Solar Energy Materials & Solar Cells 206 (2020) 110292

Available online 16 November 20190927-0248/© 2019 Elsevier B.V. All rights reserved.

Solar steam generation enabled by bubbly flow nanofluids

Guansheng Yao a, Jinliang Xu a,b,**, Guohua Liu a,b,*

a Beijing Key Laboratory of Multiphase Flow and Heat Transfer for Low Grade Energy Utilization, North China Electric Power University, Beijing, 102206, PR China b Key Laboratory of Condition Monitoring and Control for Power Plant Equipment of Ministry of Education, North China Electric Power University, Beijing, 102206, China

A R T I C L E I N F O

Keywords: Solar energy Nanofluids Bubbly flow Plasmon heating Steam generation

A B S T R A C T

Plasmonic nanofluids are recently explored to promote steam generation, showing great promise of such fluids for solar thermal applications. However, plasmonic nanoparticles are opaque and the nanofluids require high mass concentration to achieve efficient evaporation, which in turn leads to parasitic light absorption for the underlying particles. In this work, we introduce bubbles into dilute plasmonic nanofluids to enhance solar water evaporation. The dynamic bubbles not only act as light scattering centers to extend the incident light pathway and amplify solar flux, but also provide large gas-liquid interfaces for moisture capture as well as kinetic energy from bubble bursting to improve vapor diffusion. The coupling effect between plasmonic heating and bubbly- flow humidification results in a steam generation rate of 0.72 kg m� 2 h� 1 under two-sun, which is about three-time higher than that of the pure water. A series of experiments under different light intensities, con-centration of nanofluids, gas flow rates as well as photothermal materials such as carbon nanotubes (CNTs) and magnetic Fe3O4 nanoparticles are also conducted to verify the concept. It is concluded that all the nanofluids enhance the steam generation process, and the bubbly flow nanofluids can be further improved the performance. This work provides an original insight on the bubbly flow nanofluids for solar vapor generation, and stands for a basis to design scalable solar evaporators from accessible raw materials.

1. Introduction

Water and energy resources are basic elements to life that associates with human being in many aspects, such as the biological survival, economic and societal development [1,2]. With increment of population and development of industry, clean water and energy supplies will be stressed in the coming decades [3,4]. As a kind of abundant and renewable energy, solar energy is a promising alternative to conven-tional energy for water evaporation [5–7]. From the ancient times, steam generation via solar water evaporation has been used to produce clean water [8]. In these systems, heat generation occurs at the receiver surface, while steam generation releases from air-water interfaces [5]. This spatial decoupling of heat and steam generation introduces large thermal loss, which in turn leads to the relatively low evaporation ef-ficiencies [9,10].

Many efforts have been devoted to improve solar water evaporation [11–13]. Nanofluids that can efficiently absorb solar energy was earlier

proposed as volumetric heating system (Fig. 1a) to boost steam gener-ation [14]. Owing to the excellent photothermal properties, nanofluids seeded with gold nanoparticle, CNTs and graphene have been verified to achieve high-performance evaporation [15–19]. Halas et al. concluded that 80% of the absorbed sunlight can be converted into vapor and only 20% of the absorbed solar energy is converted into heating of the bulk water by using SiO2/Au nanoshells [13]. While plasmonic structures only absorb a narrow band of light around their resonance peak. To extend the light absorption spectrum, the commonly used strategies include structural engineering, dielectric environment modification as well as using hybrid nanoparticles [20–23]. For example, graphe-ne/silver hybrid nanofluids achieve a full-spectrum absorption and reach a collector efficiency of 77% [24]. Nevertheless, robust dispersion and pumping of nanofluids under strong solar radiation remain chal-lenging. Besides, the volumetric heating is inevitably heated up bulk liquid, resulting in large heat loss.

To minimize the heat loss, interfacial solar evaporation is an

* Corresponding author. Beijing Key Laboratory of Multiphase Flow and Heat Transfer for Low Grade Energy Utilization, North China Electric Power University, Beijing, 102206, PR China. ** Corresponding author. Beijing Key Laboratory of Multiphase Flow and Heat Transfer for Low Grade Energy Utilization, North China Electric Power University,

Beijing, 102206, PR China. E-mail addresses: xjl@ncepu.edu.cn (J. Xu), liuguohua126@126.com (G. Liu).

