Commercially Available Activated Carbon Fiber Felt Enables Efficient

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Commercially Available Activated Carbon Fiber Felt Enables Efficient Solar Steam Generation Haoran Li, Yurong He, Yanwei Hu, and Xinzhi Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b18071 • Publication Date (Web): 22 Feb 2018 Downloaded from http://pubs.acs.org on February 23, 2018

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ACS Applied Materials & Interfaces

Commercially Available Activated Carbon Fiber Felt Enables Efficient Solar Steam Generation Haoran Li†,‡, Yurong He*,†,‡, Yanwei Hu†,‡,§, Xinzhi Wang†,‡,§ †

School of Energy Science and Engineering, Harbin Institute of Technology, Harbin 150001,

People's Republic of China ‡

Heilongjiang Key Laboratory of New Energy Storage Materials and Processes, School of

Energy Science and Engineering, Harbin Institute of Technology, Harbin 150001, People's Republic of China Corresponding Author *

Email: [email protected]

1

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract Sun-driven steam generation is now possible and has the potential to help meet future energy needs. Current technologies often use solar condensers to increase solar irradiance. More recently, a technology for solar steam generation that uses heated surface water and low optical concentration is reported. In this work, a commercially available, activated carbon fiber felt is used to generate steam efficiently under one sun illumination. The evaporation rate and solar conversion efficiency reach 1.22 kg m-2 h-1 and 79.4%, respectively. The local temperature of the evaporator with a floating activated carbon fiber felt reaches 48 °C. Apart from the high absorptivity (about 94%) of the material, the evaporation performance is enhanced thanks to the well-developed pores for improved water supply and steam escape, and the low thermal conductivity that enables reduced bulk water temperature increase. This study helps to find a promising material for solar steam generation using a water evaporator that can be produced economically (~6 $/m2) and with long-term stability. Keywords: solar steam generation, heat localization, commercially available materials, activated carbon-fiber felt, advanced water evaporator Introduction Using solar energy to generate steam has become possible for specialized applications like water desalination, medical sterilization, and even for electricity generation.1 Recently, Zhou’s group 2 reported a hybrid solar steam/electrical generator, which creatively found that evaporation from centimetre-sized carbon black sheets can reliably generate sustained voltages of up to 1 V under ambient conditions. It is the high temperature at the heating surface that leads 2


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to rapid water evaporation, 3 and induces a high salt concentration region underlying the interface. 4 Several novel evaporators, such as porous volumetric evaporators 5 , 6 and nanofluid-based evaporators7‒9 have been proposed in the last years. Current steam production methods require the heating of large quantities of liquid. To achieve this, optical concentrators are often employed to strengthen the local solar irradiance. In 2014, a research group led by Chen10 reported a new and very efficient method to produce steam with solar energy. Their method heats mainly the water surface during low solar irradiance and keeps the majority of the water at a low temperature. Using this method the evaporation efficiency reached 85% for a solar irradiance of 10 suns (1 sun = 1 kW m-2).10 This high performance depends strongly on the basic attribute and function of the absorbing structure. In general, steam generation requires the conversion of photons into hot-carriers. The fast-growing number of hot-carriers transferred from the light-absorbing materials, increasing the substrate temperature and ultimately heating the surrounding water to produce steam.11 Common approaches to enhance the evaporation rate are to modify both the thermodynamic and optical properties, and/or to change the transport capacity of the light-absorbing materials. The specific steps are as follows:12‒15 (1) Use a material with high absorptivity across the full solar spectrum such as carbon-based materials. (2) Reduce the thermal conductivity as much as possible to limit heat conduction loss to bulk water, especially for the heat conduction at the substrate–water interface. (3) Provide a porous network and hydrophilic surface to act as primary water source and vapor escape channels. Commonly used materials for solar steam generation can be divided into two categories: plasmon generating nanoparticles, 16 ‒ 19 and carbon-based substrates.8, 20 ‒ 22 The plasmon 3



