PAN Qi, LI Jing, YAN Liangguo

(School of Water Conservancy and Environment, University of Jinan, Jinan 250022, China)

Abstract

Significan As the best inexhaustible, renewable and eco-friendly energy source of solar and the severe challenge of water resources shortage, fully utilizing the abundant solar energy and seawater to supplement the world’ s freshwater resources has become an inevitable development trend. Solar-driven interfacial evaporation has attracted enormous scientific and technical interest. It can make full use of sunlight to obtain clean and fresh water, which is a promising alternative to traditional desalination.A solar-driven interfacial evaporation system usually comprises photothermal material, substrate, water body, incident sunlight, vapor condensation and collection device, among which the evaporator composed of the photothermal material and substrate is the most critical part.

Progress In a typical solar-driven interfacial evaporation system (Fig. 1), the top layer of photothermal materials enables sunlight to thermal conversion under solar radiation. In contrast, the bottom layer of the substrate supports fluid movement and has low thermal energy loss. The water molecules evaporate at the top surface to steam and then condense to fresh water. Compared with traditional photothermal distillation, this interfacial evaporation technology can significantly reduce heat loss and acquire high efficiency in producing clean water. It also has advantages in evaporation efficiency, clean energy utilization and environmental friendliness compared to the traditional seawater desalination technology.

With the rapid development of advanced nanotechnology and material science, various photothermal materials, such as metals,semiconductors, carbon-based materials and polymers have been designed into solar interfacial evaporators. It is well known that wood has the advantages of naturally abundant microporous channels, large surface area, low thermal conductivity, good buoyancy,high mechanical strength and cost-effectiveness. Thus, wood is an ideal supporting substrate for fabricating various solar evaporators. In addition, wood-based evaporators have shown great potential in the process, cost, energy efficiency and environmental sustainability of solar-powered interfacial evaporation.

Various types of wood-based materials, from wooden membranes to wooden blocks, from solid wood to wood particles or wood fibers, have been rapidly used in solar evaporators. The evaporation performance and photothermal conversion ability can be improved by rational designing the evaporator structure, such as changing the photothermal interface and vapor transportation position. Wood-based evaporators usually comprise a photothermal conversion layer and a substrate (Fig. 2( a)). A three-layer wood-based evaporator consisting of a lower layer of wood, a middle layer of delignified wood, and an upper layer of iron-lead nanoparticles has been designed to increase the evaporation rate further. Under three solar light intensities, the evaporation rate reached 3. 28 kg·m^-2·h ^-1.

The top and bottom surfaces of wood evaporators are generally hydrophilic, and their water transfer efficiency can be improved through the capillary action inside the wood. However, the higher hydrophilicity of wood leads to the formation of a thick water film on the upper surface of the evaporator. It not only increases heat loss, but also generates salt crystals during the evaporation process and then reduces the evaporation efficiency. Drilling holes in the carbonized wood substrate to obtain a high-performance self-regenerating evaporator (Fig. 2(b)) could increase the evaporation rate and to prevent salt crystallization. The drilled channels with high hydraulic conductivities thus function as salt-rejection pathways, which rapidly exchange the salt with the bulk solution, enabling the real-time self-regeneration of the evaporator.

The absorbance of wood is much lower than that of other photothermal materials. Therefore, it is a very effective method to coat a layer of photothermal powder material on the surface of wood. Then, the top surface of these wood-based evaporators is generally black and has higher light absorption properties. All the widely studied photothermal powder materials of carbon-based materials, metals, semiconductors and polymers can be used. For example, a green evaporation system with ecological and economic advantages has been designed using lignin-derived carbon quantum dots incorporated into delignified wood (Fig. 3( a)).The evaporation rate was 1. 18 kg·m^-2·h^-1 under 1 sun (1 kW·m^-2). A novel bilayer evaporator has been fabricated by in-situ growth of carbon nanoparticles from incomplete combustion of paraffin candle flame on the wood surface (Fig. 3(b)). The resultant bilayer evaporator had a high evaporation rate of 2. 06 kg·m^-2·h^-1 with the photothermal conversion efficiency of 90% under 1 sun. It also exhibited long-term stability, self-cleaning capability and good anti-acid / base ability. Coating copper-based metal-organic framework (Cu-CAT) on wooden boards to form a wood evaporator (Fig. 4) could achieve an evaporation rate of 1. 80 kg·m^-2·h^-1. It has been reported that a wood evaporator consisted of two layers of a hydrophobic top layer (Fe₃O₄ ) and a super hydrophilic delignified wood bottom layer ( Fig. 5). The evaporation rate was 1. 3 kg·m^-2·h^-1. A bi-layered structure composite for highly efficient solar evaporation was developed based on photothermal-enhanced arginine-doped polydopamine and raw wood. The resulting wood composite had a vapor generation efficiency of 77% on the condition of 1 sun illumination and a long-time evaporation process of 100 h. Another wood evaporator prepared through the in-situ polymerization of polypyrrole (Fig.6) had a higher light absorption efficiency above 90% and a vapor generation efficiency of 72. 5%.

Conclusions and Prospects Solar-driven interfacial evaporator can make full use of sunlight, and it is a promising alternative to traditional desalination for obtaining, fresh water. Carbon-based materials, metals, semiconductors and polymers are commonly used as high-efficiency photothermal powder materials. The designed structural and functional wood evaporator utilizing the excellent performance of wood can improve the low water transmission rate, poor durability and other defects. Then the solar-driven interfacial evaporation technology based on photothermal materi l and wood provides a new idea for clean water collection and solving the shortage of freshwater resources. 

To obtain a multi-functional wood-based solar evaporator with high photothermal conversion efficiency, it is suggested that new types of photothermal conversion materials should be continuously synthesized, and the cost and difficulty of the preparation process should be reduced. The heat and mass transfer mechanisms can be further investigated and the porous network of wood is controlled to match the rate of water phase transition and steam diffusion. Finally, assembling small wood blocks into large woodbased evaporators can be encouraged to improve the expansibility of the solar-driven interfacial evaporator.

Keywords: photothermal powder material; wood; solar energy; interfacial evaporation; wood-based evaporator

Get Citation:PAN Q, LI J, YAN L G. Research progress of photothermal material-wood evaporators for solar-driven interfacial evaporation[J]. China Powder Science and Technology, 2024, 30(1): 90-102.

CLC No: TK519; TB4         Type Code:A