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Volumetric solar steam generation enhanced by reduced graphene oxide nanofluid
Applied Energy 220 (2018) 302–312
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Applied Energy journal homepage: www.elsevier.com/locate/apenergy
Volumetric solar steam generation enhanced by reduced graphene oxide nanofluid
T
⁎
Xing Liu, Xinzhi Wang, Jian Huang, Gong Cheng, Yurong He
School of Energy Science and Engineering, Harbin Institute of Technology, Harbin 150001, China Heilongjiang Key Laboratory of New Energy Storage Materials and Processes, School of Energy Science and Engineering, Harbin Institute of Technology, Harbin, Heilongjiang 150001, China
H I GH L IG H T S
G R A P H I C A L A B S T R A C T
dispersed rGO nanofluids were • Well used as volumetric broadband solarabsorber.
Solar steam generation efficiency of • rGO nanofluids could be up to ∼47%. local evaporation was realized • Rapid while the bulk fluid temperature was still low.
A R T I C LE I N FO
A B S T R A C T
Keywords: Solar energy Steam generation Reduced graphene oxide Nanofluid Evaporation
Solar steam generation is a highly efficient photo-thermal conversion method that has a wide range of applications in water purification, distillation, power plants, and seawater desalination. Low steam generation efficiency was obtained for solar steam generation using traditional working media. Therefore, reduced graphene oxide (rGO) nanofluids with good stability and light absorption capability were fabricated to achieve highly efficient volumetric solar steam generation in this work. The effects of rGO mass concentration and light intensity on solar steam generation enhancement were investigated experimentally. It was found that a hot area was formed at water–air interface due to the unique lamellar structure of rGO with good light absorption characteristic, and sunlight was absorbed by the hot area to generate steam locally, which reduced thermal loss and improved evaporation efficiency. The solar steam generation enhancement achieved by the rGO nanofluids reduced evaporation costs and expanded their applicability in seawater desalination, clean water production, sterilization of waste, etc.
1. Introduction With the depletion of traditional fossil fuels and the increasing emission of greenhouse-gases, solar energy has been identified as a green and renewable alternative [1–4]. There are two strategies to
⁎
utilize solar energy: photo-electric conversion [5] and photo-thermal conversion [6]. Despite rapid developments in photo-electric conversion applications, photo-thermal conversion is a highly efficient technique that has more applications [7] such as solar water heating [8], space heating and cooling [9], refrigeration [10], industrial process
Corresponding author at: School of Energy Science and Engineering, Harbin Institute of Technology, Harbin 150001, China. E-mail address: [email protected] (Y. He).
https://doi.org/10.1016/j.apenergy.2018.03.097 Received 14 December 2017; Received in revised form 17 February 2018; Accepted 26 March 2018 0306-2619/ © 2018 Published by Elsevier Ltd.
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generation. The mechanism of thermal and mass transfer in volumetric solar steam generation should be further addressed. In this work, graphene oxide was synthesized by the Hummers method and then high solar absorption rGO was obtained by the reduction of graphene oxide. The morphologies, structures, and properties of rGO were characterized. Furthermore, well dispersed and stable rGO based nanofluids were prepared for the volumetric solar steam generation. Finally, to explore the solar steam generation enhancement by the rGO nanofluids, solar steam generation experiments were conducted.
heating [11], and thermal power generation [12]. Solar steam generation is a typical method to achieve photo-thermal conversion and has a wide range of applications in water purification [13], distillation [14], power plants [15], and seawater desalination [16,17]. For instance, in the concentrating solar power (CSP) system, the sunlight incident is absorbed by surface absorber and converted to heat, then the heat is carried away from the absorber by heat transfer fluid (HTF), subsequently, the HTF delivers heat to a heat engine, which generates electricity [12]. In traditional solar steam generation systems, solar energy is usually received and absorbed by the tube receiver, and then used to heat the bulk fluid through thermal conduction and thermal convection. As most of absorbed solar thermal energy is used to heat the bulk fluid and heat transfer resistance between the absorber and HTF is high, the steam generation efficiency is relatively low (35–45%) [18,19]. Using volumetric absorbers such as gas–particle suspensions, molten salts, and nanofluids is a simple and effective approach to improve light absorption capacity and reduce thermal loss, which could absorb the solar energy directly [20,21]. Choi [22] first proposed that nanofluids have many special properties due to the effects of nanoparticles (NPs). A nanofluid is a fluid containing nanometer-sized particles [23]. Unlike traditional fluids, nanofluids could enhance thermal conductivities [24,25], specific heat capacities [26], heat transfer coefficients [27–29], and sunlight absorption capacities [30,31] which could help enhance the solar steam generation efficiency. Many nanofluids have been explored to enhance solar steam generation, such as noble metals and composite NPs based nanofluids. Zhang et al. [32] and Chen et al. [33] studied photo-thermal conversion with gold and silver nanofluids, respectively, and the results demonstrated high efficiency for solar heating. Neumann et al. [34,35] investigated the solar steam generation properties of Au nanofluids and demonstrated a device efficiency of 24% with a solar power of 1000 sun (1 sun = 1 kW m–2). Furthermore, Jin et al. [36] experimentally studied the steam generation performance of Au nanofluids under 220 sun irradiance and revealed that localized boiling occurred in the nanofluid. Liu et al. [37] investigated solar steam generation with liquid dispersed nanoparticles and realized highly efficient seawater purification. Guo et al. [38] explored the effect of Au NP diameters on photothermal conversion during the volumetric solar steam generation process. Amjad et al. [39] proposed a new integration method to calculate the sensible heating contribution in the Au NPs based nanofluids volumetric solar steam generation. Compared to noble metal NPs, carbon nanomaterials have better potential for solar steam generation due to the high thermal conductivities and low cost. Ni et al. [21] reported a vapor generation efficiency of up to 69% under 10 sun irradiance using a graphitized carbon black nanofluid and revealed that a global temperature rise in the fluid medium was a significant mechanism in steam generation. Wang et al. [40] obtained a high evaporation efficiency with carbon nanotube nanofluids in a direct solar steam generation experiment. Reduced graphene oxide (rGO) is a kind of new carbon material possessing superior features such as high strength, flexibility, good conduction, high thermal conductivity, and excellent optical properties. rGO has good stability even at high temperature, which enables its possible applications in many areas [41,42]. The layered structure of rGO makes it more stable than other carbon materials in evaporating water. A more concentrated hot area will form in the top surface of bulk water to heat a small portion of water for evaporation, which could reduce thermal loss to the bulk fluid and increase the evaporation efficiency. Besides, the lamellar structures intertwine in bulk water, resulting in scattering and refraction of incident light, which will contribute to light absorbance by the upper area, thus increasing the evaporation rate. Therefore, solar steam generation enhancement could be achieved using rGO nanofluid due to multiple effects. Despite some previous work on nanofluids based volumetric solar steam generation, the evaporation efficiency is still low. In addition, the stability of the nanofluids is still a problem for a long term solar steam
2. Experimental 2.1. Synthesis Graphite powder, ammonia water, L-ascorbic acid (L-AA), NaNO3, KMnO4, H2SO4, and H2O2 were supplied by Aladdin Chemical Co., Ltd. GO was synthesized via a chemical exfoliation of the graphite powder by a modified Hummers method [43] as follows: 46 mL concentrated H2SO4 was slowly added into a mixture of 2 g graphite powder and 1 g NaNO3 at 0 °C. Then, 6 g KMnO4 was added with stirring while the temperature of the mixture was maintained below 20 °C using an ice bath. Afterward, the ice bath was removed, and the temperature of the mixture was raised to 35 °C and stirred for 30 min. Then, 92 mL deionized water was added to the mixture with stirring for 15 min, followed by the addition of 60 mL H2O2 solution at 60 °C. After the suspension turned brown, it was washed with deionized water and centrifuged for several times. rGO was obtained by the reducing of GO with L-AA as reductant in aqueous solution [44]. To prepare rGO, the GO solution (100 ppm) was dispersed in deionized water and ultrasonicated for 1 h. Ammonia water was then added to regulate the pH to 10 with sonication for 30 min. L-AA (10 mg/mL) was added and the mixture was maintained at 95 °C for 3 h for the completion of reaction. The rGO solution was filtered to obtain rGO on the filter paper. Finally, rGO nanofluids were prepared by sonicating the filtered powder in a certain amount of deionized water. The entire process is shown in Fig. 1. 2.2. Solar evaporation experiments Fig. 2 shows the experimental setup for solar steam generation. Solar light was generated by a solar simulator (CEL HXF300, CEAULIGHT, Beijing, China). The light was generated by a 300W Xenon lamp, which could realize the exportation of collimated light with high energy. The Xenon lamp is one of the most common types of lamp for continuous solar simulators. It offers high intensities and an unfiltered spectrum matching reasonably well to sunlight. The spectra of the light generated by Xenon lamp is shown in Fig. S1. An acrylic tube with a thermal insulation layer was used as a solar collector, and the temperature was measured and recorded using thermocouples (TT-T-40SLE, Omega, US) and a data-acquisition system (34972A, Agilent Technology, Santa Clara, CA, US). Mass change was measured using an electric balance (Practum313–1CN, Sartorius, Göttingen, Germany). The inner height and diameter of the solar collector were 80 mm and 40 mm, respectively. The size and weight of the designed solar collector tube could be matched with the size of the solar light simulator and weighing range of the electric balance in the experiments. Seven T-type thermocouples were inserted into the solar collector at different heights (H = 10, 20, 30, 40, 50, 60, and 70 mm). 2.3. Characterization Transmission electron microscopy (TEM) images of the rGO were obtained using a field emission microscope (Tecnai G2 F30, FEI, Portland, US). Atomic force microscopy (AFM) images were obtained using a Bruker Dimension Icon with ScanAsyst (Karlsruhe, Germany) for further thickness characterization of the rGO. Ultraviolet–visible 303
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Fig. 1. Schematic graph of rGO nanofluids preparation.
