- Email: [email protected]
An experimental study on β-cyclodextrin modified carbon nanotubes nanofluids for the direct absorption solar collector (DASC): Specific heat capacity and photo-thermal conversion performance
Solar Energy Materials & Solar Cells 204 (2020) 110240
Contents lists available at ScienceDirect
Solar Energy Materials and Solar Cells journal homepage: http://www.elsevier.com/locate/solmat
An experimental study on β-cyclodextrin modified carbon nanotubes nanofluids for the direct absorption solar collector (DASC): Specific heat capacity and photo-thermal conversion performance Xiaoke Li a, c, *, Wenjing Chen b, Changjun Zou b a b c
College of Materials and Chemistry & Chemical Engineering, Chengdu University of Technology, Chengdu, 610059, China Department of Chemistry and Chemical Engineering, Southwest Petroleum University, Chengdu, 610500, PR 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: Nanofluids Specific heat capacity Photo-thermal conversion DASC Solar thermal energy
Nanofluids have an advanced application prospect in the community of heat transfer fluids and particularly for the direct absorption solar collector (DASC). But most of body of available researches gave more attention to the thermal conductivity or rheology. We recently reported on the thermal conductivity and optical properties of the β-cyclodextrin modified carbon nanotubes (CD-CNTs) nanofluids. In this paper, we focused on the specific heat capacity and photo-thermal conversion performance of ethylene glycol-based CD-CNTs nanofluids for DASC system. To this end, a series of CD-CNTs nanofluids with different volume fractions (~0.1 vol%) were prepared. The specific heat capacity decreased with the increase of CD-CNTs loadings and the highest decrement in specific heat capacity was found to be 9.07% for 0.1 vol%. In addition, the temperature had significant effect over specific heat capacity of the studied nanofluids. The volume concentration and irradiation duration time put an effect on the photo-thermal conversion efficiency. In the nanofluid with appropriate concentration, the photo thermal conversion performance was improved with the extension of light irradiation time, and the possible mechanism of carbon nanotube agglomeration was qualitatively analyzed. In summary, the promising features could ensure the CD-CNTs nanofluids application prospect to be utilized in practical DASC system.
1. Introduction Nowadays, energy is one of the most important driving forces of industrial activities and economic developments. In addition, the rapid increase in energy demand caused by economic globalization and growing population, has led to the depletion of fossil fuels. Relative to fossil energy, renewable energy holds a special place in the world economy because of its sustainable and environmentally friendly char acteristics. Among them, solar energy has become an important part of human energy use because it is unlimited forever, safe and easily accessible [1,2]. However, because of low efficiency and high cost of some solar energy utilization devices, now the economy benefits of solar energy cannot compete with conventional energy. Therefore, how to effectively store or transfer solar energy is another focus of the academic world, both now and in the near future. Nanofluid was first proposed in 1995 and was defined as colloidal suspensions of nanoparticles with dimensions of less than 100 nm [3].
Nanofluids have important application value in improving heat transfer performance related to energy utilization. In recent years, many re searches have been made on the excellent heat transfer performance of various nanofluids [4,5]. The research findings showed that the superior thermal physical properties of nanofluids ensure their potential appli cations in solar systems [6], especially the direct absorption solar col lector (DASC) system, which could absorb the solar radiation directly and volumetrically [7–9]. As most commonly used HTFs are weak ab sorbers over the ultra-violet and visible ranges of the solar spectrum, the nanofluids could considerably improve the optical and photothermal properties of these transparent fluids. On the other hand, it has been proved that the nanofluids have enhanced thermal conductivity and heat transfer coefficient, which is beneficial for DASC applications. In order to obtain more sufficient and accurate data, various nanoparticles, including oxide, metal and carbon materials and so on, were utilized for DASC systems [10–13]. Therefore, several recent researches and review papers have been dedicated to the study of different nanofluids in DASC systems, which were listed in Table 1 [12–16].
* Corresponding author. No.1, Dongsan Road, Erxianqiao, Chenghua District, Chengdu, 610059, PR China. E-mail addresses: [email protected], [email protected] (X. Li). https://doi.org/10.1016/j.solmat.2019.110240 Received 23 May 2019; Received in revised form 14 October 2019; Accepted 19 October 2019 Available online 26 October 2019 0927-0248/© 2019 Elsevier B.V. All rights reserved.
X. Li et al.
Solar Energy Materials and Solar Cells 204 (2020) 110240
application prospects in solar energy systems because the carbon nanotubes have many advantageous performance like ultra-high ther mal conductivity value and sunlight absorptive enhancement [28]. Some researchers have studied the thermal conductivity of MWCNTs nanofluids for solar applications till date [15,29,30]. Karami and his co-workers focused on the applications of MWCNTs nanofluids in volumetric solar collectors [31]. They found that the extinction coeffi cient of water-based MWCNTs nanofluids showed considerable improvement compared to the pure water even at low particle concen tration. But as far as we know, no one has studied the specific heat of carbon nanotube nanofluids. And only a few researches focused on the photo-thermal conversion (PTC) efficiency for DASC system [32]. In our former study, we prepared the β-CD modified MWCNTs (CDCNTs) nanofluids in ethylene glycol (EG) [33]. The results showed that the EG/CD-CNTs nanofluids have excellent stability, good optical properties and enhanced thermal conductivity. This paper could be considered as a follow-up study. We focused on the specific heat ca pacity and PTC efficiency of EG based CD-CNT nanofluids in this study. Based on our former study, the colloidal stability, effects of concentra tion and temperature on the SHC, and the PTC efficiency of nanofluids were detected and discussed. Based on the fact that these important parameters of surface modified carbon nanotubes nanofluids (EG based) have not been reported or applied till date as working fluids in DASC system, the authors believe that this study will provide some useful data for the lack of information in the field of nanofluid science.
Nomenclature Symbols Cp ϕv m
ρ ζ
η
Specific heat capacity [KJ/(Kg⋅K)] Volume fraction of nanofluids [%] Mass [g] Density [g/cm2] Absolute Zeta potential Photo-thermal conversion efficiency [%]
Abbreviations HTFs Heat transfer fluids TC Thermal conductivity MWCNTs Multiwalled carbon nanotubes EG Ethylene glycol SHC Specific heat capacity PTC Photo-thermal conversion Subscripts n Nanoparticle b Base fluid nf Nanofluids
However, it is noteworthy that most of the existing studies has paid more attention to thermal conductivity or rheology of nanofluids. Only a small fraction (less than 5%) focused on the specific heat capacity (SHC) of nanofluids [17]. In fact, the SHC is also an important property as it is among the basic thermos-physical properties that characterize any heat transfer fluid (HTF). In general, specific heat signifies the thermal storage capacity of a system, it is also useful to calculate the dynamic thermal conductivity, diffusion coefficient and other related quantities, which are very critical for solar energy systems or devices [17,18]. In 1998, the specific heat of water-based nanofluid was firstly investigated by Pak and Cho [19]. The specific heat of studied nanofluid was found to decrease with the increase of volume fractions. Then, Zhou and his co-workers prepared the water-based alumina (Al2O3) nanofluids and found a maximum decrease in specific heat of 45% with mass concen trations of 21.7% [20]. On the other hand, Robertis et al. [21] measured the SHC of copper/ethylene glycol (EG) nanofluids and found that the specific heat of nanofluid decreased with particle concentrations but increasing with the temperature. Elias and his co-workers reported the similar trend for Al2O3-car radiator coolant nanofluids [22]. In addition, the specific heat capacities of EG-based nanofluids with titanium dioxide [23,24], zinc oxide [25], and silica [26] have been studied by different groups. Teng et al. [27] studied the specific heat of EG/water based multi-walled carbon nanotubes (MWCNTs) nanofluids. It was found that the specific heat of the base fluid could be reduced by 2–8% by adding MWCNTs, with the larger reductions corresponding to higher particle concentration (0.4 wt% maximum). However, none of them studied or combined the SHC of water or EG based nanofluids with solar energy applications. As is known to all, the MWCNTs based nanofluids exhibit a broader
2. Materials and methods 2.1. Materials The amino functionalized multiwalled carbon nanotubes (MWCNTsNH2, diameter of 8–15 nm, purity>95%) were provided by Chengdu organic chemistry co., LTD., Chinese academy of sciences. The CD-CNTs were prepared in our lab. Details could be found in the previous study [33]. The chemical structure of CD-CNTs is shown in Fig. 1. All the chemical reagents used in this work were obtained from Kelong chem ical reagent factory (Chengdu, China). 2.2. Preparation of nanofluids It is more reasonable to treat nanofluids as a special functional colloidal suspension. In this study, the most commonly used two-step method was used to prepare the EG based CD-CNT nanofluids [34,35]. The two-step method was conducted as follows. Firstly, a certain mass of CD-CNTs was added in the base fluid to obtain the required concentra tion. Then, the sample was magnetically stirred (500 rpm) for 1 h, and a certain amount of polyvinyl pyrrolidone (PVP) was added as dispersant in the process. It should be noted that PVP-K30 was selected as disper sant due to its high temperature resistance, low foamability and low viscosity [36]. The surfactant-to-nanoparticle (SN) mass ratio between PVP and CD-CNTs nanoparticles was 0.7. Then, an ultra-sonication ho mogenizer Sonifier 250 (Branson Ultrasonics, Danbury, USA) was applied to the samples for 6 h to obtain uniformly dispersed suspensions. During the sonication, a cold-water bath was used to keep the temper ature of the nanofluids. All the mentioned processes were performed at
Table 1 Researches on nanofluids properties and applications in DASC systems. Nanofluids
Focus of study
Authors
Graphene oxide/water-ethylene glycol nanofluids Graphene oxide nanoplatelets nanofluid Review on different nanofluids Hybrid CuO-MWCNT/H2O nanofluids Different types of nanofluids
Anti-freeze property and stability Photothermal specifications and stability Optical properties Photo-thermal performance
X.Xu [12] S. khosrojerdi [13] T.B. Gorji [14] J.Qu [15]
Nusselt numbers
M. Hatami [16]
Fig. 1. The chemical structure of CD-CNTs. 2
X. Li et al.
Solar Energy Materials and Solar Cells 204 (2020) 110240
the room temperature. The volume concentrations of nanofluids in this paper were calculated using Eq. (1) (~0.1 vol%). mn =ρn ϕv ¼ ðmn =ρn Þ þ ðmb =ρb Þ
vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi! ffiffiffiffiffi u� 2 �2 � �2 � �2 δCp;nf u δmnf δms δqs δqnf t � �¼ � � þ � � þ þ �Cp;nf � �mnf � �qnf � jms j jqs j
(1)
According to the calculation results, the maximum measurement uncertainty of SHC was 1.97%.
where ϕv means the volume fraction of nanofluids (%), m stands for quantity and ρ determines the density. The subscripts n and b represent nanoparticles and base liquid, respectively.