Contents lists available at ScienceDirect

Solar Energy Materials and Solar Cells

journal homepage: http://www.elsevier.com/locate/solmat

https://doi.org/10.1016/j.solmat.2019.110292 Received 19 June 2019; Received in revised form 25 October 2019; Accepted 10 November 2019

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Fig. 1. Schematic of different solar evaporation systems. (a) The volumetric heating system, (b) the interfacial evaporation system and (c) the bubbly-flow nano-fluids system.

Fig. 2. The diagram of experimental setup.

Fig. 3. (a) The key elements in bubbly flow nanofluids system. (b–c) A bubble swarm with a mean diameter about 0.69 mm. (d–e) A bubble generator with a pore size about 1.98 μm. (f–g) Gold nanoparticles with an average diameter of 16.0 nm. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

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alternative to solve the bulk-heating issue (Fig. 1b) [25–29]. In such method, photothermal nanomaterials with mesoscopic porosity are used to localize heat at air-water interfaces [3,6]. For instance, the black gold membrane with microscale funnel structures has excellent light ab-sorption efficiency (~91% at 400–2500 nm), which leads to a thermal conversion efficiency of 57% at 20 kW/m2 [28]. A more efficient

absorber that can enable an absorbance of ~99% across the wavelengths from 400 nm to 10 μm was also developed through self-assembly of Au nanoparticles onto a porous membrane. The solar-to-vapor efficiency of this case could be over 90% under 4-sun irradiation [27]. However, the expensive plasmonic structures increase the complexity and cost of evaporator [5].

Fig. 4. The optical characterization. (a) and (b) the scattering images of bubbly flow nanofluids. (c) and (d) are the samples of Au nanofluids and bubbly flow nanofluids, respectively. (e) Absorption and (f) transmission spectrum of the Au nanofluids and the bubbly flow nanofluids.

Fig. 5. The temperature evolution in solar evaporation of (a) Au nanofluids and (b) bubbly flow nanofluids, (c–e) the temperature oscillation versus time during the evaporation of bubbly flow nanofluids.

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Despite of the solar energy harvesting and heat localization, extending the evaporation surface is another option to improve the vapor generation. The bubble column evaporator offers a good illus-tration of using gas-liquid interfaces to accelerate the evaporation [30]. This technology is first reported by Russian engineers for mineral flotation [31]. Now the concept is widely adopted for seawater desali-nation [32–34]. In such column humidifier, the carrier gas is dispersed into water column through a porous sinter to form small bubbles. One or both of gas and water heated by an external source are in direct contact, and a certain amount of vapor is extracted by the unsaturated bubbles. The releasing air with high humidity is then condensed to produce fresh water. For instance, an air bubble column humidifier, evacuated tube collectors and heat pipes are integrated for solar desalination. With the addition of oil and water in the space between the evacuated tubes and heat pipes, the daily fresh water productivity and daily efficiency are 6.275 kg/day m2 and 65%, respectively [34]. However, the application of bubble column technology for solar steam generation is still very limited.

In this work, a solar steam generation device is suggested by intro-ducing dynamic bubbles into gold nanofluids to improve the steam generation. The gold nanoparticles produce heat to enhance the evap-oration due to their strong surface plasmon resonance effect under solar irradiation. The dynamic bubbles serve as optical scattering centers to amplify solar flux, and offer large interfacial areas to enhance mass transfer at the same time, thus quickly raising the gas humidity. This

new vapor-generation mechanism is demonstrated to enhance steam generation by the coupling effect of photothermal evaporation and bubbly-flow humidification (Fig. 1c), and eventually leading to a high steam generation rate of 0.72 kg m� 2 h� 1, which is about three-time higher than that of the pure water. The findings pave a new avenue to promote steam generation, and the behind mechanism can potentially expand the use of such devices in diverse applications, such as desali-nation, wastewater treatment and catalytic slurry reactions, etc.