900W, Biosafer, Beijing, China) was used to disperse the rGO into water for preparing nanofluids, which could ensure a good dispersion and long time stability of nanofluids.
(UV–Vis) spectra were recorded on a two-beam UV–Vis spectrometer (TU1901, China). Fourier-transform infrared (FT-IR; Frontier Optica PerkinElmer, US) spectra were obtained from 500 to 4000 cm−1 at room temperature. High-power tip ultrasonoscope (Biosafer 900-92,
Fig. 2. Schematic of experimental setup for solar steam generation. 304
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Fig. 4b shows the FT-IR spectra of GO and rGO. Since GO contains a large number of C]O bonds, a corresponding peak at 1753 cm−1 appeared, while the peaks at 3424 cm−1 and 1385 cm−1 originated due to the OeH bond. The epoxy structure of CeO gave rise to a peak at 1226 cm−1, and the characteristic peak of CeO appeared at 1060 cm−1. After reduction, the peak corresponding to the epoxy structure completely disappeared, indicating that ascorbic acid treatment was effective for the removal of oxygen-containing functional groups to achieve reduction.
2.4. Error analysis The uncertainty of experimental data was determined by measurement deviation of parameters, including time t, tube diameter d, tube height h, temperature T, mass m, irradiation intensity I. when the measurement is of equal-precision and performed several times, suppose that (xk, yk, …) are n experimental realizations of (x, y, …), and that fk = f (xk, yk, …). The expectation of f and the variance of f are given respectively as follows.
μf =
σ f2 =
n
1 n
∑
1 n
∑
fk
3.2. Stability test of rGO nanofluids (1)
k=1
In the stability test, different concentrations of rGO nanofluids were placed in different cuvettes, and the cuvettes were plugged to prevent the evaporation of water. As can be seen from Fig. 5a, the rGO nanofluids have broadband absorption in the visible (400–760 nm) and near infrared regions (> 760 nm). In addition, the absorbances of rGO nanofluids increase with increasing the concentrations of rGO, which meets well with the description of Beer-Lamber law [45]. According to the Beer-Lamber law, the absorption intensity is directly proportional to the concentrations of the tested sample. When the rGO nanofluids was placed for a certain time, the UV–Vis absorption intensity would decrease if the rGO precipitates in the cuvettes. Fig. 5b shows the UV–Vis intensity (at 270 nm) changes of rGO nanofluids for 10 days of measurement. It can be seen that there is little intensity decrease during 10 days of test. This indicated that the rGO nanofluids had good stabilitiy without the addition of other dispersants and could be used in solar steam generation. Furthermore, the VU-Vis spectra of rGO nanofluids with a concentration of 10 ppm before and after the steam generation experiment was tested. For the sample after the steam generation experiment, same amount of vaporized water was added before the UV–Vis spectra test. The comparison between the VU-Vis spectra of rGO nanofluids before and after the steam generation were illustrated in Fig. S2. It can be seen that there was not much difference between the two curves. The results indicated that rGO nanofluids could keep the stability for a long term in the process of steam generation with continuous water supply.
n
(fk −μk )2
(2)
k=1
The measurement error in the mean value of f is therefore σf / n . Assuming that the function relation formula is Nk = f (N1, N2, …, Nk), taking the logarithm of this equation gives: lnN = lnf . Taking total differential on both sides of above equation yields:
∂lnf ∂lnf ∂lnf dN dNk dN1 + ⋯+ dN1 + = N ∂Nk ∂N1 ∂N1
(3)
According to the error transfer principle and in order to synthesize the total relative error E we have
E=
σN = N
2
2
2
⎛ ∂lnf σN ⎞ + ⎛ ∂lnf σN ⎞ +⋯+⎛ ∂lnf σk ⎞ 1 2 ⎝ ∂N1 ⎠ ⎝ ∂N2 ⎠ ⎝ ∂Nk ⎠ ⎜
⎟
⎜
⎟
⎜
⎟
(4)
The indirect measurement parameter is the evaporation efficiency η. The true value of η and relative error Eη are calculated by:
η=
Eη =
Δmhlv IAΔt 2 σ σ 2 σ 2 σ 2 ⎛ Δm ⎞ + ⎛ I ⎞ + ⎛ A ⎞ + ⎛ Δt ⎞ ⎝I⎠ ⎝ A⎠ ⎝ Δm ⎠ ⎝ Δt ⎠
(5)
(6)
In present work, the maximum uncertainty of solar steam generation efficiency at various tests was around 3.17% (mainly including measurement uncertainties).
3.3. Effect of solar illumination intensity
3. Results and discussion
To investigate the effect of solar illumination intensity, the solar collector was placed under different solar illumination intensities (0, 1, 3, 5, and 7 suns, where 1 sun = 1 kW m–2). Two kinds of fluids (water and 10 ppm rGO nanofluid) were used in the experiments. To better describe the evaporated amounts of different working fluids in the evaporation stabilization phase, the rate of evaporation per unit area was defined as:
3.1. Morphologies and structures of rGO The structural properties of the rGOs were characterized using AFM, as shown in Fig. 3a and b. Many rGO sheets several microns in length and width were observed on silicon substrate. The average thickness of the rGO sheets was ∼1 nm, which can be regarded as a single graphene sheet. TEM was used to study the rGO morphology, as shown in Fig. 3c. The graphene sheet appeared transparent with tulle shape with a very low contrast. This was because the thickness of monolithic graphene was very small, and the substrate was a carbon film, which results in the difficulty to distinguish them. Some dark dots ∼50 nm in size were observed in the graphene layers, which were composed of crushed graphene sheets due to ultrasonic shock. Fig. 3d shows the UV–Vis spectra of GO and rGO nanofluids. The absorption peak of the GO nanofluid appeared at ∼229 nm, while that of the rGO nanofluid appeared at ∼270 nm. It is obvious that the rGO nanofluid had a broadband absorption in the visible light range, which contributes almost 50% of the solar energy. To explore the change in structure after reduction, Raman and FT-IR spectra of the rGO and GO were obtained, as shown in Fig. 4a and b, respectively. The Raman spectrum of GO exhibits two peaks at 1350 and 1603 cm−1, which are attributed to the D and G bands of GO, respectively. The D/G intensity ratio of GO was 0.84, which was smaller than that of rGO (1.15), indicating that GO had been reduced by L-AA.
ṁ =
dm / dt A
(7) −2
−1
where ṁ is the steady-state evaporation mass flux (kg m h ), m is the evaporation amount of the stabilization phase, A is the area of evaporation (m2), and t is the total time for evaporation. In the absence of light, a certain amount of evaporation occurred in the collector. To calculate the photo-thermal evaporation ability and evaporation efficiency of the rGO nanofluid more accurately, a darkfield experiment was set up and the evaporation rate per unit area became ṁ 0 . To describe the contribution of individual nanoparticles to the overall evaporation in the solar steam generation stage, their ability to generate steam per unit mass per unit time (SVP) was defined as follows:
305
SVP = V̇ / mrGO
(8)
V̇ = (ṁ −ṁ 0) A/ ρvapor
(9)
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Fig. 3. (a) AFM images of rGO; (b) height profiles of rGO in AFM images; (c) TEM image of rGO; (d) UV–Vis spectra of rGO nanofluids.
Here, V̇ is the evaporation steam volume per unit time in the evaporation stabilization phase (m3), ρvapor is the density of water vapor at 100 °C (0.6 kg m−3), and mrGO is the amount of rGO in the nanofluid (g). In the evaporation stabilization phase, the total energy of light irradiated on the collector is defined as Qin, as shown in Eq. (4). Qnf is the thermal energy collected in the bulk fluid, as shown in Eq. (5), and Qeva is the energy for evaporation in the stabilization phase, as shown in Eq.
(6).
