurs in the solution. Because there is a direct relationship between the nanofluid stability and its thermophysical properties, the researchers investigated how an improvement in thermophysical properties affects the stability of the nanofluid. For example, Ghadimi et al. [23] and Siddiqui et al. [25] studied the influence of the ultrasonic time on TiO2[23] and Cu-Al2O3 composite [25] nanofluid stability and reported that the nanofluid thermal conductivity increased with an improvement in the stability. Because nanofluid properties such as thermal ones are strongly influenced by its stability, it is expected that this parameter also affects optical properties such as light absorption. Being at the advantage of light absorbance makes the nanofluid a suitable option for solar systems, particularly desalinating and direct steamgenerating ones, which results in a direct relationship between the nanofluid stability and amount of the steam produced. Up to now, many studies [26–31] have been carried out on nanofluid utilization in solar steam production, but researchers have yet to examine the effect of the nanofluid stability, ultrasonic time, and pH adjustment on the solar evaporation. For instance, gold (Au) nanofluids were synthesized via several methods, such as a one-step method based on a modified thermal citrate reduction method by Amjad et al. [32], a seed-growing method with some modifications by Guo et al. [33], and a one-step method by Wang et al. [34], and relying on the fact that the surface plasmonic resonance phenomenon improves photothermal conversion, these researchers investigated the effect of Au nanofluid on solar steam generation. Their results showed that due to the high price of gold NPs, it is no longer economic to utilize them in solar steam generation. Therefore, the researchers proposed carbon materials due to their high light absorption and reasonable price[35]. Accordingly, Wang et al. [36] employed a single-walled CNT nanofluid as an absorber for solar steam generation synthesized through a two-step method without addition of any surfactants for 2 hours of ultrasonic time. They succeeded in achieving 46.8% efficiency at 10 suns light intensity for a 19.04× 10-4 vol% concentration. Another effective method for generating solar steam that has recently received the attention of researchers is the use of composite nanofluids to enhance light absorption. Neumann et al. [27], Fu et al. [28], Wang et al. [29], Shi et al. [30], and Chen et al. [31] utilized nanofluids of silicium oxide/gold nanoshells (SiO2/Au) [27], graphene oxide/gold (GO/Au) [28], RGO @ Fe3O4 [29],
5
and Fe3O4 @ CNT [30,31], respectively, for the solar steam generation. They showed that the energy absorbed by the composite nanofluids improves the evaporation rate. Despite the extensive studies on solar steam production, the effects of ultrasonic time on evaporation rate have not been investigated yet [37]. Ultrasound is known as a primary source for stimulating reactions between materials [38]. Ultrasound can be used for enhancing nanofluids stability and it is expected that by optimizing the sonication time, not only the evaporation efficiency of nanofluids improves but also the cost of steam generation decreases. Therefore, in the present study, the effect of sonication time on evaporation rate is studied, for the first time. A nanocomposite (i.e., a compound of graphene nanoplates and carbon nanotubes [GNP@MWCNT]) with different concentrations (0.001, 0.002, 0.004, 0.01, 0.02 and 0.04% wt. )is suspended in seawater. Different sonication times (30, 60, 90, 120, 180 and 240 min) are applied to the samples, and then the treated nanofluids are inserted under irradiation intensities ranging from 1.6 to 3.6 suns. The main aim of the study is to find the optimal sonication time for GNP@MWCNT nanofluid to achieve the maximum evaporation rate. 2. Experimental Section (Materials and methods) To examine the effect of the ultrasonic time on evaporation rate, a composite-based nanofluid containing multi-walled carbon nanotubes/graphene nanoplates (GNP@MWCNT) is utilized in this research. In the following, the specification of the nanofluid, and experimental setup will be discussed. 2.1 Nanomaterial characteristics and nanofluid preparation To prepare the nanofluids, the graphene nanoplates with a diameter ranging from 1 to 20 μm, a thickness less than 40 nm, and a purity of 99.5%, along with MWCNTs with a diameter ranging from 20 nm to 30 nm, a length ranging from 5 to 10 µm, and a purity greater than 95% were purchased from the Vira Carbon Nanomaterial (VCN) company. To increase the stability, Arabic gum was used as the surfactant (purchased from Sigma-Aldrich Company). To synthesize the composite nanofluid using Arabic gum surfactant, the ratio of the graphene nanoplates to multiwalled carbon nanotubes was chosen as 1:1. To obtain initial stability, all nanofluids were placed in an ultrasonic bath with 200 W power and 40 kHz frequency time 30 min. Fig. 1 shows the nanofluids used in the experiments with certain concentrations compared to the seawater. TEM and SEM images of nanomaterials, taken by a LEO-150VP and LEO-1450VP, are also shown in 6
this figure. It should be noted that in the present study, the energy of the ultrasonic cannot decompose the GNP@MWCNT nanocomposite (see Fig. A.1 in appendix).
Seawater
GNP @ MWCNT 0.001% wt.
GNP @ MWCNT 0.002% wt.
GNP @ MWCNT 0.01% wt.
a
200nm
b
c
80nm
d
125nm
Fig. 1. (a) GNP@MWCNT/water samples with different concentrations in comparison with
7
the seawater; SEM image of (b) GNP@MWCNT and TEM image of (c) MWCNT (d) GNP.
2.2 Experimental Setup Figure 2 shows the experimental system designed for the present study. As shown, a solar simulator manufactured by Nanosat Co. Iran, IIIS-310 model, with a 1600 W xenon lamp and 6000 K radiation temperature, was employed for artificial radiation production. A glass beaker 70 mm in height, 38 mm in diameter, and with a plastic insulator around it was used as a solar receiver. It contained the nanofluid or the seawater. Three PT100 temperature sensors placed at heights of 10, 30, and 50 mm from the bottom of the beaker were utilized to measure the rise in fluid temperature during the experiments. The upper part of these sensors that was exposed to the radiation was whitened with a silicone coating layer to avoid light absorption and unwanted increase in temperature. To measure the evaporation rate, an accurate digital balance with a precision of 0.01 grams constructed by Kern Co., Germany, PFB2000-2 model, was used. Furthermore, a CMP3 secondary standard pyranometer with accuracy of 1w.m-2 in a wavelength range of 200-2800 nm, manufactured by Kipp & Zonen Co., was employed to adjust the light intensity in the experiment. At each stage of the test, the beaker was completely filled with the nanofluid and exposed to the light radiation for 30 min. All sensors and balance data were sent to a computer using a data logger system. It is worth noting that at the beginning of the test, the ambient temperature was 19-21°C, the relative humidity of the environment was 19%, and the air pressure was recorded as 0.9 bar.
8
Fig. 2. Experiment setup including a solar simulator, digital balance, temperature sensors, container, data logger and computer for solar steam generation.
3. Results and Discussion To investigate the ultrasonic time effect on the nanofluid stability, light absorption, and vapor generation, first, we placed a nanofluid with a 0.01% wt. concentration into the ultrasonic bath with 40 kHz frequency and 200 W power for 30, 60, 90, 120, 180 and 240 min. The results of these studies are presented in the next sections. 3.1 Study of GNP@MWCNT nanofluid stability To investigate the nanofluid stability, the zeta potential was measured using the Zeta Compact device for different samples, and the results are presented in Fig. 3 (a). As indicated, the pH of the samples produced was recorded as about 7-8 and grew gradually with the increase of the ultrasonic time by 120 min. The pH values changed directly with the variation of the surface properties and, in particular, the level of the NP surface charges. By increasing the ultrasonic time by 120 min, the pH of the samples probably got further away from the pH at the isoelectric 9
point, resulting in a rise of the repulsive force between the NPs. This caused the NPs to separate from each other and the dispersion of NPs to improve in the base fluid. This result was also found in reference [39]. As shown in Fig. 3 (b), with a rise in ultrasonic time from 30 min to 120 min, the zeta potential increases from 33 mV to 40.6 mV, falling to around 28.8 mV until 240 min. The zeta potential represents the potential difference between the stern layer, attached to the NPs’ surface with an electric charge opposite to that of the particle, and the diffuse layer around the stern layer [40]. The increase in the zeta potential values can be attributed to the pH adjustment. This is mainly because the pH and zeta potential depend on each other. Accordingly, a rise in the zeta potential and an improvement in the nanofluid stability are caused by the pH getting further away from the pH of the isoelectric point. A zeta potential value greater than 30 mV represents an almost stable nanofluid. Additionally, as expected, the increase in the concentration of NPs leads to nanofluid stability reduction (see Fig. 3c). Overall, all samples possessed reasonable stability during the test time (less than 2 hours’ ultrasonic time).
