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Investigation of photothermal heating enabled by plasmonic nanofluids for direct solar steam generation
Solar Energy 157 (2017) 35–46
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Solar Energy journal homepage: www.elsevier.com/locate/solener
Investigation of photothermal heating enabled by plasmonic nanofluids for direct solar steam generation Xinzhi Wang a, Yurong He a,⇑, Xing Liu b, Lei Shi a, Jiaqi Zhu b a b
School of Energy Science and Engineering, Harbin Institute of Technology, Harbin 150001, China Center for Composite Materials and Structures, School of Astronautics, Harbin Institute of Technology, Harbin 150001, China
a r t i c l e
i n f o
Article history: Received 22 January 2017 Received in revised form 10 May 2017 Accepted 4 August 2017
Keywords: Solar energy Surface plasmon resonance Direct solar steam generation Nanofluid
a b s t r a c t Steam production has a wide range of applications such as seawater desalination, waste sterilization, and power generation. The utilization of solar energy for this purpose has attracted much attention due to its inexhaustibility and pollution-free nature. Here, direct solar steam generation at low-concentrated solar power using plasmonic nanofluids containing gold nanoparticles (Au NPs) was investigated experimentally. The key factors required for highly efficient solar steam generation, including Au NP concentration and solar power intensity, were studied in a simulated solar system by measuring the water weight loss and system temperature change. The best evaporation performance was obtained using a plasmonic nanofluid containing 178 ppm of Au NPs under 10 sun (1 sun = 1 kW m2) illumination intensity, and the total efficiency reached 65%. However, the total efficiency of pure water was only 16%, which means that the plasmonic nanofluids reached a 300% enhancement in efficiency. Higher solar power led to a higher evaporation rate, higher specific vapor productivity (SVP), and higher Au NP concentrations resulted in better evaporation performance. Localized solar heating at the fluid-air interface was shown to contribute more to solar steam generation than to bulk fluid heating. Furthermore, the model of photothermal heating of plasmonic nanoparticle was established and the numerical results demonstrated the photothermal conversion process of plasmonic NPs from the light absorption to the heat dissipation into the bulk fluid. Ó 2017 Published by Elsevier Ltd.
1. Introduction Due to the increasing energy demand, solar energy has been selected as a promising green inexhaustible energy source (Lewis, 2007; Weinstein et al., 2015). Traditional solar harvesting technologies are photovoltaics (Atwater and Polman, 2010) and concentrated solar power (CSP) (Weinstein et al., 2015), which are applied to generate electricity. Recently, more attentions have been focused on developing novel approaches for solar energy utilization such as seawater desalination (Elimelech and Phillip, 2011; Karagiannis and Soldatos, 2008), photochemical reactions (Chen et al., 2016; Lewis, 2001; Naldoni et al., 2016), solar sterilization (Neumann et al., 2013a), etc. Current methods of solar steam generation mainly rely on solar energy collection by black surface or cavity absorbers, with the thermal energy subsequently transferred to the working fluid either directly or via a high heat capacity intermediate carrier inside the absorbing tube to heat the bulk fluid to its boiling temperature (Kundu and Lee, 2012; Lenert and ⇑ Corresponding author. E-mail address: [email protected] (Y. He). http://dx.doi.org/10.1016/j.solener.2017.08.015 0038-092X/Ó 2017 Published by Elsevier Ltd.
Wang, 2012; Weinstein et al., 2015). This kind of indirect solar steam generation usually requires highly concentrated solar energy. Moreover, it suffers from high optical loss and thermal irradiation to the environment and high heat loss during convective heat transfer. To eliminate the energy loss due to indirect absorption, direct and volumetric absorption of solar energy by the working fluid can minimize the convection heat transfer losses and achieve higher efficiency (Ni et al., 2015; Otanicar et al., 2010). Nanofluids, a kind of functional nanoparticle dispersions, have been widely studied since their discovery in 1990s (Leong et al., 2016). Recently, a number of experimental and theoretical investigations have found that nanofluid-based direct absorbing solar collectors (DASCs) exhibit excellent performance due to their photothermal characteristics (Colangelo et al., 2015, 2013; Karami et al., 2016, 2014; Liu et al., 2015; Luo et al., 2014; Otanicar et al., 2010; Tyagi et al., 2009). Moreover, Chieruzzi et al. (2017, 2015, 2016) experimentally investigated and reviewed the development of nanofluids for thermal energy storage in solar energy applications and discussed the enhancement of fundamental thermal properties of nanofluids. Shin and Banerjee (2011) reported an anomalous enhancement of specific heat capacity of
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nanofluids which was composited chloride salt eutectics and silica nanoparticles for thermal energy storage applications. Otanicar et al. (2010) studied direct absorption collectors with nanofluids containing carbon nanotubes, graphite, and silver nanoparticles, achieving an efficiency enhancement of up to 5%. Taylor et al. (2011) demonstrated that nanoparticles could absorb over 95% of incoming light by investigating the optical properties of different kinds of nanofluids. Vakili et al. (2016a, 2016b) utilized graphite and graphene nanoplatelet nanofluid for direct absorption solar collectors exhibiting good photothermal properties at low temperature. Colangelo et al. (2015, 2013) proposed a modified flat panel solar collector, and the results revealed that Al2O3-distillated water could increase the thermal efficiency by up to 11.7%, as compared to pure water. Gupta et al. (2015) experimentally investigated the volumetric absorption of thin film of Al2O3-H2O nanofluid and obtained high efficient enhancement of 22.1%. In recent decades, plasmonic nanoparticles (metal materials or doped metal oxides) have attracted much interest and have been widely used in applications such as thermal cancer treatment (O’Neal et al., 2004), magnetic recording (Challener et al., 2009), and catalysis (Linic et al., 2011; Warren and Thimsen, 2012) due to their thermal and thermo-optical properties caused by collective coherent excitation of the free electrons, which is also known as localized surface plasmon resonance (LSPR). Localized heat is generated by coupling the plasmonic nanoparticles and photons and is subsequently transferred to the surrounding medium. Zhang et al. (2014) and Chen et al. (2015) investigated the photothermal conversion ability of gold and silver nanoparticle dispersions, respectively, obtaining high photothermal efficiency and bulk liquid temperature in both cases. Du and Tang (2016, 2015) numerically investigated the optical properties of plasmonic nanofluids containing different shapes of Au NPs or agglomerated Au clusters and concluded that a blended nanofluids could enhance the solar energy harvesting. Rather than heating the bulk fluid, plasmonic NPs can also generate nanovapor under continuous laser illumination (Baffou et al., 2014) or highly concentrated solar illumination (Neumann et al., 2013b). Lombard et al. (2015) and Neumann et al. (2013b) demonstrated solar vapor generation using an Au-based dispersion, and achieved a solar steam device efficiency of 24% under the concentrated solar energy of 1000 sun (1 sun = 1 kW m2). Ni et al. (2015) reported vapor generation efficiencies of up to 69% at 10 sun using a graphitized carbon black nanofluid and numerically demonstrated that the nanofluid steam generation phenomenon results from the global heating of bulk fluid. Jin et al. (2016) performed steam generation experiments under sunlight of 220 Suns and revealed that the steam produced due to the highly non-uniform energy distribution in the system. Wang et al. (2016) experimentally obtained a high evaporation efficiency through direct solar steam generation with carbon-nanotube nanofluid. There are still few reports about the direct solar steam generation with plasmonic nanofluids, and a lack of systematic study on the effect of solar power intensities and plasmonic NP concentrations on solar steam generation performance. In this work, solar steam generation experiments were conducted with plasmonic nanofluids containing Au NPs under simulated solar light. The effect of Au NP concentration (5–178 ppm) and solar power intensity (1–10 sun) on evaporation performance were investigated by measuring the water weight loss and system temperature change. Furthermore, the optical properties of these nanofluids were studied by spectrometry, Mie theory, and finite difference time domain (FDTD) simulation. Finally, a model of heat generation through the light absorption and the heat dissipation to the surrounding medium was established and applied to demonstrate the mechanism of plasmon heating and localized heating during photothermal heating enabled by plasmonic NPs.