Qin = I AΔt
(10)
Qnf = c p,nf Mnf Δt
(11)
and,
Qeva = (ṁ −ṁ 0) Ahlv Δt
(12)
where Δt is the time of light exposure during the evaporation
Fig. 4. (a) Raman spectra of the rGO and GO; (b) FT-IR spectra of the GO, rGO and graphite. 306
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Abs (a.u.)
b
1.25 ppm 2.5 ppm 5 ppm 10 ppm
0.20
0.15
0.10
0.2
0.1
0.05
0.00 200
1.25 ppm 2.5 ppm 5 ppm 10 ppm
0.3
Abs (a.u.)
a
0.4
0.25
300
400
500
600
700
800
900
0.0
0
2
4
6
8
10
Time (Day)
Wavelength (nm)
Fig. 5. (a) UV–Vis spectra of the rGO nanofluids with different concentrations (b) the change of absorption peak with time.
stabilization phase (h), I is the power density of the light (W m−2), cp,nf is the specific heat of the nanofluid (J g−1 °C−1), ΔT is the increase in average temperature (°C) of the working fluid in the evaporation stabilization phase (°C), and hlv is the latent heat of phase transition (2.257 kJ g−1, 1 atm). The heating efficiency, ηnf, is defined as the ratio of heat consumed by the working fluid to increase the temperature against the energy of light irradiated onto the collector, as shown in Eq. (7). ηeva is the evaporation efficiency of the working fluid, which is the ratio of heat consumed by evaporation during the evaporation stabilization phase to the energy of light irradiated onto the collector, as shown in Eq. (8).
ηnf =
nanofluid, the temperature at a height of 70 mm changed from 13.8 °C to 42.8 °C when the light intensity changed from 1 sun to 7 sun, which was a threefold enhancement. It indicated that the nanofluids converted the absorbed heat to the energy required for phase change rather than raising the fluid temperature. As the light intensity increased, the rate of temperature increase at the height of 70 mm accelerated, resulting in a decrease in the time required to reach the evaporation stabilization phase. The maximum temperature difference under 7 sun was between 50 and 60 mm heights, while under 1 sun, it was between 60 and 70 mm heights. This is because at a very high light intensity, the surface rGO layers were saturated by light. Thus, the excess light went through the upper rGO layers and was absorbed by the lower rGO layers, and their temperature increased. Fig. 7c and d shows the changes in average temperatures of the rGO nanofluid and water under different light intensities. The average temperature rise of water was higher than that of the rGO nanofluid under the same light intensity. With increasing the light intensity, the average temperature difference between the rGO nanofluid and water increased. When the light intensity was 1 sun, the average temperature rise of water was 5.5 °C after 60 min irradiation, while it was 5.4 °C for the rGO nanofluid. When the light intensity reached 7 sun, the average temperature rises of water and the rGO nanofluid were 33.9 °C and 25.6 °C, respectively, which indicated that the rGO nanofluid could better concentrate the heat and reduce the thermal loss. The evaporation, heating, and total thermal efficiencies of water and rGO nanofluid could be calculated by the Eqs. (7)–(10) and are displayed in Fig. 8. Compared to pure water, the rGO nanofluid achieved higher evaporation and total thermal efficiencies but lower heating efficiency under different light intensities, which indicated that the rGO nanofluid could concentrate the heat for evaporation, thereby increasing the evaporation efficiency. Besides, the heating efficiencies of the rGO nanofluid and water decreased with increasing the light intensity. However, their evaporation and total thermal efficiencies decreased initially, and then increased with increasing the light intensity. The heating efficiency of pure water was higher than the evaporation efficiency under different light intensities. However, the trend reversed for the rGO nanofluid. Under light irradiation, the temperature of the rGO nanofluid increased drastically due to strong light absorption by the rGO layers, and the phase transition temperature was quickly reached. The temperature rise in pure water was relatively slow due to the absence of rGO, and thus, more time was required for phase transition, resulting in lower evaporation efficiency. In the rGO nanofluid, the temperature of the graphene layer gradually stabilized because evaporation removed a large amount of heat. However, water, with a lower evaporation rate, utilized only a fraction of the energy; most of the energy was used to heat water. Therefore, the average temperature
c p,nf Mnf ΔT
ηeva =
I AΔt
(13)
(ṁ −ṁ 0) hlv I
(14)
The total energy utilization efficiency (ηtotal) is defined as:
ηtotal =
Qnf + Qeva Qin
ηtotal = ηnf + ηeva
(15) (16)
The results obtained with different solar light intensities are displayed in Fig. 6. For solar steam generation of pure water, the mass change ranged from 0.548 g to 1.343 g as the solar light intensity changed from 1 kW m−2 to 7 kW m−2 (Fig. 6a). For solar steam generation of the rGO nanofluid (10 ppm), the mass change ranged from 0.971 g to 5.652 g with changes in solar light intensity (Fig. 6b). The rGO nanofluid exhibited better performance in enhancing the vapor generation process as compared to water. The enhancement increased with increasing the light intensity, as shown in Fig. 6c. The changes in SVP and ṁ could be calculated by the Eqs. (2) and (3) and are displayed in Fig. 6d. As can be seen, with increasing the light intensity, the evaporation capacity of the rGO nanofluid increased. SVP increased from 2.0 m3 g−1 h−1 to 11.3 m3 g−1 h−1, while ṁ increased from 0.9 kg h−1 m−2 to 5.4 kg h−1 m−2. With the increase in light intensity, the total energy projected on the collector as well as the light transmission capacity increased, and lower part bulk water could receive solar light. The evaporation capacity of the rGO nanofluid was thus enhanced. Fig. 7 shows the temperature curves of the rGO nanofluid at different heights and under different light intensities (temperature changes at different heights under different light intensities are shown in Fig. S3 in the Supporting Information). Fig. 7a and b shows the temperature change of water and rGO nanofluid at 7 sun, respectively. There was a larger temperature difference in the rGO nanofluid than in water. It was found that the temperatures at different heights significantly increased with increasing the light intensity. For the rGO 307
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Fig. 6. The evaporation amount of (a) water and (b) the rGO nanofluid VS time under different light intensities; (c) the enhancement effect of the rGO nanofluid compared with water; (d) the change of SVP and ṁ VS light intensity for the rGO nanofluid.
there was no obvious temperature difference from the bottom 10 mm to 70 mm. The increase rate of temperature was almost the same. This was because the absorption of light was relatively weak for water, and the light could easily reach the bottom of the collector. Since the water at different locations in the collector absorbed the same solar energy, the temperature change at each height did not differ much. However, the temperature and temperature rise changed when the working fluid in the collector was changed to rGO nanofluid. With the increase of rGO nanofluid concentration, the temperature difference at different locations increased. This indicated that rGO in water could create a more concentrated hot area to reduce the thermal loss and increase the evaporation efficiency. As displayed in Fig. 10, the infrared thermal images of water and the rGO nanofluid are significantly different. With increasing the irradiation time, the water temperature increased uniformly, while there was a distribution gradient for the rGO nanofluid. The maximum temperature gradient was at the top of bulk water, which resulted in a hot area with concentrated energy for evaporation. This reduced the thermal loss to bulk water at the bottom. The hot area was formed in the absence of the layered structure of rGO, which was similar to the evaporation of other nanofluids or membranes [21,40]. Compared to nanoparticles, the layered structure of rGO could better impede heat transfer from the top to the bottom, which favored the formation of a hot area at the top, concentrating more energy for evaporation. The rGO nanofluid employed for solar steam generation thus played a better role in strengthening the effect. Fig. 9c shows the average temperatures of different rGO concentrations under a solar irradiation of 5 sun. As can be seen, with increasing the rGO concentration, the average temperature increased first and then decreased, which was due to the faster attenuation of light along the depth direction caused by the increase of the rGO concentration. The surface nanofluid could absorb light, while the bottom
of the rGO nanofluid was much lower than that of water under the same conditions, and the heating efficiency was lower than that of water. However, due to the strong absorption capacity of the rGO layers in the nanofluids, the total absorption efficiency was higher than in water under the same conditions. 3.4. Effect of rGO concentration The effect of rGO concentration on the enhancement solar steam generation was investigated for 0 ppm to 10 ppm concentrations under a constant solar light intensity (5 kW m−2). It can be seen from Fig. 9a that the vapor generation was enhanced by the increase of rGO concentration. As the rGO concentration increased from 0 ppm to 10 ppm, the mass of the generated vapor changed from 0.8945 g to 3.587 g. In addition, the evaporation rate per unit area increased from 1.219 kg−1 h−1 m−2 to 3.58 kg−1 h−1 m−2 as the concentration increased. With the increase of rGO concentration, the ability of the unit mass of particles to produce steam per unit time decreased from 25.8 m3 g−1 h−1 to 7.5 m3 g−1 h−1, as displayed in Fig. 9b. This is because at a relatively low rGO concentration, the rGO sheets at the surface could not absorb much light energy, and the light passed through the surface layers. Therefore, the radiation depth increased and the rGO sheets in bulk water received sunlight. With the increase of the concentration, the rGO sheets at the surface absorbed the light energy completely; thus, the light could not pass through the surface, resulting in the decrease in radiation depth. Since the rGO sheets at the bottom could not absorb the sunlight, the amount of steam produced per unit mass per unit time decreased. The temperature changes under different rGO concentrations at different heights are displayed in Fig. S4 in the Supporting Information. The temperature of pure water was nearly the same at each height, and 308
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Fig. 7. The temperature change of (a) water and (b) the rGO nanofluid at 7sun; the average temperature of (c) water and (d) the rGO nanofluid VS time at different light intensities.