generating materials, such as gold and silver, can absorb solar energy thanks to their surface plasmon resonance effect.23,24 This enables intense and localized heating as long as the light wave is longer than the particle diameter and therefore able to drive the evaporation process.18 However, these regular sub-wavelength particles only possess strong absorptivity in the visible-light range. Although the absorption spectrum can be expanded by changing the shape and/or sphericity of these particles, it is still narrower than the solar spectrum.25,26 To increase the evaporation efficiency, the energy, apart from the energy-gap bandwidth of the particles, should be absorbed by the base liquid. Carbon-based materials are black and show high absorption across the full solar spectrum. By taking advantage of the better absorption, these materials can achieve higher light absorbing capacity than plasmon generating nanoparticles. In addition, the lower thermal conductivity of these materials also reduces the heat loss to bulk water.13 Carbon-based materials like graphene,27‒29 graphene oxide,30,31 carbon nanotube,32,33 and their composites34,35 have already been investigated and used for solar steam generation. However, these materials show several disadvantages compared to activated carbon-fiber (ACF) felt. For example, although the synthesis of these carbon series materials with high sensitivity and acceptable reproducibility is available these years, the price is still relatively high. In the near future, it appears difficult or impossible to manufacture a non-expensive and large membrane or 3D structure with these materials. For the carbonized woods, it is well known that the carbonized layer is easy to exfoliate, making the device fragile and hard to scale up.36 Additionally, if they were immersed in water for a long time, the secondary pollution of water would be caused. 4


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Here, for the first time, we conducted experiments at room temperature to investigate the solar steam generation capacity of a commercially available ACF felt. The evaporation enhancement mechanism, facilitated by the floated ACF, is identified and discussed. ACF felt is a promising adsorption material with many attractive properties including a high specific surface area, great adsorption, and high electrical conductivity.37‒39 Furthermore, due to its suitable absorptivity for light harvesting, low thermal conductivity (to reduce heat loss), and microchannels (for water and vapor transportation), it should be an excellent solar absorber capable of strong heat-localization. These advantages, together with the low price and long-term stable properties, make ACF felt a great candidate material that may finally enable widespread solar steam generation. The material does not float on a water surface. Fortunately, this problem can be solved by preinstalling insulation bubble wrap or insulation foam, which can reduce heat loss to the bulk water further.40 Results and Discussion To heat mainly the water surface, a bilayer structure, composed of a thick insulation foam layer and a thin ACF felt layer, was built (see Section S1 and Figure S1, Supporting Information). The diameter of the floated ACF felt is slightly smaller than the inner diameter of the chamber to ensure it can move freely during the evaporation process. The insulation foam provides buoyancy to the bilayer structure, and ensures there is little heat loss to bulk water in the evaporator (Figure S1a). The water diffuses in the sample freely, when it contacts the ACF felt directly through a narrow and annular channel (Figure S1b). The bilayer structure is a promising option for future efficient solar steam generation (Figure 1a) due to its excellent light-absorption. 5



To characterize the intrinsic optical properties of the bilayer structure, its transmission and reflection spectra were measured using the measuring system described by Tan and co-workers41 (details are available in Section S2, Supporting Information). The optical transmission of the ACF felt is close to zero for the solar spectrum, and the reflection at AM 1.5 G is as low as 5.8% thanks to its rough surface (Figure 1b). Therefore, about 94% of the incident solar energy would be absorbed by the top absorbing layer. We also measured the optical properties of the bilayer structure to understand the underlying process better. As shown in Figure 1c, the reflection of the bilayer structure is 5.7% for the measured wavelengths and there is practically no optical transmission. This confirms that the insulating foam layer has a negligible effect on the optical properties of the bilayer structure. As a result, our bilayer structure can absorb about 94% of the incoming solar energy.

