Enhancing efficiency of carbonized wood based solar steam generator for wastewater treatment by optimizing the thickness

Solar Energy 193 (2019) 434–441

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Enhancing efficiency of carbonized wood based solar steam generator for wastewater treatment by optimizing the thickness Zhen Yua, Shaoan Chenga, , Chaochao Lia, Yi Suna, Baoqiang Lib,c, ⁎

T

⁎

a

State Key Laboratory of Clean Energy, Department of Energy Engineering, Zhejiang University, Hangzhou 310027, PR China Institute for Advanced Ceramics, State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology, Harbin 150001, PR China c Key Laboratory of Advanced Structural-Functional Integration Materials & Green Manufacturing Technology, Harbin Institute of Technology, Harbin 150001, PR China b

ARTICLE INFO

ABSTRACT

Keywords: Localized heating effect Carbonized wood Optimal thickness Solar steam generator Seawater desalination Dye removal

Highly-efficient solar steam generator holds promise in seawater desalination and wastewater treatment with low energy consumption. The efficiency of generator highly depends on several factors including the kind of the absorber layer, the structure and the thickness of the generators. In this study, a model for calculating the optimal thickness was established based on the water transfer rate and thermal conduction loss. An optimal thickness of generator was determined by the calculation and experiment, taking carbonized wood based solar steam generator (CWSG) as an example. CWSG with optimal thickness about 22 mm performed a highest evaporation rate of 6.89 kg m−2 h−1, corresponding to efficiency of 87.7% under 5 sun. CWSG also served for seawater desalination and dye removal, which exhibited a stable performance even after 20 cycles. The results indicate that maximizing the efficiency of solar steam generator by thermal calculation could provide a new choice to design highly efficient solar steam generator for seawater desalination and dye removal.

1. Introduction The fresh water shortage is one of the most pressing global issues (Neoh et al., 2016; Riyajan et al., 2009). Solar steam generation could directly desalt seawater and purify sewage by collecting condensate water with low energy consumption and little climate change impact (Chen et al., 2019). It is expected to be one of the most promising solution to solve water resources problems (Liu et al., 2013; Renu et al., 2017). However, traditional solar steam generator must rely on enormous and complex concentrating systems to enhance water evaporation rate and practical efficiency, leading to large optical and systematic heat loss (El-Agouz et al., 2014; Fang et al., 2013; Neumann, O. et al., 2013). Based on capillarity and localized heating effect, Chen’s group proposed a simple and highly-efficient floated solar steam generator consisting of absorber layer, thermal insulation layer and many microchannels inside (Ni et al., 2016). After that, a lot of photothermal materials including metal plasmonic materials and carbon materials have been developed as absorber layers for efficient solar steam generation (Chen et al., 2018; He et al., 2018; Hu et al., 2017; Jiang et al., 2018). Most metal plasmonic materials such as Ag and Ag only possess

strong absorption in visible-light of full solar spectrum (Neumann, Oara et al., 2013; Richardson et al., 2006; Zhou et al., 2016). Carbon materials such as GO & CNT perform high absorption across the full solar spectrum (Guo et al., 2017; Liu, K.K. et al., 2017; Yang et al., 2017). Therefore, the porous carbon materials such as GO & CNT aerogels and graphene foams have been developed for highly efficient solar steam generation (Fu et al., 2018; Jiang et al., 2018; Sajadi et al., 2016; Yang et al., 2018). Limited to the high cost or complicated preparation of the novel carbon materials such as gmraphene oxide and CNT, developing low cost, feasible operability and stability carbon materials is considerably important for practical application. Wood has a great potential for highly efficient solar steam generation owing to good hydrophilicity, low thermal conductivity and plenty of natural microchannels (Shen et al., 2016; Wang et al., 2017; Zhu et al., 2016). Recently, Zhou et al (Xue et al., 2017) reported a woodbased solar steam generator prepared by a simple flame treatment on natural wood surface, which performed a higher efficiency over most generators reported so far (Liu, G. et al., 2017). At the same time, Hu et al (Jia et al., 2017) proposed that wood-based solar steam generator with optimal thickness would perform the highest efficiency owing to the negligible conduction loss. However, the determination of optimal

Corresponding authors at: State Key Laboratory of Clean Energy, Department of Energy Engineering, Zhejiang University, Hangzhou 310027, PR China (S. Cheng). Institute for Advanced Ceramics, State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology, Harbin 150001, PR China (B. Li). E-mail addresses: [email protected] (S. Cheng), [email protected] (B. Li). ⁎

https://doi.org/10.1016/j.solener.2019.09.080 Received 5 January 2019; Received in revised form 27 August 2019; Accepted 25 September 2019 0038-092X/ © 2019 International Solar Energy Society. Published by Elsevier Ltd. All rights reserved.