The evaporation efficiency, heating efficiency, and total energy utilization efficiency are displayed in Fig. 9d. The evaporation efficiency increased with increasing the rGO concentration, and the heating efficiency increased first and then decreased. However, the magnitude of increase in evaporation efficiency was larger than the magnitude of decrease in heating efficiency at concentrations of 2.5, 5, and 10 ppm, which resulted in an increase in the total energy utilization efficiency. This indicated that the concentration increase improved the
of the nanofluid could not. As the evaporation rate of nanofluid increased, it removed a large part of the heat. Therefore, the average temperature was not as high as that under high concentrations. The average temperature change under different concentrations tended to be linear after 30 min, and the solar steam generation process reached the stable evaporation stabilization stage. At this time, the amount of evaporation and temperature would remain unchanged due to the heat balance between absorption and dissipation.
Fig. 8. The evaporation, heating, and total thermal efficiency of (a) water and (b) the rGO nanofluid. 309
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Fig. 9. (a) The evaporation amount and (c) the average temperature with different concentrations of the rGO nanofluid VS time; the change of (b) SVP, ṁ (d) the evaporation, heating, and total thermal efficiency of the rGO nanofluid VS different concentrations.
Fig. 10. Infrared thermal image of water and the rGO nanofluid at different time. 310
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concentrations of rGO nanofluid, the light penetration depth was long and more energy was utilized to heat the working medium rather than to generate steam, resulting in a lower evaporation efficiency and higher heating efficiency. The depth of light penetration decreased with increasing the rGO concentration due to significant light absorption, resulting in a higher evaporation efficiency. The change of light intensity was similar to the change of concentration. Because high intensities enhanced the penetration of sunlight, the rGO in the upper part could not completely absorb sunlight, leading to energy being transferred to the bulk water, which resulted in lower evaporation efficiency. Thus, compared to traditional working media, the rGO nanofluid could better enhance solar steam generation at different concentrations and light intensities, which can be used in various applications.
absorptive capacity of the nanofluid. The sum of the evaporation and heating efficiencies was above 50%, which was higher than that of pure water (35%), indicating the enhancement in solar steam generation. The change in concentration was not proportional to the increase in absorbance. Initially, an increase of the concentration increased the absorption efficiency significantly. This enhancement declined with further increase in concentration. When the rGO concentration was low, the heating efficiency was higher than the evaporation efficiency. The low concentration resulted in poor absorption, leading to a small temperature rise at the surface. Thus, the evaporation efficiency was also low. The rGO at the surface absorbed only a part of the solar energy and more energy was used to heat the bulk water, resulting in a higher heating efficiency. When the rGO concentration was higher, the evaporation efficiency was higher than the heating efficiency since most of the solar energy was absorbed by rGO at the surface. In other words, most of the solar energy was used for evaporation, leading to the low temperature of bulk water and lower heating efficiency. At a certain light intensity, a low concentration of the rGO nanofluid was more conducive to improving the working temperature of the photo-heat conversion, while a high concentration of the rGO nanofluid was more conducive to evaporation. In the actual production process, the concentration should be selected according to requirements. It is also necessary to design the depth of the collector reasonably. The depths of different kinds of nanofluids could be different, which can effectively improve the efficiency of the collector. Compare to the traditional solar steam generation system, the rGO nanofluids based volumetric solar steam generation system has a higher evaporation efficiency and lower cost than the traditional one. In this work, only 10 mg rGO was required for preparing 1 L rGO based nanofluids (10 ppm). In 2009, Otanicar and Golden [46] have compared the economic and environmental features of nanofluid-based solar collectors with the conventional types. The price of nanomaterials is always an important constraint to the wide application of nanofluids in solar energy harvesting. Due to the development of nanotechnology, the cost of nanomaterial synthesis decreased. For graphene nanomaterials, the modified Hummers method is a low cost, large scale and facile approach to get high quality graphene. And the cost of rGO synthesized by the modified Hummers method was only around $6/g now. To prepare a 1 L graphene based nanofluid with a mass concentration of 10 ppm, the cost of nanomaterials could be reduced to ∼$0.06. For a volumetric solar absorption system, usage of the metal in the structure could be reduced, which is beneficial for the cost reduction. Furthermore, the operation conditions with low solar light concentration requirement could decrease the cost of the solar-harvesting system and enhance the possibility of industrial application. Right now, the nanofluids based volumetric solar steam generation has great potential application in seawater desalination, clean water production, sterilization of waste, etc. These are all low temperature solar-thermal utilizations. For other middle and high temperature solar applications, such as the solar power generation, this strategy still need to be developed for realizing high temperature and high pressure steam generation in the future. As organic Ranking cycles only require working fluid temperatures of 100–200 °C [47–49], these could be suitable power application for our small-scale device right now.
Acknowledgements This work is financially supported by the National Natural Science Foundation of China (Grant No. 51676060), the Natural Science Founds of Heilongjiang Province for Distinguished Young Scholars (Grant No. JC2016009), and the Science Creative Foundation for Distinguished Young Scholars in Harbin (Grant No. 2014RFYXJ004). Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.apenergy.2018.03.097. References [1] Duić N, Guzović Z, Kafarov V, KlemeŠ JJ, vad Mathiessen B, Yan J. Sustainable development of energy, water and environment systems. Appl Energy 2013;101:3–5. [2] Wang F, Lai Q, Han H, Tan J. Parabolic trough receiver with corrugated tube for improving heat transfer and thermal deformation characteristics. Appl Energy 2016;164:411–24. [3] Wang F, Ma L, Cheng Z, Tan J, Huang X, Liu L. Radiative heat transfer in solar thermochemical particle reactor: a comprehensive review. Renew Sustain Energy Rev 2017;73:935–49. [4] Guldentops G, Nejad AM, Vuye C, Van den bergh W, Rahbar N. Performance of a pavement solar energy collector: model development and validation. Appl Energy 2016;163:180–9. [5] Gao X, Liu J, Zhang J, Yan J, Bao S, Xu H, et al. Feasibility evaluation of solar photovoltaic pumping irrigation system based on analysis of dynamic variation of groundwater table. Appl Energy 2013;105:182–93. [6] Wang X, He Y, Liu X, Zhu J. Enhanced direct steam generation via a bio-inspired solar heating method using carbon nanotube films. Powder Technol 2017;321:276–85. [7] Yan J, Desideri U, Chou SK, Li H. Energy solutions for a sustainable world. Int J Green Energy 2016;5075:20–2. [8] Chen M, He Y, Zhu J, Wen D. Investigating the collector efficiency of silver nanofluids based direct absorption solar collectors. Appl Energy 2016;181:65–74. [9] Mateus T, Oliveira AC. Energy and economic analysis of an integrated solar absorption cooling and heating system in different building types and climates. Appl Energy 2009;86:949–57. [10] Luo H, Wang R, Dai Y. The effects of operation parameter on the performance of a solar-powered adsorption chiller. Appl Energy 2010;87:3018–22. [11] Silva R, Berenguel M, Pérez M, Fernández-Garcia A. Thermo-economic design optimization of parabolic trough solar plants for industrial process heat applications with memetic algorithms. Appl Energy 2014;113:603–14. [12] Weinstein LA, Loomis J, Bhatia B, Bierman DM, Wang EN, Chen G. Concentrating solar power. Chem Rev 2015;115:12797–838. [13] Shannon MA, Bohn PW, Elimelech M, Georgiadis JG, Mariñas BJ, Mayes AM. Science and technology for water purification in the coming decades. Nature 2008;452:301–10. [14] Sharshir SW, Peng G, Wu L, Essa FA, Kabeel AE, Yang N. The effects of flake graphite nanoparticles, phase change material, and film cooling on the solar still performance. Appl Energy 2017;191:358–66. [15] Gupta MK, Kaushik SC. Exergy analysis and investigation for various feed water heaters of direct steam generation solar-thermal power plant. Renew Energy 2010;35:1228–35. [16] Huang J, He Y, Wang L, Huang Y, Jiang B. Bifunctional Au@TiO2 core–shell nanoparticle films for clean water generation by photocatalysis and solar evaporation. Energy Convers Manage 2017;132:452–9. [17] Elimelech M, Phillip WA. The future of seawater desalination: Energy, technology, and the environment. Science 2011;333:712–8.
