Abstract
Wood-based solar steam generation holds great promise in alleviating fresh water crises due to its advantages: light absorbability, thermal management, water transpiration, and water evaporation. Although tremendous efforts have been made to improve wood-based solar steam generation devices, they mainly have focused on the optimization of photothermal materials to optimize light absorbability and thermal management to enhance efficiency of steam generation. This research demonstrates that delignified wood (DL-wood) can further improve the efficiency of steam generation via increasing both the water transportation and water evaporation. The results show that after placing carbon nanotubes (CNTs) on DL-wood, the efficiency of steam production is higher than that of natural wood coated with CNTs by 20% under ambient sunlight conditions. DL-wood with CNTs has the following advantages: (1) it is stable, available, and easy to extend; (2) it does not pollute the environment and will not cause discoloration or dregs when used; and (3) it is a promising efficiency-enhancing solution for renewable and portable solar power generation.
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Delignified Wood-based Highly Efficient Solar Steam Generation Device via Promoting Both Water Transportation and Evaporation
Yuming He,a Huayang Li,a Xuelian Guo,b and Rongbo Zheng a,*
Wood-based solar steam generation holds great promise in alleviating fresh water crises due to its advantages: light absorbability, thermal management, water transpiration, and water evaporation. Although tremendous efforts have been made to improve wood-based solar steam generation devices, they mainly have focused on the optimization of photothermal materials to optimize light absorbability and thermal management to enhance efficiency of steam generation. This research demonstrates that delignified wood (DL-wood) can further improve the efficiency of steam generation via increasing both the water transportation and water evaporation. The results show that after placing carbon nanotubes (CNTs) on DL-wood, the efficiency of steam production is higher than that of natural wood coated with CNTs by 20% under ambient sunlight conditions. DL-wood with CNTs has the following advantages: (1) it is stable, available, and easy to extend; (2) it does not pollute the environment and will not cause discoloration or dregs when used; and (3) it is a promising efficiency-enhancing solution for renewable and portable solar power generation.
Keywords: Solar steam generation; Fast water transportation; Fast water evaporation; Delignified wood
Contact information: a: Yunnan Province Key Laboratory of Wood Adhesives and Glued Products, College of Chemical Engineering, Southwest Forestry University, Kunming 650224, P. R. China; b: Wetland College, Southwest Forestry University, Kunming 650224, P. R. China;
* Corresponding author: zhengrbzy@hotmail.com
INTRODUCTION
Solar steam generation is one of the most promising, sustainable techniques for desalination and water purification, and it has attracted great attention (Ghasemi et al. 2014; Gamelas and Ferraz 2015; Ito et al. 2015). There are four key factors that affect the efficiency of solar generation devices: sunlight absorbability, thermal management, water transportation, and water evaporation (Jin et al. 2016; Ni et al. 2016; Wang et al. 2016a). Various structural materials have been designed and developed for this application (Zielinski et al. 2016), such as plasmonic metal particles for localizing heat generation (Wang et al. 2016b; Zhou et al. 2016a,b), bilayered forms with a black surface for efficient light absorption and thermal management (Li et al.2016; Shi et al. 2017), and carbonaceous materials with broadband sunlight absorptivity and hydrophilic surface for water transpiration (Ghasemi et al. 2014). More recently, inspired by the water transpiration behavior of trees, the use of carbon nanotube (CNT)-modified horizontally cut wood (Chen et al. 2017; Wang, et al. 2017), graphite-coated vertically cut wood (Li et al. 2018), grapheme oxide-coated wood (Liu et al. 2017b), surface carbonization, vertically cut wood (Liu et al. 2017a; Xue et al. 2017; Zhu et al. 2017), and Chinese ink-coated wood (Yang et al. 2018) have been demonstrated as efficient solar steam generation devices (Wright et al. 2013). This is due to the systematic optimizations of light absorbability (Jiang et al.2016), thermal management, and water transpiration enabled by the unique structure consisting of a carbonaceous surface as a light absorber, insulating the wood matrix as a thermal blocker and hierarchical micro/nanochannels as a water transportation pathway.
Although the above-mentioned wood-based solar steam generation devices have been demonstrated to greatly enhance the efficiency of solar steam generation, they mainly focused on various photothermal materials such as CNT, graphite, graphene oxide, surface carbonization, and Chinese ink combined with hierarchical porous structures to improve both the sunlight absorbability and the thermal management. There are few reports on enhancing the efficiency of solar steam generation devices via improving both the water transportation ability and water evaporation of wood-based devices (Chen et al. 2017). Wood cell walls are primarily composed of fiber tracheids and vessels with lumen diameters ranging from 10 to 100 m, which act as pathways for rapid water transport (Zhong et al. 2018a). In wood cell walls, cellulose microfibrils are embedded within lignocellulosic matrices, in which the lignin reinforces the cell walls to improve the mechanical properties and also reduces the permeability of water.
