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A mimetic transpiration system for record high conversion efficiency in solar steam generator under one-sun
Materials Today Energy 8 (2018) 166e173
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A mimetic transpiration system for record high conversion efficiency in solar steam generator under one-sun Peng-Fei Liu a, Lei Miao a, *, Ziyang Deng a, Jianhua Zhou a, Hui Su a, Lixian Sun a, Sakae Tanemura a, b, Wenjiong Cao c, Fangming Jiang c, Li-Dong Zhao d, ** a Guangxi Key Laboratory of Information Material, Guangxi Collaborative Innovation Center of Structure and Property for New Energy and Materials, School of Material Science and Engineering, Guilin University of Electronic Technology, Guilin, 541004, China b Japan Fine Ceramic Center, 2-4-1 Mutsuno, Atsuta-ku, Nagoya 456-8587, Japan c Laboratory of Advanced Energy Systems, CAS Key Laboratory of Renewable Energy, Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences (CAS), Guangzhou, 510640, China d School of Materials Science and Engineering, Beihang University, Beijing, 100191, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 December 2017 Received in revised form 22 March 2018 Accepted 9 April 2018
Currently, direct solar steam generator based on suspending plasmonic solar absorber and floating porous photothermal convertors still suffered from high optical concentrations, poor productivity rates, high expenses and complicated fabrication processes. Here we designed a mimetic transpiration system imitating the water transport and transpiration of tree, the hydrophilic airlaid paper acting as the root of tree, the commercial expanded polyethylene foam as the soil for reducing the parasitic heat loss efficiently, and black carbonized wood with tremendous vertical channels as the photothermal convertor. A proof-of-concept mimetic transpiration system generator with carbonized wood achieved a record high conversion efficiency of 91.3% under one-sun, originating from the excellent thermal management of system and >97% carbonized wood solar absorptance and green house effect of vapor. Mimetic transpiration system can be fabricated by easy and scalable processes, providing a high-efficiency solar thermal conversion system for desalination, promising their industrial applications. © 2018 Elsevier Ltd. All rights reserved.
Keywords: Solar steam generation Mimetic transpiration system Carbonized wood Photo-thermal conversion
1. Introduction As a promising, abundant and clean source of renewable energy, solar energy can meet many of the global energy needs [1,2]. The emergence of direct solar steam generators (DSSG) based on plasmonic photothermic conversion of noble metals materials [3] has attracted much attention in the last few years as an impactful quest of mankind for renewable energy applications. The DSSG technology could be developed in the fields of water purification [3,4], desalination [5,6], distillation [8], sterilization [9] and photocatalysis [10], etc. In contrast of traditional solar-driven steam generation systems that require considerable investment in optical devices and land spaces, easy-to-manufacture and scalable DSSG technology is an attractive alternative. It can be classified into * Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (L. Miao), [email protected] (L.-D. Zhao). https://doi.org/10.1016/j.mtener.2018.04.004 2468-6069/© 2018 Elsevier Ltd. All rights reserved.
suspending systems and floating systems. In the suspending DSSG system, the photothermal materials suspended in water are used to convert solar energy to thermal energy partly and cause a bulk water heating behavior, which leads to a low steam generation efficiency of 24e70%. On the other hand, the steam generation efficiency has been improved to 90% in the floating system where the assembly of the noble metal particles is loaded onto a porous template floating on the surface of the working fluids under four suns due to the heat localization [11]. Therefore, how to design the system structure and select the proper photothermal materials constitute the main challenges to overcome for putting forward the practical applications of DSSG. Designing the DSSG structure is a decisive proposition for further improvement of system efficiency. Generally, a practical DSSG system must provide the following characteristics: (a) an effective solar-thermal top layer with a high solar absorptance; (b) a super thermal insulating layer to avoid the conduction heat loss from the underneath surface of the convertor to the interfaced water surface; (c) enough pumping ability for supplying
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successively water molecules from the water interface to the absorber. Ideal photothermal materials should have broadband absorption of sunlight ranging from 380 to 2500 nm in wavelength and convert a great amount of solar energy into heat. Therefore, compared to plasmonic materials (Ag [12,13], Au [14e20] and Al [21]), carbon-based materials [7,22e30] have excellent optical performance, exhibiting nearly 97% solar absorption ability [31], and they are often chosen as the absorbing material in DSSG system. A floatable thermal insulating layer, such as insulated foam [32,33], SiO2 aerogel [34] and wood [35], can enable the light absorber to float at the surface of water and avoid parasitic heat loss to the water for improving the conversion efficiency. The system efficiency improved from 27% without thermal insulator to 80% with polystyrene foam as thermal insulator in Zhu et al. [36] study, indicating the importance of thermal management for DSSG. Chiavazzo et al. [37] reported both numerical and experimental evidence that with reasonable design of solar steam generator, efficient steam generation by solar light can be achieved through inexpensive material. Here, we have purposefully designed a new solar steam generator by imitating the process of transpiration in tree as shown in Fig. 1aec. Carbonized natural wood embedded in expanded polyethylene (EPE) foam (k ¼ 0.026 W m1 K1) and airlaid-paper wick serves as solar absorption. In the mimetic transpiration system (MTS), airlaid paper with hydrophilic property, acting as the root of tree, can uptake and transport water molecules successively from the bulk water to solar absorber. And the low-cost commercial EPE foam can reduce the parasitic heat loss from the solar absorber to bulk water efficiently. Wood monoliths with its abundant hydroxyl groups and aligned porous wood channels cannot only serve as a thermal insulator to block the conduction heat loss, but also uptake
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water from the bulk water to the solar absorber. The green house effect of steam covered on the top of the absorber reduces the emission from the absorber as shown in Fig. 1d. The steam can transmit all of the solar spectra while absorb the emittance from the absorber. The theoretical efficiency of this system under onesun (1 kW m2 solar intensity) could be 92.6% based on our thermal dynamic calculation as displayed in Fig. 1c. Recently, wood-graphene oxide composites [35], natural wood with surface carbonization [38e40] and flexible wood/carbon nanotubes membranes [41] have been studied for solar steam generation, displaying cost-efficient and scalable properties. However, the solar steam generation efficiency of the present woodbased materials system was only limited to ~80% under one-sun [35,38,41], yet restricting the practical application of DSSG system. In our newly designed MTS structure based on carbonized wood slice (CW-S) solar absorber, the solar steam generation efficiency of 91.3% under one-sun has been achieved, which is among the best compared with other reported evaporators. Firstly, the EPE foam in our MTS has an extremely low thermal conductivity (0.026 W m1 K1) which is lower than that (0.53 W m1 K1) of the wetting wood [35] and have a better capacity of thermal management. By detail analysis of heat loss and modeling of MTS under one-sun, we proved the high solar steam generation efficiency of MTS with better thermal management performance. The innovative MTS design using connection of airlaid paper wick with carbonized wood absorber offers a new approach to solar energy harvesting for high-efficiency steam generation. Further, the green house effect of water vapor covered on the top of the absorber greatly suppressed the radiation heat loss from the absorber. Importantly, this proof-ofconcept prototype MTS holds great promise for practical application
Fig. 1. Configuration and modeling of steam generation under one-sun illumination. (a) Transpiration of trees. (b) MTS under solar illumination for solar steam generation. (c) Energy flow and heat transfer in the MTS. (d) The absorption spectrum of water steam.