2.5. Photo-thermal conversion experiment The PTC experimental setup was assembled by ourselves. The sche matic diagram of the experimental device is shown as Fig. 2. Same amounts of CD-CNTs nanofluids with different volume frac tions were added into some flat bottom beakers (50 mL). The aluminized paper was pasted around and at the bottom of the beaker to effectively reduce the absorption of light. In addition, the beaker was enclosed in an adiabatic foam block (thermal conductivity�0.036 (W/(m K)) to mini mize the heat loss during the experiment. A solar simulator (SOLAR EGDE 700, Perfectlight Technology Co., Ltd., Beijing) was used as the radiative source (750 � 10 W/m2). The flux of incident radiation to the surface of the beaker was measured accurately by a solar radiation meter (AS823, SMART SENSOR, China). 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. Place another thermocouple outside the de vice for measuring ambient temperature. The maximum measuring temperature of thermocouple is 1800 � C, and the measuring error is 1.5 � C. All temperature measurements were performed by a data acquisition 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. To evaluate practical applications of EG based CD-CNTs nanofluids in DASC system, the effect of light irradiation cycle on the photothermal conversion performance was investigated by a simple test. Similar to the mentioned heating cycle, the studied nanofluids were initially irradiated by the solar simulator. The irradiated samples were then naturally cooled to room temperature and left overnight. This process was termed as one cycle of light irradiation. A total of ten cycles were performed for each sample.
2.3. Stability test In this part, a Zeta potential measurer (Brookhaven Instruments Corporation, USA) was used to predict suspension stability of nano fluids. 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. To further demonstrate the application potential of EG/CD-CNTs 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 convection, and the absorbance of sample was measured by UV–vis spectrophotometer at 980 nm subsequently. According to our previous studies, there was obvious absorption peak of ethylene glycol near 980 nm [33]. The above procedure was defined as one cycle, and each sample was repeatedly performed for thirty cycles in three days. After every 10 cycles, the samples were placed at room temperature over night. The following day, all samples will be subjected to 30 min of ul trasonic treatment to break up the soft aggregates. The samples without ultrasonic treatment were analyzed according to the same procedure as control experiments. It should be noted that even low temperature DASCs work in the temperature range of 20–100 � C. However, it was very hard to keep the temperature unchanged in the Zeta measurer and UV–vis spectropho tometer when the temperature was above 60 � C. Therefore, the tested temperature was 20–60 � C.
3. Results and discussion
2.4. Measurement of specific heat capacity
3.1. Stability
A Simultaneous Thermal Analyzer (DSC-823, Metler-Toledo, Switzerland) was employed to analyze the SHC, which was based on a standard test method ASTM E1269 [37]. First, the nanofluids samples (about 10 mg) were poured into the aluminum pans covered with pierced lids. The ramping rate was set to 10 � C/min during the measurement, and the end temperature was set to 120 � C. The nitrogen flow rate is 40 mL/min. The sample was further maintained at 120 � C for 5 min in order to ensure the accuracy of measurement data. First, a calibration experiment was run using an empty pan to obtain the baseline of heat flux. Then the heat flux of standard sapphire was measured and calibrated. The same procedure was then repeated three times for each nanofluid sample to ensure the accuracy of the results. The SHC of CD-CNTs nanofluids was determined by Eq. (2): Δqnf ⋅ms Cp;nf ¼ Cp;s ⋅ Δqs ⋅mnf
(3)
Colloidal stability is a necessary and sufficient condition for the
(2)
where Cp is the specific heat capacity, q means the heat flow and m represents the quantity of sample. Subscripts s represents standard sapphire and nf denotes nanofluids. The uncertainties of the experiment resulted from the measurement error of mass and heat flow. Therefore, the uncertainty of the experi ments could be expressed as Eq. (3) [38]:
Fig. 2. Schematic diagram of experimental apparatus. 3
X. Li et al.
Solar Energy Materials and Solar Cells 204 (2020) 110240
successful application of nanofluids. Homogeneous and stable EG-based CD-CNTs nanofluids have been prepared by the two-step method. The fresh samples with different volume fractions are shown in Fig. 3a. During the settlement experiment, no visible precipitation or stratifi cation was found even a month later (See Fig. 3b). In addition, the stability of colloidal systems is mainly the result of the interaction between particles and particle-fluid interfaces over time. Therefore, the stability of CD-CNTs nanofluids is closely related with their surface charge density. To accurately study the stability of nano fluids, the value of the Zeta potential (ζ) of nanofluids was measured at different volume concentrations. The results are shown in Fig. 4a. The Zeta potential decreased slightly with the volume fraction. But the excellent stability of CD-CNTs nanofluids was obtained at all fractions at 20 � C. Considering the application in DASC, nanofluids are always subjected to different temperature conditions. Hence, the ζ values of nanofluids at high temperature were also measured. It clearly indicated that the temperature put a side effect on the stability of nanofluids. The ζ value decreased with the increasing temperature. For example, the ζ value of 0.1 vol% CD-CNTs nanofluids decreased from 52.1 mV at 20 � C to 45.2 mV at 60 � C. This is because the high temperature will increase the probability and strength of the collision of nanoparticles and decrease the function of the surfactant, thus affecting the electrostatic balance of the suspension system. However, the CD-CNTs nanofluids generally could maintain moderate stability at high temperatures, which is beneficial for their applications in solar systems. To further test the stability of the studied nanofluid in DASC appli cations, the thermal cycling experiments were conducted and Fig. 4b shows the changes of relative absorbance (RA) of CD-CNTs nanofluids at various concentrations. As an evaluation index, RA was defined as the ratio of the absorbance of heated nanofluids to the initial absorbance of freshly prepared nanofluids. The RA values decreased with the number of thermal cycles. After 30 cycles of thermal cycling, the RA of samples without ultrasonic treatment decreased by 6.86%. This decline was reasonable and it meant the EG based CD-CNTs nanofluids could keep stable performance in DASC applications. But for the other group, sig nificant fluctuations could be observed after every 10 heating cycles. The results implied that the stability of CD-CNTs nanofluids would degrade inevitably under intermittent heated condition, but this trend could be easily reversed by ultrasonic processing. Therefore, necessary adjustments could be made easily according to the actual situation in the DASC applications. On the other hand, it was worth noting that the absorbance of heated samples was higher than that of freshly prepared nanofluids (RA>1). This is principally because the agglomerations of carbon nanotubes, which could make nanofluids show a better perfor mance in solar collector applications, especially in the long-wavelength [39]. High concentration nanofluids were more susceptible to the negative effects of cyclic heating, so the aggregates of nanoparticles
were much easier to form in nanofluids with higher volume fraction. Therefore, the absorbance of nanofluids with higher volume fraction was clearly larger in the early stage of cyclic heating experiment. In conclusion, the stability of CD-CNTs nanofluids would decrease slightly after cyclic heating, which was acceptable. But the capability of solar energy collection for CD-CNTs nanofluids will be enhanced due to the emergence of carbon nanotube flocs, which is eagerly desirable for the direct absorption solar collector in solar systems. The similar phenom enon also happened in other studies [32,39]. It should be noted that the agglomeration of nanoparticles in a nanofluid is a drawback to a solar system. But the premise is that the size of aggregates is large enough to cause precipitation or stratification. Here in our nanofluids system, we found that only tiny flocs would form in nanofluids which were invisible to the naked eye. Therefore, combining the results of stability tests, the β-cyclodextrin modified carbon nanotubes nanofluid system have potential to be used in DASC system. 3.2. Specific heat capacity The variation of SHC with respect to volume fraction of CD-CNTs loadings at room temperature is illustrated in Fig. 5a. As shown in the diagram, the SHC of nanofluid decreased with the increase of CD-CNTs loadings. This was mainly because the specific heat value of EG was much higher than that of the nanomaterials. The highest decrement was found to be 9.07% for 0.1 vol% as compared to pure EG. Teng et al. [27] found the similar trend on the SHC of multi-walled carbon nanotubes nanofluids (water-EG mixture based), which was found to decrease the SHC of the base fluid by 2–8% (0.1–0.4 wt%). However, it was reported that the SHC of 0.15 vol% of EG/Water mixture MWCNT nanofluid was higher than that of base fluid [40]. But it was decreasing with the in crease of volume fraction. Therefore, the thermal capacity of nanofluids could be decreased or increased relative to the base fluids. This depends on the types of nanoparticles and base fluids [17]. In addition, the free energy of solid-liquid interface varies with the change of suspended particles. Because of the large specific surface area of nanoparticles, the surface energy of nanoparticles has a large proportion in the system capacity, which affects the SHC of nanofluids systems. On the other hand, the grafting of β-CD onto the surface of MWCNT changed the surface hydrophobicity of carbon nanotubes, which could induce a phase transformation in the solvent material in the immediate vicinity of the nanotubes. These adhesive layers were expected to have semisolid properties, thus improving the thermal conductivity and density of nanofluids [33]. This reduced the energy demand for the same tem perature increment, thereby reducing the SHC value. Based on classical mixing theory, a simple model has been proposed to compute the SHC of nanofluids, which is expressed as follows:
Fig. 3. The EG-based CD-CNTs nanofluids with different volume fractions: (a) one day after preparation; (b) one month after preparation. 4
X. Li et al.
Solar Energy Materials and Solar Cells 204 (2020) 110240
Fig. 4. (a) The absolute value of Zeta potential of CD-CNTs nanofluids at different temperature; (b) The relative absorbance of CD-CNTs nanofluids with different cycles of heating.