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Nomenclature CWSG RWSG

carbonized wood based solar steam generator raw wood based solar steam generator thermal efficiency (%) maximum mass-transfer efficiency (%) efficiency (%) water flow rate (kg m−2 h−1) evaporation rate measured by experiments (kg m−2 h−1) the final temperature of the surface of CWSG under 1 sun (K) the initial temperature of the surroundings (K) convection heat transfer coefficient (W m−2 K−1) Stefan-Boltzmann constant (5.67 × 10−8 W m−2 K−4) the surface emissivity of the CWSG (%) total enthalpy for water evaporation (MJ kg−1) the thickness of CWSG (mm)

1

2

mf m T1

T0 heff σ hlv h

k water qwater qsolar R S λ ρ d

u

thickness was so complicated and further work was needed to determine the optimal thickness of the generator. Here we investigated the relationship between the thickness and the efficiency of the generator by using thermal calculation, taking carbonized wood based solar steam generator (CWSG) as an example. The experiment on various thickness of CWSG was also conducted for comparing with calculated results. The optimal thickness of CWSG (22 mm) was determined by both experimental and calculated results. CWSG for water or wastewater treatment was also studied. It could be inferred that CWSG with an evaporation area about 100 m2 can produce 887 kg of condensed water under 1 sun for 8 h, which means 12 persons’ daily water consumption. This study provides a new strategy by reducing the heat loss based on thermal calculation to enhance the performance of solar steam generator for wastewater treatment.

1

2

heff (T1 qsolar

T0 )

qwater qsolar

) × 100%

(T1 h kwood

h0 kwater

T0 )

(T1 (k

h

wood

+

T0) h0 kwater

) qsolar

) × 100%

mf hlv qsolar

× 100%

(4)

d 4 uh

(5)

Therefore, the maximum mass-transfer efficiency of CWSG ( 2 ) was calculated by Eq. (6) based on Eqs. (4) and (5). 2

=

d hlv 26.62 hlv · × 100% = · × 100% 4 uh qsolar h qsolar

(6)

Finally, the efficiency of CWSG should be the minimum between the thermal efficiency ( 1) and maximum mass-transfer efficiency ( 2 ). The optimal thickness of CWSG was obtained when CWSG exhibited maximum efficiency. 2.2. Preparation of CWSG

(1)

CWSG was prepared by 2 steps here. Firstly, the cylindrical wood blocks with a diameter of approximately 4 cm and a height varying from 6 mm to 30 mm were cut along the vertical direction to wood growth by a sweep. The as-obtained wood block was used as raw woodbased solar steam generator (RWSG) directly. Secondly, CWSG was prepared by carbonizing the surface of as-obtained wood blocks on a 600 °C hot plate for a certain time and then being immersed in cold water immediately. Different times for carbonizing wood formed different thickness of carbon layer in CWSG (estimated according to an empirical value of 2 mm carbon layer per 25 s).

T0) +

=

mf =

qwater was calculated by Eq. (2) (Li, H.R. et al., 2018) where h was the thickness of CWSG (mm), k wood was the thermal conductivity of CWSG (W m−1 K−1), h 0 was the distance between the bottom of CWSG and the bottom of the container (mm), k water was the thermal conductivity of water (W m−1 K−1). qwater =

qsolar

The water flow rate (mf ) of CWSG with different thickness was calculated by Eq. (5) (Jia et al., 2017; Zhu et al., 2017) where ρ was the density of the fluid (1000 kg m−3), was the surface tension (72.8 mN m−1), was the wood porosity (70%), d was the average diameter of the microchannels in the CWSG (22 μm), was the tortuosity of the microchannels in the CWSG (%), u was the viscosity of the fluid (0.8937 mPa s), h was the thickness of the CWSG (mm).

