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High-Efficiency Superheated Steam Generation for Portable Sterilization under Ambient Pressure and Low Solar Flux Chao Chang, Peng Tao, Jiale Xu, Benwei Fu, Chengyi Song, Jianbo Wu, Wen Shang, and Tao Deng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b04535 • Publication Date (Web): 03 May 2019 Downloaded from http://pubs.acs.org on May 3, 2019

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ACS Applied Materials & Interfaces

High-Efficiency Superheated Steam Generation for Portable Sterilization under Ambient Pressure and Low Solar Flux Chao Chang, Peng Tao*, Jiale Xu, Benwei Fu, Chengyi Song, Jianbo Wu, Wen Shang, and Tao Deng* State Key Laboratory of Metal Matrix Composites, School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai, 200240, China *Email: [email protected]; [email protected]

KEYWORDS: solar steam, sterilization, solar-thermal energy, interfacial evaporation, surface wettability

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ABSTRACT: Superheated solar steam generation above 100 oC is critical for many important applications such as sterilization, but is challenging to achieve under natural fluctuating low-flux solar illumination, and often requires pressurization and the usage of expensive optical concentrators. Herein, we demonstrate generation of superheated steam under ambient pressure and low-flux solar illumination by integrating recently emerged interfacial evaporation design into a solar vacuum tube. Within the tube, the water vapor, which is generated by a high-efficiency localized heating-based evaporator, is further heated by a heat exchanger into superheated steam without pressurization. The steam generator has shown tunable steam temperature from 102 oC to 165 oC and solar-to-steam conversion efficiency from 26% to 49% under one-sun illumination. Owing to the minimized heat loss from the solar vacuum tube and the interfacial evaporation design, it enables stable generation of steam above 121 oC under ambient fluctuating solar illumination with an averaged solar flux of ~600 W/m2. Effective sterilization is verified by using both the G. Stearothermophilus biological indicator and the E. coil bacteria, making portable solar steam sterilization and other steam-related applications feasible under ambient solar illumination.

INTRODUCTION Owning to its large capacity, cleanness, ubiquity and versatile conversion, solar energy is one of the most attractive renewable energy sources to propel sustainable development of human society.1, 2

Solar-thermal technology, which converts solar irradiation into thermal energy, is a facile and

efficient way to harness the abundant solar energy for a variety of heating-related applications.3-9 In these processes, steam generation often plays important roles, but the dilute natural solar flux cannot provide sufficient power density to produce steam due to the large vaporization enthalpy of water at 100 oC.10 To generate solar steam, current solar-thermal systems rely on using optical


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concentrators such as lenses, parabolic troughs and heliostats to concentrate the low-flux solar illumination by 10-1000 times to heat up the bulk water within the solar vacuum tube

6-8

or the

volumetric solar-thermal fluids within the transparent autoclave.9 These expensive concentrators, however, add significant cost to the solar-thermal system, and require extra complex supporting structure. In addition, the bulk heating-based steam generators often necessitate pressurization, which in turn raises strict requirements on the physical robustness of the steam generation system. Portable solar steam generators driven by natural solar illumination without the costly optical concentrators under ambient pressure would facilitate the broad application of solar-thermal technologies, in particular for households in the under-developed regions. In recent years, solar-driven interfacial evaporation, which localizes solar-thermal heating at the air-water interface, has emerged as an alternative way to traditional bulking heating-based evaporation for vapor and steam generation.11-14 Over the past years, the evaporation performance has been steady improved owing to the combined efforts from the development of advanced photothermal materials15, 16 including carbon-based,17, 18 polymer-based19 and plasmonic-based solar absorbers,20, 21 controlling water supply,22, 23 reducing heat loss14 and reducing evaporation enthalpy.24, 25 Currently, the interfacial evaporators have found diverse applications,11, 26-28 such as solar desalination,29-34 water purification,35-38 ground water extraction,39 distillation,40-42 and electricity generation.41-43 Most recent efforts have been made to exploring the high-efficiency solar-driven interfacial evaporation for steam sterilization applications.44, 45 Although the solar-tovapor conversion efficiency has achieved ~90% under one-sun illumination (1000 W/m2),