10
Fig. 3. (a) Change in pH value (b) Zeta potential for GNP@MWCNT (0.01% wt.) vs. different sonication time and (c) Zeta potential for GNP@MWCNT (at 120 min sonication) with different concentrations
3.2 Absorption performance investigation of the GNP@MWCNT nanofluid Because a high percentage of the radiation given off by the sun belongs to the visible area wavelengths, a UV-Vis absorbance spectrometer (made by Cecil England Co. Series 8000) was used to gain a better understanding of the light absorption by the nanofluids in this region. Fig. 4 shows the absorption spectra of the GNP@MWCNT nanofluid samples for different ultrasonic times. As observed, the rise in ultrasonic time enhances the light absorbed by the nanofluid approximately. For example, the average light absorption improves by approximately 30% when the ultrasonic time increased from 30 min to 120 min. This is mainly because a rise in ultrasonic time triggers considerable force exerted on the nanofluid to destroy the bond between the NPs. This causes Brownian motion [14] to intensify and the NPs to disperse into the base nanofluid remarkably, which is followed by an increase in the light absorption by the solution. Furthermore, it is interesting to note that there is an optimum sonication time in the light absorption. This phenomenon can be explained by the Beer-Lambert law, which is explained by equation (1) [41,42]:
I abs ( ) F ( ,w )l 1e t I 0 ( )
(1)
11
where Iabs ()=I0()-I() is the absorbed energy, I () is the transmitted light intensity at height l, I0() is the given off light intensity at l = 0, w is the weight percentage or concentration of the NPs, and Ft (λ, w) is the total extinction coefficient, which is related to the extinction coefficient of the NPs and bulk fluid in accordance with equation 2(a, b and c): 2
Ft ( ,w ) Fparticle i ( ,w i ) Fbasefluid ( )
(2.a)
i 1
Fbasefluide ( )
4 k basefluid
(2.b)
2
2
3 w i
i 1
2D
Fparticle i ( ,w i )
i 1
(2.c)
Q abs ( )
where D is the diameter of the NPs, λ is the wavelength, k is the complex component of the refractive index for the base fluid, and Qabs is the absorption efficiency of the individual NPs. It should be noted that the light dispersion in the sample remains low until
(υ = πD / λ).
Therefore, (3)
Ft ( ,w i )l dI abs ( ) 2 Ft ( ,w i )I o ( )e i 1 dl i 1 2
2 3w i 4 k basefluid i 1 Q abs ( ) 2D
2
3 w i 4 k basefluid i 1 Qabs ( ) l 2D I o ( )e
Equation 3 shows that the change rate of the absorbed energy is a function of the incident light intensity (I0), the extinction coefficient (Ft), and the depth (h). It can be seen that when the extinction coefficient increases, which depends on size of nanoparticles (D), the nanofluid concentration (w) and the wavelength (λ), a high percentage of the energy at the nanofluid surface is absorbed. In fact, by increasing the ultrasonic time until the optimum time, the stability and concentration of the nanofluid tends toward the optimal value (see Fig.3 b), following the extinction coefficient goes up, as a result of which, the rate of the absorbed energy is improved by the nanofluid, especially at the surface.
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Fig. 4. Absorbance spectra of (a) composite nanofluid GNP@MWCNT with a 0.01% wt. concentration for different sonication time (b) the seawater.
3.2 Evaporation performance investigation of the GNP@MWCNT nanofluid Figs. 5 (a) and (b) display the increase in the surface temperature along with the evaporated mass of the GNP@MWCNT composite nanofluid vs. time for a 0.01% wt. concentration and different ultrasonic times of 30, 60, 90, 120, 180 and 240 min at 3.6 suns compared to the seawater. As seen, the surface temperature rise was 22.75˚C for the seawater and 25.07˚C for the GNP@MWCNT nanofluid with the lowest ultrasonic time. Increasing the ultrasonic time and nanofluid concentration accompanied by the ultrasonic wave irradiation causes the surface temperature to increase more so that it reaches a maximum value of 26.88˚C at 120 min. Fig. 5 (b) shows the evaporation rate of the GNP@MWCNT composite nanofluid with a 0.01% wt. concentration vs. time after being exposed to radiation given off by the solar simulator for 30 min. The highest evaporation rate for the GNP@MWCNT nanofluid with a 0.01% wt. 13
concentration was 2.89 kg.m-2.h-1 after an ultrasonic time of 120 min, which is 2 times more than the seawater evaporation rate. However, the evaporation rate of the mentioned nanofluid is 1.46 times more than that of the water after an ultrasonic time of 30 min. This result demonstrates that ultrasonic time plays an effective role in absorbing the light and producing the solar steam. This can be attributed to the more appropriate distribution of the NPs in the base solution as a result of increasing the ultrasonic time (see Figs. 3 and 4).
Fig. 5. (a) Temperature rise of the top surface (b) evaporated mass of GNP@MWCNT nanofluid with a 0.01% wt. concentration at different sonication time in comparison with the seawater. To assess the performance of all nanofluids in the solar evaporation, the evaporation efficiency of the solutions with different concentrations was calculated according to equation (4) [29,31,43]:
evaporation where
fg mh
(4)
IA is the evaporation rate (kg.m-2.h-1), hfg is the enthalpy of the water phase change (2257
kJ.kg-1 at 1 atm pressure for pure water), I is the light intensity density (kW.m-2), and A is the radiation surface (m2) (the surface exposed to solar radiation). During solar evaporation, pure water just evaporates and there are no NPs in the condensed water. Therefore, in equation (4), only the enthalpy of the pure water evaporation is included. The evaporated mass rate reaches a steady state after 10 min. Fig. 6 shows the steady state evaporation efficiency and average particle size of the GNP@MWCNT composite nanofluid for 0.01% wt. concentration and 14
different ultrasonic times at 3.6 suns. As shown in Fig. 6 (a), the optimal time of sonication to reach the maximum evaporation efficiency (that is equal to 61.3%) is 120 min. The reason for this can be found in Fig. 6 (b), which displays the average particle size vs. ultrasonic time. As can be seen, there is an opposite trend between the evaporation efficiency and particle size with ultrasonic time when it changes from 30 min to 120 min. The force exerted on the particles to destroy the covalent bonds between them, causing the particles to separate from each other and disperse into the base fluid. Therefore, it will increase the fluid stability and trigger an improvement in the evaporation efficiency of the GNP@MWCNT composite nanofluid. However, after 120 min owing to agglomeration of nanoparticles which leads to the increase of average particle size the evaporation efficiency reduces. It should be mentioned that a particle size analyzer manufactured by Cordouan Co., France, Vasco3 model, was applied to determine the average particle size of the NPs.
Fig. 6. (a) Evaporation efficiency (b) average particle size for 0.01% wt. GNP@MWCNT composite nanofluid for different sonication time. 3.3 Concentration effect on the GNP@MWCNT nanofluid evaporation Figs. 7 (a) and (b) show the absorption spectrum and surface temperature increase of the GNP@MWCNT composite nanofluid, respectively. In these figures, the concentrations of 0.001, 0.002, 0.004, 0.01, 0.02 and 0.04% wt. were used and compared to the seawater. According to the Beyer-Lambert law, the increase in concentration improves the light absorption, causing the surface temperature to rise more. As shown in Fig. 7 (a), the light absorption of the nanofluids 15
did not change considerably when the MWCNT@CNT concentration was greater than 0.001% wt., which can be considered as the optimum concentration. For a 0.001% wt. concentration, the rise of the surface temperature is equal to 22.8˚C which is near to the increase of the seawater surface temperature (22.75˚C). The surface temperature increase for the solution with the highest concentration is 26.88˚C, followed by a rise in the evaporation rate (see Fig. 7c). For a 0.001% wt. concentration, the evaporation rate is 2.31 kg.m-2.h-1, which is 1.6 times more than that of the seawater, and for a 0.01% wt. concentration, the evaporation rate is 2.89 kg.m-2.h-1, which is 2 times more than that of the seawater. Fig. 7 (d) shows the evaporation efficiency for the GNP@MWCNT composite nanofluid. As observed, the rise in the concentration from 0.001 to 0.01% wt. results in a rise in the efficiency from 49.77% to 61.3%.