2. Experimental section 2.1. Materials Tetrachloroauric acid (HAuCl4; 49–50% Au basis) and sodium citrate dihydrate (HOC(COONa)(CH2COONa)22H2O, 99%, AR) were purchased from Aladdin Industrial Inc., Shanghai, China and were used without further purification. Doubly deionized water (DDI water) was produced by a Sartorius water purification system (arium Ò mini; 18.2 MX; Göttingen, Germany). 2.2. Synthesis of Au NPs Au NPs were synthesized by reduction of tetrachloroauric acid with citrate (Ji et al., 2007). HAuCl4 (180.0 mg) was dissolved in DDI water (950 mL), and the solution was vigorously stirred at 500 rpm and heated for 1 h to reach its boiling point. Sodium citrate dihydrate (510.0 mg) was dissolved in DDI water (50 mL) and kept for further use. After boiling (100 °C) the HAuCl4 solution for 20 min, the prepared citrate solution was rapidly added, inducing a color change from pale yellow to colorless and finally to wine red. Boiling was continued for another 20 min until the solution color was stable. The solution was cooled to room temperature with continuous stirring at 500 rpm. 2.3. Preparation of plasmonic nanofluids To dispose of unreacted chemicals and concentrate the Au NP dispersion, centrifugation of the reaction mixture was performed at room temperature. The as-prepared aqueous Au NP dispersions were transferred into several 50-mL centrifuge tubes and centrifuged at 10,000 rpm for 1 h using Sigma laboratory centrifuges (2-16P, Sigma, Germany). The supernatant was subsequently removed, and the Au NP sediment was dispersed in 20 mL of DDI water. Finally, the concentrated dispersion was diluted to different concentrations for the solar steam generation experiment. 2.4. Solar steam generation experiment The schematic of the localized heating of Au NPs dispersion for direct solar steam generation is shown in Fig. 1a. As Au NPs dispersion illuminated with simulated solar light on the top side of the fluid, multiparticle optical interactions in Au NPs dispersion were happened by the absorbing and scattering of incident photons. Eventually, the solar light was converted into thermal energy for the direct steam generation at the fluid-air interface. A schematic illustration of the solar steam generation setup is shown in Fig. 1b. The setup features three main parts: simulated solar light generator, weight change monitor, and temperature increase monitor. As shown in Fig. 1b, an acrylic tube with an inner diameter of 33 mm, inner height of 60 mm, and thickness of 3 mm was used as a testing chamber. In addition, five T-type thermocouples were inserted into the center of the tube to measure the temperature increase of the working fluid at different heights (start from the bottom of the tube: 10, 20, 30, 40, and 50 mm) due to light illumination. The testing chamber filled with working fluid (51.3 g) was placed on an electric balance (Practum313–1CN, Sartorius, Göttingen, Germany) that recorded its weight and exported the data to a computer every 1 min. A simulated solar light generator (CEL HXF300, CEAULIGHT, Beijing, China) illuminated the top of the testing chamber and the steam was generated due to the absorption of the solar light (at AM = 1.5). Meanwhile, the thermocouples (TT–T–40–SLE, Omega, US, accuracy of ±0.5 °C) immersed in the working fluid were connected to the data acquisition system
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Fig. 1. Schematic diagram of (a) the plasmonic nanofluid enabled direct solar steam generation and (b) the experimental setup for solar steam generation.
(34972A, Agilent Technology, US) to collect the temperature increase at 1 min intervals. In order to evaluate the plasmonic nanofluid-enabled solar steam generation, parameters of evaporation rate, specific vapor productivity (SVP), heating efficiency, evaporation efficiency, and total thermal conversion efficiency were introduced and defined (Ni et al., 2015). The weight change percentage of the working fluid was calculated by using the weight change to divide the initial mass of working fluid. The evaporation rate was calculated from the mass change during the illumination time and defined as follows:
_ ¼ Dm=ðAtÞ m
ð1Þ
_ is the evaporation rate of steam (kg m2 h1), Dm is the where m weight change of the testing fluid (kg), t is the illumination time (h) and A is the open surface area of testing chamber (m2). To further evaluate the capability (per unit mass) of Au NPs to generate solar steam, specific vapor productivity (SVP) was defined as follows:
_ SVP ¼ V=m Au NPs
ð2Þ
_ qvapor , where V_ is the volume of vapor flux (m3 g1 h1), V_ ¼ mA= qvapor is the density of water vapor at 1 atm and 100 °C (0.6 kg m3), and mAu NPs is the mass of Au NPs contained in the tested fluid (g). The solar energy absorbed by the working fluid induced the heating of bulk fluid and the generation of steam; thus, the heating efficiency gheating , evaporation efficiency gevaporation , and total thermal conversion efficiency gtotal were defined and used to describe the solar harvesting performance:
gheating ¼ ðC p mDTÞ=ðIAt 3600Þ
ð3Þ
gevaporation ¼ Dmhfg =ðIAt 3600Þ
ð4Þ
gtotal ¼ gheating þ gevaporation
ð5Þ
where C p is the specific heat capacity of water (J kg1 K1), m is the mass of residual water after evaporation (kg), DT is the temperature
Fig. 2. (a) Photographic images of plasmonic nanofluids with different Au NP concentrations; (b) TEM images of Au NPs; (c) diameter distributions of Au NPs.
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increase of bulk fluid (K), I is the power density of solar light (W m2), t is the illumination time (h), and hfg is the phase change enthalpy (2.257 106 J kg1 at 1 atm) of water. 2.5. Characterization Transmission electron microscopy (TEM) images of Au NPs were acquired using a field emission microscope (Tecnai G2 F30,
FEI, Portland, US). The mass concentrations of Au NP dispersions were assessed by elemental analysis using inductively coupled plasma atomic emission spectrometry (ICP-AES, OPTIMA7300DV, PerkinElmer, US). The optical properties of Au NP dispersions were characterized using a double-beam ultraviolet–visible (UV–Vis) spectrophotometer (TU-1901, Persee, Beijing, China) and a plastic cuvette with a 4-mm optical path length (FisherbrandTM Polymethylmetacry Semi-Micro Cuvette).
Fig. 3. Optical Properties of Au NPs: (a) Absorption coefficient of Au NPs dispersions; (b) peak intensity of Au NPs dispersions vs concentration; (c) Transmittance of Au NPs dispersions; (d) colloidal stability as a function of time; (e) normalized numerical spectral of Au NPs by Mie theory and FDTD; (f) near-field electric distribution of Au NPs at wavelength of 518 nm.
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3. Results and discussion 3.1. Characterization of Au NPs Photographic images of the prepared plasmonic nanofluids are shown in Fig. 2a. The shapes of Au NPs are characterized in Fig. 2b. The TEM images at low magnitude show the excellent dispersion and spherical structure of Au NPs. The diameters of Au NPs were determined by analyzing more than 100 particles with Nano Measurer 1.2 software. The obtained NP diameter distribution is shown in Fig. 2c. The mean diameter was determined as 13 nm with a relative standard deviation r of 8.5%, where r was defined P 1=2 as r ¼ 100 1x ðð1=ðN 1ÞÞ i ðxi xÞ2 Þ , where x is the average value of samples, xi is the value of number i sample, N is the number of samples. The results of Au NP characterization confirmed their good dispersion, near-spherical shape, and relatively uniform size. The optical properties of plasmonic nanofluids were investigated by UV–Vis spectrometry (Fig. 3a–d), which showed that their absorption coefficient increased from 400 nm onwards and reached a maximum at 518 nm. The strong absorption at 518 nm is due to the localized surface plasmon resonance of gold nanoparticles. The interaction between electromagnetic radiation and gold nanoparticles, causing free electrons to oscillate on the Au NP surface, results in high absorption of light. According to the wellknown Beer-Lambert law (Swinehart, 1962) (Eq. (6)), the relationship between absorbance and the concentration of absorber nanoparticles is linear:
A ¼ log10 ð1=TÞ ¼ log10 ðI0 =IÞ ¼ ecl
ð6Þ
where A is the measured absorbance, T is the transmittance, I0 is the initial light power, I is the power of light after passing through the sample, e is the molar extinction coefficient, c is the analyte concentration, and l is the path length. The absorption coefficient of plasmonic nanofluids at 518 nm increased linearly with the concentration of Au NPs (Fig. 3b). Inversely, the transmittance of the Au NPs dispersions decreased with the increase of Au NPs concentrations (Fig. 3c). In addition, the stability of plasmonic nanofluids was assessed by monitoring their UV–Vis spectra every day for two weeks. The dependence of plasmonic peak intensity (k = 518 nm) of six different samples on time (14 days) is shown in Fig. 3d. Since the absorption intensity is linearly related to Au NP concentration, the almost horizontal curves in Fig. 3d indicated that the prepared plasmonic nanofluids were stable for at least 14 days.