46-48

generating 100 oC steam most often still requires concentrating solar flux by ~10 times with optical lenses,13, 17, 47-49 and the solar-to-steam conversion efficiency is usually lower than 20% under onesun illumination.14, 50, 51 Compared to other applications, medical sterilization requires even higher


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steam temperatures, typically above 121 oC in order to effectively kill bacteria and ensure the health and safety.52 Generation of such high-temperature steam under low-flux solar illumination becomes more challenging. In previous solar-driven interfacial evaporation systems, it requires high optical concentration ratios more than 20

44, 45

and pressurization53 to achieve steam

temperatures higher than 121 oC. Another fact is that even the one-sun illumination flux (1000 W/m2) is often only achievable at sunny days during summer for a short period of time, and the ambient solar flux is fluctuating due to the roaming cloud.14, 33 Therefore, stable high-efficiency generation of superheated steam ( > 100 oC) under weaker fluctuating ambient solar illumination conditions is highly desired. In this work, we integrate the interfacial evaporator within a solar vacuum tube for highefficiency superheated steam generation under ambient solar illumination. Here the commercial double-walled solar vacuum tube serves as the solar collector with minimized heat loss by taking advantage of its high solar absorptance and low thermal emittance. The harvested solar-thermal energy provides heating to the porous evaporator and drives the high-efficiency interfacial evaporation to produce water vapor. The generated vapor is further heated by the copper meshbased heat exchanger inside the solar vacuum tube to form superheated steam without the necessity of pressurization. Through tuning the structure of the steam generator including the surface chemistry of the porous evaporator and the heat exchanger, and the relative length of the evaporation region over the heating region, the steam temperature can be tuned from 102 oC to 165 oC and the maximum steam generation efficiency can reach 49% under one-sun illumination. Outdoor experiments show stable generation of superheated steam above 121 oC under ambient fluctuating solar flux with an averaged solar illumination intensity of ~600 W/m2 (0.6 sun). Successful steam sterilization of a standard biological indicator and representative bacteria by the


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portable high-efficiency steam generator under ambient solar illumination was demonstrated.

EXPERIMENTAL SECTION Fabrication of Hydrophilic and Hydrophobic Copper Mesh The copper mesh (No. 40) was ultrasonically cleaned by acetone, ethanol and deionized water in turn, and then immerged within a mixed solution of 0.065M K2S2O8 and 2.5M KOH at 60 oC for 1 h. After such treatment, the hydrophilic copper mesh was obtained, and it was subsequently washed by deionized water, and dried at room temperature. The as-prepared hydrophilic copper meshes

were

further

modified

with

the

fluorinated

silane

(1H,

1H,

2H,

2H-

perfluorooctyltrichlorosilane) through a vapor deposition method by placing the hydrophilic meshes and the silane together within a vacuumed plastic desiccator to obtain the hydrophobic meshes.

Superheated Solar Steam Generator A double-walled solar vacuum tube (Shanghai Hanyan Industrial Co., Ltd.) with an inner diameter of 5.5 cm and a total length of 35 cm was used to construct the steam generator. Within the tube, it consists of a water reservoir, an evaporation region, and a steam heating region. The water reservoir is located at the sealed end of the tube (5 cm long) where the whole surface was coated with a getter coating to maintain the vacuum between the inner and outer glass tube. 100 g water was filled into the reservoir for the experiments. In the evaporation region, a layer of air-laid paper (with a thickness of 0.25 mm) and the treat copper mesh (with a thickness of 0.35 mm) were subsequently wrapped onto the surface of the polyurethane rod with a diameter of 5 cm, which was inserted into the vacuum tube. In the heating region, treated copper meshes with a diameter of 5.5 cm were used as heat exchanger, which transfers the heat from the vacuum tube to heat up the


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passing steam. The number of copper meshes was varied from 15 to 9 as the evaporation occupancy increases from 0 to 40%. The steam outlet was connected to the atmosphere by a copper tube with an inner diameter of 3 mm.