The objective of the current study was to test the hypothesis that the efficiency of solar steam generation devices can be improved by removing lignin from the wood. After the removal of hydrophobic lignin, the delignified wood (DL-wood) not only preserved the original hierarchical structure of wood, but also increased hydrophilicity and possessed more micro/nano pores in the cell walls and cell wall corners, which promotes both water transportation and evaporation from the bottom to the top surface of wood. When the sunlight illustrates the CNTs-coated DL-wood, the black surface will absorb the sunlight and generate localized heat at the water-air interface to evaporate via increasing the local temperature (Zhong and Huang 2019). With a sufficient supply of bottom water to the CNTs coated top wood surface (Zheng et al. 2016; Zhong et al. 2018b) and fast water evaporation, the efficiency of CNTs modified DL-wood device might increase compared with that of CNTs modified natural wood device under ambient sunlight illumination.
This study reports the development of CNTs-modified DL-wood solar steam generation devices by replacing the natural wood with DL-wood, which is obtained by H2O2 steam delignification. DL-wood is used as a substrate to be modified with CNTs by cutting along the direction vertical to the growth direction of trees. The water transportation and evaporation rates were characterized by measuring the transport distance of dye aqueous solutions and the mass change of water after illuminating to ambient sunlight. Compared with CNTs modified natural wood device, the efficiency of CNT modified DL-wood devices increased from 60% to 73% in this study.
EXPERIMENTAL
Materials and Chemicals
Basswood slices with a diameter of 4 cm and a thickness of 0.5 cm were used in this study. These samples have the original channels in wood aligned perpendicular to the wood plane. DL-wood was obtained by radially cutting the wood trunk, also called Ra-wood. The chemicals used in removing lignin from wood were hydrogen peroxide (30% solution, Sino pharm, Beijing, China) and CNTs (Sino pharm). The solvents used were ethanol alcohol (Sino pharm) and deionized water.
Delignification of the Wood
The precut wood slices were steam delignified by being placed on grids with the unit size of 5 mm x 5 mm, which was placed about 2 cm above an H2O2 boiling aqueous solution (30 wt%) (Li et al. 2019). When the yellow color of the sample disappeared (2 to 4 h), the samples were removed and rinsed with cold water and ethanol. The DL-wood samples were stored in ethanol.
Synthesis of DL-wood/CNTs
Commercial CNTs powder was dispersed in acetone to make the CNTs solution. These wood samples were immersed in the solution of CNTs, then taken out and dried in air. This process was repeated several times to coat CNTs to the amount of about 0.4 wt% of wood with different treatments, resulting in the final product of CNTs-coated wood (Zheng et al. 2015; Chen et al. 2017).
Measurements and Characterizations
The morphologies of the samples were characterized by a scanning electron microscope (SEM, TESCAN VEGA3, Brno, Czech Republic). The lignin contents were measured according to TAPPI T222 om-11 (2011). A universal material machine (ProLine z020tn, Zwick, Ulm, Germany) was used to measure the mechanical properties.
RESULTS AND DISCUSSION
Lignin Content and Properties
The CNTs modified DL-wood solar steam generation device is shown in Fig. 1. First, DL-wood was prepared by delignification of natural basswood via H2O2 steam. CNTs were coated on the surface of DL-wood. Finally, the device was put on the surface of water in a glass beaker to generate steam under ambient sunlight.
As shown in Fig. 2a, the lignin content of basswood can be decreased from 22.5% (natural wood) to 12.3% (half DL-wood), to 1.01% (DL-wood) when the H2O2 steam was prolonged from 2 h to 4 h, respectively. Notably, the delignified wood with 1.01% lignin content was still strong enough to be exploited in the solar steam generation device since the mechanical strength of the DL-wood in wet state and dry state is 0.42 MPa and 4.5 MPa, respectively. The microstructures of the natural and DL-wood were characterized by scanning electron microscopy (SEM). Figure 2c to 2f show the SEM images of the basswood before and after delignification. There are massive lumina with the diameter in the range from 10 μm to 50 μm (Ito et al. 2015). High-magnification SEM images reveal that the natural wood possesses dense, smooth cell wall corners and the middle lamella. Compared with natural wood, massive microscale pores were generated in the cell walls and cell wall corners after steam delignification, which promotes the water transportation and evaporation to the top surface (Fig. 2c and 2d).
To investigate the water transportation capacity, a water transportation experiment was designed. As shown in Fig. 3, 5 mm thick of natural wood and DL-wood were put into dye solution to observe the distance dye traveled after a certain time. As shown in Figs. 3c and 3d, the dye in the DL-wood were always transformed upwards by a greater distance, which indicates that the DL-wood has better water transportation capabilities than natural wood.