Fig. 5. (a) Specific heat capacity of EG based CD-CNTs nanofluids vs. volume concentration; (b) Effect of temperature on specific heat capacity of CDCNTs nanofluids.
Cp;nf ¼ ϕv ⋅ Cp;n þ ð1
ϕv Þ⋅Cp;b
nanoparticles. Vajjha et al. got the similar conclusion when they studied the SHC properties of EG/Al2O3 nanofluids [43]. However, it should be noted that, although this model has been widely used in the studies of SHC of nanofluids, its validity remains to be verified. We believe the present study could contribute to the further understanding of specific heat in nanofluid field. Effect of temperature on SHC was shown in Fig. 5b. A considerable enhancement (~27.4% on 0.1 vol% nanofluid) in the specific heat value with the temperature was found in the experiment. And most of the literatures showed the similar trend on different nanofluids [40,44,45]. Therefore, one could summarize that temperature has a significant effect on the thermal capacity or SHC of nanofluids. Specifically, the enhancement in the specific heat capacity was most likely because of the high surface area of the carbon nanotubes per unit volume, which could provide high surface energy [40]. If an application requires a wide or high temperature range, the use of high SHC fluids would be very beneficial [44]. It is highly preferred that the heat transfer fluid has higher values of SHC in order to absorb more heat from the power sources. In addition, DASC systems require long-term operation at high temperatures. Therefore, any increase in heat capacity of such nano fluids is of great importance for their practical applications in DASC system. Therefore, the CD-CNTs nanofluids are potential fluids to the DASC system.
(4)
where the subscripts nf, n and b represent nanofluids, nanoparticles and base fluid, respectively. However, recent researches have revealed the shortcomings of the model [17,41]. On the other hand, the SHC of nanofluid could be easily deduced based on the First Law of Thermodynamics if supposing that the nano particles and the base fluid were in thermal equilibrium. Therefore, an improved model for computing SHC of nanofluids was proposed by Buongiorno et al. [42]. It is expressed as Eq. (5). Cp;nf ¼
ð1
ϕv Þ⋅Cp;b ⋅ρb þ ϕv ⋅Cp;n ⋅ρn ð1 ϕv Þ⋅ρb þ ϕv ⋅ρn
(5)
where,
ρnf ¼ ϕv ⋅ ρn þ ð1
ϕv Þ⋅ρb
(6)
The comparison of measured SHC with theoretical calculation value was also shown in Fig. 5a. It was clearly seen that the predicted values using Eq. (5) was much higher than the measured specific heat capacity of CD-CNTs nanofluids. Due to higher thermal diffusivity of nano particles, the carbon nanotubes absorbed heat faster than the EG. Therefore, in fact the nanoparticles and the base liquid were not actually in thermal equilibrium. Hence, the reason of the discrepancy between predicted values and the measured values was because the model was based on the thermal equilibrium between liquid phase and 5
X. Li et al.
Solar Energy Materials and Solar Cells 204 (2020) 110240
3.3. Photo-thermal conversion performance The photo-thermal properties of nanofluids depend on the relation ship between the temperature change of nanofluids and solar radiation time to a great extent. The temperature rise curves of EG/CD-CNTs nanofluids with different volume fractions were plotted in Fig. 6. It was obvious that a considerable enhancement in the PTC performance of pure EG was obtained after the addition of CD-CNTs nanoparticles. The temperature of nanofluids rose faster than that of EG during the irra diation time. The temperature difference increased with the increase of the volume concentration of CD-CNTs nanoparticles. The temperature rise rate and the maximum equilibrium temperature of the nanofluids were higher than those of EG. For instance, the highest temperature difference of CD-CNTs nanofluid (0.1 vol%) was 79.3 � C (at the irradi ation time of 50 min), whose temperature rise rate was 1.586 � C/min. This was about 25% higher than that of base fluid. On the other hand, it took approximately 125 min for EG to reach its maximum equilibrium temperature difference (105 � C) under the illu mination of a 390 W power xenon lamp in the present experiment. But it only took 110 min for CD-CNTs nanofluids to reach its maximum equi librium temperature difference (132 � C). Therefore, the photothermal conversion performance of base fluid (EG in this paper) could be significantly improved by adding a small amount of CD-CNTs. And this was also closely related to the enhanced stability and thermal conduc tivity of CD-CNTs nanofluids [33]. Fig. 7 shows the temperature variations of EG/CD-CNTs nanofluids with different volume fractions after different light-irradiated cycles. For 0.005 vol% CD-CNTs nanofluid, it could be found that the temper ature of nanofluid increased with increasing heating cycles but decreased after six cycles of light irradiation (Fig. 7a). Compared to the first-cycle irradiation treatment, the temperatures of 0.005 vol% nano fluid were increased by 3.87 � C after being irradiated two cycles and this numerical value was 6.85 � C after being irradiated four cycles. However, the temperature of nanofluid finally decreased after a further increase of heating cycles (six and ten cycles). As mentioned and discussed in sec tion 3.1, the increase of temperature would bring instability to colloidal system, which would result in small carbon nanotube agglomerations. If the nanoparticle aggregates were limited, the resultant size reduction of aggregates could enhance the optical absorption coefficient [39]. However, the serious aggregation of carbon nanotubes will definitely aggravate the suspension instability of nanofluids. If the carbon nano tube clusters grow up to a certain size and have obvious sedimentation, the concentration of carbon nanotube aggregates will decrease from the optimum value accordingly, subsequently reducing the PTC
Fig. 7. Temperature profiles of EG/CD-CNTs nanofluids with different light irradiation times: (a) 0.005 vol%, (b) 0.05 vol%, and (c) 0.1 vol%.
Fig. 6. Temperature difference of CD-CNTs nanofluids as a function of irradi ation time. 6
X. Li et al.
Solar Energy Materials and Solar Cells 204 (2020) 110240
performance [15,32]. Therefore, appropriate carbon nanotube aggre gates in nanofluids were beneficial to improve the thermal recovery performance of solar energy. Similarly, for the 0.05 vol% EG/CD-CNTs nanofluid (see Fig. 7b), the temperature of nanofluid increased with the increase of light irradiation cycle only during the first two cycles. And for higher volume fraction (0.1 vol%), the PTC performance degraded with the increasing heating cycles, but it was still superior to the base fluid (see Fig. 7c). This was consistent with the phenomenon observed in the stability test. It could be therefore concluded that cyclic illumination would not affect the PTC efficiency of CD-CNTs nanofluids with appropriate concentration. Therefore, it is very important to choose the most suitable concentration of nanofluids in practical DASC applications. To qualitatively analyze the PTC performance of nanofluids, the PTC efficiency was calculated. The photo-thermal conversion efficiency η was given by Eq. (7).
η¼
mnf Cp;nf ΔT GAΔt
Fig. 8. The photo-thermal conversion efficiency of EG/CD-CNTs nanofluids.
(7)
nanofluids, and further quantitative analysis will be the focus of our future work.
where Δ T 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. In addition, considering the additive amount of MWCNTs was very small and many existed papers had proved the effect of MWCNTs could be ignored [46]. It could be further simplified as follows:
η¼
ρb Cp;b HΔT GΔt
4. Conclusion Since the specific heat capacity (SHC) and photo-thermal conversion (PTC) efficiency of EG based CD-CNTs nanofluids have not been re ported or applied till date as HTFs in DASC system. In this paper, experimental research and theoretical analysis were carried out in a scientific and objective way.
(8)
where H is the height of nanofluids layer. There were two assumptions about this formula. 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 PTC efficiency is shown in Fig. 8. The results clearly showed that the maximum values of PTC efficiency presented at 25 min for all volume fractions, but then it became less efficient. This was mainly because the increased heat loss during nanofluid temperature rise [32,47]. Heat loss was mainly man ifested as convection loss to the surrounding environment and radiation loss to the upper part, which was directly related to the temperature of the nanofluid. The PTC efficiency of 0.1 vol% CD-CNTs nanofluid decreased from 81.3% to 41.5% when the irradiation time duration increased from 25 to 125 min. And during this time, the temperature increased from 45 � C to 132 � C. And it should be noted that the highest PTC efficiency at 0.1 vol% was up to 81.3% at 25 min, which was about 39.1% higher than that of EG. In addition, according to Figs. 6 and 8, a decrease in the PTC effi ciency would occur if the irradiation time increased (e.g. 150 min). This phenomenon could be explained as: 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. In general, as a heat conducting and collecting medium, EG/CDCNTs nanofluids prepared in this paper have great application value and prospect in DASC system. Considering the properties of EG/CDCNTs nanofluids, we believe that 0.05–0.1 vol% nanofluids are the most suitable concentration range for DASCs applications. Although the specific heat capacity decreased with the increase of concentration, the studied nanofluids in this concentration range exhibited excellent thermal conductivity and photothermal conversion efficiency. In addi tion, the concentration could be adjusted according to the actual situa tion. However, the PTC performances of any nanofluid system is extremely complex. This section only provided a limited understanding of the underlying factors affecting PTC performance of EG/CD-CNTs
1. First, the two-step method was used to prepare EG based CD-CNTs nanofluids. Then the stability of nanofluids were evaluated through static experiment and Zeta potential measurement. In addition, the thermal cycling experiments were conducted to further test the stability of the studied nanofluid in DASC applications. 2. The SHC of nanofluids is of great importance for their practical ap plications in DASC system. Thus, the nanoparticle concentration and temperature effect on SHC of nanofluids were investigated. The SHC value decreased with the increasing of CD-CNTs loadings and the highest decrement in specific heat capacity was found to be 9.07% for 0.1 vol%. On the other hand, it was found that the temperature had significant effect over specific heat capacity of the studied nanofluids. 3. The effect of volume fraction and sunlight repetitive radiation on the PTC performance were investigated respectively. The PTC perfor mance of CD-CNTs nanofluids was largely determined by the volume concentration. In addition, compared with EG, the temperature of CD-CNTs nanofluid of 0.1 vol% was increased by 132 � C after the light irradiation of 110 min. On the other hand, the experiment re sults showed that cyclic illumination would not affect the PTC effi ciency of CD-CNTs nanofluids with appropriate concentration. The photothermal conversion efficiency increased with the increase of volume concentration and decreased with the irradiation time. The highest PTC efficiency at 0.1 vol% is up to 81.3% at 25 min, which is about 39.1% higher than that of EG. In conclusion, we believed that the promising features of suitable SHC and PTC efficiency could ensure the CD-CNTs nanofluids applica tion prospect to be utilized in practical DASC system for different tar gets. And we recommend that 0.05–0.1 vol% nanofluids are the most suitable concentration range for DASCs applications. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence 7
X. Li et al.
Solar Energy Materials and Solar Cells 204 (2020) 110240
the work reported in this paper.