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of direct solar steam generator without considerable capital, infrastructure and environmental load.
3. Results and discussion 3.1. Configuration of MTS
2. Experimental section 2.1. Material preparation CW absorber was fabricated by pyrolysis of a natural wood (Cunninghamia lanceolata) monolith at high temperature under N2 atmosphere. In a typical processing, natural wood was firstly cut into 4 4 2 cm3 monolith along the direction perpendicular to the growth of trees and dried at 100 C for 24 h. The obtained wood monolith was then carbonized at 500 C and 900 C in a tube furnace with a rate of 2 C/min for 4 h to fabricate 500CW and 900CW, respectively. Finally, the wood monolith, 500CW and 900CW were cut into a 20 20 1 mm3 slice and dried after washing with deionized water several times for the steam generation experiment. 2.2. Material characterization The morphologies and structures of the sample were characterized by scanning electron microscopy (SEM) (S-4800, Hitachi, Japan) at an acceleration voltage of 2 kV. The crystal structures of the sample were identified by using an X-ray diffraction (XRD) (D8ADBANCE, Bruker, Germany) with Cu Kɑ1 radiation. Raman scattering spectra were obtained on a Horiba LabRAM HR with a 100 objective and a 532 nm wavelength laser as an excitation source. Reflectance spectra were obtained from 250 to 2500 nm in wavelength using UV-vis-NIR spectroscopy with an integrating sphere (Lamber750, PerkinElemer, USA). Fourier transform infrared (FTIR) spectra were collected by using a spectrometer (Bruker, TENSOR27). The contact angle of the material was collected with a contact angle analyzer (JC2000D1, Powereach Co., China). Thermal images were captured by using an infrared camera (Ti32, FLUKE, USA). 2.3. Steam generation experiment The water evaporation performance of the samples was measured by a homemade system. A sample placed in the groove that was about 20 20 10 mm3 on the top of the MTS. The evaporation weight change of the MTS with a sample was measured by an electronic analytical balance (ATX 224, SHIMADZU Co., Japan) with 0.1 mg resolution, and real-time recorded by a desktop computer for 20 min at room temperature of 30 C and humility of 60%. Concentrated solar light was obtained by using a piece of Fresnel lens (diameter of 200 mm, focal length: 200 mm) to concentrate simulated solar illumination (solar-500, NBet, China). An adjustable aperture (diameter: 5 cm) was used to produce a light spot area approximate to that of the absorber. The solar flux on the absorber was measured by an optical power meter (843-R, Newport, USA) with a thermopile Sensor (919P010-16, Newport, USA). The temperature distribution of the samples under different concentrated solar light after 20 min was captured by the IR camera with emissivity of 0.98 and the SmartView 3.7 software dispose the temperature distribution patterns of the samples. The temperature of the vapor under onesun solar intensity for 20 min was measured by a thermal sensor probe (BD-PT100-3022A) which is above the surface of the absorber at a distance of 1 cm. The mass changes of 3.5 wt% saline water with MTS (500CW-S) were measured by the evaporation experiments for 20 min under one-sun. The long-time mass change of MTS on seawater was examined for 10 h. After that, the photograph of NaCl crystal was obtained by a digital camera.
As shown in Fig. 2aee, MTS is made up of three main components. Firstly, a plastic bottle is used to contain water as displayed in Fig. 2 a. The water is transported to the CW-S absorber by an airlaidpaper wick (Fig. 2b and e). Secondly, the airlaid-paper wick goes through the cap of the bottle and then a nonabsorbent expanded polyethylene (EPE) foam, reaching the underneath of the absorber (Fig. 2b). The EPE foam is on the top of the bottle-cap for blocking the conduction of heat. Thirdly, carbonized wood slice (CW-S) is selected as solar absorber (Fig. 2c). There is a scrap of airlaid paper (2 2 cm2) (Fig. 2d) between the absorber and the foam to ensure uniform water distribution. During solar steam generation experiment, the absorber is placed in the groove, which can focus the heat around the absorber, reducing the heat loss from the side wall (Fig. 2c and d). The 1 mm thick CW-S with excellent absorbance is easily obtained by using wire cutting and it stores little parasitic heat. Although the conduction of MTS with 2 mm CW-S shows 0.8 W m2 lower value that that of 1 mm, as shown in Note S1 in Supporting Information, 1 mm thick CW-S is selected in this experiment by considering water transport and steam flow. Moreover, the heat loss is suppressed by a spatial isolation between the insulator and the water. Such a MTS could realize efficient thermal management to ensure the record high efficiency in our steam generator. 3.2. Heat loss and modeling of MTS under one-sun We carried out modeling and thermal loss calculation to gain insights into DSSG efficiency under one-sun illumination. When the temperature of absorber increased after solar illumination, and the evaporation of water would start, then the heat losses of the MTS emerged, including radiative and convective heat loss of the absorber to steam, absorber to the ambient and conductive heat loss to underlying EPE foam. Therefore, it is necessary for efficient thermal management to reduce heat loss by designing rational system. As a demonstration, we mainly focus on the MTS with 500CW-S, which acts as an example to study heat losses of the system. The radiative and the convective heat losses per area were calculated using Stefan-Boltzmann formula (Equation (1)) and Newton's law of cooling (Equation (2)) respectively.
Prad ¼ εs T24 T14
(1)
Pcon ¼ hðT2 T1 Þ
(2)
where ε is the emissivity of the CW-S (i.e., 0.98), s is the StefanBoltzmann constant (i.e., 5.67 108 W m2 K4), T2 is the average temperature of absorber, T1 is the temperature of steam on top of CW-S absorber, h is convection heat transfer coefficient (assumed to be about 5 W m2 K1) [28]. Based on the theoretical estimation, when the T2 is about 50 C and T1 is about 48.6 C under one-sun solar intensity, it is estimated that the radiative heat loss from the 50 C black CW-S to the 48.6 C vapor environment is 12 W m2 and the convective heat loss is 17.4 W m2. Noted, 48.6 C is the temperature of steam transpired from MTS. The heat exchange of steam with environment is out of MTS efficiency. MTS could provide low temperature water vapor not high temperature steam. It is worth to mention that although T1 and T2 can be affected by room temperature (as shown in Table 1S, the range of ambient temperature is 20e30 C in previous works), T2 is close to
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Fig. 2. Mimic tree system. Optical photos of MTS used in this study (a) lateral view, (b) the cross profile of MTS, (c) top view with CW-S absorber, (d) top view with a groove, (e) airlaid-root in bottle.