Steam Sterilization Experiment. In the indicator tube, the purple culture medium was sealed in a glass vial and a strip paper containing the G. stearothermophilus was located at the bottom. The indicator was placed near the steam outlet for sterilization after the steam generator reached the steady evaporation state. After sterilization for 20 min, the inner glass vial was broken to release the culture medium onto the strip beneath. The obtained sample was incubated at 55 °C for 24 h. When all the bacteria were killed, the color of culture medium remained purple. Otherwise, the color changed to yellow. A glass vial containing 5 mL of E.coli bacteria suspension was placed near the steam outlet within the solar tube after reaching the steady state. The glass vial was sealed by a filter paper (Taizhou Aoke Filter Paper Factory) with a pore size of 10-15 μm that allows steam diffusion into the vial and prevents the leakage of E. coli bacteria. After sterilization for 10 min, 100 𝜇L of solution was taken out of glass vial and dropped onto a nutrient agar plate. The original unsterilized E. coli bacteria solution was diluted by 105 fold. Both the undiluted sterilized solution and the diluted unsterilized solution were incubated at 37 oC for 24 h. The sterilization performance of the E.coli bacteria was measured by the standard flat colony counting method. Measurement and Characterization The optical transmittance spectrum of the borosilicate glass at the outer wall and the absorption spectra of selective absorption coating in the inner wall of the vacuum tube were measured by an UV-Vis-NIR spectrometer (PerkinElmer Lambda 750S). The solar vacuum tube was broken into


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pieces to directly measure the optical spectra. The microstructures of the air-laid paper and copper meshes were observed by a field emission high-resolution SEM (Sirion 2000, FEI) equipped with an energy dispersive spectrometer (EDS, INCA X-Act, Oxford). An X-ray diffractometer (Smart Lab, Rigaku) was used to analyze the treated mesh. A solar simulator (TRM-PD, Jinzhou Sunshine Technology Co., Ltd.) was used to generate simulated solar illumination, and the illumination power density was checked by a power meter (CEL-NP2000, Beijing China Education Au-light Co., Ltd.). K-type thermocouples and a multichannel data acquisition system (Agilent 34972A) were applied to monitor and record the real-time temperature of the steam generator.

RESULTS AND DISCUSSION Figure.1a schematically shows the structure design and working principle of the interfacial evaporation-based superheated solar steam generator. The generator is composed of a water reservoir, an evaporation region and a steam heating region (Figure 1b). The water reservoir is located at the end of solar vacuum tube where the surface is coated with a reflective barium getter coating. In the evaporation region, the water is wicked by the capillary force of the air-laid paper to the solar-heating surface for high-efficiency evaporation. A copper mesh is laid in between the air-laid paper and the inner wall of the vacuum tube to create a narrow gap for vapor generation. A physical supporter tube made of polyurethane is placed in the center of the tube to ensure intimate contact between the evaporation structure and the vacuum tube. In the steam heating region, a string of copper meshes transfer the converted solar-thermal energy from the inner wall of the vacuum tube to further heat the passing vapor or steam into superheated steam. The generated superheated steam perfumes into atmosphere through the outlet tube without causing pressure accumulation within the steam generator. With the combination of interfacial evaporation


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structure design and mature solar vacuum tube technology, the portable generator can stably produce superheated steam under ambient solar flux for solar steam sterilization.

Figure 1. Portable interfacial evaporation-based solar steam generator for high-efficiency superheated steam generation and sterilization. (a) Schematic working principle. The ambient sunlight is converted into thermal energy by a double-walled solar vacuum tube to drive evaporation and steam generation. Within the tube, the steam generator has three regions: a water reservoir, an interfacial evaporation region, and a steam heating region. The wicked water is efficiently vaporized by the copper mesh porous evaporator, and the generated vapor is further heated by the copper mesh-based heat exchanger into superheated steam for sterilization applications under ambient pressure. The dashed lines indicate capillary wicking of water towards the evaporator. (b) Structure design of the superheated solar steam generator. The inset image shows the structure of the evaporation region.

The commercial double-walled solar vacuum tube, which consists of two concentric borosilicate glass tubes with vacuum in between, is used as the solar receiver to provide solar heating with minimized heat losses. The outer tube has a high solar transmittance of 92% within the wavelength range from 250 to 2500 nm (Figure S1), thus allowing efficient receiving of sunlight onto the inner tube surface. Figure 2a shows that the inner tube coated with the low-emissivity (0.07) selective


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absorption layer (TiNOx) has a weighted solar absorptance of 93%, which favors solar-thermal energy conversion and suppressing the radiation heat loss.