[21] E.D. Robertis, E.H.H. Cosme, R.S. Neves, A.Y. Kuznetsov, A.P.C. Campos, S. M. Landi, C.A. Achete, Application of the modulated temperature differential scanning calorimetry technique for the determination of the specific heat of copper nanofluids, Appl. Therm. Eng. 41 (2012) 10–17. [22] M.M. Elias, I.M. Mahbubul, R. Saidur, M.R. Sohel, I.M. Shahrul, S. S. Khaleduzzaman, S. Sadeghipour, Experimental investigation on the thermophysical properties of Al 2 O 3 nanoparticles suspended in car radiator coolant ☆, Int. Commun. Heat Mass Transf. 54 (2014) 48–53. [23] HwaMing Nieh, TunPing Teng, ChaoChieh Yu, Enhanced heat dissipation of a radiator using oxide nano-coolant, Int. J. Therm. Sci. 77 (2014) 252–261. [24] S.M.S. Murshed, Determination of effective specific heat of nanofluids, J. Exp. Nanosci. 6 (2011) 539–546. [25] R.S. Vajjha, D.K. Das, Experimental determination of thermal conductivity of three nanofluids and development of new correlations, Int. J. Heat Mass Transf. 52 (2009) 4675–4682. [26] A.K. Starace, J.C. Gomez, J. Wang, S. Pradhan, G.C. Glatzmaier, Nanofluid heat capacities, J. Appl. Phys. 110 (2011) 5. [27] T.P. Teng, L. Lin, C.C. Yu, Preparation and characterization of carbon nanofluids by using a revised water-assisted synthesis method, J. Nanomater. 2013 (2013) 1–12, 2013,(2013-11-10). [28] H. Xie, L. Chen, Review on the preparation and thermal performances of carbon nanotube contained nanofluids, J. Chem. Eng. Data 56 (2011) 1030–1041. [29] 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. [30] R.C. Shende, S. Ramaprabhu, Thermo-optical properties of partially unzipped multiwalled carbon nanotubes dispersed nanofluids for direct absorption solar thermal energy systems, Sol. Energy Mater. Sol. Cells 157 (2016) 117–125. [31] M. Karami, M.A.A. Bahabadi, S. Delfani, A. Ghozatloo, A new application of carbon nanotubes nanofluid as working fluid of low-temperature direct absorption solar collector, Sol. Energy Mater. Sol. Cells 121 (2014) 114–118. [32] J. Qu, M. Tian, X. Han, R. Zhang, Q. Wang, Photo-thermal conversion characteristics of MWCNT-H2O nanofluids for direct solar thermal energy absorption applications, Appl. Therm. Eng. 124 (2017) 486–493. [33] X. Li, C. Zou, W. Chen, X. Lei, Experimental investigation of β-cyclodextrin modified carbon nanotubes nanofluids for solar energy systems: stability, optical properties and thermal conductivity, Sol. Energy Mater. Sol. Cells 157 (2016) 572–579. _ [34] 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. [35] X. Li, C. Zou, A. Qi, Experimental study on the thermo-physical properties of car engine coolant (water/ethylene glycol mixture type) based SiC nanofluids, Int. Commun. Heat Mass Transf. 77 (2016) 159–164. [36] B. Wang, S. Liu, Y. Zhu, S. Ge, Influence of polyvinyl pyrrolidone on the dispersion of multi-walled carbon nanotubes in aqueous solution, Russ. J. Phys. Chem. 88 (2014) 2385–2390. [37] Y. Hu, Y. He, Z. Zhang, D. Wen, Enhanced heat capacity of binary nitrate eutectic salt-silica nanofluid for solar energy storage, Sol. Energy Mater. Sol. Cells 192 (2019) 94–102. [38] S.V. Gupta, Propagation of uncertainty, Meas. Uncertain. 27 (2012) 12–17. [39] D. Song, Y. Wang, D. Jing, J. Geng, Investigation and prediction of optical properties of alumina nanofluids with different aggregation properties, Int. J. Heat Mass Transf. 96 (2016) 430–437. [40] V. Kumaresan, R. Velraj, Experimental investigation of the thermo-physical properties of water–ethylene glycol mixture based CNT nanofluids, Thermochim. Acta 545 (2012) 180–186. [41] W. Daungthongsuk, S. Wongwises, A critical review of convective heat transfer of nanofluids, Renew. Sustain. Energy Rev. 11 (2007) 797–817. [42] J. Buongiorno, Convective transport in nanofluids, J. Heat Transf. 128 (2005) 240–250. [43] R.S. Vajjha, D.K. Das, Measurements of specific heat and density of Al2O3 nanofluid, 2008. [44] I.M. Shahrul, I.M. Mahbubul, S.S. Khaleduzzaman, R. Saidur, M.F.M. Sabri, A comparative review on the specific heat of nanofluids for energy perspective, Renew. Sustain. Energy Rev. 38 (2014) 88–98. [45] E. De Robertis, E.H.H. Cosme, R.S. Neves, A.Y. Kuznetsov, A.P.C. Campos, S. M. Landi, C.A. Achete, Application of the modulated temperature differential scanning calorimetry technique for the determination of the specific heat of copper nanofluids, Appl. Therm. Eng. 41 (2012) 10–17. [46] L. Chen, J. Liu, X. Fang, Z. Zhang, Reduced graphene oxide dispersed nanofluids with improved photo-thermal conversion performance for direct absorption solar collectors, Sol. Energy Mater. Sol. Cells 163 (2017) 125–133. [47] 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.
Acknowledgments The authors would like to acknowledge the financial support of CAS Light of West China Program (Z1113-46) and The National Natural Science Foundation of China (21576225). 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.2019.110240. References [1] O Bait, Mohamed Si–Ameur, Enhanced heat and mass transfer in solar stills using nanofluids: A review, Solar Energy 170 (2018) 694–722. [2] T.R. Shah, H.M. Ali, Applications of hybrid nanofluids in solar energy, practical limitations and challenges: a critical review, Sol. Energy 183 (2019) 173–203. [3] S.U.S. Choi, Developments and Applications of Non-newtonian Flows, vol. 231, ASME, New York, 1995, pp. 99–102. [4] V. Fuskele, R.M. Sarviya, Recent developments in nanoparticles synthesis, preparation and stability of nanofluids, Mater. Today: Proc. 4 (2017) 4049–4060. [5] D.K. Devendiran, V.A. Amirtham, A review on preparation, characterization, properties and applications of nanofluids, Renew. Sustain. Energy Rev. 60 (2016) 21–40. [6] R.A. Taylor, P.E. Phelan, T.P. Otanicar, C.A. Walker, M. Nguyen, S. Trimble, R. Prasher, Applicability of nanofluids in high flux solar collectors, J. Renew. Sustain. Energy 3 (2011) 4. [7] T.P. Otanicar, P.E. Phelan, R.S. Prasher, G. Rosengarten, R.A. Taylor, Nanofluidbased direct absorption solar collector, J. Renew. Sustain. Energy 2 (2010), 033102. [8] 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. [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 II: ZnO, CeO2, Fe2O3 nanoparticles behavior, Sol. Energy Mater. Sol. Cells 147 (2016) 321–326. [10] L. Syam Sundar, K.M. Singh, M. Hashim Farooky, S. Naga Sarada, Experimental thermal conductivity of ethylene glycol and water mixture;based low volume concentration of Al2O3 and CuO nanofluids, Int. Commun. Heat Mass Transf. 41 (2013) 41–46. [11] M. Potenza, M. Milanese, G. Colangelo, A. de Risi, Experimental investigation of transparent parabolic trough collector based on gas-phase nanofluid, Appl. Energy 203 (2017) 560–570. [12] X. Xu, C. Xu, J. Liu, X. Fang, Z. Zhang, A direct absorption solar collector based on a water-ethylene glycol based nanofluid with anti-freeze property and excellent dispersion stability, Renew. Energy 133 (2019) 760–769. [13] S. khosrojerdi, A.M. Lavasani, M. Vakili, Experimental study of photothermal specifications and stability of graphene oxide nanoplatelets nanofluid as working fluid for low-temperature Direct Absorption Solar Collectors (DASCs), Sol. Energy Mater. Sol. Cells 164 (2017) 32–39. [14] T.B. Gorji, A.A. Ranjbar, A review on optical properties and application of nanofluids in direct absorption solar collectors (DASCs), Renew. Sustain. Energy Rev. 72 (2017) 10–32. [15] J. Qu, R. Zhang, Z. Wang, Q. Wang, Photo-thermal conversion properties of hybrid CuO-MWCNT/H2O nanofluids for direct solar thermal energy harvest, Appl. Therm. Eng. 147 (2019) 390–398. [16] M. Hatami, D. Jing, Evaluation of wavy direct absorption solar collector (DASC) performance using different nanofluids, J. Mol. Liq. 229 (2017) 203–211. [17] H. Riazi, T. Murphy, G.B. Webber, R. Atkin, S.S.M. Tehrani, R.A. Taylor, Specific heat control of nanofluids: a critical review, Int. J. Therm. Sci. 107 (2016) 25–38. [18] M. Lomascolo, G. Colangelo, M. Milanese, A. de Risi, Review of heat transfer in nanofluids: conductive, convective and radiative experimental results, Renew. Sustain. Energy Rev. 43 (2015) 1182–1198. [19] B.C. Pak, Y.I. Cho, Hydrodynamic and heat transfer study OF dispersed fluids with submicron metallic oxide particles, Exp. Heat Transf. 11 (1998) 151–170. [20] S.Q. Zhou, R. Ni, Measurement of the specific heat capacity of water-based AL2O3 nanofluid, Appl. Phys. Lett. 92 (2008) 99.