Considering that efficiency of CWSG was related to the thickness (Jia et al., 2017), the optimal thickness of CWSG was obtained when CWSG exhibited maximum efficiency. The efficiency of CWSG was determined by the thermal efficiency ( 1) and maximum mass-transfer efficiency ( 2 ), together (Ghasemi et al., 2014; Tao et al., 2018). The thermal efficiency of CWSG ( 1) was calculated based on Eq. (1) (Yang, Y. et al., 2018) where was the optical absorption across the full solar spectrum of CWSG (%), qsolar was the solar irradiation (1000 W m−2), was surface emissivity of the CWSG (%), σ was the Stefan-Boltzmann constant (5.67 × 10−8 W m−2 K−4), and heff was the convection heat transfer coefficient (about 5 W m−2 K−1) (Ni et al., 2016), T1 was the final temperature of the surface of CWSG after irradiation (K), and T0 was initial temperature of the surroundings (K), qwater was the conduction heat loss of CWSG (W m−2).

(T14 T04 ) qsolar

heff (T1

The mass-transfer efficiency of CWSG ( 2 ) was calculated based on Eq. (4) (Yang, Y. et al., 2018) where mf was water flow rate (kg m−2 h−1), hlv was total enthalpy for water evaporation (MJ kg−1), qsolar was solar irradiation (W m−2)

2.1. Calculation for the optimal thickness of CWSG

=(

(T14 T04 ) qsolar

=(

(3)

2. Methods

1

the thermal conductivity of CWSG (W m−1 K−1) the distance between the bottom of CWSG and the container (mm) the thermal conductivity of water (W m−1 K−1) the conduction heat loss of CWSG (W m−2) solar irradiation (W m−2) optical absorption (%) the reflectance of the surface of CWSG (%) solar spectral irradiance (W m−2 nm−1) wavelength (nm) the density of the fluid (kg m−3) the surface tension (mN m−1) wood porosity (%) the average diameter of the microchannels in the CWSG (μm) the tortuosity of the microchannels in the CWSG (%) the viscosity of the fluid (mPa s)

k wood h0

(2)

Based on Eq. (2), Eq. (1) was simplified as Eq. (3): 435

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concentration of MB solution (g L−1).

2.3. Solar steam generation experiments

Dye removal = Ct /C0

An evaporation measurement system (Fig. 1) mainly consisting of 300 W Xenon lamp (PLS-SXE300/300UV), an IR-camera (Fluke TiS65, US) and a precision balance (Mettler Toledo, MR420) was used to compare the performance of different solar steam generator (Li, H.R. et al., 2018). Firstly, water steam generators (CWSG and RWSG) were placed into the water container before solar irradiation. Xenon lamp was positioned 10 cm away from the surface of floated generators and the light intensity of Xenon lamp was measured by a light intensity meter (Spectronics-3000 UV-AB). The temperature distribution of CWSG and RWSG was recorded by IR-camera under solar irradiation. Precision balance was used for recording the mass change of water container to calculate the evaporation rate. The efficiency of different solar vapor generator was calculated by Eq. (7) (Hong et al., 2018; Liu, K.K. et al., 2017) where m was evaporation rate calculated from the straight-line slope of the mass change curve (kg m−2 h−1), hlv was total enthalpy for water evaporation at standard atmospheric pressure (MJ kg−1), qsolar was solar irradiation, ranged from 1 sun (1 kW m−2) to 5 sun (5 kW m−2). Notably, the same temperature of the vapor generator as the water and the surroundings was kept before solar vapor generation experiments.