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Furthermore, Mie theory and finite difference time domain (FDTD, Lumerical, FDTD Solution 8.6.0) simulation were used to investigate the photo response and near-field enhancement of a single gold nanoparticle. In Mie theory (Link and El-Sayed, 2010, 1999), the cross-section extinction rext , cross-section scattering rscat , and cross-section absorption rabs of a spherical nanoparticle immersed in a fluid can be defined as
rext ¼ rscat ¼
1 2p X ð2L þ 1ÞReðaL þ bL Þ 2 jkj L¼1 1 2p X
jkj2
ð2L þ 1ÞðjaL j2 þ jbL j2 Þ
ð7Þ
ð8Þ
L¼1
rabs ¼ rext rscat
ð9Þ
where k is the wave vector, and aL ðR;kÞ and bL ðR;kÞ are the scattering coefficients expressed in terms of Ricatti-Bessel functions (WL ðxÞ and gL ðxÞ) and defined as follows:
aL ¼
mWL ðmxÞW0L ðxÞ W0L ðmxÞWL ðxÞ mWL ðmxÞg0L ðxÞ W0L ðmxÞgL ðxÞ
ð10Þ
bL ¼
WL ðmxÞW0L ðxÞ mW0L ðmxÞWL ðxÞ WL ðmxÞg0L ðxÞ mW0L ðmxÞgL ðxÞ
ð11Þ
Here, x is the size parameter (x ¼ kR), m ¼ n=nm . n is the complex refractive index of the nanoparticles, and nm is the real refractive index of the surrounding medium. Usually, the size parameter x 1 for nanoparticles, so that the electric field within the area of nanoparticle dimensions is homogeneous. Taking the first electric dipole term (L = 1 in Eqs. (7) and (8)) into account in the Mie calculation is sufficient. The behavior of a single gold nanoparticle (13 nm diameter) in water was theoretically analyzed and numerically simulated. The complex reflective index of the gold nanoparticle was obtained from the work of Johnson and Christy (Johnson and Christy, 1972), and the real reflective index of water was set to 1.33. Light propagated from the positive z-direction and induced oscillations of the electric field in the x-direction. A comparison of normalized absorption based on Mie theory, and FDTD simulation is shown in Fig. 3e. The electromagnetic field induced by the light source caused plasmon resonance at wavelength of 518 nm and led to a strong electric near-field enhancement. The near-field electric distribution of Au NPs at the plasmon resonance wavelength is shown in Fig. 3f, where E is the electric field intensity and E0 is the incident electric field intensity. A significant electric field enhancement was observed, and a
Fig. 4. (a) Evaporation mass change percentage curves and (b) evaporation enhancement for plasmonic nanofluids with different concentrations of Au NPs.
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Fig. 5. (a–g) Temperature increase at different heights of the working fluid for plasmonic nanofluids; (h) top area and bulk fluid temperature increase for plasmonic nanofluids with different Au NP concentrations at an illumination time of 60 min.
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hot area was generated near the gold nanoparticle along the xdirection due to the localized surface plasmon resonance. The electric field was strongly enhanced around the nanoparticle, with the enhancement locally reaching up to 15 times the intensity of incident light. The energy in the resonated electron cloud would be dissipated to the thermal energy in the NPs and transferred to the surrounding medium. Local heating with hot carriers contributed to the high solar-thermal conversion efficiency and enabled its application in the following solar steam generation. 3.2. Effect of Au NP concentration on solar steam generation The solar steam generation performance of plasmonic nanofluids was investigated using the experimental setup shown in Fig. 1b. Au NPs of different concentrations were dispersed in the nanofluid and illuminated at a constant solar power intensity of 10 sun (10 kW m2). The weight change percentage of the working fluid at different concentrations of the plasmonic nanofluid is illustrated in Fig. 4a. Notably, a relatively high evaporation rate was obtained even for small amounts of Au NPs. The weight change percentage of the tested fluid increased from 1.9% to 9.2% after 60 min of illumination. Increased Au NP content of the working plasmonic nanofluid caused a faster fluid weight change and led to the generation of more steam during illumination. The generation of steam was progressively enhanced with increasing Au NP concentrations, reaching almost 400% at a concentration of 178 ppm (Fig. 4b). In addition, the temperature change during solar steam generation using plasmonic nanofluids was monitored by thermocouples inserted into the testing chamber. The working fluid temperature increased with the illumination time at different heights, as shown in Fig. 5a–g. Even though water is transparent, most of nearinfrared solar energy was absorbed by water molecules and converted into thermal energy (Pegau et al., 1997). Therefore, a temperature increase of 15 K was obtained in water, not varying significantly at different heights. The plasmonic nanofluids could additionally capture visible light and convert it into thermal energy due to their high optical absorption. Therefore, plasmonic nanofluids reached much higher temperatures compared to pure water, and the temperature difference between the top and bottom parts of the working fluids increased with increasing Au NP concentrations. According to the Beer-Lambert law described above, the transmittance of the working fluid exponentially decreased with increasing optical path length and nanoparticle concentration. Thus, the fluid temperature decreased from top to
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bottom, and the temperature gap increased concomitantly with the Au NP concentration. The arithmetic average of temperatures measured at different heights of the working fluid was taken as the bulk fluid temperature. The top area temperature and bulk temperature increase for different working fluids at an illumination time of 60 min are illustrated in Fig. 5h. It can be observed that the temperature of the top area increased concomitantly with the Au NP concentration, while the bulk fluid temperature initially increased and then decreased with increasing Au NP concentration. Meanwhile, the difference between the bulk fluid and top area temperature increased concomitantly with Au NP concentration. The working fluid temperature distribution has a great effect on the evaporation performance, and we found that working fluids with a high top area temperature and a relatively low bulk temperature gave the best evaporation performance. This indicated that the temperature of the top area is the main contributor to solar steam generation. During solar illumination, a hot area was generated in the top area, enabling direct evaporation that reduces thermal loss to bulk fluid. As the solar light illuminates the plasmonic nanofluid, it is initially absorbed and scattered in the top area and partially transmitted to the underlying part. Highly concentrated and randomly dispersed Au NPs are able to concentrate light energy and generate thermal energy in a small volume close to the water-air interface, where steam can be produced and released into the air directly. Eventually, this kind of localized solar heating using plasmonic nanofluids leads to highly efficient solar steam generation. The evaporation rates and SVPs of plasmonic nanofluids with different Au NP concentrations subjected to 10-sun illumination are illustrated in Fig. 6a. The evaporation rate increased and the SVP decreased with increasing Au NP concentration. The high concentration of Au NPs led to a strong absorption of solar energy and resulted in a high evaporation rate. The low SVP values observed for high Au NP concentrations can be attributed to the low transmittance of high concentrated Au NPs dispersions (Fig. 3c) which means the lower part working fluid cannot directly interact with the incident light. Meanwhile, a large amount of Au NPs located at a lower height have small chances to convert the solar energy for steam generation. Furthermore, the heating efficiency, evaporation efficiency, and total solar energy conversion efficiency of plasmonic nanofluids with different concentrations of Au NPs were investigated under 10-sun illumination (Fig. 6b). Compared to pure water, plasmonic nanofluids are more efficient in both heating the working fluid and generating steam. Due to the optical absorption of the top part, more solar energy was absorbed and used for steam
Fig. 6. (a) Evaporation rate and SVP change as a function of Au NP concentration; (b) heating efficiency, evaporation efficiency, and total efficiency during the solar steam generation.
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generation at high Au NP concentrations. These considerations explain why the heating efficiency first increased with increasing Au NP concentration in plasmonic nanofluids and then slightly decreased. Referring to the bulk temperature increase, one can see that a further increase of Au NP concentration cannot generate more thermal energy in the working fluid, and the optimal concentration in the present experiment is 23.5 ppm. However, the evaporation efficiency increased with increasing concentration of Au NPs in plasmonic nanofluids and directly led to the same trend in total efficiency. It can thus be concluded that a high concentration of Au NPs in plasmonic nanofluids benefits the solar steam generation, while the highest heating efficiency is observed at an optimal concentration of Au NPs.
3.3. Effect of solar power intensity on steam generation As the power of solar light greatly affects solar steam generation by plasmonic nanofluids, the steam generation experiment was conducted at different solar powers (1, 3, 5, and 10 sun) with a consistent Au NP concentration of 178 ppm. For comparison, the solar steam generation experiment was also conducted with pure water under the same conditions. The comparison of testing fluid weight change percentage between water and plasmonic nanofluid under different solar power intensities is shown in Fig. 7a–d. Utilization of plasmonic nanofluids greatly enhances the performance of pure water, and the enhancement increases at higher solar powers. The integrated weight change percentage of plasmonic nanofluids
Fig. 7. Comparison of evaporation mass change percentage curves of water and plasmonic nanofluid (178.0 ppm) under four solar powers: (a) 1 sun, (b) 3 sun, (c) 5 sun, and (d) 10 sun; (e) integrated evaporation mass change curves of plasmonic nanofluids under different solar powers.