2. Methods

Fig. 2 shows the experiment setup. A porous metal sinter embedded into a glass column (40 mm inner diameter and 100 mm height) is used to produce bubbles. Air supplied from a gas cylinder flows through the sinter to continuously produce bubbles in the glass column filled with water. A pin valve is used to keep a constant flow rate and thus control the flow-pattern. The air flow rate was recorded by a rotor flow meter. The temperature of gas-liquid interface was measured by an infrared camera. A high-speed camera was installed for recording the flow- patterns. The steam generation experiments were performed using a solar simulator (CEL-PE300-3A) with a standard AM1.5 filter. The mass change of water was recorded by a high-precision electronic balance connected to computer. The experiments were carried out at controlled conditions with ambient temperature of 20 OC under atmospheric pressure. The irradiation flux was controlled at the range of 1–4 sun (1

Fig. 6. Typical flow-pattern images of solar evaporation of (a) water, (b) Au nanofluids, (c) bubbly flow and (d) bubbly flow nanofluids with the air flow rate at 40 ml/min, (e) and (f) flow pattern evolution in bubbly flow and bubbly flow nanofluids.

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sun ¼ 1 kW/m2). The bubbly flow nanofluids was utilized as volumetric solar absorber in the device.

A large contact area of the bubble-liquid interfaces can be formed by the continuous gas flow into the glass column, which is filled with the solution to a height of 85 mm (Fig. 3a). A typical flow pattern was presented in Fig. 3b. The resulted bubbles were evaluated by the Image J and Image-Pro software with a mean size of 0.69 mm (Fig. 3c). The surface structure of porous sinter (Fig. 3d) was characterized by Scan-ning electron microscopy (SEM, JEOL 7001F). It shows that the sinter surface has a mean pore size of 1.98 μm (Fig. 3e). The preparation of Au nanofluids was detailed in previous work [35]. The Au NPs were char-acterized by a transmission electron microscopy (TEM-JEOL 2011) and observed with a spherical morphology from the HRTEM image (Fig. 3f). The Au NPs was measured by the software with a mean diameter of 16.1 nm (Fig. 3g). The optical properties of nanofluids and bubbly flow

nanofluids were measured by UV–vis spectrophotometer between 300 and 900 nm and DI water was used as comparison standards. A small amount of effervescent tablet was put in the solution to release bubbles for simulating the bubbly flow condition in cuvette (Fig. 4c and d). The scattering effect of bubbly flow was tested in a darkroom with a green laser beam.

3. Results and discussion

Fig. 4 shows the optical properties of Au nanofluids and bubbly flow nanofluids. Fig. 4a and b presents the light scattering images of the bubbly flow nanofluids. There is a green light beam emitted from the laser source can be seen in the darkroom. When the bubbles rising and approaching to the light beam, the glass evaporator was fully painted by the green light due to the optical scattering effect of bubbly flow. After

Fig. 7. Characterization of solar evaporation of bubbly flow nanofluids. (a) Different irradiation intensity with Au nanofluids of 50 ppm and bubbly flow rate of 40 ml/min, (c) different concentrations from 10, 30–50 ppm and (e) different gas flow rates under two-sun irradiation. The corresponding evaporation rates and their enhancement ratios (b, d and f).

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the bubbles passing through light beam, the whole system darkens again because of the absent of light scattering effect. This phenomenon can be attributed to the optical scattering effect of bubbles, which redistributes and extends the light pathway [36]. The resulting redistribution and absorption increase the light flux and improve the light absorption by nanofluids.

Fig. 4e exhibits the optical absorption spectra of the nanofluids. Both of spectra shows the well-known behavior with an obvious absorption band at the wavelength of 520 nm owing to the surface plasmon reso-nance effect of Au [26,29]. While the absorption spectra of bubbly flow nanofluids have an enhancement range during the whole wavelength compared with Au nanofluids one. Besides, the absorption band is broadened and shifted to longer wavelength [37]. Fig. 4f shows the transmittance characteristic of two samples. The bubbly flow nanofluids are opaquer than that of Au nanofluids in the whole wavelength range. The minimum transmission occurs at the wavelength of 520 nm with a transmissivity of 30%. This also reflects that the bubbly flow nanofluids have a higher absorption ability than that of the Au nanofluids. Owing to the multiple scattering properties of bubbles, the optical path can be significantly extended in the bubbly flow nanofluids, which is beneficial to the harvesting of incident photons [38].