Figure 2. Characterization of key components for the superheated solar steam generator. (a) Absorption spectrum of the inner tube for the double-wall solar vacuum tube and comparison to solar irradiance. (b, c) SEM images of the air-laid paper at low and high magnifications. (d) A photograph of the treated hydrophilic copper mesh. (e, f) SEM images of the treated copper mesh at low and high magnifications. The inset shows a water droplet on the treated superhydrophobic copper mesh.

The air-laid paper is used as the wicking material to deliver water from the water reservoir to the evaporation region. To visually observe water movement and evaluate its wicking capacity, we placed the air-laid paper in direct contact with a dye solution (Rhodamine B). The air-laid paper with a length of 6 cm was fully wetted by the red solution within 60 s (Figure S2). The corresponding water-feeding rate was 38.25 kg/m2 h, which is much higher than the theoretical maximum evaporation rate under one-sun illumination. This implies that the air-laid paper wicking


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can provide sufficient water supply to compensate the evaporation mass loss. Previously, it was reported that the air-laid paper had good durability for solar-driven interfacial evaporation.23, 36, 54 During the steam generation experiments, it was found that the air-laid paper has maintained its mechanical integrity and good water-wicking capability after continuous tests for more than 1 month without any perceivable degradation. SEM images reveal that the air-laid paper is made up of intertwined fibers (Figure 2b), and the diameter of each fiber is ~5 𝜇m (Figure 2c). The porous structure and surface hydrophilicity of the air-laid paper lead to its good water wicking performance. The copper mesh is another important component to construct the steam generator and achieve high-performance steam generation. To investigate the influence of surface wettability of the copper mesh on the thermal performance of the steam generator, we prepared both hydrophilic and hydrophobic meshes. The hydrophilic mesh was obtained by oxidizing the pristine copper mesh with a strong base solution. Figure 2d shows that after treatment the initial copper mesh turned black and the obtained hydrophilic mesh can be cut into different sizes and shapes for further experiments. SEM characterization indicates a rough surface (Figure 2e) and needle-like nanostructure on the treated hydrophilic copper mesh after surface oxidation (Figure 2f). To obtain hydrophobic mesh, the oxidized hydrophilic copper mesh was further treated with a fluorinated silane (1H, 1H, 2H, 2H-perfluorooctyltrichlorosilane) through vapor deposition. XRD and EDS characterizations indicate the formation of copper oxide and deposition of silane on the surfaces of the treated meshes (Figure S3). Unlike the pristine untreated copper mesh that has a water contact angle of 95o, the water droplet quickly spreads on the hydrophilic mesh surface (Figure S4). The water contact angle on the hydrophobic mesh was measured to be 138o (Figure 2f).


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Figure 3. Solar steam generation by the evaporator and heater with different surface wettability under a solar flux of 1000 W/m2. (a) Schematic of experimental setup for evaluating steam generation performance. The evaporated water was condensed within a flask on a digital balance to determine the evaporation mass change. (b) Temperature evolution profiles and evaporation mass change of steam generator. (c) Steady-state evaporation flux of the steam generator with different evaporator. (d) Schematic evaporation difference between the evaporator with hydrophilic and hydrophobic copper mesh. (e) Comparison of steam temperature for the generator with different steam heater. (f) Schematic superheating of the steam by hydrophilic and hydrophobic copper mesh heater.


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To evaluate the steam generation performance, the steam generator with an evaporation occupancy (𝜀) of 20%, which is defined as the length ratio of the evaporation region to the whole vacuum tube, was placed under simulated solar radiation with tunable illumination power densities. The temperature evolution was measured by the thermocouples placed in the water reservoir, near the outlet in the evaporation region, onto the inner wall of the vacuum tube at both the top illuminated surface and the bottom non-illuminated surface, and in the outlet at the steam heating region (Figure 3a). The evaporation mass loss was determined by measuring the mass change of the flask that collected the condensed water with a digital balance, based on which the evaporation efficiency (η) is calculated by: 𝜂=