X. Wang et al. / Solar Energy 157 (2017) 35–46
under different solar powers is shown in Fig. 7e, indicating that concentrated solar power enhanced the solar steam generation process and led to a significantly higher vapor productivity. The monitored temperature increases of the working fluid at different heights under various solar powers is shown in Fig. 8a– d. The working fluid temperature gradually increased under continuous solar light illumination, and greater solar power resulted in a higher working fluid temperature and larger temperature difference. In addition, the temperature rise of the top area and bulk fluid under different solar powers are illustrated in Fig. 8e–f. Notably, the temperature increased faster under higher solar
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power. The top area temperature increase rose from 7.8 to 54.8 K, and the bulk fluid temperature increase rose from 4.5 to 35.6 K. Meanwhile, the evaporation rate and SVP increased with increasing solar power density (Fig. 9a), and the highest evaporation rate of 6.27 kg m2 h1 was obtained at a solar power of 10 kW m2. Finally, the heating efficiency, evaporation efficiency, and total solar energy conversion efficiency of plasmonic nanofluids (178 ppm) under different solar powers (1, 3, 5, and 10 sun) are demonstrated in Fig. 9b. With the increase of solar power, the heating efficiency, evaporation efficiency, and total solar energy conversion efficiency gradually decreased. The heating efficiency was
Fig. 8. Temperature increase at different heights for continuous solar light illumination at different power densities: (a) 1 sun, (b) 3 sun, (c) 5 sun, and (d) 10 sun; (e) top area and (f) bulk fluid temperature increase under different power densities.
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Fig. 9. (a) Evaporation rate and SVP changes at different solar power intensities; (b) heating efficiency, evaporation efficiency, and total efficiency changes at different solar power intensities.
25% and did not obviously decrease as solar power increased from 1 to 10 sun, while the evaporation efficiency notably decreased from 65.9 to 39.3% at increased solar power. Being a combination of heating efficiency and evaporation efficiency, the total photothermal conversion efficiency decreased with the increase of solar power. Even though the solar steam efficiency and total photothermal conversion efficiency decreased at high solar power, a high evaporation rate (Fig. 9a) could be obtained for the latter. 3.4. Heat localization mechanism Even though steam was generated under low-concentrated solar irradiation, no nanobubbles were generated around the Au NPs (Jin et al., 2016; Ni et al., 2015). Thus, a heat diffusion equation was used to describe the heat transfer between nanoparticles and surrounding media in the absence of phase change and heat convection (Baffou et al., 2010; Govorov et al., 2006):
qðrÞcðrÞ
@Tðr; tÞ ¼ rkðrÞrTðr; tÞ þ Q ðr; tÞ @t
ð12Þ
where @Tðr; tÞ is the temperature as a function of time t and coordinate r; kðrÞ, qðrÞ, and cðrÞ are the thermal conductivity, mass density, and specific heat capacity, respectively; Q ðr; tÞ is the local heat power generated by light dissipation on Au NPs. For the plasmonic nanoparticles, the heat source arises from the Joule effect:
Q ðr; tÞ ¼ jðr; tÞ Eðr; tÞ
ð13Þ
where Eðr; tÞ and jðr; tÞ represent the complex amplitudes of the electric field and the electronic current density, respectively. For a constant photothermal condition, the time-averaged heat power is set as follows:
Q ðrÞ ¼ hjðr; tÞ Eðr; tÞit ¼
1 Re½j ðrÞ EðrÞ 2
ð14Þ
where jðr; tÞ ¼ ixDðrÞ ¼ e0 eðxÞEðrÞ; DðrÞ is the electric displacement vector, eðxÞ is the dielectric constant, x is the angular frequency of light, and eðxÞ is the permittivity of Au NPs. Finally, the heat power could be expressed as a function of the electric field:
Q ðrÞ ¼
1 e0 xIm½eðxÞjEðrÞj2 2
ð15Þ
On illumination of Au NPs, their coupling with light happens in picoseconds, followed by the heat transfer from the hot Au NPs to the phonon-phonon interaction in water that establishment of a thermal equilibrium in nanoseconds. Therefore, the heat transfer
process here can be considered as a steady-state thermal equilibrium problem in the photothermal conversion during the solar steam generation process. The temperature distribution outside the Au NPs can be given as:
DTðrÞ ¼
V NP Q 1 ðr > RNP Þ 4pk0 r
ð16Þ
where V NP is the volume of Au NPs; k0 is the thermal conductivity of water, and r is the distance from the center of Au NPs. To study the photothermal heating of Au NPs under light illumination, a numerical simulation of photothermal conversion and the heat transfer process was conducted in a benchmark scenario, ie., heating of gold nanosphere of 13 nm diameter in water by an incident light propagated from the positive z-direction at wavelength of 518 nm and power of 10 kW m2. The photothermal conversion was obtained from the FDTD solution and the heat diffusion from the Au NPs to the surrounding water was achieved by finite element analysis. As the interaction with the incident light, a high heat power volume density was generation in the nanoparticle (Fig. 10a). For the periodic structure in the nanoparticle dispersion, we focused on the heat transfer in one unit lattice. To match the concentration of 178 ppm in the experiment, a nanoparticle with diameter of 13 nm was placed in a cubic box with side length of 225 nm to conduct the heat transfer from the nanoparticles to the surrounding water (Fig. 10b). The heat power volume density obtained from the FDTD solution was applied as the heat source in the nanoparticle and the boundary condition of the water box was set as symmetric. The initial temperature was set as 300 K and only transient heat conduction was considered at these small length scales (Ni et al., 2015). After one second simulation, there is a global temperature rise in the system and the temperature decrease from the nanoparticle surface to the boundary of the water box along the x-axis, meanwhile the temperature variation across the nanoparticle and water box was negligible, <0.001 K (Fig. 10c). The heat flux increases rapidly from the center of the nanoparticle to the surface of nanoparticle, and then decreases quickly along the x-axis. There is a rapid thermal equilibrium between the heated nanoparticle and surrounding water. According to the numerical analysis of photo-thermal conversion and heat diffusion of single plasmonic nanoparticle, a good understanding about the photothermal heating enabled by plasmonic nanofluids for direct solar steam generation could be achieved. As the solar light illuminates the top of plasmonic nanofluids, the Au NPs located in the top area interact with photons, generating strong coupling. Subsequently, the energy of free
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Fig. 10. (a) Heat power volume density of the Au NP under light (wavelength = 518 nm) illumination power of 10 kW m2; (b) Schematic illustrating a lattice of nanoparticles with lattice constant; (c) temperature and heat flux distribution from the central of the nanoparticle to lattice boundary along the x-axis under light (wavelength = 518 nm) illumination power of 10 kW m2 at 1 s.
electron oscillations at the Au NP surface is thermally dissipated, generating hot Au NPs. Finally, the water outside the hot Au NPs is heated to a high temperature in a short time, and the localized heating of the working fluid top area (fluid-air interface) occurs due to heat transfer from hot nanoparticles, being the largest contribution to high efficient steam generation and vapor release.
ized solar heating at the water-air interface was the main contributor to the highly efficient solar steam generation. Plasmonic nanofluids are promising materials for effective solar harvesting and conversion into thermal energy for applications such as seawater desalination, waste sterilization, and power generation. Acknowledgment
4. Conclusions In summary, plasmonic nanofluids containing Au NPs (5– 178 ppm) were used for highly efficient solar steam generation, taking advantage of the strong coupling of light with nanoparticles. High solar steam generation performance could be obtained at concentrated solar power by addition of a small amount of Au NPs, which exhibited wide solar energy applications. The optical properties of plasmonic nanofluids were investigated experimentally and numerically, exhibiting strong localized surface plasmon resonance and near-field electric enhancement. As the concentration of Au NPs and solar power intensity greatly affected the solar steam generation performance, these two factors were investigated under simulated solar light. The best solar steam generation performance was obtained for the plasmonic nanofluid containing 178 ppm of Au NPs under 10-sun illumination, reaching a total efficiency of 65%. Furthermore, the model of photothermal heating of plasmonic nanoparticle was established and the numerical results demonstrated the photothermal conversion process of plasmonic NPs from the light absorption to the heat dissipation into the bulk fluid in a short time. Based on the vapor generation experiments and theoretic photothermal analysis, it can be revealed that local-
This work was supported by the National Natural Science Foundation of China (Grant No. 51676060), Natural Science Funds of Heilongjiang Province for Distinguished Young Scholar (Grant No. JC2016009), and the Fundamental Research Funds for the Central Universities (Grant No. HIT. BRETIV. 201315). References Atwater, H.A., Polman, A., 2010. Plasmonics for improved photovoltaic devices. Nat. Mater. 9, 865. Baffou, G., Girard, C., Quidant, R., 2010. Mapping heat origin in plasmonic structures. Phys. Rev. Lett. 104, 1–4. Baffou, G., Polleux, J., Rigneault, H., Monneret, S., 2014. Super-heating and microbubble generation around plasmonic nanoparticles under cw illumination. J. Phys. Chem. C 118, 4890–4898. Challener, W.a., Peng, C., Itagi, a.V., Karns, D., Peng, W., Peng, Y., Yang, X., Zhu, X., Gokemeijer, N.J., Hsia, Y.-T., Ju, G., Rottmayer, R.E., Seigler, M.a., Gage, E.C., 2009. Heat-assisted magnetic recording by a near-field transducer with efficient optical energy transfer. Nat. Photonics 3, 303. Chen, M., He, Y., Zhu, J., Shuai, Y., Jiang, B., Huang, Y., 2015. An experimental investigation on sunlight absorption characteristics of silver nanofluids. Sol. Energy 115, 85–94. Chen, X., Li, Y., Pan, X., Cortie, D., Huang, X., Yi, Z., 2016. Photocatalytic oxidation of methane over silver decorated zinc oxide nanocatalysts. Nat. Commun. 7, 12273.