T1 due to the energy localization of MTS, leading to both of convection and radiation losses are very small [32]. To evaluate the heat losses caused by the conduction of the EPE foam, we created a numerical model in commercial software Fluent 6.3.26. The numerical domain, consists of the absorber and the EPE foam, was divided into 16,680 cells. The temperature distribution is shown in Fig. S1 in Supporting Information. We calculated heat-transfer coefficient by empirical relations for the top, side and bottom wall of the EPE foam, which are 4.64 W m2 K1, 2.31 W m2 K1 and 6.72 W m2 K1, respectively (detail calculation process shown in Note S2). Based on the numerical model, we obtained the conduction per area from absorber to EPE foam is about 44.5 W m2. The total heat losses are corresponding to ~7.4% of the total incoming solar energy, ensuring the excellent thermal management and a high efficient solar steam generation of MTS. 3.3. Structural and optical characterization The highly ordered macro-porous CW solar absorber was prepared by a simple carbonized process. A natural wood monolith was dried at 100 C for 24 h to remove the moisture and calcined at high temperature for 4 h under N2 atmospheres. Then, the fabricated CW monolith was cut into a CW-S for the following solar steam experiment. The extremely dark CW-S shown in Fig. S2 is chosen as the absorber of the solar steam generator due to three main reasons. First, the open channel has the light trapping capability to reduce the light reflection and thus it enhances the performance of light-to-heat conversion. Second, the CW-S with ordered and vertical pore channels facilitates efficiently the water transportation and steam flow. Third, natural wood is low-cost and the CW-S obtained by a simple pyrolysis method can be regarded as a blackbody. XRD patterns of 500CW-S and 900CW-S are shown in Fig. S3. The 20e26 and 41e46 broad peaks of the characteristic of amorphous carbon are observed and presented in the general vicinity of the (002) and (101) graphite reflections, indicating a low graphitization degree [41,42]. The Raman spectroscopy of the
sample is studied in Fig. 3a. Both 500CW-S and 900CW-S have defined D (1350 cm1), indicating that the resulting structure is turbostratic carbon [43]. The results of XRD and Raman spectra of the samples agree well with that of effect of pyrolyzation temperature on wood-derived carbon. The FTIR spectra of the samples were studied to verify the change of oxygen-containing functional groups of the sample in the reaction process. Fig. 3b displays the existence of abundant O-H groups in original wood, and the O-H groups decrease dramatically in both 500CW-S and 900CW-S. The wetting properties of the macro-porous samples were studied by water contact angles. As shown in the inset of Fig. 3b, the water contact of the pristine wood was 0 , which indicates excellent hydrophilicity of the wood. Both 500CW-S and 900CW-S have a higher contact angle caused by a decrease of oxygen functional groups after the pyrolysis process. However, the wettability of the CW with good adhesion to water droplets is considered as a Wenzel's wetting behavior [44], which is beneficial for the contact between the airlaid paper with water and the CW. The SEM images show the inherent structures for the obtained samples. Fig. 4aed present the top and cross-sectional SEM images of the different CW-S. Open and regular macro-porous structures with well-defined channels in the axial direction are clearly observed. The channels originate from the vessels in the natural tree (Fig. S4). Compared with that of original wood, the pore of the 500CW and 900CW are smaller due to the shrink of the wood in pyrolysis process. With the temperature increasing, the average diagonal length of pore (34.4 mm) of the 500CW is bigger than (25.0 mm) that of 900CW (Fig. S5). Such a channel can ensure the transportation of water for steam generation. Compared with existing wood-based absorber, the CW-S is carbonized overall and shows more regular pores [34,36,37]. The vertical channel with ordered porous in the CW-S not only allows effective vapor flow and pumps water, but it is also beneficial for enhancing light absorption of CW-S. The optical absorptance spectra of the samples in the wavelength range of 250e2500 nm are presented in Fig. 4e, which is calculated as unity minus reflectance and transmittance (Fig. S6).
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Fig. 3. Carbonized wood characterization. (a) Raman spectra of wood, the 500CW-S and 900CW-S, (b) FTIR spectra of the different samples (The inset figures show the contact angles of water after a water droplet has been dropped on the surface of different samples).
Fig. 4. Optical performance of carbonized wood. SEM images of top face (a) and cross-sectional view (b) of 500CW-S. SEM images of top face (c) and cross-sectional view (d) of 900CW-S. (e) Absorption spectra in the range from 250 to 2500 nm of wood (black line), 500CW-S (red line), and 900CW-S (blue line). (f) Schematic illustration of the three models of light absorption in the CW-S. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
The original wood has a poor absorptance of 53.3% weighted by standard solar spectrum of air mass 1.5 global (AM 1.5G) in the studied spectrum range. After carbonization, the excellent and broadband solar absorption of the CW has been endowed. The absorptance of the 500CW-S in the range of 250e2500 nm is 97.6%, close to that of 97.3% for 900CW-S, indicating that the rise of carbonization temperature is unobvious to the performance of the solar absorber. The excellent optical performance of the CW-S is possibly due to three models of light absorption: (a) incident light absorbed by the surface of the CW-S; (b) incident light absorbed by the inner wall of channel; and (c) reflected light from the airlaid paper absorbed by the inner wall of channel (shown in the Fig. 4f). Therefore, the CW-S is regarded as a good solar absorber for the solar steam generation. 3.4. Steam generation performance To confirm the steam-generation performance of the CW-S, the weight loss by water evaporation of different samples was measured by using a home-made test system that comprise a solar
simulator, an analytical balance, a computer and the MTS (Fig. 5a). Fig. 5b displays the water mass change in different samples under 1 kW m2 solar beam irradiation. To further confirm the DSSG ability of CW-S, the weight loss under 3 kW m2 solar intensities is shown in Fig. 5c. The evaporation rate of all the samples in different solar intensity (Fig. 5d) was obtained by fitting the mass change curves after stable stage (~300 s) (Fig. S7). The evaporation rate of 500CW-S is 1.45 kg m2 h1 at 1 kW m2 and 4.03 kg m2 h1 at 3 kW m2 solar intensity, indicating that the CW has an outstanding performance for solar steam generation. In the process of solar-driven evaporation generation, the water mass loss is caused by the natural water evaporation and solarthermal steam generation in Fig. S8. To calculate accurately the steam generation efficiency of the samples, we subtracted the water mass loss (0.039 kg m2 for 20 min) (Fig. S9) of the MTS in dark environment (i.e., at humidity of 60% and room temperature of 30 C) to obtain the solar-driven evaporation rate for different samples presented in Fig. S10. The overall steam generation efficiency (h) of the MTS for different CW samples is calculated using the Equation (3) [28]:
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Fig. 5. Enhanced solar steam generation at the watereair interface. (a) Schematic illustration of the setup for measuring solar-driven steam generation performance. (b, c) The mass loss of evaporated water of wood (black line), 500CW-S (red line) and 900CW-S (blue line) placed on the MTS under 1 kW m2 and 3 kW m2 solar intensity, respectively. The mass change reaches a stable stage after 300 s under both of one-sun and three-sun illumination (purple region). Copt is the optical concentration. (d) The evaporation rate of water mass loss of different samples on the MTS under different solar irradiations. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
_ LV mh h¼ copt qi
(3)
where m_ is the solar-driven evaporation rate of water under solar illumination, copt qi is the incident power density of solar illumination in process of steam generation experiment, and hLV made up of the sensible heat and the enthalpy of vaporization is calculated using the Equation (4):
hLV ¼ C DT þ Dhvap
(4)