8
Contents lists available at ScienceDirect
Solar Energy Materials and Solar Cells journal homepage: http://www.elsevier.com/locate/solmat
An experimental study on β-cyclodextrin modified carbon nanotubes nanofluids for the direct absorption solar collector (DASC): Specific heat capacity and photo-thermal conversion performance Xiaoke Li a, c, *, Wenjing Chen b, Changjun Zou b a b c
College of Materials and Chemistry & Chemical Engineering, Chengdu University of Technology, Chengdu, 610059, China Department of Chemistry and Chemical Engineering, Southwest Petroleum University, Chengdu, 610500, PR 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: Nanofluids Specific heat capacity Photo-thermal conversion DASC Solar thermal energy
Nanofluids have an advanced application prospect in the community of heat transfer fluids and particularly for the direct absorption solar collector (DASC). But most of body of available researches gave more attention to the thermal conductivity or rheology. We recently reported on the thermal conductivity and optical properties of the β-cyclodextrin modified carbon nanotubes (CD-CNTs) nanofluids. In this paper, we focused on the specific heat capacity and photo-thermal conversion performance of ethylene glycol-based CD-CNTs nanofluids for DASC system. To this end, a series of CD-CNTs nanofluids with different volume fractions (~0.1 vol%) were prepared. The specific heat capacity decreased with the increase of CD-CNTs loadings and the highest decrement in specific heat capacity was found to be 9.07% for 0.1 vol%. In addition, the temperature had significant effect over specific heat capacity of the studied nanofluids. The volume concentration and irradiation duration time put an effect on the photo-thermal conversion efficiency. In the nanofluid with appropriate concentration, the photo thermal conversion performance was improved with the extension of light irradiation time, and the possible mechanism of carbon nanotube agglomeration was qualitatively analyzed. In summary, the promising features could ensure the CD-CNTs nanofluids application prospect to be utilized in practical DASC system.
1. Introduction Nowadays, energy is one of the most important driving forces of industrial activities and economic developments. In addition, the rapid increase in energy demand caused by economic globalization and growing population, has led to the depletion of fossil fuels. Relative to fossil energy, renewable energy holds a special place in the world economy because of its sustainable and environmentally friendly char acteristics. Among them, solar energy has become an important part of human energy use because it is unlimited forever, safe and easily accessible [1,2]. However, because of low efficiency and high cost of some solar energy utilization devices, now the economy benefits of solar energy cannot compete with conventional energy. Therefore, how to effectively store or transfer solar energy is another focus of the academic world, both now and in the near future. Nanofluid was first proposed in 1995 and was defined as colloidal suspensions of nanoparticles with dimensions of less than 100 nm [3].
Nanofluids have important application value in improving heat transfer performance related to energy utilization. In recent years, many re searches have been made on the excellent heat transfer performance of various nanofluids [4,5]. The research findings showed that the superior thermal physical properties of nanofluids ensure their potential appli cations in solar systems [6], especially the direct absorption solar col lector (DASC) system, which could absorb the solar radiation directly and volumetrically [7–9]. As most commonly used HTFs are weak ab sorbers over the ultra-violet and visible ranges of the solar spectrum, the nanofluids could considerably improve the optical and photothermal properties of these transparent fluids. On the other hand, it has been proved that the nanofluids have enhanced thermal conductivity and heat transfer coefficient, which is beneficial for DASC applications. In order to obtain more sufficient and accurate data, various nanoparticles, including oxide, metal and carbon materials and so on, were utilized for DASC systems [10–13]. Therefore, several recent researches and review papers have been dedicated to the study of different nanofluids in DASC systems, which were listed in Table 1 [12–16].
* Corresponding author. No.1, Dongsan Road, Erxianqiao, Chenghua District, Chengdu, 610059, PR China. E-mail addresses: [email protected], [email protected] (X. Li). https://doi.org/10.1016/j.solmat.2019.110240 Received 23 May 2019; Received in revised form 14 October 2019; Accepted 19 October 2019 Available online 26 October 2019 0927-0248/© 2019 Elsevier B.V. All rights reserved.
X. Li et al.
Solar Energy Materials and Solar Cells 204 (2020) 110240
application prospects in solar energy systems because the carbon nanotubes have many advantageous performance like ultra-high ther mal conductivity value and sunlight absorptive enhancement [28]. Some researchers have studied the thermal conductivity of MWCNTs nanofluids for solar applications till date [15,29,30]. Karami and his co-workers focused on the applications of MWCNTs nanofluids in volumetric solar collectors [31]. They found that the extinction coeffi cient of water-based MWCNTs nanofluids showed considerable improvement compared to the pure water even at low particle concen tration. But as far as we know, no one has studied the specific heat of carbon nanotube nanofluids. And only a few researches focused on the photo-thermal conversion (PTC) efficiency for DASC system [32]. In our former study, we prepared the β-CD modified MWCNTs (CDCNTs) nanofluids in ethylene glycol (EG) [33]. The results showed that the EG/CD-CNTs nanofluids have excellent stability, good optical properties and enhanced thermal conductivity. This paper could be considered as a follow-up study. We focused on the specific heat ca pacity and PTC efficiency of EG based CD-CNT nanofluids in this study. Based on our former study, the colloidal stability, effects of concentra tion and temperature on the SHC, and the PTC efficiency of nanofluids were detected and discussed. Based on the fact that these important parameters of surface modified carbon nanotubes nanofluids (EG based) have not been reported or applied till date as working fluids in DASC system, the authors believe that this study will provide some useful data for the lack of information in the field of nanofluid science.
Nomenclature Symbols Cp ϕv m
ρ ζ
η
Specific heat capacity [KJ/(Kg⋅K)] Volume fraction of nanofluids [%] Mass [g] Density [g/cm2] Absolute Zeta potential Photo-thermal conversion efficiency [%]
Abbreviations HTFs Heat transfer fluids TC Thermal conductivity MWCNTs Multiwalled carbon nanotubes EG Ethylene glycol SHC Specific heat capacity PTC Photo-thermal conversion Subscripts n Nanoparticle b Base fluid nf Nanofluids
However, it is noteworthy that most of the existing studies has paid more attention to thermal conductivity or rheology of nanofluids. Only a small fraction (less than 5%) focused on the specific heat capacity (SHC) of nanofluids [17]. In fact, the SHC is also an important property as it is among the basic thermos-physical properties that characterize any heat transfer fluid (HTF). In general, specific heat signifies the thermal storage capacity of a system, it is also useful to calculate the dynamic thermal conductivity, diffusion coefficient and other related quantities, which are very critical for solar energy systems or devices [17,18]. In 1998, the specific heat of water-based nanofluid was firstly investigated by Pak and Cho [19]. The specific heat of studied nanofluid was found to decrease with the increase of volume fractions. Then, Zhou and his co-workers prepared the water-based alumina (Al2O3) nanofluids and found a maximum decrease in specific heat of 45% with mass concen trations of 21.7% [20]. On the other hand, Robertis et al. [21] measured the SHC of copper/ethylene glycol (EG) nanofluids and found that the specific heat of nanofluid decreased with particle concentrations but increasing with the temperature. Elias and his co-workers reported the similar trend for Al2O3-car radiator coolant nanofluids [22]. In addition, the specific heat capacities of EG-based nanofluids with titanium dioxide [23,24], zinc oxide [25], and silica [26] have been studied by different groups. Teng et al. [27] studied the specific heat of EG/water based multi-walled carbon nanotubes (MWCNTs) nanofluids. It was found that the specific heat of the base fluid could be reduced by 2–8% by adding MWCNTs, with the larger reductions corresponding to higher particle concentration (0.4 wt% maximum). However, none of them studied or combined the SHC of water or EG based nanofluids with solar energy applications. As is known to all, the MWCNTs based nanofluids exhibit a broader
2. Materials and methods 2.1. Materials The amino functionalized multiwalled carbon nanotubes (MWCNTsNH2, diameter of 8–15 nm, purity>95%) were provided by Chengdu organic chemistry co., LTD., Chinese academy of sciences. The CD-CNTs were prepared in our lab. Details could be found in the previous study [33]. The chemical structure of CD-CNTs is shown in Fig. 1. All the chemical reagents used in this work were obtained from Kelong chem ical reagent factory (Chengdu, China). 2.2. Preparation of nanofluids It is more reasonable to treat nanofluids as a special functional colloidal suspension. In this study, the most commonly used two-step method was used to prepare the EG based CD-CNT nanofluids [34,35]. The two-step method was conducted as follows. Firstly, a certain mass of CD-CNTs was added in the base fluid to obtain the required concentra tion. Then, the sample was magnetically stirred (500 rpm) for 1 h, and a certain amount of polyvinyl pyrrolidone (PVP) was added as dispersant in the process. It should be noted that PVP-K30 was selected as disper sant due to its high temperature resistance, low foamability and low viscosity [36]. The surfactant-to-nanoparticle (SN) mass ratio between PVP and CD-CNTs nanoparticles was 0.7. Then, an ultra-sonication ho mogenizer Sonifier 250 (Branson Ultrasonics, Danbury, USA) was applied to the samples for 6 h to obtain uniformly dispersed suspensions. During the sonication, a cold-water bath was used to keep the temper ature of the nanofluids. All the mentioned processes were performed at
Table 1 Researches on nanofluids properties and applications in DASC systems. Nanofluids
Focus of study
Authors
Graphene oxide/water-ethylene glycol nanofluids Graphene oxide nanoplatelets nanofluid Review on different nanofluids Hybrid CuO-MWCNT/H2O nanofluids Different types of nanofluids
Anti-freeze property and stability Photothermal specifications and stability Optical properties Photo-thermal performance
X.Xu [12] S. khosrojerdi [13] T.B. Gorji [14] J.Qu [15]
Nusselt numbers
M. Hatami [16]
Fig. 1. The chemical structure of CD-CNTs. 2
X. Li et al.
Solar Energy Materials and Solar Cells 204 (2020) 110240
the room temperature. The volume concentrations of nanofluids in this paper were calculated using Eq. (1) (~0.1 vol%). mn =ρn ϕv ¼ ðmn =ρn Þ þ ðmb =ρb Þ
vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi! ffiffiffiffiffi u� 2 �2 � �2 � �2 δCp;nf u δmnf δms δqs δqnf t � �¼ � � þ � � þ þ �Cp;nf � �mnf � �qnf � jms j jqs j
(1)
According to the calculation results, the maximum measurement uncertainty of SHC was 1.97%.
where ϕv means the volume fraction of nanofluids (%), m stands for quantity and ρ determines the density. The subscripts n and b represent nanoparticles and base liquid, respectively.