4. Conclusions In this work, rGO nanofluids with good stability and light absorption capability were prepared by the reduction of GO synthesized by the Hummers method to enhance solar steam generation. Different concentrations of the rGO nanofluid were utilized for solar steam generation under different light intensities. Present findings revealed that the rGO nanofluid created a hot area with concentrated energy for evaporation in the upper part of the device due to its unique lamellar structure with good light absorption. This reduced thermal loss to the bulk water and improved the evaporation efficiency. At low 311
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Applied Energy journal homepage: www.elsevier.com/locate/apenergy
Volumetric solar steam generation enhanced by reduced graphene oxide nanofluid
T
⁎
Xing Liu, Xinzhi Wang, Jian Huang, Gong Cheng, Yurong He
School of Energy Science and Engineering, Harbin Institute of Technology, Harbin 150001, China Heilongjiang Key Laboratory of New Energy Storage Materials and Processes, School of Energy Science and Engineering, Harbin Institute of Technology, Harbin, Heilongjiang 150001, China
H I GH L IG H T S
G R A P H I C A L A B S T R A C T
dispersed rGO nanofluids were • Well used as volumetric broadband solarabsorber.
Solar steam generation efficiency of • rGO nanofluids could be up to ∼47%. local evaporation was realized • Rapid while the bulk fluid temperature was still low.
A R T I C LE I N FO
A B S T R A C T
Keywords: Solar energy Steam generation Reduced graphene oxide Nanofluid Evaporation
Solar steam generation is a highly efficient photo-thermal conversion method that has a wide range of applications in water purification, distillation, power plants, and seawater desalination. Low steam generation efficiency was obtained for solar steam generation using traditional working media. Therefore, reduced graphene oxide (rGO) nanofluids with good stability and light absorption capability were fabricated to achieve highly efficient volumetric solar steam generation in this work. The effects of rGO mass concentration and light intensity on solar steam generation enhancement were investigated experimentally. It was found that a hot area was formed at water–air interface due to the unique lamellar structure of rGO with good light absorption characteristic, and sunlight was absorbed by the hot area to generate steam locally, which reduced thermal loss and improved evaporation efficiency. The solar steam generation enhancement achieved by the rGO nanofluids reduced evaporation costs and expanded their applicability in seawater desalination, clean water production, sterilization of waste, etc.
1. Introduction With the depletion of traditional fossil fuels and the increasing emission of greenhouse-gases, solar energy has been identified as a green and renewable alternative [1–4]. There are two strategies to
⁎
utilize solar energy: photo-electric conversion [5] and photo-thermal conversion [6]. Despite rapid developments in photo-electric conversion applications, photo-thermal conversion is a highly efficient technique that has more applications [7] such as solar water heating [8], space heating and cooling [9], refrigeration [10], industrial process
Corresponding author at: School of Energy Science and Engineering, Harbin Institute of Technology, Harbin 150001, China. E-mail address: [email protected] (Y. He).
https://doi.org/10.1016/j.apenergy.2018.03.097 Received 14 December 2017; Received in revised form 17 February 2018; Accepted 26 March 2018 0306-2619/ © 2018 Published by Elsevier Ltd.
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generation. The mechanism of thermal and mass transfer in volumetric solar steam generation should be further addressed. In this work, graphene oxide was synthesized by the Hummers method and then high solar absorption rGO was obtained by the reduction of graphene oxide. The morphologies, structures, and properties of rGO were characterized. Furthermore, well dispersed and stable rGO based nanofluids were prepared for the volumetric solar steam generation. Finally, to explore the solar steam generation enhancement by the rGO nanofluids, solar steam generation experiments were conducted.
heating [11], and thermal power generation [12]. Solar steam generation is a typical method to achieve photo-thermal conversion and has a wide range of applications in water purification [13], distillation [14], power plants [15], and seawater desalination [16,17]. For instance, in the concentrating solar power (CSP) system, the sunlight incident is absorbed by surface absorber and converted to heat, then the heat is carried away from the absorber by heat transfer fluid (HTF), subsequently, the HTF delivers heat to a heat engine, which generates electricity [12]. In traditional solar steam generation systems, solar energy is usually received and absorbed by the tube receiver, and then used to heat the bulk fluid through thermal conduction and thermal convection. As most of absorbed solar thermal energy is used to heat the bulk fluid and heat transfer resistance between the absorber and HTF is high, the steam generation efficiency is relatively low (35–45%) [18,19]. Using volumetric absorbers such as gas–particle suspensions, molten salts, and nanofluids is a simple and effective approach to improve light absorption capacity and reduce thermal loss, which could absorb the solar energy directly [20,21]. Choi [22] first proposed that nanofluids have many special properties due to the effects of nanoparticles (NPs). A nanofluid is a fluid containing nanometer-sized particles [23]. Unlike traditional fluids, nanofluids could enhance thermal conductivities [24,25], specific heat capacities [26], heat transfer coefficients [27–29], and sunlight absorption capacities [30,31] which could help enhance the solar steam generation efficiency. Many nanofluids have been explored to enhance solar steam generation, such as noble metals and composite NPs based nanofluids. Zhang et al. [32] and Chen et al. [33] studied photo-thermal conversion with gold and silver nanofluids, respectively, and the results demonstrated high efficiency for solar heating. Neumann et al. [34,35] investigated the solar steam generation properties of Au nanofluids and demonstrated a device efficiency of 24% with a solar power of 1000 sun (1 sun = 1 kW m–2). Furthermore, Jin et al. [36] experimentally studied the steam generation performance of Au nanofluids under 220 sun irradiance and revealed that localized boiling occurred in the nanofluid. Liu et al. [37] investigated solar steam generation with liquid dispersed nanoparticles and realized highly efficient seawater purification. Guo et al. [38] explored the effect of Au NP diameters on photothermal conversion during the volumetric solar steam generation process. Amjad et al. [39] proposed a new integration method to calculate the sensible heating contribution in the Au NPs based nanofluids volumetric solar steam generation. Compared to noble metal NPs, carbon nanomaterials have better potential for solar steam generation due to the high thermal conductivities and low cost. Ni et al. [21] reported a vapor generation efficiency of up to 69% under 10 sun irradiance using a graphitized carbon black nanofluid and revealed that a global temperature rise in the fluid medium was a significant mechanism in steam generation. Wang et al. [40] obtained a high evaporation efficiency with carbon nanotube nanofluids in a direct solar steam generation experiment. Reduced graphene oxide (rGO) is a kind of new carbon material possessing superior features such as high strength, flexibility, good conduction, high thermal conductivity, and excellent optical properties. rGO has good stability even at high temperature, which enables its possible applications in many areas [41,42]. The layered structure of rGO makes it more stable than other carbon materials in evaporating water. A more concentrated hot area will form in the top surface of bulk water to heat a small portion of water for evaporation, which could reduce thermal loss to the bulk fluid and increase the evaporation efficiency. Besides, the lamellar structures intertwine in bulk water, resulting in scattering and refraction of incident light, which will contribute to light absorbance by the upper area, thus increasing the evaporation rate. Therefore, solar steam generation enhancement could be achieved using rGO nanofluid due to multiple effects. Despite some previous work on nanofluids based volumetric solar steam generation, the evaporation efficiency is still low. In addition, the stability of the nanofluids is still a problem for a long term solar steam
2. Experimental 2.1. Synthesis Graphite powder, ammonia water, L-ascorbic acid (L-AA), NaNO3, KMnO4, H2SO4, and H2O2 were supplied by Aladdin Chemical Co., Ltd. GO was synthesized via a chemical exfoliation of the graphite powder by a modified Hummers method [43] as follows: 46 mL concentrated H2SO4 was slowly added into a mixture of 2 g graphite powder and 1 g NaNO3 at 0 °C. Then, 6 g KMnO4 was added with stirring while the temperature of the mixture was maintained below 20 °C using an ice bath. Afterward, the ice bath was removed, and the temperature of the mixture was raised to 35 °C and stirred for 30 min. Then, 92 mL deionized water was added to the mixture with stirring for 15 min, followed by the addition of 60 mL H2O2 solution at 60 °C. After the suspension turned brown, it was washed with deionized water and centrifuged for several times. rGO was obtained by the reducing of GO with L-AA as reductant in aqueous solution [44]. To prepare rGO, the GO solution (100 ppm) was dispersed in deionized water and ultrasonicated for 1 h. Ammonia water was then added to regulate the pH to 10 with sonication for 30 min. L-AA (10 mg/mL) was added and the mixture was maintained at 95 °C for 3 h for the completion of reaction. The rGO solution was filtered to obtain rGO on the filter paper. Finally, rGO nanofluids were prepared by sonicating the filtered powder in a certain amount of deionized water. The entire process is shown in Fig. 1. 2.2. Solar evaporation experiments Fig. 2 shows the experimental setup for solar steam generation. Solar light was generated by a solar simulator (CEL HXF300, CEAULIGHT, Beijing, China). The light was generated by a 300W Xenon lamp, which could realize the exportation of collimated light with high energy. The Xenon lamp is one of the most common types of lamp for continuous solar simulators. It offers high intensities and an unfiltered spectrum matching reasonably well to sunlight. The spectra of the light generated by Xenon lamp is shown in Fig. S1. An acrylic tube with a thermal insulation layer was used as a solar collector, and the temperature was measured and recorded using thermocouples (TT-T-40SLE, Omega, US) and a data-acquisition system (34972A, Agilent Technology, Santa Clara, CA, US). Mass change was measured using an electric balance (Practum313–1CN, Sartorius, Göttingen, Germany). The inner height and diameter of the solar collector were 80 mm and 40 mm, respectively. The size and weight of the designed solar collector tube could be matched with the size of the solar light simulator and weighing range of the electric balance in the experiments. Seven T-type thermocouples were inserted into the solar collector at different heights (H = 10, 20, 30, 40, 50, 60, and 70 mm). 2.3. Characterization Transmission electron microscopy (TEM) images of the rGO were obtained using a field emission microscope (Tecnai G2 F30, FEI, Portland, US). Atomic force microscopy (AFM) images were obtained using a Bruker Dimension Icon with ScanAsyst (Karlsruhe, Germany) for further thickness characterization of the rGO. Ultraviolet–visible 303
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Fig. 1. Schematic graph of rGO nanofluids preparation.