where C is the specific heat capacity of water and a constant of 4:18 J g 1 K 1 , DT is the temperature increase of water, and Dhvap is the enthalpy of vaporization on the relative temperature [45]. IR images of all samples under different solar intensities are shown in Fig. 6a. The surface temperature of the CW-S under 1 kW m2 and 3 kW m2 solar light is about 50 C and 60 C, respectively (Fig. 6b). The hLV values at different temperatures were calculated and listed in Table S2. The maximum steam generation efficiency reached a value as high as 91.3% for 500CW-S under solar intensity of 1 kW m2 (Table S3), superior to results in previous reports (Fig. 6d) [18e20,29,37], hence demonstrating a proof-of-concept for MTS exploiting photothermic mechanisms and thermal management effect under natural sunlight. Compared with 900CW-S, 500CW-S has higher steam generation efficiency possibly due to the better wettability and bigger pore of 500CW-S which can more effectively transport water to the surface of absorber in our case. With the enhancing of solar intensity to 3 kW m2, the steam generation efficiency of 500CW-S goes to 90.4% as shown in Fig. 6c, a value
higher than 90%. As previously reported [28,31], the positive correlation between the steam efficiency and solar intensity is not certain in the low optical concentration ratio region. The record high solar steam generation efficiency of the combination of CW-S with the MTS owes to the following three factors: 1) the splendid thermal management of the MTS and green house effect of vapor for efficiently reducing the heat loss; 2) the excellent light absorption throughout solar spectrum; and 3) the open pores and vertical channel to ensure the flow of the steam in the evaporation process. Additionally, the mass change and evaporation rate of the MTS on 3.5 wt% (average global seawater salinity) seawater under onesun are displayed in Fig. 7a and b. As we can see, the evaporation rate is ca. 1.38 kg m2 h1, which is lower than that of pure water due to the decrease in vapor pressure for the salt presence. To analyze long-term performance of the system for seawater, we also measured the evaporation rate for 10 h and then obtained the photograph of the CW-S surface. Fig. 7c displays the evaporation rate over 10 h in a range of 1.27e1.38 kg m2 h1, indicating stable long-term performance of the MTS. As revealed in Fig. 7d, the phenomenon that most crystal salt formed around the absorber in our case is different to the pervious report [32,46] and caused by transportation of water from the center of the CW-S to the sides (Fig. S13), suggesting the intrinsic anti-clogging ability of the MTS. Note that it is easy to remove the crystal salt on the surface, which realizes the MTS reuse in desalination. In practical application for desalination, the solar still is assembled by using transparent acrylic or glass slabs to collect distilled water. So, the solar steam generation efficiency is affected by not only the solar intensity in daytime but also the solar absorption of the slabs and the mist
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Fig. 6. Steam generation experimental result. (a) Infrared radiation thermal images of wood, 500CW-S and 900CW-S under different solar intensities. (b) The surface temperature of all samples under different solar intensities. (c) The solar steam generation efficiency of all samples under 1 kW m2 and 3 kW m2 solar irradiation. (d) Solar steam efficiency of 500CW-S compared with previous reports.
Fig. 7. The seawater desalination of the MTS under one-sun. (a) The mass change of the system with 3.5 wt% seawater under one-sun. (b) The evaporation cycle performance of the seawater with the system under one-sun. (c) The mass change (black line) and evaporation rate (red line) of 3.5 wt% seawater with the system under one-sun intensity over a 10 h evaporation period as a function of illumination time. (d) The surface photograph of 500CW-S after 10 h evaporation experiment. After 10 h evaporation experiment, the 0.15 g salt crystal (5.3 g mass change of the MTS) formation around the sides of CW-S. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
cover on the slabs surface. To improve the efficiency, we can utilize an optical lens to obtain one-sun intensity by concentrating natural light and spray nontoxic superhydrophobic materials on the surface of slabs to reduce the scattering of the mist. The optimization research on these methods is ongoing. In a word, the mimetic
transpiration of tree composed by carbonized wood absorber, coupled to low-cost and commercial airlaid paper wick and EPE foam has thus been shown to achieve record high-efficiency solar energy conversion to steam, demonstrating potential applications in desalination.
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4. Conclusion We have thus designed a prototype steam generator based on a mimetic transpiration of tree that is attractive for an affordable evaporation of water in applications such as water purification, desalination, distillation, and sterilization. The MTS integrating a thermal insulator and water path is a new system with excellent thermal management for DSSG. A highly ordered macro-porous carbonized wood is selected as a high-efficient solar absorber. With a broad absorption of the solar light, CW placed on the MTS converts solar light to heat at the interface of water and air to generate local heating of water. In our work, the evaporation rate of water and the steam generation efficiency of CW-S reached values as high as 1.45 kg m2 h1 and ~91.3% under one-sun illumination without optical concentration, respectively. The easily prepared carbonized wood with high efficient solar steam performance is attractive for DSSG, water purification and desalination. This will accelerate the solar energy for freshwater production with access to saline lakes or seawater at areas where connection to the electricity grid is either not cost-effective or not available. Acknowledgements This work was supported by National Natural Science Foundation of China (Grant No. 51572049, 51602068) and Guangxi Natural Science Foundation of China (Grant No. 2015GXNSFFA139002). This work was also supported by the Beijing Municipal Science & Technology Commission under Grant No. Z171100002017002, the Shenzhen Peacock Plan team under Grant No. KQTD201602261956591, and 111 Project under Grant No. B17002. This work was also supported by Program for Postgraduate Joint Training Base of GUET-CJYRE (No.20160513-20-Z). Appendix A. Supplementary data Supplementary data related to this article can be found at https://doi.org/10.1016/j.mtener.2018.04.004. References [1] M. Romero, A. Steinfeld, Concentrating solar thermal power and thermochemical fuels, Energy Environ. Sci. 5 (2012) 9234. [2] S. Chu, A. Majumdar, Opportunities and challenges for a sustainable energy future, Nature 488 (2012) 294e303. [3] Y. Liu, X. Wang, H. Wu, High-performance wastewater treatment based on reusable functional photo-absorbers, Chem. Eng. J. 309 (2017) 787e794. [4] J. Lou, et al., Bioinspired multifunctional paper-based rGO composites for solar-driven clean water generation, ACS Appl. Mater. Interfaces 8 (2016) 14628e14636. [5] G. Zhu, J. Xu, W. Zhao, F. Huang, Constructing black titania with unique nanocage structure for solar desalination, ACS Appl. Mater. Interfaces 8 (2016) 31716e31721. [6] Y. Liu, J. Chen, D. Guo, M. Cao, L. Jiang, Floatable, self-cleaning, and carbonblack-based superhydrophobic gauze for the solar evaporation enhancement at the airewater interface, ACS Appl. Mater. Interfaces 7 (2015) 13645e13652. [7] P. Zhang, J. Li, L. Lv, Y. Zhao, L. Qu, Vertically aligned graphene sheets membrane for highly efficient solar thermal generation of clean water, ACS Nano 11 (2017) 5087e5093. [8] G.N. Tiwari, H.N. Singh, R. Tripathi, Present status of solar distillation, Sol. Energy 75 (2003) 367e373. [9] K.G. McGuigan, et al., Solar water disinfection (SODIS): a review from benchtop to roof-top, J. Hazard Mater. 235e236 (2012) 29e46. [10] M. Gao, P.K.N. Connor, G.W. Ho, Plasmonic photothermic directed broadband sunlight harnessing for seawater catalysis and desalination, Energy Environ. Sci. 9 (2016) 3151e3160. [11] L. Zhou, et al., Self-assembly of highly efficient, broadband plasmonic absorbers for solar steam generation, Sci. Adv. 2 (2016) e1501227ee1501227. [12] H. Wang, L. Miao, S. Tanemura, Morphology control of Ag polyhedron nanoparticles for cost-effective and fast solar steam generation, Sol. RRL 1 (2017) 1600023.