Fig. 7. Effect of the concentration on (a) absorption spectra (b) surface temperature rise (c) 16
evaporated mass (d) evaporation efficiency of the GNP@MWCNT nanofluid with 120 min ultrasonic time at 3.6 suns. Table 1 compares the results of this study against the literature. As observed, Shi et al. reported a 60.30% evaporation efficiency at 10 suns for Fe3O4@MWCNT composite with a ratio of 4 to 1 and a 0.5 g. Lit-1 mass concentration [30]. Ghafurian et al. obtained a 21.7% evaporation efficiency for 0.004% wt. graphene oxide at 3.5 suns [44]. To improve the solar evaporation efficiency, Wang et al. [34] utilized gold NPs and achieved 54.45% efficiency for a 178 ppm concentration at ~3 suns. In the present study, the highest evaporation efficiency obtained was 61.3% for the GNP@MWCNT composite nanofluid with a 0.01% wt. concentration at 3.6 suns. This indicates that in addition to reasonable ultrasonic time, such a composite nanofluid can play an effective role in solar energy absorption and the steam generation compared to nanofluids consisting of single NPs (see Fig. A.2), in particular valuable ones. Furthermore, the cost analysis of the steam generate by GNP@MWCNT nanofluid, which is presented in Fig. A.3 in appendix, showed that using GNP@MWCNT with optimum sonication time and concentration can be considered as an economical method in solar steam generation.
17
Table 1. Evaporation efficiency reported in the previous studies Reference
Nanofluid type
Preparation method
[36]
SWCNT
two-step
[45] [45] [45] [46]
MWCNT GNP SWCNT MWCNT-OH
one-step one-step one-step two-step
[29]
rGO@Fe3O4
two-step
[30] [30] [30]
Fe3O4@MWCNT Fe3O4@MWCNT Fe3O4@MWCNT
two-step two-step two-step
[47]
TiO2@Fe3O4
two-step
[44] [44] [44]
GO GO GO
two-step two-step two-step
[48]
rGO
two-step
[48]
rGO
two-step
[48]
rGO
two-step
[48]
rGO
two-step
[34] [34] [34] [34] [49] [49] [49] [28]
Au Au Au Au Ag@TiO2 Ag@TiO2 Ag@TiO2 GO@Au
one-step one-step one-step one-step two-step two-step two-step two-step
[50]
Au
one-step
Present
GNP@MWCNT
two-step
Ultrasonic Radiation Evaporation time Concentration intensity efficiency (min) (Suns) (%) 19.04×10-4 120 10 46.8 vol.% 0.004% wt. 3.5 44 0.004% wt. 3.5 38 0.004% wt. 3.5 34 20 0.002% wt. 3.5 39 Not 1 mg.ml-1 1 70 mentioned 120 0.5 g.Lit-1 1 43.8 -1 120 0.5 g.Lit 3 23.3 -1 120 0.5 g.Lit 10 60.3 Not 0.1 g.Lit-1 5 22 mentioned 30 0.004% wt. 1.5 36.54 30 0.004% wt. 2.5 26.3 30 0.004% wt. 3.5 21.7 Not 10ppm 1 46.17 mentioned Not 10ppm 3 36 mentioned Not 10ppm 5 42.82 mentioned Not 10ppm 7 47.4 mentioned 178 ppm 1 66.28 178 ppm 3 54.45 178 ppm 5 42.13 178 ppm 7 39.1 30 200 ppm 1 53.6 30 200 ppm 3 45.9 30 200 ppm 10 66.9 30 15.6% wt. 16.77 59.2 Not 5.1ppm 220 54.3 mentioned 120 0.01% wt. 3.6 61.3 18
study
3.4 Sunlight intensity effect on the evaporation rate
Evaporation experiments were conducted at three power intensities of 1.6, 2.6, and 3.6 suns. Fig. 8 depicts the evaporation, sensible, and total efficiencies for the seawater and composite nanofluid with a concentration of 0.01% wt. and 120 min ultrasonic time. Full details of the total and sensible efficiency calculations, along with the effect of concentration and the ultrasonic time, are presented in Appendix (Fig. A.4).
Fig. 8. Evaporation, sensible and total efficiencies of (a) the seawater (b) 0.01% wt. GNP@MWCNT nanofluid with 120 min ultrasonic time at different light intensity Due to the heat localization, an increase in the radiation intensity results in a surface temperature rise of the nanofluid followed by an increase in the evaporation rate. At low sunlight intensities, the evaporation rate remains negligible with an intensity increase; therefore, as shown in this figure (Fig. 8), the rise in the sunlight intensity triggers the decrease of the evaporation efficiency. On the other hand, the increase in solar intensity causes the sensible efficiency to grow. This is mainly because a percentage of solar energy absorbed by the nanofluid makes the bulk fluid temperature go up. Due to increasing heat transfer to the surrounding environment, growth of the solar intensity also results in further dissipation of the energy received. Therefore, it is generally concluded that an increase of solar intensity from 1.6 to 3.6 suns causes the evaporation efficiency to decrease and the sensible evaporation to go up. Given the conditions,
19
the total efficiency will increase insignificantly. By comparing two figures, it can easily be seen that the GNP@MWCNT composite nanofluid has an effective role in the efficiencies.
4. Conclusion In this research, the solar evaporation efficiency of GNP@MWCNT composite nanofluid was investigated. For this purpose, first, six samples with a concentration of 0.01% wt. were exposed to the ultrasonic waves for six different time intervals (i.e., 30, 60, 90, 120, 180 and 240 min) and tested under 3.6 suns irradiation. It was found that the evaporation rate and efficiency are maximized at 120 min sonication. This indicates that exposure to the ultrasonic waves for 120 min causes the NP size to reduce, light absorption to enhance, and therefore, solar evaporation rate to increase. In the next step, the effects of nanofluid concentration and solar radiation intensity on the GNP@MWCNT composite nanofluid performance were investigated. To that end, the composite nanofluids were prepared with concentrations in the range of 0.001 to 0.04% wt. and tested under radiation intensities between 1.6 and 3.6 suns. The results show that the concentration of composite nanofluid plays a significant role in the evaporation rate. Rising sunlight intensity causes the evaporation rate to go up, whereas it decreases the evaporation efficiency.
20
Symbol List Evaporation surface (m2)
A
Nanofluid specific heat (kJ.kg-1.˚C-1)
Cp
Nanoparticle Diameter (nm)
D
Extinction coefficient
Ft
Enthalpy (kJ.g-1)
h
Radiation light density (kW.m-2)
I
complex component of nanofluid refractive index
k
Transmission light length(mm)
l
Evaporation rate (kg.m-2.h-1)
ṁ
Nanofluid total mass (kg)
M
Individual absorption efficiency of NPs
Q
Temperature (˚C)
T
Time (min)
t
Nanofluid concentration (% wt.)
w
Greek Symbols Variation
Δ
Efficiency
Wavelength (nm)
λ
The light dispersion in the sample
ν
Subtitle Absorbed
Abs
phase change
fg
21
Appendix. A Fig. A.1 demonstrates the SEM image of GNP@MWCNT before and after sonication time (240 min).