=

m ·hlv × 100% qsolar

(8)

The artificial seawater with 8.9 g L−1 of NaCl, 4.1 g L−1 of KCl, 7.26 g L−1 of MgCl2, and 3.3 g L−1 of CaCl2 was used as the seawater for solar desalination experiments (Yang et al., 2018). The concentrations of Na+, K+, Ca2+ and Mg2+ in seawater or condensed water were measured by a plasma atomic emission spectrometry (ICP-AES, PerkinElmer Optima 8300, USA). The cycle experiments of CWSG were conducted in 0.5 g L−1 MB solution, 5 g L−1 MB solution and seawater each for 20 cycles, respectively. In every cycle, CWSG was illuminated for 8 h. Before the next cycle, CWSG without washing was dried at 80 °C for 12 h. 2.5. Analysis The microchannels of CWSG were characterized by scanning electron microscopy (SEM, HELIOS Nano Lab 600i). The contact angle between CWSG and water was measured by a standard G10 contact angle analyzer (Krϋss Optronic GmbH, Germany). Optical absorption spectrum of CWSG and RWSG was conducted on UV-3600 spectrometer (Shimadzu, Japan) over a wavelength ranging from 300 to 2500 nm. The absorption of CWSG was calculated by the Eq. (9) (Ye et al., 2017):

(7)

=

(1

R)· S·d S ·d

(9)

where was the solar absorption (%), R was the reflectance of the surface of CWSG (%), S was solar spectral irradiance (W m−2 nm−1), and λ was the wavelength (nm). Thermal conductivity of CWSG was measured by thermal analyzer (Hot disk TPS 2500S, Sweden) at room temperature (25 °C).

2.4. CWSG for dye removal and solar desalination A homemade device was used for dye removal and solar desalination experiments (Fig. 2). The water steam generated rapidly via CWSG under solar irradiation, and then the water steam condensed on the surface of the device. The condensed water gathered by water collector was clean water, which was saved for further experiment after every experiment. Here, 0.5 g L−1 and 5 g L−1 of methylene blue (MB) solution were used as typical dye for dye removal experiments. The absorption peak at about 665 nm in UV–vis absorption spectra recorded by UV–vis spectrophotometer (UV-2600, China) was used for measuring the concentration of MB in condensed water and MB solution. The dye removal of CWSG was calculated according to Eq. (8) where Ct is the concentration of MB in condensed water (g L−1), C0 is the initial

3. Results and disscussions 3.1. Thermal mechanism analysis As Fig. 3a shown, there were plenty of hydrophilic channels for steam escaping in solar evaporation process (Liu, K.K. et al., 2017). CWSG performed the high absorption (97%) in the range from 500 nm to 2500 nm (Fig. 3b). This high optical absorption probably resulted in

Fig. 1. The schematic diagram of system for solar steam generation experiments. 436

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Fig. 2. (a) The schematic diagram and (b) photography of the homemade device for dye removal and solar desalination. Inset: The sample of the CWSG.

Fig. 3. (a) SEM image of cross-sectional views of CWSG. There were many long, well-defined and hydrophilic channels. Inset: The contact angle (17.1°) between water and the CWSG (b) Absorption spectra of CWSG and RWSG in the range from 500 nm to 2500 nm (c) IR thermal images of CWSG and the surroundings under 1 sun, showing the surface maximum temperature of CWSG about 46 °C and an average temperature of CWSG about 40 °C (d) Image of CWSG under 1 sun (e) Mass change of RWSG in dark or under 1 sun, and CWSG under 1 sun.

437

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the temperature of CWSG rising rapidly under irradiation. Considering the pretty low thermal conductivity of CWSG (0.184 W m−1 K−1), the thermal energy can’t transfer from CWSG to water in time, which resulted in water on the surface of CWSG evaporating rapidly under solar irradiation (Ni et al., 2016; Tao et al., 2018). As evidenced in Fig. 3c, the temperature of the surface of CWSG increased to 46 °C while the surroundings and bulk water maintained a low temperature about 26.1 °C under 1 sun. The vapor generated from CWSG rapidly under 1 sun, which can be observed by eyes clearly (Fig. 3d). To demonstrate the superior structure of CWSG, the performance of RWSG in dark or under 1sun, and CWSG under 1 sun was compared in Fig. 3e. For RWSG, the water mass change was only 0.11 kg m−2 in dark in 60 min while the water mass change increased to 0.42 kg m−2 under 1 sun. By contrast, CWSG showed a high water-mass change of 1.21 kg m−2, which was 1.9 times higher than that obtained by RWSG under 1 sun.