Under solar irradiation, the temperature of gas-liquid interface is a continuous increased versus time. After one-hour irradiation, the tem-perature on gas-liquid interface of Au nanofluids and bubbly flow nanofluids reach 40.5 OC and 34.3 OC, respectively (Fig. 5a and b). This reflects that the temperature of Au nanofluids is higher than bubbly flow one over the whole process. Some of the local heat brought away by the

rising bubbles leads to the surface temperature of bubbly flow nano-fluids lower than that of Au nanofluids. Fig. 5c–e shows three transient images of the interface temperature distribution of the bubbly flow nanofluids. It is found that the bubble spray process has small temper-ature gradient. The temperature oscillation can achieve 4 OC at air- liquid interface. This spatial and temporal oscillation of temperature is favorable for the steam generation due to the reduced pressure.

The flow patterns of (a) water, (b) Au nanofluids, (c) bubbly flow and (d) bubbly flow nanofluids are shown in Fig. 6. It can be observed that the color of Au nanofluids is darker than the water one, which confirms that the opaque Au nanofluids is less able to transmit light flux. The bubbly flow nanofluids (Fig. 6c) have the smaller and more evenly distributed bubbles than that without nanofluids one (Fig. 6d). Fig. 6e and f shows the sequence images of bubble rising process. The rising of bubble normally presents a spiral pathway owing to the action of different forces, which prolongs the bubble pathway and enhance heat and mass transfer process [32]. the large amount of bubbles offer large interfacial area to enhance the heat and mass transfer process and in-crease the gas humidity, which realizes an effective gas humidification [33,39]. Finally, when the rising bubble reaching to air-water surface, they will burst to project a large collection of vapors into the air [40,41].

The evaporation test was conducted in the prototype device. A series of experiments are performed under control parameters, e.g. different light intensities, concentration of nanofluids and gas flow rate. The re-sults are shown in Fig. 7. It is found that increasing of light intensity leads to an enhanced evaporation rate, while a highest enhancement ratio of bubbly flow nanofluids to pure water is observed under two-sun irradiation. This might be attributed to the competitive energy balance between the thermal evaporation and gas humidification. The increasing concentration of nanofluids amplifies the light flux and ab-sorption owing to the multi-scattering and reabsorption effects between particles, which thus results in a high evaporation rate (0.72 kg m� 2 h� 1) at a concentration of 50 ppm (Fig. 7c and d). Moreover, an increased gas- flow rate creates more bubble-water interfaces and flow turbulence for mass transfer in gas humidification process. Hereby, increasing of gas flow rate can greatly improve the gas humidification, leading to a high evaporation rate (1.02 kg m� 2 h� 1) as the gas flow rate approaching to 60 ml/min (Fig. 7e and f).

Fig. 8a shows the mass loss curves of steam generation with different fluids under two-sun irradiation. Each case was repeated under the same environmental condition to ensure the consistency of test. It can be observed that the curves decrease linearly with the extension of time. The evaporator with bubbly flow nanofluids loses the most water about 1.006 g compared with other counterparts, including water (0.335 g), bubbly flow (0.364 g) and Au nanofluids (0.582 g). The evaporation rates of all tests are calculated and listed in Fig. 8b. It can be concluded that the introducing of bubbles into both water and Au nanofluids

Fig. 8. Solar steam generation under two-sun irradiation. (a) Mass change over time with and without bubbly flow nanofluids, (b) the corresponding evaporation rates and (c) their enhancement ratios.