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A mimetic transpiration system for record high conversion efficiency in solar steam generator under one-sun Peng-Fei Liu a, Lei Miao a, *, Ziyang Deng a, Jianhua Zhou a, Hui Su a, Lixian Sun a, Sakae Tanemura a, b, Wenjiong Cao c, Fangming Jiang c, Li-Dong Zhao d, ** a Guangxi Key Laboratory of Information Material, Guangxi Collaborative Innovation Center of Structure and Property for New Energy and Materials, School of Material Science and Engineering, Guilin University of Electronic Technology, Guilin, 541004, China b Japan Fine Ceramic Center, 2-4-1 Mutsuno, Atsuta-ku, Nagoya 456-8587, Japan c Laboratory of Advanced Energy Systems, CAS Key Laboratory of Renewable Energy, Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences (CAS), Guangzhou, 510640, China d School of Materials Science and Engineering, Beihang University, Beijing, 100191, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 December 2017 Received in revised form 22 March 2018 Accepted 9 April 2018
Currently, direct solar steam generator based on suspending plasmonic solar absorber and floating porous photothermal convertors still suffered from high optical concentrations, poor productivity rates, high expenses and complicated fabrication processes. Here we designed a mimetic transpiration system imitating the water transport and transpiration of tree, the hydrophilic airlaid paper acting as the root of tree, the commercial expanded polyethylene foam as the soil for reducing the parasitic heat loss efficiently, and black carbonized wood with tremendous vertical channels as the photothermal convertor. A proof-of-concept mimetic transpiration system generator with carbonized wood achieved a record high conversion efficiency of 91.3% under one-sun, originating from the excellent thermal management of system and >97% carbonized wood solar absorptance and green house effect of vapor. Mimetic transpiration system can be fabricated by easy and scalable processes, providing a high-efficiency solar thermal conversion system for desalination, promising their industrial applications. © 2018 Elsevier Ltd. All rights reserved.
Keywords: Solar steam generation Mimetic transpiration system Carbonized wood Photo-thermal conversion
1. Introduction As a promising, abundant and clean source of renewable energy, solar energy can meet many of the global energy needs [1,2]. The emergence of direct solar steam generators (DSSG) based on plasmonic photothermic conversion of noble metals materials [3] has attracted much attention in the last few years as an impactful quest of mankind for renewable energy applications. The DSSG technology could be developed in the fields of water purification [3,4], desalination [5,6], distillation [8], sterilization [9] and photocatalysis [10], etc. In contrast of traditional solar-driven steam generation systems that require considerable investment in optical devices and land spaces, easy-to-manufacture and scalable DSSG technology is an attractive alternative. It can be classified into * Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (L. Miao), [email protected] (L.-D. Zhao). https://doi.org/10.1016/j.mtener.2018.04.004 2468-6069/© 2018 Elsevier Ltd. All rights reserved.
suspending systems and floating systems. In the suspending DSSG system, the photothermal materials suspended in water are used to convert solar energy to thermal energy partly and cause a bulk water heating behavior, which leads to a low steam generation efficiency of 24e70%. On the other hand, the steam generation efficiency has been improved to 90% in the floating system where the assembly of the noble metal particles is loaded onto a porous template floating on the surface of the working fluids under four suns due to the heat localization [11]. Therefore, how to design the system structure and select the proper photothermal materials constitute the main challenges to overcome for putting forward the practical applications of DSSG. Designing the DSSG structure is a decisive proposition for further improvement of system efficiency. Generally, a practical DSSG system must provide the following characteristics: (a) an effective solar-thermal top layer with a high solar absorptance; (b) a super thermal insulating layer to avoid the conduction heat loss from the underneath surface of the convertor to the interfaced water surface; (c) enough pumping ability for supplying
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successively water molecules from the water interface to the absorber. Ideal photothermal materials should have broadband absorption of sunlight ranging from 380 to 2500 nm in wavelength and convert a great amount of solar energy into heat. Therefore, compared to plasmonic materials (Ag [12,13], Au [14e20] and Al [21]), carbon-based materials [7,22e30] have excellent optical performance, exhibiting nearly 97% solar absorption ability [31], and they are often chosen as the absorbing material in DSSG system. A floatable thermal insulating layer, such as insulated foam [32,33], SiO2 aerogel [34] and wood [35], can enable the light absorber to float at the surface of water and avoid parasitic heat loss to the water for improving the conversion efficiency. The system efficiency improved from 27% without thermal insulator to 80% with polystyrene foam as thermal insulator in Zhu et al. [36] study, indicating the importance of thermal management for DSSG. Chiavazzo et al. [37] reported both numerical and experimental evidence that with reasonable design of solar steam generator, efficient steam generation by solar light can be achieved through inexpensive material. Here, we have purposefully designed a new solar steam generator by imitating the process of transpiration in tree as shown in Fig. 1aec. Carbonized natural wood embedded in expanded polyethylene (EPE) foam (k ¼ 0.026 W m1 K1) and airlaid-paper wick serves as solar absorption. In the mimetic transpiration system (MTS), airlaid paper with hydrophilic property, acting as the root of tree, can uptake and transport water molecules successively from the bulk water to solar absorber. And the low-cost commercial EPE foam can reduce the parasitic heat loss from the solar absorber to bulk water efficiently. Wood monoliths with its abundant hydroxyl groups and aligned porous wood channels cannot only serve as a thermal insulator to block the conduction heat loss, but also uptake
167
water from the bulk water to the solar absorber. The green house effect of steam covered on the top of the absorber reduces the emission from the absorber as shown in Fig. 1d. The steam can transmit all of the solar spectra while absorb the emittance from the absorber. The theoretical efficiency of this system under onesun (1 kW m2 solar intensity) could be 92.6% based on our thermal dynamic calculation as displayed in Fig. 1c. Recently, wood-graphene oxide composites [35], natural wood with surface carbonization [38e40] and flexible wood/carbon nanotubes membranes [41] have been studied for solar steam generation, displaying cost-efficient and scalable properties. However, the solar steam generation efficiency of the present woodbased materials system was only limited to ~80% under one-sun [35,38,41], yet restricting the practical application of DSSG system. In our newly designed MTS structure based on carbonized wood slice (CW-S) solar absorber, the solar steam generation efficiency of 91.3% under one-sun has been achieved, which is among the best compared with other reported evaporators. Firstly, the EPE foam in our MTS has an extremely low thermal conductivity (0.026 W m1 K1) which is lower than that (0.53 W m1 K1) of the wetting wood [35] and have a better capacity of thermal management. By detail analysis of heat loss and modeling of MTS under one-sun, we proved the high solar steam generation efficiency of MTS with better thermal management performance. The innovative MTS design using connection of airlaid paper wick with carbonized wood absorber offers a new approach to solar energy harvesting for high-efficiency steam generation. Further, the green house effect of water vapor covered on the top of the absorber greatly suppressed the radiation heat loss from the absorber. Importantly, this proof-ofconcept prototype MTS holds great promise for practical application
Fig. 1. Configuration and modeling of steam generation under one-sun illumination. (a) Transpiration of trees. (b) MTS under solar illumination for solar steam generation. (c) Energy flow and heat transfer in the MTS. (d) The absorption spectrum of water steam.