200nm
200nm
(a) t=0 min sonication
(b) t=240 min sonication
Fig. A.1. SEM image of GNP@MWCNT before and after 240 min sonication
Fig. A.2 depicts the evaporation efficiency for different nanofluids such as graphene nanoplates (GNPs), multi-wall carbon nanotubes (MWCNTs), and GNP@MWCNT composite nanofluid compared to that of the seawater. It was observed that evaporation efficiency improves up to 11% with the use of the composite nanofluid. This result emphasizes the importance of utilizing such a composite nanofluid compared to nanofluids consisting of just NPs.
22
Fig. A.2. Evaporation efficiencies of the nanofluids consisting of GNP, MWCNT, and the composite nanofluid compared to those of the seawater at 3.6 suns for 120 min ultrasonic time. -
Cost analysis
To assess the feasibility of using GNP@MWCNT nanofluid in solar evaporation, cost of producing 1 g/s of vapor (CPV) has been estimated [35,51]: CPV ( $
g s
)
Cos tnp m V w
(A.1)
Here Costnp is price of 1g nanoparticles, m is evaporation rate (g/s), V is nanofluid’s volume (lit) and w is nanofluid’s concentration (g/l). Fig A.3 shows the production cost of 1 g/s solar vapor by GNP@MWCNT nanofluid at different sonication times compared to other nanoparticles reported in the literature under ≈3.5 suns. As shown, the MWCNT@CNT with 120 min ultrasonic time showed the best performance in terms of economy and the production cost of 1 g/s solar vapor by MWCNT@CNT with different sonication time did not change significantly. It can be concluded that using GNP@MWCNT with optimum sonication time in evaporating can be considered as economical method.
23
Fig. A.3. The ratio of production cost of 1 g/s solar vapor via nanofluids (CPVnp) to that of GNP@MWCNT with 120 min sonication time (CPVref) - Sensible efficiency and total efficiency In the evaporation process, a percentage of thermal energy is utilized for the fluid phase change and water evaporation. A high percentage of it leads to the heating and temperature increase of the bulk fluid. The efficiency of the increase in the bulk fluid temperature, known as sensible efficiency, is obtained by equation (A.2): [44,48]
sensible
MC p T
(A.2)
t
IA
where M is the nanofluid total mass,
CP is the water-specific thermal coefficient, T is the
change in the bulk liquid temperature during the test, t is the test time, A is the surface area exposed to the radiation , and I is the light intensity density. Due to the existence of salinity, the Persian Gulf water used as the base fluid has different properties compared to distilled water. 24
Referring to the standards [52] and also considering seawater salinity to be 36 g. Lit-1, the seawater-specific heat capacity is 3.96 kJ.kg-1. K-1. The total efficiency is also obtained from the sum of the evaporation and sensible efficiencies:
total evaporation sensible
(A.3)
Fig. A.4 (a) shows the efficiencies of the GNP@MWCNT composite nanofluids with a 0.01% wt. concentration for the different ultrasonic times at 3.6 suns. An increase in ultrasonic time brings about small changes in the sensible efficiency, thereby exerting an insignificant influence on the bulk fluid temperature rise, whereas it causes the evaporation efficiency to increase and, as a result, the total efficiency to rise. Fig. A.4 (b) displays the efficiencies of the GNP@MWCNT composite nanofluids with various concentrations for 120 min ultrasonic time at 3.6 suns.
Fig. A.4. Sensible and total efficiencies of the GNP@MWCNT (0.01% wt.) composite nanofluid compared to those of the seawater for (a) different ultrasonic times and (b) different concentrations for 120 min sonication time at 3.6 suns.
25
Increasing the nanofluid concentration causes the photothermal energy transferred to the nanofluid to be used for the heat localization and the nanofluid surface temperature increase, whereas changes in the bulk fluid temperature will be negligible. This triggers slight increases in the sensible efficiency when the nanofluid concentration rises. However, the increase in light absorption followed by the rise in the fluid surface temperature triggers the rise of the evaporation rate and evaporation efficiency.
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30
There is no conflict of interest for this paper.
31
Highlights •
Sonication time effect on evaporation rate of GNP@MWCNT nanofluid was investigated.
•
Optimum sonication time was obtained at 120 min with 61.3% evaporation efficiency.
•
Effect of nanoparticle mass fraction and irradiation on efficiency was studied.
•
GNP@MWCNT nanofluid at 120 min sonication showed the best cost-efficiency.
•
The evaporated performance of MWCNT, GNP, GNP@MWCNT and pure water was compared.
32
S1350-4177(19)31167-8 https://doi.org/10.1016/j.ultsonch.2019.104817 ULTSON 104817
To appear in:
Ultrasonics Sonochemistry
Received Date: Revised Date: Accepted Date:
25 July 2019 15 September 2019 30 September 2019
Please cite this article as: M.M. Ghafurian, Z. Akbari, H. Niazmand, R. Mehrkhah, S. Wongwises, O. Mahian, Effect of sonication time on the evaporation rate of seawater containing a nanocomposite, Ultrasonics Sonochemistry (2019), doi: https://doi.org/10.1016/j.ultsonch.2019.104817
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© 2019 Published by Elsevier B.V.
Effect of sonication time on the evaporation rate of seawater containing a nanocomposite
Mohammad Mustafa Ghafuriana,b, Zohreh Akbarib, Hamid Niazmandb*, Roya Mehrkhahc, Somchai Wongwisesd, Omid Mahiana* School of Chemical Engineering and Technology, Xi'an Jiaotong University, Xi'an, Shaanxi 710049, China
a
b
Department of Mechanical Engineering, Ferdowsi University of Mashhad, Mashhad, Iran cDepartment
dFluid
of Chemistry, Ferdowsi University of Mashhad, Mashhad, Iran
Mechanics, Thermal Engineering and Multiphase Flow Research Lab (FUTURE), Faculty of
Engineering, Department of Mechanical Engineering, King Mongkuts University of Technology Thonburi, Bangmod, Bangkok 10140, Thailand
1
Abstract Sonication time has a significant contribution to the stability and properties of nanofluids (mixtures of nanoparticles and a base fluid). Finding the optimum sonication time can help to save energy and ensure optimal design. The present study deals with the sonication time effect on the evaporation rate of seawater containing a nanocomposite (i.e., a mixture of multi-walled carbon nanotubes and graphene nanoplates). For indoor experiments, a solar simulator was employed as the radiation source. At first, the nanofluid with a concentration of 0.01% wt. was sonicated in an ultrasonic bath (40 kHz frequency and 200 W power) for different times of 30, 60, 90, 120, 180, 240 min, and the associated zeta potential values were recorded to evaluate the stability. Next, the best time function was used to appraise the effect of concentration variations (0.001, 0.002, 0.004, 0.01, 0.02 and 0.04% wt.) and the light intensities (1.6, 2.6, and 3.6 suns) on the rate of solar steam generation. The results indicate that for a concentration of 0.01% wt. and under 3.6 suns, the highest evaporation efficiency of 61.3% would be achieved at 120 min sonication time. Keywords: Stability, Ultrasonic time, Composite nanofluid, Solar evaporation
2
1. Introduction Nanofluids are considered suspensions formed by the dispersion of solid nanoparticles (NPs) in a base fluid. From the practical viewpoint, the base fluid is usually a kind of conventional liquids i.e. water [1,2], oil [3], or ethylene glycol [4]. The literature shows that the base fluid properties, notably thermal [5], electrical [6], and optical ones [7] are affected when NPs are added to it. Therefore, nanofluids are a good candidate for application in various fields such as automobile industries, energy conversion and storage, solar systems, and biomedical applications [8-10]. One of the most important indices in the study of nanofluids is stability level. Because the NPs tend to adhere to each other and aggregate due to Van der Waals forces [11], the stability is highly dependent on methods of preparing the nanofluid, nanoparticle characteristics, base fluid type, surfactant, pH, and ultrasonic parameters [12]. Generally, to improve the nanofluid stability, there are two physical and chemical methods by which the surface properties of the NPs are modified. The chemical methods include surfactant addition [13], pH adjustment [14], and functionalization of NPs [12,13]. The physical methods include the use of a homogenizer [12] and force exerted on NPs by ultrasonic waves [15] to break the intermolecular bonds between NPs. On occasion, both methods (physical and chemical) are simultaneously utilized for uniform dispersion of NPs in the base fluid. The stability level of nanofluids is deeply affected by the nature of the NPs and base fluid. The NPs are divided into hydrophilic and hydrophobic groups. Hydrophilic NPs are usually stable in polar fluids (like water), whereas hydrophobic ones are stable in non-polar fluids (like oils). Given the conditions, there is no need to add an additive to stabilize the nanofluid. What is required to prepare a stable nanofluid composed of hydrophobic NPs in the polar base fluid and vice versa is the addition of a stabilizer, for example, a surfactant. In fact, as a linking agent, the surfactant establishes intermolecular bonds between the NPs and base fluid molecules [12,14]. Several researchers have investigated the surfactant effect on the stability and properties of the nanofluid. For example, Jong Choi et al. [16], Zareei et al. [11] and Soltanimehr and Afrand [16], experimentally investigated the effects of different surfactants on the stability nanofluids consisting of carbon nanotubes [16], aluminum oxide (Al2O3) [11] and MWCNT [16]. Their results indicated although the use of a surfactant is considered an economical and straightforward method to achieve desirable stability, it causes to make a resistance between NPs and base fluid, resulting in a change in NP properties, particularly thermal and optical ones. To tackle this problem, NPs are modified, thereby functionalizing their 3