2.5. Photo-thermal conversion experiment The PTC experimental setup was assembled by ourselves. The sche matic diagram of the experimental device is shown as Fig. 2. Same amounts of CD-CNTs nanofluids with different volume frac tions were added into some flat bottom beakers (50 mL). The aluminized paper was pasted around and at the bottom of the beaker to effectively reduce the absorption of light. In addition, the beaker was enclosed in an adiabatic foam block (thermal conductivity�0.036 (W/(m K)) to mini mize the heat loss during the experiment. A solar simulator (SOLAR EGDE 700, Perfectlight Technology Co., Ltd., Beijing) was used as the radiative source (750 � 10 W/m2). The flux of incident radiation to the surface of the beaker was measured accurately by a solar radiation meter (AS823, SMART SENSOR, China). 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. Place another thermocouple outside the de vice for measuring ambient temperature. The maximum measuring temperature of thermocouple is 1800 � C, and the measuring error is 1.5 � C. All temperature measurements were performed by a data acquisition 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. To evaluate practical applications of EG based CD-CNTs nanofluids in DASC system, the effect of light irradiation cycle on the photothermal conversion performance was investigated by a simple test. Similar to the mentioned heating cycle, the studied nanofluids were initially irradiated by the solar simulator. The irradiated samples were then naturally cooled to room temperature and left overnight. This process was termed as one cycle of light irradiation. A total of ten cycles were performed for each sample.
2.3. Stability test In this part, a Zeta potential measurer (Brookhaven Instruments Corporation, USA) was used to predict suspension stability of nano fluids. 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. To further demonstrate the application potential of EG/CD-CNTs 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 convection, and the absorbance of sample was measured by UV–vis spectrophotometer at 980 nm subsequently. According to our previous studies, there was obvious absorption peak of ethylene glycol near 980 nm [33]. The above procedure was defined as one cycle, and each sample was repeatedly performed for thirty cycles in three days. After every 10 cycles, the samples were placed at room temperature over night. The following day, all samples will be subjected to 30 min of ul trasonic treatment to break up the soft aggregates. The samples without ultrasonic treatment were analyzed according to the same procedure as control experiments. It should be noted that even low temperature DASCs work in the temperature range of 20–100 � C. However, it was very hard to keep the temperature unchanged in the Zeta measurer and UV–vis spectropho tometer when the temperature was above 60 � C. Therefore, the tested temperature was 20–60 � C.
3. Results and discussion
2.4. Measurement of specific heat capacity
3.1. Stability
A Simultaneous Thermal Analyzer (DSC-823, Metler-Toledo, Switzerland) was employed to analyze the SHC, which was based on a standard test method ASTM E1269 [37]. First, the nanofluids samples (about 10 mg) were poured into the aluminum pans covered with pierced lids. The ramping rate was set to 10 � C/min during the measurement, and the end temperature was set to 120 � C. The nitrogen flow rate is 40 mL/min. The sample was further maintained at 120 � C for 5 min in order to ensure the accuracy of measurement data. First, a calibration experiment was run using an empty pan to obtain the baseline of heat flux. Then the heat flux of standard sapphire was measured and calibrated. The same procedure was then repeated three times for each nanofluid sample to ensure the accuracy of the results. The SHC of CD-CNTs nanofluids was determined by Eq. (2): Δqnf ⋅ms Cp;nf ¼ Cp;s ⋅ Δqs ⋅mnf
(3)
Colloidal stability is a necessary and sufficient condition for the
(2)
where Cp is the specific heat capacity, q means the heat flow and m represents the quantity of sample. Subscripts s represents standard sapphire and nf denotes nanofluids. The uncertainties of the experiment resulted from the measurement error of mass and heat flow. Therefore, the uncertainty of the experi ments could be expressed as Eq. (3) [38]:
Fig. 2. Schematic diagram of experimental apparatus. 3
X. Li et al.
Solar Energy Materials and Solar Cells 204 (2020) 110240
successful application of nanofluids. Homogeneous and stable EG-based CD-CNTs nanofluids have been prepared by the two-step method. The fresh samples with different volume fractions are shown in Fig. 3a. During the settlement experiment, no visible precipitation or stratifi cation was found even a month later (See Fig. 3b). In addition, the stability of colloidal systems is mainly the result of the interaction between particles and particle-fluid interfaces over time. Therefore, the stability of CD-CNTs nanofluids is closely related with their surface charge density. To accurately study the stability of nano fluids, the value of the Zeta potential (ζ) of nanofluids was measured at different volume concentrations. The results are shown in Fig. 4a. The Zeta potential decreased slightly with the volume fraction. But the excellent stability of CD-CNTs nanofluids was obtained at all fractions at 20 � C. Considering the application in DASC, nanofluids are always subjected to different temperature conditions. Hence, the ζ values of nanofluids at high temperature were also measured. It clearly indicated that the temperature put a side effect on the stability of nanofluids. The ζ value decreased with the increasing temperature. For example, the ζ value of 0.1 vol% CD-CNTs nanofluids decreased from 52.1 mV at 20 � C to 45.2 mV at 60 � C. This is because the high temperature will increase the probability and strength of the collision of nanoparticles and decrease the function of the surfactant, thus affecting the electrostatic balance of the suspension system. However, the CD-CNTs nanofluids generally could maintain moderate stability at high temperatures, which is beneficial for their applications in solar systems. To further test the stability of the studied nanofluid in DASC appli cations, the thermal cycling experiments were conducted and Fig. 4b shows the changes of relative absorbance (RA) of CD-CNTs nanofluids at various concentrations. As an evaluation index, RA was defined as the ratio of the absorbance of heated nanofluids to the initial absorbance of freshly prepared nanofluids. The RA values decreased with the number of thermal cycles. After 30 cycles of thermal cycling, the RA of samples without ultrasonic treatment decreased by 6.86%. This decline was reasonable and it meant the EG based CD-CNTs nanofluids could keep stable performance in DASC applications. But for the other group, sig nificant fluctuations could be observed after every 10 heating cycles. The results implied that the stability of CD-CNTs nanofluids would degrade inevitably under intermittent heated condition, but this trend could be easily reversed by ultrasonic processing. Therefore, necessary adjustments could be made easily according to the actual situation in the DASC applications. On the other hand, it was worth noting that the absorbance of heated samples was higher than that of freshly prepared nanofluids (RA>1). This is principally because the agglomerations of carbon nanotubes, which could make nanofluids show a better perfor mance in solar collector applications, especially in the long-wavelength [39]. High concentration nanofluids were more susceptible to the negative effects of cyclic heating, so the aggregates of nanoparticles
were much easier to form in nanofluids with higher volume fraction. Therefore, the absorbance of nanofluids with higher volume fraction was clearly larger in the early stage of cyclic heating experiment. In conclusion, the stability of CD-CNTs nanofluids would decrease slightly after cyclic heating, which was acceptable. But the capability of solar energy collection for CD-CNTs nanofluids will be enhanced due to the emergence of carbon nanotube flocs, which is eagerly desirable for the direct absorption solar collector in solar systems. The similar phenom enon also happened in other studies [32,39]. It should be noted that the agglomeration of nanoparticles in a nanofluid is a drawback to a solar system. But the premise is that the size of aggregates is large enough to cause precipitation or stratification. Here in our nanofluids system, we found that only tiny flocs would form in nanofluids which were invisible to the naked eye. Therefore, combining the results of stability tests, the β-cyclodextrin modified carbon nanotubes nanofluid system have potential to be used in DASC system. 3.2. Specific heat capacity The variation of SHC with respect to volume fraction of CD-CNTs loadings at room temperature is illustrated in Fig. 5a. As shown in the diagram, the SHC of nanofluid decreased with the increase of CD-CNTs loadings. This was mainly because the specific heat value of EG was much higher than that of the nanomaterials. The highest decrement was found to be 9.07% for 0.1 vol% as compared to pure EG. Teng et al. [27] found the similar trend on the SHC of multi-walled carbon nanotubes nanofluids (water-EG mixture based), which was found to decrease the SHC of the base fluid by 2–8% (0.1–0.4 wt%). However, it was reported that the SHC of 0.15 vol% of EG/Water mixture MWCNT nanofluid was higher than that of base fluid [40]. But it was decreasing with the in crease of volume fraction. Therefore, the thermal capacity of nanofluids could be decreased or increased relative to the base fluids. This depends on the types of nanoparticles and base fluids [17]. In addition, the free energy of solid-liquid interface varies with the change of suspended particles. Because of the large specific surface area of nanoparticles, the surface energy of nanoparticles has a large proportion in the system capacity, which affects the SHC of nanofluids systems. On the other hand, the grafting of β-CD onto the surface of MWCNT changed the surface hydrophobicity of carbon nanotubes, which could induce a phase transformation in the solvent material in the immediate vicinity of the nanotubes. These adhesive layers were expected to have semisolid properties, thus improving the thermal conductivity and density of nanofluids [33]. This reduced the energy demand for the same tem perature increment, thereby reducing the SHC value. Based on classical mixing theory, a simple model has been proposed to compute the SHC of nanofluids, which is expressed as follows:
Fig. 3. The EG-based CD-CNTs nanofluids with different volume fractions: (a) one day after preparation; (b) one month after preparation. 4
X. Li et al.
Solar Energy Materials and Solar Cells 204 (2020) 110240
Fig. 4. (a) The absolute value of Zeta potential of CD-CNTs nanofluids at different temperature; (b) The relative absorbance of CD-CNTs nanofluids with different cycles of heating.
Fig. 5. (a) Specific heat capacity of EG based CD-CNTs nanofluids vs. volume concentration; (b) Effect of temperature on specific heat capacity of CDCNTs nanofluids.