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of direct solar steam generator without considerable capital, infrastructure and environmental load.
3. Results and discussion 3.1. Configuration of MTS
2. Experimental section 2.1. Material preparation CW absorber was fabricated by pyrolysis of a natural wood (Cunninghamia lanceolata) monolith at high temperature under N2 atmosphere. In a typical processing, natural wood was firstly cut into 4 4 2 cm3 monolith along the direction perpendicular to the growth of trees and dried at 100 C for 24 h. The obtained wood monolith was then carbonized at 500 C and 900 C in a tube furnace with a rate of 2 C/min for 4 h to fabricate 500CW and 900CW, respectively. Finally, the wood monolith, 500CW and 900CW were cut into a 20 20 1 mm3 slice and dried after washing with deionized water several times for the steam generation experiment. 2.2. Material characterization The morphologies and structures of the sample were characterized by scanning electron microscopy (SEM) (S-4800, Hitachi, Japan) at an acceleration voltage of 2 kV. The crystal structures of the sample were identified by using an X-ray diffraction (XRD) (D8ADBANCE, Bruker, Germany) with Cu Kɑ1 radiation. Raman scattering spectra were obtained on a Horiba LabRAM HR with a 100 objective and a 532 nm wavelength laser as an excitation source. Reflectance spectra were obtained from 250 to 2500 nm in wavelength using UV-vis-NIR spectroscopy with an integrating sphere (Lamber750, PerkinElemer, USA). Fourier transform infrared (FTIR) spectra were collected by using a spectrometer (Bruker, TENSOR27). The contact angle of the material was collected with a contact angle analyzer (JC2000D1, Powereach Co., China). Thermal images were captured by using an infrared camera (Ti32, FLUKE, USA). 2.3. Steam generation experiment The water evaporation performance of the samples was measured by a homemade system. A sample placed in the groove that was about 20 20 10 mm3 on the top of the MTS. The evaporation weight change of the MTS with a sample was measured by an electronic analytical balance (ATX 224, SHIMADZU Co., Japan) with 0.1 mg resolution, and real-time recorded by a desktop computer for 20 min at room temperature of 30 C and humility of 60%. Concentrated solar light was obtained by using a piece of Fresnel lens (diameter of 200 mm, focal length: 200 mm) to concentrate simulated solar illumination (solar-500, NBet, China). An adjustable aperture (diameter: 5 cm) was used to produce a light spot area approximate to that of the absorber. The solar flux on the absorber was measured by an optical power meter (843-R, Newport, USA) with a thermopile Sensor (919P010-16, Newport, USA). The temperature distribution of the samples under different concentrated solar light after 20 min was captured by the IR camera with emissivity of 0.98 and the SmartView 3.7 software dispose the temperature distribution patterns of the samples. The temperature of the vapor under onesun solar intensity for 20 min was measured by a thermal sensor probe (BD-PT100-3022A) which is above the surface of the absorber at a distance of 1 cm. The mass changes of 3.5 wt% saline water with MTS (500CW-S) were measured by the evaporation experiments for 20 min under one-sun. The long-time mass change of MTS on seawater was examined for 10 h. After that, the photograph of NaCl crystal was obtained by a digital camera.
As shown in Fig. 2aee, MTS is made up of three main components. Firstly, a plastic bottle is used to contain water as displayed in Fig. 2 a. The water is transported to the CW-S absorber by an airlaidpaper wick (Fig. 2b and e). Secondly, the airlaid-paper wick goes through the cap of the bottle and then a nonabsorbent expanded polyethylene (EPE) foam, reaching the underneath of the absorber (Fig. 2b). The EPE foam is on the top of the bottle-cap for blocking the conduction of heat. Thirdly, carbonized wood slice (CW-S) is selected as solar absorber (Fig. 2c). There is a scrap of airlaid paper (2 2 cm2) (Fig. 2d) between the absorber and the foam to ensure uniform water distribution. During solar steam generation experiment, the absorber is placed in the groove, which can focus the heat around the absorber, reducing the heat loss from the side wall (Fig. 2c and d). The 1 mm thick CW-S with excellent absorbance is easily obtained by using wire cutting and it stores little parasitic heat. Although the conduction of MTS with 2 mm CW-S shows 0.8 W m2 lower value that that of 1 mm, as shown in Note S1 in Supporting Information, 1 mm thick CW-S is selected in this experiment by considering water transport and steam flow. Moreover, the heat loss is suppressed by a spatial isolation between the insulator and the water. Such a MTS could realize efficient thermal management to ensure the record high efficiency in our steam generator. 3.2. Heat loss and modeling of MTS under one-sun We carried out modeling and thermal loss calculation to gain insights into DSSG efficiency under one-sun illumination. When the temperature of absorber increased after solar illumination, and the evaporation of water would start, then the heat losses of the MTS emerged, including radiative and convective heat loss of the absorber to steam, absorber to the ambient and conductive heat loss to underlying EPE foam. Therefore, it is necessary for efficient thermal management to reduce heat loss by designing rational system. As a demonstration, we mainly focus on the MTS with 500CW-S, which acts as an example to study heat losses of the system. The radiative and the convective heat losses per area were calculated using Stefan-Boltzmann formula (Equation (1)) and Newton's law of cooling (Equation (2)) respectively.
Prad ¼ εs T24 T14
(1)
Pcon ¼ hðT2 T1 Þ
(2)
where ε is the emissivity of the CW-S (i.e., 0.98), s is the StefanBoltzmann constant (i.e., 5.67 108 W m2 K4), T2 is the average temperature of absorber, T1 is the temperature of steam on top of CW-S absorber, h is convection heat transfer coefficient (assumed to be about 5 W m2 K1) [28]. Based on the theoretical estimation, when the T2 is about 50 C and T1 is about 48.6 C under one-sun solar intensity, it is estimated that the radiative heat loss from the 50 C black CW-S to the 48.6 C vapor environment is 12 W m2 and the convective heat loss is 17.4 W m2. Noted, 48.6 C is the temperature of steam transpired from MTS. The heat exchange of steam with environment is out of MTS efficiency. MTS could provide low temperature water vapor not high temperature steam. It is worth to mention that although T1 and T2 can be affected by room temperature (as shown in Table 1S, the range of ambient temperature is 20e30 C in previous works), T2 is close to
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Fig. 2. Mimic tree system. Optical photos of MTS used in this study (a) lateral view, (b) the cross profile of MTS, (c) top view with CW-S absorber, (d) top view with a groove, (e) airlaid-root in bottle.