effect on the efficiency of CWSG comparing to the heat transfer. The variation on the efficiency of the CWSG can also be explained by the effect of the heat transfer and water flow. Firstly, the water rose along the channels in floated CWSG to the surface under capillary action. The high absorption of CWSG resulted in higher temperature (about 42 °C) of the surface than the surroundings (25 °C) under 1 sun. Limited to the low thermal conductivity of CWSG (0.184 W m−1 K−1), the heat energy can’t transfer from CWSG to the bulk water in time. The water existing in the surface of CWSG evaporated rapidly by the high temperature under 1 sun. Therefore, the thermal efficiency ( 1) related to heat loss and maximum mass-transfer efficiency ( 2 ) calculated by water flow rate, both had a huge effect on the efficiency of CWSG. With the thickness of CWSG increasing from 0 mm to 60 mm, the thermal efficiency increased from 66.8% to 78.7% while the maximum masstransfer efficiency decreased from infinity to 27.8%. The thickness of CWSG varying from 0 mm to 60 mm changed the value of the maximum mass-transfer efficiency and thermal efficiency, which leaded to the efficiency of CWSG varying. CWSG performed a maximum efficiency (76.1%) with the thickness about 22 mm under 1 sun. In addition, the cost of CWSG is usually low. In this study, the cost for producing carbon layer with the area of 1 m2 and the thickness of 4 mm was around $ 0.12, while the cost of wood was around $ 0.078 per 1 m2 area and 1 mm thickness (estimated according to the purchase price in China). As the thickness increased from 5 mm to 30 mm, the cost of CWSG increased from $0.51 to $2.43 per square meter. Although the cost of CWSG always increased with the thickness, the cost of CWSG was relatively low. The CWSG costs only $ 1.84 per square meter at the optimization thickness of about 22 mm, which is highly faceable for practical application. To make it clearly, CWSG mentioned below was CWSG with maximum efficiency unless special statements.

3.2. The optimal thickness of CWSG Considering that efficiency of CWSG was related to the thickness (Jia et al., 2017), it is crucial to determine the relationship between the thickness and the efficiency of CWSG, and calculate optimal thickness for the maximum efficiency of CWSG. The efficiency and optimal thickness of CWSG were determined by the thermal efficiency ( 1) and maximum mass-transfer efficiency ( 2 ), together. The thermal efficiency of CWSG ( 1) was calculated by Eq. (3). Based on the results above where was calculated to be 97%, qsolar was 1000 W m−2, was about 0.85, σ was 5.67 × 10−8 W m−2 K−4, and heff was 5 W m−2 K−1 (Li, X. et al., 2018; Ni et al., 2016), T1 and T0 were measured to be 313 K and 298 K, respectively, h was the thickness of CWSG (mm), k wood was measured to be 0.184 W m−1 K−1, h 0 was the distance between the bottom of CWSG and the bottom of the container (60 mm), k water was measured to be 0.617 W m−1 K−1, Eq. (3) can be simplified further to be Eq. (10): 1

= (0.8225

15 h

( 0.184 + 97.2447)

) × 100%

3.3. The performance of CWSG The performance of CWSG under different solar irradiation was studied. As Fig. 5a shown, the evaporation rate of CWSG was measured to be 1.21 kg m−2 h−1 under 1 sun, ca 2.8 times that of pure water (0.43 kg m−2 h−1). As the solar irradiation increased from 1 sun to 3 sun, the evaporation rate of CWSG increased by 2.31 times from 1.21 kg m−2 h−1 to 4.01 kg m−2 h−1. As the solar irradiation further increased from 3 sun to 5 sun, the evaporation rate of CWSG increased

(10)

The maximum mass-transfer efficiency ( 2 ) was calculated by Eq. (6). Similarly, Eq. (6) can be simplified further to be Eq. (11): 2

=

d hlv 16.69 · × 100% = × 100% 4 uh qsolar h

(11)