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Figure 1. Optical properties of the bilayer structure. (a) A digital image shows solar steam generation enabled by the bilayer structure under 5 sun illumination. (b) Reflection and absorption spectra of the ACF felt in wavelengths of 300–2500 nm, and the inset shows transmission spectrum. (c) Similar to (b) but for the bilayer structure. Field-emission scanning electron microscopy (FE-SEM) was performed using a Zeiss Supra 55 (Oberkochen, Germany) electron microscope. Figures 2a,b show top- and side-views SEM 6


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images of the ACF felt. The carbon strips are randomly intertwined with big apertures, which serve as fast escape channels for the generated steam as well as paths for the transportation of water. Water enters the pores through capillary action, supplying water for local surface evaporation. During evaporation, the internal water directly contacts the strips, which are composed of coarse fibers (Figure 2c). The element mapping reveals the uniform distribution of carbon element in a carbon strip (Figure 2d). Therefore, the porosity of the material affects the water transport, which, in turn, affects the evaporation rate. We determined the porosity of the evaporator by measuring the specific weight before and after full absorption of water (Section S3, Supporting Information). As shown in Figure S2a, the calculated results show that the ACF felt evaporator has a porosity exceeding 93%.

b

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Figure 2. The porosity of the ACF felt. (a) Top-, and (b) side-views of the ACF felt. (c) SEM image of a coarse fiber. (d) The element mapping reveal uniformly overlaying of carbon element in a carbon strip. The other important property of the ACF felt is its efficient water supply when it is wetted. This can also improve the infiltration of underlying water to the top surface of the felt, as 7



schematically shown in Figure S1b. The presence of many apertures that contain air-cavities increases the air–water interfacial areas, associating with the free of any functional group, which makes the dry surface hydrophobic (Figure S2b). However, the air inside the apertures is expelled after water soaking, and the water fills the apertures through capillary action, which enables very efficient water supply. When water droplets hit the surface of a wet ACF felt, the water permeates the felt in less than half a second, which confirms the efficient water supply of the wet felt (Figure S2c). The water permeability of the wet ACF felt is revealed by the digital image in Figure 3a. When the water is in direct contact with the sample, it diffuses easily into the sample and infiltrates it completely within a few minutes. Clearly, the wetted surface uses the capillary force to channel the water flow to the hot region. The wetting process of an ACF after 2 s of touching cold water was captured by an IR thermal imager (Ti450, Fluke, USA) in dark. During this process, the sample, initially, is at a higher temperature than the cold water. As the wetting process continues, the capillary attraction facilitates longitudinal water transport, which reduces the sample temperature (Figure 3b). In other words, the capillary force in the ACF felt layer can enhance the evaporation rate of water through several process: the formation of thin films on the surface of an ACF felt, an enhanced surface area for evaporation, formation of three phase-contact lines at the edges of the capillaries.10 Thermal conductivity was measured using a laboratory-built test apparatus (Section S4 and Figure S3, Supporting Information). The measured thermal conductivity of the dry ACF felt is 0.095 W m-1 K-1 (Figure 3c), which indicates that it is a good candidate to facilitate heat-loss reduction. After soaking, the thermal conductivity increases to 0.430 W m-1 K-1 (Figure 3d). This value is also lower than that for pure 8


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water (0.609 W m-1 K-1 at room temperature42). Therefore, we confirm that a floating ACF felt in water can reduce heat-loss through non-evaporated bulk water.11,31

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Figure 3. Water transportation and thermal conductivity of the ACF felt. (a) Digital image shows a wet ACF felt after water diffusing. (b) IR images showing water infiltration process. Thermal conductivites for the (c) dry, and (d) wet ACF felts. The inset IR images showing temperature gradients at thermal equilibrium across the vertical direction of the sandwich structure. It was reported that efficient solar steam generation under natural solar irradiance without assistant heat exchangers and optical condensers will fundamentally improve the social value of solar vapor generation. 43,44 To evaluate the performance of an ACF felt-based evaporator systematically, both evaporation rate and energy conversion efficiency were determined by recording the mass change of water and temperature change of evaporator surface under one sun irradiance (Section S5 and Figure S4, Supporting Information). Figure 4a shows the recorded 9