surface. By this process, covalent bonds of the particles are destroyed and an acidic or alkali agent is attached to the NP surface, causing NPs to separate from each other, followed by improvement of the nanofluid stability. This method is widely employed in stabilizing carbon nanotubes (CNTs) [17,18]. One of the methods applied to assess the nanofluid stability is pH adjustment [19], which is deemed another chemical method for stabilizing the nanofluid in some references [12,14]. Firstly, it is necessary to define the isoelectric point to explain the pH effect. The isoelectric point is the point at which the electrical surface charge of a particle is zero. The pure charge of the nanofluid is strongly influenced by the nanofluid pH so that under various pH values, it might become negative or positive. At the isoelectric point, the suspension is completely unstable and the value of the zeta potential parameter is zero [12]. As the suspension pH gets farther away from the isoelectric point pH, the stability of the nanofluid will increase. Researchers such as Zareei [11] and Khairul [20] have studied this parameter in detail for aluminum oxide [10,19] and copper oxide NPs. In addition to chemical approaches, there are physical methods to stabilize nanofluids. One of these methods is the use of ultrasonic waves in which a high frequency is produced in the nanofluid by an ultrasonic probe (direct method) [15], ultrasonic bath (indirect method) [21], or high-pressure homogenizer [12]. There are some studies in the literature on the effect of ultrasonic waves on the stability and characteristics of nanofluids. The literature shows that the stability of some nanofluids such as water-based Mg(OH)2 in the absence of a surfactant [22] and water-based TiO2 nanofluids in the presence of anionic surfactant SDS [23] is enhanced with an increase in ultrasonic time. Other researchers investigated the effect of sonication time on nanofluid stability and found that there is an optimal time for sonication. Before the optimal time, the stability has an ascending trend with sonication time, and after that point, it follows the opposite trend. This was true for titanium oxide/water nanofluids in the absence of a surfactant [15], Cu/EG nanofluid [4], aluminum oxide/water nanofluid [24], and water-based nanofluid of magnesium hydroxide[22] in the presence of a surfactant. It has also been mentioned that increasing the ultrasonic time provides the force required to separate the NPs from each other, causing the NPs to disperse within the base fluid and their stability to increase. When the ultrasonic time is longer than the optimal time, the probability of NPs colliding with each other is 4
very high, triggering the Van der Waals attraction force between the NPs, and as a result, the particles are absorbed into each other once again. Under the mentioned circumstances, aggregation occurs in the solution. Because there is a direct relationship between the nanofluid stability and its thermophysical properties, the researchers investigated how an improvement in thermophysical properties affects the stability of the nanofluid. For example, Ghadimi et al. [23] and Siddiqui et al. [25] studied the influence of the ultrasonic time on TiO2[23] and Cu-Al2O3 composite [25] nanofluid stability and reported that the nanofluid thermal conductivity increased with an improvement in the stability. Because nanofluid properties such as thermal ones are strongly influenced by its stability, it is expected that this parameter also affects optical properties such as light absorption. Being at the advantage of light absorbance makes the nanofluid a suitable option for solar systems, particularly desalinating and direct steamgenerating ones, which results in a direct relationship between the nanofluid stability and amount of the steam produced. Up to now, many studies [26–31] have been carried out on nanofluid utilization in solar steam production, but researchers have yet to examine the effect of the nanofluid stability, ultrasonic time, and pH adjustment on the solar evaporation. For instance, gold (Au) nanofluids were synthesized via several methods, such as a one-step method based on a modified thermal citrate reduction method by Amjad et al. [32], a seed-growing method with some modifications by Guo et al. [33], and a one-step method by Wang et al. [34], and relying on the fact that the surface plasmonic resonance phenomenon improves photothermal conversion, these researchers investigated the effect of Au nanofluid on solar steam generation. Their results showed that due to the high price of gold NPs, it is no longer economic to utilize them in solar steam generation. Therefore, the researchers proposed carbon materials due to their high light absorption and reasonable price[35]. Accordingly, Wang et al. [36] employed a single-walled CNT nanofluid as an absorber for solar steam generation synthesized through a two-step method without addition of any surfactants for 2 hours of ultrasonic time. They succeeded in achieving 46.8% efficiency at 10 suns light intensity for a 19.04× 10-4 vol% concentration. Another effective method for generating solar steam that has recently received the attention of researchers is the use of composite nanofluids to enhance light absorption. Neumann et al. [27], Fu et al. [28], Wang et al. [29], Shi et al. [30], and Chen et al. [31] utilized nanofluids of silicium oxide/gold nanoshells (SiO2/Au) [27], graphene oxide/gold (GO/Au) [28], RGO @ Fe3O4 [29],
5
and Fe3O4 @ CNT [30,31], respectively, for the solar steam generation. They showed that the energy absorbed by the composite nanofluids improves the evaporation rate. Despite the extensive studies on solar steam production, the effects of ultrasonic time on evaporation rate have not been investigated yet [37]. Ultrasound is known as a primary source for stimulating reactions between materials [38]. Ultrasound can be used for enhancing nanofluids stability and it is expected that by optimizing the sonication time, not only the evaporation efficiency of nanofluids improves but also the cost of steam generation decreases. Therefore, in the present study, the effect of sonication time on evaporation rate is studied, for the first time. A nanocomposite (i.e., a compound of graphene nanoplates and carbon nanotubes [GNP@MWCNT]) with different concentrations (0.001, 0.002, 0.004, 0.01, 0.02 and 0.04% wt. )is suspended in seawater. Different sonication times (30, 60, 90, 120, 180 and 240 min) are applied to the samples, and then the treated nanofluids are inserted under irradiation intensities ranging from 1.6 to 3.6 suns. The main aim of the study is to find the optimal sonication time for GNP@MWCNT nanofluid to achieve the maximum evaporation rate. 2. Experimental Section (Materials and methods) To examine the effect of the ultrasonic time on evaporation rate, a composite-based nanofluid containing multi-walled carbon nanotubes/graphene nanoplates (GNP@MWCNT) is utilized in this research. In the following, the specification of the nanofluid, and experimental setup will be discussed. 2.1 Nanomaterial characteristics and nanofluid preparation To prepare the nanofluids, the graphene nanoplates with a diameter ranging from 1 to 20 μm, a thickness less than 40 nm, and a purity of 99.5%, along with MWCNTs with a diameter ranging from 20 nm to 30 nm, a length ranging from 5 to 10 µm, and a purity greater than 95% were purchased from the Vira Carbon Nanomaterial (VCN) company. To increase the stability, Arabic gum was used as the surfactant (purchased from Sigma-Aldrich Company). To synthesize the composite nanofluid using Arabic gum surfactant, the ratio of the graphene nanoplates to multiwalled carbon nanotubes was chosen as 1:1. To obtain initial stability, all nanofluids were placed in an ultrasonic bath with 200 W power and 40 kHz frequency time 30 min. Fig. 1 shows the nanofluids used in the experiments with certain concentrations compared to the seawater. TEM and SEM images of nanomaterials, taken by a LEO-150VP and LEO-1450VP, are also shown in 6
this figure. It should be noted that in the present study, the energy of the ultrasonic cannot decompose the GNP@MWCNT nanocomposite (see Fig. A.1 in appendix).