Cp;nf ¼ ϕv ⋅ Cp;n þ ð1
ϕv Þ⋅Cp;b
nanoparticles. Vajjha et al. got the similar conclusion when they studied the SHC properties of EG/Al2O3 nanofluids [43]. However, it should be noted that, although this model has been widely used in the studies of SHC of nanofluids, its validity remains to be verified. We believe the present study could contribute to the further understanding of specific heat in nanofluid field. Effect of temperature on SHC was shown in Fig. 5b. A considerable enhancement (~27.4% on 0.1 vol% nanofluid) in the specific heat value with the temperature was found in the experiment. And most of the literatures showed the similar trend on different nanofluids [40,44,45]. Therefore, one could summarize that temperature has a significant effect on the thermal capacity or SHC of nanofluids. Specifically, the enhancement in the specific heat capacity was most likely because of the high surface area of the carbon nanotubes per unit volume, which could provide high surface energy [40]. If an application requires a wide or high temperature range, the use of high SHC fluids would be very beneficial [44]. It is highly preferred that the heat transfer fluid has higher values of SHC in order to absorb more heat from the power sources. In addition, DASC systems require long-term operation at high temperatures. Therefore, any increase in heat capacity of such nano fluids is of great importance for their practical applications in DASC system. Therefore, the CD-CNTs nanofluids are potential fluids to the DASC system.
(4)
where the subscripts nf, n and b represent nanofluids, nanoparticles and base fluid, respectively. However, recent researches have revealed the shortcomings of the model [17,41]. On the other hand, the SHC of nanofluid could be easily deduced based on the First Law of Thermodynamics if supposing that the nano particles and the base fluid were in thermal equilibrium. Therefore, an improved model for computing SHC of nanofluids was proposed by Buongiorno et al. [42]. It is expressed as Eq. (5). Cp;nf ¼
ð1
ϕv Þ⋅Cp;b ⋅ρb þ ϕv ⋅Cp;n ⋅ρn ð1 ϕv Þ⋅ρb þ ϕv ⋅ρn
(5)
where,
ρnf ¼ ϕv ⋅ ρn þ ð1
ϕv Þ⋅ρb
(6)
The comparison of measured SHC with theoretical calculation value was also shown in Fig. 5a. It was clearly seen that the predicted values using Eq. (5) was much higher than the measured specific heat capacity of CD-CNTs nanofluids. Due to higher thermal diffusivity of nano particles, the carbon nanotubes absorbed heat faster than the EG. Therefore, in fact the nanoparticles and the base liquid were not actually in thermal equilibrium. Hence, the reason of the discrepancy between predicted values and the measured values was because the model was based on the thermal equilibrium between liquid phase and 5
X. Li et al.
Solar Energy Materials and Solar Cells 204 (2020) 110240
3.3. Photo-thermal conversion performance The photo-thermal properties of nanofluids depend on the relation ship between the temperature change of nanofluids and solar radiation time to a great extent. The temperature rise curves of EG/CD-CNTs nanofluids with different volume fractions were plotted in Fig. 6. It was obvious that a considerable enhancement in the PTC performance of pure EG was obtained after the addition of CD-CNTs nanoparticles. The temperature of nanofluids rose faster than that of EG during the irra diation time. The temperature difference increased with the increase of the volume concentration of CD-CNTs nanoparticles. The temperature rise rate and the maximum equilibrium temperature of the nanofluids were higher than those of EG. For instance, the highest temperature difference of CD-CNTs nanofluid (0.1 vol%) was 79.3 � C (at the irradi ation time of 50 min), whose temperature rise rate was 1.586 � C/min. This was about 25% higher than that of base fluid. On the other hand, it took approximately 125 min for EG to reach its maximum equilibrium temperature difference (105 � C) under the illu mination of a 390 W power xenon lamp in the present experiment. But it only took 110 min for CD-CNTs nanofluids to reach its maximum equi librium temperature difference (132 � C). Therefore, the photothermal conversion performance of base fluid (EG in this paper) could be significantly improved by adding a small amount of CD-CNTs. And this was also closely related to the enhanced stability and thermal conduc tivity of CD-CNTs nanofluids [33]. Fig. 7 shows the temperature variations of EG/CD-CNTs nanofluids with different volume fractions after different light-irradiated cycles. For 0.005 vol% CD-CNTs nanofluid, it could be found that the temper ature of nanofluid increased with increasing heating cycles but decreased after six cycles of light irradiation (Fig. 7a). Compared to the first-cycle irradiation treatment, the temperatures of 0.005 vol% nano fluid were increased by 3.87 � C after being irradiated two cycles and this numerical value was 6.85 � C after being irradiated four cycles. However, the temperature of nanofluid finally decreased after a further increase of heating cycles (six and ten cycles). As mentioned and discussed in sec tion 3.1, the increase of temperature would bring instability to colloidal system, which would result in small carbon nanotube agglomerations. If the nanoparticle aggregates were limited, the resultant size reduction of aggregates could enhance the optical absorption coefficient [39]. However, the serious aggregation of carbon nanotubes will definitely aggravate the suspension instability of nanofluids. If the carbon nano tube clusters grow up to a certain size and have obvious sedimentation, the concentration of carbon nanotube aggregates will decrease from the optimum value accordingly, subsequently reducing the PTC
Fig. 7. Temperature profiles of EG/CD-CNTs nanofluids with different light irradiation times: (a) 0.005 vol%, (b) 0.05 vol%, and (c) 0.1 vol%.
Fig. 6. Temperature difference of CD-CNTs nanofluids as a function of irradi ation time. 6
X. Li et al.
Solar Energy Materials and Solar Cells 204 (2020) 110240
performance [15,32]. Therefore, appropriate carbon nanotube aggre gates in nanofluids were beneficial to improve the thermal recovery performance of solar energy. Similarly, for the 0.05 vol% EG/CD-CNTs nanofluid (see Fig. 7b), the temperature of nanofluid increased with the increase of light irradiation cycle only during the first two cycles. And for higher volume fraction (0.1 vol%), the PTC performance degraded with the increasing heating cycles, but it was still superior to the base fluid (see Fig. 7c). This was consistent with the phenomenon observed in the stability test. It could be therefore concluded that cyclic illumination would not affect the PTC efficiency of CD-CNTs nanofluids with appropriate concentration. Therefore, it is very important to choose the most suitable concentration of nanofluids in practical DASC applications. To qualitatively analyze the PTC performance of nanofluids, the PTC efficiency was calculated. The photo-thermal conversion efficiency η was given by Eq. (7).
η¼
mnf Cp;nf ΔT GAΔt
Fig. 8. The photo-thermal conversion efficiency of EG/CD-CNTs nanofluids.
(7)
nanofluids, and further quantitative analysis will be the focus of our future work.
where Δ T 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. In addition, considering the additive amount of MWCNTs was very small and many existed papers had proved the effect of MWCNTs could be ignored [46]. It could be further simplified as follows:
η¼
ρb Cp;b HΔT GΔt
4. Conclusion Since the specific heat capacity (SHC) and photo-thermal conversion (PTC) efficiency of EG based CD-CNTs nanofluids have not been re ported or applied till date as HTFs in DASC system. In this paper, experimental research and theoretical analysis were carried out in a scientific and objective way.
(8)
where H is the height of nanofluids layer. There were two assumptions about this formula. 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 PTC efficiency is shown in Fig. 8. The results clearly showed that the maximum values of PTC efficiency presented at 25 min for all volume fractions, but then it became less efficient. This was mainly because the increased heat loss during nanofluid temperature rise [32,47]. Heat loss was mainly man ifested as convection loss to the surrounding environment and radiation loss to the upper part, which was directly related to the temperature of the nanofluid. The PTC efficiency of 0.1 vol% CD-CNTs nanofluid decreased from 81.3% to 41.5% when the irradiation time duration increased from 25 to 125 min. And during this time, the temperature increased from 45 � C to 132 � C. And it should be noted that the highest PTC efficiency at 0.1 vol% was up to 81.3% at 25 min, which was about 39.1% higher than that of EG. In addition, according to Figs. 6 and 8, a decrease in the PTC effi ciency would occur if the irradiation time increased (e.g. 150 min). This phenomenon could be explained as: 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. In general, as a heat conducting and collecting medium, EG/CDCNTs nanofluids prepared in this paper have great application value and prospect in DASC system. Considering the properties of EG/CDCNTs nanofluids, we believe that 0.05–0.1 vol% nanofluids are the most suitable concentration range for DASCs applications. Although the specific heat capacity decreased with the increase of concentration, the studied nanofluids in this concentration range exhibited excellent thermal conductivity and photothermal conversion efficiency. In addi tion, the concentration could be adjusted according to the actual situa tion. However, the PTC performances of any nanofluid system is extremely complex. This section only provided a limited understanding of the underlying factors affecting PTC performance of EG/CD-CNTs
1. First, the two-step method was used to prepare EG based CD-CNTs nanofluids. Then the stability of nanofluids were evaluated through static experiment and Zeta potential measurement. In addition, the thermal cycling experiments were conducted to further test the stability of the studied nanofluid in DASC applications. 2. The SHC of nanofluids is of great importance for their practical ap plications in DASC system. Thus, the nanoparticle concentration and temperature effect on SHC of nanofluids were investigated. The SHC value decreased with the increasing of CD-CNTs loadings and the highest decrement in specific heat capacity was found to be 9.07% for 0.1 vol%. On the other hand, it was found that the temperature had significant effect over specific heat capacity of the studied nanofluids. 3. The effect of volume fraction and sunlight repetitive radiation on the PTC performance were investigated respectively. The PTC perfor mance of CD-CNTs nanofluids was largely determined by the volume concentration. In addition, compared with EG, the temperature of CD-CNTs nanofluid of 0.1 vol% was increased by 132 � C after the light irradiation of 110 min. On the other hand, the experiment re sults showed that cyclic illumination would not affect the PTC effi ciency of CD-CNTs nanofluids with appropriate concentration. The photothermal conversion efficiency increased with the increase of volume concentration and decreased with the irradiation time. The highest PTC efficiency at 0.1 vol% is up to 81.3% at 25 min, which is about 39.1% higher than that of EG. In conclusion, we believed that the promising features of suitable SHC and PTC efficiency could ensure the CD-CNTs nanofluids applica tion prospect to be utilized in practical DASC system for different tar gets. And we recommend that 0.05–0.1 vol% nanofluids are the most suitable concentration range for DASCs applications. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence 7
X. Li et al.
Solar Energy Materials and Solar Cells 204 (2020) 110240
the work reported in this paper.