900W, Biosafer, Beijing, China) was used to disperse the rGO into water for preparing nanofluids, which could ensure a good dispersion and long time stability of nanofluids.
(UV–Vis) spectra were recorded on a two-beam UV–Vis spectrometer (TU1901, China). Fourier-transform infrared (FT-IR; Frontier Optica PerkinElmer, US) spectra were obtained from 500 to 4000 cm−1 at room temperature. High-power tip ultrasonoscope (Biosafer 900-92,
Fig. 2. Schematic of experimental setup for solar steam generation. 304
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Fig. 4b shows the FT-IR spectra of GO and rGO. Since GO contains a large number of C]O bonds, a corresponding peak at 1753 cm−1 appeared, while the peaks at 3424 cm−1 and 1385 cm−1 originated due to the OeH bond. The epoxy structure of CeO gave rise to a peak at 1226 cm−1, and the characteristic peak of CeO appeared at 1060 cm−1. After reduction, the peak corresponding to the epoxy structure completely disappeared, indicating that ascorbic acid treatment was effective for the removal of oxygen-containing functional groups to achieve reduction.
2.4. Error analysis The uncertainty of experimental data was determined by measurement deviation of parameters, including time t, tube diameter d, tube height h, temperature T, mass m, irradiation intensity I. when the measurement is of equal-precision and performed several times, suppose that (xk, yk, …) are n experimental realizations of (x, y, …), and that fk = f (xk, yk, …). The expectation of f and the variance of f are given respectively as follows.
μf =
σ f2 =
n
1 n
∑
1 n
∑
fk
3.2. Stability test of rGO nanofluids (1)
k=1
In the stability test, different concentrations of rGO nanofluids were placed in different cuvettes, and the cuvettes were plugged to prevent the evaporation of water. As can be seen from Fig. 5a, the rGO nanofluids have broadband absorption in the visible (400–760 nm) and near infrared regions (> 760 nm). In addition, the absorbances of rGO nanofluids increase with increasing the concentrations of rGO, which meets well with the description of Beer-Lamber law [45]. According to the Beer-Lamber law, the absorption intensity is directly proportional to the concentrations of the tested sample. When the rGO nanofluids was placed for a certain time, the UV–Vis absorption intensity would decrease if the rGO precipitates in the cuvettes. Fig. 5b shows the UV–Vis intensity (at 270 nm) changes of rGO nanofluids for 10 days of measurement. It can be seen that there is little intensity decrease during 10 days of test. This indicated that the rGO nanofluids had good stabilitiy without the addition of other dispersants and could be used in solar steam generation. Furthermore, the VU-Vis spectra of rGO nanofluids with a concentration of 10 ppm before and after the steam generation experiment was tested. For the sample after the steam generation experiment, same amount of vaporized water was added before the UV–Vis spectra test. The comparison between the VU-Vis spectra of rGO nanofluids before and after the steam generation were illustrated in Fig. S2. It can be seen that there was not much difference between the two curves. The results indicated that rGO nanofluids could keep the stability for a long term in the process of steam generation with continuous water supply.
n
(fk −μk )2
(2)
k=1
The measurement error in the mean value of f is therefore σf / n . Assuming that the function relation formula is Nk = f (N1, N2, …, Nk), taking the logarithm of this equation gives: lnN = lnf . Taking total differential on both sides of above equation yields:
∂lnf ∂lnf ∂lnf dN dNk dN1 + ⋯+ dN1 + = N ∂Nk ∂N1 ∂N1
(3)
According to the error transfer principle and in order to synthesize the total relative error E we have
E=
σN = N
2
2
2
⎛ ∂lnf σN ⎞ + ⎛ ∂lnf σN ⎞ +⋯+⎛ ∂lnf σk ⎞ 1 2 ⎝ ∂N1 ⎠ ⎝ ∂N2 ⎠ ⎝ ∂Nk ⎠ ⎜
⎟
⎜
⎟
⎜
⎟
(4)
The indirect measurement parameter is the evaporation efficiency η. The true value of η and relative error Eη are calculated by:
η=
Eη =
Δmhlv IAΔt 2 σ σ 2 σ 2 σ 2 ⎛ Δm ⎞ + ⎛ I ⎞ + ⎛ A ⎞ + ⎛ Δt ⎞ ⎝I⎠ ⎝ A⎠ ⎝ Δm ⎠ ⎝ Δt ⎠
(5)
(6)
In present work, the maximum uncertainty of solar steam generation efficiency at various tests was around 3.17% (mainly including measurement uncertainties).
3.3. Effect of solar illumination intensity
3. Results and discussion
To investigate the effect of solar illumination intensity, the solar collector was placed under different solar illumination intensities (0, 1, 3, 5, and 7 suns, where 1 sun = 1 kW m–2). Two kinds of fluids (water and 10 ppm rGO nanofluid) were used in the experiments. To better describe the evaporated amounts of different working fluids in the evaporation stabilization phase, the rate of evaporation per unit area was defined as:
3.1. Morphologies and structures of rGO The structural properties of the rGOs were characterized using AFM, as shown in Fig. 3a and b. Many rGO sheets several microns in length and width were observed on silicon substrate. The average thickness of the rGO sheets was ∼1 nm, which can be regarded as a single graphene sheet. TEM was used to study the rGO morphology, as shown in Fig. 3c. The graphene sheet appeared transparent with tulle shape with a very low contrast. This was because the thickness of monolithic graphene was very small, and the substrate was a carbon film, which results in the difficulty to distinguish them. Some dark dots ∼50 nm in size were observed in the graphene layers, which were composed of crushed graphene sheets due to ultrasonic shock. Fig. 3d shows the UV–Vis spectra of GO and rGO nanofluids. The absorption peak of the GO nanofluid appeared at ∼229 nm, while that of the rGO nanofluid appeared at ∼270 nm. It is obvious that the rGO nanofluid had a broadband absorption in the visible light range, which contributes almost 50% of the solar energy. To explore the change in structure after reduction, Raman and FT-IR spectra of the rGO and GO were obtained, as shown in Fig. 4a and b, respectively. The Raman spectrum of GO exhibits two peaks at 1350 and 1603 cm−1, which are attributed to the D and G bands of GO, respectively. The D/G intensity ratio of GO was 0.84, which was smaller than that of rGO (1.15), indicating that GO had been reduced by L-AA.
ṁ =
dm / dt A
(7) −2
−1
where ṁ is the steady-state evaporation mass flux (kg m h ), m is the evaporation amount of the stabilization phase, A is the area of evaporation (m2), and t is the total time for evaporation. In the absence of light, a certain amount of evaporation occurred in the collector. To calculate the photo-thermal evaporation ability and evaporation efficiency of the rGO nanofluid more accurately, a darkfield experiment was set up and the evaporation rate per unit area became ṁ 0 . To describe the contribution of individual nanoparticles to the overall evaporation in the solar steam generation stage, their ability to generate steam per unit mass per unit time (SVP) was defined as follows:
305
SVP = V̇ / mrGO
(8)
V̇ = (ṁ −ṁ 0) A/ ρvapor
(9)
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Fig. 3. (a) AFM images of rGO; (b) height profiles of rGO in AFM images; (c) TEM image of rGO; (d) UV–Vis spectra of rGO nanofluids.
Here, V̇ is the evaporation steam volume per unit time in the evaporation stabilization phase (m3), ρvapor is the density of water vapor at 100 °C (0.6 kg m−3), and mrGO is the amount of rGO in the nanofluid (g). In the evaporation stabilization phase, the total energy of light irradiated on the collector is defined as Qin, as shown in Eq. (4). Qnf is the thermal energy collected in the bulk fluid, as shown in Eq. (5), and Qeva is the energy for evaporation in the stabilization phase, as shown in Eq.
(6).
