In the past ten years, the large-scale use of clean water and the serious pollution of ecosystem have made human beings face unprecedented water crisis. Because water and energy are two crises, people hope to develop advanced water treatment technology with minimum carbon emission. In various water treatment technologies, people are making great efforts to use renewable energy (such as solar energy) to directly evaporate water.
Careful observation of the evaporation process in a conventional porous evaporator shows that the evaporation rate is actually determined by two steps (phase transformation and diffusion). In the first phase transition, water molecules evaporate from the surface of liquid water to water vapor. In the second phase transition, water vapor diffuses from porous structure to atmosphere. Previous studies have focused on increasing the evaporation area to maximize the phase transition in the first step. However, in the traditional evaporator design, due to insufficient diffusion, a large amount of water vapor remains in the hole (Figure 1a), and the diffusion of water vapor into the atmosphere has been the limiting step of evaporation.
Figure 1. Water evaporation in traditional evaporator and 3D interconnected porous evaporator
(A) The evaporation diagram in the traditional evaporator is limited by the weak diffusion of vapor in the closed hole. The vapor produced by the evaporation surface stagnates in a closed hole, resulting in ineffective diffusion and evaporation.
(B) Schematic diagram of effective evaporation of interconnected holes. The interconnected porous structure allows the effective diffusion of vapor by convection.
(C) The schematic shows the design concept of a 3D interconnected porous evaporator for high evaporation rates. Its 3D structure provides enough evaporation surface, can effectively evaporate, has excellent hydrophilicity (can be used for water supply) and high absorption of sunlight.
(D) The micro image of the interconnected porous structure ensures the effective steam diffusion under the natural convection flow.
The research team of Professor Zhu Jia of Nanjing University has proved that the carbon foam with three-dimensional interconnected porous structure can fully diffuse the steam in convection, so the evaporation rate is as high as 10.9 kg m -2 h -1, and the cumulative evaporation of outdoor 13h is 42 kg m -2. Due to the high evaporation rate and high throughput, all-weather and environmental protection, the evaporation wastewater treatment technology has a broad application prospect.
Therefore, in addition to the general considerations of optics, thermal management and water supply, the porous structure must be carefully adjusted to achieve effective phase transformation and steam diffusion (Figure 1b). There are some key requirements for evaporator design (Figure 1c). Firstly, it should have porous structure to provide enough evaporation surface. This porous structure should have excellent hydrophilicity to ensure timely water supply. Most importantly, all holes in the structure must be interconnected to ensure effective steam diffusion and natural convection (Figure 1D). Finally, to achieve a balance between the evaporation surface area and the effective diffusion space, it is necessary to optimize the pore size.
Preparation of evaporator
This design of the evaporator is realized by a unique gas assisted expansion and perforation process. As shown in Figure 2a, sucrose reacts with concentrated sulfuric acid in a cylindrical quartz mold. In this step, sucrose is dehydrated by concentrated sulfuric acid to become a carbon material with high solar absorption and excellent hydrophilicity (C6H12O6 / 6C + 6H2O). Subsequently, part of the carbon material reacts with concentrated sulfuric acid and generates a gas mixture (c + 2h2so4 / CO2 [+ 4so2 [+ 2H2O)), resulting in huge volume expansion (up to 700%) and three-dimensional (3D) structure. At the same time, since the gas generated inside always tends to escape to the surrounding environment, it will penetrate the structure and ensure that the porous structure is interconnected from the inside to the environment. As a result, three dimensional interconnected porous carbon foams (3D IPCF) were fabricated.
Figure 2. Manufacturing and characterization of 3D IPCF
(A) Manufacturing process of 3D IPCF. Sucrose is carbonated by concentrated sulfuric acid and then expanded and perforated with unique gas assistance.
(B) Photos of 3D IPCF. Scale, 1 cm.
(C) Optical micrograph of 3D IPCF. These interconnected holes range in size from tens of microns to a few millimeters.
(D) PM2.5 can pass 3D IPCF over time. This shows that the holes of the 3D IPCF are connected to each other to allow convection and air molecules to pass through.
(E) The XPS spectrum of 3D IPCF shows that it is made of carbon, oxygen and sulfur.
(F) The water contact angle of 3D IPCF shows that it has super hydrophilicity, because the water drops quickly penetrate into it.
(G) The light absorption spectrum and solar radiation spectrum of 3D IPCF.
Document link (DOI):
https://doi.org/10.1016/j.joule.2020.02.014
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