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Solar Energy journal homepage: www.elsevier.com/locate/solener
Investigation of photothermal heating enabled by plasmonic nanofluids for direct solar steam generation Xinzhi Wang a, Yurong He a,⇑, Xing Liu b, Lei Shi a, Jiaqi Zhu b a b
School of Energy Science and Engineering, Harbin Institute of Technology, Harbin 150001, China Center for Composite Materials and Structures, School of Astronautics, Harbin Institute of Technology, Harbin 150001, China
a r t i c l e
i n f o
Article history: Received 22 January 2017 Received in revised form 10 May 2017 Accepted 4 August 2017
Keywords: Solar energy Surface plasmon resonance Direct solar steam generation Nanofluid
a b s t r a c t Steam production has a wide range of applications such as seawater desalination, waste sterilization, and power generation. The utilization of solar energy for this purpose has attracted much attention due to its inexhaustibility and pollution-free nature. Here, direct solar steam generation at low-concentrated solar power using plasmonic nanofluids containing gold nanoparticles (Au NPs) was investigated experimentally. The key factors required for highly efficient solar steam generation, including Au NP concentration and solar power intensity, were studied in a simulated solar system by measuring the water weight loss and system temperature change. The best evaporation performance was obtained using a plasmonic nanofluid containing 178 ppm of Au NPs under 10 sun (1 sun = 1 kW m2) illumination intensity, and the total efficiency reached 65%. However, the total efficiency of pure water was only 16%, which means that the plasmonic nanofluids reached a 300% enhancement in efficiency. Higher solar power led to a higher evaporation rate, higher specific vapor productivity (SVP), and higher Au NP concentrations resulted in better evaporation performance. Localized solar heating at the fluid-air interface was shown to contribute more to solar steam generation than to bulk fluid heating. Furthermore, the model of photothermal heating of plasmonic nanoparticle was established and the numerical results demonstrated the photothermal conversion process of plasmonic NPs from the light absorption to the heat dissipation into the bulk fluid. Ó 2017 Published by Elsevier Ltd.
1. Introduction Due to the increasing energy demand, solar energy has been selected as a promising green inexhaustible energy source (Lewis, 2007; Weinstein et al., 2015). Traditional solar harvesting technologies are photovoltaics (Atwater and Polman, 2010) and concentrated solar power (CSP) (Weinstein et al., 2015), which are applied to generate electricity. Recently, more attentions have been focused on developing novel approaches for solar energy utilization such as seawater desalination (Elimelech and Phillip, 2011; Karagiannis and Soldatos, 2008), photochemical reactions (Chen et al., 2016; Lewis, 2001; Naldoni et al., 2016), solar sterilization (Neumann et al., 2013a), etc. Current methods of solar steam generation mainly rely on solar energy collection by black surface or cavity absorbers, with the thermal energy subsequently transferred to the working fluid either directly or via a high heat capacity intermediate carrier inside the absorbing tube to heat the bulk fluid to its boiling temperature (Kundu and Lee, 2012; Lenert and ⇑ Corresponding author. E-mail address: [email protected] (Y. He). http://dx.doi.org/10.1016/j.solener.2017.08.015 0038-092X/Ó 2017 Published by Elsevier Ltd.
Wang, 2012; Weinstein et al., 2015). This kind of indirect solar steam generation usually requires highly concentrated solar energy. Moreover, it suffers from high optical loss and thermal irradiation to the environment and high heat loss during convective heat transfer. To eliminate the energy loss due to indirect absorption, direct and volumetric absorption of solar energy by the working fluid can minimize the convection heat transfer losses and achieve higher efficiency (Ni et al., 2015; Otanicar et al., 2010). Nanofluids, a kind of functional nanoparticle dispersions, have been widely studied since their discovery in 1990s (Leong et al., 2016). Recently, a number of experimental and theoretical investigations have found that nanofluid-based direct absorbing solar collectors (DASCs) exhibit excellent performance due to their photothermal characteristics (Colangelo et al., 2015, 2013; Karami et al., 2016, 2014; Liu et al., 2015; Luo et al., 2014; Otanicar et al., 2010; Tyagi et al., 2009). Moreover, Chieruzzi et al. (2017, 2015, 2016) experimentally investigated and reviewed the development of nanofluids for thermal energy storage in solar energy applications and discussed the enhancement of fundamental thermal properties of nanofluids. Shin and Banerjee (2011) reported an anomalous enhancement of specific heat capacity of
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nanofluids which was composited chloride salt eutectics and silica nanoparticles for thermal energy storage applications. Otanicar et al. (2010) studied direct absorption collectors with nanofluids containing carbon nanotubes, graphite, and silver nanoparticles, achieving an efficiency enhancement of up to 5%. Taylor et al. (2011) demonstrated that nanoparticles could absorb over 95% of incoming light by investigating the optical properties of different kinds of nanofluids. Vakili et al. (2016a, 2016b) utilized graphite and graphene nanoplatelet nanofluid for direct absorption solar collectors exhibiting good photothermal properties at low temperature. Colangelo et al. (2015, 2013) proposed a modified flat panel solar collector, and the results revealed that Al2O3-distillated water could increase the thermal efficiency by up to 11.7%, as compared to pure water. Gupta et al. (2015) experimentally investigated the volumetric absorption of thin film of Al2O3-H2O nanofluid and obtained high efficient enhancement of 22.1%. In recent decades, plasmonic nanoparticles (metal materials or doped metal oxides) have attracted much interest and have been widely used in applications such as thermal cancer treatment (O’Neal et al., 2004), magnetic recording (Challener et al., 2009), and catalysis (Linic et al., 2011; Warren and Thimsen, 2012) due to their thermal and thermo-optical properties caused by collective coherent excitation of the free electrons, which is also known as localized surface plasmon resonance (LSPR). Localized heat is generated by coupling the plasmonic nanoparticles and photons and is subsequently transferred to the surrounding medium. Zhang et al. (2014) and Chen et al. (2015) investigated the photothermal conversion ability of gold and silver nanoparticle dispersions, respectively, obtaining high photothermal efficiency and bulk liquid temperature in both cases. Du and Tang (2016, 2015) numerically investigated the optical properties of plasmonic nanofluids containing different shapes of Au NPs or agglomerated Au clusters and concluded that a blended nanofluids could enhance the solar energy harvesting. Rather than heating the bulk fluid, plasmonic NPs can also generate nanovapor under continuous laser illumination (Baffou et al., 2014) or highly concentrated solar illumination (Neumann et al., 2013b). Lombard et al. (2015) and Neumann et al. (2013b) demonstrated solar vapor generation using an Au-based dispersion, and achieved a solar steam device efficiency of 24% under the concentrated solar energy of 1000 sun (1 sun = 1 kW m2). Ni et al. (2015) reported vapor generation efficiencies of up to 69% at 10 sun using a graphitized carbon black nanofluid and numerically demonstrated that the nanofluid steam generation phenomenon results from the global heating of bulk fluid. Jin et al. (2016) performed steam generation experiments under sunlight of 220 Suns and revealed that the steam produced due to the highly non-uniform energy distribution in the system. Wang et al. (2016) experimentally obtained a high evaporation efficiency through direct solar steam generation with carbon-nanotube nanofluid. There are still few reports about the direct solar steam generation with plasmonic nanofluids, and a lack of systematic study on the effect of solar power intensities and plasmonic NP concentrations on solar steam generation performance. In this work, solar steam generation experiments were conducted with plasmonic nanofluids containing Au NPs under simulated solar light. The effect of Au NP concentration (5–178 ppm) and solar power intensity (1–10 sun) on evaporation performance were investigated by measuring the water weight loss and system temperature change. Furthermore, the optical properties of these nanofluids were studied by spectrometry, Mie theory, and finite difference time domain (FDTD) simulation. Finally, a model of heat generation through the light absorption and the heat dissipation to the surrounding medium was established and applied to demonstrate the mechanism of plasmon heating and localized heating during photothermal heating enabled by plasmonic NPs.