Fig. 9. The evaporation efficiency (black bar) and thermal efficiency (red bar) of solar steam systems with different fluids under two-sun irradiation. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

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significantly improve the evaporation performance. The evaporation rate of the bubbly flow nanofluids is 0.72 (kg m� 2 h� 1), which is three times higher than that 0.24 (kg m� 2 h� 1) of the water (Fig. 8b and c). This illustrates that the case with bubbly flow nanofluids have the best ability to enhance solar steam generation, even at a relative lower

temperature than the nanofluids case. To examine the performance and energy consumption, the vapor

generation efficiency and thermal efficiency are defined as: ηevaporation¼ hfg�r=Qsolar, ηheating ¼ ðcmΔt þ Δm�hfgÞ=Qsolar, respectively, where r is the evaporation rate of steam ðkg m� 2 h� 1Þ, hfg is the phase change enthalpy ð2:26 � 106J kg� 1 at 1 atmÞ, Qsolar is the irradiation flux of solar simulator, c is the specific heat capacity of water ðJ kg� 1 K� 1Þ, Δm is the weight change of testing fluid (kg), Δtis the temperature increase of bulk fluid (K), mis the total mass of testing fluid (kg). The steam generation efficiency and thermal efficiency of pure water are 7.5% and 40.7%, respectively (Fig. 9). In contrast to the pure water, the Au nanofluids show a better efficiency in both fluid heating and steam generation, with the corresponding efficiency of 11.3% and

Fig. 10. Comparison of the light absorption and the evaporation performance of three kinds of nanofluids. (a) Absorption spectrum, (b) photograph showing the response of Fe3O4 nanofluids to an external magnet, (c) TEM image of CNTs, (d) infrared images showing the evaporation temperature at air-water interface, (e) mass changes over time, (f) the corresponding enhancement compared to pure water, and (g) the enhancement induced from bubbly flow.

Table 1 Solar thermal nanofluids comparison table [45].

Type Graphite Al Copper Silver Au

Particle, vol% 0.0004 0.001 0.004 0.004 0.004 Approximate cost ($/L) 0.52 0.64 1.85 3.65 233

Assumes pure water base - where water þ stabilizers ¼ 0.5$/L.

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54.6%. This is attributed to the nanofluids enhances the solar thermal conversion at the bulk liquid. The bubbly-flow case presents a higher evaporation efficiency than the pure water and a little bit lower effi-ciency than the Au nanofluids. While its thermal efficiency is lower than that of the nanofluids. This can be explained that the bubbly flow tur-bulence for mass transfer takes away some of the heat energy, resulting in a low thermal efficiency. But the bubble bursting carries out a large collection of vapors at a low temperature. Due to multi-scattering and reflecting of light energy, a uniform bulk temperature is achieved in bubbly flow nanofluids, which thus presents the highest evaporation rate with an efficiency of 22.6%. The thermal efficiency of bubbly flow nanofluids case is 61.3%, which is about 20% enhancement over the base liquid.

To further extend the feasibility of our concept in variety of condi-tions, carbon nanotubes (CNTs) and semiconductor Fe3O4 nanofluids are also employed for the evaporation under two-sun irradiation. The both nanofluids show a broadband light absorption in the wavelength of 300–900 nm (Fig. 10a), even at a low concentration of 0.1 vol%. Fe3O4 has unique electric and magnetic properties based on the transfer of electrons between Fe2þ and Fe3þ in octahedral sites, which is often used for wastewater treatment and heavy metal removal process [42]. After the evaporation, Fe3O4 nanoparticles can be easily separated from the solution due to the magnetic property and thus can be cycling for further use (Fig. 10b) [43]. CNTs composed of a mixture of sp2 and sp3 bonding also has broad-band light absorption property due to their loosely-held π electrons (Fig. 10c), which is widely used in solar evaporation due to their facile preparation process with low cost [16,44]. Fig. 10d presents the evaporation temperatures at air-water interface of gold, Fe3O4 and CNT nanofluids, respectively. The evaporation tests show that all the nanofluids enhance the evaporation process. The bubbly flow nanofluids further improve the performance compared to that of the nanofluids (Fig. 10e and f). Especially, the bubbly-flow, Au nanofluids exhibit the highest evaporation rate with a best enhancement ratio of 1.73 (Fig. 10g). This could be attributed to a better physical coupling between the plasmonic heating and bubble dynamics. However, the physical coupling between nanofluids heating and bubble dynamics are very complex. Modelling of the multiphasic bubbly flow remains challenging due to the difficult in formulating the modified bubble behaviors induced by the adsorbed nanoparticles. As to cost consideration of the materials, although the gold nanofluids shows the best evaporation rate even when its concentration is 20 times lower than that of the other nanofluids, the cost of Au nanofluids achieves 233 $/L at 0.004 vol% (Table 1) [45]. It is thus not ready for large-scale application. In contrast, the cost of carbon materials is relatively low, only 0.52 $/L for 0.0004 vol%. This kind of nanofluids with good economic benefits should be proposed as solar absorbing materials. In the future, the optimized hybrid nanostructures with broadband light harvesting properties, facile preparation and low cost, easily recovery from the solution should be developed for commercial application.