Seawater
GNP @ MWCNT 0.001% wt.
GNP @ MWCNT 0.002% wt.
GNP @ MWCNT 0.01% wt.
a
200nm
b
c
80nm
d
125nm
Fig. 1. (a) GNP@MWCNT/water samples with different concentrations in comparison with
7
the seawater; SEM image of (b) GNP@MWCNT and TEM image of (c) MWCNT (d) GNP.
2.2 Experimental Setup Figure 2 shows the experimental system designed for the present study. As shown, a solar simulator manufactured by Nanosat Co. Iran, IIIS-310 model, with a 1600 W xenon lamp and 6000 K radiation temperature, was employed for artificial radiation production. A glass beaker 70 mm in height, 38 mm in diameter, and with a plastic insulator around it was used as a solar receiver. It contained the nanofluid or the seawater. Three PT100 temperature sensors placed at heights of 10, 30, and 50 mm from the bottom of the beaker were utilized to measure the rise in fluid temperature during the experiments. The upper part of these sensors that was exposed to the radiation was whitened with a silicone coating layer to avoid light absorption and unwanted increase in temperature. To measure the evaporation rate, an accurate digital balance with a precision of 0.01 grams constructed by Kern Co., Germany, PFB2000-2 model, was used. Furthermore, a CMP3 secondary standard pyranometer with accuracy of 1w.m-2 in a wavelength range of 200-2800 nm, manufactured by Kipp & Zonen Co., was employed to adjust the light intensity in the experiment. At each stage of the test, the beaker was completely filled with the nanofluid and exposed to the light radiation for 30 min. All sensors and balance data were sent to a computer using a data logger system. It is worth noting that at the beginning of the test, the ambient temperature was 19-21°C, the relative humidity of the environment was 19%, and the air pressure was recorded as 0.9 bar.
8
Fig. 2. Experiment setup including a solar simulator, digital balance, temperature sensors, container, data logger and computer for solar steam generation.
3. Results and Discussion To investigate the ultrasonic time effect on the nanofluid stability, light absorption, and vapor generation, first, we placed a nanofluid with a 0.01% wt. concentration into the ultrasonic bath with 40 kHz frequency and 200 W power for 30, 60, 90, 120, 180 and 240 min. The results of these studies are presented in the next sections. 3.1 Study of GNP@MWCNT nanofluid stability To investigate the nanofluid stability, the zeta potential was measured using the Zeta Compact device for different samples, and the results are presented in Fig. 3 (a). As indicated, the pH of the samples produced was recorded as about 7-8 and grew gradually with the increase of the ultrasonic time by 120 min. The pH values changed directly with the variation of the surface properties and, in particular, the level of the NP surface charges. By increasing the ultrasonic time by 120 min, the pH of the samples probably got further away from the pH at the isoelectric 9
point, resulting in a rise of the repulsive force between the NPs. This caused the NPs to separate from each other and the dispersion of NPs to improve in the base fluid. This result was also found in reference [39]. As shown in Fig. 3 (b), with a rise in ultrasonic time from 30 min to 120 min, the zeta potential increases from 33 mV to 40.6 mV, falling to around 28.8 mV until 240 min. The zeta potential represents the potential difference between the stern layer, attached to the NPs’ surface with an electric charge opposite to that of the particle, and the diffuse layer around the stern layer [40]. The increase in the zeta potential values can be attributed to the pH adjustment. This is mainly because the pH and zeta potential depend on each other. Accordingly, a rise in the zeta potential and an improvement in the nanofluid stability are caused by the pH getting further away from the pH of the isoelectric point. A zeta potential value greater than 30 mV represents an almost stable nanofluid. Additionally, as expected, the increase in the concentration of NPs leads to nanofluid stability reduction (see Fig. 3c). Overall, all samples possessed reasonable stability during the test time (less than 2 hours’ ultrasonic time).
10
Fig. 3. (a) Change in pH value (b) Zeta potential for GNP@MWCNT (0.01% wt.) vs. different sonication time and (c) Zeta potential for GNP@MWCNT (at 120 min sonication) with different concentrations
3.2 Absorption performance investigation of the GNP@MWCNT nanofluid Because a high percentage of the radiation given off by the sun belongs to the visible area wavelengths, a UV-Vis absorbance spectrometer (made by Cecil England Co. Series 8000) was used to gain a better understanding of the light absorption by the nanofluids in this region. Fig. 4 shows the absorption spectra of the GNP@MWCNT nanofluid samples for different ultrasonic times. As observed, the rise in ultrasonic time enhances the light absorbed by the nanofluid approximately. For example, the average light absorption improves by approximately 30% when the ultrasonic time increased from 30 min to 120 min. This is mainly because a rise in ultrasonic time triggers considerable force exerted on the nanofluid to destroy the bond between the NPs. This causes Brownian motion [14] to intensify and the NPs to disperse into the base nanofluid remarkably, which is followed by an increase in the light absorption by the solution. Furthermore, it is interesting to note that there is an optimum sonication time in the light absorption. This phenomenon can be explained by the Beer-Lambert law, which is explained by equation (1) [41,42]:
I abs ( ) F ( ,w )l 1e t I 0 ( )
(1)
11
where Iabs ()=I0()-I() is the absorbed energy, I () is the transmitted light intensity at height l, I0() is the given off light intensity at l = 0, w is the weight percentage or concentration of the NPs, and Ft (λ, w) is the total extinction coefficient, which is related to the extinction coefficient of the NPs and bulk fluid in accordance with equation 2(a, b and c): 2
Ft ( ,w ) Fparticle i ( ,w i ) Fbasefluid ( )
(2.a)
i 1
Fbasefluide ( )
4 k basefluid
(2.b)
2
2
3 w i
i 1
2D
Fparticle i ( ,w i )
i 1
(2.c)
Q abs ( )
where D is the diameter of the NPs, λ is the wavelength, k is the complex component of the refractive index for the base fluid, and Qabs is the absorption efficiency of the individual NPs. It should be noted that the light dispersion in the sample remains low until
(υ = πD / λ).
Therefore, (3)
Ft ( ,w i )l dI abs ( ) 2 Ft ( ,w i )I o ( )e i 1 dl i 1 2
2 3w i 4 k basefluid i 1 Q abs ( ) 2D
2
3 w i 4 k basefluid i 1 Qabs ( ) l 2D I o ( )e
Equation 3 shows that the change rate of the absorbed energy is a function of the incident light intensity (I0), the extinction coefficient (Ft), and the depth (h). It can be seen that when the extinction coefficient increases, which depends on size of nanoparticles (D), the nanofluid concentration (w) and the wavelength (λ), a high percentage of the energy at the nanofluid surface is absorbed. In fact, by increasing the ultrasonic time until the optimum time, the stability and concentration of the nanofluid tends toward the optimal value (see Fig.3 b), following the extinction coefficient goes up, as a result of which, the rate of the absorbed energy is improved by the nanofluid, especially at the surface.
12
Fig. 4. Absorbance spectra of (a) composite nanofluid GNP@MWCNT with a 0.01% wt. concentration for different sonication time (b) the seawater.