2. Experimental section 2.1. Materials Tetrachloroauric acid (HAuCl4; 49–50% Au basis) and sodium citrate dihydrate (HOC(COONa)(CH2COONa)22H2O, 99%, AR) were purchased from Aladdin Industrial Inc., Shanghai, China and were used without further purification. Doubly deionized water (DDI water) was produced by a Sartorius water purification system (arium Ò mini; 18.2 MX; Göttingen, Germany). 2.2. Synthesis of Au NPs Au NPs were synthesized by reduction of tetrachloroauric acid with citrate (Ji et al., 2007). HAuCl4 (180.0 mg) was dissolved in DDI water (950 mL), and the solution was vigorously stirred at 500 rpm and heated for 1 h to reach its boiling point. Sodium citrate dihydrate (510.0 mg) was dissolved in DDI water (50 mL) and kept for further use. After boiling (100 °C) the HAuCl4 solution for 20 min, the prepared citrate solution was rapidly added, inducing a color change from pale yellow to colorless and finally to wine red. Boiling was continued for another 20 min until the solution color was stable. The solution was cooled to room temperature with continuous stirring at 500 rpm. 2.3. Preparation of plasmonic nanofluids To dispose of unreacted chemicals and concentrate the Au NP dispersion, centrifugation of the reaction mixture was performed at room temperature. The as-prepared aqueous Au NP dispersions were transferred into several 50-mL centrifuge tubes and centrifuged at 10,000 rpm for 1 h using Sigma laboratory centrifuges (2-16P, Sigma, Germany). The supernatant was subsequently removed, and the Au NP sediment was dispersed in 20 mL of DDI water. Finally, the concentrated dispersion was diluted to different concentrations for the solar steam generation experiment. 2.4. Solar steam generation experiment The schematic of the localized heating of Au NPs dispersion for direct solar steam generation is shown in Fig. 1a. As Au NPs dispersion illuminated with simulated solar light on the top side of the fluid, multiparticle optical interactions in Au NPs dispersion were happened by the absorbing and scattering of incident photons. Eventually, the solar light was converted into thermal energy for the direct steam generation at the fluid-air interface. A schematic illustration of the solar steam generation setup is shown in Fig. 1b. The setup features three main parts: simulated solar light generator, weight change monitor, and temperature increase monitor. As shown in Fig. 1b, an acrylic tube with an inner diameter of 33 mm, inner height of 60 mm, and thickness of 3 mm was used as a testing chamber. In addition, five T-type thermocouples were inserted into the center of the tube to measure the temperature increase of the working fluid at different heights (start from the bottom of the tube: 10, 20, 30, 40, and 50 mm) due to light illumination. The testing chamber filled with working fluid (51.3 g) was placed on an electric balance (Practum313–1CN, Sartorius, Göttingen, Germany) that recorded its weight and exported the data to a computer every 1 min. A simulated solar light generator (CEL HXF300, CEAULIGHT, Beijing, China) illuminated the top of the testing chamber and the steam was generated due to the absorption of the solar light (at AM = 1.5). Meanwhile, the thermocouples (TT–T–40–SLE, Omega, US, accuracy of ±0.5 °C) immersed in the working fluid were connected to the data acquisition system
X. Wang et al. / Solar Energy 157 (2017) 35–46
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Fig. 1. Schematic diagram of (a) the plasmonic nanofluid enabled direct solar steam generation and (b) the experimental setup for solar steam generation.
(34972A, Agilent Technology, US) to collect the temperature increase at 1 min intervals. In order to evaluate the plasmonic nanofluid-enabled solar steam generation, parameters of evaporation rate, specific vapor productivity (SVP), heating efficiency, evaporation efficiency, and total thermal conversion efficiency were introduced and defined (Ni et al., 2015). The weight change percentage of the working fluid was calculated by using the weight change to divide the initial mass of working fluid. The evaporation rate was calculated from the mass change during the illumination time and defined as follows:
_ ¼ Dm=ðAtÞ m
ð1Þ
_ is the evaporation rate of steam (kg m2 h1), Dm is the where m weight change of the testing fluid (kg), t is the illumination time (h) and A is the open surface area of testing chamber (m2). To further evaluate the capability (per unit mass) of Au NPs to generate solar steam, specific vapor productivity (SVP) was defined as follows:
_ SVP ¼ V=m Au NPs
ð2Þ
_ qvapor , where V_ is the volume of vapor flux (m3 g1 h1), V_ ¼ mA= qvapor is the density of water vapor at 1 atm and 100 °C (0.6 kg m3), and mAu NPs is the mass of Au NPs contained in the tested fluid (g). The solar energy absorbed by the working fluid induced the heating of bulk fluid and the generation of steam; thus, the heating efficiency gheating , evaporation efficiency gevaporation , and total thermal conversion efficiency gtotal were defined and used to describe the solar harvesting performance:
gheating ¼ ðC p mDTÞ=ðIAt 3600Þ
ð3Þ
gevaporation ¼ Dmhfg =ðIAt 3600Þ
ð4Þ
gtotal ¼ gheating þ gevaporation
ð5Þ
where C p is the specific heat capacity of water (J kg1 K1), m is the mass of residual water after evaporation (kg), DT is the temperature
Fig. 2. (a) Photographic images of plasmonic nanofluids with different Au NP concentrations; (b) TEM images of Au NPs; (c) diameter distributions of Au NPs.
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increase of bulk fluid (K), I is the power density of solar light (W m2), t is the illumination time (h), and hfg is the phase change enthalpy (2.257 106 J kg1 at 1 atm) of water. 2.5. Characterization Transmission electron microscopy (TEM) images of Au NPs were acquired using a field emission microscope (Tecnai G2 F30,
FEI, Portland, US). The mass concentrations of Au NP dispersions were assessed by elemental analysis using inductively coupled plasma atomic emission spectrometry (ICP-AES, OPTIMA7300DV, PerkinElmer, US). The optical properties of Au NP dispersions were characterized using a double-beam ultraviolet–visible (UV–Vis) spectrophotometer (TU-1901, Persee, Beijing, China) and a plastic cuvette with a 4-mm optical path length (FisherbrandTM Polymethylmetacry Semi-Micro Cuvette).
Fig. 3. Optical Properties of Au NPs: (a) Absorption coefficient of Au NPs dispersions; (b) peak intensity of Au NPs dispersions vs concentration; (c) Transmittance of Au NPs dispersions; (d) colloidal stability as a function of time; (e) normalized numerical spectral of Au NPs by Mie theory and FDTD; (f) near-field electric distribution of Au NPs at wavelength of 518 nm.
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3. Results and discussion 3.1. Characterization of Au NPs Photographic images of the prepared plasmonic nanofluids are shown in Fig. 2a. The shapes of Au NPs are characterized in Fig. 2b. The TEM images at low magnitude show the excellent dispersion and spherical structure of Au NPs. The diameters of Au NPs were determined by analyzing more than 100 particles with Nano Measurer 1.2 software. The obtained NP diameter distribution is shown in Fig. 2c. The mean diameter was determined as 13 nm with a relative standard deviation r of 8.5%, where r was defined P 1=2 as r ¼ 100 1x ðð1=ðN 1ÞÞ i ðxi xÞ2 Þ , where x is the average value of samples, xi is the value of number i sample, N is the number of samples. The results of Au NP characterization confirmed their good dispersion, near-spherical shape, and relatively uniform size. The optical properties of plasmonic nanofluids were investigated by UV–Vis spectrometry (Fig. 3a–d), which showed that their absorption coefficient increased from 400 nm onwards and reached a maximum at 518 nm. The strong absorption at 518 nm is due to the localized surface plasmon resonance of gold nanoparticles. The interaction between electromagnetic radiation and gold nanoparticles, causing free electrons to oscillate on the Au NP surface, results in high absorption of light. According to the wellknown Beer-Lambert law (Swinehart, 1962) (Eq. (6)), the relationship between absorbance and the concentration of absorber nanoparticles is linear:
A ¼ log10 ð1=TÞ ¼ log10 ðI0 =IÞ ¼ ecl
ð6Þ
where A is the measured absorbance, T is the transmittance, I0 is the initial light power, I is the power of light after passing through the sample, e is the molar extinction coefficient, c is the analyte concentration, and l is the path length. The absorption coefficient of plasmonic nanofluids at 518 nm increased linearly with the concentration of Au NPs (Fig. 3b). Inversely, the transmittance of the Au NPs dispersions decreased with the increase of Au NPs concentrations (Fig. 3c). In addition, the stability of plasmonic nanofluids was assessed by monitoring their UV–Vis spectra every day for two weeks. The dependence of plasmonic peak intensity (k = 518 nm) of six different samples on time (14 days) is shown in Fig. 3d. Since the absorption intensity is linearly related to Au NP concentration, the almost horizontal curves in Fig. 3d indicated that the prepared plasmonic nanofluids were stable for at least 14 days.