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mass change of bulk water as a function of irradiation time for pure water evaporator and ACF felt-based evaporator under one sun as well as in the dark. The evaporation capacity during the initial period increases gradually as more incident photons are collected and transformed to thermal energy. This provides the latent heat enthalpy for the phase transition of water. However, after a specific, transient, initial period, the evaporation rate remains stable, which indicates that thermal equilibrium was reached between absorption, evaporation, and heat losses. The solar thermal efficiency was calculated using Equation (1):4,10

η=

& lv mh × 100% q

(1)

where m& is the evaporation rate of water under steady-state conditions (kg m-2 s-1). The value of m& in this work was derived from the absolute value of the straight-line gradient of the mass

change curve for irradiation times between 30 and 60 min, q = 1 kW m-2, which is the total incident solar irradiance at the surface of the evaporator, and hlv is the sum of enthalpy for liquid-vapor phase change, including sensible and latent heat enthalpy, can be calculated as: hlv = λlv + C p ∆ T

(2)

where λlv is the latent heat of vaporization of water at standard atmospheric pressure (2.257 MJ kg-1), Cp is specific heat capacity of water (4.2 kJ kg-1 K-1), and ∆T is the temperature rise of the water (Figure 4b). The fitting results show that the evaporation rates for water in the dark and under one sun are 0.22, and 0.55 kg m-2 h-1, respectively. When an ACF felt was floated, the evaporation rates decrease to 0.20 kg m-2 h-1 in dark and increase to 1.22 kg m-2 h-1 under one sun illumination 10


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(Figure 4c). Therefore, the solar thermal efficiencies for water in the dark and under one sun are 14.0% and 35.0%. When a floating ACF felt was used, the values drop to 12.6% in the dark and increase to 79.4% under one sun. To ensure that the produced steam is only the result of solar illumination, the evaporation rate of the evaporator in the dark at room temperature was subtracted. Therefore, under one sun, the solar thermal efficiency of the ACF felt evaporator with the floated structure is 65.4%, which is ~3.11 times of that for the pure water evaporator (21.0%). This confirms that the ACF felt can enable high-efficiency solar steam generation under one sun illumination. This improvement is facilitated by the higher absorbance, larger evaporation areas, high pore density, and the fibrous microstructures of the ACF felt. In the dark, the heat barrier (lower thermal conductivity) of the ACF felt can reduce the heat transfer between bulk water and the evaporation layer during the spontaneous evaporation of water, which leads to relatively low evaporation rates.45 In addition, it will increase the mass transfer resistance of steam due to the decrease of evaporation areas. Moreover, there is a continuous progression from multi-layer adsorption to capillary condensation in which the narrower capillaries became completely filled with liquid.46 In a narrow capillary the saturation vapor pressure is reduced, suppressing the evaporation process. Under one sun illumination, the evaporation rate using our new approach is only ≈3.9% lower than for the recently reported porous carbon black/graphene oxide bilayer body (1.27 kg m-2 h-1).45 Compared to the carbon foam supported evaporators,10 the insulation layer in our work has lower thermal conductivity and is waterproof, and the water could only contact the ACF felt via a 2-mm annular channel. It is clear that water can fill the pores of the carbon foam and more heat will transfer to bulk water and surroundings. Therefore, the 11