Fig. 1. (a) The preparation process of DL-wood/CNTs. (b) A schematic drawing of an DL-wood/CNTs device that absorbs sunlight and heat to generate water vapor on the surface of the wood and approximate working drawing of a vessel channel, after the water molecules reach the top, the water molecules release rapidly through the local heat provided by the CNTs
To verify the superiority of DL-wood compared with natural wood in the evaporation of water, water-saturated DL-wood and natural wood were placed under 1 sun conditions for self-evaporation. Microholes and smoother vessel channels were generated via the removal of hydrophobic lignin, leading to faster evaporation of water under sunlight conditions than natural wood. DL-wood and natural wood were subjected to water self-volatization for 8 cycles under sunny (condition 1) conditions. The water evaporation of DL-wood was 0.261 kg m-2 h-1, 0.245 kg m-2 h-1, 0.276 kg m-2 h-1, 0.267 kg m-2 h-1, 0.259 kg m-2 h-1, 0.268 kg m-2 h-1, 0.250 kg m-2 h-1, and 0.254 kg m-2 h-1, while that of natural wood is 0.228 kg m-2 h-1, 0.220 kg m-2 h-1, 0.234 kg m-2 h-1, 0.231 kg m-2 h-1, 0.225 kg m-2 h-1, 0.234 kg m-2 h-1, 0.225 kg m-2 h-1, 0.223 kg m-2 h-1, respectively. As shown in Fig. 4b, the enhancement factor of evaporation of DL-wood vs. natural wood is 12% to 18%.
Under ambient sunlight (light intensity of 0.98 kW m-2, ambient temperature of 30 C) for 10 min, the water loss for the blank water control group, the natural wood/CNTs group, the half DL-wood/CNTs group, and the DL-wood/CNTs group were 0.08 kg/m2, 0.14 kg/m2, 0.17 kg/m2, and 0.19 kg/m2, respectively. As illustrated in Fig. 4c, after 1 h, the water loss of the blank water control group, the natural wood/CNTs group, the half DL-wood/CNTs group, and the DL-wood/CNTs group were 0.79 kg/m2, 1.24 kg/m2, 1.31 kg/m2, and 1.40 kg/m2, respectively.
Fig. 2. (a) Lignin content of natural wood, half DL-wood and DL-wood. (b) Experimental stress-strain curves for the DL-wood in wet state. SEM images of (c, e) natural wood and (d, f) DL-wood.
The water evaporation caused by the wind factor was subtracted since the outdoor experiment was influenced by the wind. The steam generation rate of the last four control groups was summarized as: 0.48 kg m-2 h-1, 0.94 kg m-2 h-1, 1.01 kg m-2 h-1, and 1.12 kg m-2 h-1. The enhancement factor of evaporation in the last three groups was 1.9 times, 2.1 times, and 2.3 times of that of the blank water group, shown in Fig. 4d.
Fig. 3. (a) The experimental sketch and (b) brief comment of Methylene blue solution transport in DL- and natural wood. (c) The solution transport distances between DL- and natural wood for 0 min, 2 min, and 5 min. (d) The average solution transport speed between DL- and natural wood at the end of the 5th min.
The amount of steam production of DL-wood/CNTs was higher than that of natural wood/CNTs by 20% under the same ambient sunlight. It can be seen that under the daytime sun illumination (1 sun), DL-wood/CNTs exhibited good performance for solar steam generation.
Thus, DL-wood/CNTs generated more steam than natural wood/CNTs, which greatly improves the efficiency of solar energy utilization. Under normal sunlight illumination (1 sun), the solar energy efficiency of DL-wood/CNTs devices reached 73%, which was calculated using the following equation (Chen et al. 2017),
η = mhLV/Copt P0 (1)
where m (kg m-2 h-1) denotes the evaporation rate, Copt is the optical concentration, and hLV is the total enthalpy of the liquid-vapor phase transition including the sensible heat (Chen et al. 2017). Compared with the previously reported solar energy utilization rate of 65% under sunlight (1 sun), DL-wood/CNTs increased the efficiency by 8%. This provides a potential, alternative device either steam power production or transmission, and it also opens up new paths for global energy issues that need to be solved. It should be noted that when the light intensity continuously increased from 1 to 10 suns, the steam generation efficiency gradually improved, and the solar energy utilization efficiency gradually improved (Chen et al. 2017).
Fig. 4. (a) The volatilization rate (V.R.) of the material under normal daylight (1 sun); (b) the enhancement factor of evaporation of DL-wood vs natural wood; (c) the steam generation efficiency; and (d) the enhancement factor of DL-wood, half DL-wood, natural wood, and blank water control group.