39
Furthermore, Mie theory and finite difference time domain (FDTD, Lumerical, FDTD Solution 8.6.0) simulation were used to investigate the photo response and near-field enhancement of a single gold nanoparticle. In Mie theory (Link and El-Sayed, 2010, 1999), the cross-section extinction rext , cross-section scattering rscat , and cross-section absorption rabs of a spherical nanoparticle immersed in a fluid can be defined as
rext ¼ rscat ¼
1 2p X ð2L þ 1ÞReðaL þ bL Þ 2 jkj L¼1 1 2p X
jkj2
ð2L þ 1ÞðjaL j2 þ jbL j2 Þ
ð7Þ
ð8Þ
L¼1
rabs ¼ rext rscat
ð9Þ
where k is the wave vector, and aL ðR;kÞ and bL ðR;kÞ are the scattering coefficients expressed in terms of Ricatti-Bessel functions (WL ðxÞ and gL ðxÞ) and defined as follows:
aL ¼
mWL ðmxÞW0L ðxÞ W0L ðmxÞWL ðxÞ mWL ðmxÞg0L ðxÞ W0L ðmxÞgL ðxÞ
ð10Þ
bL ¼
WL ðmxÞW0L ðxÞ mW0L ðmxÞWL ðxÞ WL ðmxÞg0L ðxÞ mW0L ðmxÞgL ðxÞ
ð11Þ
Here, x is the size parameter (x ¼ kR), m ¼ n=nm . n is the complex refractive index of the nanoparticles, and nm is the real refractive index of the surrounding medium. Usually, the size parameter x 1 for nanoparticles, so that the electric field within the area of nanoparticle dimensions is homogeneous. Taking the first electric dipole term (L = 1 in Eqs. (7) and (8)) into account in the Mie calculation is sufficient. The behavior of a single gold nanoparticle (13 nm diameter) in water was theoretically analyzed and numerically simulated. The complex reflective index of the gold nanoparticle was obtained from the work of Johnson and Christy (Johnson and Christy, 1972), and the real reflective index of water was set to 1.33. Light propagated from the positive z-direction and induced oscillations of the electric field in the x-direction. A comparison of normalized absorption based on Mie theory, and FDTD simulation is shown in Fig. 3e. The electromagnetic field induced by the light source caused plasmon resonance at wavelength of 518 nm and led to a strong electric near-field enhancement. The near-field electric distribution of Au NPs at the plasmon resonance wavelength is shown in Fig. 3f, where E is the electric field intensity and E0 is the incident electric field intensity. A significant electric field enhancement was observed, and a
Fig. 4. (a) Evaporation mass change percentage curves and (b) evaporation enhancement for plasmonic nanofluids with different concentrations of Au NPs.
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Fig. 5. (a–g) Temperature increase at different heights of the working fluid for plasmonic nanofluids; (h) top area and bulk fluid temperature increase for plasmonic nanofluids with different Au NP concentrations at an illumination time of 60 min.
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hot area was generated near the gold nanoparticle along the xdirection due to the localized surface plasmon resonance. The electric field was strongly enhanced around the nanoparticle, with the enhancement locally reaching up to 15 times the intensity of incident light. The energy in the resonated electron cloud would be dissipated to the thermal energy in the NPs and transferred to the surrounding medium. Local heating with hot carriers contributed to the high solar-thermal conversion efficiency and enabled its application in the following solar steam generation. 3.2. Effect of Au NP concentration on solar steam generation The solar steam generation performance of plasmonic nanofluids was investigated using the experimental setup shown in Fig. 1b. Au NPs of different concentrations were dispersed in the nanofluid and illuminated at a constant solar power intensity of 10 sun (10 kW m2). The weight change percentage of the working fluid at different concentrations of the plasmonic nanofluid is illustrated in Fig. 4a. Notably, a relatively high evaporation rate was obtained even for small amounts of Au NPs. The weight change percentage of the tested fluid increased from 1.9% to 9.2% after 60 min of illumination. Increased Au NP content of the working plasmonic nanofluid caused a faster fluid weight change and led to the generation of more steam during illumination. The generation of steam was progressively enhanced with increasing Au NP concentrations, reaching almost 400% at a concentration of 178 ppm (Fig. 4b). In addition, the temperature change during solar steam generation using plasmonic nanofluids was monitored by thermocouples inserted into the testing chamber. The working fluid temperature increased with the illumination time at different heights, as shown in Fig. 5a–g. Even though water is transparent, most of nearinfrared solar energy was absorbed by water molecules and converted into thermal energy (Pegau et al., 1997). Therefore, a temperature increase of 15 K was obtained in water, not varying significantly at different heights. The plasmonic nanofluids could additionally capture visible light and convert it into thermal energy due to their high optical absorption. Therefore, plasmonic nanofluids reached much higher temperatures compared to pure water, and the temperature difference between the top and bottom parts of the working fluids increased with increasing Au NP concentrations. According to the Beer-Lambert law described above, the transmittance of the working fluid exponentially decreased with increasing optical path length and nanoparticle concentration. Thus, the fluid temperature decreased from top to
41
bottom, and the temperature gap increased concomitantly with the Au NP concentration. The arithmetic average of temperatures measured at different heights of the working fluid was taken as the bulk fluid temperature. The top area temperature and bulk temperature increase for different working fluids at an illumination time of 60 min are illustrated in Fig. 5h. It can be observed that the temperature of the top area increased concomitantly with the Au NP concentration, while the bulk fluid temperature initially increased and then decreased with increasing Au NP concentration. Meanwhile, the difference between the bulk fluid and top area temperature increased concomitantly with Au NP concentration. The working fluid temperature distribution has a great effect on the evaporation performance, and we found that working fluids with a high top area temperature and a relatively low bulk temperature gave the best evaporation performance. This indicated that the temperature of the top area is the main contributor to solar steam generation. During solar illumination, a hot area was generated in the top area, enabling direct evaporation that reduces thermal loss to bulk fluid. As the solar light illuminates the plasmonic nanofluid, it is initially absorbed and scattered in the top area and partially transmitted to the underlying part. Highly concentrated and randomly dispersed Au NPs are able to concentrate light energy and generate thermal energy in a small volume close to the water-air interface, where steam can be produced and released into the air directly. Eventually, this kind of localized solar heating using plasmonic nanofluids leads to highly efficient solar steam generation. The evaporation rates and SVPs of plasmonic nanofluids with different Au NP concentrations subjected to 10-sun illumination are illustrated in Fig. 6a. The evaporation rate increased and the SVP decreased with increasing Au NP concentration. The high concentration of Au NPs led to a strong absorption of solar energy and resulted in a high evaporation rate. The low SVP values observed for high Au NP concentrations can be attributed to the low transmittance of high concentrated Au NPs dispersions (Fig. 3c) which means the lower part working fluid cannot directly interact with the incident light. Meanwhile, a large amount of Au NPs located at a lower height have small chances to convert the solar energy for steam generation. Furthermore, the heating efficiency, evaporation efficiency, and total solar energy conversion efficiency of plasmonic nanofluids with different concentrations of Au NPs were investigated under 10-sun illumination (Fig. 6b). Compared to pure water, plasmonic nanofluids are more efficient in both heating the working fluid and generating steam. Due to the optical absorption of the top part, more solar energy was absorbed and used for steam
Fig. 6. (a) Evaporation rate and SVP change as a function of Au NP concentration; (b) heating efficiency, evaporation efficiency, and total efficiency during the solar steam generation.
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generation at high Au NP concentrations. These considerations explain why the heating efficiency first increased with increasing Au NP concentration in plasmonic nanofluids and then slightly decreased. Referring to the bulk temperature increase, one can see that a further increase of Au NP concentration cannot generate more thermal energy in the working fluid, and the optimal concentration in the present experiment is 23.5 ppm. However, the evaporation efficiency increased with increasing concentration of Au NPs in plasmonic nanofluids and directly led to the same trend in total efficiency. It can thus be concluded that a high concentration of Au NPs in plasmonic nanofluids benefits the solar steam generation, while the highest heating efficiency is observed at an optimal concentration of Au NPs.