The minimum between the 1and 2 was the efficiency of CWSG (Fig. 4). As Fig. 4 shown, the efficiency of CWSG increased from 66.8% to 75.9% with the thickness of CWSG increasing from 0 mm to 22 mm while the efficiency of CWSG decreased from 75.9% to 27.8% with the thickness of CWSG further increasing to 60 mm. To further verify the relationship above between the thickness and the efficiency of CWSG, the performance of CWSG with different thickness under 1 sun was studied. As the thickness of CWSG increased from 6 mm to 22 mm, the efficiency of CWSG under 1 sun increased from 68.1% to 76.1%, ca 2.5 times to 2.8 times that of pure water (27.1%). As the thickness of CWSG increased further to 50 mm, the efficiency of CWSG decreased to 21.1%, even lower than that of pure water (27.1%). These experimental results revealed that the maximum efficiency of CWSG about 76.1% was achieved under 1 sun when the thickness of CWSG was 22 mm, which was consistent with the calculated results within the margin of error. It should be noted that here only one parameter of optimization has been conducted for improvement. Future works should focus on multiparameter optimization to further increase the efficiency of solar steam generator. Furthermore, as the thickness increased from 5 mm to 22 mm, the efficiency increased 0.35% per mm of thickness (from 70.2% to 76.1%), while the efficiency decreased 1.27% per mm of thickness (from 76.1% to 27.9%) when the generator thickness increased from 22 mm to 60 mm. These results might indicate that mass transfer had much great

Fig. 4. The relationship between the thickness and the efficiency of CWSG. The red line in Fig. 4 was the maximum efficiency of CWSG calculated based on the thermal efficiency (green line) and the maximum mass-transfer efficiency (cyan line). The blue plots in Fig. 4 were the efficiency of CWSG measured in experiment. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) 438

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Fig. 5. Performance of CWSG under different solar irradiation. (a) The evaporation rate (ER, blue) and efficiency (red) under 1 sun, 3 sun and 5 sun. (b) The temperature of water and CWSG under 1 sun, 3 sun and 5 sun. The original temperature of CWSG and water were both 25 °C before irradiation. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Although the high efficiency was achieved by CWSG under solar irradiation, the higher temperature of CWSG than the surroundings brought about the inevitable convection loss and radiation loss. To enhance the performance of solar steam generator, the novel thermal structure with less convection loss and radiation loss would be designed in future. The new physical and thermal mechanisms should be developed, too.

Table 1 The performance of wood-based solar steam generator in literatures and our work. Materials

Power density

Efficiency (%)

Ref

Bilayer wood F-Wood/CNTs F-wood CWSG

1 kW m−2 1 kW m−2 1 kW m−2 1 kW m−2

57.3 65 72 76.1

Zhu et al. (2017) Chen et al. (2017) Xue et al. (2017) Our work

3.4. Solar desalination and dye removal

by 71.8% from 4.01 kg m−2 h−1 to 6.89 kg m−2 h−1. Similarly, the efficiency of CWSG remarkably increased from 76.1% to 84.4% with the solar irradiation increasing from 1 sun to 3 sun while the efficiency of CWSG slightly increased from 84.4% to 87.7% as the solar irradiation increased from 3 sun to 5 sun. As recorded in Fig. 5b, the average temperature of the heated surface of CWSG under 1 sun increasing from 25 °C to 40 °C while the average temperature of water under 1 sun only increased 1.3 °C from 25 °C to 26.3 °C. With the solar irradiation increasing further, the temperature difference between the heated surface of CWSG and the water increased significantly. The heated surface temperature of CWSG increased to 67 °C and 79 °C under 3 sun and 5 sun, respectively while the temperature of water only increased to 29.2 °C and 34.3 °C under 3 sun and 5 sun, respectively. These results revealed that CWSG can localize the solar energy absorbed by CWSG on the surface effectively and minimize heat loss from CWSG to the water, which resulted in the efficiency of CWSG at least 1.3 times that of the other solar steam generators reported previously (Table 1).

CWSG was used for solar desalination and dye removal experiments under 1 sun. As Fig. 6a shown, the evaporation rates of CWSG in seawater were 1.18 kg m−2 h−1 under 1 sun and 6.42 kg m−2 h−1 under 5 sun, respectively, which were only 2.5% and 6.8% lower than those in water. In 20 cycles, CWSG exhibited a stable evaporation rate about 1.18 ± 0.05 kg m−2 h−1 under 1 sun while about 6.42 ± 0.09 kg m−2 h−1 under 5 sun, which implied the outstanding cycling stability of CWSG in seawater. The concentrations of Na+, K+, Mg2+, Ca2+ in condensed water decreased 6 orders magnitude than those of seawater (Table 2), which were much lower than those of the limitation of World Health Organization (WHO) and United States Environmental Protection Agency (USEPA) for drinking water (Li et al., 2017). As Fig. 6b shown, the evaporation rates of CWSG in 0.5 g L−1 MB solution and 5 g L−1 were 1.16 kg m−2 h−1 and 1.11 kg m−2 h−1 under 1 sun, respectively, which were 4.1% and 8.3% respectively, lower than those in water (1.21 kg m−2 h−1) under 1 sun. In 20 cycles, CWSG