𝑚ℎLV 𝑞solA

where 𝑚 is the evaporation mass flux, ℎLV is the total evaporation enthalpy change of water including both the sensible heat and latent heat of vaporization, 𝑞sol is the solar flux, A is the area of evacuated tube that receives solar illumination. Figure 3b shows that under one-sun solar illumination the top inner wall of vacuum tube has a quick temperature rise to ~140 oC in the first 30 min, then the temperature gradually increases to a temperature around 180 oC after 2 h. By contrast, the non-illuminated side showed much slower temperature rise and it reached a stabilized temperature at 100 oC after 100 min (Figure S5). Similarly, the water in the reservoir was slowly warmed up due to the low solar absorption of the getter surface. Temperature measurement at the outlets of the evaporation region and the steam heating region confirms that the produced water vapor was further heated into hot steam above 120 oC after passing through the heating region. It is worth noting that the temperature difference between the illuminated side and the non-illuminated side would cause non-uniform heating of the steam, but the superheated steam was homogenously remixed within the outlet tube before leaving


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the generator. The corresponding evaporation mass showed similar small change during the initial 60 min as it takes time for the vapor to diffuse out of generator. After that, the evaporation mass flow reached a steady state and the evaporation mass loss linearly increased with prolonged illumination time. Large-flux vapor generation at the evaporation region is the pre-requisite for high-efficiency production of superheated steam. We investigated the effect of surface wettability of the copper mesh evaporator on the evaporation performance. Figure 3c presents that the hydrophobic evaporator has the largest steady-state evaporation flux. When the steam generator reached the steady-state, the evaporation mass loss of hydrophobic evaporator reached 0.5 kg/m2, which is much larger than the hydrophilic evaporator (0.33 kg/m2) and the untreated evaporator (0.38 kg/m2). The significantly increased evaporation flux from the hydrophobic evaporator should be ascribed to the created narrow gap between the air-laid paper and the inner wall of the vacuum tube, which enlarges the evaporation surface and allows the generated vapor to perfume out. In the hydrophilic evaporator, both the air-laid paper and copper mesh are wetted by the wicked water, thus the evaporation mainly occurs at the edge of the copper mesh and air-laid paper (Figure 3d). Similarly, if without the copper mesh, the evaporation would mainly occur at the edge of the porous air-laid paper surface, which leads to even lower evaporation flux (0.28 kg/m2) (Figure 3c). Consequently, the hydrophobic evaporator was applied in the evaporation region for further experiments. In the steam heating region, the copper mesh heat exchanger was applied to transfer the heat from the inner surface of the vacuum tube to generate superheated steam. Figure 3e shows that without the copper mesh heat exchanger, the steam temperature was only 102 oC after one-sun illumination for 120 min, meaning that the steam was not effectively heated in the heating region.


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It was found that the temperature of the superheated steam is also affected by the surface wettability of heat exchanger. Under the same illumination conditions, the hydrophobic heat exchanger heated the steam to 135 oC, but the steam heated by hydrophilic heat exchanger and the untreated heat exchanger only reached 110 oC and 117 oC, respectively (Figure 3e). As schemed in Figure 3f, the water steam can condensed into droplets forming a layer of water film on the surface of hydrophilic copper mesh, which severely decreases the heat transfer coefficient of copper mesh heat exchanger. In comparison, it is difficult for the water steam to nucleate on the surface of hydrophobic copper mesh heat exchanger. Therefore, hydrophobic copper meshes were used as the heat exchanger in the steam heating region to generate superheated steam. It was also noticed that unlike pristine or hydrophilic copper mesh, which can be easily corroded in the hightemperature steam, the fluorosilane-coated hydrophobic copper mesh has maintained its surface morphology and water wettability after continuous operation of the steam generator for 7 days (Figure S6). As schemed in Figure 4a, we further investigated the influence of evaporation occupancy (𝜀) on the steam generation performance. It can be seen that if the 𝜀 is too large, there will be no enough heating region to heat the steam to a high temperature; if the 𝜀 is too small, the evaporation efficiency will be low because of limited evaporation area. By varying the evaporation occupancy from 0 to 40%, experimental measurements indeed confirm that the steam temperature gradually decreases with the occupancy (Figure 4b), and the steady-state evaporation flux has the opposite dependence on the occupancy (Figure 4c). Figure 4d presents that with increasing occupancy the steam temperature almost linearly decreased from 165 oC to 102 oC, and the steady-state evaporation efficiency increased from to 26% to 49% under one-sun illumination. For medical sterilization applications, the occupancy around 20% is a good selection to achieve balanced solar-


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to-steam conversion efficiency of 42% and a high steam temperature of 132 oC.