3.2 Evaporation performance investigation of the GNP@MWCNT nanofluid Figs. 5 (a) and (b) display the increase in the surface temperature along with the evaporated mass of the GNP@MWCNT composite nanofluid vs. time for a 0.01% wt. concentration and different ultrasonic times of 30, 60, 90, 120, 180 and 240 min at 3.6 suns compared to the seawater. As seen, the surface temperature rise was 22.75˚C for the seawater and 25.07˚C for the GNP@MWCNT nanofluid with the lowest ultrasonic time. Increasing the ultrasonic time and nanofluid concentration accompanied by the ultrasonic wave irradiation causes the surface temperature to increase more so that it reaches a maximum value of 26.88˚C at 120 min. Fig. 5 (b) shows the evaporation rate of the GNP@MWCNT composite nanofluid with a 0.01% wt. concentration vs. time after being exposed to radiation given off by the solar simulator for 30 min. The highest evaporation rate for the GNP@MWCNT nanofluid with a 0.01% wt. 13
concentration was 2.89 kg.m-2.h-1 after an ultrasonic time of 120 min, which is 2 times more than the seawater evaporation rate. However, the evaporation rate of the mentioned nanofluid is 1.46 times more than that of the water after an ultrasonic time of 30 min. This result demonstrates that ultrasonic time plays an effective role in absorbing the light and producing the solar steam. This can be attributed to the more appropriate distribution of the NPs in the base solution as a result of increasing the ultrasonic time (see Figs. 3 and 4).
Fig. 5. (a) Temperature rise of the top surface (b) evaporated mass of GNP@MWCNT nanofluid with a 0.01% wt. concentration at different sonication time in comparison with the seawater. To assess the performance of all nanofluids in the solar evaporation, the evaporation efficiency of the solutions with different concentrations was calculated according to equation (4) [29,31,43]:
evaporation where
fg mh
(4)
IA is the evaporation rate (kg.m-2.h-1), hfg is the enthalpy of the water phase change (2257
kJ.kg-1 at 1 atm pressure for pure water), I is the light intensity density (kW.m-2), and A is the radiation surface (m2) (the surface exposed to solar radiation). During solar evaporation, pure water just evaporates and there are no NPs in the condensed water. Therefore, in equation (4), only the enthalpy of the pure water evaporation is included. The evaporated mass rate reaches a steady state after 10 min. Fig. 6 shows the steady state evaporation efficiency and average particle size of the GNP@MWCNT composite nanofluid for 0.01% wt. concentration and 14
different ultrasonic times at 3.6 suns. As shown in Fig. 6 (a), the optimal time of sonication to reach the maximum evaporation efficiency (that is equal to 61.3%) is 120 min. The reason for this can be found in Fig. 6 (b), which displays the average particle size vs. ultrasonic time. As can be seen, there is an opposite trend between the evaporation efficiency and particle size with ultrasonic time when it changes from 30 min to 120 min. The force exerted on the particles to destroy the covalent bonds between them, causing the particles to separate from each other and disperse into the base fluid. Therefore, it will increase the fluid stability and trigger an improvement in the evaporation efficiency of the GNP@MWCNT composite nanofluid. However, after 120 min owing to agglomeration of nanoparticles which leads to the increase of average particle size the evaporation efficiency reduces. It should be mentioned that a particle size analyzer manufactured by Cordouan Co., France, Vasco3 model, was applied to determine the average particle size of the NPs.
Fig. 6. (a) Evaporation efficiency (b) average particle size for 0.01% wt. GNP@MWCNT composite nanofluid for different sonication time. 3.3 Concentration effect on the GNP@MWCNT nanofluid evaporation Figs. 7 (a) and (b) show the absorption spectrum and surface temperature increase of the GNP@MWCNT composite nanofluid, respectively. In these figures, the concentrations of 0.001, 0.002, 0.004, 0.01, 0.02 and 0.04% wt. were used and compared to the seawater. According to the Beyer-Lambert law, the increase in concentration improves the light absorption, causing the surface temperature to rise more. As shown in Fig. 7 (a), the light absorption of the nanofluids 15
did not change considerably when the MWCNT@CNT concentration was greater than 0.001% wt., which can be considered as the optimum concentration. For a 0.001% wt. concentration, the rise of the surface temperature is equal to 22.8˚C which is near to the increase of the seawater surface temperature (22.75˚C). The surface temperature increase for the solution with the highest concentration is 26.88˚C, followed by a rise in the evaporation rate (see Fig. 7c). For a 0.001% wt. concentration, the evaporation rate is 2.31 kg.m-2.h-1, which is 1.6 times more than that of the seawater, and for a 0.01% wt. concentration, the evaporation rate is 2.89 kg.m-2.h-1, which is 2 times more than that of the seawater. Fig. 7 (d) shows the evaporation efficiency for the GNP@MWCNT composite nanofluid. As observed, the rise in the concentration from 0.001 to 0.01% wt. results in a rise in the efficiency from 49.77% to 61.3%.
Fig. 7. Effect of the concentration on (a) absorption spectra (b) surface temperature rise (c) 16
evaporated mass (d) evaporation efficiency of the GNP@MWCNT nanofluid with 120 min ultrasonic time at 3.6 suns. Table 1 compares the results of this study against the literature. As observed, Shi et al. reported a 60.30% evaporation efficiency at 10 suns for Fe3O4@MWCNT composite with a ratio of 4 to 1 and a 0.5 g. Lit-1 mass concentration [30]. Ghafurian et al. obtained a 21.7% evaporation efficiency for 0.004% wt. graphene oxide at 3.5 suns [44]. To improve the solar evaporation efficiency, Wang et al. [34] utilized gold NPs and achieved 54.45% efficiency for a 178 ppm concentration at ~3 suns. In the present study, the highest evaporation efficiency obtained was 61.3% for the GNP@MWCNT composite nanofluid with a 0.01% wt. concentration at 3.6 suns. This indicates that in addition to reasonable ultrasonic time, such a composite nanofluid can play an effective role in solar energy absorption and the steam generation compared to nanofluids consisting of single NPs (see Fig. A.2), in particular valuable ones. Furthermore, the cost analysis of the steam generate by GNP@MWCNT nanofluid, which is presented in Fig. A.3 in appendix, showed that using GNP@MWCNT with optimum sonication time and concentration can be considered as an economical method in solar steam generation.
17
Table 1. Evaporation efficiency reported in the previous studies Reference
Nanofluid type
Preparation method
[36]
SWCNT
two-step
[45] [45] [45] [46]
MWCNT GNP SWCNT MWCNT-OH
one-step one-step one-step two-step
[29]
rGO@Fe3O4
two-step
[30] [30] [30]
Fe3O4@MWCNT Fe3O4@MWCNT Fe3O4@MWCNT
two-step two-step two-step
[47]
TiO2@Fe3O4
two-step
[44] [44] [44]
GO GO GO
two-step two-step two-step
[48]
rGO
two-step
[48]
rGO
two-step
[48]
rGO
two-step
[48]
rGO
two-step
[34] [34] [34] [34] [49] [49] [49] [28]
Au Au Au Au Ag@TiO2 Ag@TiO2 Ag@TiO2 GO@Au
one-step one-step one-step one-step two-step two-step two-step two-step
[50]
Au
one-step
Present
GNP@MWCNT
two-step
Ultrasonic Radiation Evaporation time Concentration intensity efficiency (min) (Suns) (%) 19.04×10-4 120 10 46.8 vol.% 0.004% wt. 3.5 44 0.004% wt. 3.5 38 0.004% wt. 3.5 34 20 0.002% wt. 3.5 39 Not 1 mg.ml-1 1 70 mentioned 120 0.5 g.Lit-1 1 43.8 -1 120 0.5 g.Lit 3 23.3 -1 120 0.5 g.Lit 10 60.3 Not 0.1 g.Lit-1 5 22 mentioned 30 0.004% wt. 1.5 36.54 30 0.004% wt. 2.5 26.3 30 0.004% wt. 3.5 21.7 Not 10ppm 1 46.17 mentioned Not 10ppm 3 36 mentioned Not 10ppm 5 42.82 mentioned Not 10ppm 7 47.4 mentioned 178 ppm 1 66.28 178 ppm 3 54.45 178 ppm 5 42.13 178 ppm 7 39.1 30 200 ppm 1 53.6 30 200 ppm 3 45.9 30 200 ppm 10 66.9 30 15.6% wt. 16.77 59.2 Not 5.1ppm 220 54.3 mentioned 120 0.01% wt. 3.6 61.3 18
study
3.4 Sunlight intensity effect on the evaporation rate
Evaporation experiments were conducted at three power intensities of 1.6, 2.6, and 3.6 suns. Fig. 8 depicts the evaporation, sensible, and total efficiencies for the seawater and composite nanofluid with a concentration of 0.01% wt. and 120 min ultrasonic time. Full details of the total and sensible efficiency calculations, along with the effect of concentration and the ultrasonic time, are presented in Appendix (Fig. A.4).