T1 due to the energy localization of MTS, leading to both of convection and radiation losses are very small [32]. To evaluate the heat losses caused by the conduction of the EPE foam, we created a numerical model in commercial software Fluent 6.3.26. The numerical domain, consists of the absorber and the EPE foam, was divided into 16,680 cells. The temperature distribution is shown in Fig. S1 in Supporting Information. We calculated heat-transfer coefficient by empirical relations for the top, side and bottom wall of the EPE foam, which are 4.64 W m2 K1, 2.31 W m2 K1 and 6.72 W m2 K1, respectively (detail calculation process shown in Note S2). Based on the numerical model, we obtained the conduction per area from absorber to EPE foam is about 44.5 W m2. The total heat losses are corresponding to ~7.4% of the total incoming solar energy, ensuring the excellent thermal management and a high efficient solar steam generation of MTS. 3.3. Structural and optical characterization The highly ordered macro-porous CW solar absorber was prepared by a simple carbonized process. A natural wood monolith was dried at 100 C for 24 h to remove the moisture and calcined at high temperature for 4 h under N2 atmospheres. Then, the fabricated CW monolith was cut into a CW-S for the following solar steam experiment. The extremely dark CW-S shown in Fig. S2 is chosen as the absorber of the solar steam generator due to three main reasons. First, the open channel has the light trapping capability to reduce the light reflection and thus it enhances the performance of light-to-heat conversion. Second, the CW-S with ordered and vertical pore channels facilitates efficiently the water transportation and steam flow. Third, natural wood is low-cost and the CW-S obtained by a simple pyrolysis method can be regarded as a blackbody. XRD patterns of 500CW-S and 900CW-S are shown in Fig. S3. The 20e26 and 41e46 broad peaks of the characteristic of amorphous carbon are observed and presented in the general vicinity of the (002) and (101) graphite reflections, indicating a low graphitization degree [41,42]. The Raman spectroscopy of the
sample is studied in Fig. 3a. Both 500CW-S and 900CW-S have defined D (1350 cm1), indicating that the resulting structure is turbostratic carbon [43]. The results of XRD and Raman spectra of the samples agree well with that of effect of pyrolyzation temperature on wood-derived carbon. The FTIR spectra of the samples were studied to verify the change of oxygen-containing functional groups of the sample in the reaction process. Fig. 3b displays the existence of abundant O-H groups in original wood, and the O-H groups decrease dramatically in both 500CW-S and 900CW-S. The wetting properties of the macro-porous samples were studied by water contact angles. As shown in the inset of Fig. 3b, the water contact of the pristine wood was 0 , which indicates excellent hydrophilicity of the wood. Both 500CW-S and 900CW-S have a higher contact angle caused by a decrease of oxygen functional groups after the pyrolysis process. However, the wettability of the CW with good adhesion to water droplets is considered as a Wenzel's wetting behavior [44], which is beneficial for the contact between the airlaid paper with water and the CW. The SEM images show the inherent structures for the obtained samples. Fig. 4aed present the top and cross-sectional SEM images of the different CW-S. Open and regular macro-porous structures with well-defined channels in the axial direction are clearly observed. The channels originate from the vessels in the natural tree (Fig. S4). Compared with that of original wood, the pore of the 500CW and 900CW are smaller due to the shrink of the wood in pyrolysis process. With the temperature increasing, the average diagonal length of pore (34.4 mm) of the 500CW is bigger than (25.0 mm) that of 900CW (Fig. S5). Such a channel can ensure the transportation of water for steam generation. Compared with existing wood-based absorber, the CW-S is carbonized overall and shows more regular pores [34,36,37]. The vertical channel with ordered porous in the CW-S not only allows effective vapor flow and pumps water, but it is also beneficial for enhancing light absorption of CW-S. The optical absorptance spectra of the samples in the wavelength range of 250e2500 nm are presented in Fig. 4e, which is calculated as unity minus reflectance and transmittance (Fig. S6).
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Fig. 3. Carbonized wood characterization. (a) Raman spectra of wood, the 500CW-S and 900CW-S, (b) FTIR spectra of the different samples (The inset figures show the contact angles of water after a water droplet has been dropped on the surface of different samples).
Fig. 4. Optical performance of carbonized wood. SEM images of top face (a) and cross-sectional view (b) of 500CW-S. SEM images of top face (c) and cross-sectional view (d) of 900CW-S. (e) Absorption spectra in the range from 250 to 2500 nm of wood (black line), 500CW-S (red line), and 900CW-S (blue line). (f) Schematic illustration of the three models of light absorption in the CW-S. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
The original wood has a poor absorptance of 53.3% weighted by standard solar spectrum of air mass 1.5 global (AM 1.5G) in the studied spectrum range. After carbonization, the excellent and broadband solar absorption of the CW has been endowed. The absorptance of the 500CW-S in the range of 250e2500 nm is 97.6%, close to that of 97.3% for 900CW-S, indicating that the rise of carbonization temperature is unobvious to the performance of the solar absorber. The excellent optical performance of the CW-S is possibly due to three models of light absorption: (a) incident light absorbed by the surface of the CW-S; (b) incident light absorbed by the inner wall of channel; and (c) reflected light from the airlaid paper absorbed by the inner wall of channel (shown in the Fig. 4f). Therefore, the CW-S is regarded as a good solar absorber for the solar steam generation. 3.4. Steam generation performance To confirm the steam-generation performance of the CW-S, the weight loss by water evaporation of different samples was measured by using a home-made test system that comprise a solar
simulator, an analytical balance, a computer and the MTS (Fig. 5a). Fig. 5b displays the water mass change in different samples under 1 kW m2 solar beam irradiation. To further confirm the DSSG ability of CW-S, the weight loss under 3 kW m2 solar intensities is shown in Fig. 5c. The evaporation rate of all the samples in different solar intensity (Fig. 5d) was obtained by fitting the mass change curves after stable stage (~300 s) (Fig. S7). The evaporation rate of 500CW-S is 1.45 kg m2 h1 at 1 kW m2 and 4.03 kg m2 h1 at 3 kW m2 solar intensity, indicating that the CW has an outstanding performance for solar steam generation. In the process of solar-driven evaporation generation, the water mass loss is caused by the natural water evaporation and solarthermal steam generation in Fig. S8. To calculate accurately the steam generation efficiency of the samples, we subtracted the water mass loss (0.039 kg m2 for 20 min) (Fig. S9) of the MTS in dark environment (i.e., at humidity of 60% and room temperature of 30 C) to obtain the solar-driven evaporation rate for different samples presented in Fig. S10. The overall steam generation efficiency (h) of the MTS for different CW samples is calculated using the Equation (3) [28]:
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Fig. 5. Enhanced solar steam generation at the watereair interface. (a) Schematic illustration of the setup for measuring solar-driven steam generation performance. (b, c) The mass loss of evaporated water of wood (black line), 500CW-S (red line) and 900CW-S (blue line) placed on the MTS under 1 kW m2 and 3 kW m2 solar intensity, respectively. The mass change reaches a stable stage after 300 s under both of one-sun and three-sun illumination (purple region). Copt is the optical concentration. (d) The evaporation rate of water mass loss of different samples on the MTS under different solar irradiations. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
_ LV mh h¼ copt qi
(3)
where m_ is the solar-driven evaporation rate of water under solar illumination, copt qi is the incident power density of solar illumination in process of steam generation experiment, and hLV made up of the sensible heat and the enthalpy of vaporization is calculated using the Equation (4):
hLV ¼ C DT þ Dhvap
(4)