insulation foam supported evaporator in this work shows higher conversion efficiency. In general, thanks to its high evaporation rate, low price and long-term stable properties (see Section S5 and Figure S5, Supporting Information), ACF felt may enable commercially viable solar steam generation in the future. It is the geometry of the evaporator that helps reduce one-dimensional heat conduction losses. Our results confirm that the ACF felt evaporator provides localized heating, which makes it possible to maintain a lower temperature of the bulk water. To provide additional evidence, an IR camera was used to measure the temperature during irradiating under one sun. The surface temperatures, without and with ACF felt, are shown in Figure 4b as a function of illumination time. The initial surface temperatures of the evaporators, without and with the bilayer structure are 25.8 and 26.3 °C, respectively. Temperatures of the evaporator without ACF felt slightly increase during illumination, because that water cannot absorb the abundant solar energy in the visible light wavelength, and the light propagation distance in water is much longer than that in ACF felt. While for the evaporator with ACF felt, once after the light has been turned on, a rapid temperature increase is observed. Considering the increasing temperature, about 20 min are needed for the ACF felt-based evaporator to reach a quasi-static temperature. As expected, after illumination for 60 min, the surface temperature for the evaporators without and with ACF felt reach about 33 and 48 °C, respectively. Figure 4d shows typical IR images of the evaporators for one sun. Both evaporators show clear bulk water temperature increase, which suggests severe longitudinal heat conduction deterioration. However, the bulk water temperature is reduced when floating the ACF structure. This is because the absorbed solar energy is transferred preferentially 12


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into latent heat enthalpy and consumed to produce steam instead of heating the bulk water.8,47 It is important that our evaporator still does not operate optimally and a large amount of heat loss occurs. The heat loss includes inevitable convection and reflection losses (11.5% and 5.7%) from the top surface to the surroundings, the conduction loss to bulk water (15.8%), and the convection loss from the lateral and bottom surfaces of the chamber to the surroundings (3.2%) (more details for the calculation of heat losses are available in Section S6, Supporting Information). Future studies may focus on the structural optimization of the evaporator to minimize the avoidable heat loss to bulk water. One could also reduce the convection loss to the surroundings through the lateral and bottom surfaces of the evaporator. Additionally, to achieve the commercial and large-scale applications of the evaporators, some critical problem (including but not limited: how to use the steam generated by this kind of technology? How to avoid the condition that the sunlight does not directly illuminate to the heating material at the surface of water? How does the scale-up of evaporators affect the evaporation process) should also be addressed.

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Figure 4. Solar steam generation enabled by ACF felt evaporator. (a) Mass change of water as a function of irradiation time under one sun illumination and in the dark. (b) Surface water temperature profiles during illumination under one sun. (c) Steady-state evaporation rate (left) and solar thermal efficiency (right). (d) IR images for the evaporators without and with ACF felt. Conclusions In conclusion, a commercially available ACF felt was used for efficient solar steam generation under one sun illumination. An evaporation rate and efficiency of up to 1.22 kg m-2 h-1 and 79.4%, respectively, were achieved. While eliminating the effect of room temperature, the evaporation efficiency of the ACF felt evaporator under one sun was 3.11 times of that for pure water evaporator. In addition, during illumination, the top surface temperature for the ACF evaporator was higher than that for the water evaporator, which confirms that the ACF felt evaporator provides more localized heating. Overall, the higher evaporation efficiency of the 14


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ACF felt evaporator could be achieved thanks to the strong light absorptivity of the used materials, the well-developed pores that enable better water supply and steam escape, and the low thermal conductivity for reducing bulk water temperature increase. This study will help future solar steam generation through commercially available water evaporators with long-term stability. ASSOCIATED CONTENT Supporting Information. Design and optical image of the evaporator, schematic for the water flow process, optical properties, porosity, contact angle, and thermal conductivity measurement methods, specific weight change of the sample, details for the evaporation experiment, and energy balance analysis for the bilayer evaporator. AUTHOR INFORMATION Corresponding Author *Email: [email protected]. Author Contributions H. Li and Y. He conceived, designed and performed the experiments, and wrote the paper. Y. Hu and X. Wang helped to design the experiments and provided many useful ideas. All authors discussed the results and revised the manuscript. §

Y. Hu and X. Wang contributed equally.

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Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work is jointly supported by the National Natural Science Foundation of China (Grant No. 51676060), the Natural Science Founds of Heilongjiang Province for Distinguished Young Scholars (Grant No. JC2016009), and the Fundamental Research Funds for the Central Universities (Grant No. HIT. BRETIV. 201315).

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