Fig. 6. Evaporation rate (ER) of CWSG (a) in seawater under 1 or 5 sun (b) in 0.5 g L−1 MB solution or 5.0 g L−1 MB solution under 1 sun. 439

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Table 2 The concentration of Na+, K+, Mg2+, Ca2+ in seawater, condensed water collected under 1 sun and under 5 sun.

Seawater Condensed water (under 1 sun) Condensed water (under 5 sun)

Na+

K+

Mg2+

Ca2+

8900 mg L−1 0.233 mg L−1

410 mg L−1 0.217 mg L−1

726 mg L−1 0.171 mg L−1

336 mg L−1 0.523 mg L−1

0.177 mg L−1

0.089 mg L−1

0.271 mg L−1

0.273 mg L−1

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Table 3 MB removal of CWSG for different MB solution under 1 sun.

1st cycle 5th cycle 10th cycle 15th cycle 20th cycle

0.5 g L−1 MB

5.0 g L−1 MB

100 100 100 100 100

100 100 100 100 100

performed a stable evaporation rate about 1.16 ± 0.08 kg m−2 h−1 in 0.5 g L−1 MB solution while 1.11 ± 0.03 kg m−2 h−1 in 5.0 g L−1 MB solution under 1 sun. As Table 3 shown, the MB removals of CWSG were all 100%, implying MB can’t be detectable in condensed water. These results indicated CWSG has a great potential for solar desalination and dye removal. Based on the results mentioned above, the minimum evaporation rate of CWSG about 1.11 kg m−2 h−1 was obtained in seawater and dye. Therefore, 888 kg of condensed water can be collected by CWSG with the evaporation area about 100 m2 under 1 sun for 8 h, which means 12 persons’ daily water consumption (Zhang et al., 2018). Considering the recyclability of CWSG for practical application, the performance of CWSG in seawater with high salinity would be studied in the future. And further study of CWSG for industry dyeing wastewater treatment was also needed. 4. Conclusion The carbonized wood based solar steam generator (CWSG) with high efficiency for seawater desalination and dye removal was successfully developed. The optimal thickness of CWSG was determined by calculations and confirmed by solar evaporation experiments. CWSG with optimal thickness about 22 mm performed the highest evaporation rate of 6.89 kg m−2 h−1 with solar conversion efficiency of 86.4% under 5 sun. CWSG showed the stable performance for dye removal and seawater desalination even after 20 cycles, indicating that our work provides a new strategy to design highly efficient solar steam generator for seawater desalination and dye removal. Acknowledgments This work was supported by National Science Foundation of China (No. 51778562, No. 51478414 No. 51972086 and No. 51621091), State Key Laboratory of Urban Water Resource and Environment of Harbin Institute of Technology, China (2019TS01), and Scientific Research Fund of Zhejiang Provincial Education Department. References Chen, C., Kuang, Y., Hu, L., 2019. Challenges and opportunities for solar evaporation. Joule 3 (3), 683–718. Chen, C.J., Li, Y.J., Song, J.W., Yang, Z., Kuang, Y., Hitz, E., Jia, C., Gong, A., Jiang, F., Zhu, J.Y., Yang, B., Xie, J., Hu, L.B., 2017. Highly flexible and efficient solar steam generation device. Adv. Mater. 29 (30), 1701756. Chen, M., He, Y., Wang, X., Hu, Y., 2018. Numerically investigating the optical properties of plasmonic metallic nanoparticles for effective solar absorption and heating. Sol. Energy 161, 17–24. El-Agouz, S.A., Abd El-Aziz, G.B., Awad, A.M., 2014. Solar desalination system using

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