Qin = I AΔt
(10)
Qnf = c p,nf Mnf Δt
(11)
and,
Qeva = (ṁ −ṁ 0) Ahlv Δt
(12)
where Δt is the time of light exposure during the evaporation
Fig. 4. (a) Raman spectra of the rGO and GO; (b) FT-IR spectra of the GO, rGO and graphite. 306
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Abs (a.u.)
b
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0.20
0.15
0.10
0.2
0.1
0.05
0.00 200
1.25 ppm 2.5 ppm 5 ppm 10 ppm
0.3
Abs (a.u.)
a
0.4
0.25
300
400
500
600
700
800
900
0.0
0
2
4
6
8
10
Time (Day)
Wavelength (nm)
Fig. 5. (a) UV–Vis spectra of the rGO nanofluids with different concentrations (b) the change of absorption peak with time.
stabilization phase (h), I is the power density of the light (W m−2), cp,nf is the specific heat of the nanofluid (J g−1 °C−1), ΔT is the increase in average temperature (°C) of the working fluid in the evaporation stabilization phase (°C), and hlv is the latent heat of phase transition (2.257 kJ g−1, 1 atm). The heating efficiency, ηnf, is defined as the ratio of heat consumed by the working fluid to increase the temperature against the energy of light irradiated onto the collector, as shown in Eq. (7). ηeva is the evaporation efficiency of the working fluid, which is the ratio of heat consumed by evaporation during the evaporation stabilization phase to the energy of light irradiated onto the collector, as shown in Eq. (8).
ηnf =
nanofluid, the temperature at a height of 70 mm changed from 13.8 °C to 42.8 °C when the light intensity changed from 1 sun to 7 sun, which was a threefold enhancement. It indicated that the nanofluids converted the absorbed heat to the energy required for phase change rather than raising the fluid temperature. As the light intensity increased, the rate of temperature increase at the height of 70 mm accelerated, resulting in a decrease in the time required to reach the evaporation stabilization phase. The maximum temperature difference under 7 sun was between 50 and 60 mm heights, while under 1 sun, it was between 60 and 70 mm heights. This is because at a very high light intensity, the surface rGO layers were saturated by light. Thus, the excess light went through the upper rGO layers and was absorbed by the lower rGO layers, and their temperature increased. Fig. 7c and d shows the changes in average temperatures of the rGO nanofluid and water under different light intensities. The average temperature rise of water was higher than that of the rGO nanofluid under the same light intensity. With increasing the light intensity, the average temperature difference between the rGO nanofluid and water increased. When the light intensity was 1 sun, the average temperature rise of water was 5.5 °C after 60 min irradiation, while it was 5.4 °C for the rGO nanofluid. When the light intensity reached 7 sun, the average temperature rises of water and the rGO nanofluid were 33.9 °C and 25.6 °C, respectively, which indicated that the rGO nanofluid could better concentrate the heat and reduce the thermal loss. The evaporation, heating, and total thermal efficiencies of water and rGO nanofluid could be calculated by the Eqs. (7)–(10) and are displayed in Fig. 8. Compared to pure water, the rGO nanofluid achieved higher evaporation and total thermal efficiencies but lower heating efficiency under different light intensities, which indicated that the rGO nanofluid could concentrate the heat for evaporation, thereby increasing the evaporation efficiency. Besides, the heating efficiencies of the rGO nanofluid and water decreased with increasing the light intensity. However, their evaporation and total thermal efficiencies decreased initially, and then increased with increasing the light intensity. The heating efficiency of pure water was higher than the evaporation efficiency under different light intensities. However, the trend reversed for the rGO nanofluid. Under light irradiation, the temperature of the rGO nanofluid increased drastically due to strong light absorption by the rGO layers, and the phase transition temperature was quickly reached. The temperature rise in pure water was relatively slow due to the absence of rGO, and thus, more time was required for phase transition, resulting in lower evaporation efficiency. In the rGO nanofluid, the temperature of the graphene layer gradually stabilized because evaporation removed a large amount of heat. However, water, with a lower evaporation rate, utilized only a fraction of the energy; most of the energy was used to heat water. Therefore, the average temperature
c p,nf Mnf ΔT
ηeva =
I AΔt
(13)
(ṁ −ṁ 0) hlv I
(14)
The total energy utilization efficiency (ηtotal) is defined as:
ηtotal =
Qnf + Qeva Qin
ηtotal = ηnf + ηeva
(15) (16)
The results obtained with different solar light intensities are displayed in Fig. 6. For solar steam generation of pure water, the mass change ranged from 0.548 g to 1.343 g as the solar light intensity changed from 1 kW m−2 to 7 kW m−2 (Fig. 6a). For solar steam generation of the rGO nanofluid (10 ppm), the mass change ranged from 0.971 g to 5.652 g with changes in solar light intensity (Fig. 6b). The rGO nanofluid exhibited better performance in enhancing the vapor generation process as compared to water. The enhancement increased with increasing the light intensity, as shown in Fig. 6c. The changes in SVP and ṁ could be calculated by the Eqs. (2) and (3) and are displayed in Fig. 6d. As can be seen, with increasing the light intensity, the evaporation capacity of the rGO nanofluid increased. SVP increased from 2.0 m3 g−1 h−1 to 11.3 m3 g−1 h−1, while ṁ increased from 0.9 kg h−1 m−2 to 5.4 kg h−1 m−2. With the increase in light intensity, the total energy projected on the collector as well as the light transmission capacity increased, and lower part bulk water could receive solar light. The evaporation capacity of the rGO nanofluid was thus enhanced. Fig. 7 shows the temperature curves of the rGO nanofluid at different heights and under different light intensities (temperature changes at different heights under different light intensities are shown in Fig. S3 in the Supporting Information). Fig. 7a and b shows the temperature change of water and rGO nanofluid at 7 sun, respectively. There was a larger temperature difference in the rGO nanofluid than in water. It was found that the temperatures at different heights significantly increased with increasing the light intensity. For the rGO 307
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Fig. 6. The evaporation amount of (a) water and (b) the rGO nanofluid VS time under different light intensities; (c) the enhancement effect of the rGO nanofluid compared with water; (d) the change of SVP and ṁ VS light intensity for the rGO nanofluid.
there was no obvious temperature difference from the bottom 10 mm to 70 mm. The increase rate of temperature was almost the same. This was because the absorption of light was relatively weak for water, and the light could easily reach the bottom of the collector. Since the water at different locations in the collector absorbed the same solar energy, the temperature change at each height did not differ much. However, the temperature and temperature rise changed when the working fluid in the collector was changed to rGO nanofluid. With the increase of rGO nanofluid concentration, the temperature difference at different locations increased. This indicated that rGO in water could create a more concentrated hot area to reduce the thermal loss and increase the evaporation efficiency. As displayed in Fig. 10, the infrared thermal images of water and the rGO nanofluid are significantly different. With increasing the irradiation time, the water temperature increased uniformly, while there was a distribution gradient for the rGO nanofluid. The maximum temperature gradient was at the top of bulk water, which resulted in a hot area with concentrated energy for evaporation. This reduced the thermal loss to bulk water at the bottom. The hot area was formed in the absence of the layered structure of rGO, which was similar to the evaporation of other nanofluids or membranes [21,40]. Compared to nanoparticles, the layered structure of rGO could better impede heat transfer from the top to the bottom, which favored the formation of a hot area at the top, concentrating more energy for evaporation. The rGO nanofluid employed for solar steam generation thus played a better role in strengthening the effect. Fig. 9c shows the average temperatures of different rGO concentrations under a solar irradiation of 5 sun. As can be seen, with increasing the rGO concentration, the average temperature increased first and then decreased, which was due to the faster attenuation of light along the depth direction caused by the increase of the rGO concentration. The surface nanofluid could absorb light, while the bottom
of the rGO nanofluid was much lower than that of water under the same conditions, and the heating efficiency was lower than that of water. However, due to the strong absorption capacity of the rGO layers in the nanofluids, the total absorption efficiency was higher than in water under the same conditions. 3.4. Effect of rGO concentration The effect of rGO concentration on the enhancement solar steam generation was investigated for 0 ppm to 10 ppm concentrations under a constant solar light intensity (5 kW m−2). It can be seen from Fig. 9a that the vapor generation was enhanced by the increase of rGO concentration. As the rGO concentration increased from 0 ppm to 10 ppm, the mass of the generated vapor changed from 0.8945 g to 3.587 g. In addition, the evaporation rate per unit area increased from 1.219 kg−1 h−1 m−2 to 3.58 kg−1 h−1 m−2 as the concentration increased. With the increase of rGO concentration, the ability of the unit mass of particles to produce steam per unit time decreased from 25.8 m3 g−1 h−1 to 7.5 m3 g−1 h−1, as displayed in Fig. 9b. This is because at a relatively low rGO concentration, the rGO sheets at the surface could not absorb much light energy, and the light passed through the surface layers. Therefore, the radiation depth increased and the rGO sheets in bulk water received sunlight. With the increase of the concentration, the rGO sheets at the surface absorbed the light energy completely; thus, the light could not pass through the surface, resulting in the decrease in radiation depth. Since the rGO sheets at the bottom could not absorb the sunlight, the amount of steam produced per unit mass per unit time decreased. The temperature changes under different rGO concentrations at different heights are displayed in Fig. S4 in the Supporting Information. The temperature of pure water was nearly the same at each height, and 308
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Fig. 7. The temperature change of (a) water and (b) the rGO nanofluid at 7sun; the average temperature of (c) water and (d) the rGO nanofluid VS time at different light intensities.