[21] E.D. Robertis, E.H.H. Cosme, R.S. Neves, A.Y. Kuznetsov, A.P.C. Campos, S. M. Landi, C.A. Achete, Application of the modulated temperature differential scanning calorimetry technique for the determination of the specific heat of copper nanofluids, Appl. Therm. Eng. 41 (2012) 10–17. [22] M.M. Elias, I.M. Mahbubul, R. Saidur, M.R. Sohel, I.M. Shahrul, S. S. Khaleduzzaman, S. Sadeghipour, Experimental investigation on the thermophysical properties of Al 2 O 3 nanoparticles suspended in car radiator coolant ☆, Int. Commun. Heat Mass Transf. 54 (2014) 48–53. [23] HwaMing Nieh, TunPing Teng, ChaoChieh Yu, Enhanced heat dissipation of a radiator using oxide nano-coolant, Int. J. Therm. Sci. 77 (2014) 252–261. [24] S.M.S. Murshed, Determination of effective specific heat of nanofluids, J. Exp. Nanosci. 6 (2011) 539–546. [25] R.S. Vajjha, D.K. Das, Experimental determination of thermal conductivity of three nanofluids and development of new correlations, Int. J. Heat Mass Transf. 52 (2009) 4675–4682. [26] A.K. Starace, J.C. Gomez, J. Wang, S. Pradhan, G.C. Glatzmaier, Nanofluid heat capacities, J. Appl. Phys. 110 (2011) 5. [27] T.P. Teng, L. Lin, C.C. Yu, Preparation and characterization of carbon nanofluids by using a revised water-assisted synthesis method, J. Nanomater. 2013 (2013) 1–12, 2013,(2013-11-10). [28] H. Xie, L. Chen, Review on the preparation and thermal performances of carbon nanotube contained nanofluids, J. Chem. Eng. Data 56 (2011) 1030–1041. [29] 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. [30] R.C. Shende, S. Ramaprabhu, Thermo-optical properties of partially unzipped multiwalled carbon nanotubes dispersed nanofluids for direct absorption solar thermal energy systems, Sol. Energy Mater. Sol. Cells 157 (2016) 117–125. [31] M. Karami, M.A.A. Bahabadi, S. Delfani, A. Ghozatloo, A new application of carbon nanotubes nanofluid as working fluid of low-temperature direct absorption solar collector, Sol. Energy Mater. Sol. Cells 121 (2014) 114–118. [32] J. Qu, M. Tian, X. Han, R. Zhang, Q. Wang, Photo-thermal conversion characteristics of MWCNT-H2O nanofluids for direct solar thermal energy absorption applications, Appl. Therm. Eng. 124 (2017) 486–493. [33] X. Li, C. Zou, W. Chen, X. Lei, Experimental investigation of β-cyclodextrin modified carbon nanotubes nanofluids for solar energy systems: stability, optical properties and thermal conductivity, Sol. Energy Mater. Sol. Cells 157 (2016) 572–579. _ [34] 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. [35] X. Li, C. Zou, A. Qi, Experimental study on the thermo-physical properties of car engine coolant (water/ethylene glycol mixture type) based SiC nanofluids, Int. Commun. Heat Mass Transf. 77 (2016) 159–164. [36] B. Wang, S. Liu, Y. Zhu, S. Ge, Influence of polyvinyl pyrrolidone on the dispersion of multi-walled carbon nanotubes in aqueous solution, Russ. J. Phys. Chem. 88 (2014) 2385–2390. [37] Y. Hu, Y. He, Z. Zhang, D. Wen, Enhanced heat capacity of binary nitrate eutectic salt-silica nanofluid for solar energy storage, Sol. Energy Mater. Sol. Cells 192 (2019) 94–102. [38] S.V. Gupta, Propagation of uncertainty, Meas. Uncertain. 27 (2012) 12–17. [39] D. Song, Y. Wang, D. Jing, J. Geng, Investigation and prediction of optical properties of alumina nanofluids with different aggregation properties, Int. J. Heat Mass Transf. 96 (2016) 430–437. [40] V. Kumaresan, R. Velraj, Experimental investigation of the thermo-physical properties of water–ethylene glycol mixture based CNT nanofluids, Thermochim. Acta 545 (2012) 180–186. [41] W. Daungthongsuk, S. Wongwises, A critical review of convective heat transfer of nanofluids, Renew. Sustain. Energy Rev. 11 (2007) 797–817. [42] J. Buongiorno, Convective transport in nanofluids, J. Heat Transf. 128 (2005) 240–250. [43] R.S. Vajjha, D.K. Das, Measurements of specific heat and density of Al2O3 nanofluid, 2008. [44] I.M. Shahrul, I.M. Mahbubul, S.S. Khaleduzzaman, R. Saidur, M.F.M. Sabri, A comparative review on the specific heat of nanofluids for energy perspective, Renew. Sustain. Energy Rev. 38 (2014) 88–98. [45] E. De Robertis, E.H.H. Cosme, R.S. Neves, A.Y. Kuznetsov, A.P.C. Campos, S. M. Landi, C.A. Achete, Application of the modulated temperature differential scanning calorimetry technique for the determination of the specific heat of copper nanofluids, Appl. Therm. Eng. 41 (2012) 10–17. [46] L. Chen, J. Liu, X. Fang, Z. Zhang, Reduced graphene oxide dispersed nanofluids with improved photo-thermal conversion performance for direct absorption solar collectors, Sol. Energy Mater. Sol. Cells 163 (2017) 125–133. [47] 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.
Acknowledgments The authors would like to acknowledge the financial support of CAS Light of West China Program (Z1113-46) and The National Natural Science Foundation of China (21576225). 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.2019.110240. References [1] O Bait, Mohamed Si–Ameur, Enhanced heat and mass transfer in solar stills using nanofluids: A review, Solar Energy 170 (2018) 694–722. [2] T.R. Shah, H.M. Ali, Applications of hybrid nanofluids in solar energy, practical limitations and challenges: a critical review, Sol. Energy 183 (2019) 173–203. [3] S.U.S. Choi, Developments and Applications of Non-newtonian Flows, vol. 231, ASME, New York, 1995, pp. 99–102. [4] V. Fuskele, R.M. Sarviya, Recent developments in nanoparticles synthesis, preparation and stability of nanofluids, Mater. Today: Proc. 4 (2017) 4049–4060. [5] D.K. Devendiran, V.A. Amirtham, A review on preparation, characterization, properties and applications of nanofluids, Renew. Sustain. Energy Rev. 60 (2016) 21–40. [6] R.A. Taylor, P.E. Phelan, T.P. Otanicar, C.A. Walker, M. Nguyen, S. Trimble, R. Prasher, Applicability of nanofluids in high flux solar collectors, J. Renew. Sustain. Energy 3 (2011) 4. [7] T.P. Otanicar, P.E. Phelan, R.S. Prasher, G. Rosengarten, R.A. Taylor, Nanofluidbased direct absorption solar collector, J. Renew. Sustain. Energy 2 (2010), 033102. [8] 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. [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 II: ZnO, CeO2, Fe2O3 nanoparticles behavior, Sol. Energy Mater. Sol. Cells 147 (2016) 321–326. [10] L. Syam Sundar, K.M. Singh, M. Hashim Farooky, S. Naga Sarada, Experimental thermal conductivity of ethylene glycol and water mixture;based low volume concentration of Al2O3 and CuO nanofluids, Int. Commun. Heat Mass Transf. 41 (2013) 41–46. [11] M. Potenza, M. Milanese, G. Colangelo, A. de Risi, Experimental investigation of transparent parabolic trough collector based on gas-phase nanofluid, Appl. Energy 203 (2017) 560–570. [12] X. Xu, C. Xu, J. Liu, X. Fang, Z. Zhang, A direct absorption solar collector based on a water-ethylene glycol based nanofluid with anti-freeze property and excellent dispersion stability, Renew. Energy 133 (2019) 760–769. [13] S. khosrojerdi, A.M. Lavasani, M. Vakili, Experimental study of photothermal specifications and stability of graphene oxide nanoplatelets nanofluid as working fluid for low-temperature Direct Absorption Solar Collectors (DASCs), Sol. Energy Mater. Sol. Cells 164 (2017) 32–39. [14] T.B. Gorji, A.A. Ranjbar, A review on optical properties and application of nanofluids in direct absorption solar collectors (DASCs), Renew. Sustain. Energy Rev. 72 (2017) 10–32. [15] J. Qu, R. Zhang, Z. Wang, Q. Wang, Photo-thermal conversion properties of hybrid CuO-MWCNT/H2O nanofluids for direct solar thermal energy harvest, Appl. Therm. Eng. 147 (2019) 390–398. [16] M. Hatami, D. Jing, Evaluation of wavy direct absorption solar collector (DASC) performance using different nanofluids, J. Mol. Liq. 229 (2017) 203–211. [17] H. Riazi, T. Murphy, G.B. Webber, R. Atkin, S.S.M. Tehrani, R.A. Taylor, Specific heat control of nanofluids: a critical review, Int. J. Therm. Sci. 107 (2016) 25–38. [18] M. Lomascolo, G. Colangelo, M. Milanese, A. de Risi, Review of heat transfer in nanofluids: conductive, convective and radiative experimental results, Renew. Sustain. Energy Rev. 43 (2015) 1182–1198. [19] B.C. Pak, Y.I. Cho, Hydrodynamic and heat transfer study OF dispersed fluids with submicron metallic oxide particles, Exp. Heat Transf. 11 (1998) 151–170. [20] S.Q. Zhou, R. Ni, Measurement of the specific heat capacity of water-based AL2O3 nanofluid, Appl. Phys. Lett. 92 (2008) 99.
8