Fig. 8. Evaporation, sensible and total efficiencies of (a) the seawater (b) 0.01% wt. GNP@MWCNT nanofluid with 120 min ultrasonic time at different light intensity Due to the heat localization, an increase in the radiation intensity results in a surface temperature rise of the nanofluid followed by an increase in the evaporation rate. At low sunlight intensities, the evaporation rate remains negligible with an intensity increase; therefore, as shown in this figure (Fig. 8), the rise in the sunlight intensity triggers the decrease of the evaporation efficiency. On the other hand, the increase in solar intensity causes the sensible efficiency to grow. This is mainly because a percentage of solar energy absorbed by the nanofluid makes the bulk fluid temperature go up. Due to increasing heat transfer to the surrounding environment, growth of the solar intensity also results in further dissipation of the energy received. Therefore, it is generally concluded that an increase of solar intensity from 1.6 to 3.6 suns causes the evaporation efficiency to decrease and the sensible evaporation to go up. Given the conditions,
19
the total efficiency will increase insignificantly. By comparing two figures, it can easily be seen that the GNP@MWCNT composite nanofluid has an effective role in the efficiencies.
4. Conclusion In this research, the solar evaporation efficiency of GNP@MWCNT composite nanofluid was investigated. For this purpose, first, six samples with a concentration of 0.01% wt. were exposed to the ultrasonic waves for six different time intervals (i.e., 30, 60, 90, 120, 180 and 240 min) and tested under 3.6 suns irradiation. It was found that the evaporation rate and efficiency are maximized at 120 min sonication. This indicates that exposure to the ultrasonic waves for 120 min causes the NP size to reduce, light absorption to enhance, and therefore, solar evaporation rate to increase. In the next step, the effects of nanofluid concentration and solar radiation intensity on the GNP@MWCNT composite nanofluid performance were investigated. To that end, the composite nanofluids were prepared with concentrations in the range of 0.001 to 0.04% wt. and tested under radiation intensities between 1.6 and 3.6 suns. The results show that the concentration of composite nanofluid plays a significant role in the evaporation rate. Rising sunlight intensity causes the evaporation rate to go up, whereas it decreases the evaporation efficiency.
20
Symbol List Evaporation surface (m2)
A
Nanofluid specific heat (kJ.kg-1.˚C-1)
Cp
Nanoparticle Diameter (nm)
D
Extinction coefficient
Ft
Enthalpy (kJ.g-1)
h
Radiation light density (kW.m-2)
I
complex component of nanofluid refractive index
k
Transmission light length(mm)
l
Evaporation rate (kg.m-2.h-1)
ṁ
Nanofluid total mass (kg)
M
Individual absorption efficiency of NPs
Q
Temperature (˚C)
T
Time (min)
t
Nanofluid concentration (% wt.)
w
Greek Symbols Variation
Δ
Efficiency
Wavelength (nm)
λ
The light dispersion in the sample
ν
Subtitle Absorbed
Abs
phase change
fg
21
Appendix. A Fig. A.1 demonstrates the SEM image of GNP@MWCNT before and after sonication time (240 min).
200nm
200nm
(a) t=0 min sonication
(b) t=240 min sonication
Fig. A.1. SEM image of GNP@MWCNT before and after 240 min sonication
Fig. A.2 depicts the evaporation efficiency for different nanofluids such as graphene nanoplates (GNPs), multi-wall carbon nanotubes (MWCNTs), and GNP@MWCNT composite nanofluid compared to that of the seawater. It was observed that evaporation efficiency improves up to 11% with the use of the composite nanofluid. This result emphasizes the importance of utilizing such a composite nanofluid compared to nanofluids consisting of just NPs.
22
Fig. A.2. Evaporation efficiencies of the nanofluids consisting of GNP, MWCNT, and the composite nanofluid compared to those of the seawater at 3.6 suns for 120 min ultrasonic time. -
Cost analysis
To assess the feasibility of using GNP@MWCNT nanofluid in solar evaporation, cost of producing 1 g/s of vapor (CPV) has been estimated [35,51]: CPV ( $
g s
)
Cos tnp m V w
(A.1)
Here Costnp is price of 1g nanoparticles, m is evaporation rate (g/s), V is nanofluid’s volume (lit) and w is nanofluid’s concentration (g/l). Fig A.3 shows the production cost of 1 g/s solar vapor by GNP@MWCNT nanofluid at different sonication times compared to other nanoparticles reported in the literature under ≈3.5 suns. As shown, the MWCNT@CNT with 120 min ultrasonic time showed the best performance in terms of economy and the production cost of 1 g/s solar vapor by MWCNT@CNT with different sonication time did not change significantly. It can be concluded that using GNP@MWCNT with optimum sonication time in evaporating can be considered as economical method.
23
Fig. A.3. The ratio of production cost of 1 g/s solar vapor via nanofluids (CPVnp) to that of GNP@MWCNT with 120 min sonication time (CPVref) - Sensible efficiency and total efficiency In the evaporation process, a percentage of thermal energy is utilized for the fluid phase change and water evaporation. A high percentage of it leads to the heating and temperature increase of the bulk fluid. The efficiency of the increase in the bulk fluid temperature, known as sensible efficiency, is obtained by equation (A.2): [44,48]
sensible
MC p T
(A.2)
t
IA
where M is the nanofluid total mass,
CP is the water-specific thermal coefficient, T is the
change in the bulk liquid temperature during the test, t is the test time, A is the surface area exposed to the radiation , and I is the light intensity density. Due to the existence of salinity, the Persian Gulf water used as the base fluid has different properties compared to distilled water. 24
Referring to the standards [52] and also considering seawater salinity to be 36 g. Lit-1, the seawater-specific heat capacity is 3.96 kJ.kg-1. K-1. The total efficiency is also obtained from the sum of the evaporation and sensible efficiencies:
total evaporation sensible
(A.3)
Fig. A.4 (a) shows the efficiencies of the GNP@MWCNT composite nanofluids with a 0.01% wt. concentration for the different ultrasonic times at 3.6 suns. An increase in ultrasonic time brings about small changes in the sensible efficiency, thereby exerting an insignificant influence on the bulk fluid temperature rise, whereas it causes the evaporation efficiency to increase and, as a result, the total efficiency to rise. Fig. A.4 (b) displays the efficiencies of the GNP@MWCNT composite nanofluids with various concentrations for 120 min ultrasonic time at 3.6 suns.
Fig. A.4. Sensible and total efficiencies of the GNP@MWCNT (0.01% wt.) composite nanofluid compared to those of the seawater for (a) different ultrasonic times and (b) different concentrations for 120 min sonication time at 3.6 suns.
25
Increasing the nanofluid concentration causes the photothermal energy transferred to the nanofluid to be used for the heat localization and the nanofluid surface temperature increase, whereas changes in the bulk fluid temperature will be negligible. This triggers slight increases in the sensible efficiency when the nanofluid concentration rises. However, the increase in light absorption followed by the rise in the fluid surface temperature triggers the rise of the evaporation rate and evaporation efficiency.
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There is no conflict of interest for this paper.
31
Highlights •
Sonication time effect on evaporation rate of GNP@MWCNT nanofluid was investigated.
•
Optimum sonication time was obtained at 120 min with 61.3% evaporation efficiency.
•
Effect of nanoparticle mass fraction and irradiation on efficiency was studied.
•
GNP@MWCNT nanofluid at 120 min sonication showed the best cost-efficiency.
•
The evaporated performance of MWCNT, GNP, GNP@MWCNT and pure water was compared.
32