The evaporation efficiency, heating efficiency, and total energy utilization efficiency are displayed in Fig. 9d. The evaporation efficiency increased with increasing the rGO concentration, and the heating efficiency increased first and then decreased. However, the magnitude of increase in evaporation efficiency was larger than the magnitude of decrease in heating efficiency at concentrations of 2.5, 5, and 10 ppm, which resulted in an increase in the total energy utilization efficiency. This indicated that the concentration increase improved the
of the nanofluid could not. As the evaporation rate of nanofluid increased, it removed a large part of the heat. Therefore, the average temperature was not as high as that under high concentrations. The average temperature change under different concentrations tended to be linear after 30 min, and the solar steam generation process reached the stable evaporation stabilization stage. At this time, the amount of evaporation and temperature would remain unchanged due to the heat balance between absorption and dissipation.
Fig. 8. The evaporation, heating, and total thermal efficiency of (a) water and (b) the rGO nanofluid. 309
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Fig. 9. (a) The evaporation amount and (c) the average temperature with different concentrations of the rGO nanofluid VS time; the change of (b) SVP, ṁ (d) the evaporation, heating, and total thermal efficiency of the rGO nanofluid VS different concentrations.
Fig. 10. Infrared thermal image of water and the rGO nanofluid at different time. 310
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concentrations of rGO nanofluid, the light penetration depth was long and more energy was utilized to heat the working medium rather than to generate steam, resulting in a lower evaporation efficiency and higher heating efficiency. The depth of light penetration decreased with increasing the rGO concentration due to significant light absorption, resulting in a higher evaporation efficiency. The change of light intensity was similar to the change of concentration. Because high intensities enhanced the penetration of sunlight, the rGO in the upper part could not completely absorb sunlight, leading to energy being transferred to the bulk water, which resulted in lower evaporation efficiency. Thus, compared to traditional working media, the rGO nanofluid could better enhance solar steam generation at different concentrations and light intensities, which can be used in various applications.
absorptive capacity of the nanofluid. The sum of the evaporation and heating efficiencies was above 50%, which was higher than that of pure water (35%), indicating the enhancement in solar steam generation. The change in concentration was not proportional to the increase in absorbance. Initially, an increase of the concentration increased the absorption efficiency significantly. This enhancement declined with further increase in concentration. When the rGO concentration was low, the heating efficiency was higher than the evaporation efficiency. The low concentration resulted in poor absorption, leading to a small temperature rise at the surface. Thus, the evaporation efficiency was also low. The rGO at the surface absorbed only a part of the solar energy and more energy was used to heat the bulk water, resulting in a higher heating efficiency. When the rGO concentration was higher, the evaporation efficiency was higher than the heating efficiency since most of the solar energy was absorbed by rGO at the surface. In other words, most of the solar energy was used for evaporation, leading to the low temperature of bulk water and lower heating efficiency. At a certain light intensity, a low concentration of the rGO nanofluid was more conducive to improving the working temperature of the photo-heat conversion, while a high concentration of the rGO nanofluid was more conducive to evaporation. In the actual production process, the concentration should be selected according to requirements. It is also necessary to design the depth of the collector reasonably. The depths of different kinds of nanofluids could be different, which can effectively improve the efficiency of the collector. Compare to the traditional solar steam generation system, the rGO nanofluids based volumetric solar steam generation system has a higher evaporation efficiency and lower cost than the traditional one. In this work, only 10 mg rGO was required for preparing 1 L rGO based nanofluids (10 ppm). In 2009, Otanicar and Golden [46] have compared the economic and environmental features of nanofluid-based solar collectors with the conventional types. The price of nanomaterials is always an important constraint to the wide application of nanofluids in solar energy harvesting. Due to the development of nanotechnology, the cost of nanomaterial synthesis decreased. For graphene nanomaterials, the modified Hummers method is a low cost, large scale and facile approach to get high quality graphene. And the cost of rGO synthesized by the modified Hummers method was only around $6/g now. To prepare a 1 L graphene based nanofluid with a mass concentration of 10 ppm, the cost of nanomaterials could be reduced to ∼$0.06. For a volumetric solar absorption system, usage of the metal in the structure could be reduced, which is beneficial for the cost reduction. Furthermore, the operation conditions with low solar light concentration requirement could decrease the cost of the solar-harvesting system and enhance the possibility of industrial application. Right now, the nanofluids based volumetric solar steam generation has great potential application in seawater desalination, clean water production, sterilization of waste, etc. These are all low temperature solar-thermal utilizations. For other middle and high temperature solar applications, such as the solar power generation, this strategy still need to be developed for realizing high temperature and high pressure steam generation in the future. As organic Ranking cycles only require working fluid temperatures of 100–200 °C [47–49], these could be suitable power application for our small-scale device right now.
Acknowledgements This work is financially supported by the National Natural Science Foundation of China (Grant No. 51676060), the Natural Science Founds of Heilongjiang Province for Distinguished Young Scholars (Grant No. JC2016009), and the Science Creative Foundation for Distinguished Young Scholars in Harbin (Grant No. 2014RFYXJ004). Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.apenergy.2018.03.097. References [1] Duić N, Guzović Z, Kafarov V, KlemeŠ JJ, vad Mathiessen B, Yan J. Sustainable development of energy, water and environment systems. Appl Energy 2013;101:3–5. [2] Wang F, Lai Q, Han H, Tan J. Parabolic trough receiver with corrugated tube for improving heat transfer and thermal deformation characteristics. Appl Energy 2016;164:411–24. [3] Wang F, Ma L, Cheng Z, Tan J, Huang X, Liu L. Radiative heat transfer in solar thermochemical particle reactor: a comprehensive review. Renew Sustain Energy Rev 2017;73:935–49. [4] Guldentops G, Nejad AM, Vuye C, Van den bergh W, Rahbar N. Performance of a pavement solar energy collector: model development and validation. Appl Energy 2016;163:180–9. [5] Gao X, Liu J, Zhang J, Yan J, Bao S, Xu H, et al. Feasibility evaluation of solar photovoltaic pumping irrigation system based on analysis of dynamic variation of groundwater table. Appl Energy 2013;105:182–93. [6] Wang X, He Y, Liu X, Zhu J. Enhanced direct steam generation via a bio-inspired solar heating method using carbon nanotube films. Powder Technol 2017;321:276–85. [7] Yan J, Desideri U, Chou SK, Li H. Energy solutions for a sustainable world. Int J Green Energy 2016;5075:20–2. [8] Chen M, He Y, Zhu J, Wen D. Investigating the collector efficiency of silver nanofluids based direct absorption solar collectors. Appl Energy 2016;181:65–74. [9] Mateus T, Oliveira AC. Energy and economic analysis of an integrated solar absorption cooling and heating system in different building types and climates. Appl Energy 2009;86:949–57. [10] Luo H, Wang R, Dai Y. The effects of operation parameter on the performance of a solar-powered adsorption chiller. Appl Energy 2010;87:3018–22. [11] Silva R, Berenguel M, Pérez M, Fernández-Garcia A. Thermo-economic design optimization of parabolic trough solar plants for industrial process heat applications with memetic algorithms. Appl Energy 2014;113:603–14. [12] Weinstein LA, Loomis J, Bhatia B, Bierman DM, Wang EN, Chen G. Concentrating solar power. Chem Rev 2015;115:12797–838. [13] Shannon MA, Bohn PW, Elimelech M, Georgiadis JG, Mariñas BJ, Mayes AM. Science and technology for water purification in the coming decades. Nature 2008;452:301–10. [14] Sharshir SW, Peng G, Wu L, Essa FA, Kabeel AE, Yang N. The effects of flake graphite nanoparticles, phase change material, and film cooling on the solar still performance. Appl Energy 2017;191:358–66. [15] Gupta MK, Kaushik SC. Exergy analysis and investigation for various feed water heaters of direct steam generation solar-thermal power plant. Renew Energy 2010;35:1228–35. [16] Huang J, He Y, Wang L, Huang Y, Jiang B. Bifunctional Au@TiO2 core–shell nanoparticle films for clean water generation by photocatalysis and solar evaporation. Energy Convers Manage 2017;132:452–9. [17] Elimelech M, Phillip WA. The future of seawater desalination: Energy, technology, and the environment. Science 2011;333:712–8.
4. Conclusions In this work, rGO nanofluids with good stability and light absorption capability were prepared by the reduction of GO synthesized by the Hummers method to enhance solar steam generation. Different concentrations of the rGO nanofluid were utilized for solar steam generation under different light intensities. Present findings revealed that the rGO nanofluid created a hot area with concentrated energy for evaporation in the upper part of the device due to its unique lamellar structure with good light absorption. This reduced thermal loss to the bulk water and improved the evaporation efficiency. At low 311
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