Figure 4. Impact of evaporation occupancy on solar steam generation. (a) Schematic of superheated steam generators with different evaporation occupancy. (b) Steam temperature evolution of generators with different evaporation occupancy. (c) Steady-state evaporation flux of steam generators with different evaporation occupancy. (d) Efficiency and steam temperature evolution.

To theoretically analyze the steady-state solar-to-steam conversion efficiency, we also built an analytical model (Note S1). According to the energy conservation principle: 𝑞sol ∙ 𝑇glass ∙ 𝛼𝑎𝑏𝑠 ∙ A = 𝑞rad ∙ A′ + 𝑞conv ∙ A′ + 𝑞evap

where 𝑇glass is the transmittance of borosilicate glass (0.92), 𝛼𝑎𝑏𝑠 is the absorptance of the selective absorber (0.93), A is the effective solar absorption area (259.05 cm2), A′ is the heat-dissipating area (518.1 cm2), 𝑞rad and 𝑞conv are the radiation and convection heat loss of the generator, respectively.


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It should be noted that for the vacuum tube only half of the surface can receive solar illumination, but the gained heat can be lost from the whole surface through convection and radiation. Fortunately, the double-walled structure of the vacuum tube leads to a low temperature (50 oC) at the outmost surface on the illuminated side (Figure S7), which in turn helps reduce the convection and radiation heat losses. The calculated solar-to-steam conversion efficiency increases with the evaporation occupancy, and the calculated results are in good agreement with experimental measurement (Figure S8). The solar vacuum tube-based steam generator also enables high-temperature steam generation under fluctuating solar illumination intensities lower than 1000 W/m2, which is more often for natural sunlight. By choosing the evaporation occupancy of 20 %, we further studied the influence of solar illumination flux on the steam generation performance. Figure 5a shows that with a solar flux of 600 W/m2 (0.6 sun), the stable steam temperature is 123 oC and further decreasing the illumination solar flux to 400 W/m2 (0.4 sun), the steam temperature dropped to 105 oC. To simulate fluctuating solar illumination, we periodically switched on and switched off the solar simulator after the steam generator reached the steady state. Figure 5b presents that repeatedly shining the tube for 2 min and turning off the one-sun illumination for 1 min the steam temperature is maintained between 127 oC and 131 oC. Careful examination shows that the drop and rise of the steam temperature profile has the same pace with the switching on-and-off of the solar illumination. This implies that the system has a fast response, which should be due to the advantageous interfacial evaporation design and minimized heat losses from the vacuum tube. When changing the switching-on and switching-off period to 30 s (on) and 30 s (off), and 1 min (on) and 1 min (off), the steam temperature can also be maintained above 121 oC (Figure S9). Different from the observed stable steam temperature in Figure 5b, the steam temperature gradually declines with


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prolonged time when the switching-on and switching-off periods are same. This is because only half surface of the vacuum tube can receive solar illumination, but the gained heat can be lost from the whole surface through convection and radiation. We further demonstrated that by choosing a switching-on period of 90 s and a switching-off period of 30 s the generated steam temperature can be kept between 128 oC and 131 oC during the whole 30-min testing period (Figure S10), thus enabling stable sterilization operation under natural sunlight with fluctuating solar flux. At last, we carried out outdoor experiments to evaluate the steam generation performance under the ambient solar illumination, and explored its application for sterilization. The outdoor experiment was conducted during August 2018 on the roof of the MSE Building H at Shanghai Jiao Tong University in Shanghai, China. Figure 5c shows the recorded solar flux from 11:00 am to 2:00 pm. To test sterilization performance, a commercial biological sterilization indicator was placed within the evacuated tube near the steam outlet (Figure 5d). Although the solar illumination intensity varied between 350 W/m2 and 700 W/m2, the steam reached a stable temperature of 121 oC

after solar irradiation for 75 min (Figure 5e). In the indicator vial, a strip containing spores (G.