where C is the specific heat capacity of water and a constant of 4:18 J g 1 K 1 , DT is the temperature increase of water, and Dhvap is the enthalpy of vaporization on the relative temperature [45]. IR images of all samples under different solar intensities are shown in Fig. 6a. The surface temperature of the CW-S under 1 kW m2 and 3 kW m2 solar light is about 50 C and 60 C, respectively (Fig. 6b). The hLV values at different temperatures were calculated and listed in Table S2. The maximum steam generation efficiency reached a value as high as 91.3% for 500CW-S under solar intensity of 1 kW m2 (Table S3), superior to results in previous reports (Fig. 6d) [18e20,29,37], hence demonstrating a proof-of-concept for MTS exploiting photothermic mechanisms and thermal management effect under natural sunlight. Compared with 900CW-S, 500CW-S has higher steam generation efficiency possibly due to the better wettability and bigger pore of 500CW-S which can more effectively transport water to the surface of absorber in our case. With the enhancing of solar intensity to 3 kW m2, the steam generation efficiency of 500CW-S goes to 90.4% as shown in Fig. 6c, a value
higher than 90%. As previously reported [28,31], the positive correlation between the steam efficiency and solar intensity is not certain in the low optical concentration ratio region. The record high solar steam generation efficiency of the combination of CW-S with the MTS owes to the following three factors: 1) the splendid thermal management of the MTS and green house effect of vapor for efficiently reducing the heat loss; 2) the excellent light absorption throughout solar spectrum; and 3) the open pores and vertical channel to ensure the flow of the steam in the evaporation process. Additionally, the mass change and evaporation rate of the MTS on 3.5 wt% (average global seawater salinity) seawater under onesun are displayed in Fig. 7a and b. As we can see, the evaporation rate is ca. 1.38 kg m2 h1, which is lower than that of pure water due to the decrease in vapor pressure for the salt presence. To analyze long-term performance of the system for seawater, we also measured the evaporation rate for 10 h and then obtained the photograph of the CW-S surface. Fig. 7c displays the evaporation rate over 10 h in a range of 1.27e1.38 kg m2 h1, indicating stable long-term performance of the MTS. As revealed in Fig. 7d, the phenomenon that most crystal salt formed around the absorber in our case is different to the pervious report [32,46] and caused by transportation of water from the center of the CW-S to the sides (Fig. S13), suggesting the intrinsic anti-clogging ability of the MTS. Note that it is easy to remove the crystal salt on the surface, which realizes the MTS reuse in desalination. In practical application for desalination, the solar still is assembled by using transparent acrylic or glass slabs to collect distilled water. So, the solar steam generation efficiency is affected by not only the solar intensity in daytime but also the solar absorption of the slabs and the mist
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Fig. 6. Steam generation experimental result. (a) Infrared radiation thermal images of wood, 500CW-S and 900CW-S under different solar intensities. (b) The surface temperature of all samples under different solar intensities. (c) The solar steam generation efficiency of all samples under 1 kW m2 and 3 kW m2 solar irradiation. (d) Solar steam efficiency of 500CW-S compared with previous reports.
Fig. 7. The seawater desalination of the MTS under one-sun. (a) The mass change of the system with 3.5 wt% seawater under one-sun. (b) The evaporation cycle performance of the seawater with the system under one-sun. (c) The mass change (black line) and evaporation rate (red line) of 3.5 wt% seawater with the system under one-sun intensity over a 10 h evaporation period as a function of illumination time. (d) The surface photograph of 500CW-S after 10 h evaporation experiment. After 10 h evaporation experiment, the 0.15 g salt crystal (5.3 g mass change of the MTS) formation around the sides of CW-S. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
cover on the slabs surface. To improve the efficiency, we can utilize an optical lens to obtain one-sun intensity by concentrating natural light and spray nontoxic superhydrophobic materials on the surface of slabs to reduce the scattering of the mist. The optimization research on these methods is ongoing. In a word, the mimetic
transpiration of tree composed by carbonized wood absorber, coupled to low-cost and commercial airlaid paper wick and EPE foam has thus been shown to achieve record high-efficiency solar energy conversion to steam, demonstrating potential applications in desalination.
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4. Conclusion We have thus designed a prototype steam generator based on a mimetic transpiration of tree that is attractive for an affordable evaporation of water in applications such as water purification, desalination, distillation, and sterilization. The MTS integrating a thermal insulator and water path is a new system with excellent thermal management for DSSG. A highly ordered macro-porous carbonized wood is selected as a high-efficient solar absorber. With a broad absorption of the solar light, CW placed on the MTS converts solar light to heat at the interface of water and air to generate local heating of water. In our work, the evaporation rate of water and the steam generation efficiency of CW-S reached values as high as 1.45 kg m2 h1 and ~91.3% under one-sun illumination without optical concentration, respectively. The easily prepared carbonized wood with high efficient solar steam performance is attractive for DSSG, water purification and desalination. This will accelerate the solar energy for freshwater production with access to saline lakes or seawater at areas where connection to the electricity grid is either not cost-effective or not available. Acknowledgements This work was supported by National Natural Science Foundation of China (Grant No. 51572049, 51602068) and Guangxi Natural Science Foundation of China (Grant No. 2015GXNSFFA139002). This work was also supported by the Beijing Municipal Science & Technology Commission under Grant No. Z171100002017002, the Shenzhen Peacock Plan team under Grant No. KQTD201602261956591, and 111 Project under Grant No. B17002. This work was also supported by Program for Postgraduate Joint Training Base of GUET-CJYRE (No.20160513-20-Z). Appendix A. Supplementary data Supplementary data related to this article can be found at https://doi.org/10.1016/j.mtener.2018.04.004. References [1] M. Romero, A. Steinfeld, Concentrating solar thermal power and thermochemical fuels, Energy Environ. Sci. 5 (2012) 9234. [2] S. Chu, A. Majumdar, Opportunities and challenges for a sustainable energy future, Nature 488 (2012) 294e303. [3] Y. Liu, X. Wang, H. Wu, High-performance wastewater treatment based on reusable functional photo-absorbers, Chem. Eng. J. 309 (2017) 787e794. [4] J. Lou, et al., Bioinspired multifunctional paper-based rGO composites for solar-driven clean water generation, ACS Appl. Mater. Interfaces 8 (2016) 14628e14636. [5] G. Zhu, J. Xu, W. Zhao, F. Huang, Constructing black titania with unique nanocage structure for solar desalination, ACS Appl. Mater. Interfaces 8 (2016) 31716e31721. [6] Y. Liu, J. Chen, D. Guo, M. Cao, L. Jiang, Floatable, self-cleaning, and carbonblack-based superhydrophobic gauze for the solar evaporation enhancement at the airewater interface, ACS Appl. Mater. Interfaces 7 (2015) 13645e13652. [7] P. Zhang, J. Li, L. Lv, Y. Zhao, L. Qu, Vertically aligned graphene sheets membrane for highly efficient solar thermal generation of clean water, ACS Nano 11 (2017) 5087e5093. [8] G.N. Tiwari, H.N. Singh, R. Tripathi, Present status of solar distillation, Sol. Energy 75 (2003) 367e373. [9] K.G. McGuigan, et al., Solar water disinfection (SODIS): a review from benchtop to roof-top, J. Hazard Mater. 235e236 (2012) 29e46. [10] M. Gao, P.K.N. Connor, G.W. Ho, Plasmonic photothermic directed broadband sunlight harnessing for seawater catalysis and desalination, Energy Environ. Sci. 9 (2016) 3151e3160. [11] L. Zhou, et al., Self-assembly of highly efficient, broadband plasmonic absorbers for solar steam generation, Sci. Adv. 2 (2016) e1501227ee1501227. [12] H. Wang, L. Miao, S. Tanemura, Morphology control of Ag polyhedron nanoparticles for cost-effective and fast solar steam generation, Sol. RRL 1 (2017) 1600023.
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