3.3. Effect of solar power intensity on steam generation As the power of solar light greatly affects solar steam generation by plasmonic nanofluids, the steam generation experiment was conducted at different solar powers (1, 3, 5, and 10 sun) with a consistent Au NP concentration of 178 ppm. For comparison, the solar steam generation experiment was also conducted with pure water under the same conditions. The comparison of testing fluid weight change percentage between water and plasmonic nanofluid under different solar power intensities is shown in Fig. 7a–d. Utilization of plasmonic nanofluids greatly enhances the performance of pure water, and the enhancement increases at higher solar powers. The integrated weight change percentage of plasmonic nanofluids
Fig. 7. Comparison of evaporation mass change percentage curves of water and plasmonic nanofluid (178.0 ppm) under four solar powers: (a) 1 sun, (b) 3 sun, (c) 5 sun, and (d) 10 sun; (e) integrated evaporation mass change curves of plasmonic nanofluids under different solar powers.
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under different solar powers is shown in Fig. 7e, indicating that concentrated solar power enhanced the solar steam generation process and led to a significantly higher vapor productivity. The monitored temperature increases of the working fluid at different heights under various solar powers is shown in Fig. 8a– d. The working fluid temperature gradually increased under continuous solar light illumination, and greater solar power resulted in a higher working fluid temperature and larger temperature difference. In addition, the temperature rise of the top area and bulk fluid under different solar powers are illustrated in Fig. 8e–f. Notably, the temperature increased faster under higher solar
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power. The top area temperature increase rose from 7.8 to 54.8 K, and the bulk fluid temperature increase rose from 4.5 to 35.6 K. Meanwhile, the evaporation rate and SVP increased with increasing solar power density (Fig. 9a), and the highest evaporation rate of 6.27 kg m2 h1 was obtained at a solar power of 10 kW m2. Finally, the heating efficiency, evaporation efficiency, and total solar energy conversion efficiency of plasmonic nanofluids (178 ppm) under different solar powers (1, 3, 5, and 10 sun) are demonstrated in Fig. 9b. With the increase of solar power, the heating efficiency, evaporation efficiency, and total solar energy conversion efficiency gradually decreased. The heating efficiency was
Fig. 8. Temperature increase at different heights for continuous solar light illumination at different power densities: (a) 1 sun, (b) 3 sun, (c) 5 sun, and (d) 10 sun; (e) top area and (f) bulk fluid temperature increase under different power densities.
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Fig. 9. (a) Evaporation rate and SVP changes at different solar power intensities; (b) heating efficiency, evaporation efficiency, and total efficiency changes at different solar power intensities.
25% and did not obviously decrease as solar power increased from 1 to 10 sun, while the evaporation efficiency notably decreased from 65.9 to 39.3% at increased solar power. Being a combination of heating efficiency and evaporation efficiency, the total photothermal conversion efficiency decreased with the increase of solar power. Even though the solar steam efficiency and total photothermal conversion efficiency decreased at high solar power, a high evaporation rate (Fig. 9a) could be obtained for the latter. 3.4. Heat localization mechanism Even though steam was generated under low-concentrated solar irradiation, no nanobubbles were generated around the Au NPs (Jin et al., 2016; Ni et al., 2015). Thus, a heat diffusion equation was used to describe the heat transfer between nanoparticles and surrounding media in the absence of phase change and heat convection (Baffou et al., 2010; Govorov et al., 2006):
qðrÞcðrÞ
@Tðr; tÞ ¼ rkðrÞrTðr; tÞ þ Q ðr; tÞ @t
ð12Þ
where @Tðr; tÞ is the temperature as a function of time t and coordinate r; kðrÞ, qðrÞ, and cðrÞ are the thermal conductivity, mass density, and specific heat capacity, respectively; Q ðr; tÞ is the local heat power generated by light dissipation on Au NPs. For the plasmonic nanoparticles, the heat source arises from the Joule effect:
Q ðr; tÞ ¼ jðr; tÞ Eðr; tÞ
ð13Þ
where Eðr; tÞ and jðr; tÞ represent the complex amplitudes of the electric field and the electronic current density, respectively. For a constant photothermal condition, the time-averaged heat power is set as follows:
Q ðrÞ ¼ hjðr; tÞ Eðr; tÞit ¼
1 Re½j ðrÞ EðrÞ 2
ð14Þ
where jðr; tÞ ¼ ixDðrÞ ¼ e0 eðxÞEðrÞ; DðrÞ is the electric displacement vector, eðxÞ is the dielectric constant, x is the angular frequency of light, and eðxÞ is the permittivity of Au NPs. Finally, the heat power could be expressed as a function of the electric field:
Q ðrÞ ¼
1 e0 xIm½eðxÞjEðrÞj2 2
ð15Þ
On illumination of Au NPs, their coupling with light happens in picoseconds, followed by the heat transfer from the hot Au NPs to the phonon-phonon interaction in water that establishment of a thermal equilibrium in nanoseconds. Therefore, the heat transfer
process here can be considered as a steady-state thermal equilibrium problem in the photothermal conversion during the solar steam generation process. The temperature distribution outside the Au NPs can be given as:
DTðrÞ ¼
V NP Q 1 ðr > RNP Þ 4pk0 r
ð16Þ
where V NP is the volume of Au NPs; k0 is the thermal conductivity of water, and r is the distance from the center of Au NPs. To study the photothermal heating of Au NPs under light illumination, a numerical simulation of photothermal conversion and the heat transfer process was conducted in a benchmark scenario, ie., heating of gold nanosphere of 13 nm diameter in water by an incident light propagated from the positive z-direction at wavelength of 518 nm and power of 10 kW m2. The photothermal conversion was obtained from the FDTD solution and the heat diffusion from the Au NPs to the surrounding water was achieved by finite element analysis. As the interaction with the incident light, a high heat power volume density was generation in the nanoparticle (Fig. 10a). For the periodic structure in the nanoparticle dispersion, we focused on the heat transfer in one unit lattice. To match the concentration of 178 ppm in the experiment, a nanoparticle with diameter of 13 nm was placed in a cubic box with side length of 225 nm to conduct the heat transfer from the nanoparticles to the surrounding water (Fig. 10b). The heat power volume density obtained from the FDTD solution was applied as the heat source in the nanoparticle and the boundary condition of the water box was set as symmetric. The initial temperature was set as 300 K and only transient heat conduction was considered at these small length scales (Ni et al., 2015). After one second simulation, there is a global temperature rise in the system and the temperature decrease from the nanoparticle surface to the boundary of the water box along the x-axis, meanwhile the temperature variation across the nanoparticle and water box was negligible, <0.001 K (Fig. 10c). The heat flux increases rapidly from the center of the nanoparticle to the surface of nanoparticle, and then decreases quickly along the x-axis. There is a rapid thermal equilibrium between the heated nanoparticle and surrounding water. According to the numerical analysis of photo-thermal conversion and heat diffusion of single plasmonic nanoparticle, a good understanding about the photothermal heating enabled by plasmonic nanofluids for direct solar steam generation could be achieved. As the solar light illuminates the top of plasmonic nanofluids, the Au NPs located in the top area interact with photons, generating strong coupling. Subsequently, the energy of free
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Fig. 10. (a) Heat power volume density of the Au NP under light (wavelength = 518 nm) illumination power of 10 kW m2; (b) Schematic illustrating a lattice of nanoparticles with lattice constant; (c) temperature and heat flux distribution from the central of the nanoparticle to lattice boundary along the x-axis under light (wavelength = 518 nm) illumination power of 10 kW m2 at 1 s.
electron oscillations at the Au NP surface is thermally dissipated, generating hot Au NPs. Finally, the water outside the hot Au NPs is heated to a high temperature in a short time, and the localized heating of the working fluid top area (fluid-air interface) occurs due to heat transfer from hot nanoparticles, being the largest contribution to high efficient steam generation and vapor release.
ized solar heating at the water-air interface was the main contributor to the highly efficient solar steam generation. Plasmonic nanofluids are promising materials for effective solar harvesting and conversion into thermal energy for applications such as seawater desalination, waste sterilization, and power generation. Acknowledgment
4. Conclusions In summary, plasmonic nanofluids containing Au NPs (5– 178 ppm) were used for highly efficient solar steam generation, taking advantage of the strong coupling of light with nanoparticles. High solar steam generation performance could be obtained at concentrated solar power by addition of a small amount of Au NPs, which exhibited wide solar energy applications. The optical properties of plasmonic nanofluids were investigated experimentally and numerically, exhibiting strong localized surface plasmon resonance and near-field electric enhancement. As the concentration of Au NPs and solar power intensity greatly affected the solar steam generation performance, these two factors were investigated under simulated solar light. The best solar steam generation performance was obtained for the plasmonic nanofluid containing 178 ppm of Au NPs under 10-sun illumination, reaching a total efficiency of 65%. Furthermore, the model of photothermal heating of plasmonic nanoparticle was established and the numerical results demonstrated the photothermal conversion process of plasmonic NPs from the light absorption to the heat dissipation into the bulk fluid in a short time. Based on the vapor generation experiments and theoretic photothermal analysis, it can be revealed that local-
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