Bubbles play an important role in heat transfer, humidification, electricity generation and other aspects of industry [32,33,46–49]. For example, a temporal sequence of bubbles has been introduced in silicon microchannels to improve heat transfer performance and stabilize flow boiling instability. The so-called seed bubbles not only stabilize the boiling flow and heat transfer, but also reduce the heating surface temperatures by decreasing the thermal non-equilibrium between liquid and vapor phases [50,51]. Injection of gas can lead to the creation of a bubbly mixture near the flow surface, which significantly modifies the flow within the turbulent boundary layer. This turbulent layer can act as a drag-reducing air layer beneath a solid surface [46,47]. Besides, the periodically nucleation and collapsing of thermal bubbles has been used as power source to drive microfluidic pump. The thermal bubbles generated in the pump are driven by pulsive electrical signals with various frequencies. The measured pumping flow rate can reach 5 μl/min and the highest pumping pressure is 377 Pa [52]. Moreover, the bubbly flow can be used to manipulate the electrical double layer on

hydrophobic surfaces. With the advantages of its geometry and hydro-dynamics, the ascending air bubbles in water has been applied to pro-duce electricity [48,49]. In contrast to the above applications, we, for the first time, introduce bubbles in nanofluids for solar steam genera-tion. The dynamic bubbles not only extend the pathway of incident light and amplify the light flux, but also provide large gas-liquid interfaces for mass transfer, which thus promotes the steam generation. This study offers a good illustration of the use of a gas-liquid interface to drive solar steam generation that involving heat and mass transfer processes.

4. Conclusion

In this work, we experimentally studied a solar evaporation system that using dynamic bubbles within nanofluids to enhance the water vapor generation. A series of experiments using different photothermal materials such as gold, carbon nanotube and magnetic Fe3O4 nanofluids under various light intensities, concentration of nanofluids and gas flow rates are implemented to verify the concept. It demonstrates that all the nanofluids enhance the steam generation, and the bubbly flow nano-fluids can be further improved the performance. A high steam genera-tion rate of 0.72 kg m� 2 h� 1 is achieved by using bubbly-flow gold nanofluids at a relatively low-concentration of 50 ppm, which is about three-time higher than that of the pure water. A better performance also can be expected by increasing the light intensity, concentration of nanofluids and gas flow rate. This high performance can be ascribed to the following contributions: 1) the well-dispersed gold nanoparticles in solution generate heat in the presence of electromagnetic radiation. 2) the dynamic bubbles as optical scattering centers extend the path of incident light and increase solar flux. 3) the continuously rising bubbles offer a large interfacial area to enhance mass transfer for gas humidifi-cation. 4) the bubble bursting process promotes the vapor generation at air/water interface due to the reduced pressure. With further optimi-zation of the materials and device structure, the bubbly flow nanofluids system may be used for diverse industrial applications, such as pollution abatement, evaporation-driven power generation and sea water desalination.

Declaration of competing interest

The authors declare no competing financial interests.

Acknowledgements

The research was supported by grants from the Natural Science Foundation of China (51576002 and 51436004).

Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi. org/10.1016/j.solmat.2019.110292.

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Solar steam generation enabled by bubbly flow nanofluids1 Introduction2 Methods3 Results and discussion4 ConclusionDeclaration of competing interestAcknowledgementsAppendix A Supplementary dataReferences