Stearothermophilus) is located at the bottom section and another small glass vial containing the purple-colored culture medium is placed above the indicator strip (Figure S11). After the steam generator reaching the steady state, the indicator vial was placed near the steam outlet for 20 min for sterilization. Both the sterilized and the control indicator vials were compressed to release the culture medium onto the indicator strip. The inset images in Figure 5e show that after culturing at 56 oC for 24 h the sterilized vial has maintained the purple color, but the control sample changed into yellow, which proves successful sterilization.


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Figure 5. Solar steam sterilization under ambient solar illumination. (a) Steam temperature profiles under different solar fluxes. (b) Solar steam generation under periodically switching-on and switching-off solar illumination. (c) Solar flux of outdoor ambient solar irradiation. (d) Schematic of sterilization by the superheated solar steam generator. (e) Steam temperature evolution of the steam generator under ambient solar illumination. The inset photographs show the appearance of G. Stearothermophilus biological indicator before (yellow) and after sterilization (pink). (f) Photographs of bacteria E. coli on the agar plate before and after one-sun solar steam sterilization.


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We also demonstrated that the steam generator could be used for sterilization of bacteria. Figure 5f presents that after treatment within the steam generator under one-sun illumination for 10 min the E. coli bacteria were all killed. Complete sterilization of the E. coli bacteria was also observed when the solar flux was decreased to 600 W/m2 and 400 W/m2 (Figure S12). Although the steam temperature only reached 104 oC under the low illumination flux of 400 W/m2, it is sufficient for sterilizing the E. coli bacteria. The superheated steam with such temperatures could be generated during cloudy days (Figure S13), which would facilitate the wide application of the superheated steam generator for medical sterilization. In contrast to direct solar steam generation by using concentrated sunlight, here we achieved generation of superheated steam through reheating the low-temperature vapor or steam by the copper mesh heat exchangers. Such strategy avoids the usage of expensive optical concentrators (US200$/m2)14. We have shown that the superheated steam enables effective sterilization of both spores and bacteria. Although pressurization is beneficial for further improving the steam temperature and sterilization performance, it necessitates the usage of expensive pressure-resistant autoclaves. By comparison, the steam outlet in our device can help timely release the pressurized steam into ambient air and thus enables construction of the sterilization device with low-cost solar vacuum tubes. Such design also provides the possibility to make use of the escaped steam for other applications by connecting the outlet of steam sterilizer with other equipment. We previously reported that the air-laid paper-based solar-driven interfacial evaporator could generate clean water out of seawater and contaminated water sources.23, 36 This water purification capability would enable the widespread adoption of solar steam sterilization even in undeveloped regions where clean water is not available.


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CONCLUSIONS In summary, we have demonstrated high-efficiency stable generation of superheat solar steam under ambient solar illumination by fusing the mature solar vacuum tube technology and recently emerged interfacial evaporation design. Effective sterilization of both spores and bacteria by the generated steam were demonstrated. Considering that the steam temperature can be broadly tuned by varying the evaporation occupancy, thus the steam generator can be readily implemented for a broad range of applications that have different requirements on steam temperature. The reported interfacial evaporation-based superheated solar steam generator eliminates the need for solar concentration. This will help expand the usage of ambient solar-thermal technology into sterilization and other fields that involve the usage of superheated steam.

ASSOCIATED CONTENT Supporting Information Supporting information available: Transmittance spectrum of transparent outer glass; Wicking capacity test of the air-laid paper; XRD spectrum and EDS analysis of the treated meshes; Contact angle measurements for the copper mesh; Temperature evolution of different components in the superheated steam generator; Durability test of hydrophobic copper mesh; Steam temperature evolution under periodically switched solar illumination; Schematic structure of biological indicator for sterilization and outdoor experimental setup; Steam temperature evolution of the steam generator and solar steam sterilization performance under low solar fluxes; Heat transfer analysis of the superheated steam generator.


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AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] *E-mail: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT We thank the support from National Key R&D Program of China (2017YFB0406100), National Natural Science Foundation of China (Grant No: 51873105, 51521004 and 51420105009), Shanghai Rising-Star Program (Grant No: 18QA1402200), Innovation Program of Shanghai Municipal Education Commission (Grant No: 2019-01-07-00-02-E00069), Science and Technology on Monolithic Integrated Circuits and Modules Laboratory (614280303020317), and Interdisciplinary Program of Shanghai Jiao Tong University (YG2017QN68).

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